OS3.1 | Response of ocean biogeochemical cycles to past, present and future climate change
EDI
Response of ocean biogeochemical cycles to past, present and future climate change
Convener: Alessandro Tagliabue | Co-conveners: Yael Kiro, Charlotte Laufkötter, Netta ShalevECSECS, Christopher Somes, Camille RichonECSECS
Orals
| Thu, 27 Apr, 14:00–17:55 (CEST)
 
Room L2
Posters on site
| Attendance Wed, 26 Apr, 16:15–18:00 (CEST)
 
Hall X5
Orals |
Thu, 14:00
Wed, 16:15
Climate induced alterations to net primary production act alongside changes to biogeochemical cycling of oxygen and nutrients to affect marine ecosystem structure and function, as well as the ocean carbon cycle on decadal to centennial timescales. Climate change is driving alterations to these key components of ocean health, both via long term changes and the emergence of extremes. The 6th Climate Model Intercomparison Project provides new opportunities to analyze the long-term changes in biogeochemistry under different emissions scenarios, as well as to explore the emergence and potential impacts of extremes. Additionally, historical variability linked to climate oscillations such as ENSO and the Southern Annular Mode provide an opportunity to bring insights from observed changes and impacts. Moreover, isotope systems and proxies are often used in paleoclimate and paleoceanography across geologic timescales of climate change to interpret past environmental changes in Earth's history. Their interpretation relies heavily on these isotope systems' budget in the ocean.



This session invites submissions, from both observations and modelling efforts, that address the impact of climate change operating over multiple timescales on net primary production, biogeochemical cycling of nutrients and oxygen, and the ocean carbon cycle, including cascading effects for marine ecosystems to modulate biodiversity and ecosystem services.

Orals: Thu, 27 Apr | Room L2

Chairpersons: Charlotte Laufkötter, Christopher Somes, Alessandro Tagliabue
14:00–14:05
Ocean Biogeochemistry in a Changing Climate
14:05–14:25
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EGU23-14178
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solicited
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Highlight
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On-site presentation
Tatiana Ilyina

The ocean plays an essential role in regulating Earth’s climate; it is also essential for regulating the Earth’s biogeochemical cycles of carbon, nitrogen, and oxygen. Long-term (at the end of this century) changes in ocean biogeochemical cycles will be determined by the pace of anthropogenic emissions and resulting climate change. For the ocean carbon cycle, Earth system models (ESMs) within the 6th Climate Model Intercomparison Project project that while the ocean carbon sink continues to grow with rising emissions, the fraction of emissions that is taken up declines as atmospheric CO2 rises, resulting in a positive carbon–climate feedback. By contrast, near-term (until 2040) changes will be masked by internal climate variability. ESMs in concert with observations are key to constrain the response of ocean biogeochemical cycles to ongoing climate change. Yet, predictive understanding of how ocean biogeochemical cycles would respond to rapid and strong changes in emissions is currently missing. Such knowledge is critical in support of monitoring and verification of political actions for strong and rapid decarbonization. I will talk about recent progress and challenges in our understanding of the long-term and near-term fate of the ocean biogeochemical cycles under changing climate.

How to cite: Ilyina, T.: The changing ocean biogeochemistry in the Earth system, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14178, https://doi.org/10.5194/egusphere-egu23-14178, 2023.

14:25–14:35
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EGU23-958
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ECS
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On-site presentation
Pradeebane Vaittinada Ayar, Jerry Tjiputra, Laurent Bopp, Jim Christian, Tatiana Ilyina, John Krasting, Roland Séférian, Hiroyuki Tsujino, Michio Watanabe, and Andrew Yool

The El Niño–Southern Oscillation (ENSO) widely modulates the global carbon cycle. More specifically, it alters the net uptake of carbon in the tropical ocean. Over the tropical Pacific, less carbon is released during El Niño, while the opposite is the case for La Niña. Here, the skill of Earth system models (ESMs) from the latest Coupled Model Intercomparison Project (CMIP6) to simulate the observed tropical Pacific CO2 flux variability in response to ENSO is assessed. The temporal amplitude and spatial extent of CO2 flux anomalies vary considerably among models, while the surface temperature signals of El Niño and La Niña phases are generally well represented. Under historical conditions followed by the high-warming Shared Socio-economic Pathway (SSP5-8.5) scenarios, about half the ESMs simulate a reversal in ENSO–CO2 flux relationship. This gradual shift, which occurs as early as the first half of the 21st century, is associated with a high CO2-induced increase in the Revelle factor that leads to stronger sensitivity of partial pressure of CO2 (pCO2) to changes in surface temperature between ENSO phases. At the same time, uptake of anthropogenic CO2 substantially increases upper-ocean dissolved inorganic carbon (DIC) concentrations (reducing its vertical gradient in the thermocline) and weakens the ENSO-modulated surface DIC variability. The response of the ENSO–CO2 flux relationship to future climate change is sensitive to the contemporary mean state of the carbonate ion concentration in the tropics. We present an emergent constraint between the simulated contemporary carbonate concentration with the projected cumulated CO2 fluxes. Models that simulate shifts in the ENSO–CO2 flux relationship simulate positive bias in surface carbonate concentrations.

How to cite: Vaittinada Ayar, P., Tjiputra, J., Bopp, L., Christian, J., Ilyina, T., Krasting, J., Séférian, R., Tsujino, H., Watanabe, M., and Yool, A.: Response of the ENSO-driven CO2 flux variability in the equatorial Pacific under high-warming scenario, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-958, https://doi.org/10.5194/egusphere-egu23-958, 2023.

14:35–14:45
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EGU23-1356
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ECS
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On-site presentation
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Amber Boot, Anna von der Hyedt, and Henk Dijkstra

The Atlantic Meridional Overturning Circulation (AMOC) is thought to be a tipping element in the Earth System with two stable states. Currently, the AMOC is in a state of strong overturning, but studies have shown that climate change might tip the AMOC to a state of weak overturning. Changing the state of the AMOC changes the global climate, but especially the climate in the North Atlantic. Due to disrupted heat transport the Northern Hemisphere is expected to cool, while the Southern Hemisphere is expected to warm. Besides effects on the climate, the AMOC also influences the carbon cycle by transporting nutrients and Dissolved Inorganic Carbon. Deep water formation in the North Atlantic is for example an important pathway of carbon from the surface to the deep ocean. It can therefore be expected that a weakening of the AMOC affects the marine carbon cycle and therefore also atmospheric pCO2. Here, we investigate the effect of a forced AMOC weakening on atmospheric pCO2 using simulations performed with the Community Earth System Model v2 (CESM2). We force the simulations with the emission driven SSP5-8.5 scenario and additionally, force the simulations with freshwater forcing in the North Atlantic Ocean. This so-called hosing weakens the AMOC on top of a weakening caused by the greenhouse gas emissions. We use these simulations to determine how much and through what mechanisms, an AMOC weakening influences atmospheric pCO2

How to cite: Boot, A., von der Hyedt, A., and Dijkstra, H.: The effect of forced Atlantic Meridional Overturning Circulation weakening on atmospheric pCO2, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1356, https://doi.org/10.5194/egusphere-egu23-1356, 2023.

14:45–14:55
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EGU23-6639
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On-site presentation
Chia-Te Chien, Markus Pahlow, Markus Schartau, Na Li, and Andreas Oschlies

The similarity of the average nitrogen-to-phosphorus ratios (N:P) in marine dissolved-inorganic and particulate-organic matter indicates tight links between those pools in the World Ocean. Here we analyse the sensitivity of marine biogeochemistry to variations in phytoplankton N and P subsistence quotas in an optimality-based ecosystem model coupled to the UVic Earth system model. Our results reveal distinct feedbacks between changes in the N and P quotas, N2 fixation, and denitrification that loosen the coupling between dissolved and particulate N:P.  We demonstrate the importance of particulate N:C and P:C for regulating dissolved N:P on the global scale, with oxygen concentration being an important mediator. Our analysis also reveals a potential interdependence between phytoplankton stoichiometry and global equilibrium climate conditions.

How to cite: Chien, C.-T., Pahlow, M., Schartau, M., Li, N., and Oschlies, A.: Phytoplankton physiology controls global ocean biogeochemistry (and climate), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6639, https://doi.org/10.5194/egusphere-egu23-6639, 2023.

14:55–15:05
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EGU23-6828
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ECS
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On-site presentation
Miriam Seifert, Cara Nissen, Björn Rost, Meike Vogt, Christoph Völker, and Judith Hauck

Phytoplankton growth is controlled by environmental drivers such as nutrients and light availability, temperature, and the carbonate system. Thereby, changes in one driver can modify the response towards another driver. These interactive effects are usually not considered in large-scale ocean biogeochemistry models, potentially leading to incomplete projections of future phytoplankton biomass. In the presented work, we first parameterized growth sensitivities to changes in the carbonate system. We then used the results of a meta-analysis on interactive driver effects in published phytoplankton laboratory studies to develop model parameterizations for dual driver interactions (carbonate system versus temperature, carbonate system versus light). The parameterizations were tested in the biogeochemistry and phytoplankton functional type model REcoM under present-day and future conditions. While future phytoplankton biomass decreases by a similar amount with and without driver interactions (5-6%), interactive driver effects become visible on a group-specific level. Once driver interactions are considered, the biomass of diatoms and small phytoplankton decreases by -8.1% and -5.0%, respectively, and the biomass of coccolithophores increases by +33.2% from present-day to future conditions on a global scale. In comparison, the biomass of diatoms, small phytoplankton, and coccolithophores changes by 0.0%, -9.0%, and -10.8%, respectively, in simulations without driver interactions. Hence, projections of the global future phytoplankton community shift towards a larger share of small phytoplankton and coccolithophores and a smaller share of diatoms if interactive driver effects are taken into account. Regionally, the effect of driver interactions is largest in the Southern Ocean, where diatom biomass decreases (-7.5%) instead of increases (+14.5%). In conclusion, our study reveals that model projections of future phytoplankton biomass may miss out important information on the future phytoplankton community composition and group-specific direction of change if driver interactions are not considered.

How to cite: Seifert, M., Nissen, C., Rost, B., Vogt, M., Völker, C., and Hauck, J.: Look ahead! Future projections of phytoplankton communities are altered by interactive effects of environmental drivers, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6828, https://doi.org/10.5194/egusphere-egu23-6828, 2023.

15:05–15:15
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EGU23-14290
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On-site presentation
Olaf Duteil and Wonsun Park

Climate change impacts atmospheric properties and circulation at different time scales, ranging from daily to millenial. We specifically assess here the impact of a change in atmospheric synoptic variability (ASV) (0-1 month) on mean upper ocean properties. In a first step, we disentangle the ASV and low frequency part in atmospheric fields originating from a climate change experiment performed by the Kiel Climate Model. In a second step, we use these fields to perform a set of sensitivity experiments to the change in ASV by employing a NEMO-PISCES configuration. We show that a decrease in ASV results in a slowdown of the mean ocean circulation and a global decrease in primary productivity. Our study highlights the need for more precise quantifications of the atmospheric synoptic variability in climate models and observations.

How to cite: Duteil, O. and Park, W.: Future change in atmospheric synoptic variability : impact on ocean circulation and primary productivity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14290, https://doi.org/10.5194/egusphere-egu23-14290, 2023.

15:15–15:25
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EGU23-11625
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On-site presentation
Dominik Hülse, Katharina Six, Daniel Burt, Fatemeh Chegini, Lennart Ramme, and Tatiana Ilyina

Nitrogen (N) plays a central role in marine biogeochemistry by regulating biological productivity, influencing the cycles of carbon, oxygen, and other nutrients, and controlling oceanic emissions of the potent greenhouse gas nitrous oxide (N2O). Although the marine N cycle consists of multiple chemical species, global biogeochemical models often employ simple parameterizations of N transformations and omit key tracers and processes. Here we present an extended numerical representation of the marine N cycle that includes explicit tracers for nitrate, dinitrogen, nitrous oxide, ammonium, and nitrite. The extended model simulates heterotrophic denitrification, DNRN, DNRA, anammox, and nitrification of ammonium and nitrite and thus allows for a detailed representation of a step-wise reduction of fixed nitrogen in hypoxic zones and oxidation of reduced N-species in oxic waters. The updated biogeochemical model is included in the new global ICOsahedral Non-hydrostatic Ocean model ICON-O developed at the Max Planck Institute for Meteorology. ICON-O features a flexible, triangular grid created by recursively subdividing the original 20 triangles of the icosahedron resulting, in our configuration, in an average resolution of 40 km and 235,403 triangles. We describe the tuning and spin-up of a pre-industrial control simulation and compare model results with global and local estimates to quantify the N cycle (e.g., rates of primary production, N2 fixation, nitrification, DNRN, DNRA, anammox). We further report results from a historical transient simulation focusing on N dynamics within the oxygen minimum zone of the Eastern Tropical South Pacific.

How to cite: Hülse, D., Six, K., Burt, D., Chegini, F., Ramme, L., and Ilyina, T.: An extended N cycle in the eddy-permitting global ocean model ICON-O, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11625, https://doi.org/10.5194/egusphere-egu23-11625, 2023.

15:25–15:35
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EGU23-2513
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ECS
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On-site presentation
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Xi Ruan, Damien Couespel, Marina Levy, Jiying Li, Julian Mak, and Yan Wang

Earth System Models utilized to predict the ocean physical and biogeochemical responses under anthropogenic climate change do not yet routinely resolve mesoscale eddies due to computational resource constraints, and ocean mesoscale eddies are often parameterized. Mesoscale eddy parameterizations are known to affect the large-scale ocean circulation through impacts on the stratification, leading to for example changes of the nutrient stream, which is known to have large-scale impacts for the nutrient supply. Here we examine numerical ocean models with different representations of the mesoscale eddies and their combined physical and biogeochemical response in a hierarchy of models differing in the horizontal resolution, ranging from a non-eddying model with eddy parameterization, eddy permitting models without and with eddy parameterization, and eddy resolving models that serves as the model truth. 

In the case of non-eddying models, we find that existing prescriptions can result in a "better" bulk biogeochemical response but for the wrong physical reasons, relative to the model truth. On the other hand, more recent parameterization schemes can improve the physical response in terms of the sensitivity to changes in forcing, but the biogeochemical response is more subtle. It is confirmed here that the biggest change in the biophysical response stems from a model becoming eddy permitting. However, the eddy permitting model without parameterization is found to overshoot and be "too good" relative to the model truth in the biogeochemical response, attributed to the explicit eddies being too weak. Results are presented from a new approach that supplements the action of explicit eddies in eddy permitting model via eddy parameterization, but without adversely damping the explicit eddies. Combined with more recent parameterizaton schemes, the approach can lead to a biogeochemical response that compares very favorably to the model truth, attributed entirely to the improvement in the physical response in the eddy permitting model, so that we are getting the right answer for the right reasons.

How to cite: Ruan, X., Couespel, D., Levy, M., Li, J., Mak, J., and Wang, Y.: Assessment of the impact of mesoscale eddies on the large-scale physical and biogeochemical responses under climate change, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2513, https://doi.org/10.5194/egusphere-egu23-2513, 2023.

15:35–15:45
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EGU23-1001
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ECS
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Virtual presentation
Prima Anugerahanti and Alessandro Tagliabue

The importance of iron in driving net primary production (NPP) and the biological carbon pump across the Southern Ocean has been explored in numerous studies. However, the potential role for manganese, essential to oxygen production and combating oxidative stress, has not received the same attention despite the noted physiological inter-dependencies between iron-manganese and that both are strongly depleted in Southern Ocean. In the sixth climate model intercomparison project, earth system models (ESMs)  project increasing NPP in the Southern Ocean due to supply of additional iron, while the global trend shows a decline. Similar mechanisms also describe the role of the ocean carbon cycle during the last glacial maximum. However, under increasing iron supply, more manganese is required to fulfil phytoplankton growth, and the neglect of manganese limitation in ESMs can further increase the uncertainty of future NPP in the Southern Ocean under future or past climate change.

Here we use a hierarchy of experiments with the state-of-the-art global ocean biogeochemical model PISCES-QUOTA, including explicit manganese limitation, to explore how the physiological traits govern iron and manganese stress in response to a changing climate. Our results show that manganese is deficient throughout much of the Southern Ocean, but iron is generally the limiting resource. Explicitly representing iron and Mn co-limitation through oxidative stress enhances the extent of manganese deficiency, especially for diatoms. Traits associated with photophysiological adaptation and management of oxidative stress may be unique in Antarctic plankton and are critical in modulating the footprint of both iron and manganese stress and hence the impacts on the carbon cycle in a changing climate. Overall, our results indicate that both iron and manganese are key determinants of the impact of climate change on the Southern Ocean, with a notable role for region-specific adaptive and acclimatory responses that require further constraint.

How to cite: Anugerahanti, P. and Tagliabue, A.: The footprint of iron-manganese limitation of the biological carbon cycle in a changing climate, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1001, https://doi.org/10.5194/egusphere-egu23-1001, 2023.

Coffee break
Chairpersons: Yael Kiro, Netta Shalev
16:15–16:25
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EGU23-14222
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Virtual presentation
Fatemeh Chegini, Lucas Casaroli, Mariana Salinas, David Nielsen, Joeran Maerz, Moritz Mathis, Dominik Hülse, Lennart Ramme, Hongmei Li, and Tatiana Ilyina

Increasing computational power enables Earth System Models (ESMs) to be run at higher resolutions than on conventional grids with spacings of O(100km). The new generation of ESMs running at resolutions of O(5-10km) are able to resolve phenomena such as mesoscale eddies in the ocean and convective storms in the atmosphere. Resolving these features can be a step towards reducing uncertainties in carbon cycle modeling as they directly affect the ocean uptake of anthropogenic carbon (Harrison et al. 2018). However, the spin-up of ocean biogeochemistry in globally high resolution ESMs remains computationally challenging. Traditionally, the spin-up duration of ocean biogeochemical components (e.g., in CMIP5/CMIP6 models) ranges from one hundred to several thousand years to avoid model drifts in the euphotic, mesopelagic and even deeper ocean that has an overturning time of O(1000 years). This long spin-up time is, however, not yet computationally affordable in high resolution ESMs despite recent advances in improving their scalability (Linardakis et al. 2022). Therefore, different spin-up strategies are required and need to be explored.

We here present our strategy to run the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013, Jungclaus et al. 2022) in the high resolution ICON-Sapphire ESM (Hohenegger et al. 2022) configuration. We discuss the steps we take from tuning and spin-up of HAMOCC in a cascade of resolutions and configurations: initially in a 40km ocean only setup, subsequently in a 10km ocean-only and eventually a 5km ESM setup. Furthermore, we examine the possibility of replacing interpolated results (used as initialization for the next higher resolution) with available observations (e.g., nutrients, alkalinity, dissolved inorganic carbon) and its consequence on biogeochemical drifts of key tendencies such as CO2 surface fluxes. Finally, we discuss the scientific questions that can be addressed using this spin-up strategy and its limitations.

 

References:

Harrison et al. 2018: https://doi.org/10.1002/2017GB005751

Hohenegger et al. 2022: https://doi.org/10.5194/gmd-2022-171

Ilyina, T., et al. 2013: https://doi.org/10.1029/2012MS000178.

Jungclaus et al. 2022: https://doi.org/10.1029/2021MS002813

Linardakis et al. 2022: https://doi.org/10.5194/gmd-2022-214

How to cite: Chegini, F., Casaroli, L., Salinas, M., Nielsen, D., Maerz, J., Mathis, M., Hülse, D., Ramme, L., Li, H., and Ilyina, T.: Spin-up strategy for ocean biogechemistry in a high resolution Earth System Model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14222, https://doi.org/10.5194/egusphere-egu23-14222, 2023.

16:25–16:35
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EGU23-15521
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Virtual presentation
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Anjaneyan Panthakka and Jayanarayanan Kuttippurath

High concentrations of Chlorophyll-a (Chl-a) appear in the Arabian Sea (AS) during the winter and early spring seasons (November–March), known as the winter blooms, primarily due to the reversing monsoon winds and associated changes in the ocean. The onset, duration, intensity and peak period of the seasonal blooms show distinctive regional characteristics in AS. Recent changes in ocean dynamics and plankton composition in AS have adverse effects on the distribution of Chl-a concentration there. Here, we examine the long-term spatio-temporal changes in winter blooms, and assess the impact of wind, mesoscale eddies, surface currents, sea surface temperature (SST), mixed layer depth (MLD) and sea surface salinity on the blooms. We observe a significant decrease in these blooms, which started in the early 2000s and intensified in recent decade (2010–2020), with a prominent decline in the adjascent Gulfs (Gulf of Aden: -1.38 ± 0.7 x 10-5 mg m-3 year-1, Gulf of Oman: -4.71 ± 1.35 x 10-6 mg m-3 year-1), and the West coast of India (-6.71 ± 2.85 x 10-6 mg m-3 year-1). The major factors that control blooms in the Gulf of Oman and open waters are MLD and ocean temperature. On the other hand, in the Gulf of Aden, blooms are largely driven by the coastal upwelling and eddies. The bloom is primarily driven by winter cooling along the north-western Indian coast, but it is inhibited to south by the inter-basin exchange of surface waters carried by the West Indian Coastal Current. This study thus reveals particular mechanisms that trigger and regulate the winter blooms in AS. These seasonal blooms may continue to decline as a result of the ongoing ocean warming, which would be a threat for the regional marine productivity and food security.

How to cite: Panthakka, A. and Kuttippurath, J.: Spatial and temporal changes of the winter bloom in the Arabian Sea during the past two decades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15521, https://doi.org/10.5194/egusphere-egu23-15521, 2023.

Global ocean budgets
16:35–16:45
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EGU23-7079
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ECS
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solicited
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On-site presentation
Jianghui Du and Derek Vance

Rare Earth Elements (REE) are among the key tracers in modern chemical oceanography. Yet their budgetary imbalance in the modern ocean has limited our understanding of their marine biogeochemical cycling and their applications as paleo-proxies. The flux of REE across the sediment-water interface appears to be the dominant source of REE to the ocean. Most studies on the marine and sedimentary cycling of REE focus particularly on neodymium (Nd), which has a radiogenic isotope system that helps to constrain its ocean budget. Recently we developed a reactive transport model to study the diagenesis of Nd at a deep sea site on the Oregon margin, which successfully explained the distributions of Nd and its radiogenic isotope in pore water and authigenic sediment, and the diagenetic control of the benthic Nd flux to the ocean. Here we extend this model to include the whole REE series. We show that the transformation and fractionation of REE in sediment can be adequately modelled only via reactions between pore water, authigenic Fe/Mn oxides and phosphates. Using the model result we can scale the sedimentary sources of the rest of the REE elements to that of Nd, providing better constraints on the ocean budgets of all REE.

How to cite: Du, J. and Vance, D.: Modelling the sedimentary source and diagenetic fractionation of Rare Earth Elements in the ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7079, https://doi.org/10.5194/egusphere-egu23-7079, 2023.

16:45–16:55
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EGU23-8433
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ECS
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On-site presentation
Ben Cala, Olivier Sulpis, Mariette Wolthers, and Matthew Humphreys
Carbonate mineral dissolution has the potential to neutralise anthropogenic CO2 and act as a buffer against ocean acidification. To accurately quantify these effects and predict how this process will respond to a changing climate, we need to know what its drivers are. Still, our current understanding of dissolution is incomplete: for instance, excess alkalinity production in the mesopelagic suggests that carbonates dissolve where seawater is thought to be supersaturated with respect to calcite. In situ measurements of the dissolution rate can help to determine which other environmental factors - apart from the saturation state - drive dissolution. However, those measurements are scarce and even though they measure the same phenomenon, the resulting rates differ by up to two magnitudes between the available studies. Additionally, the dissolution patterns with depth are also not consistent with each other. Possible explanations for these variances include differences in methodologies and sample types, or the respective physical and chemical environments. This work aims to disentangle those factors and determine the real qualitative and quantitative value of the existing datasets. This is achieved through review of the literature and the training and interpretation of a supervised regression model (XGBoost), exploring the blind spots in our current conception of carbonate mineral dissolution. Based on these results, changes to the implementation of calcite and aragonite dissolution in Earth System Models are recommended.

How to cite: Cala, B., Sulpis, O., Wolthers, M., and Humphreys, M.: Dissolving better: what can Earth System models learn from 60 years of in situ carbonate mineral dissolution measurements, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8433, https://doi.org/10.5194/egusphere-egu23-8433, 2023.

16:55–17:05
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EGU23-1394
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ECS
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On-site presentation
Louise Delaigue, Olivier Sulpis, Gert-Jan Reichart, and Matthew P. Humphreys

Although global marine anthropogenic CO2 inventories have typically highlighted the North Atlantic as being the main feature of interest, southern hemisphere processes also play a key role in the changing marine carbon cycle. The South Subtropical Convergence (SSTC) in the South Atlantic, where low-macronutrient subtropical gyre waters intersect high-macronutrient Antarctic Circumpolar Current waters, is a key location for studying key processes such as the effects of climate change on the biological carbon pump. Here, we present a time series consisting of marine carbonate chemistry measurements from recent expeditions and global marine observations (GLODAPv2.2022 and BGC-ARGO) at 40°S in the Atlantic Ocean. We calculate the rates of change in dissolved inorganic carbon (DIC) and other related variables and use these to disentangle the apparent drivers of DIC change both natural (carbonate pump, Ccarb and soft tissue pump, Csoft) and anthropogenic (Canth). An increase in DIC is observed throughout the water column. The relative contribution of the Csoft and Canth components to the DIC change varies, however, significantly depending on whether we use nutrients or dissolved oxygen in the analysis. We discuss the causes of this discrepancy, using a water mass analysis to investigate the effect of water mass composition shifts in formation regions, and exploring the impact of reduced oxygenation and increased remineralization in the Southern Ocean, aiming to understand which tracer more accurately represents the changing carbon pump.

How to cite: Delaigue, L., Sulpis, O., Reichart, G.-J., and Humphreys, M. P.: Multidecadal change in natural carbon dynamics at the interface between Atlantic and Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1394, https://doi.org/10.5194/egusphere-egu23-1394, 2023.

17:05–17:15
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EGU23-16325
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On-site presentation
David Evans, Yair Rosenthal, Jonathan Erez, Hagar Hauzer, Laura Cotton, Xiaoli Zhou, Romi Nambiar, Peter Stassen, Paul Pearson, Willem Renema, Pratul Kumar Saraswati, Jonathan Todd, Wolfgang Müller, and Hagit Affek

The drawdown of CO2 via the temperature-dependent weathering of silicate minerals is thought to be one of the key processes acting to maintain Earth’s climate within narrow bounds over geologic time. However, the climatic responsiveness of weathering on multi-million-year timescales is, to our knowledge, yet to be demonstrated. If other factors dominate climate regulation on geologic timsecales, previously unexplored factors may be important in driving long-term carbon cycle changes. Here, we present the first continuous Cenozoic record of the concentration of calcium in seawater ([Ca2+sw]). Our record is based on the Na/Ca of exceptionally well-preserved foraminiferal calcite, a methodology which leverages the extremely long seawater Na+ residence time (>40 Myr) to interpret such changes predominantly in terms of [Ca2+sw] fluctuation. We show that a 12 mM decrease in [Ca2+sw] occurred over the last ~50 Ma, with a close correspondence to the timing of atmospheric CO2 changes, potentially implying a common driver. Using a carbon cycle box model, we demonstrate that, if the relationship between silicate weathering is shallower than commonly assumed, then this change in [Ca2+sw] can mechanistically explain the majority of the Cenozoic CO2 decrease, via the effect that Ca2+ has on CaCO3 burial rates. Given the recently identified major change in the global sea floor spreading rate, this finding shifts the key driver of long-term climate from the terrestrial to marine realm. Conversely, if there is a steep relationship between silicate weathering and climate, the climatic responsiveness of weathering is such that the system would rebalance before [Ca2+sw] can drive a major CO2 change. Our results therefore highlight the need to determine whether silicate weathering is responsive to climate change on geologic timescales before the long-term drivers of CO2 can be determined.

How to cite: Evans, D., Rosenthal, Y., Erez, J., Hauzer, H., Cotton, L., Zhou, X., Nambiar, R., Stassen, P., Pearson, P., Renema, W., Saraswati, P. K., Todd, J., Müller, W., and Affek, H.: The seawater calcium concentration may be a driver of long-term changes in CO2, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16325, https://doi.org/10.5194/egusphere-egu23-16325, 2023.

17:15–17:25
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EGU23-9965
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ECS
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On-site presentation
Celeste Cunningham, Simona Ruso, and William Arnott

Levees in modern deep-marine systems have been shown to sequester significant amounts of organic carbon, due largely to their wide expanse and high rates of sedimentation. However, relatively few studies have examined organic carbon sequestration in ancient deep-marine leveed slope channel systems. Examining the distribution of organic material in ancient levee deposits could provide insight into paleoenvironmental conditions and the evolution of ancient ocean and climate systems.

Deep marine levee deposits of the Neoproterozoic Windermere Supergroup are exceptionally well-exposed in the Southern Canadian Cordillera of western Canada, where detailed physical description and stratigraphic logging were combined with total organic carbon (TOC) and X-ray fluorescence (XRF) to evaluate trends in the distribution of organic carbon and elemental composition within a 300 m-thick succession. These geochemical analyses were then used to reconstruct paleoenvironmental conditions such as primary productivity, ocean redox, weathering intensity, and detrital flux. In this succession, TOC ranges from < 0.1% to 4.04% (uncorrected for the effects of greenschist metamorphism). Organic-rich strata, taken to be ≥ 1% TOC, are principally confined to a single 60 m-thick stratigraphic interval, where they typically occur as anomalously thick, mud-rich sandstone turbidites. Organic matter in these beds occurs mostly as micro-scale carbon sorbed onto the surface of clay grains but can also occur as uncommon sand-sized organomineralic aggregates or discrete sand-sized amorphous grains.

In this same interval, trends in elemental data indicate an increase in primary productivity, weathering intensity, and detrital influx, and a decrease in ocean oxygenation levels. These data suggest that intense continental weathering, high terrigenous input, elevated sea level, and relatively low oxygenation conditions all act to enhance organic matter production in shallow marine environments and organic matter accumulation and preservation in the deep marine. However, although all these components contributed to increased organic production, accumulation, and preservation on their own, the results of this study suggest that it is the temporal coincidence of all of them in a “perfect storm” that is required for significant organic carbon enrichment. Additionally, because these strata are Neoproterozoic in age the preserved organic matter is exclusively marine in origin, which then raises the possibility that the conditions described here are unique to deep-sea turbidite systems before the evolution of metazoans or terrestrial plants. 

By studying the geochemical trends of both organic-rich and organic-poor rocks in this ancient outcrop, this study helps to elucidate the role of various paleoenvironmental factors in deep-marine organic matter enrichment throughout geologic time. This will ultimately improve our understanding of the complex interplay of physical, chemical, and biological processes that govern marine sedimentation and their relationship with the carbon cycle and past global climate, particularly in systems that pre-date terrestrial vegetation.

How to cite: Cunningham, C., Ruso, S., and Arnott, W.: Paleoenvironmental Factors Controlling Organic Carbon Sequestration in Neoproterozoic Deep-Marine Levees, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9965, https://doi.org/10.5194/egusphere-egu23-9965, 2023.

17:25–17:35
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EGU23-15115
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ECS
|
Virtual presentation
Rong Hu

Deep-sea oxygen concentrations reflect combined effects of air-sea exchange in high-latitude surface waters, ventilation through ocean circulation and the organic carbon remineralization at depth. Reconstruction of past bottom water oxygen (BWO) concentrations has been challenging due to limitations of each existing BWO proxy whose fidelity may be complicated by diagenetic or depositional factors. Therefore, evaluations on BWO changes with multiple-proxy approach are always preferred. In this study, we exploit the authigenic uranium content on mixed planktonic foraminiferal coatings as a BWO proxy by presenting new foraminiferal U/Ca and U/Mn ratios of the Holocene and last glacial maximum (LGM) sediments from 54 sites throughout the Pacific Ocean, covering a range of modern BWO from 8-210 μmol/kg. As expected, foraminiferal U/Ca and U/Mn are influenced by sedimentation rates and organic carbon fluxes. Nevertheless, we also observe a negative correlation of Holocene U/Ca and U/Mn with BWO, with decreasing sensitivities towards higher BWO, suggesting the control of BWO on foraminiferal U/Ca and U/Mn variations. Based on the comparison of our foraminiferal U/Ca and U/Mn ratios between the Holocene and LGM and existing redox proxy data, we provide new constraints on Equatorial and South Pacific oxygenation changes during the LGM. First, the boundary between better oxygenated upper ocean and less oxygenated deeper ocean in the Eastern Equatorial Pacific was limited to a narrower water depth range between ~0.6 and 0.7 km. Second, our data imply better oxygenation in the upper and bottom waters of the Pacific Ocean and mid-depth deoxygenation, which contrasts with findings in the deep Atlantic and Indian Oceans. After excluding influences from other factors such as sedimentation rates and productivity, our study demonstrates foraminiferal U/Ca and U/Mn provide a useful proxy for BWO reconstruction in the Pacific, thus helping to constrain the glacial-interglacial oceanic carbon cycle.

How to cite: Hu, R.: U/Ca and U/Mn in foraminiferal coatings as a proxy for ocean oxygenation changes: new calibration and constraints on glacial oxygenation changes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15115, https://doi.org/10.5194/egusphere-egu23-15115, 2023.

17:35–17:45
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EGU23-15760
|
ECS
|
On-site presentation
Cornelia Mertens and Jordon Hemingway

The most important sulfur sink from a global redox-perspective is diagenetic pyrite produced in marine sediments. The amount and isotopic composition of this pyrite is thought to reflect environmental and physical properties of the ocean. This includes sulfate reduction rate, sulfate concentration, sedimentation rates, organic carbon and reactive iron concentrations and reactivities, porosity of the sediment etc. Our goal is to identify the main drivers that can explain the majority of observed sulfur isotopic composition in pyrite. To this end, we use a diagenetic model and calculate theoretical profiles for organic carbon, reactive iron, and a number of sulfur species and their isotopes in marine sediments. We calibrate our model using 216 sedimentary cores from a wide range of environmental conditions and locations from across the world. The model allows us to calculate burial rates and isotopic composition of pyrite in marine sediments on a global scale as well as infer drivers of the Phanerozoic pyrite d34S record. We show that isotopic composition of pyrite is determined by only three variables: the ratio of sulfate to organic carbon, the ratio of reactive iron to organic carbon, and the porosity. Based on this, we reinterpret Phanerozoic d34S trends as recording a shift in the locus and environmental conditions where pyrite is formed, rather than a change in microbial sulfate reducer fractionation.

How to cite: Mertens, C. and Hemingway, J.: Drivers of Sedimentary Pyrite δ34S Values - at Present and in the Past, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15760, https://doi.org/10.5194/egusphere-egu23-15760, 2023.

17:45–17:55
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EGU23-16545
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ECS
|
On-site presentation
Nurit Weber and Yael Kiro

Ocean chemistry is dictated by weathering and transporting elements from the land to the ocean and their removal through precipitation and adsorption. While the role of rivers was established many decades ago, other sources of elements such as mid-ocean ridge hydrothermal systems, discharge from forearcs of subduction zones, and submarine groundwater discharge (SGD) have been recognized more recently. SGD may release large amounts of trace metals, nutrients, carbon, and other dissolved species to the coastal ocean. The element fluxes may be comparable to surface water flow due to groundwater interaction with the aquifer sediments and the high ratio between rock and water. To better understand the coastal water budgets, it is crucial to assess all sources and sinks, including SGD. Recent attempts to calculate the dissolved inorganic carbon (DIC) and alkalinity ocean budgets have shown that riverine DIC input and marine carbonate burial cannot be balanced by alkalinity delivered via submarine groundwater. This deficit in the ocean's DIC and alkalinity mass balance may be attributed to insufficient knowledge of the carbonate and alkalinity contributions through SGD, particularly seawater circulation in the aquifer.

The DIC and alkalinity fluxes are influenced by the mechanisms driving groundwater flow in the subsurface (fresh and saline water), affecting the flow paths, residence times, and redox states. Therefore, we expect DIC enrichment or depletion to vary among different environmental settings. Thus, it is unclear if coastal aquifers serve as a source or a sink for DIC, depending on the settings. Our study shows a significant contribution of alkalinity by fresh groundwater discharge and also during long-term seawater circulation in the East Mediterranean coastal aquifer. Even on a relatively short distance like the Israeli Mediterranean coastline (~150 km), we observed differences in alkalinity and DIC derived from the shift in the aquifer's rocks as carbonate amounts drop and sand levels increase from north to south. To comprehensively and globally understand alkalinity fluxes through SGD, we generated an extensive data archive on coastal aquifers worldwide. This data is used to characterize the interactions between groundwater and country rocks depending on the type of rock and how they may impact groundwater alkalinity and DIC. Most of the groundwater samples lie below the 1:1 Alkalinity-DIC ratio line, which may suggest that the major processes affecting the carbonate system contribute more DIC than Alkalinity. Many sandy and carbonate aquifers have a DIC greater than alkalinity, which indicates a significant amount of CO2 and low pH levels. In contrast, alluvial aquifers have a minor trend, and basaltic aquifers usually have DICs equal to alkalinities (the most common species is bicarbonate). To better understand climate feedback mechanisms, it is crucial to know the ocean's alkalinity buffer system budget and its carbon sources and sinks.

How to cite: Weber, N. and Kiro, Y.: The role of submarine groundwater discharge in the ocean carbon budget, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16545, https://doi.org/10.5194/egusphere-egu23-16545, 2023.

Posters on site: Wed, 26 Apr, 16:15–18:00 | Hall X5

X5.369
|
EGU23-8227
Christopher Somes, Angela Landolfi, and Andreas Oschlies

Nitrogen and iron are the main limiting nutrients on phytoplankton growth in the ocean. Their nutrient budgets contain processes that are sensitive to environmental change including low oxygen thresholds. In this study, we use a global ocean biogeochemical model coupled within an Earth system model of intermediate complexity to quantify anthropogenic controls on marine nitrogen and iron cycling under warming and atmospheric nutrient pollutant scenarios. We performed model sensitivity simulations to isolate the individual and combined effects of marine nitrogen and iron cycle feedbacks on ocean productivity and deoxygenation. Our model simulations demonstrate strong stabilizing feedbacks when considering either the marine nitrogen and iron cycle individually. However, when the full set of marine nitrogen-iron feedbacks were included, enhanced nitrogen and iron source inputs outweighed sinks under anthropogenic scenarios. These marine nitrogen-iron biogeochemical feedbacks were responsible for driving a projected 4% increase in productivity and 27% expansion in the volume of oxygen deficient zones by year 2100 in the model, whereas a sensitivity simulation without these feedbacks resulted in a 9% decrease in productivity and 16% reduction in the volume of ODZs. Our model study suggests that positive amplifying feedbacks between the marine nitrogen and iron cycles may already be playing an important role increasing ocean productivity and deoxygenation in the Anthropocene.

How to cite: Somes, C., Landolfi, A., and Oschlies, A.: Simulating marine nitrogen and iron biogeochemical feedbacks and drivers of ocean productivity and deoxygenation in the Anthropocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8227, https://doi.org/10.5194/egusphere-egu23-8227, 2023.

X5.371
|
EGU23-11823
Akitomo Yamamoto, Tomohiro Hajima, Dai Yamazaki, Maki Noguchi-Aita, Akinori Ito, and Michio Kawamiya

Nutrient inputs from the atmosphere and rivers to the ocean are increased substantially by human activities. These increasing inputs of nutrients from human activities promote oceanic NPP, potentially partially counteracting decreases caused by climate change. Then, increases in export of organic matter to the ocean interior and its decomposition consumes dissolved oxygen. Therefore, nutrient inputs to the ocean promote carbon uptake and amplify climate-driven ocean deoxygenation. However, the previous generation of Earth system models that participated in the Coupled Model Intercomparison Project Phase 5 (CMIP5), which contributed substantially to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, did not account for anthropogenic nutrient inputs to the ocean. Several CMIP phase 6 (CMIP6) Earth system models do consider anthropogenic nutrient inputs to the ocean for the historical period, but their impact on ocean biogeochemical cycles has not been fully assessed, even for individual Earth system models. Therefore, our understanding of the impact of such perturbations on ocean biogeochemistry is even less complete than that associated with climate change. In particular, the quantitative relationship between the effects of climate change on ocean biogeochemical cycles and those of anthropogenic nutrient inputs remains poorly understood.

In this study, using historical simulations by one of the CMIP6 models (MIROC-ES2L) that considers anthropogenic nutrient inputs, we demonstrate that the contribution of anthropogenic nutrient inputs to past changes in global oceanic productivity, carbon uptake, and deoxygenation is of similar magnitude to the effect of climate change. In particular, two noteworthy results are obtained: (1) that anthropogenic fertilization could more than counteract the expected decrease in NPP caused by ocean warming and stratification for the historical period, and (2) that it could accelerate climate-driven deoxygenation in the upper ocean, helping to close the gap between models and observations. Additionally, current estimation of the imbalance in the carbon budget could be explained partially by increase in oceanic carbon uptake associated with anthropogenic nutrient inputs to the ocean. These improvements provide support regarding the significant contribution of anthropogenic nutrient inputs to global changes in ocean biogeochemistry. Considering the effects of both nutrient inputs and climate change is crucial in assessing anthropogenic impacts on ocean biogeochemistry.

How to cite: Yamamoto, A., Hajima, T., Yamazaki, D., Noguchi-Aita, M., Ito, A., and Kawamiya, M.: Response of the ocean carbon and oxygen cycles to climate change and anthropogenic nutrient inputs, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11823, https://doi.org/10.5194/egusphere-egu23-11823, 2023.

X5.372
|
EGU23-3348
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ECS
Chelsey Baker, Stuart Painter, Alessandro Tagliabue, Paul Halloran, Alban Planchat, Abigail McQuatters-Gollop, and Stephanie Henson

Biotic processes in the ocean play a crucial role in driving and mediating natural long-term ocean carbon storage. IPCC assessment exercises find high uncertainty, and therefore low confidence, around the magnitude and sign of change in future ocean carbon storage. This uncertainty is due to a lack of mechanistic understanding of relevant biological processes and/ or a paucity of observational data which limits robust parameterisations in global ocean biogeochemical models. Our study aims to identify and prioritise the processes that have a strong impact on future ocean carbon storage, with tractability from both a modelling and observational perspective. These processes could be the focus of future studies that aim to improve parameterisations in global biogeochemical models used in Earth System Models. We undertook a gap analysis to identify key processes and highlight future research priorities around three areas: net primary production (NPP), interior remineralisation and alkalinity. Here we evaluate CMIP6 model projections to 2100 under the high emissions SSP5-8.5 scenario to determine both the spread and uncertainty in NPP, particulate organic carbon transfer efficiency through the ocean interior and surface salinity-normalised alkalinity. We undertook a model interrogation of which processes are represented, their level of parameterised complexity and the variability in the parameterisation approach. Our analysis shows that CMIP6 models generally agree on the sign of change for transfer efficiency, but display a wide spread for NPP and salinity-normalised alkalinity by the end of the 21st century. Combining our analysis of CMIP6 models and gaps in knowledge allows the potential key processes and uncertainties driving future changes in key biological components of the ocean carbon cycle to be identified. By highlighting the potential gaps that require attention, the representation of biological processes in global ocean biogeochemical models can be improved in future modelling efforts. 

How to cite: Baker, C., Painter, S., Tagliabue, A., Halloran, P., Planchat, A., McQuatters-Gollop, A., and Henson, S.: Insights into biology’s role in future ocean carbon storage from CMIP6 models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3348, https://doi.org/10.5194/egusphere-egu23-3348, 2023.

X5.373
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EGU23-11212
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ECS
Helen Stewart, Rin Irie, Aya Suzuki, Masaki Hisada, and Keiko Takahashi

Marine phytoplankton play a vital role in global biogeochemical cycles, accounting for roughly half of global primary production (Beardall 2009). Climate change is expected to alter physical conditions in the ocean leading to a loss of functional diversity in phytoplankton (Dutkiewicz 2021). However, due to the complexity and scale of phytoplankton communities and the physical processes that shape them, the details of these changes are poorly understood. Ocean mixing, which governs nutrient and organism transport essential to phytoplankton communities, is thought to be of particular importance to plankton community evolution and divergence (Coles 2017). In this work, we aim to examine the effect of submesoscale processes on phytoplankton community productivity and divergence with computer simulation experiments. As a first step, we will represent physical mechanisms for submesoscale eddy formation by using MIT-GCM, because primary mechanisms for submesoscale eddy formation are thought to be baroclinic instability and drag vorticity generation due to ocean topography (McWilliams 2019).

Previous simulation experiments showed that increasing spatial resolution from mesoscale-resolving (~10 km) to submesoscale resolving scales (~2 km), allowed for the emergence of a denser vortex populations, resulting in an increased phytoplankton abundance (Levy et al 2012). In this study, the validity of this barotropic model assumption is examined at varying spatial horizontal resolution (~50 km, 10 km, 2 km). MIT-GCM simulations are performed in a baroclinic rectangular basin (3180 km x 2180 km) with a depth of 4000 m, representing an idealized portion of the North Atlantic Ocean. Furthermore, simulation results for a basin with idealized flat-bottom topography and more realistic topography are compared. The validity of barotropic model assumptions, significance of topography and potential effects on marine phytoplankton are discussed. In the near future, we have plans to extend this model simulation approach to a range of topographies, such as coastlines and continental shelfs, in order to discuss interaction mechanisms between oceanic physical processes and plankton distribution in those regions.

References
Beardall, John, Slobodanka Stojkovic, and Stuart Larsen. "Living in a high CO2 world: impacts of global climate change on marine phytoplankton." Plant  Ecology & Diversity 2.2 (2009): 191-205.
Coles, V. J., et al. "Ocean biogeochemistry modeled with emergent trait-based genomics." Science 358.6367 (2017): 1149-1154. 
Dutkiewicz, Stephanie, Philip W. Boyd, and Ulf Riebesell. "Exploring biogeochemical and ecological redundancy in phytoplankton communities in the global ocean." Global change biology 27.6 (2021): 1196-1213.
Lévy, Marina, et al. "Large-scale impacts of submesoscale dynamics on phytoplankton: Local and remote effects." Ocean Modelling 43 (2012): 77-93.
McWilliams, James C. "A survey of submesoscale currents." Geoscience Letters 6.1 (2019): 1-15.

How to cite: Stewart, H., Irie, R., Suzuki, A., Hisada, M., and Takahashi, K.: Effect of submesoscale dynamics and baroclinic instabilities on phytoplankton, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11212, https://doi.org/10.5194/egusphere-egu23-11212, 2023.

X5.374
|
EGU23-3474
Yael Kiro

Submarine groundwater discharge (SGD) is important to coastal aquifers' biogeochemistry and ecology. Most SGD is comprised of circulating seawater in the coastal aquifer. The circulating seawater is driven by several mechanisms with different spatial and temporal scales, from short-term/small-scale circulation driven by tides and waves through seasonal exchange driven by the sea- or groundwater-level changes and up to long-term/large-scale circulation driven by density differences. Although short-term circulation has been shown to affect groundwater chemistry and potentially modify the composition of seawater for some elements, long-term processes have the potential to affect elements (e.g., Na, Ca, K, Mg, Sr) that are controlled by long-term geochemical processes. These are not affected by the short-term/small-scale processes, thus allowing differentiation and quantifying the long-term density-driven circulation only. Being able to differentiate the different circulating seawater components is a critical step toward quantifying major elements fluxes from the coast into the ocean.

Our study presents a new compilation of worldwide coastal aquifers' data, which allows for determining the major elements' end-member composition of coastal aquifer groundwaters (see figure). Based on these compositions and their uncertainties, we could quantify the SGD flux of the long-term component due to density-driven circulation. Based on the Ca2+, K+, Sr2+, and 87Sr/86Sr ocean budgets, the calculated long-term SGD flux is 1117±487 km3/y (see figure). Although this flux is small compared to the global SGD water fluxes, it yields elemental fluxes of 8.0±3.5 Tmol Ca, -1.9±0.98 Tmol K, and 0.19±0.036 Tmol Sr per year, which are on the same order of magnitude as the fluxes through rivers.

End-member enrichment (+) and depletion (-)
Ca2+ - 13±8 mM
K+ - -1.48±1.11 mM
Sr2+ - 0.169±0.081 mM
87Sr/86Sr - 0.7089±0.00001

 

Figure 1: A global compilation of major elements enrichment and depletion in coastal aquifers’ groundwaters. The enrichment/depletion is calculated as the addition/deficit of a particular element compared to its expected concentration due to conservative mixing only.

Figure 2: Monte-Carlo simulation results of the long-term circulation (LTC) flux based on the different element and isotope budgets (up) and the combined distribution (bottom).

Major results:

Tmol/y

LTC

Rivers

Ca2+

8.0±3.5

13.2±1.3

K+

-1.9±0.98

1.9±0.4

Sr2+

0.19±0.036

0.033±0.006

How to cite: Kiro, Y.: The role of coastal aquifers in the composition of major elements in the ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3474, https://doi.org/10.5194/egusphere-egu23-3474, 2023.

X5.375
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EGU23-4034
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ECS
Netta Shalev, Boaz Lazar, Ludwik Halicz, Ittai Gavrieli, Tomaso Bontognali, and Derek Vance

Recent studies of seawater compositions of some ‘non-traditional’ stable isotope systems, such as 26Mg/24Mg (reported as δ26Mg), have uncovered great potential to enhance our understanding of the past Earth. However, differences between existing oceanic records, and the scarcity of such record data, currently limit this approach. Thus, new archives for these isotope compositions, independent of the commonly-used carbonate archives, are required. Marine evaporites have been widely used to decipher the chemical and ‘traditional’ isotope compositions (such as 87Sr/86Sr, S isotopes, etc.) history of past oceans. In several studies [1, 2, 3], we investigate the Mg isotope composition of different evaporite minerals and brines, presenting two examples: an experimental study of marine Mg-K (potash) salts and an in-situ study of pore-water and sediment from a modern sabkha environment.

The δ26Mg value of marine-derived brines and precipitating Mg-salts during the evaporation path of seawater were determined experimentally, up to a degree of evaporation (DE) of ca. 500. The sequence of Mg-salts included epsomite, kainite, carnallite, kieserite, and bischofite. We identify a mineral-dependent Mg isotope fractionation in both directions (i.e., some minerals are enriched in 26Mg relative to the brine, whereas others are depleted in 26Mg). Due to the precipitation of multi-mineral assemblages having opposite fractionations, the δ26Mg value of the brines changed only slightly throughout the evaporation path, despite considerable Mg removal.

The Mg concentrations and δ26Mg values of all pore-water samples extracted from the sabkha sediments are elevated relative to modern seawater and the closest evaporitic lagoon. Evaporation (DE range between 6.5 and 11), mixing, and Mg loss into dolomite are the three processes that determine the Mg concentration. Dolomite formation and mixing with ‘fresh’ lagoon water determine the δ26Mg values of pore-water. This shows that, in such evaporitic environments, the evaporitic minerals may precipitate from an already altered solution (with δ26Mg different than seawater), leading to a somewhat more complicated interpretation of the ancient record.

Based on these data, we suggest that by taking into account the complexity of evaporitic systems, the δ26Mg values of evaporites preserved in the geological record may be used to 1) quantify geochemical processes that fractionate Mg-isotopes within the basin, such as dolomite formation; and 2) complete the secular variations curve of the marine δ26Mg record using well-established evaporitic sequences.

 

[1] Shalev N., Lazar B., Halicz L., and Gavrieli I. (2021), The Mg isotope signature of marine Mg-evaporites. Geochimica et Cosmochimica Acta, 301, 30-47, https://doi.org/10.1016/j.gca.2021.02.032.

[2] Shalev N., Bontognali T.R.R. and Vance D. (2021), Sabkha Dolomite as an Archive for the Magnesium Isotope Composition of Seawater. Geology, 49 (3), 253–257, https://doi.org/10.1130/G47973.1.

[3] Shalev N., Lazar B., Köbberich M., Halicz L., and Gavrieli I. (2018), The chemical evolution of brine and Mg-K-salts along the course of extreme evaporation of seawater - An experimental study. Geochimica et Cosmochimica Acta, 241, 164-179, https://doi.org/10.1016/j.gca.2018.09.003.

How to cite: Shalev, N., Lazar, B., Halicz, L., Gavrieli, I., Bontognali, T., and Vance, D.: Exploring Mg-evaporites and sabkha dolomite as archives for seawater Mg isotope composition, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4034, https://doi.org/10.5194/egusphere-egu23-4034, 2023.

X5.376
|
EGU23-15878
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ECS
Maayan Yehudai, Lucy E. Tweed, Sean Ridge, Yingzhe Wu, and Steven L. Goldstein

Geochemical reconstructions of past deep water-mass structure show that the deep Atlantic Ocean basin changed dramatically over glacial-interglacial timescales and that these changes were tightly linked to the climate system and the carbon cycle. Over the past two decades, Neodymium-isotopes (εNd) has emerged as one of the major tracers of past oceanic water-mass structure. In today’s Atlantic basin, εNd patterns trace the deep water-mass structure, similar to conservative tracers such as salinity, showing more negative values in the North Atlantic and more positive, Pacific-like values in the South Atlantic. This spread of εNd values between two end-members reflects the age differences between the Archean-to-Paleoproterozoic-aged continental-cratonic sources eroding into the North Atlantic, and inputs into the Pacific from mantle-derived volcanics around the Pacific rim. Therefore, to estimate past changes in the mixing proportions of the Pacific and the North Atlantic end-members, one can theoretically use a simple binary mixing equation. However, although the end-member εNd-values can be traced through time by εNd analyses of appropriate deep sea core samples, two major issues plague the εNd proxy, one being variable and largely unknown non-conservative effects such as submarine groundwater discharge, boundary exchange reactions and sedimentary benthic fluxes and the second being unknown past Nd concentrations of the Atlantic and Pacific endmembers. Here we address the latter “paleo-[Nd] problem” with a Bayesian analysis that examines the sensitivity of Nd-isotope ratios to Nd concentration changes in the end-member water masses over glacial-interglacial time scales. Results show that even substantial variability in end-member Nd-concentrations ratio likely has little impact on Nd-isotope ratios at intermediate locations, indicating that Nd-isotope changes reflect the water-mass mixtures throughout the Pleistocene, thus supporting its use to reconstruct Atlantic water-mass structure in the past. This finding is an important step in validating the use of εNd as a past ocean-mixing proxy as it indicates that given the right location choice, where non-conservative effects are minor, the fraction of the different end-members can be quantitatively estimated using the binary mixing equation and the modern end-member Nd-concentration ratio.

How to cite: Yehudai, M., Tweed, L. E., Ridge, S., Wu, Y., and Goldstein, S. L.: The effect of Neodymium-Concentration Changes in Ocean Water Mass End-members on Neodymium-Isotopes at Mixing Locations: A Bayesian Approach, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15878, https://doi.org/10.5194/egusphere-egu23-15878, 2023.

X5.377
|
EGU23-4772
|
ECS
Haejin Kim, Hanna Kim, Kyeong Ok Kim, and Naoki Hirose

The East Sea (ES, or East/Japan Sea) is a semi-enclosed marginal sea where deep convection occurs and is therefore appropriate to identify the signals of climate changes. The substantial decrease in dissolved oxygen (DO) concentration has been observed in the deep layers of the ES compared to the globally averaged condition. In addition, in-situ and satellite-based measurement datasets as well as model results showed the enhancement of biological production in the upper layers. As such, changes in biogeochemical environments have been recorded in the ES due to the climate change, which also has a clear impact on long-term changes in the DO concentration.

This study investigates the alternations of biogeochemical environments, from the surface to the deep layer of the ES, using a coupled physical-biogeochemical model. This modelling study also allowed a quantitative estimation of the biological effect on the DO concentration.

How to cite: Kim, ., Kim, H., Kim, K. O., and Hirose, N.: Long-term changes in biogeochemical environments and their effects on dissolved oxygen concentrations in the East Sea, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4772, https://doi.org/10.5194/egusphere-egu23-4772, 2023.

X5.378
|
EGU23-7945
|
ECS
Yining Li, Wenhui Liu, and Netta Shalev

As a significant sink of seawater magnesium, δ26Mg values of syndepositional marine dolomites can be utilized to reconstruct seawater Mg isotope composition in the past. This, in turn, may serve as a good tracer for the Mg cycle throughout Earth’s history. Furthermore, δ26Mg values of ancient dolomites precipitated in different depositional environments may help in reconstructing the paleo-conditions that prevailed in these environments, in particular, the hydrology and restriction conditions of ancient sedimentary basins.

In this study, we collected Middle Ordovician marine dolomite samples from five profiles distributed in the different locations of the Ordos Basin, North China Plate. During the Ordovician, marine carbonates were deposited in the whole area of the Ordos Basin under different sedimentary settings that can be generally divided into open marine settings, a dolomite platform, and a saline gypsum lake. After systematic petrology observation (optic and cathode luminescence microscope), we microdrilled fine-crystallized dolomite to conduct chemistry and Mg isotope analyses. The dolomites from profile #1, which were deposited in an open marine environment, exhibit the lowest δ26Mg values between -2.31‰ to -2.21‰. The δ26Mg values of dolomite samples are generally heavier toward the more evaporitic center of the basin. The highest δ26Mg values (-1.84‰ to -1.70‰) were measured in samples from profile #5 located in the saline gypsum lake area.

We suggest that this gradient of δ26Mg values from the outer parts of the basin toward the center is resulting from a prior formation of dolomite in open versus gradually more restricted settings. Dolomite is enriched in the lighter isotope, 24Mg, relative to its precipitating solution. Thus, under restricted settings, dolomite formation will increase the δ26Mg value of the remaining dissolved Mg in the solution and consequently, also the δ26Mg values of further precipitating dolomites. This further suggests that the Mg isotope composition of dolomite can be used to reconstruct the enclosure degree of ancient evaporitic basins. Together with the observation that δ26Mg values of dolomite samples from profile #1 vary in a narrow range of 0.10‰ between different formations, this also suggests that profile #1 located in open marine settings can be utilized to reconstruct the Mg isotope signature of seawater during the Middle Ordovician.

How to cite: Li, Y., Liu, W., and Shalev, N.: Magnesium isotope signature of Middle Ordovician dolomites from the Ordos Basin, China , EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7945, https://doi.org/10.5194/egusphere-egu23-7945, 2023.

X5.379
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EGU23-12512
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ECS
Yana Kirichenko, Jörg D. Rickli, Tomaso R.R. Bontognali, and Netta Shalev

The geochemical cycle of strontium (Sr) is closely tied to the long-term inorganic cycle of carbon since the budgets of both elements are controlled by similar natural processes such as volcanism, continental weathering, and carbonates precipitation. As carbon dioxide is an important greenhouse gas, new insights into its cycle are fundamental for understanding climate variations in the past. Sr isotopes are a promising tool to advance our knowledge of the oceanic budget of Sr and, ultimately, the processes controlling long-term changes in atmospheric carbon dioxide and climate. While the widely used radiogenic strontium isotopes (87Sr/86Sr) can trace the oceanic Sr input fluxes, the stable-Sr isotope system (δ88/86Sr) is responsive to both oceanic input and output fluxes of Sr. Gypsum (CaSO4∙2H2O) is an evaporitic mineral containing significant concentrations of Sr reaching up to thousands ppm. The geological record contains information about numerous events of the formation of marine evaporitic giants, in which gypsum is one of the most volumetrically important rocks. Given the remarkable sizes of evaporite reservoirs and the considerable content of Sr in gypsum, this rock can play an important role in the cycling of Sr and its oceanic isotope budget. This study focuses on quantifying the fractionation of stable Sr isotopes during gypsum precipitation to address open questions regarding 1) the suitability of gypsum as an archive for seawater δ88/86Sr, 2) the potential impact of the evaporite sink on the global seawater δ88/86Sr, and 3) the role of marine evaporite rocks in the continental cycle of Sr.

Gypsum samples were produced experimentally in the laboratory and outdoors by evaporating natural seawater, and Sr isotope fractionation was found empirically by analyzing δ88/86Sr values of the precipitated solids and their respective solutions. Additionally, the experimental results were confirmed by studying natural samples, including modern gypsum and associated pore water from Dohat Faishakh Sabkha in Qatar and Messinian gypsum from Sicily.

The estimated typical Sr isotope fractionation in gypsum is 0.22±0.02‰. This positive value has the opposite direction compared to the negative Sr isotope fractionation in Ca-carbonate precipitation. However, the conducted experiments revealed a high variability of isotope fractionation values, ranging between 0.04‰ and 0.23‰ depending on the stirring environment. Therefore, the use of gypsum as an archive for δ88/86Sr in past seawater must be approached with caution since robust reconstruction would require careful investigation of the studied natural samples and their precipitation environment. Furthermore, given the found isotope fractionation, during periods of intense evaporite formation, the removal of Sr to gypsum can serve as leverage for a detectable global seawater δ88/86Sr change. Finally, weathering of gypsum is significantly affecting riverine δ88/86Sr and can explain at least 25% of the gap between the δ88/86Sr values of rivers (0.32‰ [3]) and source lithologies: carbonate (0.16-0.22‰ [2,4]) and silicate (~0.30‰ [1]) rocks.

[1] Charlier et al. (2012). EPSL. 329–330, 31–40.

[2] Krabbenhöft et al. (2010). GCA. 74, 4097–4109.

[3] Pearce et al. (2015). GCA. 157, 125–146.

[4] Vollstaedt et al. (2014). GCA. 128, 249–265.

 

 

How to cite: Kirichenko, Y., Rickli, J. D., Bontognali, T. R. R., and Shalev, N.: New insights into the Sr isotope budget from stable-Sr isotope fractionation in gypsum, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12512, https://doi.org/10.5194/egusphere-egu23-12512, 2023.