Displays

Palaeo-oceanographic reconstructions indicate that the distribution of global ocean water masses has undergone major glacial–interglacial rearrangements over the past ~2.5 million years. Given that the ocean is the largest carbon reservoir, such circulation changes were probably key in driving the variations in atmospheric CO₂ concentrations observed in the ice-core record. However, we still lack a mechanistic understanding of the ocean’s role in regulating CO₂ on these timescales. Here, we show that glacial ocean–sea ice numerical simulations with a single-basin general circulation model, forced solely by atmospheric cooling, can predict ocean circulation patterns associated with increased atmospheric carbon sequestration in the deep ocean. Under such conditions, Antarctic bottom water becomes more isolated from the sea surface as a result of two connected factors: reduced air–sea gas exchange under sea ice around Antarctica and weaker mixing with North Atlantic Deep Water due to a shallower interface between southern- and northern-sourced water masses. These physical changes alone are sufficient to explain ~40 ppm atmospheric CO₂ drawdown—about half of the glacial–interglacial variation. Our results highlight that atmospheric cooling could have directly caused the reorganization of deep ocean water masses and, thus, glacial CO₂ drawdown. This provides an important step towards a consistent picture of glacial climates.

How to cite: Marzocchi, A. and Jansen, M.: Global cooling linked to increased glacial carbon storage via changes in Antarctic sea ice, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1338, https://doi.org/10.5194/egusphere-egu2020-1338, 2020.

While paleoproxy records and modelling studies consistently suggest that North Atlantic  Deep Water (NADW) was shallower at the Last Glacial Maximum (LGM) than during pre-industrial times, its strength is still subject to debate partly due to different signals across the North Atlantic. Here, using a series of LGM experiments performed with a carbon isotopes enabled Earth system model, we show that proxy records are consistent with a shallower and weaker NADW. A significant equatorward advance of sea-ice over the Labrador Sea and the Nordic Seas shifts the NADW convection sites to the south of the Norwegian Sea. While the deep western boundary current in the Northwest Atlantic weakens with NADW, a change in density gradients strengthens the deep southward flow in the Northeast Atlantic. A shoaling and weakening of NADW further allow penetration of Antarctic Bottom Water in the North Atlantic despite its transport being reduced. This resultant globally weaker oceanic circulation leads to an increase in deep ocean carbon of ~500 GtC, thus significantly contributing to the lower LGM atmospheric CO₂ concentration.

How to cite: Menviel, L., Spence, P., Skinner, L., Tachikawa, K., Friedrich, T., Missiaen, L., and Yu, J.: A weaker Atlantic Meridional Overturning Circulation at the Last Glacial Maximum led to a greater deep ocean carbon content, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2920, https://doi.org/10.5194/egusphere-egu2020-2920, 2020.

We use a free running Last Glacial Maximum (LGM) setup of CESM1 with its full ecosystem model to understand which processes are responsible for the large difference in atmospheric CO2 concentration between the LGM  and 1850 CE.
Just by accounting for the changed orbital forcing  and replacing today's bathymetry and icesheet orography with their Peltier et al. (2015) LGM reconstructions, leads to a 55 ppm difference in atmospheric CO2.  Additional experiments with increased aolian iron fluxes make it plausible that IPCC class ESMs can reproduce the processes that were hypothesized to be important for the observed low LGM CO2 concentration.

A second focus of our study is the connection between sea level, ocean turbulence and the strengths of the various carbon pumps. Including the full amount of the suggested increase in ocean mixing during the LGM would lead to a 20 ppm larger CO2 concentration.This suggests that either mixing during the LGM is not understood yet, or that ESMs may indeed misrepresent one or more aspects of the various carbon pumps.

We conclude with a discussion of uncertainties within the model setup, in particular with regards to the assumed structure of ocean mixing.

How to cite: Jochum, M., Vettoretti, G., Chase, Z., and Nuterman, R.: Climate, Mixing, and Carbon Budgets in a LGM set-up of CESM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4837, https://doi.org/10.5194/egusphere-egu2020-4837, 2020.

The Southern Ocean westerly winds are hypothesised to play a key role in regulating atmospheric CO₂ over glacial-interglacial cycles; constraints on the paleo-latitude of the westerly winds have, however, remained allusive.  Here we use changes in the spatial pattern of planktic foraminiferal ∂¹⁸O to track changes in the latitude of the Southern Ocean polar and subtropical fronts over the last deglaciation, which are closely tied to the position of the westerly winds. We find a ~5° equator-ward shift in the position of the fronts (and thus westerlies) during the last glacial maximum relative to their Holocene position. Our reconstruction shows the poleward shift in the westerlies over deglaciation closely mirrors the sub-millennial scale variability seen in the rise in atmospheric CO₂. We propose that changes in the position of the westerly winds modulate CO₂ via changes in the extent of Southern Ocean sea ice and circulation of the abyssal ocean. Using climate model simulations, we explore the possibility of a feedback loop by which these CO₂/climatic changes may lead to further changes in the position of the westerly winds.

How to cite: Gray, W., Wills, R., Michel, E., and Kageyama, M.: Poleward shift in the Southern Ocean westerlies synchronous with the deglacial rise in atmospheric CO2, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9899, https://doi.org/10.5194/egusphere-egu2020-9899, 2020.

Increased carbon sequestration in the ocean subsurface is commonly assumed to have been one of the main causes responsible for lower glacial atmospheric CO₂ concentrations. This carbon must have been stored away from the atmosphere for thousands of years, yet the water mass structure accommodating such increased carbon storage continues to be debated. Here we present new sediment derived bottom water neodymium isotope data that allow fingerprinting of water masses and their mixtures and provide a more complete picture of the Atlantic overturning circulation geometry during the Last Glacial Maximums. These results suggest that the vertical and meridional structure of the Atlantic deep water mass distribution only experienced minor changes since the last ice age. In particular, we find no compelling evidence supporting glacial southern sourced water substantially expanding to shallower depths and farther into the northern hemisphere than today, which has been inferred from stable carbon isotope reconstructions. We argue that depleted δ¹³C values observed in the deep Northwest Atlantic do not necessarily indicate the presence of southern sourced water. Instead, these values may represent a northern sourced water mass with lower than modern preformed δ¹³C values that were further modified downstream by increased sequestration of remineralized carbon, facilitated by a more sluggish glacial deep circulation. If proven to be correct, the glacial water mass structure inferred from Nd isotopes has profound implications on our understanding of the deep ocean carbon storage during the Last Glacial Maximum.

How to cite: Pöppelmeier, F., Blaser, P., Gutjahr, M., Jaccard, S., Frank, M., Max, L., and Lippold, J.: Northern Sourced Water dominated the Atlantic Ocean during the Last Glacial Maximum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1353, https://doi.org/10.5194/egusphere-egu2020-1353, 2020.

The leading hypotheses proposed to explain the rise in atmospheric CO2 during the last glacial to interglacial transition proposes enhanced carbon transfer from the intermediate and deep oceans to the atmosphere via the intensification of southern ocean upwelling. To test this scenario, we generated a high resolution record of boron isotopes (d¹¹B) and B/Ca (proxies for pH and carbonate ion concentration, respectively) measured on shells of the benthic foraminifera C. wuellestorfi from a marine sedimentary core located at intermediate depth (1536m) on the Chilean margin. Our records confirm the link between changes in ocean circulation and variations in the carbonate chemistry at this site. The data also reveal the increase of intermediate water pH at the very late LGM, before the beginning of the deglaciation and the rise in atmospheric pCO₂. To account for this observation, we suggest the existence of an early release of carbon from the intermediate ocean to the atmosphere in response to sea ice retreat occurring at the same time. The lack of any clear increase in atmospheric CO2 suggests that this release of intermediate ocean carbon was compensated by enhanced biological pumping.

How to cite: Euverte, R., Michel, E., Bassinot, F., Rae, J., Gray, W., and Trudgill, M.: New pH evidence for changes in intermediate South East Pacific carbon storage during the last deglaciation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8652, https://doi.org/10.5194/egusphere-egu2020-8652, 2020.

For over 100 years scientists have puzzled over the mechanisms responsible for the repeated climate changes known as Ice Ages. A breakthrough was achieved when ice cores and marine archives revealed that the Ice Ages were paced at 100kyr intervals in alignment with Earth’s eccentricity cycle for the past million years. A second breakthrough was achieved when ice core records revealed that the Ice Ages were accompanied by ~80-90ppm variations in atmospheric pCO₂. But after decades of research the mechanisms responsible for those atmospheric pCO₂ variations remains an open and unresolved puzzle.

Here we present new findings that challenge the long-standing paradigm that geologic processes that regulate carbon exchange between the Earth’s interior and exterior act too slowly to have influenced the ocean and atmosphere carbon budgets on glacial time scales. The evidence includes large Δ¹⁴C excursions found in biogenic sediments in each of the Ocean basins at the last glacial termination. These excursions point to a sustained release of ¹⁴C-dead carbon spanning several thousand years.  In the Atlantic, Pacific and Indian Ocean the excursions are found near seafloor deformation features, including pockmarks that are indicative of gas-rich fluid release from sub-surface reservoirs. In the eastern equatorial Pacific, the Δ¹⁴C excursions are associated with enhanced hydrothermal metal concentrations including Fe, and Z that point to a hydrothermal source. Our ongoing research seeks to identify the storage and release mechanisms that operate on these carbon reservoirs on glacial time scales and to put constraints on the amount of carbon released at the last glacial termination. While the amount of carbon released from these geologic sources remains an open question for now, it is clear that geologic processes have affected changes in the global carbon budget on glacial time scales.

How to cite: Stott, L., Shao, J., Harazin, K., Davy, B., Pecher, I., Coffin, R., Reiss, L., and Suckale, J.: Storage/Release of Geologic Carbon Influenced Pleistocene Glacial/Interglacial Atmospheric pCO2 Cycles, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4241, https://doi.org/10.5194/egusphere-egu2020-4241, 2020.

The deglacial atmospheric CO2 increase has been attributed to a combination of mechanisms, many of which relate to the ocean outgassing triggered by changing marine physical and biogeochemical states. To quantify the impact of proposed processes and feedback on the deglacial CO2 rise, previous modelling studies mostly conducted time-slice sensitivity experiments. Here, we present results from a transient deglaciation simulation (24 kB.P. - 1850) using the comprehensive Max Planck Institute Earth System Model (MPI-ESM). We force the model with the deglacial atmospheric greenhouse gases (CO2, CH4, N2O) concentrations, obital parameters, ice sheet reconstruction and transient dust deposition. The ocean biogeochemical component of MPI-ESM is using the same automatical adjustment of bathymetry and land-sea mask in response to deglacial continental runoff and melt water discharge. In and around the areas of changing land-sea mask, we redistribute the marine biogeochemical tracers in accord with the simulated salinity. Terrestrial organic matter is transferred from flooded land areas to the ocean, which guarantees mass conservation with respect to carbon. We also include 13C tracers in the ocean biogeochemical component to evaluate the simulated ocean state against proxy data. The initial marine nutrients and carbon inventories are set the same as those in the present-day ocean. 
During the first 3 kyr, the climate and ocean state show, as expected, only modest variations. Some flooding events of coastal areas bring terrestrial organic matter to the ocean and lead locally to CO2 outgassing for several decades. Terrestrial organic matter has a higher carbon to nutrient stoichiometry as compared to marine organic matter, thus its remineralization favours CO2 outgassing. Additionally, the accumulation of terrestrial organic matter in the top layers of the marine sediment reduces the replenishment of the water-column nutrients by the re-flux of remineralization products from marine sediment. Consequently, the strength of the local biological pump decreases. Further results will be presented and discussed. 

How to cite: Liu, B., Six, K., and Ilyina, T.: Ocean carbon cycle during the last deglaciation in the Max Planck Institute Earth System Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3433, https://doi.org/10.5194/egusphere-egu2020-3433, 2020.

Marine plankton play a key role in climatic transitions through their ability to transfer atmospheric carbon dioxide (CO₂) to the deep ocean via the biological pump. It has been suggested that the lower atmospheric CO₂ concentrations during the Last Glacial Maximum (LGM) might have resulted from enhanced export production triggered by higher micronutrient (Fe, Si) availability from continental dust, particularly in the Southern Ocean. Such a scenario is consistent with higher sediment accumulation rates observed during the LGM.

In this study we use a new competition-driven ecosystem model that includes four major plankton types (diazotrophs, coccolithophores, diatoms and other general phytoplankton) to investigate their response to LGM climatic boundary conditions and to reconstructed micronutrient (Fe, Si) availability. We apply different dust fluxes, based on two plausible reconstructions (Mahowald et al., 2006 and Ohgaito et al., 2018). We compare LGM simulations with preindustrial simulations and disentangle the simulated ecosystem response due to climate forcing from the response due to micronutrient availability. We find that the ecosystem responses are complex and spatially heterogenic.

How to cite: Saini, H., Kvale, K. F., Meissner, K. J., Menviel, L., and Missiaen, L.: Modelled response of marine ecosystems to Last Glacial Maximum forcing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1370, https://doi.org/10.5194/egusphere-egu2020-1370, 2020.

The processes leading to the observed atmospheric CO2 variations of ~80 ppm between glacial and interglacial times associated with the glacial cycles of the past million years are still not fully understood. Computationally efficient Earth system models are a unique tool to help elucidate the mechanisms behind the CO2 variations. Here we use the newly developed Earth system model of intermediate complexity CLIMBER-X to explore the effect of different processes on the atmospheric CO2 evolution since the last glacial maximum using transient simulations.

CLIMBER-X includes the frictional-geostrophic 3D ocean model GOLDSTEIN coupled to the HAMOCC ocean and sediment carbon cycle model, the semi-empirical statistical-dynamical atmosphere model SESAM and the land model PALADYN. The model also includes the ice sheet model SICOPOLIS, but for in presented experiments the ice sheets are prescribed from reconstructions. CLIMBER-X can simulate ~10,000 model years per day.

In transient experiments of the last 20,000 years we test the sensitivity of simulated atmospheric CO2 to changes in ocean circulation, ocean temperature, sea level, atmospheric dust deposition and the model representation of crucial ocean biogeochemistry and land carbon cycle processes.

How to cite: Willeit, M. and Ganopolski, A.: Transient simulations of the last deglaciation with interactive carbon cycle using CLIMBER-X, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13761, https://doi.org/10.5194/egusphere-egu2020-13761, 2020.

Fluxes of particles and solutes between deep ocean and marine sediment are essential in the biogeochemical cycles of carbon and nutrients, such as nitrogen, silicon and iron. On a millennial time scale, sediment accumulation connects the ocean with the surface lithosphere which impacts the climate through weathering. Despite the importance of sediments in the climate system, fluxes between ocean and sediment are poorly constrained and most of the ocean models use very simplified parameterisation based on some measurements on shelves.

Here we like to present the coupling of the marine biogeochemical model REcoM2 (Regional Ecosystem Model, version2) coupled with the sediment model MEDUSA (Model of Early Diagenesis in the Upper Sediment with Adaptable complexity) for a better understanding of the role of sediments in the marine carbon cycle. MEDUSA resolves chemical reactions and physical processes within the marine sediments. As REcoM allows deviations from the Redfield C:N ratio both in phytoplankton production and remineralisation, the molar ratio of carbon and nitrogen in sinking fluxes vary with time and depth. Our MEDUSA set-up is made to be able to deal with flexible stoichiometry in sinking fluxes by resolving two classes of organic matter with different C:N ratios and degradation rates. We performed model-data comparisons of calcite, opal and particulate organic matter in sediment for present-day to constrain the biological productivity and sinking behaviour of particles in water column, and studied the role of the marine carbon cycle for glacial carbon storage and the drawdown of atmospheric CO2 in simulations under glacial climate conditions. 

How to cite: Völker, C., Ye, Y., Butzin, M., Köhler, P., and Munhoven, G.: Role of sediment in the marine C cycle—insights from a coupled ocean-sediment model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18562, https://doi.org/10.5194/egusphere-egu2020-18562, 2020.

More and more climate models now include the carbon cycle, but multi-models studies of climate-carbon simulations within the Climate Model Intercomparison Project (CMIP) are limited to present and future time periods. In addition, the carbon cycle is not considered in the simulations of past periods analysed within the Paleoclimate Modelling Intercomparison Project (PMIP). Yet, climate-carbon interactions are crucial to anticipate future atmospheric CO₂ concentrations and their impact on climate. Such interactions can change depending on the background climate, it is thus necessary to compare model results among themselves and to data for past periods with different climates such as the Last Glacial Maximum (LGM).

The Last Glacial Maximum, around 21,000 years ago, was about 4°C colder than the pre-industrial, and associated with large ice sheets on the American and Eurasian continents. It is one of the best documented periods thanks to numerous paleoclimate archives such as marine sediment cores and ice cores. Despite this period having been studied for years, no consensus on the causes of the lower atmospheric CO₂ concentration at the time (around 180 ppm) has been reached and models still struggle to simulate these low CO₂ values. The ocean, which contains around 40 times more carbon than the atmosphere, likely plays a key role, but models tend to simulate ocean circulation changes in disagreement with proxy data, such as carbon isotopes.

This new project aims at comparing, for the first time, the carbon cycle representation at the Last Glacial Maximum from general circulation models and intermediate complexity models. We will explain the protocol and present first results in terms of carbon storage in the main reservoirs (atmosphere, land and ocean) and their link to key climate variables such as temperature, sea ice and ocean circulation. The use of coupled climate-carbon models will not only allow to compare changes in the carbon cycle in models and analyse their causes, but it will also enable us to better compare to indirect data related to the carbon cycle such as carbon isotopes.

How to cite: Bouttes, N., Ivanovic, R., Abe-Ouchi, A., Kobayashi, H., Menviel, L., Oka, A., and Yamamoto, A. and the PMIP-carbon members: PMIP-carbon: towards a multi-models comparison of climate-carbon interactions at the Last Glacial Maximum, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13279, https://doi.org/10.5194/egusphere-egu2020-13279, 2020.

The Last Glacial Maximum (LGM) was characterized by increased carbon storage in the deep ocean, as well as extremely poorly ventilated southern-sourced deep water (AABW) compared to northern-sourced deep water (NADW).

Here we analyse benthic (Cibicidoides wellerstorfi) d¹³C, and compare 3 sites sitting on the deep floor at 5 km water depth: MD13-3473 in the Tore inside basin; MD03-2698 in the Iberian margin; and TN057-21 in the South Atlantic. The Tore Seamount is a geological structure 300 km off the West Iberian margin at 40°N latitude. It has a crater-like morphology with a 5500 m deep basin in its middle, where calypso core MD13-3473 was collected, confined from the open ocean by a summit rim at 2200 m water depth (wd). The only connection between the deepest Tore Seamount basin and the Atlantic circulation is a NE gateway down to 4300 mwd.

The results for the LGM show similar values around -1.0 ‰ for the South Atlantic and the Iberian margin, in other words these sites were both bathed by AABW. However, the Tore basin record exhibits values around 0 ‰, similarly to open sites in the Iberian margin at 3.5 km depth. This seems to indicate a remarkable isolation of the Tore inside basin from the Atlantic deep bottom waters influence.

Among other things, we plan to examine the residence time of the Tore basin bottom water by measuring the radiocarbon age difference between benthic and planktonic foraminifera. 

Our results confer to this enclosed environment the status of an in-situ deep ocean laboratory where to test hypotheses of past ocean circulation changes like the role of deep waters in sequestering glacial CO₂. Core MD13-3473 covers 430 thousands of years, therefore 5 deglacial cycles (Spanish project “TORE5deglaciations”, CTM2017-84113-R, 2018-2020).

How to cite: Antón, L., Lebreiro, S., Nave, S., Skinner, L., Michel, E., Waelbroeck, C., and Sierro, F.: How well is the deep Tore seamount basin ventilated?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11702, https://doi.org/10.5194/egusphere-egu2020-11702, 2020.

Mediterranean Outflow Water (MOW) acts as a net source of salt and heat into North Atlantic intermediate depths that ultimately contributes to the Atlantic Meridional Overturning Circulation. On this basis, it has been hypothesised that MOW variability might influence global climate. Although several studies have documented major glacial-interglacial changes in deep- and intermediate Mediterranean circulation patterns, little is known about associated impacts on MOW properties, in particular its residence time and geochemical signature. Using a set of cold-water coral samples from along the ‘pre-MOW’ and MOW path, i.e. from the Alboran Sea to the northern Galician Bank including the Strait of Gibraltar and the Gulf of Cadiz, we aim to identify changes in both the ventilation state of the water masses flowing out of the Mediterranean and the distribution of coral growth.With this purpose, paired Uranium-series and AMS radiocarbon ages have been obtained in the same coral samples allowing any potential change in the reservoir age to be inferred accurately, as well as allowing a spatio-temporal ‘coral map’ to be created. Furthermore, these results have been complemented by trace element measurements in benthic foraminifera from the Alboran coral mound sediment core.

Our results show a particular spatio-temporal coral distribution with glacial presence only at the deepest sites of the Gulf of Cadiz (~1000m), followed by ~300m Western Mediterranean (WMed) coral appearance across the deglaciation/mid Holocene (14-4 kyr), to end with a proliferation at the Strait of Gibraltar and Galicia Bank from ~6 kyr towards the present. We hypothesise 1) that ~300m WMed area might have been bathed in Atlantic waters inflow during the glacial due to sea-level drop, returning to LIW (Levantine Intermediate Water) influence over the deglaciation, and 2) that MOW reached deeper areas outside of the Mediterranean Sea in the Gulf of Cadiz during the glacial period. Regarding the reservoir age, little change at the WMed is observed at 150-450m across the deglaciation as compared to the large ventilation excursion detected in the Iberian Margin at ~1000m. However, a ventilation age gradient of ~300 yr related to water depth is observed within WMed corals when appearing at the Bølling-Allerød, in synchrony with significant changes in hydrographical parameters inferred from foraminiferal trace element from the same area. Overall, our results suggest a water mass reorganization at the surface-intermediate layer of the WMed during the deglaciation and early Holocene, but the ultimate nature of these changes needs yet to be explored by further analysis of Nd isotopes as well as of trace elements beyond the deglaciation.

How to cite: de la Fuente, M., Skinner, L., Ercilla, G., d'Acremont, E., Somoza, L., González Sanz, F. J., Lo Iacono, C., Corbera, G., Pena, L. D., Sadekov, A., Scott, P., Zhang, P., Cheng, H., and Cacho, I.: Inferring deglacial ventilation ages in Western Mediterranean waters using cold-water corals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20171, https://doi.org/10.5194/egusphere-egu2020-20171, 2020.

The release of old carbon via the Southern Ocean has been thought to contribute to the last deglacial atmospheric CO₂ rise, but underlying processes are not fully understood, in part, due to insufficient high-fidelity radiocarbon (Δ¹⁴C) reconstructions minimally complicated by age models and release of “dead carbon”. Here, we present a new deep-water Δ¹⁴C record for a core located at 3.3 km water depth from the Southwest Pacific, based on a robust age model using planktonic Mg/Ca along with co-existing benthic ¹⁴C measurements. Our results confirm previous records that suggest enhanced ventilation in the Southern Ocean during Heinrich Stadial 1 and the Younger Dryas. For the first time, our data show a large Δ¹⁴C decline during the Antarctic Cold Reversal, indicating strengthened stratification in the deep South Pacific. Our results strongly support that the deep ocean supplied old carbon to the atmosphere during the last deglaciation.

How to cite: Dai, Y., Yu, J., and Rafter, P.: Release of old carbon from the deep South Pacific during the last deglaciation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4230, https://doi.org/10.5194/egusphere-egu2020-4230, 2020.

Atmospheric carbon dioxide (CO₂) concentrations during the last glacial period (70,000 – 23,000 years ago) fluctuated on millennial timescales closely following variations in Antarctic temperature. This close coupling has suggested that the sources and sinks driving millennial scale CO₂ changes are dominated by processes in the Southern Ocean. However, recent work revealed centennial-scale increases in CO₂ during abrupt climate events of the last deglaciation which may represent a second mechanism of carbon cycle variability. 

Here we analyze a high resolution CO₂ record from the last glacial period from the West Antarctic Ice Sheet (WAIS Divide) that precisely defines the timing of CO₂ changes with respect to Antarctic ice core proxies for temperature, dust delivery, and sea-ice extent down to the centennial-timescale. Although CO₂ closely tracks all these proxies over millennia, peak CO₂ levels most often lag behind all proxies by a few hundred years. This decoupling from Antarctic climate variability is most prominent during the onset of DO interstadial events when CO₂, CH₄ and Greenland temperature all increase simultaneously. Regression analysis suggests that the CO₂ variations can be explained by a combination of two mechanisms: one operating on the time scale of Antarctic climate variability, and a second responding on the Dansgaard-Oeschger time scale.  

Recent δ¹³C-CO₂ data from the last glacial period support our finding that CO₂ variability is the sum of multiple mechanisms.  The Antarctic climate variability is likely associated with the release of respired organic carbon from the deep ocean.  Superimposed on these oscillations are two types of centennial-scale changes: CO₂ increases and δ¹³C-CO₂ minima in the middle of Heinrich stadials and ii) CO₂ increases and small changes in δ¹³C-CO₂ that at the onset of DO interstadial event.

To provide a comprehensive and quantitative constraint on the mechanisms of CO₂ variability during the last glacial period, we run a large suite of transient box model experiments (n = 500) forced with varying combinations of forcings based on proxy time-series (e.g. AABW formation, NADW formation, ocean temperature, dust delivery, and sea-ice extent).  Using data constraints from the ice core records of CO₂, δ¹³C-CO₂ and mean ocean temperature, we arrive at an ensemble of scenarios that can explain a large amount of the centennial and millennial-scale variability observed in the ice core record. Parsing this into a series of factorial experiments we find that Southern Hemisphere processes can explain 80% of the observed variability and Northern Hemisphere processes account for the remaining 20%.  A further breakdown on the level of individual mechanisms is marred by the high degree of correlation between carbon cycle forcings likely operating in the Southern Hemisphere.  None-the-less, our results highlight how multiple mechanisms operating over multiple timescales may have interacted during the last glacial period to drive changes in atmospheric CO₂. 

How to cite: Bauska, T., Marcott, S., and Brook, E.: The ice core record of atmospheric CO2 variability during the Last Glacial Period: new insights from timing and isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7292, https://doi.org/10.5194/egusphere-egu2020-7292, 2020.

It has been known since the 1980s that as the Last Glacial Period ended carbon dioxide (CO₂) rose from ~190 ppm to 280 ppm, but the source of this carbon is still unknown. Here it is proposed that the reason why this problem is still unsolved is that the current carbon cycle models are based on outdated chemistry. For instance, many geologists and oceanographers believe that CO₂ is drawn down out of the atmosphere by silicate weathering. This idea originated in the 19th Century when it was believed that CO₂ was an acid.  Now we know that acids are proton donors and that only when CO₂ reacts with water does it form weak carbonic acid (H₂CO₃). Silicate weathering is the result of the protons (H⁺ ions) from the carbonic acid increasing the solubility of the insoluble silicate rocks, with the carbon (HCO₃⁻ and CO₃²⁻) acting purely as spectator ions in those reactions.

Here a new carbon cycle is presented where:

• 1) a new reservoir, the ‘aquasphere’, is incorporated in the inorganic carbon system, which is the hydrosphere less the oceans, i.e. freshwater including rainwater;
• 2) CO₂ is drawn down from the atmosphere into the aquasphere by dissolution in rainwater, rather than by silicate weathering;
• 3) CO₂ is also drawn down from the atmosphere by photosynthesis, some of which is respired into the aquasphere;
• 4) carbonate weathering is a source of dissolved inorganic carbon to the aquasphere and from there to the oceans, rather than being a neutral player in the carbon system;
• 5) carbonate sediments in the ocean, which provides a major sink for inorganic carbon, are produced by biotic activity, not chemical precipitation, thus no CO₂ is generated by their formation;
• 6) the carbon sediment sink can also become an inorganic carbon source if the lysocline shoals, e.g. when oceanic pH falls or sea level rises.

With this model, it can be shown that the sea-level rise, caused by melting ice sheets, will shoal the lysocline, which explains both the source and the cause of increased atmospheric CO₂ during glacial terminations. This implies that there will be a further increase in CO₂ from the ocean sediments caused by sea-level rise when the Greenland and West Antarctic ice sheets melt as a result of anthropogenic global warming. Moreover, since ocean acidification also causes the lysocline to shoal, producing more atmospheric CO₂ in a positive feedback loop, then we may have a repeat of the PETM (Paleo-Eocene Thermal Maximum) event when runaway global warming was caused by an increase in atmospheric CO₂.

How to cite: McDonald, A.: Increase in CO2 during the Last Termination explained by a new inorganic carbon cycle. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19047, https://doi.org/10.5194/egusphere-egu2020-19047, 2020.

Northern peatlands store large amounts of carbon (C) and have played an important role in the global carbon cycle since the Last Glacial Maximum. Most northern peatlands have established since the end of the deglaciation and accumulated C over the Holocene, leading to a total present-day stock of 500 ± 100 GtC. This is a consolidated estimate, emerging from a diversity of methods using observational data. Recently, Nichols and Peteet (2019 Nature Geoscience 12: 917-921) presented an estimate of the northern peat C stock of 1055 GtC—exceeding previous estimates by a factor of two. Here, we will review various approaches and estimates of northern peatlands C storage in the literature and consider peat C storage in the context of the Holocene global C budget. We argue that the estimate by Nichols and Peteet is an overestimate, caused by systematic bias introduced by their inclusion of data that are representative for the major peatland regions and of records that lack direct measurements of C density. In particular, some “peatland” sites and data that were included in their synthesis were likely from lacustrine sediments prior to the onset of peat deposits. Furthermore, we argue that their estimate cannot be reconciled within the constraints offered by ice-core and marine records of stable C isotopes and estimated contributions from other processes that affected the terrestrial C storage during the Holocene.

How to cite: Yu, Z., Joos, F., Bauska, T., Stocker, B., Fischer, H., Loisel, J., Brovkin, V., Hugelius, G., Nehrbass-Ahles, C., Kleinen, T., and Schmitt3, J.: Revisiting Carbon Storage in Northern Peatlands: Ground-Based Estimates and Top-Down Constraints from Holocene Global Carbon Budget Reconstructions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10779, https://doi.org/10.5194/egusphere-egu2020-10779, 2020.

Environmental archives and carbon cycle models suggest that climate warming during the last deglaciation (the transition from the last glacial to the Holocene) caused large-scale thaw of Arctic permafrost, followed by the release of previously freeze-locked carbon. In addition to changing oceanic circulation and outgassing of CO₂ trapped in the deep glacial ocean, organic carbon (OC) release from thawing permafrost might have contributed to the rise in atmospheric CO₂ by 80 ppmv or ~200 Pg C between 17.5 and 11.7 kyr before present (BP). The few Arctic sediment cores to date, however, lack either temporal resolution or reflect only regional catchments, leaving most of the permafrost OC remobilization of the deglaciation unconstrained.

Our study explores the flux and fate of OC released from permafrost to the Siberian Arctic Seas during the last deglaciation. The Arctic Ocean is the main recipient of permafrost material delivered by river transport or collapse of coastal permafrost, providing an archive for current and past release of OC from thawing permafrost. We studied isotopes (Δ¹⁴C-OC, δ¹³C-OC) and terrestrial biomarkers (CuO-derived lignin phenols, n-alkanes, n-alkanoic acids) in a number of sediment cores from the Siberian Shelf and Central Arctic Ocean to reconstruct source and fate of OC previously locked in permafrost.

The composite record of three cores from the Laptev, East Siberian and Chukchi Seas suggest a combination of OC released by deepening of permafrost active layer in inland Siberia and by thermal collapse of coastal permafrost during the deglaciation. Coastal erosion of permafrost during the deglaciation suggests that sea-level rise and flooding of the Siberian shelf remobilized OC from permafrost deposits that covered the dry shelf areas during the last glacial. A sediment core from the Central Arctic Ocean demonstrates that this occurred in two major pulses; i) during the Bølling-Allerød (14.7-12.9 kyr BP), but most strongly ii) during the early Holocene (11-7.6 kyr BP). In the early Holocene, flooding of 80% of the Siberian shelf amplified permafrost OC release to the Arctic Ocean, with peak fluxes 10-9 kyr BP one order of magnitude higher than at other times in the Holocene.

It is likely that the remobilization of permafrost OC by flooding of the Siberian shelf released climate-significant amounts of dormant OC into active biogeochemical cycling and the atmosphere. Previous studies estimated that a pool of 300-600 Pg OC was held in permafrost covering Arctic Ocean shelves during the last glacial maximum; one can only speculate about its whereabouts after the deglaciation. Present und future reconstructions of historical remobilization of permafrost OC will help to understand how important permafrost thawing is to large-scale carbon cycling.

How to cite: Martens, J., Wild, B., Tesi, T., Muschitiello, F., O’Regan, M., Jakobsson, M., Semiletov, I., Dudarev, O. V., and Gustafsson, Ö.: Sediment archives from the Arctic Ocean provide evidence for massive remobilization of permafrost carbon in Siberia during the last glacial termination, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18643, https://doi.org/10.5194/egusphere-egu2020-18643, 2020.