Displays

There is compelling evidence of a strong relation between the Atlantic Meridional Overturning Circulation (AMOC) and millennial scale climate variability during the last glacial period. Part of the advances in understanding the underlying mechanisms rely on the analysis of the sedimentary Pa/Th ratio, which can be used to qualitatively infer past flow rates in the Atlantic. The compilation of existing North Atlantic records indicates repeated, consistent and significant Pa/Th increases across millennial-scale events, indicating significant reductions of deep-water formation in the Northwest Atlantic. However, the use of sedimentary Pa/Th as a pure kinematic circulation proxy is challenging because Pa and Th are also highly sensitive to changes in particulate flux intensity and composition that have probably occurred across these millennial scale events. A primary control of particles on the available Pa/Th records has been ruled out ensuring the absence of correlation between the reconstructed particle fluxes (e.g. Th-normalized opal fluxes) and the sedimentary Pa/Th. However, quantitative estimates of the impact of particles on the available paleo Pa/Th are still missing.

In this study, we use the Pa/Th enabled iLOVECLIM Earth System Model of Intermediate Complexity to decipher the impact of particles on the sedimentary Pa/Th. We evaluate the impact of imposed changes in biogenic particle flux intensity and composition on the Atlantic Pa/Th in a 3-D geographical perspective. We find that up to 30% of the observed Pa/Th increase across Heinrich Stadial 1 could be explained by changes in particle fluxes and composition. Besides, changes in the Particulate Organic Carbon (POC) most efficiently affects the sedimentary Pa/Th, followed by biogenic opal. Last but not least, the global Atlantic sedimentary Pa/Th response is very sensitive to shifts in the geographical distribution of particles and high scavenging areas. In our simulations, a decrease of the opal production in the Northwest Atlantic can induce a far field Pa/Th increase in a large part of the North Atlantic basin, suggesting that a local monitoring of the particle fluxes might not be enough to rule out any influence of the particles on paleo sedimentary Pa/Th records.

How to cite: Missiaen, L., Menviel, L., Meissner, K. J., Bouttes, N., Roche, D. M., Dutay, J.-C., Quiquet, A., Lhardy, F., Waelbroeck, C., and Pichat, S.: Modelling the impact of biogenic particle flux intensity and composition on sedimentary Pa/Th, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-290, https://doi.org/10.5194/egusphere-egu2020-290, 2020.

²³⁰Th-normalization is a valuable paleoceanographic tool for reconstructing high-resolution sediment fluxes during the late Pleistocene (last ~500,000 years). As its application has expanded to ever more complex marine environments, the nuances of ²³⁰Th systematics, with regards to particle type, particle size, lateral advective/diffusive redistribution, and other processes, have emerged. We synthesized over 1000 sedimentary records of ²³⁰Th from across the global ocean at two time slices, the Late Holocene (0-5000 years ago, or 0-5 ka) and the Last Glacial Maximum (18.5-23.5 ka), and investigated the spatial structure of ²³⁰Th-normalized mass fluxes. On a global scale, sedimentary mass fluxes were significantly higher during the Last Glacial Maximum (1.79-2.17 g/cm²kyr, 95% confidence) relative to the Holocene (1.48-1.68 g/cm²kyr, 95% confidence). We then examined the potential confounding influences of boundary scavenging, nepheloid layers, hydrothermal scavenging, size dependent sediment fractionation, and carbonate dissolution on the efficacy of ²³⁰Th as a constant flux proxy. Anomalous ²³⁰Th behavior is sometimes observed proximal to hydrothermal ridges and in continental margins where high particle fluxes and steep continental slopes can lead to the combined effects of boundary scavenging and nepheloid interference. Notwithstanding these limitations, we found that ²³⁰Th-normalization is a robust tool for determining sediment mass accumulation rates in the majority of pelagic (> 1000 m) marine settings.

How to cite: Costa, K. and the GEOTRACES Working Group 3: Particle Fluxes: 230Th normalization: New insights on an essential tool for quantifying sedimentary fluxes in the modern and Quaternary ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2186, https://doi.org/10.5194/egusphere-egu2020-2186, 2020.

The Atlantic Meridional Overturning Circulation (AMOC) brings heat from the tropics to the high latitudes, and its temporal variability has major impacts on climatic cycles. We have constructed north-south profiles using deep sea cores from the North Atlantic to the Southern Ocean, covering the past ~1.5 Ma or so, including the interval prior to and including the Mid-Pleistocene Transition (MPT), the interval of ‘lukewarm interglacials’ following the MPT, and to the present-day, using Nd isotopes in Fe-Mn oxide encrusted foraminifera and fish debris. Some important observations show that our Nd isotope records indeed reflect the AMOC variability, rather than regional Nd sources or alteration effects. Firstly, throughout the time interval and at all sites, the εNd-values show glacial-interglacial ‘zig-zags’, indicating stronger AMOC during interglacials and weaker AMOC during glacials. Secondly, going from north to south the data show increasingly weaker NADW signals at all points in time. Thirdly, all of the εNd-values are those expected from seawater Nd sources. The εNd-values at North Atlantic DSDP Site 607 during interglacials are almost always between -13 and -14.5, similar to present-day NADW both before and after the AMOC-crisis, thus indicating that the normal NADW range during interglacials has remained similar since the middle Pleistocene. Fourthly, at all times, the εNd-values throughout the transect remain sandwiched by the global North Atlantic and North Pacific end-member values. These observations are what are required if the data reflect the glacial-interglacial waxing and waning of the AMOC, but are unexpected for virtually any other scenario.

How to cite: Yehudai, M., Goldstein, S., Pena, L. D., Kim, J., Jaume-Segui, M., Basak, C., Knudson, K., Hartman, A. E., and Lupien, R.: The Atlantic Meridional Overturning Circulation Over Time From Nd Isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13297, https://doi.org/10.5194/egusphere-egu2020-13297, 2020.

Deep water formation in the North Atlantic represents an integral link between the atmosphere, cryosphere, and the deep ocean: heat loss from warm surface waters supplies moisture to the high latitudes and their subsequent sinking ventilates the deep ocean and sequesters greenhouse gases from the atmosphere. This moisture supply supported the formation of immense ice sheets in the region during the last glacial, which in turn affected climate. While many studies have improved our understanding of these processes for past glacials, a comprehensive picture including the significance and variation of deep water export from the Nordic Seas is still missing. Furthermore, recent observations suggested the export of a previously unknown bottom water mass from the glacial subpolar North Atlantic.

In this study we investigate the distribution and sourcing of water masses in the subpolar North Atlantic since MIS3 with the help of authigenic Nd isotopes. This method benefits from the large heterogeneity in Nd isotopic compositions of source rocks in this region, but the post-depositional dissolution of detritus within the sediments can also impede interpretations of individual records. We thus compare several Nd isotope records from the subpolar North Atlantic and Nordic Seas in order to define distinct deep water mass end members and estimate their prevalence and mixing in the subpolar North Atlantic during the last 30 ka. Our observations suggest that Nordic Seas deep water overflowing the Greenland-Scotland Ridge during MIS2 reached into the deep subpolar North Atlantic. Furthermore, its spatial distribution implies that overflow across Denmark Strait into the Irminger Basin was more pronounced than overflow into the Iceland Basin further south. The hydrographic configuration during the Last Glacial Maximum thus appears to have been more complex and more similar to today than previously thought.

How to cite: Blaser, P., Pöppelmeier, F., Frank, M., Gutjahr, M., and Lippold, J.: North Atlantic deep water sources and export since MIS3: implications from Nd isotopes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11092, https://doi.org/10.5194/egusphere-egu2020-11092, 2020.

The stable oxygen isotope ratio (δ¹⁸O) of planktic foraminifers, the C₃₇ ketone unsaturation ratio (U^K′₃₇) and the TetraEther indeX of tetraethers consisting of 86 carbon atoms (TEX₈₆) are three well-known examples of paleothermometric proxies. These established proxies are in the realism phase of the Elderfield proxy curve (2002 Geochim. Cosmochim. Acta 66 Suppl. 1, 213, DOI: 10.1016/S0016-7037(02)01009-8), which means that their advantages and shortcomings are relatively well evidenced, though not fully understood. By contrast, the Ring Index of hydroxylated tetraethers (RI-OH′) is an example of novel paleothermometer. RI-OH′ is still in the optimism phase, so its potential in paleothermometry remains to be further explored.

Here, we present new high-resolution temperature records over the interval 160–50 ka BP using four organic proxies (RI-OH′, RI-OH, TEX₈₆ and U^K′₃₇) from three deep-sea sediment cores located in a north-south transect along the Iberian Margin. RI-OH′, RI-OH and TEX₈₆ are based on LC-MS analyses of individual tetraethers with a two-column HPLC and improved mass spectrometric method. We analyzed all organic proxies in the same organic extracts to optimize proxy-proxy comparisons and phase relationship studies.

Our main results strengthen the optimism concerning the novel RI-OH′ proxy for five reasons. 1/, the only existing global core-top calibration to date allows to reconstruct realistic sea surface temperature (SST) from RI-OH′ in comparison to those derived from U^K′₃₇ and TEX₈₆. 2/, RI-OH′ allows to establish plausible latitudinal temperature gradients, which are reasonably coherent with those based on U^K′₃₇ and TEX₈₆. 3/, RI-OH′ records resemble those from established paleothermometers, especially U^K′₃₇ and δ¹⁸O of planktic foraminifers that better reflect SST than does TEX₈₆. 4/, RI-OH′ responds to Dansgaard-Oeschger and Heinrich events as expected for North Atlantic SST proxies, which supports a direct relationship with Greenland temperature records. 5/, the outputs of a bipolar seesaw model forced with the RI-OH′ record are well correlated with Antarctic paleotemperatures as expected from theoretical considerations.

Overall, our main findings support a continued interest on the novel hydroxylated tetraether paleothermometer RI-OH′ so that it can progress along the Elderfield proxy curve. This work complements our first promising attempt based on a RI-OH record for a shallow core from the western Mediterranean Sea, located in a complex sedimentary setting much less favorable than the Iberian Margin (Davtian et al., 2019 Paleoceanography and Paleoclimatology 34, 616–634, DOI: 10.1029/2018PA003452).

How to cite: Davtian, N., Bard, E., Rostek, F., Ménot, G., and Darfeuil, S.: The novel hydroxylated tetraether index RI-OH′ as a sea surface temperature proxy for the period 160–50 ka BP off the Iberian Margin, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9988, https://doi.org/10.5194/egusphere-egu2020-9988, 2020.

Whether or not Arctic regions remain(ed) a carbon sink or source to the atmosphere during rapidly warming climates (in the past) is a fundamental question with regards to future global warming and ocean acidification. The boron isotopic composition of planktonic foraminiferal shell calcite (δ¹¹B_Cc) can potentially provide valuable information of past seawater pH if information on a second carbonate system parameter, temperature, and salinity is available. However, most applications of palaeoceanographic proxies to the cold polar oceans are limited due to a paucity of calibration data, limited information on the calcification habitat, and secondary effects of the carbonate system on the temperature recorded by Mg/Ca values measured in the dominant Arctic species Neogloboquadrina pachyderma sinistral (NPS). Here we present a new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) δ¹¹B dataset measured on live NPS collected via plankton tows from the Labrador Sea and Baffin Bay. We compare our results with δ¹¹B_borate derived from pH measurements, δ¹³C DIC seawater values, temperature and salinity collected at the time and depth the foraminifera calcified. To quantify the control of low carbonate ion concentration on Mg/Ca derived temperatures we measured B/Ca alongside Mg/Ca in the calibration dataset. We are thus able to present a new geochemical correction scheme that can isolate non-thermal controls on the Mg/Ca-temperature relationship for NPS, allowing us for the first time the reconstruction of carbonate system parameters in the Arctic Ocean.

How to cite: Morley, A., Raitzsch, M., Bijma, J., Karancz, S., and Kucera, M.: Deciphering the Signal of Arctic Climate Change , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19481, https://doi.org/10.5194/egusphere-egu2020-19481, 2020.

Ocean circulation plays an essential role in Earth’s climate and the global carbon cycle. A prerequisite for improving confidence in future climate projections is the accurate numerical modeling of past deep ocean circulation changes. Unfortunately our understanding of such changes is impeded by ambiguities in the data-based reconstructions which heavily rely on radiocarbon dating of marine samples. Central to this method is the knowledge of the reservoir age (the age difference between the surface ocean and the atmosphere). Concomitant changes in atmospheric levels, air-sea exchange rates, and ocean circulation have the potential to drive large temporal and spatial changes of this reservoir age over the deglaciation. However these changes are not well constrained by field evidence. In consequence large uncertainties affect the dating of main climate events. Model studies allow complementing field data while also providing the means of assessing the sensitivity to different processes.

Here, we investigate the sensitivity of the radiocarbon reservoir ages and reconstructed calendar ages over the last termination. For this purpose we take advantage of a set of transient simulations performed with the Max Planck Institute Earth System Model (MPI-ESM) with interactive calculation of river runoff and automatic adjustment of model topography. The experiments, starting at 26 ka BP, are constrained with prescribed time varying ice sheets and topography in addition to variations of the Earth orbital parameters and reconstructed atmospheric greenhouse gases concentrations. Changes in ice sheet volume naturally result in freshwater surges which affect the global circulation and water masses distribution. Ocean radiocarbon is included in the model. The atmospheric ¹⁴C follows the INTCAL13 reconstruction while the impacts of varying wind speed, sea-ice cover, and atmospheric CO₂ on air-sea exchange rates are explicitly included.

Different ice-sheets reconstructions (ICE-6G_C and GLAC-1D) and model configurations (addressing vertical mixing, bathymetry and land-sea mask) provide a range of ocean responses. The impact on reservoir ages of uncertainties related to planktonic foraminifer species-specific habitat is also considered. Together with the suite of model states this provides a range of reservoir ages over time. A calibration step allows then obtaining an estimate of the temporal evolution over the deglaciation of the time resolution of the radiocarbon dating method. Regional and global evolutions are examined and discussed.

How to cite: Mouchet, A., Mikolajewicz, U., and Voelker, A.: The ocean radiocarbon reservoir age over the last termination and the calendar age uncertainty of marine samples: a sensitivity study with a coupled climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19759, https://doi.org/10.5194/egusphere-egu2020-19759, 2020.

The Indian-Atlantic interocean exchange (IAIE), occurring through Agulhas current and its leakage around the southern tip of Africa, is one of the return flows of global thermohaline circulation that contributes to the temperate climate in Europe. The IAIE affects the transport of heat and salt to the zone of deep-water formation in the N Atlantic, influencing the variability of Atlantic Meridional Overturning Circulation (AMOC). During the last 600 kyr, significant climatic events took place such as the Mid-Bruhnes event (MBE) (~430 ka) that marks a transition towards more intense interglacial periods.

The main objective of our study is to assess the impact of climate forcing on the strength of both surface and deep water IAIE during the last 600 kyr. For this purpose, we examined the variability of a group of warm-water planktonic foraminiferal species for tracing surface water circulation. We combined published and unpublished data from 3 cores along an Indian-Atlantic transect: two cores in the Indian Ocean, core MD96-2048 (26°10’S, 34°01’E, 660 m) in the source of the Agulhas current and our study core MD96-2077 (33º10’S, 31º14’E, 3781 m) in the middle of the Agulhas current; and one core in the Atlantic Ocean, core ODP1087 (31°27’S, 15°18’E 1372 m) recording the Agulhas leakage.

Since Globorotalia menardii and Globorotalia tumida are frequently used to trace Agulhas leakage, their variability in Agulhas current in the Indian Ocean is of our interest. Therefore, we compared the relative abundances of the warm-water planktonic G. menardii and G. tumida species with a group of warm-water planktonic foraminiferal species to record the strength of Agulhas current in core MD96-2077. Our results show that the group of warm-water planktonic species reflects increased Agulhas current strength at glacial terminations coinciding with stronger Agulhas leakage (Atlantic core ODP1087) as observed in previous studies. However, in core MD96-2077, both G. menardii and G. tumida relative abundances increase during interglacial periods. This indicates that production of these species in the Agulhas current source region is unlikely to trace Agulhas leakage in the Atlantic Ocean. The analyses of deep-water circulation proxies (Nd isotopes, benthic O and C stable isotopes) are in progress, and they will allow us to assess the response of deep circulation to changes in Agulhas current and leakage over the last 600 kyr.

How to cite: Pérez-Asensio, J. N., Tachikawa, K., de Garidel-Thoron, T., Vidal, L., Sonzogni, C., Guihou, A., and Chen, M.-T.: Foraminiferal tracers of Indian-Atlantic interocean exchange during the last 600 kyr, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3037, https://doi.org/10.5194/egusphere-egu2020-3037, 2020.

Over the past two decades, an impressive amount of radiocarbon age measurements on samples of fossil benthic foraminifera and deep-sea corals have been published in the literature. These measurements are commonly used to draw inferences about changes in the ventilation of deep oceanic basins during the last deglacial period. Lacking in most previous studies, however, are quantitative estimates of deep-ocean paleo-ventilation rates and quantitative estimates of their errors, leading to potential over-interpretation and sterile debate. Moreover, most previous studies were concerned with the interpretation of individual records with low or no regard for other records available for the same time interval.

Here we present an effort to go beyond the qualitative interpretation of single radiocarbon records by analyzing an updated compilation of ¹⁴C age data using recursive least-squares (RLS) methods (a Kalman filter and a related smoother). In stark contrast with other methods of data analysis, RLS methods can provide an estimate of the history of the state of the physical system of interest and an estimate of the error in this history, which are consistent (in the least-squares sense) with times series of data and with a dynamical model, given estimates of the statistics of the errors in the data and in the model. Our current compilation includes 1,698 deep water ¹⁴C age data for the past 40 kyr based on fossil samples of benthic foraminifera, deep‐sea corals, deep‐dwelling planktonic foraminifera, bivalves, and spiral shells. The geographic distribution of the samples is very irregular, with most of them originating from near the margins and with large regions devoid of any data. The depths of the samples vary from about 250 m to about 5,000 m. In our study, the potential of RLS methods to estimate the history of deep-ocean ventilation rates and their errors from deep water ¹⁴C age data is explored for a number of abyssal layers in the Atlantic Ocean during the deglacial interval from 20 to 10 kyr BP. The approach used to apply the powerful but computationally expensive RLS methods to the analysis of geologic time series is described, the least-squares estimates of ventilation rate history in different layers are reported, and their significance in the light of their error estimates is discussed.

How to cite: Marchal, O., Zhao, N., and Duffy, F.: Can Radiocarbon Records lead to Quantitative Estimates of Deep-Ocean Ventilation Rates with Error Estimates in the Geologic Past? , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3642, https://doi.org/10.5194/egusphere-egu2020-3642, 2020.

The naturally-occurring particle-reactive radionuclides protactinium-231 (²³¹Pa) and thorium-230 (²³⁰Th) are used as tracers of a variety of oceanic processes, both at present and in the past. Most notably, the sediment ²³¹Pa/²³⁰Th ratio has been used to infer changes in the Atlantic Meridional Overturning Circulation over the last (de)glaciation. However, recent measurements along the U.S. GEOTRACES North Atlantic transect (GA03) revealed two features which are at odds with current understanding about ²³¹Pa and ²³⁰Th behaviour in the ocean: (i) a sharp decrease in dissolved ²³¹Pa and ²³⁰Th activities with depth below 2000-4000 m and (ii) very large particulate ²³¹Pa and ²³⁰Th activities near the bottom, at a number of stations between the New England continental shelf and Bermuda. Concomitant measurements of particulate matter concentration and potential temperature showed that both features are associated with the benthic nepheloid layer (BNL) and the bottom mixed layer (BML) that are present at these stations.

Here we develop and apply a simplified model of the exchange of particles, ²³¹Pa, and ²³⁰Th between the BNL and the upper sediment, to explore the extent to which the radionuclide anomalies observed near the bottom at a number of GA03 stations can be explained by local sediment resuspension. We find that the model can broadly reproduce the observed anomalies at two stations where samples for radionuclide analyses were collected near the seafloor. Sensitivity tests with the model show that the ²³¹Pa/²³⁰Th ratio of particles in the BML and the sediment varies by a factor of 3 as the sediment resuspension rate fluctuates within a range consistent with observational estimates. The modelled variability is comparable to the spatial variability of ²³¹Pa/²³⁰Th of suspended particles in the modern North Atlantic and to the variability of Atlantic sediment ²³¹Pa/²³⁰Th records across the last (de)glacial period. Two factors are found to contribute to the modelled sensitivity of the sediment ²³¹Pa/²³⁰Th to sediment resuspension rate: the vertical turbulent mixing in the BML and the differential scavenging intensity of Pa and Th due to variation in particle concentration. Overall, our study indicates that the exchange of material between the BNL and the upper sediment can affect the particulate ²³¹Pa/²³⁰Th ratio in the bottom water and the sediment, which may complicate the use of sediment ²³¹Pa/²³⁰Th as a palaeoceanographic tracer.

How to cite: Chen, S.-S., Marchal, O., Lerner, P., McCorkle, D., and Rutgers van der Loeff, M.: On the influence of sediment resuspension on deep-ocean Pa-231 and Th-230 cycling: Roles of turbulent mixing and differential scavenging, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4247, https://doi.org/10.5194/egusphere-egu2020-4247, 2020.

         Cold-water corals represent an intriguing paleoceanographic archive with a great potential to reconstruct high-resolution paleoenvironmental changes. Compared to those of shallow-water corals, proxies derived from cold-water corals have been complicated by biologically mediated vital effects. The oxygen and carbon stable isotope compositions of cold-water coral skeletons are more depleted than the expected carbonate-seawater equilibrium values by ~4–6‰ and ~10‰, respectively. Therefore, it is necessary to correct for the vital effect before using δ¹⁸O as a temperature proxy. δ¹⁸O and δ¹³C of cold-water corals exhibit strong linear correlations after adjusting for ambient seawater δ¹⁸O and δ¹³C values. The δ¹⁸O intercepts of this linear regression were found to be correlated with water temperatures. This so-called ‘intercept method’ can therefore be used to reconstruct temperatures variations of intermediate and deep oceans. Moreover, sampling along the growing bands of cold-water corals can provide samples to generate temperature sequences. After that, three geochemical models have been proposed to explain the δ¹⁸O and δ¹³C depletion of cold-water corals. However, none of them can explain the behavior of all geochemical parameters. In future, more analyses and experiments at micro-scales are required to adjust these geochemical models or propose new ones.

How to cite: Kong, L.: Oxygen and Carbon Isotopes of Cold-water Corals——Reconstructing Paleotemperature changes and Calcification Mechanism, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6500, https://doi.org/10.5194/egusphere-egu2020-6500, 2020.

In 2004 McManus et al. published their famous ²³¹Pa/²³⁰Th record from the Bermuda Rise revealing millennial-scale changes in circulation strength back to the Last Glacial Maximum. This record marks the boost of this proxy as a kinematic circulation change proxy for the Atlantic Ocean and the initial rising slope on the ‘Elderfield-Curve’. However, the up-to-date data base of Atlantic sedimentary ²³¹Pa/²³⁰Th records gives a rather inconsistent picture of changes in the circulation strength in the Atlantic throughout the past 25 ka (Ng et al., 2018). Since both radioisotopes are strongly particle reactive it is obvious that scavenging processes may play a major role in their cycling as well. At ocean margins such processes do have a major impact on ²³¹Pa/²³⁰Th, leading to increased values and thus potentially overprinting the circulation signal. In contrast, records from open ocean sites are assumed to show a less biased circulation signal. In addition, the GEOTRACES program (Schlitzer et al., 2018) provided valuable seawater data allowing for examining the cycling of both radioisotopes under today’s circulation regime in more detail. A transect across the North Atlantic by Hayes et al. (2015) revealed that nepheloid layers contribute to strong bottom scavenging of ²³¹Pa and ²³⁰Th in the northwestern Atlantic basin. Surprisingly, sedimentary core-top values do not mirror predominant scavenging effects but rather indicate a strong export of ²³¹Pa and therefore a circulation signal. With our modern proxy toolbox, it is impossible to reconstruct the occurrence and intensity of past nepheloid layers and hence their potential effect on recorded ²³¹Pa/²³⁰Th variations. Therefore, isotope-enabled models may help to better decipher the interwoven processes controlling ²³¹Pa/²³⁰Th (Rempfer et al., 2017; Lerner et al., 2019). Here an up-to-date compilation of northwestern Atlantic ²³¹Pa/²³⁰Th data will be presented. Our findings base on records covering the last 25 ka and will be interpreted in the context of recent model simulations as well as compared to seawater data. Thus, we aim for a deeper understanding of ²³¹Pa and ²³⁰Th cycling in the northwestern Atlantic.

References:

Hayes, C., et al. (2015), Deep-Sea Res. Pt. II, 116, 29-41.
Lerner et al. (2020), Deep Sea. Res. Pt. I, 155, 1-41.
McManus, J. F., et al. (2004), Nature, 428, 834-837.
Ng, H., et al. (2018), Nat. Comm., 9, 1-10.
Rempfer et al. (2017), EPSL, 468, 27-37.
Schlitzer, R., et al. (2018), Chem. Geol., 493, 210-223.

How to cite: Süfke, F., Pöppelmeier, F., Blaser, P., and Lippold, J.: 231Pa/230Th in the northwestern Atlantic: circulation versus particles?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6725, https://doi.org/10.5194/egusphere-egu2020-6725, 2020.

Current climatic trends are expected to lead to expansion of oxygen minimum zones and an overall decrease in oxygen concentration [O₂] in the oceans. In order to improve predictions of future trends we need to create a better understanding of the natural oxygen cycle. The iodine to calcium ratio (I/Ca) of planktonic foraminifera is an increasingly popular proxy to assess upper water column oxygenation. Recent studies suggest that this ratio is mainly controlled by subsurface water dissolved oxygen concentrations. A thorough assessment of the proxy has been carried out for the South Atlantic, but is currently lacking for the Indian and Pacific Oceans, which contain the worlds’ most intense and large oxygen minimum zones. Here we present results of recent (Holocene) planktonic foraminifera (mixed layer and deep dwelling species) I/Ca measurements across a range of oceanographic conditions ([O₂] varies between < 10 µmol/kg to > 200 µmol/kg) from the Indian and Pacific Ocean to further refine the proxy, using sample material provided by Lamont-Doherty Core Repository.

How to cite: Winkelbauer, H. A., Chenery, S., Hamilton, E. M., Leng, M., and Hoogakker, B.: Planktic foraminiferal I/Ca from Holocene sediments of the Pacific and Indian Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9634, https://doi.org/10.5194/egusphere-egu2020-9634, 2020.

Subsurface nitrate in the Labrador Sea (NW Atlantic) and Baffin Bay is provided by North Pacific water flowing through Bering Strait and the Canadian Arctic as well as by advection from the North Atlantic. Both these nitrate sources are distinct in their isotopic signature (δ¹⁵N), owing to benthic denitrification on the Bering, Chukchi and east Siberian shelves and nitrogen fixation in the North Atlantic, respectively. Accordingly, water column profiles of δ¹⁵N_(nitrate) collected off Greenland in the eastern Labrador Sea show low δ¹⁵N_(nitrate), which mixes with more ¹⁵N-enriched nitrate flowing through Baffin Bay into the northern Labrador Sea. The Labrador Current carries this mixture southward along the western Labrador Sea, toward Newfoundland. The δ¹⁵N of surface sediments in the Labrador Sea closely mirrors these water column signals, suggesting that sediments can be used to trace changes in both the source signature of Atlantic versus Pacific-derived nitrate as well as in the admixture of the two source waters.

Two downcore sedimentary δ¹⁵N records from the NE and NW Labrador Sea coast both show high δ¹⁵N values of ca. 7‰ during the early Holocene (9-7 kyrs BP). In the NE Labrador Sea, this is followed by a long-term decrease toward δ¹⁵N of ca. 4.5‰ at the core top, in contrast to a much more subtle decrease in the NW Labrador Sea (surface sediment δ¹⁵N of ca. 6.5‰). The decreasing δ¹⁵N values along the eastern Labrador Sea are consistent with a Holocene increase in nitrogen fixation in the North Atlantic or an increasing advection of isotopically light nitrate. In turn, an increasing admixture of North-Pacific-derived nitrate, or intensified denitrification on the Bering Shelf would be required to explain the much subdued Holocene δ¹⁵N decrease in the NW Labrador Sea.

How to cite: Kienast, M., Davin, S., Doering, K., Hebbeln, D., Kienast, S., Lehmann, N., Schneider, R., Sherwood, O., and Weiser, J.: Isotopic evidence for changes in the origin and cycling of nitrogen in the Labrador Sea during the last 8,000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12134, https://doi.org/10.5194/egusphere-egu2020-12134, 2020.

Despite growing interest in ocean-climate interactions in response to recent anthropogenic warming, historical hydrographic data with which to assess changes in the deep ocean over the last century are limited. With their robust calcium carbonate skeletons, deep-sea corals, especially long-lived bamboo corals, serve as a potential archive for reconstructing continuous high-resolution paleoceanographic records extending back hundreds to even thousands of years.

Here we use deep-sea bamboo corals collected between 800 and 2000 m water depth in the eastern equatorial Atlantic to reconstruct the ventilation history over the last century. Deep-sea bamboo corals have a jointed axis consisting of organic nodes and internodes composed of calcium carbonate. The radiocarbon content of the organic nodes documents the radiocarbon of surface water and likely records the distinctive bomb ¹⁴C signal that can be used to generate a chronology for each coral specimen. By contrast, the radiocarbon content of calcite internodes records the radiocarbon signature of deep water over the lifetime of the coral. The reconstructed calcite radiocarbon record shows a quasi-periodic cycle of about two-decades, which is likely linked to multidecadal fluctuations in North Atlantic climate influencing the ventilation state of the water mass. In addition to radiocarbon records, we show that trace metal compositions of bamboo coral also provides key information with regard to both biomineralization processes, past environmental conditions, and chemistry of seawater. By combining radiocarbon and elemental composition of bamboo coral, we are building a set of tools with which to reconstruct deep ocean dynamics over the last century.

How to cite: Liu, Q., Robinson, L. F., Stewart, J. A., Knowles, T., Hendy, E., Li, T., and Samperiz Vizcaino, A.: Equatorial Atlantic ventilation over the last century revealed by deep-sea bamboo coral radiocarbon records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17827, https://doi.org/10.5194/egusphere-egu2020-17827, 2020.

Most paleoceanographic studies using planktic foraminifera focus on annual means, but seasonal signals buried by the analyses of lumped specimens could be very valuable. Surface ocean feedbacks on climate change may be more significant in the seasonal realm than annual mean in the northern South China Sea, a region being strongly affected by Asian monsoons and tropical cyclones. Here we use oxygen isotope measurements on individual specimens of surface and subsurface planktic foraminiferal species to reconstruct surface seasonality and seasonal upper ocean stratification in this region. Many studies have shown that the thermocline was deeper in the tropical Pacific during the Pliocene than the Pleistocene, but the mechanism remains unclear. Several processes could lead to changes in the upper ocean stratification, such as changes in sea surface temperature and upper ocean mixing by tropical cyclones. Our results show that the upper ocean stratification was weaker during the Late Pliocene than the Early Pleistocene, with the change more significant in summer than winter, while no systematic offset is observed in the surface seasonality. The observations suggest that enhanced mixing by tropical cyclones might be the major cause of the deeper thermocline during the Pliocene.

How to cite: Zhao, N., Vonhof, H., Giosan, L., Schiebel, R., and Haug, G.: Oxygen isotopes of individual planktic foraminifers reveal Pliocene-Pleistocene change of seasonal upper ocean stratification in the northern South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20172, https://doi.org/10.5194/egusphere-egu2020-20172, 2020.

How to cite: Conley, D. and Hendry, K.: Using the silica isotope composition of biogenic materials in marine sediments to reconstruct ocean chemistry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22504, https://doi.org/10.5194/egusphere-egu2020-22504, 2020.