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At present, it is well-known that the fast increase in atmospheric carbon dioxide (CO2) concentrations resulting from human activities (Cant), drives the dramatic changes observed in our environment such as global warming and ocean acidification. The Arctic Ocean has been identified as one of the fastest-changing regions of the world ocean and can therefore be considered as a sentinel for future global scenarios.Here, Cant-rich waters coming from the Atlantic Ocean become isolated from the atmospheric input of CO2 as they flow at an intermediate depth below the mixed layer, making the Arctic Ocean a key region for intermediate-to-long-term storage of Cant. Despite having such an important role, the magnitude of the Cant inventory and its change over time in the region is yet not fully understood, particularly if we are to consider future changes in ice coverage and therefore ocean circulation.A way of estimating oceanic Cant inventories is by applying the so-called Transit Time Distribution (TTD) method, which implies the use of transient tracers such as the anthropogenically produced CFC-12 and SF6.In this work we present a new estimate of Cant inventory for the Arctic Ocean in 2015 assessed with the TTD method using both well-established tracers (CFC-12 and SF6, both having a global source) as well as novel ones (anthropogenic radionuclides 129I and 236U, both having primarily a point-like source represented by European nuclear reprocessing plants, as well as a global one represented by the global fallout from nuclear bomb testing).The TTD was here applied following a relatively novel approach to infer the statistical parameters that describe the age distribution within a water sample, the mean (G) and the width (D). Unlike the “classical TTD” approach, the one used in this study allows the statistical parameters of the TTD to be constrained for each individual sample rather than finding values that are most representative of the region and time studied. We first show a comparison of the two TTD approaches by comparing mean and mode ages as well D/G ratios of this study (new TTD method) to those presented in Rajasakaren et al. 2019 (classical TTD method), using CFC-12 and SF6 as our tracers’ pair. We then compare TTD results obtained from the two tracers’ pairs, CFC-12/SF6 and 129I-/236U, using the new TTD method.Finally, we estimate and compare Cant concentrations and inventories obtained with the two pairs of transient tracers to one-another as well as to previous estimates of Cant in the region by Rajasakaren et al (2019) obtained with the “classical TTD”. This study demonstrates for the first time the feasibility of using anthropogenically produced radionuclides with input functions and chemical properties different than CO2 as proxies for Cant estimates.

The mean cooling of the global ocean during the Last Glacial Maximum (LGM) was recently estimated to 2.6°C using noble gases trapped in ice cores (1). The ocean, however, is highly heterogeneous with respect to its internal temperature varying both in latitude and water depth. While temperature changes in the deep ocean are small at about 2 - 3 °C (1,2), the upper ocean is more dynamic. Regional temperature anomalies of up to 7°C are predicted during the LGM compared to modern interior ocean temperature by global ocean circulation models (3). Due to the temperature drop to near freezing conditions and the global increase in salinity from ice sheet growth, the oceans’ deep interior became strongly haline stratified (2). Temperatures of the glacial ocean thermocline are, however, less well constrained.Here, thermocline temperature reconstructions since the last glacial based on the Li/Mg ratio in cold-water coral skeletons are presented. The coral samples, collected from 300 - 1200 m water depths from different sites in the Atlantic (43°N to 25°S), reveal synchronous 5 - 7°C cooling during the last glacial period compared to today, as well as a dramatic shoaling of the thermocline. At the end of the LGM, warming of the upper thermocline ocean occurred early in the southern hemisphere followed by a fluctuating warming and thermocline deepening in the northern Hemisphere. This supports the oceanic climate seesaw proposed by Stocker and Johnson in 2003 (4). We thus propose dramatic changes in the export of polar waters towards the Equator and an enhanced subsurface ocean stratification leading to a mostly polar Atlantic with a shallow permanent thermocline during the glacial. References:1) Bereiter et al., Nature 553, 39-44 (2018).2) Adkins et al., Science 298, 1769-1773 (2002).3) Ballarotta et al., Clim. Past 9, 2669-2686 (2013).4) Stocker and Johnsen, Paleoceanography 18, 1087 (2003).

ISSN:0148-0227 ISSN:2169-9275

The inflow of Atlantic Waters to the Arctic Ocean is a crucial determinant for the future trajectory of this ocean basin with regard to warming, loss of sea-ice and ocean acidification. Yet many details of the fate and circulation of these waters within the Arctic remain unclear. Here, we use the two long-lived artificial radionuclides 129I and 236U together with two tracer age models to constrain the pathways and circulation times of Atlantic waters in the surface and in the mid-depth Atlantic layer (250–800 m depth). We thereby benefit from the unique time-dependent tagging of Atlantic waters by these two isotopes. In the surface layer, a binary mixing model yields tracer ages of Atlantic Waters between 9–16 years in the Amundsen Basin, 12–17 years in the Fram Strait (East Greenland Current) and up to 20 years in the Canada Basin, reflecting the pathways of Atlantic Waters through the Arctic and their exiting through Fram Strait. In the mid-depth Atlantic layer (250 to 800 m), the transit time distribution (TTD) model yields mean ages in the central Arctic ranging between 15 and 65 years, while the mode ages representing the most probable ages of the TTD range between 2 and 30 years. The estimated mean ages are overall in good agreement with previous studies using artificial radionuclides or ventilation tracers. Although we find the overall flow to be dominated by advection, the shift of the mode age towards a younger age compared to the mean age reflects also the presence of a substantial amount of lateral mixing. For applications interested in how fast signals are transported into the Arctic's interior, the mode age appears to be a suitable measure. The short mode ages obtained in this study suggest that changes in the properties of Atlantic Waters will quickly spread through the Arctic Ocean and can lead to relatively rapid changes throughout the upper water column in future years.

Carbon isotopic measurements in oceanic dissolved inorganic carbon (DIC) contribute to many oceanographic fields. For instance, radiocarbon (14C) has been essential to elucidate aspects related to ocean circulation, air-sea exchange, carbon cycling and biogeochemistry. Despite its importance as a tracer in oceanography, oceanic 14C has been less well studied than other tracers (e.g. CFCs) as disentangling the natural from the artificial component is not trivial. Another major limitation was the large volume seawater samples required for the decay counting of 14C. Advances in Accelerator Mass Spectrometry (AMS) allowed the reduction of the sample volume to a couple of liters, permitting to obtain spatially better resolved distributions of oceanic 14C during repeated GO-SHIP sections. Yet, methods for sample preparation were borrowed from decay counting and not optimized for AMS. Here, we present a method that we recently developed in the Laboratory of Ion Beam Physics (ETHZ) that allows the rapid (14C in small seawater samples with unprecedented precision (14C from 50 - 60 ml samples, by sparging the acidified seawater with helium gas to extract CO2. The fully automated method is controlled via a LabVIEW program that runs through all consecutive steps: catalyst preconditioning, CO2 extraction, CO2 trapping and thermal CO2 release from the trap into the reactor for graphitization, which is performed simultaneously for 7 samples. The method is optimized by introducing a Cu-Ag furnace that improves and accelerates the graphitization to less than 2 hours. As a proof of principle, we will show two sections of 14C corresponding to two recent expeditions carried out in the North Atlantic (OVIDE section) and the Fram Strait in 2018. The high precision of the results allows for the characterization of different water masses in the subpolar North Atlantic Ocean, which reflect the export of anthropogenic carbon to the abyssal waters as a result of deep-water formation in the Iceland-Scotland Overflow Water and the Denmark Strait Overflow Water. Results will be also compared to previously published oceanic Δ14C data in those regions. These studies already demonstrate the potential to use Δ14C as a powerful and cost-efficient tool to resolve oceanic circulation patterns, especially with respect to ventilation of the water column.Casacuberta, N., Castrillejo, M., Wefing, A.-M., Bollhalder, S., & Wacker, L. (2019). High Precision 14C Analysis in Small Seawater Samples. Radiocarbon, 00(00), 1–12. https://doi.org/10.1017/rdc.2019.87

The Last Glacial Maximum (LGM) is marked by significant cooling of the global ocean, which was recently estimated to 2.6°C using noble gases trapped in ice cores (1). This cooling is not equally distributed throughout the world oceans, since global ocean circulation models predict regional temperature anomalies during the LGM of up to 7°C (annually and zonally averaged) when compared to modern interior ocean temperature (2). The oceans deep interior thus became haline stratified (3) due to the drop in temperature to near freezing and the global increase in salinity from ice sheet growth. In contrast to a deepening of the modern thermocline as a result of anthropogenic global warming, cooling causes the thermocline to rise in the sub-tropics as more polar waters enter the mid-depth ocean.Here we present glacial thermocline temperature reconstructions since the LGM based on the Li/Mg ratio in aragonite skeletons of precisely dated cold-water corals. Corals have been collected from 300-1000m water depths from sites in the northern and southern Atlantic (62°N to 25°S) and demonstrate synchronous 5 - 7°C glacial cooling, and a dramatic shoaling of the thermocline. Through the deglaciation the warming of the upper thermocline ocean occurs early in the southern hemisphere followed by fluctuating warming and thermocline deepening in the northern Hemisphere, which supports the oceanic climate seesaw proposed by Stocker and Johnson in 2003 (4). We thus propose dramatic changes in export of polar waters towards the Equator and augmented subsurface ocean stratification leading to a mostly polar Atlantic with a shallow permanent thermocline. This shoaling possibly increased the rate of nutrient recycling causing higher biological surface ocean activity and the cooling promoted carbon storage. During the glacial, we assume an atmospheric forcing, such as equatorward displacement of the Hadley circulation, to steer the glacial polar water advance as mid-depth boundary currents in the northern and southern hemisphere to effectively spread the cold water through the entire mid-depth Atlantic.References:Bereiter et al.: Mean global ocean temperatures during the last glacial transition. Nature 553, 39-44 (2018). Ballarotta et al.: Last Glacial Maximum world ocean simulations at eddy-permitting and coarse resolutions: do eddies contribute to a better consistency between models and palaeoproxies?, Clim. Past 9, 2669-2686 (2013). Adkins et al.: The Salinity, Temperature, and d18O of the Glacial Deep Ocean. Science 298, 1769-1773 (2002). Stocker and Johnsen: A minimum thermodynamic model for the bipolar seesaw, Paleoceanography 18, 1087 (2003).

A new method to extract CO2 in seawater samples for the determination of F14C has been developed in the Laboratory of Ion Beam Physics at ETH Zurich. The setup consists of an automated sampler designed to extract dissolved inorganic carbon (DIC) from 7 samples in a row, by flushing the seawater with He gas to extract CO2. The fully automated method is controlled via a LabVIEW program that runs through all consecutive steps: catalyst preconditioning, CO2 extraction, CO2 trapping, thermal CO2 release from the trap into the reactor and finally the graphitization reaction which is performed simultaneously in the 7 reactors. The method was optimized by introducing a Cu-Ag furnace that was placed between the water and zeolite traps, which resulted in a better and faster graphitization performance (14C) as water mass tracer.

Anthropogenic chemical tracers are powerful tools to study ocean circulation timescales, water mass provenance and mixing regimes. In the Arctic Ocean, the releases of artificial radionuclides from European nuclear reprocessing plants (RPs) act as valuable transient tracers as they label the inflowing Atlantic Waters with a distinct anthropogenic signal. In recent years, the combination of the two long-lived radionuclides 129I and 236U has emerged as a new tracer pair and several studies have shown their potential to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait.The circulation times of Atlantic-origin waters in the Arctic Ocean that were inferred using this tracer pair (in combination with the naturally occurring 238U) agree to those obtained by means of other transient tracers. Moreover, the combination of 129I and 236U promises to be a useful marker of water mass mixing regimes both in the surface waters and the subsurface Atlantic layer. In particular, the interface between Atlantic and Pacific Waters in the polar surface layer of the Arctic Ocean can be easily identified as these two water masses are labelled by very different 129I/236U and 236U/238U atom ratios.Here we present a compilation of 129I and 236U in a quasi-synoptic pan-arctic section including the Fram Strait and we show how this data can be used to gain information about circulation patterns. We discuss timescales and transport characteristics of Atlantic Water flow, the position and variability of the front between Atlantic and Pacific Waters and the temporal variability of Pacific Waters in the Fram Strait.

ISSN:0148-0227 ISSN:2169-9275

High-precision U-series dating of scleractinian cold-water corals is a key chronological tool for studies of past environmental and climate conditions. Here, we tested and optimized an automated chemical extraction system (ESI prepFAST-MC equipped with an Eichrom TRU-resin chromatographic column) for its ability to purify U and Th isotopes for mass spectrometric U-series dating at the sub-‰ precision level. Chemical yields are constantly high, on average around 90% for both U and Th. Analytical blanks are comparable to manual purification (< 0.15 pg U, 0.15 pg Th for a typical sample of 50 mg) and memory effects due to the recycling of the column are mostly insignificant as the carry-over fraction is about 10− 5 for Th and 10− 3 for U isotopes. However, it was found that the built-in UTh (TRU-resin) column must be pre-conditioned for analysis using > 1 μg g− 1 Th and U in order to achieve the above mentioned chemical yields. This conditioning has no impact on the Th/U data. The automated chemical preparation protocol described here is compared to conventional high precision U-series dating with manual sample purification. For the 34 cold-water corals extracted from a sediment core collected from a coral mound off Angola, the differences between 230Th/238U- and 234U/238U-ratios and U-series ages measured with the two analytical methods are smaller than the respective analytical uncertainty of < 3.0‰, 0.8‰ and 3.0‰, respectively. Overall, ages of the studied corals span 34,000 years and perfectly meet quality control constrains, such as initial seawater δ234U0. Finally, our record of coral ages indicates vigorous coral growth under warm and cold climate conditions in the temperate south-eastern Atlantic, contrasting climate influenced coral occurrences in the north-eastern Atlantic. ISSN:0009-2541 ISSN:1872-6836

Geophysical Research Abstracts, 21, ISSN:1029-7006, ISSN:1607-7962

The Nd isotopic composition (eNd) of seawater and cold-water coral (CWC) samples from the Gulf of Cadiz and the Alboran Sea, at a depth of 280–827 m were investigated in order to constrain middepth water mass dynamics within the Gulf of Cadiz over the past 40 ka. eNd of glacial and Holocene CWC from the Alboran Sea and the northern Gulf of Cadiz reveals relatively constant values (−8.6 to −9.0 and −9.5 to −10.4, respectively). Such values are similar to those of the surrounding present-day middepth waters from the Mediterranean Outflow Water (MOW; eNd ∼ −9.4) and Mediterranean Sea Water (MSW; eNd ∼ −9.9). In contrast, glacial eNd values for CWC collected at thermocline depth (550–827 m) in the southern Gulf of Cadiz display a higher average value (−8.9 ± 0.4) compared to the present-day value (−11.7 ± 0.3). This implies a higher relative contribution of water masses of Mediterranean (MSW) or South Atlantic origin (East Antarctic Intermediate Water, EAAIW). Our study has produced the first evidence of significant radiogenic eNd values (∼ −8) at 19, 23–24, and 27 ka, which are coeval with increasing iceberg discharges and a weakening of Atlantic Meridional Overturning Circulation (AMOC). Since MOW eNd values remained stable during the last glacial period, it is suggested that these radiogenic eNd values most likely reflect an enhanced northward propagation of glacial EAAIW into the eastern Atlantic Basin.

Anthropogenic chemical tracers are powerful tools to study pathways, water mass provenance and mixing processes in the ocean. Releases of the long-lived anthropogenic radionuclides 129I and 236U from European nuclear reprocessing plants label Atlantic Water entering the Arctic Ocean with a distinct signal that can be used to track pathways and timescales of Atlantic Water circulation in the Arctic Ocean and Fram Strait. Apart from their application as transient tracers, the difference in anthropogenic radionuclide concentrations between Atlantic- and Pacific-origin water provides an instrument to distinguish the interface between both water masses. In contrast to classically used water mass tracers such as nitrate-phosphate (N:P) ratios, the two radionuclides are considered to behave conservatively in seawater and are not affected by biogeochemical processes occurring in particular in the broad shelf regions of the Arctic Ocean. Here we present a time-series of 129I and 236U data across the Fram Strait, collected in 2016 (as part of the GEOTRACES program) and in 2018 and 2019 (by the Norwegian Polar Institute). While the overall spatial distribution of both radionuclides was similar among the three sampling years, significant differences were observed in the upper water column of the EGC, especially between 2016 and 2018. This study is the first attempt to investigate the potential of 129I and 236U as water mass composition tracers in the East Greenland Current (EGC). We discuss how the 129I - 236U tracer pair can be applied to estimate fractions of Atlantic and Pacific Water, especially considering their time-dependent input into the Arctic Ocean., EGUsphere