Search

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).

The Northeast Atlantic is crucial regarding the northward heat and carbon export into the Arctic Ocean. At the surface and at mid-depth (0 -1000 m), however, most of the water re-circulates through two basin scale gyres, with warm and salty waters in the sub-tropical gyre (STG) and significantly fresher and colder waters in the sub-polar gyre (SPG). In addition, the Azores Front (AF) separates the northeastern branch from the southeastern branch of the STG, which is today positioned around 34.5°N in the east Atlantic. Underneath both gyres newly formed deep waters from the Arctic Ocean and Labrador Sea are spread into the Atlantic basin. Here we investigate, whether these water masses and recirculation patterns reveal distinct histories of carbon uptake and advection in the eastern North Atlantic. Therefore, water samples spanning the entire water column have been collected at 6 stations along a north-south transect at 25°W spanning from 42° to 61°N in 2012 (N/O Thalassa ICE-CTD cruise). In 2018 (RV Meteor M151 ATHENA cruise) samples further south were collected spanning until 29.5°N thus including the present day AF.The radiocarbon content of Dissolved Inorganic Carbon (DIC) from 8 profiles (N>60, 30° to 61°N, 50-3000m depth) were measured at the AMS facility at CEZA facility in Mannheim following CO2 extraction from seawater at Heidelberg University. Δ14C values range from +50 ‰ in the upper layers of the subtropical Atlantic to -100 ‰ in 3000 m depth also in the subtropical Atlantic. Three main feature appear in the radiocarbon distribution. The surface shows a moderate difference between SPG and STG Δ14C values of 14C values of nearly 0‰ are found identical to the modern Northern Hemisphere atmosphere. In contrast, the STG (30°-43°N) reveals up to 50‰ enriched water reflecting limited carbon uptake from the atmosphere. Thus this layer will act as a source of radiocarbon to the polar seas and atmosphere in the near future. Below, between 1000 and 2000 m water masses north of the AF reveal a nearly homogeneous Δ14C value of -10‰ with a moderate decreasing trend with depth. South of the AF Δ14C values show a strong decrease with depth from 0 to -75‰, hence water masses remain still little affected by the advection of bomb radiocarbon (thus anthropogenic carbon). Thus, as expected the AF and the mid-depth gyre play a crucial role in distributing carbon throughout the east Atlantic.

The climatic controls of stalagmite radiocarbon (14C) remain one focus of modern paleoclimatology due to recent efforts and achievements in 14C calibration. The Hulu cave 14C record (Cheng et al., 2018) has proven the potential of stalagmites from temperate climate zones for atmospheric 14C reconstruction. However, a constant dead carbon fraction (DCF) in stalagmites over long periods is rather exceptional. In our study, a high-resolution 14C record (N=111) of a precisely U-Series dated stalagmite from Sofular Cave (Northern Turkey) with elemental Mg/Ca ratio data is presented. A phase of low and constant DCF (12.5% ± 1.6%, N=20) between 10 and 14 kyr BP, together with relatively stable Mg/Ca ratios suggest stable hydrological soil/karst conditions above the cave. However, we observe unstable soil conditions for the period before 14 kyr BP where DCF is strongly variable between a lower threshold of approximately 5% and an upper limit of 25%. Near a phase of slow growth at ~17 kyr BP DCF as high as 38% is observed on sub-centennial timescales. The combination of stable isotopes, element ratios, radiocarbon and U-series data allows for multi-proxy analysis of the impact of rapid climate changes like D/O events on the incorporation of 14C into stalagmites. Between 15 and 27 kyr BP, hydrological changes have a large impact on limestone dissolution systematics which is reflected in fast DCF variations on sub-centennial timescales. A growth stop between 21 and 23 kyr BP is resolved. Although a comprehensive reconstruction of atmospheric 14C variations is not possible for the entire growth period, the stalagmite reproduces the deviation from the IntCal13 record (Reimer et al. 2013) seen in the Hulu 14C data at ~40 kyr BP during the Laschamp geomagnetic reversal and provides further inside on the climate dependency of 14C incorporation in stalagmites.ReferencesCheng, H., Lawrence Edwards, R., Southon, J., et al.: Atmospheric 14C/12C changes during the last glacial period from Hulu cave, Science, 362(6420), 1293–1297, doi:10.1126/science.aau0747, 2018.Reimer, P. J., Bard, E., Bayliss, A., et al.: IntCal13 and Marine13 Radiocarbon Age Calibration Curves 0–50,000 Years cal BP, Radiocarbon, 55(4), 1869–1887, doi:10.2458/azu_js_rc.55.16947, 2013.

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).