ALBERT

Description: An important element of the global ocean thermohaline circulation is the oceanic connection between the Indian and South Atlantic Oceans off South Africa. Variable amounts of warm, salt-enriched South Indian Ocean waters enter the South Atlantic, the so-called ‘warm water return route’, and provide a source for heat and salt to the Atlantic thermocline that ultimately preconditions the Atlantic meridional overturning circulation for convection in the north, the formation of North Atlantic Deep Water (NADW). This eastward surface return flow is compensated at depth by a westward setting deep flow into the southern Indian Ocean that consists of NADW exiting the South Atlantic and Southern Source Waters (SSW), influenced by the Antarctic Circumpolar Current (ACC). Here we present a high-resolution multi-proxy record of deep water variability from sediment core MD02-2588 (2907 m water depth) and IODP Site U1475 (2669 m water depth) both recovered from the southern flank of the Agulhas Plateau in the southernmost South Atlantic. The location is close to the interface between NADW and SSW in the Southern Ocean enabling the reconstruction of the timing and amplitude of changes in southward advection of NADW and Southern Ocean circulation. We concentrate on identifying the phasing between changes in ice volume, the location of surface ocean fronts, deep ventilation and near-bottom flow speeds over the past 1.5 Ma – across the Middle Pleistocene transition. Our benthic carbon isotope record from MD02-2588/Site U1475 strongly suggest that there was a continued mid-depth northern source water influence over the southern Agulhas Plateau during glacial periods of the past 1.5 Ma. Nonetheless, significantly increased near bottom flow speeds, ~5–10 cm s−1 (3–7 μm coarser), during glacial periods indicates that there must be additional controls on physical ventilation. We suggest that vigor of near bottom currents on the Southern Agulhas Plateau is likely influenced by the orbital scale meridional expansion and contraction of the ACC and its associated surface fronts.

Description: Glacial climate is marked by abrupt, millennial-scale climate changes known as Dansgaard–Oeschger cycles. The most pronounced stadial coolings, Heinrich events, are associated with massive iceberg discharges to the North Atlantic. These events have been linked to variations in the strength of the Atlantic meridional overturning circulation. However, the factors that lead to abrupt transitions between strong and weak circulation regimes remain unclear. Here we show that, in a fully coupled atmosphere–ocean model, gradual changes in atmospheric CO2 concentrations can trigger abrupt climate changes, associated with a regime of bi-stability of the Atlantic meridional overturning circulation under intermediate glacial conditions. We find that changes in atmospheric CO2 concentrations alter the transport of atmospheric moisture across Central America, which modulates the freshwater budget of the North Atlantic and hence deep-water formation. In our simulations, a change in atmospheric CO2 levels of about 15 ppmv—comparable to variations during Dansgaard–Oeschger cycles containing Heinrich events—is sufficient to cause transitions between a weak stadial and a strong interstadial circulation mode. Because changes in the Atlantic meridional overturning circulation are thought to alter atmospheric CO2 levels, we infer that atmospheric CO2 may serve as a negative feedback to transitions between strong and weak circulation modes.

Description: Some studies suggest that specific equilibrium climate sensitivity S might be state-dependent. Reanalyzing existing paleodata of global mean surface temperature ∆Tg and radiative forcing ∆R of CO2 and land ice albedo for the last 800,000 years we show that this state-dependency of S is only found if ∆Tg is based on reconstructions, and not when ∆Tg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land-ice sheet growth and sea level fall) the multi-millennial component of reconstructed ∆Tg is diverging from atmospheric CO2, while in simulations both variables vary more synchronously. For a reconstruction-based extrapolation of S to the future we eliminate these periods due to an expected sea level rise. Consequently, S determined from proxy-based reconstructions without these data with strong ∆Tg-CO2 divergence is less state-dependent or even constant (state-independent), and yields into an equilibrium warming for 2 × CO2 of 1.9–3.8 K.

Description: The Southern Ocean is involved in setting the state of global climate through its role in redistributing heat and salt through the world ocean and its control on atmospheric CO2. Utilising sediment core sites on the southern Agulhas Plateau (AP) in the southwest Indian Ocean, we present new records of ice-rafted debris mass accumulation rate (IRDMAR), intermediate and benthic oxygen and carbon isotope, sortable silt mean grain size and bulk sediment chemistry (XRF) spanning the past 2 Ma. The AP is situated at the southern extent of the Indian-Atlantic Ocean Gateway (I-AOG); the upper water column is dominated by Indian Ocean waters not leaked into the South Atlantic and instead flowing eastward as the Agulhas Return Current. South of the AP, the relatively cold and fresh waters of the Sub-Antarctic Zone (SAZ) meet their northern limit and steep meridional property gradients occur. The AP region is therefore highly sensitive to variations in both the Sub-Antarctic Zone (SAZ) to the south and the Agulhas Current System to the north. IODP Site U1475 (41°25.61’S; 25°15.64’E, 2669 m water depth), was recovered from a contourite drift deposit on the southern AP, situated close to the modern-day subtropical front. Together with complementary data from sediment core MD02-2588 from the same location, our results indicate that during glacial periods there was a persistent influence of a well-ventilated water mass within the I-AOG with a carbon isotope signature similar to present-day Northern Component Water (NCW). The records of chemical ventilation and near-bottom flow vigour closely reflect changes in the advection of NCW and meridional variability in the location of the Antarctic Circumpolar Current and its associated fronts, as recorded by IRDMAR. We suggest that equatorward expansions of the circum-Antarctic frontal system, occurring relatively early in the glacial sequence, are central in triggering this glacial overturning circulation, hence modulating global climate. On orbital timescales, the SAZ represents a window through which external forcing may be translated into the global climate system; likely relevant for the enigmatic Mid-Pleistocene Transition.

Description: To understand how the relationship between ice sheets, oceans and climate will respond to continued anthropogenic warming, it is crucial to examine its evolution in the geological past. A key way that we can reconstruct glacio-marine processes is to study the deposition of sediment transported by free-floating ice to the open ocean. Here, we combine paleo-iceberg trajectory modelling with Pleistocene records of ice-rafted debris (IRD), lithogenic grain size distributions and clay mineralogy from the Indian-Atlantic Ocean Gateway, at the northern limit of the modern Sub-Antarctic Zone (SAZ), on the southern Agulhas Plateau (AP). The records we present are from a continuous splice of sediment core sites MD02-2588 and IODP Site U1475 41°25.61’S; 25°15.64’E, 2669 m water depth), spanning 0 – 1.65 Ma at an average of 1.5-kyr resolution. Given the distal location of the AP from the Antarctic continent, the sustained delivery of IRD is indicative of (likely massive) icebergs, traversing the Southern Ocean before depositing entrained sediment. Both our model analyses and IRD data show that SAZ iceberg rafting was generally higher during Pleistocene glacial periods, facilitated by increased transport and survivability. We characterise the signature of this IRD by mineralogical, geochemical, and grain size analysis. By determining the provenance of this material, it is possible to gain insight into the past export of icebergs from the Antarctic Ice Sheet, in particular identifying the response of marine-terminating glaciers to the range of climate conditions associated with Pleistocene glacial-interglacial cycles. SEM and EDS analysis of the IRD reveal mineralogies indicative of basement crystalline rock, and an absence of volcanic glass. This demonstrates an Antarctic origin for the sediment, as opposed to volcanic inputs from sub-Antarctic island arcs. Furthermore, the presence of garnet bearing an Almandine end-member signature indicates that the IRD deposited on the AP are of Weddell Sea origin. This is coherent with modelled iceberg trajectories showing a high export of icebergs from the Weddell Sea gyre into the Antarctic Circumpolar Current. An equatorward expansion of the SAZ likely increased the proximity of iceberg trajectories to the AP as well as improving the survivability of icebergs through surface cooling. We suggest that this process plays an important role in global climate by modulating the distribution of the SAZ freshwater budget and influencing the mode and intermediate water masses that connect the Southern Ocean to the upper limb of Atlantic Overturning Circulation.

Description: We reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land ice sheet growth and sea level fall) the multimillennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future, we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.

Description: The import of relatively salty water masses from the Indian Ocean to the Atlantic is considered to be important for the operational mode of the Atlantic Meridional Overturning Circulation (AMOC). However, the occurrence and the origin of changes in this import behavior on millennial and glacial/interglacial timescales remains equivocal. Here we reconstruct multiproxy paleosalinity changes in the Agulhas Current since the Last Glacial Maximum and compare the salinity pattern with records from the Indian-Atlantic Ocean Gateway (I-AOG) and model simulations using a fully coupled atmosphere-ocean general circulation model. The reconstructed paleosalinity pattern in the Agulhas Current displays coherent variability with changes recorded in the wider I-AOG region over the last glacial termination. We infer that salinities simultaneously increased in both areas consistent with a quasi interhemispheric salt-seesaw response, analogous to the thermal bipolar seesaw in response to a reduced cross-hemispheric heat and salt exchange during times of weakened AMOC. Interestingly, these hydrographic shifts can also be recognized in the wider Southern Hemisphere, which indicates that salinity anomalies are not purely restricted to the Agulhas Current System itself. More saline upstream Agulhas waters were propagated to the I-AOG during Heinrich Stadial 1 (HS1). However, the salt flux into the South Atlantic might have been reduced due to a decreased volume transport through the I-AOG during the AMOC slowdown associated with HS1. Hence, our combined data-model interpretation suggests that intervals with higher salinity in the Agulhas Current source region are not necessarily an indicator for an increased salt import via the I-AOG into the South Atlantic.

Description: Paleo-climate records and geodynamic modelling indicate the existence of complex interactions between glacial sea level changes, volcanic degassing and atmospheric CO2, which may have modulated the climate system’s descent into the last ice age. Between ∼85 and 70 kyr ago, during an interval of decreasing axial tilt, the orbital component in global temperature records gradually declined, while atmospheric CO2, instead of continuing its long-term correlation with Antarctic temperature, remained relatively stable. Here, based on novel global geodynamic models and the joint interpretation of paleo-proxy data as well as biogeochemical simulations, we show that a sea level fall in this interval caused enhanced pressure-release melting in the uppermost mantle, which may have induced a surge in magma and CO2 fluxes from mid-ocean ridges and oceanic hotspot volcanoes. Our results reveal a hitherto unrecognized negative feedback between glaciation and atmospheric CO2 predominantly controlled by marine volcanism on multi-millennial timescales of ∼5,000–15,000 years.

Description: Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of American Association for the Advancement of Science for personal use, not for redistribution. The definitive version was published in Science 349 (2015): 706-710, doi:10.1126/science.aaa9554.

Description: Changes in the formation of dense water in the Arctic Ocean and Nordic Seas (the ‘Arctic Mediterranean’, AM) likely contributed to the altered climate of the last glacial period. We examine past changes in AM circulation by reconstructing 14C ventilation ages of the deep Nordic Seas over the last 30,000 years. Our results show that the deep glacial AM was extremely poorly ventilated (ventilation ages of up to 10,000 years). Subsequent episodic overflow of aged water into the mid-depth North Atlantic occurred during deglaciation. Proxy data also suggest the deep glacial AM was ~2-3°C warmer than modern; deglacial mixing of the deep AM with the upper ocean thus potentially contributed to melting sea-ice and icebergs, as well as proximal terminal ice-sheet margins.

Description: Funding was provided by a WHOI OCCI scholarship and OCCI grant 27071264 (DJRT); WHOI OCCI and NSF grants OIA-1124880 and OCE-1357121 (GG); the WHOI J. Lamar Worzel Assistant Scientist Fund, the Penzance Endowed Fund in Support of Assistant Scientists and NSF ANT-1246387 (WG); EU FP7 Marie Curie grant No. 298513 (MZ), grant DP140101393 (JY); and NERC grant NE/J008133/1 (SB).