ALBERT

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.