ALBERT

Description: Contourite deposits found in the Indian-Atlantic ocean gateway hold detailed information on past changes in the bottom water flow history over long time intervals of the Cenozoic. Until IODP Exp. 361 only late Pleistocene paleoceanographic studies for the region were carried out using sediment samples obtained from piston cores. We present preliminary results from Site U1475 (Agulhas Plateau), a location proximal to the entrance of North Atlantic Deep Water (NADW) to the Southern Ocean and South Indian Ocean. The site is located over a sediment drift in 2669 m water depth and comprises a complete stratigraphic section of the last ∼7 Ma. The whole spliced sediment record (292 meters) of Site U1475 was measured using an X-ray fluorescence (XRF) core scanner to derive multi-centennial resolution records of major element intensities. Based on these measurements it is possible to derive of biogenic (e.g. %CaCO3) and siliciclastic (e.g. TiO2, K2O) mineral phases. Because the Ca counts almost exclusively stem from biogenic carbonate formed by microfossil shells the ratio of biogenic components vs. terrigenous is reflected in e.g. the Ca/Ti or Ca/Fe records. On the other hand, changes of elemental ratios such as K/Fe or Al/Ti show the variability within the terrigenous sediment fraction. While long-term changes in elemental ratios can be linked to the seismic reflection pattern associated with deep water circulation changes, short-term cyclicities in sediment provenance reflect orbital scale Plio-Pleistocene climate variations. E. g. power spectra performed on the ln(Ca/Ti) and ln(K/Fe) records for a peculiar Pliocene (~5.7 - 4.1 Ma) high sedimentation rate interval reveal significant spectral density peaks at periods close to the Milankovitch precession band (19 – 23 kyr). Such a high variability in the precession band is also evident in a number of other element ratios while changes in physical properties (e.g. density, seismic impedance) for the same interval seem to be dominated by eccentricity. We present evolutionary spectral analyses revealing how the orbital response of the different parameters have changed over time and derive an improved age model based on cyclostratigraphy.

Description: Contourites are deposits formed by along-slope bottom currents and are therefore sensitive to changes in current velocity, sediments supply and paleoceanographic conditions. They are typically associated with high accumulation rates making these archives ideal for paleoenviromental reconstructions. Nevertheless, they are also occasionally affected by winnowing of fine particles and erosion/deposition of allochthonous material, which alters the grain-size and mineralogy. These processes can, as such, promote significant bias in proxy interpretation compared with other pelagic deposits. X-ray fluorescence (XRF) core-scanning is ideal to assess elemental variations in these high accumulation rate sequences. The comparison between lithological changes, Natural Gamma Ray and other parameters with XRF scanning data, along with statistical analysis can provide very useful information to support improved proxy interpretation. Using this approach at Site U1387, (detrital contourite system at Gulf of Cadiz), results indicate that the Zr/Al ratio represents a promising proxy for bottom current speed and show the transition from a hemipelagic to a contouritic system during the Miocene/Pliocene transition. Carbonate content and Ba/Al ratio appear to represent paleo-productivity variations and later to be completely overprinted by current activit y. At Site U1475 (carbonate contourite system at Agulhas Plateau) Zr content is just one artifact associated with high Sr content and the Ca/Sr ratio appears to be a more promising proxy for contourite reconstruction that is influenced by carbonate dissolution by deep corrosive waters. Comparing both locations we can conclude that proxies associated with the continuous background sediment settling over the seafloor (e.g. planktonic foraminifera) do not appear to be severely biased in countourite systems.

Description: Atmosphere-ocean interactions play an important role for understanding processes and feedbacks in the Southern Ocean (SO) and are relevant for changes in Antarctic ice-sheets and atmospheric CO2 concentrations. The most important atmospheric forcing at high and mid-latitudes of the Southern Hemisphere is the westerly wind belt (SWW), which strongly affects the strength and extension of the Antarctic Circumpolar Current (ACC), upwelling of deep-water masses, and controls the back-flow of intermediate waters to the tropics. In order to address orbital and millennial-scale changes of the SWW and the ACC, we present sediment proxy records from the Pacific SO including the Chilean Margin and the Drake Passage. The Drake Passage (DP) represents the most important oceanic gateway along the ACC. Based on grain-size and geochemical properties of sediment records from the southernmost continental margin of South America, we reconstruct changes in DP throughflow dynamics over the past 65,000 years. In combination with published sediment records from the Scotia Sea and preliminary sediment records from the central Drake Passage (Polarstern cruise PS97, 2016), we argue for a considerable total reduction of DP transport and reveal an up to ~40% decrease in flow speed along the northernmost ACC pathway entering the DP during glacial times. Superimposed on this long-term decrease are high-amplitude millennial-scale variations, which parallel Southern Ocean and Antarctic temperature patterns. The glacial intervals of strong weakening of the ACC entering the DP imply a reduced Pacific-Atlantic exchange via the DP (“cold-water route”). The reduced Drake Passage glacial throughflow was accompanied by a pronounced northward extension of the Antarctic cold-water sphere in the Southeast Pacific sector and stronger export of northern ACC water into the South Pacific gyre. These oceanographic changes are consistent with reduced SWW within the modern maximum wind strength zone over the subantarctic ACC and reduced wind forcing due to extended sea-ice further south. Despite this reduction in winds in the core of the westerlies, we observe 3-fold higher dust deposition during glacial periods in Past Antarctic Ice Sheet Dynamics (PAIS) Conference September 10-15th 2017, Trieste - Italy the Pacific Southern Ocean (SO). This observation may be explained by a combination of factors including more expanded arid dust source areas in Australia and a northward extent or enhancement of the SWW over Southeast Australia during glacials that would plausibly increase the dust uptake and export into the Pacific SO. Such scenario would imply stronger SWW at the present northernmost margin of the wind belt coeval with weaker core westerlies in the south and reduced ACC strength, including Drake Passage throughflow during glacials. We conclude that changes in DP throughflow play a critical role for the global meridional overturning circulation and interbasin exchange in the Southern Ocean, most likely regulated by variations in the westerly wind field and changes in Antarctic sea-ice extent. Keywords: Pelagic Southern Ocean, Antarctic Circumpolar Current, Southern Westerlies, Teleconnections.

Description: The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled. An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50°E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.

Description: The paleoclimate and ecosystem variability in Africa during the Plio/Pleistocene has received considerable attention due to its potential links to hominid evolution. However, the reconstruction of this variability hinges critically upon highly temporally resolved proxy data from continuous, well-dated sediment archives. In light of these requirements we use a new XRF core-scanning record from International Ocean Discovery Program (IODP) Site U1478 off the Limpopo River mouth (Mozambique Channel, SW Indian Ocean) spanning the past c. 4 Ma to identify the climate variability in SE Africa. Our results show that the elemental distribution in the Site U1478 cores is mainly controlled by the rate of terrigenous input and – to a lesser extent – by bottom-current transport and post-depositional processes such as propagation of paleoredox boundaries and diagenesis across some intervals. The log(Ti/Ca) ratio, which is used as a tracer of terrigenous sediment input, shows quasi-cyclical variability across the entire record that closely matches the periods of orbital parameters. However, the cyclical behaviour of the log(Ti/Ca) signal varies through time, with the uppermost 106 m of the sequence (0–1.07 Ma) displaying a mix of precession and obliquity signals, the intervals 106–223 m (1.07–2.80 Ma) and 240–257 m (3.68–4.05 Ma) being dominated by precession, and the interval 223–240 m (2.80–3.68 Ma) being controlled by eccentricity. To refine the available chronology for Site U1478, which is based on shipboard biostratigraphic and paleomagnetic data, we have tuned the log(Ti/Ca) record to the LR04 benthic oxygen isotope record, summer insolation at 25° S, and orbital eccentricity depending on the dominant cyclicities in the XRF dataset across individual time intervals. The resulting chronology enables us to evaluate the XRF data as well as the previously available shipboard sedimentological and geochemical datasets within a regional and global climatic context. This allows the connection of a c. 7-m-thick contourite deposit and a prominent paleoredox boundary to hydroclimate and ocean-circulation changes during the early Pleistocene and across the Mid-Pleistocene Transition, respectively. Moreover, a decoupling of the log(Ti/Ca) and the log(Ti/K) records, with the latter indicating the degree of sediment weathering, from 3.2 to 2.8 Ma points to an increased delivery of highly weathered sediments to Site U1478. We attribute this to temporarily wetter and warmer conditions in the catchment of the Limpopo River and/or a change in the sediment source, perhaps associated with the tectonically driven enlargement of the Zambezi River catchment during the late Pliocene/early Pleistocene.

Description: Efforts to understand long-term Indian Ocean dynamics and land-sea linkages in southeast Africa during periods of significant global and regional climate change have been inhibited by a lack of high-resolution climate records, particularly during the Plio-Pleistocene. Here we present new biomarker and pollen records from International Ocean Discovery Program (IODP) Site U1478, located at the Upper Agulhas Confluence near the Limpopo River mouth, to establish environmental conditions at the southeast African margin between 4 and 1.8 Ma and address this spatiotemporal gap. Compound-specific hydrogen isotopes of terrestrial leaf waxes (δDwax) and TEX86, using marine archaeal lipids, document hydroclimate variability and sea surface temperature (SST), respectively, permitting an onshore-offshore climate comparison. The U1478 records establish the Limpopo catchment response to the switch in Indonesian Throughflow source waters, the mid-Pliocene Warm Period, and intensification of Northern Hemisphere glaciations at ~2.7 Ma. Broad coherence between the δDwax and SST records supports a linkage between Indian Ocean temperatures and southeast African hydroclimate. We hypothesize that additional mechanisms including Indian Ocean cross-basin SST gradients (ΔSST) and high latitude glaciation acted as hydroclimate controls during the Plio-Pleistocene. We use ΔSST to evaluate ocean-atmosphere patterns similar to the Indian Ocean Dipole (IOD) and establish generally wetter conditions in the region associated with positive IOD-like phases. Additionally, an obliquity signal evident in the δDwax record indicates that glacial-interglacial variability likely influenced the tropical rain belt position and also controlled rainfall. Hydroclimate and environmental conditions across the Plio-Pleistocene in southeast Africa may have important implications for regional hominin evolution.

Description: The teleconnections existing between low- and high-latitude water masses are a critical component of the Earth’s climate system. One region of global significance is the Indian-Atlantic Ocean gateway (I-AOG) that lies off the southern tip of Africa. Today, the regional oceanography is dominated by the Agulhas Current and its leakage into the South Atlantic, the so-called “warm water route,” the Agulhas Current Retroflection and the Agulhas Return Current’s interaction with the subtropical front (STF). However, the connections between these frontal systems and links to high latitude climate dynamics are not well understood. On glacial/interglacial timescales, biogenic silica (BSi) production in the I-AOG, close to the STF, has been linked to transport of high-latitude silica rich intermediate waters to this site throughout the Pleistocene (Romero et al., 2015). Here we present BSi accumulation rates, diatom assemblage data, and bulk sediment chemistry (XRF) for Pliocene age sediments from IODP Site U1475 (41°25.61’S; 25°15.64’E, 2669 m water depth). While the overall BSi content remains low (0.5 to 4.18 weight %), increases in diatom accumulation are seen at four distinct intervals (4.6 to 4.3 Ma, 3.3 Ma, 3.14 Ma, and 2.8 to 2.6 Ma). Additionally, XRF records of Si/Al and Ba/Al appear to coincide with the BSi record and we infer that nutrient-rich intermediate waters “leaked” from the Southern Ocean and resulted in increased BSi production. Additionally, an increase in opal accumulation at the Site between 4.6 to 4.3 Ma, is likely tied to a reorganization of global nutrient pools associated with the shoaling of waters at the Panama Canal. During the shoaling event BSi production increases at Site U1475 are coincident with a reduction in BSi production in the Galapagos Basin (e.g. ODP Site 846), thus suggesting a potential connection between Pacific and Atlantic Ocean basins during the Pliocene. An increase in abundance of Southern Ocean diatoms from 3.9 Ma to 2.6 Ma may also indicate water mass cooling and increased transport of high-latitude intermediate waters. The Site U1475 record of microfossils highlights a trend of intermediate water input throughout the Pliocene. Additionally, comparisons with the so-called “cold water route” will allow a better view of global intermediate water changes during the Pliocene.

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.