During the Cenozoic (66–0 Ma) Tasmania has continuously been at a crucial geographic location. It represented the final tectonic connection between Australia and Antarctica before complete separation of both continents in the late Eocene, and therefore a barrier for circumpolar flow. Since the Eocene-Oligocene transition, the northward drifting Tasmania was bathed by the throughflow of the subtropical front, but remained an obstacle of the ideal flow path of strengthening ocean currents. The sedimentary record around Tasmania thus represents a perfect archive to record the oceanographic consequences of this regional tectonic change. We here present a new TEX₈₆ and U_K’³⁷-based SST compilation from 4 sediment cores: ODP Site 1172 (East Tasman Plateau), Site 1170 and 1171 (South Tasman Rise) and Site 1168 (western Tasman margin). We paired these reconstructions with microplankton (dinoflagellate cyst) assemblage data which reflect qualitatively the surface water conditions: nutrients, temperature, salinity. Together, the >1.300 samples portray the SST evolution around the island, from the time it was still connected to the Antarctic continent in the Paleocene to its near-subtropical location today. Trends in the SST compilation broadly follow those in benthic foraminiferal stable isotope compilations, but with some interesting deviations. Differences in SSTs on either side of the Tasmanian Gateway are small in the early Paleogene (66–34 Ma), even when the Tasmanian Gateway is considered closed. Widening of the Tasmanian Gateway around the Eocene-Oligocene transition (34Ma) immediately allows throughflow of what later becomes the Leeuwin Current, which warms the sw Pacific. Oligocene and Neogene SST trends follow those of the benthic d¹⁸O, and with continuous influence of the proto-subtropical front. While the SST evolution of Tasmania is remarkably stable in most of the Oligocene, prominent cooling steps are inferred in the Late Oligocene (26 Ma), at the MMCT (~14 Ma), in the mid-to-late Miocene (9 Ma, 7 Ma and 5.3 Ma) and in the Pliocene (2.8 Ma). The remarkably strong Neogene cooling of the subtropical front implies expansion of subpolar temperate conditions and probably gradual strengthening of the Antarctic circumpolar current. Pliocene-Pleistocene SST variability is strong over glacial-interglacial cycles. Taken together, the sites portray a complete overview of local environmental change of the subtropical front area, and provides crucial context to the history of Southern Ocean heat transport and regional climate.

How to cite: Bijl, P., Hoem, F., Hou, S., Thöle, L., Sauermilch, I., and sangiorgi, F.: The Cenozoic sea surface temperature evolution offshore Tasmania, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2802, https://doi.org/10.5194/egusphere-egu23-2802, 2023.

Earth’s climate during the Neogene period changed in several steps from a planet with unipolar ice sheets to today’s bipolar configuration. Yet, time-continuous and well-preserved sedimentary archives from this time interval are scarce. This is especially true for those records that can be used for tracing the role of astronomical climate forcing. Ocean Drilling Program (ODP) Site 752 was drilled on Broken Ridge (Indian Ocean) and provides a time-continuous sedimentation history since the early Miocene in its upper portion.  To date, no astronomical-scale paleoclimate research has been conducted on this legacy ODP site. Here, we use X-ray fluorescence (XRF) core scanning data and benthic foraminifera (BF) taxonomic and quantitative analyses to reconstruct the paleoceanographic changes in the Indian Ocean since 23 Ma. Productivity-related elements from the XRF dataset, show higher productivity during the early Miocene and late Pliocene/early Pleistocene. Moreover, we found strong 405-kyr and ~110-kyr eccentricity imprints in the spectral analysis result of this XRF-derived paleoproductivity proxy. Although the precession signal is also quite remarkable in the spectral analysis results, the 4-cm resolution may not be adequate to further test the precession contribution. Bottom water oxygenation reconstructed using BF, suggest no oxygen minimum zone conditions for the late Miocene on site 752. Dissolved oxygen concentrations (DOC) indicate low oxic conditions (⁓ 2 ml/L) during this time, and relatively low stress species distribution (< 32%) along with abundant oxic species like H. boueana, C. mundulus, L. pauperata and Gyroidinoides spp. suggest predominantly high oxic conditions during the late Miocene (DOC > 2 ml/L). Meanwhile, the grain size (> 425µm) record shows an increasing trend at ~5 Ma, which indicates more current winnowing. Therefore, we argue that the drop in Mn is the result of the increase in the current winnowing, instead of the OMZ expansion. On the other hand, high-amplitude changes in Fe content from the lower Miocene to the middle Miocene, cannot be explained by eolian input, suggesting the source might be the neighbor-distanced Amsterdam-St. Paul hot spot. The source of Fe might be the neighbor-distanced Amsterdam-St. Paul hot spot. We conclude that the legacy ODP Site 752 constitutes an excellent paleoceanographic archive that allows us to reconstruct Indian Ocean dynamics since the early Miocene. New drillings on Broken Ridge with state-of-the-art scientific ocean drilling techniques will provide more detailed information and be highly beneficial for paleoclimatic and paleoceanographic research.

How to cite: Lyu, J., Bar­ra­gán-Mon­til­la, S., Auer, G., Bialik, O., Christensen, B., and De Vleeschouwer, D.: Astronomically-paced changes in paleoproductivity, winnowing, and mineral flux over Broken Ridge (Indian Ocean) since the Early Miocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5655, https://doi.org/10.5194/egusphere-egu23-5655, 2023.

Chemical weathering of silicate rocks is a well recognized method by which carbon dioxide is removed from the atmosphere and fixed as calcium carbonates in the sedimentary record. For many years the long term cooling of the Earth during the Cenozoic has been linked to uplift, erosion and weathering of the Himalayas and Tibetan Plateau, however following scientific ocean drilling of the submarine fans in the Asian marginal seas it now seems that this region could not be responsible for cooling, at least during the Neogene. Although other factors such as burial of organic carbon and the rates of degassing during seafloor spreading may also be important, erosion and weathering of other regions may also be important in controlling global CO2 concentrations. In this study we focus on the role of New Guinea, the large (>2500 km long) orogen formed as Australia collided with Indonesia since the Mid Miocene. New Guinea comprises slices of arc and ophiolite rocks that are susceptible to weathering, and is located in the tropics where warm, wet conditions favor rapid weathering. Rainfall exceeds >4 m annually in the island center. Analyses of sediment from Deep Sea Drilling Project Sites 210 and 287 in the Gulf of Papua now allow the weathering and erosion history of the island to be reconstructed. A trend to more continental erosion since 15 Ma reflects uplift and erosion of tectonics slices of the Australian plate. At the same time chemical weathering shows increasing intensity, especially since 5 Ma, as proxied by major element ratios (K/Rb, K/Al) and clay minerals. Greater proportions of kaolinite point to more tropical weathering since the Mid Miocene. Trends to more weathering contrast with Himalayan records that show the reverse, and suggest that New Guinea may be an important component in controlling global climate in the past 15 Ma.

How to cite: Clift, P. and Mohtadi, M.: Chemical Weathering in New Guinea since the Mid Miocene, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1997, https://doi.org/10.5194/egusphere-egu23-1997, 2023.

The mid-Pliocene (3.3-3.0 Ma) is a time when the Earth's climate fluctuated between cold glacial conditions (marine isotope stage M2; 3.3 Ma) and periods when global temperatures were ~3°C warmer than the pre-industrial (Mid-Pliocene warm period; 3.3-3.025 Ma) when CO₂ concentrations reached ~400 ppm. Thus, the paleoclimate reconstruction of this time interval provides an analogue of the present-day and near-future climate change in a moderate pCO₂ increase scenario. Although fluctuations in benthic δ¹⁸O in the mid-Pliocene were predominantly associated with Northern Hemisphere glacial dynamics, the contribution of Antarctic ice to mid-Pliocene glacial-interglacial cyclicity is unknown. Moreover, the surface oceanographic response of the Southern Ocean to Pliocene glacial-interglacial climate change is poorly documented

We studied 2 sedimentary records from offshore west Tasmania (ODP Site 1168) and the Agulhas Plateau (IODP Site U1475), both located close to the modern position of the subtropical front (STF) in the Southern Ocean and encompassing the mid-Pliocene. The STF is a crucial surface water mass boundary separating cold, fresher subantarctic waters and warm, more saline subtropical waters and plays a key role in global ocean circulation, ocean-atmosphere CO₂ exchange and meridional heat transport.

We use lipid biomarkers, dinoflagellate cyst assemblages and benthic foraminiferal clumped isotopes to reconstruct surface and bottom oceanographic conditions over the mid-Pliocene including the M2 glaciation. We identify similar sea surface temperature (SST) changes at the two sites. Site 1168 SST cools from 18°C to 12°C and at Site U1475 from 21°C to 18°C across the M2 glaciation. Dinoflagellate cyst assemblages suggest strong latitudinal shifts of the subtropical front associated to Pliocene glacial-interglacial climate changes. However, the most profound assemblage shift occurs at the M2 deglaciation stage at both sites, suggesting strong and unprecedented surface water freshening. Preliminary clumped isotope results suggest bottom water temperatures at Site 1168 are stable around 9°C between M2 and mid-Piacenzian warm period, indicating that the enrichment in δ¹⁸O across the M2 is mainly contributed by large ice volume changes. We interpret the surface water freshening of the subantarctic zone as signaling major iceberg calving following the M2 glaciation, suggesting that the Antarctic contribution to the M2 glaciation was profound.

How to cite: Hou, S., Stockhausen, M., Toebrock, L., Sangiorgi, F., Starr, A., Berke, M., Ziegler, M., and Bijl, P.: Mid-Pliocene subtropical front variability in the Southern Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7924, https://doi.org/10.5194/egusphere-egu23-7924, 2023.

The Agulhas leakage is an important contributor to the global thermohaline conveyor system, adding warm and saline subtropical waters from the Indian Ocean to the South-east Atlantic Ocean. It has been proposed that weaker Agulhas leakage occurred during glacial stages, but that leakage was reinvigorated during deglaciations and was, in turn, potentially important for the development of interglacial warmth.

Little is known about the longer-term evolution of Agulhas leakage during the Pliocene and Pleistocene (the last 5.3 Ma). In the Pliocene, the continental ice sheets were smaller in size, and the position and strength of key ocean and atmosphere circulation systems in the South Atlantic region were different. The Pliocene is also characterised by a series of gateway changes which are argued to have affected North Atlantic climate, but the response of the Agulhas leakage system remains unclear. It is also unclear whether the ‘early deglaciation’ signal is a specific component of the late Pleistocene 100 kyr cycles. Identifying how and when this signal developed could have important implications for understanding the impact of ocean circulation changes on the development of the mid-Pleistocene climate transition (MPT) ~1.2-0.6 Ma, when the period of the glacial-interglacial cycles shifted from ~41 kyr to ~100 kyr.

Here we present initial results from a new Cape Basin site (Site U1479, 35°03.53′S; 17°24.06′E), which was recovered by IODP Expedition 361 in 2016 from the western slope of the Agulhas Bank (Hall et al., 2016). We combine reconstructions of sea surface temperatures (using the alkenone-derived U^K₃₇’ index) and sea surface salinity (from alkenone dD analysis) with details of planktonic foraminifera assemblages, to identify and understand variability in Agulhas leakage operating across both orbital and longer timescales. There is an overall cooling of ~4°C since the Pliocene, but it is focussed around ~2 Ma and from 1.2 Ma. Orbital scale and longer-term variability in SST, sea surface salinity and Agulhas leakage fauna are also determined, demonstrating that the Agulhas leakage system has evolved across a range of timescales through the Plio-Pleistocene, especially in association with the MPT.

References

Hall, I.R., Hemming, S.R., LeVay, L.J., and the Expedition 361 Scientists, 2016. Expedition 361 Preliminary Report: South African Climates (Agulhas LGM Density Profile). International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.pr.361.2016

How to cite: McClymont, E., Caley, T., Charles, C., Starr, A., Sanchez Montes, M. L., West, M., Rossignol, L., Hall, I., and Hemming, S.: Pliocene-Pleistocene evolution of the Agulhas leakage to the Atlantic Ocean, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7477, https://doi.org/10.5194/egusphere-egu23-7477, 2023.

The Mid-Pleistocene Transition (MPT) between ~1200 and ~800 ka was associated with a major shift in global climate and was marked by a change in glacial/interglacial periodicity from ~41 to ~100 kyr that resulted in higher-amplitude sea-level variations and intensified glacial cooling. The Indonesian Throughflow (ITF), which controls the exchange of heat between the Pacific and Indian Oceans, is a major component of the global climate system. On the other hand, Asian-Australian Monsoon dynamics play a key role in regional primary productivity. Therefore, reconstruction of ITF and Asian-Australian Monsoon variability during the MPT could potentially clarify the impact of the glacio-eustatic sea level changes on the climate and ecosystem of Northwest Australia. The International Discovery Program (IODP) Expedition 363 retrieved an extended, continuous hemipelagic sediment succession spanning the past two million years at Site U1483 on the Scott Plateau off Northwestern Australia.

In this study, we analyzed radiolarian assemblages in core top samples retrieved during the RV Sonne Expedition 257 and downcore samples from IODP Site U1483 to estimate the variability in regional sea surface temperatures (SSTs) during the MPT, and to explore ITF dynamics in relation to glacio-eustatic sea-level variations and tropical monsoon strength. We suggest that glacio-eustatic sea-level variations have been a key factor affecting changes in SSTs at Site U1483, primarily because the shallow and hydrogeographically complex nature of the sea means that SSTs are highly sensitive to glacio-eustatic sea-level variation. Based on comparisons with SST data from the mid latitudes off Northwest Australia and the South China Sea, we suggest that the SSTs at Site U1483 are highly dependent on prevailing climate changes in the northern hemisphere rather than changes in the climate of the Southern hemisphere. In addition, comparisons of radiolarian total abundances with X-ray fluorescence-scanning elemental data suggested that, until the onset of the MPT (~1200 ka), radiolarian productivity was higher during strong summer monsoons during interglacial periods, probably because of the high riverine runoff generated by heavy summer monsoonal precipitation. However, since ~900 ka, there appears to have been a shift in the mode of radiolarian productivity that has resulted in increased radiolarian productivity during glacial periods when the delivery of nutrients is increased due to the enhanced mixing of the upper water column in the shallow sea caused by strong trade winds. 

How to cite: Matsuzaki, K., Holbourn, A., Kuhnt, W., Gong, L., and Ikeda, M.: Variability of the Indonesian Throughflow and Australian monsoon dynamism across the Mid Pleistocene Transition (IODP 363, Site U1483), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-41, https://doi.org/10.5194/egusphere-egu23-41, 2023.

Late Quaternary clay mineral assemblages, radiogenic isotope, and siliciclastic grain size records collected from high sedimentation Site U1483 of the International Ocean Discovery Program (IODP), beneath the path of the modern-day Indonesian Throughflow (ITF) and Leeuwin Current of northwest Australia are studied to reconstruct sediments provenance, transport processes and ocean current behavior, and to evaluate the Australian summer monsoon over the last 500 kyr. Clay minerals are primarily composed of smectite (41–70 %), followed by kaolinite (10–28 %), illite (13.5–25 %), and minor chlorite (3–14 %). Our reconstructed model based on the clay minerals source comparison and radiogenic isotope (Sr-Nd-Pb) records suggest the Victoria and Ord rivers of the Kimberley region as the source over the past 500 kyr for Site U1483. Smectite is mainly derived from the mafic volcanic and smectite-rich Bonaparte Gulf, whereas kaolinite and illite are primarily derived from felsic igneous and metamorphic rocks, respectively, found in the drainage areas of these rivers. Chlorite is primarily contributed by the Indonesian Throughflow (ITF), with a minor contribution from the northwest Australian rivers. Variations in the clay mineral assemblages and grain size records indicate strong glacial-interglacial cyclicity, with small grain size, high smectite, and low kaolinite and illite during glacial periods, while interglacial intervals are marked by a relative increase in kaolinite and illite, mean grain size, and decrease in smectite content. (Kaolinite+illite+chlorite)/smectite and kaolinite/smectite ratios are adopted as proxies for the ITF strength and Australian summer monsoon, respectively. High values of kaolinite/smectite and (kaolinite+illite+chlorite)/smectite ratios during the interglacial intervals indicate a wet summer monsoon with high river discharge and a strong ITF and Leeuwin Current, which has the capacity to transport a relatively high percentage of large-size kaolinite and illite sediments to Site U1483. In contrast, during glacials, the low values of kaolinite/smectite and (kaolinite+illite+chlorite)/smectite ratios imply a dry summer monsoon with low sediment discharge and weak ITF and Leeuwin Current, which can majorly carry the small smectite size particles in its suspension. The mean grain size and clay/silt ratio also indicate that the strength of ITF and Leeuwin Current was weak during glacials and gained high strength during the interglacials. The proxy records’ spectral analysis indicates a strong eccentricity period of 100-kyr, an obliquity period of 41-kyr, and a precession period of 23-kyr, implying that the clay mineral input along the northwest Australian margin is influenced by both high-latitude ice sheet forcing and low-latitude tropical processes.

How to cite: Sarim, M. and Xu, J.: Late Quaternary glacial-interglacial variability of the Indonesian Throughflow and Australian summer monsoon: Evidences from clay mineral and grain size records at IODP Site U1483 of northwest Australia, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3430, https://doi.org/10.5194/egusphere-egu23-3430, 2023.

The Indo-Pacific Warm Pool (IPWP) is the warmest and most dynamic ocean-atmosphere-climate system on Earth and has undergone significant climatic changes during the Pleistocene glacial periods (De Deckker et al., 2012; Lea et al., 2000; Russell et al., 2014). During the Last Glacial Maximum, the latitudinal position of the Southern Ocean fronts, both south of Africa and Australia, was shown to be critical in controlling the outflow of warm water of the Agulhas Current from the Indian Ocean and the IPWP area. Yet, there is no direct evidence for such oceanic change on the scale of the Late Pleistocene glacial-interglacial transitions.

Here, we combine sea surface temperature proxies (d¹⁸O and Mg/Ca) with the neodymium (Nd) isotopic signature to reconstruct changes in climate and oceanic circulation in the eastern tropical Indian Ocean over the last 500 ka. The most striking feature of our dataset is the oscillating Nd signal that mimics the glacial-interglacial cycles. While interglacial periods are characterized by a more significant contribution from the less radiogenic Antarctic intermediate water mass (AAIW, ~ -7 ε_Nd), glacial periods are marked by more radiogenic water mass of Pacific origin (~ -5 ε_Nd). We argue that under global cooling, the northward penetration of the AAIW has weakened due to the general slowdown of the global thermohaline circulation. Furthermore, the oscillating pattern is also recorded in the sea surface temperature and salinity, indicating the settlement of cooler and more saline surface water masses probably linked to a less expanded IPWP and weaker Leeuwin Current during glacial intervals.

We suggest that under low AAIW a less intense advective mixing occurred, allowing a deepening of both halocline and thermocline in the tropical eastern Indian Ocean. Our new proxy-derived dataset confirms results from models (DiNiezo et al., 2018), suggesting that these ocean conditions could amplify the externally forced climate changes resulting from drier atmospheric conditions and weaken the monsoon during glacial periods in the Indonesian region.

How to cite: Le Houedec, S., Tremblin, M., Champion, A., and Samankassou, E.: Changes in intermediate circulation waters along the tropical eastern Indian Ocean during quaternary climatic oscillations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2105, https://doi.org/10.5194/egusphere-egu23-2105, 2023.

A collapse of the Atlantic Meridional Overturning Circulation (AMOC) could drive widespread changes in tropical rainfall, but the underlying physical mechanisms are poorly understood. Numerical simulations validated against hydroclimate changes during Heinrich Stadial 1(HS1) – the most recent, best-documented AMOC collapse – show a global response driven by cooling over the tropical North Atlantic. This pattern of ocean cooling is key to link changes in rainfall across the tropics with the reductions in AMOC strength. Cooling over the tropical North Atlantic drives changes over the Pacific and Indian oceans that uniquely explain the paleoclimatic evidence. A similar response is active in simulations of future greenhouse warming, but model disagreement regarding the pattern of AMOC-induced tropical cooling produces divergent rainfall predictions across the tropics. Models with responses consistent with the paleodata predict more pronounced rainfall reductions across the tropics, revealing a heightened risk of drought over vulnerable societies and ecosystems worldwide.

How to cite: DiNezio, P.: The tropical response to a collapse of the Atlantic Meridional Overturning Circulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10410, https://doi.org/10.5194/egusphere-egu23-10410, 2023.