Paleobathymetry of the Southern Ocean and its role in paleoclimate variations
Geophysics
Final Report Abstract
The development of the circum-Antarctic Southern Ocean has a key significance in changes of the atmospheric, oceanographic and cryospheric fabric of the Earth. The Southern Ocean developed from a series of isolated basins to a single circum-Antarctic ocean system with deepwater connections between the Pacific, Atlantic and Indian Ocean in the Cenozoic. Especially since the Eocene-Oligocene boundary at 34 million years ago (Ma), the development of the Southern Ocean has acted as one of the main controlling but also sensitive factors of the Earth’s climate system. We constructed a suite of new paleobathymetric grids of the Southern Ocean from 34 Ma to recent by compiling and analyzing records of 150 DSDP/ODP/IODP drill sites and more than 500,000 km of seismic profiles. Establishing a unified seismic stratigraphy and applying a quantitative backstripping technique to map paleo-water depths of relevant past time slices was a time-consuming but rewarding effort. The intermediate data of sediment thicknesses and volumes from this ‘Big Data’ analysis were already utilized to study the growth of Antarctic continental shelves due to glacially induced progradational sediment deposition, to contribute to a global compilation and study of sediment thicknesses in the oceans, and to derive a series of paleotopography models of the Antarctic mainland for the last 34 million years. These Antarctic paleotopography grids provide important boundary conditions for models seeking to understand past behaviour of the Antarctic ice sheets and the implications for changes in global ice volume, temperature and sea level across major climate transitions of the Cenozoic. Further in-depth analyses of specific paleo-climatic aspects of the sedimentary patterns in the Southern Ocean are followed. The entire suite of paleobathymetric grids encompasses the transition from isolated ocean basins to deep-water exchange between interconnected Southern Ocean basins, the establishment and modes of the intensity of the Antarctic Circumpolar Current, the varying transport of sediments by waxing and waning of the Antarctic ice sheets as well as the periods of ice sheet re-advance after warm periods. Throughout the glacial history of the Antarctic continent, the main sedimentation has been located in similar depositional areas. In East Antarctica, changes in regional main glacial outlets can be observed, and today’s main regional outlets only gained importance after the complete transition into full glacial conditions during the late Miocene. We observe two main phases of glacial deposition in the Southern Ocean which are related to the first occurrence of the continental ice sheet in the early Oligocene and during the late Miocene after the reestablishment of full glacial conditions after the Mid-Miocene Climatic Optimum. Submarinebased portions of both ice sheets recovered back to full glacial conditions at a slower rate. After the Miocene, changes in sedimentation are most noticeable along the outer Antarctic shelves with shelf progradation enlarging the shelves towards modern bathymetry. An unusually thick sedimentary package with very large sedimentation rates of up to 30 cm/kyr during the Early Oligocene was observed on the abyssal plain offshore Wilkes Land. We infer that this massive accumulation of potentially biogenic material is the result of spatial and temporal unique oceanographic and climatic conditions in the Australian-Antarctic corridor. The combination of a newly glaciated continent increased nutrient supply and a clock-wise ocean circulation that transported warmer waters and created an upwelling cell, lead to enhanced biogenic sedimentation in the deep sea. The increased productivity and carbon sequestration potentially impacted the reduction of atmospheric CO2 concentrations, tipping the climatic scales towards colder conditions. The role of the Southern Ocean gateways for Antarctic glacial developments was analyzed based on the new paleobathymetric grids. The result shows strong indications on the relevance on glaciation depending on the widths and depths of only one or both major gateways between the Pacific, Atlantic and Indian Ocean. The datasets generated as part of this DFG funded project are an invaluable data treasure that will be further utilized for a number of additional analysis and studies by various national and international groups, including ours at AWI, working on paleoclimate and paleo-ice sheet issues.
Publications
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(2019). GlobSed: Updated total sediment thickness in the World’s oceans. Geochemistry Geophysics Geosystems, 20, 1756-1772
Straume, E.O., Gaina, C., Medvedev, S., Hochmuth, K., Gohl, K., Whittaker, J.M., Abdul Fattah, R., Doornenbal, J.C., Hopper, J.R.
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(2019). Reconstructions of Antarctic topography since the Eocene- Oligocene boundary. Palaeogeography Palaeoclimatology Palaeoecology, 535
Paxman, G.J., Jamieson, S.S.R., Hochmuth, K., Gohl, K., Bentley, M.J., Leitchenkov, G., Ferraccioli, F.
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(2019). Seaward growth of Antarctic continental shelves since establishment of a continent-wide ice sheet: Patterns and mechanisms. Palaeogeography Palaeoclimatology Palaeoecology, 520, 44-54
Hochmuth, K., Gohl, K.
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(2020). The evolving paleobathymetry of the circum-Antarctic Southern Ocean since 34 Ma – a key to understanding past cryosphere-ocean developments. Geochemistry Geophysics Geosystems
Hochmuth, K., Gohl, K., Leitchenkov, G., Sauermilch, I., Whittaker, J.M., Uenzelmann- Neben, G., Davy, B., De Santis, L.
