CL4.25

The Lomonosov Ridge (LR) and Fram Strait (FR) represent prominent morphologic features in the Arctic Ocean. Their tectonic evolution control ocean circulation, sedimentation environment, glacial processes and ecosystem through time. We present findings of a 300 km long seismic transect from the Gakkel Deep through the southeastern Amundsen Basin (AB), and onto the LR. The data image an up to 3 km thick sedimentary sequence that can be subdivided into six major seismic units.

The two lower units AB-1 and AB-2 consist of syn-rift sediments of Paleocene to early Eocene age likely eroded off the Barents-Kara and Laptev Sea shelves, and the subsiding LR.

AB-2 includes the time interval of the “Azolla event,” which is regarded as an era of a warm Arctic Ocean punctuated by episodic incursions of fresh water. The connection to North Atlantic waters via the Fram Strait was not yet established, and anoxic conditions prevailed in the young, still isolated Eurasian Basin. Also, the LR still was above or close to sea level and posed an obstacle for water exchange between the Eurasian and Amerasian basins.

The top of AB-2 onlaps the acoustic basement at magnetic anomaly C21o (∼47.3 Ma). Its contact with unit AB-3 above is marked by a striking loss in reflection amplitudes. This prominent interface can be traced through the AB, indicating widespread changes in tectonic and deposition conditions in the Arctic Ocean since the middle Eocene. For younger crust the depth of acoustic basement rises significantly, as well as the deformation of the surface. Both are probably linked to a reorganization of tectonic plates accompanied by a significant decrease in spreading rates.

Units AB-3 and AB-4 indicate the accumulation of sediments between the middle Eocene and the earliest Miocene. Erosional, channel-like interruptions indicate these layers to reflect the stage when Fram Strait opened and continuously deepened. Incursions of water masses from the North Atlantic probably led to first bottom currents and produced erosion, slumping, and subsequent mixing of deposits.

The upper units AB-5 to AB-6 show reflection characteristics and thicknesses similar all over the Arctic Ocean indicating that basin-wide pelagic sedimentation prevailed at least since late Oligocene. Drift bodies, sediment waves, and erosional structures indicate the onset of a modern ocean circulation system and bottom current activity in the early Miocene in the Amundsen Basin. At that time, the FR was developed widely, and also the LR no longer posed an obstacle between the Amerasia and Eurasia Basins. Lastly, unit AB-6 indicates pronounced variations in the sedimentary layers, and is associated with the onset of glacio-marine deposition since the Pliocene (5.3 Ma).

How to cite: Weigelt, E., Gaedicke, C., and Jokat, W.: Coupled evolution of tectonic, ocean circulations, and depositional regime in the southeastern Amundsen Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5190, https://doi.org/10.5194/egusphere-egu21-5190, 2021.

The Kerguelen Plateau, southern Indian Ocean rises up 2000 m above the surrounding seafloor and hence forms an obstacle for the flow of the Antarctic Circumpolar Current (ACC) and the Antarctic Bottomwater (AABW). The ACC is strongly deviated in its flow towards the north. A branch of the AABW flows northwards along the eastern flank of the plateau and in its path is steered by several basement highs and William’s Ridge. Seismic data collected during RV Sonne cruise SO272 image sediment drifts shaped in the Labuan Basin, which document onset and variabilities in pathway and intensity of this AABW branch in relation to the development of the Antarctic ice sheet and tectonic processes, e.g., the opening of the Tasman gateway. The eastern flank of the Kerguelen shows strong erosion of the post-mid Eocene sequences. In places, the Paleocene/early Eocene sequences are also affected by thinning and erosion. A moatcan be observed along the Kerguelen Plateau flank indicating the flowpath of the north setting AAWB branch. Sediment drifts and sediment waves are formed east of the moat. Similar features are observed at the inner, western flank of William’s Ridge thus outlining the recirculation of the AABW branch in the Labuan Basin. The chronological and spatial will be reconstructed via the analysis of those sedimentary structures to provide constraints on climate and ocean circulation variability.

How to cite: Uenzelmann-Neben, G., Schneider, M., Westerhold, T., and Zirks, E.: Sediment drifts at the eastern Kerguelen Plateau: Achives of climate and circulation development, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-141, https://doi.org/10.5194/egusphere-egu21-141, 2021.

Formation of Antarctic Bottom Water (AABW) plays an essential role within Meridional Overturning Circulation and is widely accepted to be the engine of global Thermohaline Circulation (THC), which is sensitive to climate changes. Studying paleo conditions and changes of AABW distribution during warm and cold periods is fundamental to gain knowledge about its interaction and response to climate changes, which helps to understand recent and future changes of THC due to global warming.

West of Prydz Bay, along MacRobertson Land Shelf area, a recent production of dense shelf water in the Cape Darnley Polynya and outflow as so-called Cape Darnley Bottom Water (CDBW) along the Wild Canyon has been recognized. CDBW contributes around 6-13% to the total circumpolar AABW. In order to understand the paleo conditions of AABW it is necessary to investigate the paleo-evolution of CDBW. To do this, we have studied the formation history of a 200 km long sediment drift (Darnley Drift herein) at the western flank of the Wild Canyon. We utilized more than 13.000 km of multi-channel seismic reflection data and lithological Data of ODP Site 1165.  

We characterized Darnley Drift to be a mixed turbiditic-contourite drift formed by an interplay of downslope and along-slope processes. During the Oligocene, turbiditic outflow dominated along the later formed Wild Canyon. An onset of CDBW can be inferred during the early Miocene, forming an asymmetric channel-levee system along the Wild Canyon. After the mid-Miocene Climatic Optimum a major climate change occurred, resulting in a strong intensification of bottom currents and major growth of the drift with simultaneous areas of non-deposition and erosion. This was followed by a sharp reduction of sedimentation rates. Since the late Miocene the growth of Darnley Drift is further dominated by contourite bottom currents.

How to cite: Nielsen, R. and Uenzelmann-Neben, G.: Cenozoic reconstruction of Cape Darnley Bottom Water paleo-distribution imprinted in the drift formation history off MacRobertson Land Shelf, East Antarctica, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8061, https://doi.org/10.5194/egusphere-egu21-8061, 2021.

The South Orkney Microcontinent (SOM) is located in the central sector of the South Scotia Arc, at the Weddell Sea northern edge. The SOM is the largest continental block in the southern Scotia Arc with a surface of more than 70.000 km². Its current location is the result of the continental break-up from the Antarctic Peninsula related to the Powell Basin opening, considered one of the first steps in the formation of the Drake Passage during the Eocene-Oligocene.

In this work we present a 3D geological model of the SOM built with Geomodeller® using free-air gravity anomaly data from Topex and magnetic data from WDMAM. To obtain a reliable result, some constrains have been taken into account: (1) GEBCO data are used to establish the bathymetric level, (2) basement depth and geometry is calculated from multi-channel seismic profiles over the study area obtained from the Seismic Data Library System (SDLS), and (3) the analytic signal of total field magnetic anomalies has been used to limit the extension of the bodies that cause the PMA (Pacific Margin Anomaly).

All these data, together with additional geological and geophysical interpretation, have allowed to build the 3D model. The characterization of the sedimentary basins shape, the deep crust structure and Moho geometry, the volume of the magnetic bodies and the nature and geometry of the SOM margins will provide a better understanding of the complex SOM structure resulting from different tectonic phases since the Mesozoic and related to the Scotia-Drake opening.

The preliminary result shows a good fit between the observed and calculated gravimetric anomaly. We are currently working on the gravimetric inversion to obtain an optimal adjustment.

How to cite: Morales-Ocaña, C., Bohoyo, F., Escutia, C., Marín-Lechado, C., Druet, M., Rey-Moral, C., and Galindo-Zaldívar, J.: 3D geological modelling of the South Orkney Microcontinent (southern Scotia Arc, Antarctica) from seismic and potential field data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10699, https://doi.org/10.5194/egusphere-egu21-10699, 2021.

The possible causes of the onset of Antarctic glaciation around the Eocene-Oligocene Transition (EOT), approximately 34 million years ago (~34Ma), are poorly understood. Uncertainties particularly remain over the role of the Drake Passage opening on the development of the Antarctic Circumpolar Current (ACC), and how this affected both marine and terrestrial environments. A major obstacle in understanding the role of the opening Drake Passage and ACC in Cenozoic climate changes has been the lack of continuous records spanning the EOT from the region. Here we present new palynomorph data from ODP Leg 113 Site 696 Hole B, recording changes in terrestrial environments and paleoclimate across the EOT. The sporomorph assemblage reveals the presence of Nothofagus-dominated forests with secondary Podocarpaceae and an understory of angiosperms and cryptogams growing across much of the Northern Antarctic Peninsula and South Orkney Microcontinent during the late Eocene (~37.60-34.95 Ma). Palaeoclimate reconstructions show that these forests grew under wet temperate conditions, with mean annual temperature and precipitation around 12°C and 1650mm, respectively. Today, similar temperate Nothofagus-dominated mixed-podocarp forests occur in the temperate Valdivian region of southern Chile. At the onset of the EOT, the palynomorph assemblage indicates an unusual expansion of gymnosperms and cryptogams, accompanied by a rapid increase in taxa diversity between ca. 34 and 32 Ma. Sporomorph based climate reconstructions do not provide evidence for an abrupt cooling at the EOT but reveal the onset of prolonged cooling phases throughout the early Oligocene. A contemporaneous increase in reworked Mesozoic sporomorphs at the EOT is likely to be linked to frequent glacial advances from the Antarctic Peninsula and South Orkney Microcontinent, although iceberg-rafted debris from Antarctica cannot be ruled out. We conclude that climate instability and glacial related disturbance at the onset of the EOT facilitated the suppression of Nothofagus and the expansion of a more diverse vegetation with many pioneer taxa that were able to quickly colonise during glacial retreat cycles.

How to cite: Thompson, N., Salzmann, U., López Quirós, A., Escutia, C., Bijl, P., Hoem, F., Etourneau, J., Sicre, M.-A., Roignant, S., and Amoo, M.: Southern high latitude vegetation change across the Drake Passage region linked to prolonged intervals of climate cooling during the early Oligocene, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9503, https://doi.org/10.5194/egusphere-egu21-9503, 2021.

Considered as one of the most significant climate reorganisations of the Cenozoic period, the Eocene-Oligocene (E/O) Transition (ca. 33.9-33.5 Ma) is characterised by global cooling coupled with glacial advance on Antarctica. Combined micropalaeontological (diatom and dinoflagellate) and sedimentological evidence hint of regional reorganisation of ocean currents around Antarctica, in association with the Eocene-Oligocene transition. The late Eocene to early Oligocene deepening of the Tasman Gateway resulted in the flow of warm surface waters from the Australo-Antarctic gulf into the southwestern Pacific Ocean.  However, the extent and effect of these changes in ocean circulation on regional terrestrial climate and vegetation across the E/O Transition is not readily known. Here, we present new well-dated, high resolution palynological (sporomorph) data from the East Tasman Plateau (ODP Site 1172) to reconstruct climate and vegetation dynamics from the late Eocene through to the early Oligocene. Results from our sporomorph data point to four vegetation communities occupying Tasmania under different precipitation and temperature regimes: (i) Paratropical rainforest along the coastlines and temperate rainforests at higher altitude of the hinterlands from 37.97-37.52 Ma; (ii) cool temperate forest expanding into areas previously occupied by the paratropical forests between 37.306-35.60 Ma; (iii) a complex mix of paratropical associations coexisting with frost-tolerant taxa, followed by a period of relative stability shown in the dominance of cold-temperate taxa from 35.50-33.36 Ma; (iv) a warm temperate forest present in the early Oligocene (33.25-33.06 Ma). Our sporomorph record showed a general cooling trend from the latest-middle Eocene to the late Eocene (37.97-35.60 Ma), fluctuations between warm and cold climates (35.50 – 34.19 Ma), a period of relative stable cooling across the E/O transition (33.94-33.5 Ma), and a rather unusual rapid warming right after the E/O transition (earliest Oligocene; 33.36 - 33.06 Ma). Our quantitative estimates of terrestrial temperature change and palaeoecological reconstructions show a close link with previously published dinoflagellate cyst data from this same study site, suggesting a possible vegetation and climate response to tectonic changes (most likely the tectonic opening and deepening of the Tasman Gateway ca. 35.5 Ma) and relative short-term regional reorganisation of ocean currents.

Keywords: Antarctica, Eocene-Oligocene Transition, sporomorph, temperate rainforest, Tasman Gateway

How to cite: Amoo, M., Salzmann, U., Bijl, P. K., and Thompson, N.: New palynological data from the East Tasman Plateau (ODP Site 1172) indicate rapid earliest Oligocene warming., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9831, https://doi.org/10.5194/egusphere-egu21-9831, 2021.

The repeated proximity of West Antarctic Ice Sheet (WAIS) ice to the Ross Sea continental shelf break has been inferred to directly influence sedimentary processes occurring on the continental slope. Sediment delivery to the shelf edge by grounded ice sheets during past glacials may have influenced turbidity current and debris flow activity, thus the records of these processes can be used to study the past history of the WAIS. However, the continental slope record may also be affected by density-driven or geostrophic oceanic bottom currents, therefore additionally providing an archive on their history and interplay with depositional mechanisms that are driven by ice sheets. Here, we investigate the upper 120.94m of one sediment core (length: 208.58mbsf) from Hole U1525A collected by International Ocean Discovery Program (IODP) Expedition 374 in 2018. Hole U1525A is located on the south-western levee of the Hillary Canyon (Ross Sea, Antarctica) and the depositional lobe of the nearby trough-mouth fan. Using core descriptions, grain size analysis, and physical properties datasets, we develop a lithofacies scheme that allows construction of a detailed depositional model and environmental history of past ice sheet-ocean interaction at the eastern Ross Sea continental shelf break/slope for the past 2.4 Ma. The earliest Pleistocene interval (2.4-1.35 Ma) is interpreted as a largely hemipelagic environment dominated by ice-rafting and reworking/deposition by relatively persistent bottom current activity. Microfossil barren, finely interlaminated sediments are interpreted as contourites deposited under the presence of multi-year sea-ice. During the latter part of the early Pleistocene (1.35-0.8 Ma), bottom current activity was weaker and turbiditic processes more common, likely related to the increased proximity of grounded ice at the shelf edge. Much of the fine-grained sediments were probably deposited via gravitational settlement from turbid plumes, and a sustained nepheloid layer. The thickest interval of turbidite interlamination occurs after ~1 Ma, following the onset of the “Mid-Pleistocene Transition” (MPT), interpreted as a time when most terrestrial ice sheets increased in size and glacial periods were longer and more extreme. Sedimentation in the mid-late Pleistocene (< ~0.8 Ma) was dominated by glacigenic debris flow deposition, as the trough mouth fan that dominates the eastern Ross Sea continental shelf prograded and expanded over the site. More frequent and longer-lasting fully-extended glacial conditions allowed the continued progradation of the trough-mouth fan across the core site. These findings will help to improve estimations of WAIS ice extent in future Ross Sea shelf-based modelling studies, and provide a basis for more detailed analysis of the formation and growth of the WAIS under distinct oceanographic conditions.

How to cite: King, M., Gales, J., Laberg, J. S., McKay, R., De Santis, L., Kulhanek, D., Hosegood, P., Morris, A., Rebesco, M., and Expedition 374 Scientists, I.: Timing, frequency and nature of sedimentary processes operating on the eastern Ross Sea continental slope during the Pleistocene- a record from IODP Expedition 374, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3009, https://doi.org/10.5194/egusphere-egu21-3009, 2021.

Modern observations of the West Antarctic Ice Sheet (WAIS) show relatively warm ocean water causing negative changes in ice-sheet mass. The largest ice mass loss in the WAIS occurs in the Amundsen Sea region, where warm water flows onto the shelf and melts the marine-based ice shelves, a process with the potential to lead to full collapse of the WAIS. Geologic records from similar and warmer climate conditions than today are required to understand the role of changes affecting the Amundsen Sea drainage sector in steering past WAIS dynamics. International Ocean Discovery Program (IODP) Expedition 379 successfully recovered sediment drill cores from two sites on the continental rise in the Amundsen Sea, West Antarctica. Both sites are located on a large sediment drift that provides a continuous, long-term record of glacial history in a drainage basin that is fed exclusively by the WAIS. Sediments at both sites are associated with depositional processes related to glacial extent on the shelf. Repeated alternations of two major facies groups composed of dark-gray laminated silty clay and massive/bioturbated greenish-gray, clast-bearing mud are interpreted to represent cycles of glacial and interglacial periods. High-resolution sedimentological analyses define characteristics that vary within the two broad sedimentary facies, helping provide constraints on depositional processes of the sediments and controlling WAIS dynamics.

Detailed investigations were conducted on Miocene to Pliocene strata using grain size and shape analysis, combined with X-Ray Fluorescence data and computer tomography scans, as well as detailed thin section analysis. Laminated silty clay intervals contain consistently fine-grained sediments dominated by terrigenous components that were supplied by downslope transport during glacial periods. Massive/bioturbated muds with ice rafted debris (IRD) display variable grain size trends accompanied by changes in particular elemental ratios indicating increased supply of biogenic components and possibly reduced delivery of terrigenous detritus during interglacial periods. The boundaries between massive, interglacial facies and laminated, glacial facies are usually sharp; although occasionally, a more gradual interglacial-glacial transition is observed. Different sedimentation patterns suggest fluctuations in downslope transport and bottom current intensities that are connected to ice sheet extent on the West Antarctic continental shelf. Further analysis may reveal facies characteristics that vary with glacial-interglacial cycles and allow improved interpretation of past WAIS dynamics and Southern Ocean circulation.

How to cite: Robinson, D. E., Wellner, J. S., Gohl, K., and Reinardy, B. T. I. and the IODP Expedition 379 Scientists: Sedimentary signature of past West Antarctic Ice Sheet and ocean dynamics from deep sea drill cores in the Amundsen Sea (IODP Expedition 379), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13136, https://doi.org/10.5194/egusphere-egu21-13136, 2021.

The earliest Cenozoic evolution of the Mid-Norwegian and Lofoten-Vesterålen continental margin (~65-70^o N) involved rifting, opening and finally seafloor spreading, initiating the Norwegian-Greenland Sea. These events resulted in large morphological and structural variations along the margin, creating accommodation space in a deep- to shallow-marine setting that allowed for accumulation of the Miocene sediments of the Kai- and Molo formations. The Cenozoic seismic stratigraphic correlation between the wide Mid-Norwegian and the narrow Lofoten-Vesterålen margin is poorly established. We therefore here analyze a large database of seismic data and exploration wellbores to give new insights on the sedimentary processes and paleo-environments during the Miocene evolution of this complex continental margin segment.

Steeply dipping clinoforms of the Molo Formation testify to a Miocene coastal outbuilding on the eastern part of the northern Mid-Norwegian margin. West of this, elongated sediment accumulations oriented in an along-slope SSW-NNE direction characterize the palaeo-slope. These are up to ~200 km long, between 40 and 110 km wide and up to ~520 m thick. An internal divergent reflection configuration characterize the elongated accumulations and they typically display a progressive upslope onlapping relationship onto an overall gently westward-dipping underlying morphology that includes domes, highs and ridges. Small incisions are frequently observed in association with the upslope onlap. These characteristics are altogether typical of contourites deposited from ocean currents. In the Vøring Basin, the internal seismic configuration can be described as consisting of low to moderate amplitude parallel-layered reflections, which are interpreted to represent a deep-water hemipelagic setting.

On the much narrower Lofoten-Vesterålen margin, parts of the Kai Formation show a seismic reflection configuration similar to what is observed on the northern Mid-Norwegian margin (e.g. elongated character, divergent internal reflections). These sediments are therefore also interpreted to be contouritic- and hemipelagic deposits. In contrast to the northern Mid-Norwegian contourites, the Lofoten-Vesterålen contourites are generally thinner, and they onlap onto an underlying steeply dipping continental slope, a slope which is also characterized by submarine canyons. Downslope of these, depocenters oriented perpendicular to the margin (i.e. slope-parallel), suggest influence of downslope processes through the canyons.

Our preliminary results show the presence of several contourite build-ups on the investigated margin, indicating the occurrence of a well-established ocean circulation with a persistent current direction along the Norwegian margin during deposition of the Kai Formation. The main source area for these sediments were likely south of the Mid-Norwegian margin. Coastal outbuilding in the Molo Formation and canyon-fed sediment input also testify to a sediment input from the east in the Miocene, and some of these were likely also re-distributed by ocean currents.

How to cite: Bjordal Olsen, S., Rydningen, T. A., Laberg, J. S., Lasabuda, A. P. E., and Knutsen, S.-M.: Miocene continental margin growth dominated by deposits from ocean currents – an example from offshore Norway, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8688, https://doi.org/10.5194/egusphere-egu21-8688, 2021.

The Eurasian Ice Sheet Complex was the world’s third largest ice mass during the last glacial maximum (LGM), and included the British, Fennoscandian and Svalbard–Barents Sea ice sheets. Of these three, the mostly marine-based Svalbard-Barents Sea Ice Sheet (SBIS) is the least well constrained in terms of ice sheet dynamics and deglacial retreat patterns. Improving the understanding of the behavior and decay of this marine paleo-ice sheet can provide knowledge that is relevant to understanding the future evolution of the marine terminating ice margins in Greenland and Antarctica, which are today undergoing rapid retreat and thinning.

We present high-resolution TOPAS sub-bottom profiler data and multi-proxy analyses of four sediment gravity cores (1.15 to 5.05 m long) retrieved from water depths of c. 250-550 m in a trough south of Kvitøya, NW Barents Sea. The data were collected during the Nansen Legacy (https:/arvenetternansen.com/) Paleo-cruise in 2018, with the aim of reconstructing the patterns and timing of deglaciation of the SBIS and postglacial environmental changes in the northern Barents Sea. The data show a succession of up to 10 m high and 400 m wide ridges, interpreted to be recessional push-moraines, representing small still-stands or re-advances of the ice front during its retreat in southwesterly direction. An up to 40 m high and 20 km long sedimentary wedge in the central and western part of the study area buries some of these moraines. This wedge is interpreted to be a grounding zone wedge representing a major still-stand or re-advance during the deglaciation.

The gravity cores are located distal to, on the distal slope and on top of the grounding zone wedge. A muddy diamict defines the lowermost unit in each core. It is interpreted to be primarily subglacial till. This till is covered by laminated mud, interpreted to represent sedimentation from meltwater plumes that emanated from the nearby ice margin. Massive marine mud containing scattered clasts (the clasts are interpreted to be ice rafted debris) define the uppermost unit in all cores. This is suggested to represent deposition from suspension settling and ice rafting in a glacier-distal environment at the end of the last glacial, as well as during modern conditions.

Radiocarbon dates (submitted for dating) will provide a minimum age for the formation of the grounding zone wedge and the recessional moraines in front of it. This will improve the chronology on the deglacial events forming these deposits and landforms. Together with detailed multi-proxy analyses of the sedimentary units, this will also provide new knowledge about the development from glacial conditions to a glacier-proximal and –distal, and an open marine environment from the last glacial to the present.

How to cite: Trælvik Eilertsen, V., Tom Arne, R., Forwick, M., Winsborrow, M., and Laberg, J. S.: Depositional environments in the northern Barents Sea, from the last glacial to the present — preliminary results, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12617, https://doi.org/10.5194/egusphere-egu21-12617, 2021.

The ice cap of the sub-Antarctic island South Georgia is influenced by the Antarctic Circumpolar Current and is hence more sensitive to changing climate than the significantly larger and more isolated Antarctic ice sheets. Furthermore, the sediment deposits in fjords and glacially eroded troughs around the island have superbly archived glacier behavior, environmental and climatic changes since the late Pleistocene. This makes South Georgia an attractive target to study past climate variability in the Southern Hemisphere. Nevertheless, the ice sheet’s extents and dynamics during the Last Glacial Maximum (LGM), the Antarctic Cold Reversal (ACR), and the Holocene deglaciation phase are still poorly understood. Although several studies on land and in marine near-shore areas of South Georgia have addressed this, only few studies are based on marine sediment cores from the continental shelf. In this study, we use ten gravity cores from three different troughs on the southern and northwestern shelf to further investigate the climatic and glaciological evolution of South Georgia during and since the LGM.

Multi-proxy sedimentological analyses carried out in this study include core logging, XRF geochemical profiling, XRD analyses on bulk sediment and clay fraction, measurements of physical properties, magnetic susceptibility, grain size distribution and shear strength. For the Drygalski Trough on the southern shelf, lithofacies description reveals the deposition of stratified, predominantly sandy diamicton and greenish-grey massive to laminated, sometimes bioturbated mud with variable amounts of clasts. First radiocarbon ages from benthic foraminifera constrain the deposition of the diamicton, interpreted as waterlain till, on the outer shelf to the LGM. Inferred linear sedimentation rates attest to low sediment input on the outer shelf during the LGM (34 cm/ka) and the Holocene (23-32 cm/ka). In contrast, a higher sedimentation rate (114 cm/ka) between 14.7 and 13.7 cal ka BP is likely associated with enhanced erosion due to a possible re-advance of South Georgia’s glaciers during the ACR’s colder and wetter climate. For island-proximal cores, sedimentation rates are generally higher than on the outer shelf with rates of 80-2300 cm/ka during the Mid- to Late Holocene. This stronger fluctuation of sedimentation rates is due to higher temporal resolution of the dated sediments compared to the outer shelf. Grain-size distribution on the outer shelf shows a gravel content of 1-28 wt% in the diamicton facies from the LGM and 1-5 wt% in a sediment interval dated to 16.8 cal ka BP. This sediment interval is also characterized by a high content of pebbles, likely reflecting an increased input of IRD. The overlying ACR and Holocene show a low gravel content of 0-0.7 wt%. The diamicton suggests that ice-proximal conditions prevailed on the outer shelf during the LGM and therefore supports the theory of a shelf-wide glaciation. The combination of a low-resolution sediment core from the outer shelf and island-proximal high-resolution sediment cores has the potential to give new insights into South Georgia’s climate history from the Late Pleistocene to the Late Holocene.

How to cite: Lešić, N.-M., Streuff, K., Kuhn, G., Bohrmann, G., and von Dobeneck, T.: The late Pleistocene and Holocene glaciation history of sub-Antarctic South Georgia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6763, https://doi.org/10.5194/egusphere-egu21-6763, 2021.

During the Holocene, 180 m of diatom ooze sediments were deposited in the Antarctic Wilkes Land margin continental shelf at site U1357A (Integrated Ocean Drilling Program Expedition 318, Escutia et al., 2011). Holocene sediments are dominated by rhythmic laminated deposits above a poorly sorted gravelly siltstone diamicton from the Last Glacial Maximum (LGM). CT-scans reveal three events of gravel/sand/silt sediments interbedded within the laminated sediments and interpreted as ice rafted debris (IRD).  Two of these events (from 185,1 to 185,45 and 174, 8 to 175,37 meters below seafloor, mbsf) are characterized by dispersed large clasts (1-5cm) within a muddy matrix at the base, transitioning to the top to millimetre-size clasts that are either aligned with the dark and light laminae or dispersed. A third event (176,2 to 177,2 mbsf) is characterized by a structureless sediment sequence with high concentrations of dispersed clasts that are up to 1-2 cm size. We used ImageJ/Fiji software, to conduct a quantitative analysis of grains bigger than 1mm in CT Scan 3D images. Measured parameters include grain size (Feret length), grain orientation (Feret angle), circularity and roundness, among other. In addition, grey scale profiles have been created from the sediment CT-scan images as a density proxy. Quantitative data and density profiles have been used to aid the sedimentological characterization of the Holocene deglaciation section and to infer depositional environment and patterns of deglaciation.

Escutia, C., Brinkhuis, H., Klaus, A., and the Expedition 318 Scientists, Proc. IODP, 318: Site 1357. Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.318.105.2011

How to cite: Gutierrez-Pastor, J., Escutia, C., Röhl, U., Salabarnada, A., and Jimenez-Espejo, F.: Sedimentological characterization of coarse sediments within the deglaciation sequence in the east Antarctic Wilkes Land margin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11181, https://doi.org/10.5194/egusphere-egu21-11181, 2021.

Ice rafted debris (IRD) in marine sedimentary sequences provide critical information about the evolution of ice sheets. These include enigmatic phases of ice sheet instability like Heinrich events, Dansgaard-Oeschger cycles or Bond events. Higher sampling resolution and greater spatial coverage in IRD records can help gain a better understanding of paleoaclimate, and help predict the future behavior of ice sheets. However, creating high-resolution IRD-records from marine sediment cores is a manual time- and labor-intensive laboratory procedure. By allowing for rapid and non-destructive quantification of micrometer (µm) scale particles, Computed Tomography (CT) holds the potential to increase both resolution and the pace of analysis. We demonstrate the potential of this approach with results from both experimental results and application on conventionally analyzed records. By using basic image processing tools on CT-imagery of phantom-boxes (replicating marine sediment cores with IRD) we counted sand-particles (>150 µm) of different mineralogies ranging from 25-2000 particles/g. The CT-results proved to match the manual counts with a r² of up to 0.99 and a P-value of 0.00. Further, when applying the method on segments of natural marine sediment cores with published IRD-records, we were able to reconstruct the same trends as continuous counts with a 5 times higher spatial resolution. In addition, this non-destructive method gave further information on the impact of bioturbation, grainsize distribution and the sedimentary structure of IRD-deposits. In conclusion, this work can help the field to gain an even better understanding of the behavior of ice sheets by optimizing the efficiency and spatial resolution of IRD-records, while at the same time gaining a better understanding on the processes affecting the IRD-deposits.

How to cite: Cederstrøm, J. M., Rutledal, S., Støren, E. W. N., and van der Bilt, W. G. M.: Semi-automated quantification of Ice Rafted Debris in sediment archives with Computed Tomography., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10455, https://doi.org/10.5194/egusphere-egu21-10455, 2021.