Seismic characteristics of sediment drifts: An example from the Agulhas Plateau, southwest Indian Ocean

Sediment drifts provide information on the palaeoceanographic development of a region. Additionally, they may represent hydrocarbon reservoirs. Because of this, sediment drift investigation has increased over the last few years. Nevertheless, a number of problems remain regarding the processes controlling their shape, the characteristic lithological and seismic patterns and the diagnostic criteria.As an example, sediment drifts from the Agulhas Plateau, southwest Indian Ocean, are presented here. They show a variety of seismic features and facies including an asymmetric mounded geometry, changes in internal reflection pattern, truncation of internal reflectors at the seafloor and discontinuities. This collection of observations in combination with the local oceanography appears to comprise a diagnostic tool for sediment drifts

The Agulhas Ridge, South Atlantic: the peculiar structure of a fracture zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridges location between 80 Ma and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridges structure.The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic.In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material

Kerguelen Plateau Drift Deposits: outstanding high-resolution chronicle of Cenozoic climatic and oceanographic changes in the southern Indian Ocean

Cruise Leg SO272 with RV SONNE, leaving Port Louis, Mauritius, on January 11 2020, returning to Cape Town, South Africa, on March 4 2020, comprised seismic reflection studies and geological sampling of the Kerguelen Plateau in the southern part of the Indian Ocean. The Kerguelen Plateau 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 thereby shaping sediment drifts. A detailed study and analysis of the structure of the Labuan Basin and the central Kerguelen Plateau via seismic data and a correlation with results from DSP Leg 120 Sites 748, 750, and 751 was needed to supply information on the Cretaceous and Tertiary development of the AABW and its influence on the path of the Antarctic Circumpolar Current, This in turn will allow conclusions on the development of the East Antarctic Ice Sheet. Seismic profiles were gathered, which capture the structure of the Labuan Basin and the Kerguelen Plateau to basement and image sediment drifts. In total ~4000 km of high resolution seismic reflection data were recorded. Bathymetric and Parasound data were recorded parallel to the seismic profiling. To complement the seismic studies and provide ages of the outcropping sediment geological samples were retrieved at 11 locations using a gravity corer and multi-corer. Both datasets will form the base for an IODP proposal

The Indian-Atlantic Ocean gateway during the Pliocene: current dynamics and changing sediment provenance

The Pliocene epoch represents a discrete interval which reversed a long-term trend of late Neogene cooling and is also the most recent geological interval in which global temperatures were several degrees warmer than today. It is therefore often considered as the best analogue for a future anthropogenic greenhouse world. However, there is growing evidence that the Pliocene was not a stable period but can rather be subdivided in several distinct climate phases. Our understanding of Pliocene climate variability in the Southern Hemisphere, and especially in the Atlantic-Indian ocean gateway, is limited by scarce marine records and poor age control on existing terrestrial climate archives. At five from six drilling locations of IODP Exp. 361 (Jan. – March 2016) high resolution complete Plio-/Pleistocene sections have been recovered (see IODP Expedition 361 – Southern African Climates and Agulhas LGM Density Profile by Gruetzner et al., this Volume). Our new research proposal focuses on three of these sites forming a latitudinal transect in the Atlantic-/Indian Ocean gateway and combines chemical, physical property and seismic methods. Primary site for our investigations is Site U1475 with the focus on the interplay between northern and southern sourced deep water masses at the Agulhas Plateau. This will be augmented by investigations at Sites U1479 (Cape Basin) and U1474 (Natal Valley), both located in the pathway of modern NADW. Our research is driven by three main working hypotheses: Seismic stratigraphies for the last 6 Ma and sediment drift growth in the Atlantic-Indian gateway are mainly controlled by bottom water flow changes Using the new sediment archives and physical property records from IODP Exp. 361 (Hall et al., 2016) we aim to construct detailed seismic stratigraphies for the Agulhas Plateau, the Natal valley and the Cape basin for the last 6 Ma. At all Exp. 361 sites P-wave velocity and density records are of sufficient quality to enable detailed correlations of drilling results and site survey data through the calculation of synthetic seismograms. Our working hypothesis implies that seismic reflection patterns and sediment accumulation during the Pliocene are closely linked to deep water circulation changes associated with climate Pliocene phases. Furthermore four distinct high latitude Pliocene glaciation events have been identified. We speculate that these phases and events have led to deep water circulation changes in Agulhas region, have altered the sediment physical properties and thus may be recognized as reflectors in the seismic profiles. How did the sediment input of terrigenous vs. biogenic sediment components in the gateway change during these events? Are these changes driven by dilution, dissolution, or productivity? We strive to answer these questions by interpreting the physical and chemical (XRF) core scanning records. Trajectories and intensities of deep water masses in the Agulhas region during the Pliocene were influenced by Antarctic ice volume rather than by the closure of the Central American Seaway. The Exp. 361 drill sites offer the possibility to inter-correlate different flow speed proxies and to derive a detailed picture of flow changes during the Pliocene. By comparing core-measurements of sortable silt (S̅S̅), physical properties and XRF-core scanning data with seismic features we will tie the major flow speed changes to our seismic grid covering the Agulhas Plateau such that changing current intensities and pathways can be mapped together. Here we hypothesize that these changes are mainly driven by climate (Antarctic ice volume). What were the main changes associated with the Pliocene instability of Antarctic ice sheets and was the production of Antartic Bottom Water (AABW) reduced or enhanced during these intervals? How have the sedimentation patterns changed under the growing influence of North Atlantic Deep Water (NADW)? Was there also a potential influence of tectonic processes on the flow changes in the Agulhas region? Especially the closure of the Centarl American Seaway CAS between ~14 and ~2.7 Ma is thought to have had a profound impact on climate. The Pliocene variability in sediment provenance on millennial timescales is subdued when compared to the Pleistocene. Understanding the mechanisms and causes of abrupt climate change is one of the major challenges in global climate change research today and there is growing evidence that millennial scale climate variability was enhanced during times when a critical threshold in continental ice volume was surpassed. Dramatic millennial scale climate shifts are well documented for the “glacial world” of the late Pleistocene but are examined to a much lesser extent for earlier time periods. We suggest testing the potential threshold behaviour for the Atlantic-Indian gateway by comparing short term fluctuations in sediment composition and siliciclastic provenance in the Agulhas region before and after the onset of the Northern Hemisphere glaciation (NHG) at ~2.7 Ma. Time series of sediment provenance dated by “orbital tuning” will be analysed in the time and frequency domain to investigate at what times during the interval 2 – 6 Ma millennial scale climate variability was enhanced or subdued

The structure and classification of the Mozambique Ridge based on seismic reflection data

The Mozambique Ridge (MozR), a prominent basement high in the southwestern Indian Ocean, consists of four major geomorphological units associated with numerous phases of volcanic activity between 140 Ma and 122 Ma. Over the last decades nature and origin of the Mozambique Ridge have been intensely debated with one hypothesis suggesting a Large Igneous Province (LIP) origin. This would have had immense influence on climate during the early Cretaceous with the emission of gases and heat into atmosphere and ocean but also implications on the development of the South African gateway with the formation of obstacles for surface and deep circulation. An extensive seismic survey was conducted over the Mozambique Ridge with the aim of solving the questions about its origin and evolution. High-resolution seismic reflection data reveals a number of magmatic centers with a random distribution. Intra-basement reflections can be identified up to several hundred ms TWT below top of basement. The internal reflections generally dip away from their magmatic centers and individual reflections can typically be traced for 5-15 km. These are interpreted to represent massive lava flow units, which are characteristic of oceanic plateau eruptions. Additionally to primary volcanic features associated with the initial emplacement of the individual segments of the Mozambique Ridge we identify secondary volcanic features indicating magmatic reactivation after its initial build-up. The total volume of the southern Mozambique Ridge is estimated to be 2.2 x 106 km3. We use this estimation to obtain a more precise reconstruction for the emplacement of the Mozambique Ridge. Based on our results we propose an oceanic LIP origin of the southern Mozambique Ridge and show that our data points toward a sequential development of its segments

Fossilized silica diagenetic fronts: Implications for palaeoceanographic evolution across the Falkland/Malvinas plateau

A set of newly collected 2D seismic reflection data allows the mapping of two distinct cross-cutting reflectors across the Falkland/Malvinas Plateau. Reflector XR-F/MB in the Falkland/Malvinas Basin appears as a bottom simulating reflector that mimics the geometry of the present seafloor, whereas reflector XR-F/MT in the Falkland/Malvinas Trough is a non-bottom simulating reflector that mimics a shallower reflector representing theEarly-Middle Miocene unconformity. The discordant geometry of these two reflectors with respect to the hoststratigraphyis argued to be associated with Opal-A to Opal-CT diagenesis, which is primarily a function of temperature. However, the estimated temperature at the present depth for reflector XR-F/MB lies below the minimum temperature for the onset of silica diagenesis. Based on their geometry and seismic characteristics, the two reflectors are interpreted to be fossilized silica diagenetic fronts, formed under palaeo-thermal conditions different from today. We hypothesize that the erosional action of intensified deep and bottom water masses subsequent to Antarctic glaciations during the Early-Middle Miocene may have driven the fossilization of the diagenetic front in the study area. It is estimated that erosion of a minimum of 270 m of overburden would account for the temperature drop driving the fossilization of the silica diagenetic fron

On the paleo footprint of Cape Darnley Bottom Water off MacRobertson Land Shelf, East Antarctica

Cape Darnley Bottom Water (CDBW) is a major contributor to Antarctic Bottom Water (AABW) formation in the MacRobertson Land continental shelf area, East Antarctica. As the production of CDBW is dependent on intense sea ice formation in the Cape Darnley Polynya Region, it is sensitive to climatic changes, such as global warming. Studying paleo-conditions of CDBW during Antarctica’s transition from coolhouse to icehouse during the middle to late Miocene allows to gain knowledge about the onset of bottom water production in the Cosmonaut/Prydz Bay region as well as changes in the strength and outflow path of the CDBW. In order to study the paleo-conditions of the CDBW, we have investigated the formation history of a 200 km long, 70 km wide sediment drift (Darnley Drift herein) at the western flank of Wild Canyon. In the early Miocene, the upper rise was dominated by turbiditic channel-levee growth and large continental sediment supply off the MacRobertson Land continental shelf. Therefore, no indications of CDBW formation were observed. During the middle Miocene, the dominant sediment transport regime transformed from turbiditic to contouritic mode, mirroring Antarctica’s climatic transition into an icehouse world. This climatic transformation caused the initiation of CDBW, which is inferred from the onset of Darnley Drift formation as a levee-drift and its growth to a maximum areal extent of 60,000 km2, four times the size of today. Since the late Miocene/early Pliocene the sedimentation rate has been strongly reduced and bottom current controlled deposition dominated. The growth of mixed levee-drifts along the continental slope-rise transition parallel to Darnley Drift suggests an intensifi- cation of paleo CDBW generation and outflow to be comparable to observations of recent CDBW, in addition to an intensification of the Antarctic Slope Front and Circumpolar Deep Water

Prydz Bay sediment drifts: Archives of modifications in East Antarctic climatic and oceanographic conditions

The detailed onset of the Antarctic glaciation during the Eocene/Oligocene and the later ice sheet dynamic in response to warm phases during the Miocene and Pliocene is still under discussion. Attempts to solve the open questions by scientific drilling have been limited by the fact that early Oligocene to early Miocene sediments, which bear witness to the onset of glaciation and early dynamics of the ice sheet, have been eroded from the continental shelf or are buried below thick Neogene sequences and could thus not be sampled during ODP Legs 119 and 188. Several hypotheses place the onset of bottom water formation as the result of down welling due to strong cooling into the Miocene, the late Oligocene, or the late Eocene, which shows the range of uncertainty in dating this event. The dynamical response, e.g., of the Lambert Glacier-Amery Ice Shelf drainage system to climate variability is recorded in the sediments of Prydz Bay and the adjacent slope and rise of the Cooperation Sea. Thus a study of sedimentary features and structures and the prevailing sediment transport patterns can help to understand the development of this system and its sensitivity to climate change. The analysis of seismic reflection data allows to reconstruct sediment input and sediment transport patterns. This represents an important tool, even if an indirect one, to infer past changes in climate and oceanography in the absence of direct information from drilled geological samples. A large dataset of high-quality seismic lines has been acquired along the Prydz Bay margin, is available via the SCAR seismic data library system and will be analysed with respect to documents of down-slope, i.e., the result of material input via advancing the ice sheet, and along-slope, i.e., features resulting from the shaping of bottom and deep water, to infer past changes in climate and oceanography in combination with results from ODP Leg 119 and 188. This way we also intend to close the gap, which could not be sampled by drilling (the early Oligocene to early Miocene)

Onset and modifications in intensity and pathways of water mass exchange between the Southeast Pacific and the South Atlantic with focus on the Falkland Plateau, Northern Scotia Ridge and the West Georgia Basin, Cruise No. MSM81, February 2 2019-March 15 2019, Valparaiso (Chile) - Montevideo (Uruguay)

Cruise Leg MSM81 with RV MARIA S. MERIAN, leaving Valparaiso, Chile, on February 2 2019, returning to Montevideo, Uruguay, on March 15 2019, comprised seismic reflection studies of the Falkland Plateau, the westernmost part of the Agulhas-Falkland Fracture Zone in the South Atlantic. The Falkland Plateau rises up 1500 m above the surrounding seafloor and hence forms an obstacle for the exchange of water masses between high and lower latitudes. A water mass exchange between the Pacific and Atlantic oceans has been enabled with the opening of Drake Passage. In this way heat and energy could be transferred between the two oceans. A detailed study and analysis of the structure of the Falkland Plateau and channel in the south via seismic data and a correlation with results from DSDP Leg 36 Sites 327, 329, and 330 as well as Leg 71 Site 511 was needed to supply information on the Cretaceous and Tertiary development of the Falkland Plateau and its influence on the path of the Antarctic Circumpolar Current, Upper and Lower Circumpolar Deepwater, South Pacific Deepwater, and Weddell Sea Deepwater. Seismic profiles were gathered, which capture the structure of the Falkland Plateau to basement and possible sediment drifts. In total ~5200 km of high resolution seismic reflection data were recorded. Bathymetric and Parasound data were recorded parallel to the seismic profiling. To complement the seismic studies SVP and XSV measurements at six locations and ADCP measurements across the whole working area were carried out