The Bengal Fan on different temporal and spatial scales : Integrating seismoacoustic and IODP Expedition 354 data to examine internal and external controls on depositional processes

Submarine fans are regarded as excellent archives for long- and short-term climate variations and the continental uplift/erosion history. The Bengal Fan has attracted particular scientific attention as is it the primary sink of Himalayan material and, thus, recorder of one of the key areas in the global climate system. A comprehensive understanding of the fana s complex depositional dynamics is an essential prerequisite to ultimately connect changes in the sedimentary sink with changes in the source. The here presented thesis examines the depositional dynamics of the Bengal Fan. A unique, integrated dataset of echosounder and multichannel seismic data, results from IODP Expedition 354 drilling, and a novel, system-specific age-depth model enables an overarching study of the architecture, stratigraphy, and depositional processes of the Bengal Fan and channel-levee systems at various scales. The outcome of this thesis provides new insights into the architectural and stratigraphic evolution of the lower Bengal Fan and marks a significant advancement towards an extensive understanding of the sedimentary archive in is full complexity

Der Bengalfächer auf verschiedenen räumlichen und zeitlichen Skalen : Die Integration seismoakustischer Daten mit IODP Expedition 354 Daten zur Untersuchung interner und externer Einflüsse auf Ablagerungsprozesse

Submarine fans are regarded as excellent archives for long- and short-term climate variations and the continental uplift/erosion history. The Bengal Fan has attracted particular scientific attention as is it the primary sink of Himalayan material and, thus, recorder of one of the key areas in the global climate system. A comprehensive understanding of the fana s complex depositional dynamics is an essential prerequisite to ultimately connect changes in the sedimentary sink with changes in the source. The here presented thesis examines the depositional dynamics of the Bengal Fan. A unique, integrated dataset of echosounder and multichannel seismic data, results from IODP Expedition 354 drilling, and a novel, system-specific age-depth model enables an overarching study of the architecture, stratigraphy, and depositional processes of the Bengal Fan and channel-levee systems at various scales. The outcome of this thesis provides new insights into the architectural and stratigraphic evolution of the lower Bengal Fan and marks a significant advancement towards an extensive understanding of the sedimentary archive in is full complexity

Middle to Late Pleistocene Architecture and Stratigraphy of the Lower Bengal Fan—Integrating Multichannel Seismic Data and IODP Expedition 354 Results

Utilizing a novel data set of integrated high-resolution multichannel seismic data with IODP Expedition 354 drilling results, a Middle to Late Pleistocene stratigraphy for the lower Bengal Fan is developed. The study reveals a high lateral and temporal variability of deposition expressed by lateral shifts (often exceeding 100 km) between successive channel-levee systems (CLSs), which occurred on average every ~15 kyr independent from sea-level changes. The CLSs are embedded in sheeted sediments deposited out of unchannelized turbidity currents, which represent almost two thirds of the lower Bengal Fan sediments. On 100-kyr timescales, CLSs and sheeted/unchannelized sediments build up subfans, which alternately occupied the western and the eastern Bengal Fan, while the remaining area was draped by ~10 to 20 m-thick layers of background/hemipelagic sediments. Three subfans have been reconstructed: Subfan B (1.24–0.68 Ma) formed concurrently with the Middle Pleistocene Hemipelagic Layer, Subfan C (0.68–0.25 Ma) covered the entire study area, and Subfan D (0.25 Ma to recent) deposited concomitant with the Late Pleistocene Hemipelagic Layer. The continuous succession of subfans indicates an uninterrupted fan activity independent from sea-level cycles at least since the Middle Pleistocene. This remarkable independent behavior in terms of sediment supply has not been observed at the Amazon Fan but is in agreement with observations from the Congo Fan. Finally, the analysis of a complete cross section through the lower Bengal Fan reveals that almost half of the sediment represents sands, indicating that the lower Bengal Fan may not generally be classified as “mud rich” (≤30% sand)

Multichannel reflection seismic data from the Lower Bengal Fan, Profile GeoB97-020-027

Utilizing a novel dataset of integrated high‐resolution multichannel seismic data with IODP Expedition 354 drilling results, a Middle to Late Pleistocene stratigraphy for the lower Bengal Fan is developed. The study reveals a high lateral and temporal variability of deposition expressed by lateral shifts (often exceeding 100 km) between successive channel‐levee systems (CLSs), which occurred on average every ~15 kyrs independent from sea‐level changes. The CLSs are embedded in sheeted sediments deposited out of unchannelized turbidity currents, which represent almost two thirds of the lower Bengal Fan sediments. On 100‐kyrs time‐scales, CLSs and sheeted/unchannelized sediments build up subfans, which alternately occupied the western and the eastern Bengal Fan, while the remaining area was draped by ~10‐20 m thick layers of background/hemipelagic sediments. Three subfans have been reconstructed: Subfan B (1.24‐0.68 Ma) formed concurrently with the Middle Pleistocene Hemipelagic Layer (MPHL), Subfan C (0.68‐0.25 Ma) covered the entire study area, and Subfan D (0.25 Ma – recent) deposited concomitant with the Late Pleistocene Hemipelagic Layer (LPHL). The continuous succession of subfans indicates an uninterrupted fan activity independent from sea‐level cycles at least since the Middle Pleistocene. This remarkable independent behavior in terms of sediment supply has not been observed at the Amazon Fan but is in agreement with observations from the Congo Fan. Finally, the analysis of a complete cross‐section through the lower Bengal Fan reveals that almost half of the sediment represents sands, indicating that the lower Bengal Fan may not generally be classified as 'mud‐rich' (≤30% sand)

Middle to Late Pleistocene Evolution of the Bengal Fan: Integrating Core and Seismic Observations for Chronostratigraphic Modeling of the IODP Expedition 354 8° North Transect

International audienceWe investigate chronology and age uncertainty for the middle to upper Pleistocene lower Bengal Fan using a novel age-depth modeling approach that factors lithostratigraphic, magnetostratigraphic, biostratigraphic, cyclostratigraphic, and seismic stratigraphic constraints, based on results from the International Ocean Discovery Program Expedition 354 Bengal Fan and analysis of the GeoB97-020/027 seismic line. The initial chronostratigraphic framework is established using regionally extensive hemipelagic sediment units, and only age-depth models of fan deposits that respect the superposition of channel-levee systems between sites are accepted. In doing so, we reconstruct signals of regional sediment accumulation rate and lithogenic sediment input through the perspective of a two-dimensional~320 km transect at 8°N that are consistent with more distal and more ambiguous regional records. This chronology allows us to discuss the depositional history of the middle to upper Pleistocene lower Bengal Fan within the context of sea level, climate, and tectonic controls. We hypothesize, based on the timing of accumulation rate changes, that progradation and intensification of the Bengal Fan's channel-levee system at 8°N was largely driven by increases in sea level amplitude during this time. However, it is also possible this progradation was influenced by changes in Pleistocene climate and increased Himalayan erosion rates, driving greater sediment flux to the fan. Plain Language Summary Deep sea fans are sediment deposits in the ocean that often form near river systems offshore continental margins. The largest of these, the Bengal Fan in the northern Indian Ocean, contains the most complete record of materials eroded from the Himalayan Mountains and can be used to study the climate and tectonic history of the region. Sediments are moved from the river mouth to the fan in a series of ever-changing channels that distribute sediments across the fan surface, making it impossible to obtain a complete and continuous record of Himalayan erosion at any one location. International Ocean Discovery Program Expedition 354 drilled a series of seven locations in a transect across the fan to capture a more complete record of where sediment was deposited over the last 1.25 Myr, a time characterized by major changes in Earth's climate system. Here we discuss statistics of sediment deposition from a computer model constrained by observations from those seven sites. The results indicate that the Bengal Fan grew rapidly during a time when global sea level changes, caused by the growth and decay of continental ice sheets, became more intense