Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan

We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and 230Th/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at ∼ 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at ∼ 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (∼ 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (∼ 1 km2) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since ∼ 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of ‘fossil’ carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope

Branched glycerol dialkyl glycerol tetraethers and crenarchaeol record post-glacial sea level rise and shift in source of terrigenous brGDGTs in the Kara Sea (Arctic Ocean)

This study evaluates the glycerol dialkyl glycerol tetraether (GDGT) distribution and provenance in sediments(spanning a minimum of 13.3 ka) from the St. Anna Trough (Northern Kara Sea). The site has experiencedextensive fluctuation in the delivery of river-derived organic matter (OM), caused by a eustaticchange in sea level. This is in line with the record of the concentration of the isoprenoid GDGT, crenarchaeol,produced by marine Thaumarchaota, which was low at the bottom of the core, increasing graduallyin the most shallow unit. The concentration of branched (br)GDGTs showed an opposite trend and amarked shift in distribution. The deepest sediments (>10 ka), with a distribution currently encounteredin surface sediments in front of the Yenisei River, are characterized by terrigenous brGDGTs, whereas thedistribution in the shallowest unit (<10 ka) is strongly influenced by marine, in-situ brGDGTs. During theshift from terrigenous to marine-sourced brGDGTs, there was one horizon where a pronounced shift inthe brGDGT distribution was observed and the brGDGT concentration significantly decreased. As thebrGDGTs delivered to the current Kara Sea system are derived from several sources, we postulate thata temporary change in the relative importance of the brGDGT sources happened during this interval.Both in-situ production and changing brGDGT provenance have implications for paleoclimate reconstructionusing brGDGTs. In-situ production of marine brGDGTs results in a higher reconstructed pH. However,these in-situ produced brGDGTs did not influence the reconstructed mean annual air temperature (MAT),when the MATmrs calibration was used. Changes in the relative contribution of brGDGT sub-pools wereshown, however, to influence both soil pH reconstruction and MAT reconstruction

Branched GDGT, crenarchaeol, TOC and bulk δ¹³C in SPM of the Selenga River and its outflow in Lake Baikal, and Lake Baikal outflow in the Yenisei River

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids found in several environments, including soils, rivers and lakes, whose distribution varies with temperature and pH, although this dependence is apparently not the same for the different environments. Mixing of brGDGT sources may thus complicate palaeoenvironmental reconstruction. The extent to which brGDGTs in a lake outflow reflect the distribution delivered by upstream rivers was studied for Lake Baikal (Russia), one of the largest freshwater lakes worldwide. Fifteen brGDGTs in suspended particulate matter (SPM) of the Selenga River and its outflow from the lake were quantified. The river and lake SPM had rather different distributions. The riverine distribution was still apparent in the SPM of the lake surface water 5 km from the river mouth, but shifts in the distribution were already apparent in the SPM of the surface water after 1 km. Based on the brGDGT distributions of the SPM of the Selenga outflow and that of the lake, conservative mixing between the river and the lake brGDGT distributions could not fully explain the observed shifts in distributions. Both preferential degradation and in situ production of brGDGTs in the surface and, especially, bottom water of the river outflow were potentially responsible. This implies that a riverine lipid distribution delivered to a lake can be modified prior to being transported downstream. The lacustrine brGDGT distribution, that possibly could have reflected a mixture of mountainous and Selenga River SPM, was not recognized in downstream Yenisei River SPM. The watershed of Lake Baikal thus does not seem to contribute to the brGDGTs transported to the marine system. As many large rivers have major lakes in their watershed, this has implications for palaeoclimate reconstruction from river fan sediments globally

Branched GDGT and crenarchaeol lipids, and bulk parameters (TOC, TN, C/N, δ¹³C and d15N) of a 13.3 ka Kara Sea (St. Anna Trough; Arctic Ocean) core

Sediments (spanning a minimum of 13.3 ka) from the St. Anna Trough (Northern Kara Sea). The site has experienced extensive fluctuation in the delivery of river-derived organic matter (OM), caused by a eustatic change in sea level. This is in line with the record of the concentration of the isoprenoid GDGT, crenarchaeol, produced by marine Thaumarchaota, which was low at the bottom of the core, increasing gradually in the most shallow unit. The concentration of branched (br)GDGTs showed an opposite trend and a marked shift in distribution. The deepest sediments (>10 ka), with a distribution currently encountered in surface sediments in front of the Yenisei River, are characterized by terrigenous brGDGTs, whereas the distribution in the shallowest unit (<10 ka) is strongly influenced by marine, in-situ brGDGTs. During the shift from terrigenous to marine-sourced brGDGTs, there was one horizon where a pronounced shift in the brGDGT distribution was observed and the brGDGT concentration significantly decreased. As the brGDGTs delivered to the current Kara Sea system are derived from several sources, we postulate that a temporary change in the relative importance of the brGDGT sources happened during this interval. Both in-situ production and changing brGDGT provenance have implications for paleoclimate reconstruction using brGDGTs. In-situ production of marine brGDGTs results in a higher reconstructed pH. However, these in-situ produced brGDGTs did not influence the reconstructed mean annual air temperature (MAT), when the MATmrs calibration was used. Changes in the relative contribution of brGDGT sub-pools were shown, however, to influence both soil pH reconstruction and MAT reconstruction

Lipid biomarkers for anaerobic oxidation of methane and sulphate reduction in cold seep sediments of Nyegga pockmarks (Norwegian margin): discrepancies in contents and carbon isotope signatures

International audienc

Palaeo methane-seepage history traced by biomarker patterns in a carbonate crust, Nile deep-sea fan (Eastern Mediterranean Sea)

International audienc