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Wehrmann L.M.,Max Planck Institute for Marine Microbiology | Arndt S.,University of Bristol | Marz C.,AG Mikrobiogeochemie | Ferdelman T.G.,Max Planck Institute for Marine Microbiology | Brunner B.,Max Planck Institute for Marine Microbiology
Geochimica et Cosmochimica Acta | Year: 2013

Transient pore-water and solid-phase signatures in deep subseafloor marine sediments, resulting from changes in both the amount and the quality of the organic matter input, are common but often difficult to interpret. We combined high-resolution pore-water and solid-phase data from Integrated Ocean Drilling Program (IODP) Expedition 323 Site U1341 (Bowers Ridge, Bering Sea) with inverse reaction-transport modeling to examine the evolution and potential preservation of diagenetic signals in these deep subseafloor sediments. We explore how these signals reflect major changes in the deposition and reactivity of organic matter to the seafloor at Bowers Ridge. Results of the inverse model approach reveal that 2.51-2.58. Ma ago a high deposition flux of extremely labile organic matter, probably linked to increased surface water primary productivity, affected this site. Associated elevated organoclastic sulfate reduction rates facilitated low sulfate concentrations, the onset of methanogenesis, and consequently sulfate reduction coupled to the anaerobic oxidation of methane (AOM). Sulfate depletion caused the dissolution of biogenic barite reflected by a sedimentary interval with low Ba/Al ratios. Two sulfate-methane transition zones (SMTZs) evolved where high rates of AOM controlled sulfate consumption which was sustained by the influx of sulfate from seawater above and a deep source below. The positions of both SMTZs shifted non-synchronously over the subsequent ∼130,000. yrs, until methanogenesis and AOM declined. The present-day sulfate concentration and sulfur isotope profiles still reflect the impact of the reactive organic matter pulse. They also record a period of very low reactive organic matter deposition during the middle to late Pleistocene, probably linked to very low primary productivity, resulting in little microbial carbon turnover in the sediment. Our study shows that combining biogeochemical signatures recorded in the solid-phase of deep subseafloor sediments with the analysis of transient pore-water signals by inverse reaction-transport modeling yields new insights into past deep biosphere processes and the paleoproductivity of marine basins. © 2013 Elsevier Ltd. Source


Marz C.,AG Mikrobiogeochemie | Marz C.,Northumbria University | Schnetger B.,AG Mikrobiogeochemie | Brumsack H.-J.,AG Mikrobiogeochemie
Paleoceanography | Year: 2013

Intergrated Ocean Drilling Program Expedition 323 recovered a sediment record covering the last ∼4.3 Ma from the Bering Sea (Integrated Ocean Drilling Program Site U1341, Bowers Ridge, 2177 m water depth). To resolve Pliocene-Pleistocene paleoenvironmental changes in this marginal basin, ∼190 sediment samples were analyzed for their bulk element composition. Aluminium contents in Bowers Ridge sediments are variable but overall higher toward younger sediments, probably related to the intensification of the Northern Hemispheric Glaciation and increasing sea ice transport in the Bering Sea. The gradual increase of terrigenous input is mirrored by decreasing SiO2 and excess Si (Sixs) contents, but the overall Si enrichment of the deposits reflects continuous opal deposition since the Pliocene at Bowers Ridge. Unlike in the North Pacific, the Sixs record at Site U1341 does not support a dramatic decrease in opal export following the onset of the Northern Hemispheric Glaciation around 2.7 Ma, but SiO2xs have higher accumulation rates (up to ∼8 g/cm2/ka) between ∼2.6 and ∼1.8 Ma BP. During this period, the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas. We here discuss how the onset of North Pacific stratification at ∼2.7 Ma BP may have caused leakage of nutrient-rich deep/intermediate North Pacific water into the Bering Sea via the deep Kamtchatka Strait, leading to increased opal deposition - and likely reactive organic matter export - at Bowers Ridge. As a result, magnetic and geochemical records were overprinted by intensified diagenesis, significantly affecting their applications as paleoceanographic proxies. © 2013 American Geophysical Union. All Rights Reserved. Source

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