The evolution of early diagenetic signals in Bering Sea subseafloor sediments in response to varying organic carbon deposition over the last 4.3 Ma

Abstract

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 2.51–2.58Ma 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 ∼130,000yrs, 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.