Reconstruction of ancient sulfate‒methane transition depths using the concentrations and sulfur isotopic compositions of authigenic sulfide minerals in marine sediments

Abstract

On the basis of a one-dimensional transport-reaction model, we propose a novel approach to identify the relationship between the depth of the sulfate‒methane transition (SMT) and the stratigraphic distribution of authigenic sulfide minerals in terms of both concentration and sulfur isotopic composition. We apply this approach to Ocean Drilling Program Sites 994 and 995 at the Blake Ridge, offshore southeastern North America. Within the present-day sulfate reduction zone, our numerical simulation suggests that sulfide mineralization is basically at a steady state, the concentrations of which depend largely on the reactivity of sedimentary organic carbon. The observed extreme ³⁴S-depletion of sulfide minerals (e.g., δ³⁴S values as low as −45‰ VCDT) can occur without the process of sulfur disproportionation. We then derive a series of theoretical profiles by changing the SMT depth from 4 m to 30 m below the seafloor and construct a contour plot of the theoretical SMT depth as a function of the concentration and sulfur isotopic composition of solid-phase sulfides after complete mineralization under steady-state conditions. Below the present-day SMT, six stratigraphic locations of the ancient SMT are identified and their depths below each contemporaneous seafloor are interpolated. Theoretical estimates suggest that the SMT gradually subsided from 8.4 to ca. 24 m below the contemporaneous seafloor (mbcsf) over the past 643 to 6.6 ka. At present, the SMT has been occurring steadily at depths of ca. 21 to 22 mbcsf for at least 2.9 kyr. This is the first time that the evolution of the ancient SMT depth has been quantitatively reconstructed, by which the attainment of knowledge can be used to research the early diagenetic evolution history of the pore water sulfur species and will arouse much interest and concern from other researchers.