Enhanced Late Miocene Organic Carbon Preservation and Sulfur Sequestration Beneath the Oxygen Deficient Zone in the Eastern Arabian Sea

Abstract

The ocean represents the largest sulfur pool on Earth, with 1.3 x 10⁹ Tg of sulfur present in seawater. Marine sediment sulfur sequestration is the primary sink for seawater sulfate, responsible for the burial and storage of 100 Tg of sulfur per year. The sequestration of sulfur in marine sediments is largely driven by the formation of iron monosulfides, metastable iron sulfide minerals, and pyrite, however the different paths driving the degree of pyritization within marine sediments require different environmental conditions and carbon sources. In this thesis research, the possible diagenetic pathways resulting in sulfur sequestration are investigated in a 34 Myr sedimentary record at National Gas Hydrate Program (NGHP) Expedition 01 Site-01A, off western peninsular India in the Kerala-Konkan Basin of the eastern Arabian Sea. To constrain the abundance and isotopic signature of the total sulfur throughout the NGHP Site-01A record, an elemental analyzer and mass spectrometer were used to measure bulk sediment total sulfur (TS) and δ³⁴S. These results were integrated with previously published total organic carbon (TOC), total nitrogen (TN), CaCO₃, and δ¹³CTOC measurements, core sedimentology, and a biostratigraphic age model. The integrated results show an abrupt increase in both TOC and TS at 12 Ma that coincides with a shift from enriched to increasingly depleted δ34S of TS. Observed increases in TOC, sedimentation rate, TS and δ³⁴S depletion from 12 Ma to recent suggest OSR was the dominant diagenetic process driving pyritization. We attribute the increasing level of TOC from 12 ma to recent to be linked to the eastward expansion of the Arabian Sea oxygen deficient zone (ODZ), documented elsewhere in the eastern Arabian Sea at approximately 13 Ma. The presence of the ODZ would allow for both increased primary productivity in the surface waters of the Arabian Sea and the increased preservation of organic carbon as it sinks through ODZ in the water column. From 12 Ma to recent, enhanced TOC preservation under high sedimentation rates provided microbes with an energy source for OSR and subsequent pyritization. Prior to 12 Ma, TOC preservation in the sediments was extremely limited, likely due to water column and seafloor consumption in a well oxygenated water column and slow sedimentation rates, minimizing pyritization. Although measurements of enriched δ³⁴S are consistent with AOM from 18 to 12 Ma, the lack of measured methane or any other indicator of methane seepage or AOM-driven diagenesis throughout the 34 Ma record suggests AOM was not a significant path for pyritization. Instead, we interpret these enriched δ³⁴S values to reflect sulfide oxidation. These results emphasize that in the absence of methane, TOC preservation is a first order control on OSR and subsequent pyritization and highlight the import role of the overlying oceanic environment in enhancing the preservation of organic carbon and the degree of sulfur sequestration in marine sediments.