HOLOCENE SEDIMENTARY RECORD PRESERVES SULFUR SYSTEM DYNAMICS IN LOCH DUART (NW SCOTLAND); EVIDENCE FOR VARIABILITY IN δ34S DRIVEN BY THE INTERPLAY OF POST-GLACIAL EUSTATIC RISE AND ISOSTATIC REBOUND

Loch Duart Marsh, an isolation basin in NW Scotland, has experienced intermittent oceanic connection through the Holocene. This connection has been controlled via two primary, competing forcings: eustatic rise due to glacial meltwater and relative sea level (RSL) fall due to isostatic rebound associated with the loss of the British and Irish Ice Sheet. At Loch Duart, this balancing act has modulated both sedimentation and water column geochemistry and can be evaluated via facies, elemental, and isotopic analyses. Over the last 15 kyr, Loch Duart has transitioned between (1) marine (when eustatic rise due to meltwater > isostatic rebound), (2) non-marine (when isostatic rebound > eustatic rise), and (3) brackish-water settings (when the magnitude of these two effects were balanced and marine intrusion occurred only during high tides).

The facies changes, largely identified by lithology and foraminiferal assemblage, are coincident with marked perturbations in the local sulfur (S) cycle. The marine facies possess relatively stable and light δ³⁴S_py values (average = -27.2 ‰), the non-marine interval shows an abrupt positive excursion (~30 ‰ change; average = 9‰), and the brackish interval records intermediate values (average = -16.2 ‰). While δ³⁴S_bulk values covary with δ³⁴S_py throughout the record, the relationship of this covariance flips between facies. The marine and brackish sections preserve bulk δ³⁴S values heavier than coeval δ³⁴S_pyr, whereas the nonmarine section exhibits δ³⁴S_bulk < ^δ34S_py, suggesting that RSL modulates the isotopic composition of non-pyrite phases in the bulk S pool. The sedimentary d³⁴S record is a primary tool for reconstructing Earth’s surface redox history. However, there is strong evidence that local δ³⁴S_pyr records are influenced by changes in depositional setting. In this study we analyze the S-isotopic composition and wt. % of elemental-S, acid-volatile-S, pyrite, and organic-S to evaluate sulfur system dynamics in a location characterized by clear changes in marine influence and facies. In doing so, we seek to improve our understanding of the controls on Earth’s surface S-cycle and our ability to interpret variability in the S-isotopic signature obtained from the geologic record.