Microbial sulfate reduction regulated by relative sea level change in a Pleistocene – Holocene sedimentary record: Insights from Loch Duart, Scotland, UK

A Pleistocene–Holocene-aged sediment core recovered near Loch Duart, located in the coastal Assynt region of NW Scotland, UK, provides new insight into the relationship between the sulfur isotope composition of iron sulfides (pyrite) and organic sulfur under rapidly changing environmental conditions. Since the Late Glacial period, shifts in local marine connectivity at Loch Duart have been driven by the competition between two fundamental Earth surface processes: eustatic sea level rise due to post-glacial meltwater contributions since the Last Glacial Maximum and relative sea level (RSL) fall associated with glacial isostatic rebound. These processes, imprinted on the sedimentary record, have been evaluated via lithology, microfossil assemblages, elemental analysis, and isotopic measurements. Over the last 17 kyr, Loch Duart has transitioned from (1) marine conditions, when eustatic rise due to deglaciation exceeded glacial isostatic rebound, to (2) non-marine conditions, where land uplift caused by isostatic rebound exceeded eustatic rise, to (3) marine conditions, as the eustatic contribution outpaced isostatic rebound, followed by (4) brackish-water conditions, as the eustatic contribution reduced while glacial isostatic uplift continued, with marine inundation limited to part of the tidal cycle at the present day. Here, we evaluate marked perturbations in the local sulfur (S) cycle related to the aforementioned environmental changes. The marine interval coincides with relatively stable and low δ³⁴S_sulfide values (average ∼ −27.2 ‰), the non-marine interval records an abrupt positive δ³⁴S_sulfide excursion of over 30 ‰ (average ∼ 9 ‰), and the brackish interval preserves intermediate values (average ∼ −16.2 ‰). The δ³⁴S_org values shift sympathetically with δ³⁴S_sulfide, although the magnitude of δ³⁴S_org change is nominal by comparison, particularly during the transition from freshwater to marine facies. As expected, marine and brackish sections preserve higher δ³⁴S_org values than coeval δ³⁴S_sulfide. Interestingly, this relationship is reversed in the freshwater facies, where sulfides are ³⁴S-enriched relative to organic S by as much as 20 ‰, suggesting that RSL modulates the isotopic composition of non-pyrite phases in the bulk S pool. We hypothesize that this inverse relationship (δ³⁴S_org < δ³⁴S_sulfide) may arise from a spatial decoupling of pyrite and organic S formation within the water column and/or sediments in a system with low sulfate concentrations. Evaluating shifts in the local S-cycle associated with RSL changes allows for a novel comparison between S and osmium isotope records, demonstrating that these proxies may have joint applications for paleoenvironmental investigations in shallow coastal systems. We offer new perspectives on the interplay between eustasy, RSL, and the S-cycle by assessing these relationships in a coastal isolation basin.