Tracking past sea-level changes via isotope geochemistry: Insights from a Holocene post-Last Glacial Maximum record, NW Scotland

Abstract

Previous paleoenvironmental and sea-level reconstructions from nearshore environments have typically required the preservation of biological proxies, however, in regions of poor preservation such reconstructions are severely hampered. Thus, this research utilises an isotopic tracer (osmium-isotope system - 187Os/188Os) that is not reliant on microfossil preservation but is captured by sedimentary organic matter during deposition. Here we apply a multi-element geochemical approach coupling osmium with carbon, sulphur and nitrogen elemental and isotopic analysis together with X-ray fluorescence, radiocarbon dating and foraminiferal analysis to Loch Duart Marsh, NW Scotland, an isolation basin containing Late Glacial to Holocene sediments. This study assesses the suitability of these geochemical proxies for palaeoenvironmental reconstructions and combines them with the established biostratigraphic method of foraminifera analysis to reconstruct a detailed palaeoenvironmental and sea-level reconstruction of Loch Duart Marsh.
The Loch Duart Marsh sediment core contains a sedimentary interval deposited under freshwater conditions, the 187Os/188Os signature of which is distinctly more radiogenic (up to 4.3) than during marine deposition (~1). This revealed that periods of inundation and basin isolation can be distinguished in the osmium isotope record. The freshwater interval is further supported by a distinct increase in C/N (~6 units). Radiocarbon dating indicates a duration of freshwater sedimentation of 4.1 ka, with marine ingression following the freshwater phase occurring at 10.2 ka. The complex relative sea-level record of NW Scotland due to glacial isostatic adjustment provides a unique history that is reflected in the 187Os/188Os signature of the sedimentary organic matter, where 187Os/188Os values of ~1 reflect marine settings and more radiogenic values indicate freshwater deposition. The application of 187Os/188Os geochemistry to this site further displays its suitability as a technique to reconstruct relative sea-level, a technique that could be applied to basins globally.