A MULTI-GEOCHEMICAL APPROACH TO TRACKING RELATIVE SEA LEVEL DURING THE LATE GLACIAL TO HOLOCENE

Paleoenvironmental reconstructions from nearshore environments typically rely on the preservation of biological proxies. Thus, the absence or poor preservation of biological proxies hamper the reconstruction of relative sea level (RSL). Here we apply a multi-element geochemical approach, not reliant on microfossil preservation, to a sediment core from Loch Duart Marsh, NW Scotland, UK. This site is an isolation basin recording Late Glacial to Holocene sedimentation preserving palaeoenvironmental changes associated with the deglaciation of the last British-Irish Ice Sheet (BIIS). Our approach couples osmium with carbon, sulphur and nitrogen elemental and isotopic analysis together with X-ray fluorescence, radiocarbon dating and foraminiferal analysis. One of our aims is to assess the suitability of osmium isotopes as an independent method of RSL reconstruction. NW Scotland has a complex RSL record due to the combination of glacial isostatic adjustment and glacio-eustatic sea-level change. This provides a unique history that is reflected in the ¹⁸⁷Os/¹⁸⁸Os signature of the sedimentary organic matter. Within the sediment core is an interval with a distinctly more radiogenic (up to 4.89) ¹⁸⁷Os/¹⁸⁸Os signature bracketed by sediments with a ¹⁸⁷Os/¹⁸⁸Os signature of ~1 (typical open ocean values). We interpret this more radiogenic interval as indicating a period when the basin was isolated from the open ocean, allowing an increase in weathered, more radiogenic material to dominate the signal. This freshwater interval is further supported by a distinct increase in C/N ratios and the absence of foraminifera (only found in sediments with some marine influence). Radiocarbon dating indicates a duration of freshwater sedimentation/basin isolation of 4.2 ka, with marine ingression following the freshwater phase occurring at 10.2 ka. This research shows that periods of inundation and basin isolation can be identified through the analysis of the sediment’s associated osmium signature where ¹⁸⁷Os/¹⁸⁸Os values of ~1 reflect marine settings and more radiogenic values indicate freshwater deposition. The application of ¹⁸⁷Os/¹⁸⁸Os coupled with other (C, S, N) geochemistry to this site displays its suitability as a technique to reconstruct RSL, a methodology that could be applied to basins globally.