PALAEOENVIRONMENTAL RECONSTRUCTION OF A FJORD CATCHMENT NW SCOTLAND, UK SINCE THE LAST GLACIAL MAXIMUM: A MULTIGEOCHEMICAL APPROACH

In this study, using a multi-geochemical approach not reliant on microfossil preservation, the deglacial history of the British-Irish Ice Sheet (BIIS) is reconstructed using a sediment core recovered from a fjord on the NW Scottish coast (UK). Specifically, the osmium-isotope (¹⁸⁷Os/¹⁸⁸Os) system is employed as a tool for paleo-oceanographic reconstruction. The ¹⁸⁷Os/¹⁸⁸Os of water reflects the balance between riverine, continent-derived, radiogenic Os inputs (¹⁸⁷Os/¹⁸⁸Os ~1.4) and mantle-derived, unradiogenic Os (¹⁸⁷Os/¹⁸⁸Os ~0.12). The very distinct character of these two sources facilitates use of ¹⁸⁷Os/¹⁸⁸Os values as a proxy for degree of marine influence on a local water column. Here, we establish values distinct from the contemporaneous open ocean (~1), which we interpret to reflect glacial retreat and subsequent readvance following the Last Glacial Maximum and Younger Dryas.

The basal section of the core (>13.1 cal ka BP) preserves distinctly radiogenic ¹⁸⁷Os/¹⁸⁸Os values (~1.3) that trend upsection toward slightly less radiogenic values (~1.1). This trend reflects a proposed period of increased glacial meltwater discharge associated with deglaciation of BIIS. The nadir in ¹⁸⁷Os/¹⁸⁸Os values (~1.1) is followed by a trend toward more radiogenic values (1.2 – 1.3). We associate this positive shift with glacial readvance during the Younger Dryas and increased sediment delivery to the fjord catchment. Notably, this period of elevated ¹⁸⁷Os/¹⁸⁸Os values persists beyond the Younger Dryas. The “overshoot” likely reflects continued significant freshwater supply to the fjord drainage area after glacial retreat, and subsequent water column stratification. We estimate that these conditions persisted from 8 to 6 cal ka BP. Finally, ¹⁸⁷Os/¹⁸⁸Os values approach ~1 (from 6 cal ka BP to present day), suggesting renewed exchange with seawater. This interpretation is supported by a distinct increase in wt% C wt% N and δ¹³C.

This study shows the utility of ¹⁸⁷Os/¹⁸⁸Os, especially when coupled with other established stable isotope systems (C, S, N), as a powerful technique for reconstructing glacially influenced paleoenvironmental records, particularly in cases where poor preservation, or absence of biological proxies, have hampered such reconstructions.