T86 Advances and New Voices in Marine and Coastal Geoscience

Monday, 16 October 2023
8:00 AM - 12:00 PM
325

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

Paper number: T86-12
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 55, No. 6

doi: 10.1130/abs/2023AM-390908

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