Palaeoenvironmental reconstruction of a fjord catchment NW Scotland, UK since the Last Glacial Maximum: a multi-geochemical approach

Loch Sunart, a fjord in NW Scotland, UK records Late Glacial to Holocene sedimentation preserving palaeoenvironmental changes associated with the deglaciation of the British-Irish Ice Sheet (BIIS). This study combines a multi-proxy approach (osmium isotope analysis, carbon, sulphur and nitrogen elemental and isotopic analysis together with X-ray fluorescence, alkenone biomarkers and benthic foraminifera) to contribute to our understanding of the regional nature of BIIS deglaciation and enable the reconstruction of palaeoclimate variations since the last glacial. The observed patterns in the applied proxies collectively suggest that between ~18 and 12.9 cal ka BP as the BIIS retreated Loch Sunart experienced increased glacial meltwater discharge. Throughout this period, ¹⁸⁷Os/¹⁸⁸Os values are more radiogenic than the contemporaneous open ocean waters (~1.04 – 1.06), which are interpreted to reflect an increase in glacially eroded sediment flux directly associated with glacial retreat following the Last Glacial Maximum. During the Younger Dryas, the fjord experienced a pause in glacially derived sediment reflected by a minimum in ¹⁸⁷Os/¹⁸⁸Os (~1.1), which was followed by an increase in ¹⁸⁷Os/¹⁸⁸Os to more radiogenic values (~1.3) at the end of the Younger Dryas. Coincident with changes in ¹⁸⁷Os/¹⁸⁸Os values, alkenone-based %C_37:4 values increase (40-60%), δ¹³C_org values decrease (-24‰) and foraminifera species indicative of restricted water renewal increased in abundance, suggesting a period of water stratification. A rapid shift in ¹⁸⁷Os/¹⁸⁸Os values toward a marine signature of ~1 indicates that the basin experienced a breakdown in water stratification and renewed mixing from 5 cal ka BP. This interpretation is further supported by a coincident decrease in %C_37:4 to <15%, and by distinct increases in wt% C, wt% N and δ¹³C_org. This multi-proxy approach, specifically integrating Os, provides additional insight to the BIIS in NW Scotland, in particular how such fjords can respond to glacial readvance during the Younger Dryas, which was not captured by previous studies.