Geochemical records of Holocene environmental change in a high latitude coastal basin

Climatic warming, eutrophication, and sea level rise are impacting coastal systems worldwide. High latitude coastal regions are particularly vulnerable to such environmental changes. Here, we assess temporal and spatial trends in salinity, bottom water redox conditions, and organic matter supply in the Bothnian Sea throughout the Holocene. To do so, we use sediment geochemical records for five sites based on discrete sampling and whole core X-ray-fluorescence (XRF) scanning. Our results reveal a basin-wide response to environmental changes, i.e. an abrupt shift from an oligotrophic and well‑oxygenated freshwater lake to a eutrophic and euxinic brackish/marine system that was followed by a return to oxygenated waters and a period of gradual freshening. Bottom water deoxygenation, as recorded by sedimentary enrichments in trace metals such as molybdenum, uranium, copper, arsenic (As), and elevated organic carbon to phosphorus (P) ratios, varied in intensity across the Bothnian Sea basin, and generally was more intense than in the Baltic Proper. Recent sediments show increased anthropogenic impact in the Bothnian Sea, reflected by increased deposition of the pollutants lead and As, and the nutrient P. High P burial aligns with recent signs of eutrophication and associated cyanobacterial blooms. Given the susceptibility for mobilization of sediment P to the water column, there is a potential risk for a positive feedback-loop between increased P recycling and primary productivity upon further eutrophication if there is also a decline in bottom water oxygen. Our work provides insight into the response of a high latitude coastal basin to both abrupt and gradual environmental changes, which is relevant for future projections of anthropogenic impacts in such regions.