Early human impact in a 15,000-year high-resolution hyperspectral imaging record of paleoproduction and anoxia from a varved lake in Switzerland

Makri, Stamatina; Rey, Fabian; Gobet, Erika; Gilli, Adrian; Tinner, Willy; Grosjean, Martin (2020). Early human impact in a 15,000-year high-resolution hyperspectral imaging record of paleoproduction and anoxia from a varved lake in Switzerland. Quaternary science reviews, 239, p. 106335. Elsevier 10.1016/j.quascirev.2020.106335

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20th century eutrophication and global spread of anoxia is a threat for freshwater ecosystems. Little isknown about Holocene anoxia and meromixis events when anthropogenic impacts were weaker andnatural ecosystem variability played the dominant role. In this study, we examine the relationship be-tween lake mixing and lake production, climate variability, vegetation cover, catchment erosion and (pre)historic anthropogenic impacts in Moossee (Switzerland), over the last 15,000 years. We use sub-annually resolved calibrated hyperspectral imaging data (total chlorophyll for paleoproduction, bacter-iopheophytin for anoxia and meromixis) combined with X-rayfluorescence and pollen data. Productionshows afirst increase at 14,500 cal yr BP, a further increase after 7500 cal BP, relative maxima in the lateBronze, Iron and Middle Ages, and the unprecedented peak in the 20th Century. Until 7500 cal BP, thelake was well mixed with only scarce phases of seasonal to multiannual anoxia. Repeated meromixisevents occurred between 7500 and 2500 cal BP when temperatures were high, forests closed, and lakeproduction was already enhanced. After the forests were cleared (2500 cal BP) the lake remained mostlyholomictic. Holocene meromixis events were systematically terminated by local deforestation related toNeolithic and Bronze Age lakeshore settlements: charcoal peaked, tree pollen dropped below a thresholdof 80%, soil erosion and lake production increased and bacteriopheophytin disappeared. Meromixis re-established after the termination of lakeshore settlements and the onset of afforestation with tree pol-len exceeding 80%. These repeated cycles unambiguously document how even early human societiesaffected the mixing regime and biogeochemical cycling in this lake.

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