Sediment isotope tracers from Lake Saarikko, Finland, and implications for Holocene hydroclimatology

Abstract

Multi-component isotopic analyses on a sediment core from Lake Saarikko, Finland, give insight into Holocene climate changes in northern Europe. Oxygen isotopic records from aquatic cellulose and authigenic carbonate reflect consistent, similar trends in hydrological balance, and are employed jointly to infer relative changes in lake water temperature. Other geochemical and isotopic tracers shed light on fluctuations in lake level and nutrient cycling, facilitating the interpretation of the oxygen isotopic archives. In the early Holocene (∼10 000–9000 cal yr BP), the lake level was low and dry conditions prevailed despite low temperatures. Higher lake levels and depleted oxygen isotopic values ∼9000–7500 cal yr BP were probably caused by several factors: higher effective humidity, an isotopically depleted precipitation source, a cold event around 8300 cal yr BP, and increased spring snowmelt. In contrast, higher summer temperatures, lower effective humidity, lower lake levels and enriched oxygen isotopic values were experienced during the mid-Holocene ∼7500–3300 cal yr BP. The occurrence of the warmest inferred lake water and the most enriched cellulose oxygen-isotope values ∼6000–5000 cal yr BP is consistent with a summer temperature maximum inferred from other proxy sources from the region. The late Holocene was characterized by higher effective humidity and higher lake levels. Increase in spring snowmelt and colder temperatures, most likely related to the Little Ice Age, induced a drastic change in the lake hydrology and probably caused the formation of its present outlet ∼800 cal yr BP. Changes in the lake hydrological balance and climate variables inferred from oxygen isotopic records can be related to atmospheric circulation, most clearly to changes in the dominance and strength of westerly air-flow as well as summertime anticyclonic circulation.