Holocene records of carbon and hydrogen isotope ratios of organic matter in annually laminated sediments of Lake Korttajarvi, central Finland

An 11.6 m long continuous succession of annually laminated sediments from Lake Korttajarvi in central Finland was investigated for the isotopic composition of carbon and hydrogen in organic matter. The sequence covers a time period of 9590 years, and the varve chronology has been thoroughly described in earlier studies. From 7100 to 4400 BC the lake was part of the Ancient Lake Paijanne, but in 4400 BC it became separated and formed the present independent lake system. Two organic fractions were investigated. One fraction obtained by HCl-treatment was analyzed for delta C-13 and another HCl-HF-digested organic fraction was analyzed for both delta C-13 and delta D. The isotopic data were compared to atomic C/N ratios, carbon contents, diatom-inferred pH values and other environmental parameters. The diatom-inferred pH values and organic carbon contents provide evidence for a long-term change towards more acidic conditions and lower productivity in Lake Korttajarvi. The inferred pH values decrease from 7.0 to 6.1, followed by a slight increase during the last millennia. Variations in pH are accompanied by an increase in the delta C-13(HCl-HF) values of organic matter from -31.6 to -29.2 parts per thousand, followed by a subtle decrease to -29.4 parts per thousand. The changes in pH and delta C-13(HCl-HF) are closely related (r = -0.91, P < 0.01), and apparently reflect changing environmental conditions in the lake and in its catchment area. delta D values show a marked shift to higher values during the early Holocene, which may be partly related to a climatic amelioration leading to the Holocene Climatic Optimum in 6000-2500 BC. The Medieval Warm Period in AD 980-1250 is associated with a local maximum in delta D, lending support for a significant warming during that time.