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We present here radiocarbon dating results from two boreal lakes In Finland, which are permanently (meromictic) or seasonally stratified and contain continuous sequences of annually laminated sediments that started to form In the early Holocene. The radiocarbon dating results of different organic components were compared with the varve-based sediment chronologies. The deviation between the Lake Valkiajarvi varve chronology (8400 varve years ±2-3% error estimate) and 33 14C dates taken from insoluble and soluble organic phases vary inconsistently throughout the Holocene. In extreme cases mean calibrated radiocarbon dates with 95.4% confidence levels (2σ) are -2350 and +2040 years offset when compared with the varve chronology. On average, the radiocarbon dates are offset by ca. +550 years. The deviation between the Lake Nautajarvl varve chronology (9898 varve years ±1% error estimate) and 26 14C dates analyzed with conventional and AMS methods indicates that radiocarbon dates are systematically older by 500-1300 years (about 900 years on average). This significant offset mean that radiocarbon dates obtained from organic bulk sediment of meromictic and seasonally hypoxic lakes must be cautiously interpreted because of the reservoir effect and carbon cycling at the sediment-water Interface. Direct evidence was obtained from the dating of soluble fraction and insoluble organic matter from near bottom water In the monimolimnion of Lake Valkiajarvi, which yielded 14C ages of 560 ±80 BP and 2070 ±140 BP, respectively. Our study reinforces previous results that age-depth models based on bulk sediment radiocarbon dates obtained on sediments of stratified lakes are of limited value for accurate dating of changes in land use and especially the commence of agriculture. [ABSTRACT FROM AUTHOR] more...

Model-based estimates suggest that lake sediments may be a significant, long-term sink for organic carbon (C) at regional to global scales. These models have used various approaches to predict sediment storage at broad scales from very limited data sets. Here, we report a large-scale direct assessment of the standing stock and sedimentation rate of C for a representative set of lakes in Finland. The 122 lakes were selected from the statistically selected Nordic Lake Survey database, they cover the entire country and the water quality represents the average lake water quality in Finland. Unlike all prior estimates, these data use sediment cores that comprise the entire sediment record. The data show that within Finland, aquatic ecosystems contain the second largest areal C stocks (19 kg C m−2) after peatlands (72 kg C m−2), and exceed by significant amounts stocks in the forest soil (uppermost 75cm; 7.2 kg C m−2) and woody biomass (3.4 kg C m−2). Kauppiet al.(1997). The Finnish estimate extrapolated over the boreal region gives a total C pool in lakes 19–27 Pg C, significantly lower than the previous model-based estimates. [ABSTRACT FROM AUTHOR] more...