Organic lacustrine sediment varves as indicators of past precipitation changes: a 3,000-year climate record from Central Finland.

Annually laminated (varved) sediments from Lake Kallio-Kourujärvi, Central Finland, provide high-resolution sedimentological data for the last three millennia. These varves consist of two laminae that represent (1) deposition during the spring-to-autumn growing season, composed of degraded organic matter and a variety of microfossils, and (2) deposition during winter, composed of fine-grained homogenous organic matter. Because of the absence of a clastic lamina, these varves differ from the typical, well-described, clastic-organic varve sequences in Fennoscandian lakes. Such organic varves in Finnish lakes have not been studied in detail before. Three thousand varves were counted and their seasonal deposition was distinguished. Comparison of varve thickness with meteorological data revealed a positive correlation between organic varve thickness and precipitation. This suggests that catchment erosion processes and consequent organic matter and nutrient inputs are important factors in organic varve formation. The correlation between temperature and growing-season lamina thickness varied from insignificant, to positive, to negative during different time spans. This suggests that organic matter accumulation can sometimes have a significant, but unpredictable role in organic varve formation, via organic matter production and degradation, processes that are influenced strongly by water column temperature. The organic varves of Lake Kallio-Kourujärvi enable a unique, high-resolution approach for the study of past climate and environment. Our results suggest that decadal periods of increased precipitation occurred during BP 2150-2090, 1710-1620, 1410-1360, 920-870 (1030-1080 AD), and after 370 BP (1580 AD). Dryer intervals occurred during BP 2750-2720, 1900-1850, 1800-1740, 1600-1500, and 780-700 (1170-1250 AD), 590-520 (1360-1430 AD). [ABSTRACT FROM AUTHOR]