Abstract
Total organic carbon content (TOC) of lake sediments has been used recently as a useful paleoclimate proxy. But, no study has shown the genuine relationship between TOC content of lake sediments and climate parameters. Therefore, we try to examine modern relationships among TOC in lake sediments, biological productivity of lake water and weather factors using limnological observation data conducted monthly in Lake Kizaki since AD 1981.
The sediment cored on 11 December 2003 was cut into 0.5 cm interval, and apparent density and TOC content were measured. Apparent density peaks can be correlated with the known flood events. Sedimentation rates were determined precisely on the basis of flood ages, and were used for the age determination of the cored sediment. Then, annual TOC content can be determined from AD 1981 to 2002. Monthly measurements of chlorophyll a amount in water column were integrated to annual chlorophyll a amount which can be a representative of annual biological productivity. Meteorological data at Omachi City, 4 km south to the lake were used as weather factors during the same period. As a result, we have found a correlation (n = 21, r = 0.47) between winter temperature and annual chlorophyll a amount. The TOC of sediment also shows some concordance with chlorophyll a (n = 21, r = 0.50) and winter temperature (n = 23, r = 0.44). When we compare TOC content in sediments with 3-year moving average of winter temperature and annual chlorophyll a, the correlations between them become much better. These results suggest that organic carbon of lake sediment reflects the lake biological productivity which is controlled by winter temperature, namely, shortness of the coldest season. It is concluded that the TOC content of lake sediments can be used as a useful paleoclimate proxy when sedimentation rate is almost constant.
