Coupling between 210Pb ex and organic matter in sediments of a nutrient-enriched lake: An example from Lake Chenghai, China

Abstract

Sediment cores were collected from deep-water areas of Lake Chenghai, China in June 1997. The vertical profile of 137Cs activity gives reliable geochronological results. The results also indicate that sediment accumulation rates in deep-water areas of Lake Chenghai were relatively constant in recent decades, averaging 0.43 g cm − 2 y − 1 , despite a variable organic carbon influx. 210Pb eq (= 226Ra) activity was relatively constant also, with an average value of 54.3 ± 3.2 Bq kg − 1 . Vertical profiles of 210Pb ex (= 210Pb total − 226Ra) decreased exponentially, resulting in somewhat lower sediment accumulation rates (0.3 g cm − 2 y − 1 ). These lower rates are likely less reliable, as the relatively large fluctuations in 210Pb ex activities correlate closely to the organic carbon (C org) content of the sediments. For example, the vertical profile of 210Pb ex activity displays peaks at mass depths of 3.7–4.7 g cm − 2 (10–12 cm) and 10–11 g cm − 2 (25–28 cm), similar to the maxima in the vertical profile of C org. This phenomenon must be related to the delivery of particulate organic matter (POM) from the water to the sediments, or to watershed soil erosion. Since the mean atomic ratios of H org / C org and C org / N org in Lake Chenghai sediments are 5.5 and 7.0, respectively, indicating that POM was predominantly derived from the remains of authigenic algae, this eliminates watershed erosion rates as a primary control on lake sedimentation rates as resolved by 210Pb ex. Sedimentation fluxes ( F(C org)) of particulate organic carbon since 1970 varied between 60 to 160 g m − 2 y − 1 , and appeared to closely influence variations in 210Pb ex concentrations. For example, sedimentation fluxes of 210Pb ex (F( 210Pb ex)) showed maxima in the years 1972–1974 and 1986–1989, likely reflecting historical variations of lake biological productivity or carbon preservation.