Annually laminated sediments from Onondaga Lake, New York (USA) provide a basis for high-resolution studies of lake degradation and recovery

摘要

The chronology of a 184-cm-long sediment core from Onondaga Lake, New York (USA), is known with annual certainty from 2006 to the 1880s. Sediment age is known with similar precision, but less confidence, from the 1880s to the 1820s. Paired dark and light laminations are visible throughout most of the core, and are confirmed as annual varves by individual particle analysis and well-defined Cs-137 age horizons. The varves can be correlated with historical impacts on the lake to within 1-2 years, stratigraphic resolution that is comparable to the best obtained from other varved lakes in which minimally disruptive sampling techniques such as freeze coring were used. Seven stratigraphic units and four sub-units were defined using visual description and the Sr/Ca profile. Onset of laminations correlates to an 1822 change in lake level. The core contains evidence of past water-quality changes, including: (1) shortened extent of seasonal calcite supersaturation in the lake during the 1800s, (2) salt industry activity during the mid-1800s, (3) calcite-precipitation (whiting) events during the 1900s, and (4) greater impact of local hydrology on lake sedimentation prior to 1900. During the late 1800s, varves were interrupted by an approximately 12-year interval of non-laminated sediment that corresponds to a time of major alteration and industrial development in the watershed. There are no apparent turbidite layers in the sediment column. Onondaga Lake owes its remarkably preserved, 200-year sediment record largely to its high rate of sediment accumulation. The rate rises from 0.1 to 0.3 cm/year just after artificial lowering of the lake water level in 1822, to about 1 cm/year during the 1900s. This study shows that anthropogenically altered lakes can contain stratigraphic records in their bottom sediments with temporal resolution comparable to the annual time steps of mechanistic water-quality models. It also demonstrates the utility of the Sr/Ca ratio profile as a sediment descriptor in carbonate-rich lake systems and provides a detailed stratigraphic basis for continued historical water-quality assessment in what used to be one of the world's most polluted lakes.