HISTORICAL TRACE ELEMENT TRENDS IN ANADROMOUS AND NON-ANADROMOUS LAKES IN SOUTHWEST ALASKA AS DECIPHERED FROM LAKE SEDIMENT CORES

A study of trace element concentrations from five lake sediment cores from southwest Alaska was undertaken to establish historical trends in trace elements from anadromous and non-anadromous lake systems. Sediment cores were collected within the last ten years by various coring methods. All cores were sub-sampled at 1.0 cm intervals for standard core analyses. Additionally, samples were analyzed for Hg by standard methods by AA. A separate sub-sample of the sediment was digested in ultra-pure H₂O₂ and HNO₃ and analyzed by ICP-MS for major and trace elements. It appears that the anadromous lakes contain higher (up to 250 ppb in Brooks Lake) Hg concentrations than the non-anadromous lakes (averaging ~ 25 ppb Hg). However, both major and trace element accumulation rate trends indicate that all five lakes are dominated by landscape processes which seem to be the primary contributors of most elements to the lakes. The concentrations of trace elements vary from lake to lake depending on the overall landscape contributions (ie. glacial melt-water fed or not). One end-member lake (Idavain Lake, non-anadromous) core appears to have preserved the best undisturbed record of trace element accumulation over time and also indicates an increase in Hg from 24 ppb to 45 ppb at ~mid-1700s A.D. Other trace elements including Zn, Cu, Pb, and Cr also indicate an approximate 50% increase at that time possibly from atmospheric contributions. Trace element trends also nearly double around the time of the Mount Katmai 1912 eruption but then recover to levels similar to those around mid-1700s. The other end-member lake cores (Naknek Lake and Lake Clark, anadromous and glacial fed) indicate an overall approximate 50% increase in most trace elements since pre-1800 A.D. but also show this trend for major elements likely indicating that these systems are dominated by landscape weathering over time with periodic contributions from volcanic ash including the Mount Katmai eruption. The sediment core from Kontrashibuna Lake (non-anadromous) shows significant variability in trace and major element concentrations throughout the core, likely the result of the frequent landslide disruptions that were identified in the core.