Historical trace element trends recorded in lake sediment cores from the Southwest Alaska Network

Munk, LeeAnne; Cohn, Brian; Finney, Bruce

A study of trace element concentrations and accumulation rates in sediment cores from five lakes from southwest Alaska (Southwest Alaska Network (SWAN) of the National Park Service) 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 0.5 to 1.0 cm intervals for standard core analyses. Additionally, samples were analyzed for mercury by atomic absorption (AA). A sub-sample of the sediment was digested in ultra-pure H2O2 and HNO3 and analyzed by inductively coupled plasma mass spectrometery (ICP-MS) for major and trace elements including aluminum (Al), arsenic (As), chromium (Cr), copper (Cu), lead (Pb), mercury (Hg), molybdenum (Mo), nickel (Ni), and zinc (Zn). The longest record is preserved in the sediment core from Naknek Lake which extends back to 1284 AD. 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 contributor 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). Idavain Lake, which is non-anadromous and non-glacial appears to have the least disturbed sediment 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 probably from volcanic ash sources but other atmospheric contributions cannot be ruled out. Trace element concentrations also nearly double around the time of the Mount Katmai 1912 eruption but then recover to levels similar to those around mid-1700s. The lake sediment cores from Naknek Lake and Lake Clark (anadromous and glacial fed) indicate an approximate 50% increase in most major and trace elements since pre-1800 A.D. likely indicating that these systems are dominated by landscape weathering over time. The sediment core from Kontrashibuna Lake (non-anadromous and glacial) 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.

Historical trace element trends recorded in lake sediment cores from the Southwest Alaska Network

Series

Collections