TRACE ELEMENT AND LEAD ISOTOPIC SIGNATURES IN ALPINE LAKE SEDIMENTS, FRONT RANGE, COLORADO

Sediment cores from two alpine lakes in the Colorado Front Range were collected, split and analyzed to determine the nature and timing of atmospheric deposition. To minimize influences of non-atmospheric anthropogenic sources, alpine lakes in relatively remote locations were chosen. Black Lake (BL), elevation 3230 m, is located within Rocky Mountain National Park, 5 km from a park road, and is underlain by 1420 Ma Silver Plume Granite. Green Lake 5 (GL5), elevation 3605 m, is located within the restricted City of Boulder watershed, 5 km from a seasonal road, and receives sediment derived primarily from Precambrian schists and gneisses and the Tertiary-age Audubon-Albion stock. Twenty-seven cm of BL core was recovered, and using ²¹⁰Pb dating method, the base was dated at ~ the year 1250. Twenty-three cm of GL5 core was recovered, and the base was dated at ~ the year 1740. Other layers within each core were dated to quantify variations in sedimentation rates. Samples were taken every 0.5 to 1 cm, dried, and digested using HNO₃, HF, and HCl by microwave digestion. Trace element chemistry was determined by ICPMS and Pb isotopic compositions by MC-ICPMS.

From the base to the late 1800's, samples from both cores display fairly uniform chemistry; Pb concentrations in lower BL core ranged from 29-33 ppm and GL5 core from 28-32 ppm. Between the late 1800's and the 1920's, concentrations of some metals (including Bi, Cd, Pb, and Sb) increase by a factor of 2 to 3, Pb concentrations in BL increase to 91 ppm and in GL5 to 95 ppm. Above this level, concentrations of these metals remain elevated but to varying degrees. At the chemical transition, preliminary Pb isotopic results display a sharp change from relatively constant isotopic ratios consistent with geologic background sources to ratios with a substantial component of potentially anthropogenic Pb. Possible sources of anthropogenic Pb within or adjacent to the Front Range include smelter emissions from processing of hardrock mining ore, emissions from coal-burning power plants, and leaded gas usage. Additional isotopic analyses, utilization of trace element signatures, and comparison with the relative timing of source outputs should enable the identification of the primary source or sources of atmospheric deposition in the Front Range alpine ecosystem.