PHOSPHATE AND TRACE METAL RECORDS FROM AN IRISH MARL LAKE: TRACING ANTHROPOGENIC INFLUENCE OVER SHORT AND LONG TIME SCALES

Lake sediments have long been used as historiographical records for natural and anthropogenic environmental changes. Evidence for lead smelting and copper mining from Grecian times has been found in deep sediment cores, and eutrophication from recent decades can likewise be traced by analyzing phosphate profiles in shallower depths. In this study we examine a lake in western Ireland within a catchment whose bedrock is entirely comprised of Carboniferous limestone. Lough Carra is a polymictic marl lake in Mayo County, Ireland, and at 16km²is the largest marl lake in the country. Its average depth is only 1.8m, but numerous deeper sections exist, to a maximum depth of ~18m.

Several short (<1m) cores and one long (8m) core were taken from varied locations and depths around the lake. Sediments are being analyzed for trace metal concentrations through sequential-step acid extractions, ICP-MS, and Hydra (DMR) mercury analysis. Phosphate is being analyzed by sequential-step acid digestion, total acid digestion, and measured by ascorbic-acid spectrophotometry. Samples are being dated by ²¹⁰Pb and ¹⁴C analyses.

Initial phosphate concentrations and profiles agree with data from a previous study in Lough Carra. Soluble reactive phosphate (SRP) levels are around 7-13 ppm, the majority of which is bound to Fe-Mn hydroxides. Labile phosphate and phosphate bound to CaCO₃ each account for only about 1ppm of total concentration. Initial total organic carbon (TOC) results show average levels around ~5% in deep sites, and ~7% in shallower sites, and correlate with SRP levels. Unlike a previous study, TOC levels do not appear to correlate inversely with %CaCO₃.

Trace metal profiles constructed from the long core are expected to document historical trace metal levels through the past few thousand years, and may show evidence for Bronze and Iron Age mining in Western Ireland. Phosphate concentrations will be used to assess paleoproductivity and recent eutrophication, and should reflect regional land use and agricultural development.