Does organic matter degradation affect the reconstruction of pre-industrial atmospheric mercury deposition rates from peat cores? — A test of the hypothesis using a permafrost peat deposit in northern Canada

Abstract

Peat bogs generally exhibit significantly lower Hg accumulation rates during the pre-industrial (pre-1800 AD) period than lake sediments. Organic matter (OM) humification and losses of associated Hg from the deeper sections of peat profiles were recently proposed as an explanation for this widely-observed discrepancy. Here, this hypothesis is tested by examining the state of OM preservation and Hg accumulation rates in a dated, 165 cm (6270 yr) long permafrost peat core from northern Canada. Pyrolytic speciation of organic carbon using RockEval 6 analysis, and organic petrology of the OM, showed that OM throughout most of the core was better preserved than is typical of many peat deposits, although below ∼ 120 cm depth the initial stages of humification resulted in losses of labile organic compounds and increases in the proportion of refractory carbon. However, through most of the core there was little evidence of significant humification which could have caused large OM mass losses. Mercury flux averaged 0.82 ± 0.36 µg/m2/yr over the six millennia of peat accumulation, well within the range previously reported from other peat bogs. Thus, this paper adds to other studies, using a variety of humification indicators, which found no relationship between OM preservation/humification and pre-industrial Hg flux rates in peat bogs. Other explanations must be sought to account for the observed differences between pre-industrial peat and lake sediment Hg histories.