Assessing the Stability of Mercury and Methylmercury in a Varved Lake Sediment Deposit

Abstract

Using lake sediments to infer past total mercury and methylmercury loading to the environment requires that diagenetic processes within the sediment do not significantly affect the concentrations or net accumulation rates of the mercury species. Because carbon is lost during early sediment diagenesis, the close link between carbon and mercury raises the question of how reliable lake sediments are as archives of total mercury and methylmercury loading. In this study we used a series of freeze cores taken in a lake with varved (annually laminated) sediment to assess the stability of total mercury and methylmercury over time. By tracking material deposited in specific years in cores collected in different years, we found that despite a 20--25% loss of carbon in the first 10--15 years, there was no apparent loss of total mercury over time; hence, lake sediments can be considered as reliable archives. However, over the first 5--8 years after sedimentation, about 30--40% of the methylmercury was lost (a decrease of 0.025--0.030 microg MeHg m(-2) yr(-1)), suggesting that sediment profiles showing increasing methylmercury concentrations toward the sediment surface are in large part an artifact of diagenetic processes (net demethylation), rather than a record of changes in methylmercury loading.

Over three millennia of mercury pollution in the Peruvian Andes

“…Consequently, these increases cannot be explained by a rapid influx of catchment material or enhanced Hg scavenging by organic matter. Moreover, lake sediment burdens of total Hg are largely unaffected by diagenetic processes (12,13), and represent minimum estimates of total Hg deposited to a lake surface because of reductive losses before final burial (14). Because we know of no natural mechanism capable of replicating such large and synchronized increases in Hg deposition in 2 adjacent lakes, and given their proximity to a major cinnabar deposit, we attribute with confidence the early increases of Hg observed at LY1 and LY2 to the emergence of regional-scale cinnabar mining at Huancavelica.…”

The impact of over 100 years of wildfires on mercury levels and accumulation rates in two lakes in southern California, USA

“…Previous work indicated Hg chronologies may be confounded by biological reworking of sediments and sorption of Hg onto redox sensitive metals, such as Fe (Rasmussen 1994). However, Lockhart et al (2000) analyzed Hg in sediment cores from polluted sites and verified sediment total mercury profiles were stable over time, while Rydberg et al (2008) reported diagenesis did not significantly obfuscate Hg trends in sediment cores over a 30-year period. In core BBL-1, there was a lack of correlation between Fe and Hg levels (r 2 = 0.00, p [ 0.95, n = 8), while there was a strong correlation between Al and Fe levels (r 2 = 0.88, p \ 0.01, n = 17), suggesting a low probability that diagenetic remobilization impacted Hg levels.…”

Mercury dynamics in lake sediments

“…7 and their average sediment accumulation rate value of 0.013 g cm À2 year À1 , a demethylation rate constant of 4 Â 10 À5 d À1 can be calculated. Rydberg et al (2008) also reported a decline in {MeHg} with sediment age in the varved sediments of Lake Nylandssjö n; using the data shown in their Fig. 3a, we calculate a demethylation rate constant of 6 Â 10 À5 d À1 .…”

“…Surface sediments are enriched in {MeHg} in Basin A, but not in Basin B. Surface sediment enrichments in MeHg were also observed in other lake sediments and were attributed to MeHg production and/or deposition at the sediment surface and subsequent demethylation with sediment burial (Hines et al, 2004;He et al, 2007;Rydberg et al, 2008). In the Fe-rich deposits collected with the Teflon sheets in Basin A, the average (n = 7) {MeHg}:{Fe} molar ratio was 6.5 ± 1.9 Â 10 À10 , a much lower value than that found in the top 0.5-cm sediment layer (4.9 Â 10 À8 ; Table 3).…”

“…1016/j.gca.2011.02.007 it focuses on Hg dynamics in recent sediment deposits which are considered to be a key location for MeHg formation (Krabbenhoft et al, 1998;Kainz et al, 2003;Hammerschmidt et al, 2006). Current inferences on in situ Hg T and MeHg mobility and on the processes involving these species in modern sediments are mostly derived from measurements in the solid phase alone (Lockhart et al, 2000;Rydberg et al, 2008) or from laboratory experiments involving bacterial cultures (Compeau and Bartha, 1985) or incubations of Hg-spiked pure solid phases, sediments or lake water (Gunneriusson et al, 1995;Tiffreau et al, 1995;Ramlal et al, 1986;Hintelmann et al, 2000;Eckley et al, 2005;Miller, 2006). We submit that measuring Hg T , MeHg and ancillary parameters in sediments and porewaters, combined with thermodynamic and kinetic modeling, provides an alternative to constrain in situ Hg T and MeHg mobility, physico-chemical processes involving Hg T and MeHg and their reaction kinetics.…”