ICMGP - Conference Presentation

Abstract Information :

Weathering is a key process transferring materials from continents to the hydrosphere, and plays a critical role in the cycling of trace elements. Mercury (Hg) is a toxic heavy metal and well-known global pollutant. Soil is the largest reservoir of Hg on Earth surface, and thus the behavior of Hg during soil weathering significantly affects how Hg transports from terrestrial to aquatic environment and biota. However, the transport and fate of Hg during soil pedogenesis are rarely studied. The fractionation of Hg stable isotopes has been widely used to trace Hg cycling. Here we measured Hg concentration and isotope ratios in a latosol profile in Zhanjiang, Guangdong Province, South China, formed through intense weathering of basalt, in order to understand how Hg isotope fractionate during weathering. All soil samples showed significantly negative ?202Hg (?2.97? to ?2.54?)??199Hg (-0.68? to -0.43?) and ?200 Hg(-0.01? to -0.06?), which are dramatically different from those of the bedrock (?202Hg = -7.30?, ?199Hg = -0.05?, ?200 Hg = +0.01?), suggesting that throughout the profile Hg was dominantly from atmospheric deposition rather than from bedrock. The Hg enrichment and isotope compositions both show distinct patterns along the whole profile. There was significant weathering leaching loss of the geological source of Hg and a significant positive shrift ~0.8 in ?202Hg. This is likely because lighter Hg isotope was absorbed by Fe hydro(oxide) and clay colloid, which were massively weathered and migrated, resulting in significant isotope fractionation. As the main source of mercury in the latosol profile, the soil ?202Hg deposited from litter showed a strong pH dependence in the downward migration and transformation of the profile. pH affected the degree and species of Hg adsorbed in soils. More HgOH+/HgCl+ species carrying lighter isotopes was adsorbed when the pH of latosol increased from 4.8/4.9 to 5.1/5.2, causing soil ?202 Hg to shift negatively. The soils also show much more negative ?199Hg than the average ?199Hg of atmospheric Hg(0) and foliage, which are the dominant sources of Hg in our profile, and a downward positive shift of ?199Hg. We suggest that the significantly more negative ?199Hg is produced by photoreduction of Hg(II) complexed by thiols in organic matters on the soil surface, followed by subsequently partial loss of these thiol-bound Hg(II) during runoff. The remainder Hg(II) was leached downward and mixed with geologically sourced Hg with near-zero ?199Hg, causing the positive shift of ?199Hg downward. Overall, our results suggest that pedogenetic processes such as weathering can cause significant fractionation of Hg isotopes, although the Hg isotope compositions in latosol profile soils are primarily controlled by atmospheric deposition and photoreduction on the surface layer.