Mercury speciation and Hg stable isotope ratios in sediments from Amazon floodplain lakes—Brazil

Abstract

Hg concentrations and isotope ratios were measured to better characterize the mercury dynamics related to Hg cycling in Amazon floodplain lakes. We collected sediments, suspended particulate material (SPM), and plankton from floodplain lakes and compared then to sediments from rivers and soils of the central Amazon basin by measuring concentrations of total Hg (THg) and methylmercury (MMHg), and mercury isotope ratios. Concentrations of THg and MMHg in the lake sediments ranged to 69–109 ng g−1 and 0.62–4.78 ng g−1, respectively. A positive correlation between THg and MMHg in soils and sediments suggest that inorganic Hg in this system is highly bioavailable and readily converted to MMHg. δ202Hg values ranged from −1.40‰ to −0.89‰ and Δ199Hg from −0.34‰ to −0.18‰. These values were comparable to those measured in riverine sediments (−2.14‰ to −1.23‰ and −0.51‰ to −0.05‰), suggesting a contribution of riverine sediments to lake sediments, at least during the season of rising waters, during which lake samples were collected. SPM on the other hand was much elevated in THg (590–1066 ng g−1) and showed more negative δ202Hg (−3.00 to −2.15), similar to those found in soils (−2.99‰ to −2.17‰), suggesting that Hg in SPM may originate from erosion of floodplain soils.

Mercury Isotopic Fractionation during Pedogenesis in a Tropical Forest Soil Catena (French Guiana): Deciphering the Impact of Historical Gold Mining

“…Foliage and soil litter (Oi and Oe horizons) have the lowest [THg] (123 ± 67 ng g −1 ) and exhibit intermediary MIF odd and MDF values (Δ 199 Hg = −0.22 ± 0.27‰; δ 202 Hg = −2.22 ± 0.52‰; N = 14) that are in the range of reported data for foliage and forest floor in the United States. 22,36 Finally, soil horizons (A, B, and C) from the undisturbed soil catena (N = 35) have intermediary [THg] values (291 ± 97 ng g −1 ) but exhibit low Δ 199 Hg (−0.55 ± 0.22‰) and δ 202 Hg (−2.43 ± 0.55‰) values that are similar to those of other Amazonian forested soils (mean Δ 199 Hg = −0.55 ± 0.10‰; δ 202 Hg = −2.15 ± 0.14‰) 18,37 but much lower than reported values for soils in the United States, 22 in a boreal region, 25 or in subtropical China. 38 These particularly low Hg isotopic values are most probably inherited from fractionation processes that lasted for millions of years during the development of these soils.…”