Mercury distribution in a toposequence of sub-Antarctic forest soils of Tierra del Fuego (Argentina) as consequence of the prevailing soil processes

Abstract

The total content, distribution and assessment of Hg sources in five soils of a toposequence of sub-Antarctic forest soils in Tierra del Fuego were explained as a combination of pedogenetic processes and the interactions between Hg and the soil compounds. The soils of the toposequence were mainly podzols and podzolic soils, strongly acidic (water pH<5.0), organic matter rich in O horizons (239-444gkg-1) and non-crystalline compounds dominate Al and Fe distribution. The total Hg (HgT) values ranged between 12 and 375ngg-1 and diminished with soil depth although the secondary peaks of the HgT were observed in illuvial (Bhs or Bs) or buried A horizons. The total Hg was significantly correlated to the total contents of C, N and S (r>0.71) and to the contents of the non-crystalline Al and Fe compounds (0.44-0.71), whereas the Hg/C ratio increases with soil depth up to values of 11.3μgHgg-1 C. Exogenic Hg, i.e. Hg deposited from the atmosphere (HgExo), constitutes over 87% of the HgT in the O and A horizons, whilst lithogenic Hg (HgLit) is 36% of the average HgT.Podzolization seems to control the distribution of the HgT in the mid-slope and upslope soils (P480, P590, P630), as suggested by the HgT peaks in the Bhs or Bs horizons, where illuviated organic matter and non-crystalline Al and Fe compounds favour Hg retention. In the downslope soils (P340 and P220), both podzolization and polycyclism (that result in soils formed by two or more different and contrasting processes derived from a substantial change in soil formations factors, that lead to the accumulation of features over their lifetime), seem to influence the HgT depth pattern in light of the high values observed in the A and Bhs horizons of the buried soils. The significant correlation between the Hg/C and Cp/C ratios strengthens the efficiency of humified organic matter to complex Hg, whereas the peaks of the Hg/C ratio in the Bhs and Bs horizons also indicate that Al and Fe compounds are also involved in Hg retention. Polycyclic events, i.e. those contrasting soil processes that lead to different chemical and morphological signatures within the soil profile, could justify high HgLit values in the deeper horizons of the downslope soils, whilst the peaks of HgExo in the Bhs or Bs horizons could be considered to be a consequence of Hg mobilization and the subsequent retention by Al and Fe compounds during podzolization which is supported by the significant correlations of these soil compounds with HgExo.Environmentally, illuvial horizons and buried A horizons could act as a safety mechanism delaying the arrival of Hg to stream waters.