Tracking recurring drought/wet intervals and human impact over subtropical South America since the end of the Little Ice Age using Hg content in sediments

D. ARIZTEGUI; S. GUEDRON; L. COIANIZ; E. PIOVANO

Foz de Iguazú

Congreso; AGU-The meeting of the Americas; 2010

AGU

ABSTRACT BODY: Recent investigations have shown that variations of Hg content in lacustrine sediments can reflect natural- and anthropogenically-induced changes in the lake catchment. Various natural processes, including volcanic eruptions, atmospheric escapes from the ocean and rock weathering can release Hg into water bodies, soils and the atmosphere. More recently, Hg atmospheric emissions have substantially increased as a result of industrialization and ore mining (i.e., gold and silver). Additional sources such as forest burning remains important anthropogenic causes for atmospheric Hg emissions. Due to its long residence time in the atmosphere (i.e., at least one year), atmospheric Hg deposition can be recorded worldwide even in remote areas. Once deposited, Hg is incorporated into the biosphere (trapped into organic matter) and, thus, measurable Hg content in lake sediments can also be attributed to vegetation, forest fires and soils erosion. Laguna Mar Chiquita, a saline terminal lake located in the central north Pampean plains of Argentina, contains a sedimentary record of lake level fluctuations. These historically documented and instrumentally recorded shifts have been induced by substantial changes in rainfall affecting the watershed of this subtropical region during the last two centuries. Lake level changes have resulted in large variations in both surface and volume of the basin. The latter has dramatically affected mineral precipitation, sediment distribution, and primary productivity as well as associated biological processes. These most recent lake level variations are also shown by substantial changes in the Hg content of the bulk sediment in concert with changes in the nature and type of the sedimentary organic matter. Laguna Mar Chiquita sediments show a clear increase in Hg content that correlates well with increasing percentages of organic matter coinciding with lake level highstands. Conversely, they both display an antiphase behavior with carbonate content highlighting the affinity of Hg with the sulfur associated with the sedimentary organic fraction. The Laguna Mar Chiquita excellent chronological model allows us identifying human-induced changes such as World War II and enhanced industrial activity during the second half of the 20th century. These anthropogenic changes, however, do not mask the natural trend resulting from changes in the E/P ratio. Ongoing analyses will apply this combined biogeochemical approach to older sediments aiming to reconstruct - and eventually disentangle - the history of natural and anthropogenically-induced environmental changes throughout the Holocene.