A peat core Hg stable isotope reconstruction of Holocene atmospheric Hg deposition at Amsterdam Island (37.8oS)
Chuxian Li, Maxime Enrico, Oliver Magand, Beatriz Ferreira Araújo, Gaël Le Roux, Stefan Osterwalder, Aurélien Dommergue, Yann Bertrand, J. Brioude, François De Vleeschouwer, Jeroen E. Sonke
Abstract
a b s t r a c t Mercury (Hg) stable isotopes have been broadly used to investigate the sources, transformation and deposition of atmospheric Hg during the industrial era thanks to the multiple isotope signatures deriving from mass-dependent (represented by d 202 Hg) and mass-independent fractionation (represented by D xxx Hg) in the environment. Less is known about the impact of past climate change on atmospheric Hg deposition and cycling, and whether Hg isotopes covary with past climate. Here, we investigate Hg concentration and Hg isotope signatures in a 6600-year-old ombrotrophic peat record from Amsterdam Island (AMS, 37.8 o S), and in modern AMS rainfall and gaseous elemental Hg (Hg 0 ) samples. Results show that Holocene atmospheric Hg deposition and plant Hg uptake covary with dust deposition, and are both lower under a high humidity regime associated with enhanced Southern Westerly Winds. Modern AMS gaseous Hg 0 and rainfall Hg II isotope signatures are similar to those in the Northern Hemisphere (NH). Holocene peat D 199 Hg and D 200 Hg are significantly correlated (R 2 = 0.67, P < 0.001, n = 58), consistently oscillating between the modern Hg 0 and rainfall Hg II end-members. Peat D 200 Hg and D 199 Hg provide evidence of plant uptake of Hg 0 as the dominant pathway of Hg deposition to AMS peatland, with some exceptions during humid periods. In contrast to NH archives generally documenting a modern increase in D 199 Hg, recent peat layers (post-1900CE) from AMS show the lowest D 199 Hg in the peat profile (0.42 0.27 , 1r, n = 8). This likely reflects a significant change in the post-depositional process on deposited anthropogenic Hg in 20 th century (i.e. dark abiotic reduction), enabling more negative D 199 Hg to be observed in AMS peat. We further find that the oscillations of Hg isotopes are consistent with established Holocene climate variability from dust proxies. We suggest peat Hg isotope records might be a valid rainfall indicator.