Mercury accumulation in the sediment of the Western Mediterranean abyssal plain: A reliable archive of the late Holocene
Résumé
Temporal reconstruction of Hg deposition from sediment archives is relatively
straightforward in organic-rich or high sedimentation rate environments, such as lakes
and ocean margins. To retrieve long-term records at regional or global scales, deepsea
sediments are more appropriate, but such records are scarce and their reliability
has been questioned because of possible post-depositional Hg diagenetic
remobilization. Here, we investigated the accumulation of Hg in the Balearic Abyssal
Plain (2850 m deep) of the Western Mediterranean through a comprehensive
characterization of the chemical and isotopic composition (organic carbon, nitrogen,
sulfur, major and redox-sensitive elements) of sediment trap material and sediment
cores. The analysis of material collected in the sediment traps, deployed at 250, 1440,
and 2820 m, indicates that Hg is (i) partially re-emitted to the atmosphere and
mobilized in the twilight zone and that (ii) the Hg downward flux depends on the
primary production in surface waters, suggesting that organic matter (OM) acts as the
main Hg-carrier phase. As the Hg concentrations of material collected in the traps vary
little with depth but the Hg:C org ratio of the settling particulate matter decreases with
depth, Hg must be re-adsorbed onto the more refractory fraction of the settling OM.
Results of selective chemical extractions of the sediment indicate that Hg is very
weakly coupled to the iron cycle but strongly associated with sulfur, supporting the
assumption that its vertical distribution was only weakly altered by diagenetic
remobilization. In addition, the distributions of S and δ 34 S in the sedimentary column
exclude the possibility that local volcanism impacted on Hg enrichment of the
sediments. Accordingly, a reconstruction of Hg accumulation rates (Hg-AR) during the
Late Holocene is readily achieved. Biological mixing and smoothing of the sediment
record, as revealed by the distribution of radionuclides in surface sediments, was
considered in the interpretation of the Hg-AR record. The first anthropogenic Hg signal
recorded in the studied cores corresponds to the Iron Age and the Roman Empire
period, as Hg-ARs rose from the baseline (0.7 ± 0.2 μg m -2 yr -1 ) to an average
value of 2.2 ± 0.5 μg m -2 yr -1 . The Hg-ARs return to baseline values at the decline
of the Roman Empire, display a small increase during the Medieval Period (1.5 ± 0.5
μg m -2 yr -1 ), increase abruptly at the onset of the Industrial Era, leading to a ~10-
fold increase in Hg deposition in the last 120 years ( 8.9 ± 1.4 μg m -2 yr -1 ), and
retreat progressively over the past 50 years.
Domaines
Sciences de l'environnement
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