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Accumulation rates and predominant atmospheric sources of natural and anthropogenic trace elements on the Falkland Islands

Periodic Reporting for period 1 - SOUTHERNCHANGE (Accumulation rates and predominant atmospheric sources of natural and anthropogenic trace elements on the Falkland Islands)

Reporting period: 2016-10-10 to 2018-10-09

Atmospheric deposition derived from long-range transport is a key process supplying essential (e.g. Fe, Zn, Cu) and toxic (e.g. Pb, As) trace elements to remote ecosystems such as ocean islands or open oceans. Any present or future change in atmospheric deposition rates can be only understood in the context of natural variations and controls derived from geological archives such as peat bogs and their record of natural and anthropogenic inputs. While atmospheric deposition of trace elements in the Northern Hemisphere is relatively well understood during the Holocene, our knowledge regarding past atmospheric deposition in the southern South Atlantic Ocean region is very limited due to the lack of high quality records. Quantifying atmospheric element deposition in the western South Atlantic Ocean is of key importance to the sustainable development of the Falkland Islands as increasing emissions of toxic metals stemming from the economic development of South America and from planned oil exploration may pose a significant threat to the marine ecosystem health.

The aim of the SOUTHERNCHANGE project was to improve our understanding of the atmospheric cycling of dust and trace elements in the Falkland Islands (FI) as representative of the western Southern Atlantic Ocean (wSAO). The objectives of the project were (i) to characterise the chemical and isotopic composition of natural aerosols deposited in the wSAO and the FI during the Holocene, (ii) to reconstruct the natural atmospheric deposition rates during the Holocene, (iii) to identify the main sources of the natural atmospheric input of dust and trace metals critical to marine and terrestrial ecosystem dynamics and environmental health in the wSAO and the FI and (iv) to assess the extent of human impact (via local activities or via long range transport) on current atmospheric trace element accumulation in the Falkland Islands.
1. Natural accumulation rates and predominant atmospheric sources of dust and trace elements in the Falklands during the Holocene
We established a high-resolution record of atmospheric deposition of dust since the Late Glacial period (i.e. the last 15 000 years) in the wSAO. This was achieved by studying the inorganic geochemistry of a peat core taken in the Falkland Islands, and included the characterization of elemental concentrations (major, trace elements and rare earth elements REE) and of radiogenic isotope ratios of lead (Pb) and neodymium (Nd).
Two main natural origin of atmospheric deposition were identified in the peat record based on the variations of dust accumulation rates and its geochemical composition: (i) mineral dust originating from the southernmost part of South America (Patagonia, Tierra del Fuego) and (ii) volcanic ash emitted during large explosive eruption of southern Andean volcanoes (Reclus at ~14.0 kyr BP, Monte Burney at 9.5 kyr BP, Hudson at 8.1 kyr BP and 3.6 kyr BP and Aguilera at 3.0 kyr BP). The deposition rates of mineral dust were four times higher during the Antarctic Cold Reversal (14.5-13.0 kyr BP) than during the Holocene due to the sustainment of dust production and transport in relation with glacier advance in Patagonia and stronger westerlies during the ACR. During the Holocene, mineral dust accumulation rates declined and only small variations were recorded. However, variations of the geochemical composition (REE signature) of atmospheric deposition were recorded and suggest that the dominant source areas within Patagonia changed in relation with major changes in the position and intensity of the Southern Westerly Wind belt during the Early and Late Holocene.
The geochemical behaviour of trace elements within the peat deposit was investigated showing that only the atmospheric record of Pb, Sb and Zn was preserved. The concentration profiles of other elements such as As, Cd and Cu are disturbed by post-depositional processes and thus do not reflect changes in atmospheric deposition. The accumulation rates of Pb, Sb and Zn were calculated and the results demonstrate that they are controlled both by mineral dust and volcanic ash deposition.
2. Anthropogenic influence of atmospheric trace element deposition in the Falkland Islands
The study of the most recent section of the peat core allowed to estimate the onset of anthropogenic influence on trace metal deposition in the Falkland Islands. The rise of Pb enrichment in atmospheric deposition started at ca. 980 AD and predated the colonisation of the Falkland Islands in the 18th century, demonstrating that Pb originated from long-range emissions. In addition, a change in Pb isotopic signature is observed from 1350 AD reflecting a change in Pb source. Our results suggest, for the first time, that Pb emitted by early Andean metallurgy reached the southwest South Atlantic Ocean and confirm the wide dispersion of emitted contaminants into the atmosphere.
The current level of metal enrichment in atmospheric deposition in the Falkland Islands was assessed by analysis metal concentration in lichens, aerosols and surface peat collected during the project. The results show that As, Cd, Cu, Pb, Sb and Zn are significantly enriched, relatively to the average composition of the upper continental crust, in all the studied samples demonstrating that atmospheric deposition in the Falkland Islands are affected by metal contamination from human activities. Lead isotopic signature was determined in these samples and it differed from the natural background signature, reflecting the contribution of Pb emitted by human activities. We showed that local emissions participate to metal enrichment in atmospheric deposition but the contribution of local and distant sources could not be determined due to similar Pb isotopic signature of anthropogenic emissions in South America.
The project SOUTHERNCHANGE established for the first time a high quality record of dust deposition in the western South Atlantic Ocean since the Late Glacial period (<15 kyr BP). We demonstrated that atmospheric deposition of dust and trace elements (Pb, Sb, Zn) in the Falkland Islands are primarily controlled by changes in climate and in the Southern Westerly Wind (SWW) belt in southern South America and occasionally by large explosive eruptions of Andean volcanoes.
We observed that Pb emitted by Pre-Hispanic and Spanish metallurgy in South America reached the Falkland Islands and that current atmospheric deposition in the Falkland Islands are significantly enriched in metal(loid)s originating from human activities despite their remote location.
Finally, this work provides a unique reference and baseline data for future environmental and climatic studies in the SAO.
Peatland landscape in the Falkland Islands