Mercury (Hg) is a metal of environmental concern due to its high volatility, long atmospheric residence time (1–2 years), and the bioaccumulation of its methylated forms. It is released and dispersed in the atmosphere by natural emission sources such as volcanoes, geothermal vents, and Hg-enriched soil, as well as by anthropogenic activities such as mining, coal-fired plants, and chlor-alkali plants [1
]. Its transport and deposition in different environments on Earth depends on the type of Hg.
Gaseous elemental mercury (Hg(0)) is the dominant form of atmospheric Hg, and it is efficiently transported around the globe by long-range atmospheric transport. It can be oxidized into highly reactive and water soluble divalent species (Hg(II)), and/or particle-bound Hg (Hg(p)) that can be deposited through wet and dry processes onto surfaces. Therefore, atmospheric oxidation of Hg(0) to divalent Hg enhances its deposition to environmental surfaces [1
]. Particle-bound Hg (Hgp
) represents a minor fraction of total Hg in the atmosphere and can be dispersed over tens to hundreds of kilometers [1
Natural archives such as mires, lake sediments, glacial ice, and marine sediments have been widely used to reconstruct Hg accumulation at the local, regional and global scale [6
]. These archives have allowed for the identification of processes and factors that control the deposition and accumulation of Hg over time. Most recently, a factor between a three and five-fold increase in the accumulation of Hg was observed in different parts of the world since the Industrial Revolution, suggesting a worldwide increase in atmospheric Hg deposition [6
]. This anthropogenic input overlaps the signal driven by natural processes of deposition and accumulation. Organic matter degradation and mass loss related to peat evolution enhance Hg accumulation in peatlands [12
]. Short- and long-term climate oscillations seem to play an important role in Hg cycling through (i) controlling the re-emission of some of the accumulated Hg [14
]; (ii) inducing Hg depletion events in polar environments during glacial periods [15
]; (iii) producing algal or diatom scavenging in lakes or marine environments [10
] or by (iv) releasing Hg from permafrost mires in warm periods [19
]. Finally, processes such as volcanism and fires may play a role in releasing Hg into the atmosphere or in the landscape, but the effects are only visible in very specific cases [20
Nevertheless, most of these considerations are almost exclusively based on Holocene records, and only a few studies extend their conclusions back to the Pleistocene. In contrast to the Northern Hemisphere, there is a limited amount of quite heterogeneous information available for the Southern Hemisphere. The works focused on Hg reconstruction in the South Hemisphere are almost exclusively from the Patagonian area. Most of these studies are based on lake sediments [20
] and a few on peat [12
]. More recently, a lacustrine record from NE Brazil has shown the climate influence driving Hg accumulation rates during the past 20,000 years [28
In a recent work, we studied Hg accumulation in Pinheiros (18° S, 43° W) [29
], a Pleistocene age (last ~57 ka) tropical mire located in a valley in Serra do Espinhaço Meridional (state of Minas Gerais, Brazil). In this work, we found that three of the four main processes controlling Hg concentration depended on climate: wet and dry Hg deposition (rainfall and dustfall, respectively), as well as local catchment soil erosion owing to precipitation events [29
]. The effect of long-term peat decomposition, the only autogenic process identified, was confined to the Holocene section of the peat [29
Although some of the mentioned works provided information from Pleistocene sections (~11.2, 14.6, ~57 and 20 cal BP, [12
]), more long-term records from a wider geographical area are needed in order to fully understand the various processes that can influence emission, deposition, and accumulation of Hg in continental ecosystems. In particular, the role of climate through controls on both deposition and accumulation is one of the major issues that needs to be addressed.
In this study, we analyzed two peat cores from Rano Aroi (ARO), a small mire located in Easter Island, which covers the last ~71 ka BP. We determined the Hg content in the peat samples and in selected vegetation samples of present-day flora located in the watershed. Data from two previous studies on the same records [31
] were used to reconstruct the environment in Rano Aroi between ca. 71.0 ka BP and ca. 9.0 ka cal BP in the ARO 06 01 core, and the last ca. 39.0 ka cal BP in the ARO 08 02 core. Thus, PC1 and PC2 from a Principal Component Analysis (PCA) performed in ARO 06 01 represent the long-term background fluxes of inorganic particulate material and the delivery of large amounts of terrigenous particles transported during wet events, respectively [32
]. Nitrogen concentration (TN) and the C/N ratio were used to attribute the contribution of the organic matter source; for example, algae or terrestrial and aquatic plants [32
]. Isotopic organic matter data, δ13
C (‰), was used to indicate the C3
vegetation dominance and thus the main climatic conditions. The objectives of the present research were (i) to reconstruct the Pleistocene Hg flux variability at millennial to centennial resolution and (ii) to determine the main factors that controlled peat Hg concentrations over long time scales.
The Hg record in the Rano Aroi cores shows a huge range of values between 35–1000 ng g−1
, mostly driven by wet precipitation—directly on the mire or through the catchment circuits—essentially during high-rainfall events. Two large maxima occurred at the end of the LGM (~20 ka cal BP) and at ~8.5 ka cal BP, both periods of general cold climatic conditions and wetness in Rano Aroi. Low temperature would accelerate the atmospheric oxidation of Hg(0) to divalent Hg, and if combined with high precipitation this would result in very efficient surface deposition of atmospheric Hg. Thus, both colder and humid conditions would have favored Hg accumulation in Rano Aroi, since Hg deposition is controlled by temperature and humidity variations [14
Other processes such as mire vegetation, peat decomposition, aeolian dust, oceanic evasion, and volcanic activity (direct Hg emission and Hg oxidation) may have played a secondary role in Hg content. Oceanic evasion might have played a role as a general background Hg source, and increases in the fire regimen during the LGM at the tropical latitudes of South America might have been an extra Hg emission to the atmosphere that was eventually deposited on Easter Island during heavy rainfall episodes. Moreover, wind-borne dust particles derived from remote sources with higher Hg concentrations might also play a role in enhanced Hg concentrations during the LGM, and especially during the mid-Holocene Hg peak (4.2 ka cal BP), in agreement with previous works.
The combinations of these processes cause a range of variation of Hg concentrations of 28-fold in a remote and undisturbed area, at least during the studied period. The maximum values are higher than those recorded in most peat records belonging to the Industrial Period, highlighting that natural factors played a significant role in local Hg accumulation—sometimes even more so than anthropogenic sources.