The role of climate: 71 ka of atmospheric mercury deposition in the southern hemisphere recorded by Rano Aroi Mire, Easter Island (Chile)

geosciences Article The Role of Climate: 71 ka of Atmospheric Mercury Deposition in the Southern Hemisphere Recorded by Rano Aroi Mire, Easter Island (Chile) Marta Pérez-Rodríguez 1,2,*, Olga Margalef 3,4, Juan Pablo Corella 5, Alfonso Saiz-Lopez 5, Sergi Pla-Rabes 4, Santiago Giralt 6and Antonio Martínez Cortizas 2 1Institut für Geoökologie, Abt. Umweltgeochemie, Technische Universität Braunschweig, Langer Kamp, 38106 Braunschweig, Germany 2Departamento de Edafoloxía e Química Agrícola, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] 3Spanish National Research Council (CSIC), Global Ecology Unit CREAF-CEAB-UAB, Cerdanyola del Vallès, 08193 Catalonia, Spain; omar[email protected] 4Ecological Research Center and Forestry Applications (CREAF), Campus de Bellaterra (UAB), Cerdanyola del Vallès, 08193 Barcelona, Spain; [email protected] 5Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, Spanish National Research Council (CSIC), Serrano 119, 28006 Madrid, Spain; pablo.cor[email protected] (J.P.C.); [email protected] (A.S.-L.) 6Institute of Earth Sciences Jaume Almera (ICTJA-CSIC), Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain; [email protected] *Correspondence: m.per[email protected]; Tel.: +49-(0)-531-391-7242 Received: 27 August 2018; Accepted: 7 October 2018; Published: 11 October 2018   Abstract: The study of mercury accumulation in peat cores provides an excellent opportunity to improve the knowledge on mercury cycling and depositional processes at remote locations far from pollution sources. We analyzed mercury concentrations in 150 peat samples from two cores from Rano Aroi (Easter Island, 27 ◦ S) and in selected vegetation samples of present-day flora of the island, in order to characterize the mercury cycling for the last ~71 ka BP. The mercury concentrations showed values ranging between 35 and 200 ng g −1 , except for a large maxima (~1000 ng g −1 ) which occurred at the end of the Last Glacial Maximum (LGM, ~20 ka cal BP) in both peat cores. Low temperatures during the LGM would accelerate the atmospheric oxidation of Hg(0) to divalent mercury that, coupled with higher rainfall during this period, most likely resulted in a very efficient surface deposition of atmospheric mercury. Two exceptional short-lived Hg peaks occurred during the Holocene at 8.5 (350 ng g −1 ) and 4.7 (1000 ng g −1 ) ka cal BP. These values are higher than those recorded in most peat records belonging to the industrial period, highlighting that natural factors played a significant role in Hg accumulation—sometimes even more so than anthropogenic sources. Our results suggest that wet deposition, linked to atmospheric oxidation, was the main process controlling the short-lived Hg events, both in the mire and in the catchment soils. Keywords: mercury cycle; Easter Island; Pleistocene; Holocene; climate; peatland 1. Introduction 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 – 3 ]. Its transport and deposition in different environments on Earth depends on the type of Hg. Geosciences 2018,8, 374; doi:10.3390/geosciences8100374 www.mdpi.com/journal/geosciences Geosciences 2018,8, 374 2 of 19 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 , 4 , 5 ]. Particle-bound Hg (Hg p ) 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 – 10 ]. 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 , 10 , 11 ]. 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 , 13 ]. 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 , 16 – 18 ] 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 [20–24]. 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 , 21 , 25 , 26 ] and a few on peat [ 12 , 27 ]. 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 , 27 – 30 ]), 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 , 32 ] 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 C 3 and C 4 vegetation dominance and thus the main climatic conditions. The Geosciences 2018,8, 374 3 of 19 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. 2. Materials and Methods 2.1. Regional Setting Easter Island (27 ◦ 07 0 S, 109 ◦ 22 0 W) is a Chilean volcanic island, Miocene in age, located in the southern Pacific Ocean. The nearest continental point from the South American continent lies 3512 km away. The island has a triangular shape and the topography is characterized by more than 70 volcano craters and the rolling surfaces of lava flows [ 33 – 35 ]. The climate is humid subtropical, with average monthly temperatures between 18 ◦ C (August) and 24 ◦ C (February) [ 36 ] and highly variable annual rainfall ranging from 500–1800 mm (mean of 1130 mm). Rano Aroi is a small mire located in an ancient Pleistocene volcano crater close to the most elevated area of the island (27 ◦ S, 108 ◦ W, 430 m elevation, 0.13 km 2 ; Figure 1). The crater slopes form a small catchment area (0.158 km 2 , maximum dimension 0.21 km North to South). The watershed lithology mainly consists of very porphyritic olivinic tholeite, basalt and hawaiite lava flows, which are rich in iron [ 34 ]. The mire is covered by Scirpus californicus,Polygonum acuminatum,Asplenium polyodon var. squamulosum,Vittaria elongata and Cyclosorus interruptus [ 37 ], while the catchment area is covered by grassland and planted eucalyptus. There is a small ravine acting as a natural outflow that infiltrates before reaching the coast. An artificial outlet built in the 1960s has been used to control the maximum outflow from the peatland since then. Geosciences 2018, 8, x FOR PEER REVIEW 3 of 19 source; for example, algae or terrestrial and aquatic plants [32]. Isotopic organic matter data, δ 13 C (‰), was used to indicate the C 3 and C 4 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. 2. Materials and Methods 2.1. Regional Setting Easter Island (27°07′ S, 109°22′ W) is a Chilean volcanic island, Miocene in age, located in the southern Pacific Ocean. The nearest continental point from the South American continent lies 3512 km away. The island has a triangular shape and the topography is characterized by more than 70 volcano craters and the rolling surfaces of lava flows [33–35]. The climate is humid subtropical, with average monthly temperatures between 18 °C (August) and 24 °C (February) [36] and highly variable annual rainfall ranging from 500–1800 mm (mean of 1130 mm). Rano Aroi is a small mire located in an ancient Pleistocene volcano crater close to the most elevated area of the island (27° S, 108° W, 430 m elevation, 0.13 km 2 ; Figure 1). The crater slopes form a small catchment area (0.158 km 2 , maximum dimension 0.21 km North to South). The watershed lithology mainly consists of very porphyritic olivinic tholeite, basalt and hawaiite lava flows, which are rich in iron [34]. The mire is covered by Scirpus californicus, Polygonum acuminatum, Asplenium polyodon var. squamulosum, Vittaria elongata and Cyclosorus interruptus [37], while the catchment area is covered by grassland and planted eucalyptus. There is a small ravine acting as a natural outflow that infiltrates before reaching the coast. An artificial outlet built in the 1960s has been used to control the maximum outflow from the peatland since then. Figure 1. (a) Map showing the location of Rano Aroi on the island. (b) Photo of the Rano Aroi mire, indicating the location of the studied cores (ARO 06 01, center of the mire and ARO 08 02, margins of the mire). 2.2. Sampling Two sediment cores (ARO 06 01, 13.9 m long and ARO 08 02, 4 m long) were retrieved from Rano Aroi on March 2006 and October 2008, respectively. The core ARO 06 01 was sampled with a UWITEC corer from the central part of the mire, and the ARO 08 02 with a Russian corer. The uppermost two meters of the core ARO 06 01 were rejected at the field to avoid potential anthropic remobilization, as described in previous studies [31,32]. Meanwhile, ARO 08 02 was retrieved with a Russian corer from the eastern boundary in order to recover the previously discarded sediments, since the mire’s margins were potentially not affected by recent human activities [38,39]. None of the cores retrieved in either campaign reached the bedrock. The cores were sealed, transported to a core repository, and stored in a cold room at +4 °C until sampling. Both cores were lithologically described and sampled for smear slides every 5 cm [31]. Figure 1. ( a ) Map showing the location of Rano Aroi on the island. ( b ) Photo of the Rano Aroi mire, indicating the location of the studied cores (ARO 06 01, center of the mire and ARO 08 02, margins of the mire). 2.2. Sampling Two sediment cores (ARO 06 01, 13.9 m long and ARO 08 02, 4 m long) were retrieved from Rano Aroi on March 2006 and October 2008, respectively. The core ARO 06 01 was sampled with a UWITEC corer from the central part of the mire, and the ARO 08 02 with a Russian corer. The uppermost two meters of the core ARO 06 01 were rejected at the field to avoid potential anthropic remobilization, as described in previous studies [ 31 , 32 ]. Meanwhile, ARO 08 02 was retrieved with a Russian corer from the eastern boundary in order to recover the previously discarded sediments, since the mire’s margins were potentially not affected by recent human activities [ 38 , 39 ]. None of the cores retrieved in either campaign reached the bedrock. The cores were sealed, transported to a core repository, and stored in a cold room at +4 ◦ C until sampling. Both cores were lithologically described and sampled for smear slides every 5 cm [31]. Geosciences 2018,8, 374 4 of 19 2.3. Age Model The chronology for both cores was fully described by Margalef, et al. [ 31 ]. A total of 27 AMS 14 C dates were obtained from pollen concentrates of the ARO 06 01 and ARO 02 08 cores in the Poznan Radiocarbon Laboratory (Poland). Only 18 dates were used for the age-depth model. The AMS ages were calibrated using CALIB 6.02 software, and the INTCAL 98 curve [ 40 ] and CalPal [ 41 ] for samples older than 20,000 radiocarbon year BP. The age model was built by simple linear interpolation [31]. 2.4. Geochemical Analysis Both cores were sampled every 5 cm (286 samples) for total carbon and nitrogen (TC and TN) and stable isotope ( δ13 C) analyses. Samples were dried at 60 ◦ C for 48 h, frozen with liquid nitrogen, and ground in a ring mill. Analyses were performed using a Finnigan delta Plus EA-CF-IRMS spectrometer, located at the Centres Tècnics i Tecnològics of the Universitat de Barcelona (CCiTUB) [ 31 ]. Mercury concentrations were determined in 150 samples of the two cores collected in Rano Aroi and in selected vegetation samples of present-day flora located in the island. Dried, milled samples were analysed for Hg using a Milestone DMA-80 analyser hosted at the Departamento de Bioloxía Funcional (University of Santiago de Compostela). The analysis of one in every five samples was undertaken in duplicate to judge precision. A standard reference material of the moss Pleurozium schreberi [ 42 ] M3 was run with each set of samples. The quantification limit [average of the blanks + (10 × standard deviation of the blanks)] was 1.34 ng g −1 , and the mean recovery of the reference material was 90 ±4%. 3. Results 3.1. Organic Fraction: Total Carbon, Total Nitrogen, C/N and δ13C The Rano Aroi deposits are mostly organic, with TC concentrations between 40% and 70% [ 31 ]. Total N levels of core ARO 06 01 vary between 0.4% and 2%, with C/N ratios ranging from 40–110 (Figure 2). ARO 08 02 shows similar values with respect to TC (50–60%), TN (0.8–2%, peaking to 2.5% at 140 and 80 cm) and C/N (20–80, with a marked decrease from 2 m upward) (Figure 3). In ARO 06 01, δ13 C shows a constant value around − 14 ‰ from depths of 14 m to 9 m, whereas from 9 m to 6 m, δ13 C values gradually shift from − 14 ‰ to − 26 ‰ (Figure 2). In the upper five meters, δ13 C values vary around − 26 ‰ . In ARO 08 02, a gentle shift of δ13 C from − 19 ‰ to − 23 ‰ occurs from depths of 4 m to 3.5 m (Figure 3). δ13 C curves show high-frequency changes (dips) within the long-term trend [31]. Geosciences 2018,8, 374 5 of 19 Geosciences 2018, 8, x FOR PEER REVIEW 5 of 19 Figure 2. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 06 01 core. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: total nitrogen and total carbon (TN, C/N) ratios and δ13C, are indicative of the origin of organic matter; and δ13C (‰) and Facies C [31] are wet event indicators. Periods of wet events and drought conditions are marked. Figure 3. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 08 02 core versus depth. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: TN (C/N ratios and δ13C, are indicative of the origin of organic matter; Figure 2. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 06 01 core. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: total nitrogen and total carbon (TN, C/N) ratios and δ13 C, are indicative of the origin of organic matter; and δ13 C ( ‰ ) and Facies C [ 31 ] are wet event indicators. Periods of wet events and drought conditions are marked. Geosciences 2018, 8, x FOR PEER REVIEW 5 of 19 Figure 2. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 06 01 core. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: total nitrogen and total carbon (TN, C/N) ratios and δ13C, are indicative of the origin of organic matter; and δ13C (‰) and Facies C [31] are wet event indicators. Periods of wet events and drought conditions are marked. Figure 3. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 08 02 core versus depth. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: TN (C/N ratios and δ13C, are indicative of the origin of organic matter; Figure 3. Depth profiles of the main paleoenvironmental proxies and Hg concentration analyzed in the ARO 08 02 core versus depth. Lithology and radiocarbon age samples are indicated in the column. Geochemical proxies: TN (C/N ratios and δ13 C, are indicative of the origin of organic matter; Facies C [31] is wet event indicator. Periods of high rainfall events and drought conditions are marked. Geosciences 2018,8, 374 6 of 19 3.2. Mercury Mercury concentrations in the Rano Aroi samples varied in the same range for both cores (Figures 2 and 3). Core ARO 06 08 has values that range between 35 and 1023 ng g −1 (147 ± 49, mean ± standard deviation), with the maximum concentration in the sample at 370 cm of core depth. The Hg minimum is located at 580 cm (Figure 2). Core ARO 08 02 displays Hg values that range between 37 and 1088 ng g −1 (104 ± 56 ng g −1 ), and two maxima (1088 and 910 ng g −1 ) are located at 195 and 285 cm of core depth. According to Hg concentrations, this profile shows three distinct sections: (i) from the base of the core to 290 cm, values increase with depth but with small oscillations (122 ± 26 ng g −1 ); (ii) from 280 to 202 cm, values are low and almost constant (73 ± 14 ng g −1 ); and (iii) the upper 190 cm have intermediate values (101 ± 56 ng g −1 ), with two relative maxima of 177 and 192 ng g −1 at 130 and 75 cm, respectively (Figure 3). Mercury in the vegetation samples had a large concentration range (Table 1). The lowest value (341 ng g −1 ) corresponds to the rhizome of Scirpus californicus and the maximum value (11,175 ng g −1 ) to an unidentified rush sample. Table 1. Mercury concentrations in vegetation samples collected in Easter Island. Local/English Name Scientific Name Hg (ng g−1) Kikuyu/Kikuyu grass Pennisetum clandestinum 1945 Nga’atu/Totora (duster) Scirpus californicus (duster) 1804 Nga’atu/Totora (rhizome) Scirpus californicus (rhizome) 341 /Rush no identified 11,175 Maku Piro/molassesgrass Melinis minutiflora 1470 4. Discussion 4.1. Mercury Concentrations in Rano Aroi Records The two Rano Aroi records (ARO 08 02 and ARO 06 01) cover different temporal ranges (38.7 ka cal BP-present day and 71 ka BP–8.5 ka cal BP, respectively) and were retrieved less than 50 m apart (Figure 1). Previous studies using multiple short cores from the same peatland showed that spatial variability can be a problem for an accurate Hg study, since vegetation and micro-topography changes can affect the interception and retention of atmospheric deposition over relatively short timescales [ 43 , 44 ]. However, the short distance between the cores, the small size of the mire (0.13 km 2 ), and the age-depth model based on both cores [ 31 ] seem to have largely solved this problem. Therefore, we discuss the processes that controlled Hg concentrations in Rano Aroi using both cores together; the differences between them are illustrated based on other proxies (as TC, TN and δ13C). To the best of our knowledge, there is only one comparable record—the Pleistocene peat record from a tropical-sub tropical area—to Rano Aroi in the literature. This is the Pinheiros mire, located in Minas Gerais state, Brazil. The range of Hg concentrations in Rano Aroi records (~35 and 333 ng g −1 ) is, in general, comparable with that of Pinheiros mire (~36–370 ng g −1 ) [ 29 ], although the minima found in the Brazilian mire are considerably lower than those in Rano Aroi (<2 vs. 35 ng g −1 ), probably due to the large quartz content of the corresponding peat sections in the former [ 29 ]. On the other hand, the ARO cores show two extraordinary Hg peaks of ~1000 ng g −1 at ~20.0 ka cal BP and ~4.7 ka cal BP (Figures 4and 5) that are more than two-fold the maximum found in the Brazilian record. Geosciences 2018,8, 374 7 of 19 Geosciences 2018, 8, x FOR PEER REVIEW 7 of 19 Figure 4. Mercury concentration and main proxies; from bottom to top: PC1, PC2, Hg concentration, C/N, TN and δ13C 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. PC1 and PC2 data are from previous research [32] and represent the long-term background fluxes of inorganic particulate material and the delivery of large amounts of terrigenous particles transported during wet events, respectively. The δ13C (‰) general trend indicates C3 and C4 vegetation dominance. Wet events according to δ13C (‰) excursions and PC2 peaks that match with Hg concentration increases (blue bars); the dashed line indicates the position of the δ13C (‰) excursion in agreement with the literature (see text). A red discontinuous bar indicates a highly oxidized peat section. Discontinuous trend lines indicate sections with chronological uncertainty. Cold periods are indicated (*). Figure 4. Mercury concentration and main proxies; from bottom to top: PC1, PC2, Hg concentration, C/N, TN and δ13 C 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. PC1 and PC2 data are from previous research [ 32 ] and represent the long-term background fluxes of inorganic particulate material and the delivery of large amounts of terrigenous particles transported during wet events, respectively. The δ13 C ( ‰ ) general trend indicates C 3 and C 4 vegetation dominance. Wet events according to δ13 C ( ‰ ) excursions and PC2 peaks that match with Hg concentration increases (blue bars); the dashed line indicates the position of the δ13 C ( ‰ ) excursion in agreement with the literature (see text). A red discontinuous bar indicates a highly oxidized peat section. Discontinuous trend lines indicate sections with chronological uncertainty. Cold periods are indicated (*). Geosciences 2018,8, 374 8 of 19 Geosciences 2018, 8, x FOR PEER REVIEW 8 of 19 Figure 5. Mercury concentration and main proxies uses to reconstruct the environment in Rano Aroi in the last ca. 39.0 ka cal BP in the ARO 08 02 core; from bottom to top: Hg concentration, C/N, TN and δ13C) [32]. Wet events according to δ13C (‰) excursions that match with Hg concentration increases (blue bars); the dashed line indicates the position of the δ13C (‰) excursion in agreement with the literature (see text). Cold periods are indicated (*). Maxima Hg values in the Pleistocene and Holocene sections of Rano Aroi are comparatively high with respect to other peatlands worldwide, even in sections affected by anthropogenic emissions. For example, for the Industrial Period, maxima of ~800 ng g−1 were reported for the Czech Republic [45], >400 ng g−1 for Scotland [46,47], ca. 600 ng g−1 in Northeast China [48], ca. 130 ng g−1 for Canada [49], and ca. 300 ng g−1 in Spain [43]. Our two maxima of natural Hg concentrations are, however, similar to those (up to 1100 ng g−1) found in four peat cores from Belgium, dating to AD Figure 5. Mercury concentration and main proxies uses to reconstruct the environment in Rano Aroi in the last ca. 39.0 ka cal BP in the ARO 08 02 core; from bottom to top: Hg concentration, C/N, TN and δ13 C) [ 32 ]. Wet events according to δ13 C ( ‰ ) excursions that match with Hg concentration increases (blue bars); the dashed line indicates the position of the δ13 C ( ‰ ) excursion in agreement with the literature (see text). Cold periods are indicated (*). Maxima Hg values in the Pleistocene and Holocene sections of Rano Aroi are comparatively high with respect to other peatlands worldwide, even in sections affected by anthropogenic emissions. For example, for the Industrial Period, maxima of ~800 ng g −1 were reported for the Czech Republic [ 45 ], >400 ng g −1 for Scotland [ 46 , 47 ], ca. 600 ng g −1 in Northeast China [ 48 ], ca. 130 ng g −1 for Canada [ 49 ], and ca. 300 ng g −1 in Spain [ 43 ]. Our two maxima of natural Hg concentrations Geosciences 2018,8, 374 9 of 19 are, however, similar to those (up to 1100 ng g −1 ) found in four peat cores from Belgium, dating to AD 1930–1980, which were interpreted to be associated with coal burning and smelter emissions [ 50 ]. Pre-Industrial Hg concentrations in the previously mentioned records are quite heterogeneous (such as 40–50 ng g −1 in the peatlands studied in the Czech Republic [ 45 ] or 111 ± 64 ng g −1 in the records from Northeast China [ 48 ]), but are in the range of the ARO records. These comparisons suggest that, under natural conditions, as expected in Easter Island at ~20 and ~4.7 ka cal BP, large amounts of Hg can accumulate over relatively low background values. 4.2. Environmental Conditions in Rano Aroi and Related Proxies In previous studies on Rano Aroi cores, a combination of proxies was used to improve the knowledge on tropical and subtropical peat dynamics and the climatic history of Easter Island. The peatland demonstrated itself to be a good archive for reconstructing the environmental history of the area, which was mainly driven by climate [ 31 , 32 ], although without ruling out the effect of human activity in the last few millennia [51]. The δ13 C variations indicated two main climatic periods [ 31 , 32 ]. From ~70 to ~55 ka BP, high δ13 C values (mean − 15 ‰ , C 4 -type plant remains) suggest the predominance of drier-colder climatic conditions, while from ~47 to ~8.5 ka cal BP low δ13 C values ( − 26 ‰ , C 3 -type plant remains) indicate a general dominance of wetter climate conditions (Figure 4). A gradual transition between both climates took place between ~55 ka BP and ~47 ka cal BP, and this is reflected by progressively lighter values of δ13 C (Figure 4). ARO 08 02 does not show this transition (Figure 5) because it only covers the last ~38 ka cal BP, but the δ13 C values shifted to lower values in agreement with the ARO 06 01 tendency. The long-term δ13 C trend was interrupted by negative excursions, interpreted as short wetter periods. The same general pattern of δ13 C data is followed by PC1, the first component of a Principal Component Analysis performed on the inorganic fraction of Rano Aroi [ 32 ]. PC1 is characterized by large positive loadings (>0.7) of typically lithogenic elements and some metals. This component was identified as a proxy of the long-term fluxes of inorganic particulate material to the peatland. Its similarity with the δ13 C background trend suggests higher erosion during drier periods (~70 to ~55 ka BP)—probably due to lower vegetation cover—and an increasing mineral input during the short rainfall events previously mentioned (Figure 4). Another process, identified by the second component (PC2), is the occurrence of enhanced precipitation events related to the strong runoff and delivery of large amounts of terrigenous particles (probably rich in organic matter) from the soils of the catchment to the mire. Matching with these events, the C/N ratio and total nitrogen (TN) values show abrupt decreases and increases, respectively. Higher TN values and lower C/N ratio were attributed to a higher contribution of lacustrine algal material, in contrast to high C/N values that indicate higher proportions of terrestrial or aquatic plant (versus algae) organic matter [32]. 4.3. Processes Controlling Hg Content 4.3.1. Climate Temperature Temperature has a significant effect on Hg oxidation. Atmospheric oxidation is currently thought to occur through a two-step oxidation mechanism, favored at cold temperatures, and based on an initial recombination of Hg(0) and Br followed by the addition of a second radical (e.g., Br, I, Cl, BrO, ClO, OH, NO 2 or HO 2 ) [ 4 , 52 ] in competition with the thermal dissociation of the HgBr intermediate [ 53 ]. Low temperatures stabilize the intermediate HgBr species, allowing the second oxidation step that yields divalent Hg to occur. As a result, the oxidation, and associated surface deposition, of Hg is accelerated during cold periods. 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