Source and geochemistry of acidic waters in King George Island, Antarctic peninsula

Abstract

Acid rock drainage (ARD), a natural global phenomenon, occurs worldwide in various geochemically distinct settings such as high-alpine, periglacial environments in the Central Eastern Alps, in the foreland of glaciers in Peru, the Rocky Mountains and in the shoreline of King George Island (KGI) in Maritime, Antarctica. In general, when sulfide minerals, in particular pyrite, are exposed to gaseous or aqueous O2, or any oxidizing agents (e.g. Fe(III)aq,) a complex set of chemical weathering reactions occurs, producing Fe and sulfate-rich acidic effluent. If the buffering capacity of the bedrock is insufficient to neutralize generated acidity over an extended time, ARD develops, even in extreme Antarctic conditions. The resulting sulfuric acid, a strong weathering agent, enhances the weathering of rocks that interact with and cause the liberation of potentially key elements such as nutrients (e.g., Fe) as well as environmentally toxic elements (e.g. Pb, Zn, Cu, and Cd) into the environment. Biological reactions can significantly contribute to ARD generation. In general, Fe and S-oxidizing bacteria, A. ferroxidans, is involved in the formation of ARD, acting as a catalytic. In the Antarctic continent, generation of ARD is limited due to the presence of ice-cover areas. The existence of the extensive sulfide mineralization on the Barton Peninsula, KGI, together with the increasing global warming strongly suggests that ARD occurrences may even become more frequent in the region. To date, no systematic studies have comprehensively examined the sources, geochemical characteristics, and potential environmental impacts of ARD formation on the Barton Peninsula, KGI, in Maritime Antarctica. With increased global warming, it is even more critical and necessary to elucidate the rock weathering reactions coupled with sulfide oxidation and their contribution to ARD formation on the peninsula. This information is vital for assessing the future hazards posed by naturally occurring ARD on surface water quality, not only on the Peninsula but also in similar settings worldwide. This study aims explaining sources, formation mechanism, and geochemical characteristics of acid rock drainage on the Barton Peninsula, King George Island, by adapting field and experimental data including isotopes, water and sediment geochemistry. The study area, the Barton Peninsula, is located in King George Island, maritime, Antarctica, is largely covered by basalt to basaltic andesite. A granodiorite stock is located in the central northern part of the peninsula and diorite mostra is observed in the northern part of the peninsula. The lowermost stratigraphic unit, the Sejong Formation, is largely composed of volcanicclastics and observed along almost the entire southern coastline. The field excursion and sampling were carried out at the end of the austral summer of 2019. Rock (n=25), sediment (n=30) and water (n=29) samples were collected and kept at ITU Geomicrobiology and Biogeochemistry Laboratory until further analysis. In-situ pH values range between 3.6 and 6.7 while the average EC value is ca. 112.2 μS/cm. Geochemical analysis of water samples revealed that cations and anions of water samples follow the decreasing order of Mg+2> Na+2> Ca+2> K+ and Cl- >SO4-2>HCO3-, respectively. According to Piper diagram, the waters are classified as calcium and sodium chlorite rich and mixed type waters. A number of different indexes is used to evaluate the degree of alteration of the sediments obtained from acidic and non-acidic waters. The calculated chemical index of alteration (CIA) values are further discussed in the accompanying ternary diagram, A-CN-K, which allows for the graphical interpretation of the proportional chemical changes. Most igneous rocks in different compositions will plot between a CIA value of 35 and 50, with mafic rocks placing the lower values. The chemical index of alteration (CIA) values calculated for each sediment sample range from 37.6 to 74 % with the highest values associated with acidic waters. For example, the sediments from the acidic lake L7 have the highest CIA values. Moreover, these samples are plotted on the moderate to high degree of weathering trend on the A-CN-K diagram along with AI2O3 enrichment. In addition to the CIA values, the index of compositional variability (ICV) was calculated. Since sediments with a high percentage of non-clay silicate minerals, or unaltered minerals, will have ICV values greater than one. In contrast, sediment samples with mostly clay minerals under intense weathering conditions should have ICV values lower than one. Accordingly, ICV values of the sediments are between 1.05 and 2.88 with the lowest values belong to those from the acidic waters. The ICV values of the sediments obtained from the acidic lakes (L7 and L2) range from 1 to 1.2, and 0.96 to 1.13, respectively. The rest of the sediments ranges from 1.25 to 3.55. These calculated indexes suggest that non-acidic water associated sediments are largely originated from the physical weathering of the bedrocks while acidic water associated sediments result in both the chemical and physical weathering processes A. ferrooxidans bacteria were adapted to 10°C in the laboratory conditions. After the optimum growth conditions of the bacteria were provided, biotic and abiotic experimental sets were established with pyrite mineral in waters with 3 different oxygen isotope values (δ18O; -10.66‰ (Van), -7.21‰ (UP), and 8.5 ‰ (ISO)) with an initial pH=2. At the end of the biotic and abiotic pyrite experiments, solution chemistry showed significant differences. The concentrations of Fe (II)aq, Fe(tot), and SO4(aq) are at least 20-fold higher in the biotic experiments. In addition to the pyrite experiments, rock experiments (basaltic andesite) obtained from the Barton Peninsula (KSJ-20) were set up under the exact similar conditions to pyrite. At the end of the experiments, in general, cation compositions in the biotic experiments showed higher values. In particular, concentrations of Na, Mg, K and Ca are notably higher in the biotic experiments. These labile elements are readily leached from the rocks due to the biotic activity, producing acidity via enhanced pyrite oxidation. Therefore, the quantity of elements (SO4, Fe+2, Fe(tot)) released to the environment was higher in the biotic experimental set. Accordingly, with increasing rock dissolution pH of the biotic experiments raised compared to the abiotic ones. Leaching experiments conducted with pyrite and rock showed that dissolution rate is influenced by the presence of bacteria. The concentration of elements such as Al, K, Mg, and Ca which were released into the environment from the rock is slightly higher in the biotic ones compared to the abiotic ones. Pyrite oxidation is significantly higher in the presence of bacteria, releasing high sulfate, Fe and acidity. These experimental results suggest that dissolution of the bedrocks with pyrite contents at the Barton Peninsula is influenced by A. ferrooxidans. The role of the bacteria is to oxidize pyrite and produce acidity which in turn react with the bedrocks, releasing cations and metals into the environment. The high CIA and low ICV values of the sediments from the acidic waters support this mechanim. Therefore, the low ICV and high CIA values around lakes L2 and L7 in the north of the island with low pH may be attributed to the presence of biological activity. High concentration of metals such as Pb, Zn, Cu and As in the acidic waters compared to those of the non-acidic waters, further suggest ARD enhanced metal release into the environment. The δ18OSO4 and δ34SSO4 values of the field and rock experiments are consistent with the pyrite oxidation experiments. The δ18OSO4 and δ34SSO4 values of dissolved sulfate collected from the acidic waters on the Barton Peninsula closely plotted on the experimental lines of pyrite oxidation, suggesting that pyrite is the source of ARD at the peninsula and predominantly oxidized under biotic and oxic conditions. The δ34SSO4 values obtained from the pyrite experiments closely reflect the δ34S value of pyrite and therefore, the δ18OSO4 and δ34SSO4 values can be used to elucidate biotic and abiotic pyrite oxidation as well as source of sulfate under a glaciated environment.