Arsenic Mobility in Lake Sediments during Late-Holocene Climate Warming: Implications for Geochemical Baselines and Contaminant Stability in a Changing Northern ClimateTuesday, November 20, 2018 - 09:20 to 09:39 Theatre 2
Climate change is influencing the biogeochemistry of high northern latitude lake systems. These changes may have implications for the mobility of naturally occurring metal(loid)s and anthropogenic contaminants. Arsenic (As) mobility is highly susceptible to limnologic changes, seasonal fluctuations, and increases in organic matter due to its redox behaviour. However, the influence of regional climate change on long-term As stability in lake environments is poorly understood. Lake sediments provide archives of past climate changes and are widely used to reconstruct biogeochemical responses to regional climate trends. Through the examination of changes in sediment and porewater As speciation during mid- to late-Holocene (5,000 yr cal BP to present) regional warming cycles, this study provides insight as to how climate warming affects sedimentary As concentrations. This information is important for interpretation of metal(loid) concentrations in modern-day sediments, especially in areas disturbed by mining activities. Arsenic geochemistry is combined with multivariate analysis of paleoclimate proxies (particle size, organic matter type and quantity) and radiometric dating (14C and 210Pb) to determine the influence of climate warming on the distribution of As in lake sediment. Sediment cores were collected from four lakes in the Courageous Lake Greenstone Belt, central NT, in proximity to the former Tundra gold mine. Paleoclimate reconstructions of two of these lakes document increases in As concentrations in both sediments and porewater coincident with known periods of regional climate warming. During the Medieval Warm Period (ca. 5,000 to ca. 4,000 yrs BP; 110 yr/cm) sediment As concentrations are significantly (rs = 0.75, p < 0.05, n = 15) related to the labile organic matter fraction (S2 as determined by Rock Eval® pyrolysis); conversely, a significant (p < 0.05) but negative (rs = - 0.8) relationship between sedimentary As concentrations and labile organic matter is evident over the shorter duration warming event centred at ca. 2,040 yrs BP (47 yrs/cm). These data suggest that changes in primary productivity may affect the remobilization and sequestration of As in lake sediment; however the effects on As mobility vary between lakes. The prevalence of both arsenopyrite and secondary, authigenic As-bearing pyrite (as determined by SEM, EMPA and bulk XANES) during these regional climate warming intervals provides evidence to support the interpretation that active remobilization of geogenic As occurred in lake sediments. However, in mining-impacted sediment, the relationship between labile organic matter and sedimentary As is not consistent in the four lakes studied. This study demonstrates that past regional warming cycles have played a role in influencing As mobility in sub-Arctic mineralized regions and thus provide a useful tool to better predict future geochemical change.