Investigating the causes of organic and inorganic pollution in high Andean lakes (Junín region, Peru) through nitrogen isotope and metals analysis of sediment cores

Analyzing lake sediment cores provides an opportunity to observe changing characteristics of sediment deposition over time. Interpreting these changes allows us to extrapolate variations of climate and pollution over the history of the lake. In this study we analyzed sediments for d15N values and metal concentrations to determine a timeline of organic and inorganic pollution in five remote high Andean lakes in Peru (provinces of Junín and Pasco). We have also developed an age model for the five lakes using correlations of well-dated 210Pb concentrations from a proximal lake in the region. Anthropogenic emissions of nitrogen through the burning of fossil fuels, the Haber-Bosch process, and other agricultural techniques can have adverse effects on ecosystems. Although adverse effects of N additions are often focused in areas of high agricultural productivity due to its use in fertilizers, recent studies suggest that atmospheric deposition of N from the burning of fossil fuels occurs in remote areas far from agricultural activity. We have analyzed N isotopic signatures in surface cores collected at the lakes in order to determine the origin of the nitrogen. Due to the fact that there is little agriculture in the region, our preliminary hypothesis is that excess N is present as a result of either atmospheric deposition or animal waste from grazers. Based on previous work using N isotopes, we expect that atmospheric deposition will yield decreasing δ15N values through the 19th and 20th century intervals in the cores, while inputs of animal waste would yield higher δ15N values. Our analyses have yielded varying results amongst the five lakes. δ15N values of three lakes, Alcacocha, Punrun, and Bosque de Piedras, decrease with time, by 0.7‰, 1.82‰, and 3.28‰ respectively. We attribute these trends to the atmospheric deposition of NOx. Paucartamba exhibited an initial decrease by 2.33‰, with a subsequent increase by 2.19‰ starting at 10 cm in the core. We attribute the initial decrease to atmospheric deposition of nitrogen and the following increase to an input of animal waste. The final lake, Chacacancha, displayed a trend of consistently increasing δ15N values over time, increasing by 1.32‰. Chacacancha has the highest δ15N values, with a range of 5-9‰, whereas the next highest value among the lakes is less than 4‰. We do not have a great explanation for the elevated values in Chacacancha. Metal concentrations provide further insight into the levels of pollution in these lakes. Building on previous work in the region, it is clear that these lakes have been polluted through eolian transport of metals from incorrect disposal of mining waste in the region. Our results consistently show that the lakes down wind of the Cerro de Pasco mine have elevated heavy metal concentrations. Based on our age model the concentrations peaked in the early 1800s and have declined since then. Decreasing concentrations could be a result of better mining practices or from a dilution by sediments as the lakes have become more productive over time. Overall, analyzing N isotopes and metal concentrations within sediment cores provides an idea as to how anthropogenic activity affects the trophic status and overall health of lakes