BIOGEOCHEMISTRY AND SEDIMENT TRANSPORT THROUGH A TROPICAL ANDEAN PATERNOSTER LAKE SYSTEM

We sampled paternoster lakes and their connecting streams in the Queshque Valley of the Cordillera Blanca, Peru in June and July 2014. The objectives of this study are to understand modern biogeochemical and sedimentological dynamics though the lake system. These analyses will serve as a modern proxy calibration for limnologically-based paleoclimate reconstructions. Water samples were taken throughout the system and analyzed for temperature, pH, total alkalinity, total suspended material (TSM), dissolved inorganic carbon (DIC), dissolved oxygen (DO), pCO₂, specific conductivity, and turbidity. Sediment traps were placed in two lakes, and surface sediments were collected and measured for magnetic susceptibility (MS), total organic carbon (TOC), total inorganic carbon (TIC), and elemental composition. Preliminary data show a decrease in specific conductivity with increasing distance from the glacier (from 0.088 to 0.062 mS/cm), indicating a glacial source of dissolved material. TSM values, however, are highly variable (mean 1.102 ± 0.581 mg/L) with distance from the glacier, which suggests either an influx of organic material or a non-glacial inorganic sediment source. DO increases slightly with distance from the glacier, illustrating higher primary productivity possibly due to lower light attenuation. In one stream, samples were taken at ~6 hour intervals over a 24 hour period to decipher any possible trends in a diurnal cycle of glacial erosion and biological activity. A correlative sinusoidal trend of temperature and TSM was recorded, suggesting higher temperature caused increased glacial melting and subsequent erosion. An opposite trend was seen for specific conductivity, which illustrates the dilution of dissolved material with increased melting. Mean Upper Lake surface sediment grain size (19.54 μm) is significantly larger than that of Lower Lake (3.40 μm), suggesting a glacially-derived sediment source. However, there is no clear trend of grain size, biogenic silica, TOC, or TIC with distance from the main inflow of each lake, indicating that there are likely significant secondary inflows. These data show the importance of considering secondary, non-glacially-derived sediment inflows in paleoclimate reconstructions.