Abstract: Pore waters extracted from lake sediments provide valuable information about environmental conditions, nutrient flow, climate history, and biogeochemical processes occurring in the sediment, as well as the overlying water column. While the chemistry of marine sediments has been studied extensively, there have been decidedly fewer studies examining early diagenesis of lake sediments, despite the important role it plays in ecosystem health and the significant impact it can have on global methane emissions. Pore water samples have been collected for Lake Wononscopomuc (Connecticut) and Lough Carra (Ireland), and profiles have been determined for pH, alkalinity, and dissolved inorganic carbon (DIC), as well as the concentration of various dissolved species (Ca+2, sulfate, methane, etc). In addition, profiles of stable carbon isotope (13C/12C) ratios have been generated for DIC and methane. This summer, our work centered on developing explanatory models to understand the geochemical processes that produce observed DIC concentration profiles. We used the software PROFILE, developed for Berg et. al (1998), to model distinct zones of DIC and Ca+2 production and consumption. The program uses Fickโs First Law to calculate diffusive flux and produces a very close fit with a generally small number of zones. Using our measured ๐ฟ13C-DIC values, we calculated 13C concentrations at measured depths and ran that data through PROFILE, which generated production rates and concentrations for 13C which then could be compared to our ๐ฟ13C data. For some cores, only one zone of production of isotopically enriched DIC was required to produce a close fit to our measured values. However, it is possible that there multiple sources of DIC occurring at different depths and with different ๐ฟ13C values that produce a similar fit. In most cores measured, ๐ฟ13C-DIC increases with depth to levels above what would be expected from decomposition of bulk organic matter. In Lough Carra, dissolution of calcium carbonate (CaCO3), which produces DIC with higher 13C levels, may account for some of this isotopic enrichment, as porewaters are consistently supersaturated with respect to CaCO3, but even there, the ๐ฟ13C-DIC values are too high to be explained by carbonate dissolution alone. As such, some other factor is likely at play, such as methanogenesis, which produces isotopically depleted CH4 and isotopically enriched CO2, a DIC species. Further study is required to determine the exact causes of the observed isotope profile.
Hawthorne_RIS_Poster_Final_tk.pptx-Skye-Hawthorne-taggedLive Poster Session: Zoom Link
Thursday, July 30th 1:15-2:30pm EDT