THE CARBON ISOTOPE RECORD AND EARLY DIAGENESIS OF MARL LAKE SEDIMENTS, LOUGH CARRA, IRELAND

Carbonate and organic carbon isotope measurements of lacustrine sediments yield excellent paleoenvironmental records provided that the processes influencing sediment d¹³C values are well understood. While lake sedimentary carbon isotope records are commonly interpreted in terms of allochthonous inputs and water column processes, relatively few studies have examined the effects of early diagenetic processes such as methanogenesis or carbonate mineral recrystallization on sedimentary d¹³C values. Here, we present lake water, pore water, and sediment chemistry from Lough Carra, a shallow marl lake in western Ireland to 1) evaluate the effect of early diagenesis on lacustrine sedimentary d¹³C records and 2) document the paleoenvironmental history of Lough Carra. In July 2012, four ~1-meter cores and one 8-meter core were collected from Lough Carra. Overlying lake water and pore water chemistries (pH, T, Ca²⁺, DIC, d¹³C_DIC, CH₄, d¹³C_CH4) indicate rapid groundwater or spring inputs to various basins within the lake. In pore waters significant benthic methane production creates supersaturation with respect to calcite and high d¹³C_DIC values, up to +10‰. In some short cores, age constraints (²¹⁰Pb, ¹³⁷Cs, Hg) and sediment chemistries show a decrease in d¹³C_OrgC and d¹³C_CaCO3 and an increase in D¹³C_CaCO3-OrgC of several per mil since approximately the late 1800s. The 8-meter core contains laminated marl sediments from ~1.5 – 8 m depth and the d¹³C data is divided into two basic zones. From 8 to 2.2 m sediment depth, d¹³C_OrgC and d¹³C_CaCO3 values slightly increase while D¹³C_CaCO3-OrgC is largely invariant. From 2.2 to 0 m depth, d¹³C_OrgC and d¹³C_CaCO3 values decrease rapidly and D¹³C_CaCO3-OrgC is variable. Pore water d¹³C_DIC mass balance models constrain possible DIC carbon sources and the potential shift of bulk carbonate d¹³C due to precipitation of high d¹³C calcite after deposition. Along with diagenetic effects, the Lough Carra carbon isotope records reflect land-use changes, paleoproductivity, carbon inputs, and the Suess Effect. This study demonstrates that lacustrine sedimentary d¹³C interpretations should consider diagenetic effects and be cognizant of potential significant spatial variability within a relatively small lake basin.