VARIABLE LIQUID DECARBONATION METHODS FOR Δ13CORG ANALYSES IN SEDIMENTARY SECTIONS AND ON MODERN SOILS AND RIVER SEDIMENTS

Excursions in the isotope composition of organic carbon in sedimentary rocks (δ¹³C_org) are recorded throughout Earth’s history. These excursions help to correlate between sedimentary sections and provide information about past changes in the global carbon cycle. Accurately reconstructing these changes requires robust measurement of δ¹³C_org. An important step in sample preparation for organic carbon isotope analyses is decarbonation, or removal of inorganic carbon from the rest of the sample. Complete removal of carbonate is critical to δ¹³C_org analyses because carbonate phases have a heavier isotopic signature, leading to a more positive δ¹³C_org value if unintentionally left in a sample. Incomplete decarbonation due to variabilities in laboratory methods can thus lead to differing interpretations regarding the carbon cycle in deep time.

Carbonate is typically removed by treating samples with acid, but questions remain about this method. Since different carbonate phases (e.g., calcite, dolomite, siderite) may be more or less resistant to removal via acid soaking, we examined the effect of variable time, temperature, and grain size on the decarbonation process, focusing on repeatability of isotopic and % organic carbon (%TOC) values produced by the different procedures. Specifically, we tested variable liquid decarbonation temperatures (20 and 70ºC), lengths of time (10, 60, 120, 240, and 480 minutes), and sample grain sizes (motorized disk crusher vs. handheld mortar and pestle) for several geological samples. These samples include various rocks (some of which contain dolomite), soils, and sediments. Preliminary results suggest insignificant effects on measured δ¹³C from altering liquid decarbonation time and temperature, although we see some variability in %TOC values with increasing decarbonation temperature and time, especially for soil samples. We will report these analyses and discuss possible solubilization of organic carbon in soil and sediment samples, and well as grain size effects in decarbonation of rock samples.