Characterization of organic matter in the Yegua Formation, East-Central Texas: Relation to microbial processes and depositional environment

Volatile organic acids (VOA) and lipids were analyzed in sediments, and VOAs and neutral sugars were analyzed in groundwaters to characterize sedimentary and dissolved organic carbon (DOC).; Geochemical and microbiological evidence indicate viable microorganisms which produce or consume VOAs. Acetic and propionic acids in mudstones range from 200–1270 and 20–38 nmol.gdw−1 . In contrast, acetic and propionic acids in sands are 50–200 and <20 nmol.gdw−1. VOA analyses and laboratory incubations suggest production of VOAs in mudstones, and consumption of VOAs by sulfate reducing bacteria (SRB) in sands. Assuming that SRB completely respire the acetate transported into the aquifer (5.4 μmol.1 −1.m.yr−1), the CO2 production rate is 9.2 μmol.1−1.m.yr−1. This value is within the range for deep aquifers, but 103 to 105 times slower than near-surface microbial processes. This slow mineralization of organic matter (OM) and a loose commensalism between the microbial communities probably accounts for their long-term survival.; Sedimentological and geochemical evidence indicate depositional cycles represented as parasequences separated by exposure surfaces. Organic petrography and geochemistry indicate presence of terrestrial and marine OM in transgressive sediments. In contrast, regressive sediments contain only terrestrial OM. These differences relate to contrasting OM sources and depositional styles on the shelf. The study suggests that organic geochemistry data combined with sedimentology provide greater resolution in distinguishing stratigraphic boundaries.; High, intermediate, and low molecular weight (HMW, IMW, and LMW) DOC, VOAs, and neutral sugars were analyzed. DOC ranges from 1–6 mg.l−1, with IMW and HMW DOC as major fractions. VOAs comprise 3–20% and neutral sugars 0.8–6.7% of DOC. Acetic acid ranges from 1.0–4.1 μmol.l−1 in shallow versus 0.37–0.52 μmol.l−1 in deeper groundwaters. Neutral sugars range in concentration from 0.6 to 2.7 μmol.l−1. Neutral sugars in shallow groundwaters are more diagenetically altered as indicated by high mole% fucose+rhamnose and low neutral sugar yield. Bulk δ13C of DOC and HMW DOC ranges from −24 to −32‰, and LMW DOC ranges from −32 to −34‰ In contrast, soil-zone OM δ 13C is higher (−16 to −19‰). Moreover, the carbon isotopes imply a predominantly C₃ OM source for DOC.