Abstract
The ca. 2.0 Ga Zaonega Formation in NW Russia preserves exceptionally 13C-depleted organic matter (δ13CVPDB as low as −40‰ to −45‰), which many have interpreted as reflecting global-scale processes. Here, we report the first compound-specific carbon and hydrogen isotope compositions of hydrocarbons and CO2 in gases trapped as fluid inclusions in pyrobitumen-rich rocks from a 500-m-long core through the formation. Our data indicate a thermogenic origin for C2-C4 gases and yield the first quantitative depth dependent CO2-CH4 apparent temperature gradient for this section, ranging from 72 °C to 350 °C. Our data show that hotter temperatures occur low in the core adjacent to a gabbro sill and decline systematically upward. They also support previous interpretations that thermogenic hydrocarbon seepage, generated during syndepositional emplacement of gabbro, fueled seafloor methanotrophy to form biomass with anomalously low δ13Corg values. Our data can be explained entirely by local basinal mechanisms, but we cannot rule out potential contributions by global carbon-cycle perturbations.