Using U–Pb dating of Cenozoic seep carbonates to assess periods of hydrocarbon seepage in sedimentary basins

Constraining the age of authigenic carbonate precipitation is crucial to track the timing of hydrocarbon migration in (paleo-) seep systems. The age of ancient seep carbonates is usually indirectly inferred using biostratigraphic proxies obtained from the surrounding sediments or from fossils preserved within the carbonates. In-situ carbonate U–Pb geochronology is an established method for tracing the timing of low temperature fluid flow processes. This method has the potential to provide important absolute ages for seepage events. Here, a combined approach using carbonate U–Pb geochronology and stable isotope analyses has been applied to seep carbonates from the Tertiary Piedmont Basin (NW Italy) to constrain the age of authigenic carbonate precipitation from different localities across the basin. The specimens record negative δ13C values (> −44.3 ‰ Vienna PeeDee Belemnite) indicating anaerobic oxidation of methane as the main source of inorganic carbon. ²⁰⁶Pb/²³⁸U lower intercept ages range from 16.6 ± 5.1 to 3.8 ± 1.0 Ma, marking two distinct periods of seepage activity. Significant age differences between hydrocarbon-derived cements from the seep carbonates of the Tertiary Piedmont Basin and their surrounding sediments (ca. 12 Ma) demonstrate that the commonly used biostratigraphic approach can lead to incorrect conclusions regarding the timing of hydrocarbon migration and consequent carbonate precipitation. Such age discrepancies have important consequences when studying past mass extinction events or hyperthermal periods potentially linked with gas hydrate destabilization.