ORIGIN OF DEPLETED SIGNATURES IN CARBONATE-CEMENTED DEEP-WATER SANDSTONES (NEOPROTEROZOIC, BRITISH COLUMBIA): EVIDENCE FOR METHANOGENESIS?

The role of methane in the generation of depleted isotopic signatures of cap carbonates in Neoproterozoic post-glacial succession remains a hotly contested issue. We have investigated the isotopic composition of carbonate-cemented sandstones in the Windermere turbidite system, part of a deep-water setting, in order to test for the possible influence of methanogenesis as well as to evaluate the utility of their isotopic compositions for chemostratigraphic correlation.

The samples examined come from basin-floor sand-rich turbidites of the Upper Kaza Group and represent an interval of deposition that post-dates a glacial event (~608 Ma; Re-Os method).  The sandstones comprise thick-bedded turbidites, up to 1 m, that feature dune cross-bedded sandstones at the tops of graded Bouma Ta beds.  The cross-beds are up to 50 cm in thickness, composed of poorly-sorted coarse sandstone and commonly host brown-weathering calcite cement.  The presence of rip-up clasts of carbonate-cemented sandstone present in Bouma Ta beds attests to the early timing of these cements.  The d¹⁸O isotopic values of the calcites range between -18.74 ‰ and -16.82 ‰ and their depleted composition and homogeneity may be a result of re-equilibration due to low-grade metamorphism.  In contrast, the d¹³C-values for the cross-stratified sandstones are significantly more varied, ranging from -20.15 ‰ to -9.98 ‰.  The d¹³C-values also vary as a function of stratigraphic height. Given the unlikely possibility of meteoric diagenetic effects in these rocks, we suggest that the negative d¹³C-values may be explained by mixing at the sediment-seawater interface between Neoproterozoic seawater and porewaters rich in carbon generated by methanogenic bacteria.  The vertical variation in the isotopic composition of carbon in the cements holds promise for future chemostratigraphic applications. More importantly, these cements may provide evidence for a previously unrecognized flux of methanogenic carbon in Neoproterozoic seawater.