The Carboniferous carbon isotope record from sedimentary organic matter: can we disentangle the carbon cycle?

Abstract:

A comprehensive analysis of the δ¹³C composition of sedimentary organic matter from Euramerican Carboniferous successions indicates there are significant shifts in δ¹³C through this key time interval. Our studies have revealed that, at an individual location, the source and delivery mechanism of the sediment contribute to the type of organic matter preserved and, in turn this influences the measured δ¹³C values from bulk sedimentary organic matter of organic matter. In general, where marine-derived organic matter is dominant in these Carboniferous successions then δ¹³C values are characteristically lower compared to the higher values encountered where terrestrial plant-derived material is most abundant. The implication of these observations is that an apparent carbon isotope excursion identified from the bulk organic matter may reflect a change in transport processes, or depositional environment, rather than a perturbation in the global carbon cycle. In our most recent studies, however, we compare δ¹³C values from specific wood fragments and bulk sedimentary organic matter from non-marine, marine basinal, and marine shelfal successions from the earliest Mississippian through to the early Pennsylvanian. These data indicate that early Mississippian δ¹³C of organic matter is far less negative (around –22%₀) than material of Late Mississippian age (around –26%₀), however by the early Pennsylvanian, δ¹³C values return to –22%₀. There are some δ¹³C data from brachiopod carbonate from this time interval and similar shifts are indicated. Our data are beginning to address whether we can identify a primary carbon cycle signal from the Carboniferous record using δ¹³C from a range of sedimentary environments. If we can, there are still questions around what the record is telling us about the global carbon cycle during a period when plant groups, including lycopods and seed ferns, rapidly diversified.