Abstract:
In pelagic carbonate sediments, the degree to which the delta (super 13) C values of inorganic and organic fractions co-vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200 Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well-known pitfalls to using shallow marine carbonate materials, including diagenesis, semi-isolation of depositional environments and input of different types of sediments with varying inorganic delta (super 13) C (delta (super 13) C (sub inorganic) ) values, which can obscure any global signatures. One method used to assess whether global changes in delta (super 13) C are accurately represented by delta (super 13) C (sub inorganic) records is to examine variations in the delta (super 13) C of co-occurring organic material (delta (super 13) C (sub organic) ). If a delta (super 13) C (sub organic) record co-varies with a co-occurring delta (super 13) C (sub inorganic) record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150 m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The delta (super 13) C (sub organic) values measured in this study were compared to previously published delta (super 13) C (sub inorganic) records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between delta (super 13) C (sub inorganic) and delta (super 13) C (sub organic) increased from the proximal location (Site 1005, r (super 2) =0.1), to the distal site (Site 1006, r (super 2) =0.63). The importance of platform-derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co-variation between inorganic and organic delta (super 13) C records, which exhibit no correlation on the platform itself. In contrast, the co-variance in delta (super 13) C values at the basinal location, Site 1006, is explained by a two-point mixing model, which demonstrates the importance of both pelagic and platform-derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic delta (super 13) C values; this results in a correlation between delta (super 13) C (sub inorganic) and delta (super 13) C (sub organic) records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using delta (super 13) C (sub organic) values to support the ability of delta (super 13) C (sub inorganic) values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk delta (super 13) C signal. Abstract Copyright (2011), International Association of Sedimentologists.