Abstract:
Covariance of delta (super 13) C values of inorganic and organic fractions of pelagic carbonate sediments has been used to interpret rates of organic carbon production, burial and decomposition through time. This relationship is relatively consistent in deep sea sediments, which permits estimates of organic carbon production and preservation. However, the majority of pelagic sediments older than 200 myr have since been subducted, therefore carbonate sediments deposited in epeiric seas and platforms are often substituted in studies of the Paleozoic carbon cycle. There are several well known pitfalls in using such sediments including diagenesis, semi-isolation of the depositional environment, and input of varying types of sediments with differing delta (super 13) C values. One method to assess that global changes in delta (super 13) C are accurately represented by inorganic delta (super 13) C is to examine variations in the delta (super 13) C of co-occurring organic material. If the delta (super 13) C (sub organic) covaries with delta (super 13) C (sub inorganic) it is argued that the signals must be related to the global delta (super 13) C values. This assumption was investigated by analyzing periplatform sediments drilled along a slope transect off the western margin of Great Bahama Bank. The delta (super 13) C (sub inorganic) was shown to correlate between all sites along the transect; however, these records were not related to the global carbon cycle. Instead, these variations relate to global sea-level changes which can initiate or terminate production of isotopically distinct sediment on the platform. At the sites studied, the correlation between delta (super 13) C (sub inorganic) and delta (super 13) C (sub organic) increased from the proximal site (1005, r (super 2) =0.1) to the more distal site (1006, r (super 2) =0.63). The absence of a covariance between delta (super 13) C (sub inorganic) and delta (super 13) C (sub organic) values at the proximal Site 1005 reflects the predominance of bank top sediment and organic material, which exhibit no correlation of delta (super 13) C (sub organic) and delta (super 13) C (sub inorganic) values. In contrast, distal Site 1006 is a two point mixing model consisting of platform sediments with enriched delta (super 13) C values, and relatively depleted ocean derived material. This results in a strong positive correlation between delta (super 13) C (sub inorganic) and delta (super 13) C (sub organic) records at Site 1006 that is unrelated to the global carbon cycle. Such data question the use of the delta (super 13) C (sub organic) values to support the global nature of the delta (super 13) C (sub inorganic) values.