THE RELATIONSHIP BETWEEN THE δ13C OF INORGANIC AND ORGANIC COMPONENTS IN NEOGENE PERIPLATFORM SEDIMENTS: IMPLICATIONS FOR THE PROTEROZOIC

Covariance of δ¹³C 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 δ¹³C values. One method to assess that global changes in δ¹³C are accurately represented by inorganic δ¹³C is to examine variations in the δ¹³C of co-occurring organic material. If the δ¹³C_organic covaries with δ¹³C_inorganic it is argued that the signals must be related to the global δ¹³C values. This assumption was investigated by analyzing periplatform sediments drilled along a slope transect off the western margin of Great Bahama Bank. The δ¹³C_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 δ¹³C_inorganic and δ¹³C_organic increased from the proximal site (1005, r²=0.1) to the more distal site (1006, r²=0.63). The absence of a covariance between δ¹³C_inorganic and δ¹³C_organic values at the proximal Site 1005 reflects the predominance of bank top sediment and organic material, which exhibit no correlation of δ¹³C_organic and δ¹³C_inorganic values. In contrast, distal Site 1006 is a two point mixing model consisting of platform sediments with enriched δ¹³C values, and relatively depleted ocean derived material. This results in a strong positive correlation between δ¹³C_inorganic and δ¹³C_organic records at Site 1006 that is unrelated to the global carbon cycle. Such data question the use of the δ¹³C_organic values to support the global nature of the δ¹³C_inorganic values.