The nature of the δ13C of periplatform sediments: Implications for stratigraphy and the global carbon cycle

The carbon-isotopic composition (δ13C) of bulk carbonates, obtained from a transect of sites drilled through platform and periplatform sediments of Holocene to Early Miocene age, has been compared to ascertain whether changes in the δ13C can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the δ13C of the dissolved inorganic carbon in the oceans over this time period. Five of the sites were drilled during Leg 166 of the Ocean Drilling Project (1003-1007) in a transect ranging from five km to 25 km away from the platform margin and penetrating sediments of Holocene to Oligocene age that are contained in 17 depositional sequences (A-Q). Two shallow-water sites, Clino and Unda were situated on a extension of the same transect on Great Bahama Bank in a water depth of 10-15 m. With the exception of Unda and Clino, the δ13C of the carbonates ranges from +5‰ in the younger sequences to +1‰ in the Early Miocene. In each of the sites, the δ13C is strongly positively correlated with the percentage of aragonite. As a consequence, the δ13C of sequences A through F is strongly correlated, reflecting the decreasing amount of aragonite with increasing depth. In the two platform sites, the δ13C is significantly lower in the younger portions of the cores as a result of the influences of meteoric diagenesis during repeated exposure during the Pleistocene. Although the δ13C of the individual sequences can be correlated in most instances between the ODP holes, the changes are not related to global changes in the δ13C of the oceans which in contrast to the δ13C of the platform sediments become isotopically lower towards the present day. Instead variations in the δ13C appear to be related to varying mixtures of δ13C-rich banktop sediments and pelagic material.