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

The carbon-isotopic composition (delta(13)C) 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 delta(13)C can be correlated between sediments of equivalent ages and whether such changes can be related to global changes in the delta(13)C 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 kin 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 delta(13)C of the carbonates ranges from +5 parts per thousand in the younger sequences to +1 parts per thousand in the Early Miocene. In each of the sites, the delta(13)C is strongly positively correlated with the percentage of aragonite. As a consequence, the delta(13)C of sequences A through F is strongly correlated, reflecting the decreasing amount of aragonite with increasing depth. In the two platform sites, the delta(13)C 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 delta(13)C of the individual sequences can be correlated in most instances between the ODP holes, the changes are not related to global changes in the delta(13)C of the oceans which in contrast to the delta(13)C of the platform sediments become isotopically lower towards the present day. Instead variations in the delta(13)C appear to be related to varying mixtures of delta(13)C-rich banktop sediments and pelagic material. (c) 2005 Elsevier B.V. All rights reserved.