Global synchronous changes in the carbon isotopic composition of carbonate sediments unrelated to changes in the global carbon cycle

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Abstract
The carbon isotopic (δ 13 C) composition of bulk carbonate sediments deposited off the margins of four carbonate platforms/ramp systems (Bahamas, Maldives, Queensland Plateau, and Great Australian Bight) show synchronous changes over the past 0 to 10 million years. However, these variations are different from the established global pattern in the δ 13 C measured in the open oceans over the same time period. For example, from 10 Ma to the present, the δ 13 C of open oceanic carbonate has decreased, whereas platform margin sediments analyzed here show an increase. It is suggested that the δ 13 C patterns in the marginal platform deposits are produced through admixing of aragonite-rich sediments, which have relatively positive δ 13 C values, with pelagic materials, which have lower δ 13 C values. As the more isotopically positive shallow-water carbonate sediments are only produced when the platforms are flooded, there is a connection between changes in global sea level and the δ 13 C of sediments in marginal settings. These data indicate that globally synchronous changes in δ 13 C can take place that are completely unrelated to variations in the global carbon cycle. Fluctuations in the δ 13 C of carbonate sediments measured during previous geological periods may also be subject to similar processes, and global synchroniety of δ 13 C can no longer necessarily be considered an indicator that such changes are related to, or caused by, variations in the burial of organic carbon. Inferences regarding the interpretation of changes in the cycling of organic carbon derived from δ 13 C records should be reconsidered in light of the findings presented here.