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

Abstract

The carbon isotopic (␦ 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 ␦ 13 C measured in the open oceans over the same time period. For example, from 10 Ma to the present, the ␦ 13 C of open oceanic carbonate has decreased, whereas platform margin sediments analyzed here show an increase. It is suggested that the ␦ 13 C patterns in the marginal platform deposits are produced through admixing of aragonite-rich sediments, which have relatively positive ␦ 13 C values, with pelagic materials, which have lower ␦ 13 C values. As the more isotopically positive shallowwater carbonate sediments are only produced when the platforms are flooded, there is a connection between changes in global sea level and the ␦ 13 C of sediments in marginal settings. These data indicate that globally synchronous changes in ␦ 13 C can take place that are completely unrelated to variations in the global carbon cycle. Fluctuations in the ␦ 13 C of carbonate sediments measured during previous geological periods may also be subject to similar processes, and global synchroniety of ␦ 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 ␦ 13 C records should be reconsidered in light of the findings presented here.isotope ͉ organics ͉ shallow-water platform V ariations in the stable carbon isotopic composition (␦ 13 C) of calcium carbonate sediments throughout geological time have been an invaluable aid in unraveling changes in the global carbon cycle (1, 2). In addition, such variations in ␦ 13 C have provided information on the transfer of carbon between reservoirs during major geological events such as bolide impacts or catastrophic climate changes (3, 4). Generally speaking, increases in the ␦ 13 C of carbonates are considered indicative of increased burial and preservation of isotopically negative organic carbon, whereas decreases in the ␦ 13 C suggest transfer of carbon from the organic into the inorganic reservoir (5-7). Hence, variation in the ␦ 13 C of carbonate material should be a proxy for the amount of buried organic carbon. For example, the ␦ 13 C of the oceanic record has become increasingly more negative over the past 50 million years (Myr) (8, 9), suggesting that as much as 10 20 g of organic carbon have been transferred from the organic to the inorganic carbon reservoir (6).The most reliable archives of changes in the ␦ 13 C of the oceans over the past 100 Myr to 200 Myr are contained in the skeletons of planktic and benthic foraminifera (9, 10). Not only are these organisms relatively diagenetically stable, but the ␦ 13 C values can be corrected for any species-spe...

Circum-Tethyan carbonate platform evolution during the Palaeogene: the Prebetic platform as a test for climatically controlled facies shifts

“…A trend towards δ 13 C depleted ratios and negative maxima in δ 13 C correspond to phases of sediment erosion and reworking during sea level lowstands and regressions (Hilbrecht et al 1986;Voigt & Hilbrecht 1997). Swart (2008) demonstrated a strong relationship between Late Cenozoic sea level changes and the δ 13 C trends in carbonate platform systems. Aragoniteproducing organisms proliferate at carbonate platforms especially during transgressions and high sea levels.…”

“…Microfacies indicate the predominance of aragonitic coralline red algae and calcitic larger benthic foraminifera ( Figure 4). Following the approach of Swart (2008), aragonitic organisms, which flourish during times of relatively high sea level, favour the production of 13 C-enriched carbon isotopes. Global sea level and regional sequence stratigraphic reconstructions point to a flooding of the Prebetic platform prior to the Eocene-Oligocene boundary ( Figure 1; Geel 2000; Miller et al 2005).…”

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“…In addition, the age of the last platform sequence will provide the age of the platform drowning that potentially is linked to the onset of the current system. Site U1466 cores will also address another objective of Expedition 359: the periplatform sediments deposited at this site offer the opportunity to retrieve a complete record of δ 13 C though the lower and middle Miocene that, together with other Expedition 359 sites, will provide another data set of the carbon isotopic record that is needed to calibrate the periplatform platform margin record against the pelagic record (Swart, 2008).…”

Volume 359: Maldives Monsoon and Sea Level

“…In addition, the age of the last platform sequence will provide the timing of the platform drowning that potentially is linked to the onset of the current system. Site U1466 cores will also address another objective of Expedition 359: the periplatform sediments deposited at this site offer the opportunity to retrieve a complete record of δ 13 C though the lower and middle Miocene that, together with other Expedition 359 sites, will provide another data set of the carbon isotopic record that is needed to calibrate the periplatform platform margin record against the pelagic record (Swart, 2008).…”