The stable carbon isotopic composition of organic material in platform derived sediments: Implications for reconstructing the global carbon cycle

Amanda M. Oehlert, Kathryn A. Lamb-Wozniak, Quinn B. Devlin, Greta J. Mackenzie, John J.G. Reijmer, Peter K. Swart

Research output: Contribution to journalArticle

43 Scopus citations

Abstract

In pelagic carbonate sediments, the degree to which the δ 13C values of inorganic and organic fractions co-vary has been used to interpret rates of production, burial and decomposition of organic carbon. This relationship is relatively consistent through time, permitting estimates of organic carbon production and preservation. However, as the majority of pelagic sediments older than 200Myr have been subducted, carbonate sediments deposited in epeiric seas and platforms are often substituted for pelagic carbonates in analyses of ancient global carbon cycling. There are well-known pitfalls to using shallow marine carbonate materials, including diagenesis, semi-isolation of depositional environments and input of different types of sediments with varying inorganic δ 13C (δ 13C inorganic) values, which can obscure any global signatures. One method used to assess whether global changes in δ 13C are accurately represented by δ 13C inorganic records is to examine variations in the δ 13C of co-occurring organic material (δ 13C organic). If a δ 13C organic record co-varies with a co-occurring δ 13C inorganic record, it is argued that the signals must be related to variations in the global carbon cycle. This assumption has been investigated by analysing the isotopic composition of the organic carbon preserved in the uppermost 150m of periplatform sediments recovered during ODP Leg 166 from the western margin of Great Bahama Bank. The δ 13C organic values measured in this study were compared to previously published δ 13C inorganic records measured on identical samples, thus allowing a study of the correlation between the two records through time. These analyses showed that the correlation coefficient between δ 13C inorganic and δ 13C organic increased from the proximal location (Site 1005, r 2=0·1), to the distal site (Site 1006, r 2=0·63). The importance of platform-derived carbonate and organic material at the proximal location, Site 1005, is reflected in the absence of a co-variation between inorganic and organic δ 13C records, which exhibit no correlation on the platform itself. In contrast, the co-variance in δ 13C values at the basinal location, Site 1006, is explained by a two-point mixing model, which demonstrates the importance of both pelagic and platform-derived carbonate and organic carbon in generating the positive correlation between the organic and inorganic δ 13C values; this results in a correlation between δ 13C inorganic and δ 13C organic records at Site 1006 that is unrelated to global carbon cycling. Such data question the applicability of using δ 13C organic values to support the ability of δ 13C inorganic values to record global carbon cycling in carbonates recovered from environments where multiple sources of carbonate and organic carbon contribute to the bulk δ 13C signal.

Original languageEnglish (US)
Pages (from-to)319-335
Number of pages17
JournalSedimentology
Volume59
Issue number1
DOIs
StatePublished - Jan 1 2012

Keywords

  • Carbon cycle
  • Carbon isotopes
  • Carbonate platforms
  • Organic matter
  • PCO

ASJC Scopus subject areas

  • Geology
  • Stratigraphy

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