Carbon isotope records from periplatform sediments: What do they tell us?
In pelagic carbonate sediments, the degree to which the δ13C values of inorganic and
organic fractions covary is used to determine how accurately the 13C values record changes
in the global carbon cycle. Variations in the difference between the organic and inorganic
13C values are thought to reflect changes in the pCO2 of the atmosphere. However, since the
majority of pelagic sediments older than 200 Ma have been subducted, carbonate sediments
deposited in epeiric seas and platforms are often substituted for pelagic materials. There are
many pitfalls in using shallow marine carbonate materials for the reconstruction of global
13C records including diagenesis, semi-isolation of depositional environments, and input of
different types of sediments with varying inorganic 13C (δ13Cinorganic) values. These pitfalls
were investigated by analyzing the 13C value of organic carbon (13Corganic) preserved in the
upper 150m of periplatform sediments recovered from the western margin of Great Bahama
Bank. The 13Corganic values measured in this study were compared to published 13C inorganic
records that used identical samples, allowing an evaluation of the correlation between the
records through time.
The results of this study showed that the correlation between 13Cinorganic and 13Corganic
increased from an insignificant value at the sites proximal to the platform (Site 1005, r2=0.09,
Site 1003, r2= 0.04), to significant correlations at the more distal sites (Site 1007, r2= 0.41 and
Site 1006, r2=0.63). This pattern reflects the major contributions of carbon to the sediments.
At Site 1005 the bulk sediment is dominated by platform derived material, and this study
shows that there is no correlation between 13Cinorganic and 13Corganic values in the bank top
sediments. In contrast, Site 1006 receives variable mixtures of platform and open ocean
material through time, which have distinct 13C values. These strong correlations can
therefore be explained by a two-point mixing model between a pelagic source and a platform
source. Thus, variations in the distal records are controlled by sea level fluctuations that
control platform top contributions to the periplatform environment and not by changes in
global carbon cycling. These results question the assumption that a positive correlation
between 13Cinorganic and 13Corganic records confirms the global nature of the 13Cinorganic
values, especially when the carbonates are recovered from environments where multiple
sources of carbonate and organic carbon contribute to the 13C signals. This finding has
important implications for reconstructions of ancient global carbon cycling in periplatform
settings, epeiric seas, and carbonate ramp environments.