Sulfurized carbohydrates: an important sedimentary sink for organic carbon?

In contrast to the general belief that carbohydrate carbon (CCHO) is preferentially degraded and is not extensively preserved in the sedimentary record, it is shown here that CCHO forms a large fraction of the organic matter (OM) of the total organic carbon (TOC)-rich upper Jurassic Kimmeridge Clay Formation as a result of early diagenetic sulfurization, a previously unrecognized pathway of OM preservation. This is evident from both changes in the molecular composition of the insoluble OM and from δ13CTOC shifts of 6‰ with varying CCHO contents. Furthermore, experiments simulating the natural sulfurization of the CCHO-rich alga Phaeocystis spp. demonstrated that sulfurization can indeed lead to a substantial preservation of CCHO with a molecular fingerprint identical to that of the Kimmeridge Clay and many other Recent and ancient marine OM-rich sediments. These results imply that preservation of CCHO can exert a fundamental control on δ13CTOC in OM-rich sediments, complicating the interpretation of δ13CTOC records with regard to estimating terrestrial versus aquatic OM fractions, reconstruction of past atmospheric CO2 levels and global carbon budget models.