Sources and fate of terrigenous organic matter in modern and ancient sediments from the northern Gulf of Mexico

Coastal marine sediments are the predominant long-term sink for organic matter (OM) in the ocean. The delineation between OM sources deposited in marine sediments, particularly between land and ocean contributions, is essential for the quantitative understanding of the global carbon cycle. The goal of this research is to trace terrigenous carbon in modern and ancient sediments of the Gulf of Mexico (GMex). The specific research objectives are to examine the sources and fate of terrigenous OM (OMterr) in coastal sediments, to quantify OMterr in modern and ancient sediments, and to evaluate long-term, climatically induced changes in North America's vegetation.; Terrigenous OM has historically been assumed to be compositionally homogenous. Such an assumption has led to an underestimate of terrigenous contributions to sedimentary OM. Results from this research indicate that OMterr is in fact heterogeneous, composed of at least two chemically distinct endmembers (vascular plant matter and soil-derived OM). Results of a three-endmember mixing calculation, used to account for this heterogeneous composition, indicate that OMterr accounts for ca. 65--80% of OM deposited as shelf sediments. Importantly, the OMterr abundance is 40--85% higher using this approach than indicated by traditional binary mixing. Such a calculation demonstrates how OMterr has been previously underestimated, and highlights the need for its requantification given the contribution from two distinct components.; The overall goal of this research is to distinguish between C3 (cool-season) and C4 (warm season) sources of terrigenous vegetation in modern and ancient marine sediments to ascertain the effect of climate change on North America's vegetation. Expansion of C4 vegetation is observed following deglaciation in the early-Holocene, while a decline in C4 is observed during late-Holocene cooling. These results are similar to those from North American continental records of Holocene vegetation changes. Expansion of C4 vegetation is also observed during warmer, interglacial periods relative to colder, glacial periods. Glacial marine OM accumulation estimates that are based on bulk isotope records that did not account for continental vegetation variability overestimate paleoproductivity, and need re-evaluation.