What happens to terrestrial organic matter in the ocean?

Abstract

The title of this session, What happens toterrestrial organic matter in the ocean? comesdirectly from the title of a paper published by Hedgeset al. (1997) and continues to be a topic of greatdiscussion. The annual flux of organic matter (~0.5 PgC) from the terrestrial to marine environment issubstantial and an important component of the globalcycles of carbon and other bioactive elements. JohnHedges first pondered the fate of terrestrial organicmatter in the ocean in the early 1970s as a graduatestudent at the University of Texas Marine ScienceInstitute in Port Aransas, Texas. As a student, Johndeveloped methods for using lignin-derived phenolsas unique biomarkers of terrestrial plants (Hedges,1975; Hedges and Parker, 1976). He investigated theabundance and distribution of terrestrial organiccarbon (TOC) in Gulf of Mexico sediments usinglignin phenols in combination with bulk stable Cisotopes as tracers of TOC. His pioneering applicationof lignin phenols as biomarkers of terrestrial materialwas the beginning of a lifelong pursuit. John’s studies,as well as those of many others using a variety ofapproaches, lead to the general conclusion that a largefraction of the TOC discharged from rivers isremineralized within ocean margins (Hedges et al.,1997).1. What is terrestrial organic carbon?The term terrestrial organic carbon has variousconnotationsanddefinitions.Tosome,thetermimpliesorganic carbon that is bfixedQ or biosynthesized in theterrestrial environment. To others, the term implies allorganic carbon discharged from rivers to the coastalocean. Organic carbon in seawater and marine sedi-mentsisoftenconsideredtobeofterrestrialoriginonlyif it has characteristic chemical or isotopic signaturesthat are indicative of a vascular plant origin. However,thebroadestdefinitionsofTOCincludeorganiccarbonthat spans the entire spectrum of biological sources,physical forms, and chemical and isotopic composi-tions. This spectrum of biological sources includesvascularplants,nonvascularplants(bryophytes),algae,fungi, protozoans, bacteria, archaea, viruses andanimals, as well as an equally dizzying array ofbiochemical and isotopic diversity. In addition to thiscomplexity, relic forms of organic carbon, such askerogen,coalandpetroleum,andcombustionproducts,