A Coupled Molecular Isotopic Approach to Trace Sources of Organic Carbon Preserved in the Sedimentary Record

Abstract

Organic compounds synthesized by organisms are subject to post-formation biological and physicochemical processes that alter their chemical composition, and complicate the recognition and quantitation of these materials in downstream organic carbon (OC) reservoirs such as soils and sediments. Moreover, the time-scales over which organic matter is processed prior to burial may vary substantially. As a result of these factors, contemporaneously deposited material may exhibit a range of ages and labilities. For example, vascular plant debris may be directly deposited in aquatic sediments (via eolian transport), or it may enter a protracted soil cycle that both modifies composition and delays delivery. On the other extreme, relict organic carbon eroding from uplifted sedimentary rocks may contribute ancient, highly refractory OC to the depositional environment. Thus, the differing extents of 'pre-conditioning' that organic matter undergoes prior to burial may greatly modify its chemical signature and strongly dictate its reactivity in the sub-surface. For the above reasons, in seeking to quantify the proportions of organic matter preserved in the subsurface that stem from different sources it is important to find tracer properties that are largely independent of degradation, and to assign ages to specific source materials. We have applied methodologies to measure the stable carbon isotopic composition and radiocarbon content of individual organic compounds in order to investigate the origin of OC buried in soils and sediments. ~13C measurements (Goni and Eglinton, 1996) are used to establish source characteristics, and molecular 14C measurements (Eglinton et al., 1996) are used to determine apparent ages for assessment of the residence times and cycling rates within (and between) carbon reservoirs. Focus has been placed on the molecular isotopic characteristics of terrestrially-derived organic matter in continental margin sediments. Preliminary measurements of hydrocarbon lipids in surficial marine sediments using this new coupled isotopic approach indicated considerable age variability that could be attributed to specific source inputs (Eglinton et al., 1997). Here, we extend these measurements to other compound classes, including dissociation products of refractory aliphatic and phenolic macromolecules that form a larger proportion of the organic matter preserved in sediments and soils.