Kerogen in the Eel River system of northern California - Characterization and analytical approaches for its study
Kerogen, ancient organic matter formed over geologic time in sedimentary deposits, is the most abundant form of organic carbon (OC) on Earth. Continental margins are important sites for OC burial and the subsequent sequestering of C out of the atmosphere. Study of dynamics of OC in the global carbon cycle are important to understanding and predicting atmospheric gas levels, especially oxygen and CO2. In the Eel River system in northern California kerogen-laden bedrock is subject to rapid erosion and transport to a marine shelf with high sediment accumulation rates. This area presents an opportunity to study kerogen dynamics in both terrestrial and marine settings. The objective of this study was to characterize changes in kerogen and modern OC associated with fine-grained sediment as particles move from a bedrock source to the seabed in the Eel River system. As a starting point, the organic matter of the kerogen-laden bedrock was characterized using elemental and isotopic analyses, FTIR, solid state NMR and Rock-Eval pyrolysis. The effectiveness of the various analytical tools for tracking the kerogen and studying OC transformations in the Eel River system was investigated. Characterization of the kerogen found in the clay-rich melange bedrock of the Eel River watershed identified it as a mature Type III kerogen. More then 70% of the OC in the bulk bedrock sediment and about 90% of the OC in the clay-sized fraction of the bedrock was due to kerogen. Low-density particles in the bedrock included fossilized wood fragments. The FTIR spectrum of the bitumen fraction isolated from the bedrock had a relatively strong aromatic character compared to other published bitumen spectra. The kerogen had low oxygen, nitrogen and sulfur contents and Rock-Eval analysis classified it as highly mature. The FTIR spectrum of the demineralized kerogen had strong aromatic and ether bands and lacked any distinct carbonyl peaks around 1710 cm-1. 13C CP-MAS NMR showed that about 70% of the OC in the kerogen was aromatic. Overall, the kerogen was found to be highly aromatic, un-weathered and with a significant terrestrial source component. Microbial nitrogen processing complicates the use of C/N ratios for identifying source organic matter in the clay-sized fraction. In soils, C/N ratios reflect the degree of microbial organic matter alteration and are not good OC source indicators. In marine sediments on the Eel shelf, C/N ratios are potentially insensitive to organic matter changes in fine sediment fractions and are better suited for bulk sediment and coarse fraction analysis. FTIR spectra can provide information about the molecular composition of the organic matter in the clay-sized fraction. Similarity in IR spectra did not consistently correspond to similarity in other characteristics, such as C/N or del 13C. Microbial processing may cause organic matter with similar characteristics to be spectrally variable, although microbial alteration is insufficient to explain differences in all the spectra analyzed. Dual isotope analysis provided the best method for identifying and quantifying kerogen in modern sediments. Fractional kerogen contents were calculated using del 13C and delta 14C values. Kerogen persists in soils of the watershed and in the sediments of the Eel shelf. The clay-sized particles in all the sediments sampled appear to retain a kerogen OC load unaltered from bedrock levels. In the clay-sized fraction of the soils analyzed, generally 50% or more of the total OC was due to kerogen. Similar kerogen contents were also seen in surface sediments and recently buried sediments of the Eel shelf.
Advisor:Neal E. Blair
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Keywords:marine earth and atmospheric sciences
Date of Publication:08/10/2005