Biomineralization of organic matter in cobalt-rich crusts from the Marcus-Wake Seamounts of the western Pacific Ocean

Abstract

Organic matter in cobalt-rich crust (CRC) from the Marcus-Wake Seamounts of the western Pacific Ocean, Sample CM1D03, has been analyzed to understand the source, geochemistry and mineralization of organic matter, and the mineralization environment. Biomarkers, including n-alkanes, isoprenoids, terpanes and sterols, have been detected in various layers of the CRC sample, using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The content of organic carbon (OC) and its stable isotope (δ13C), and the combined features of the biomarkers show that the mineralized organic matter in CM1D03 CRC was mainly derived from microorganisms and lower plankton (e.g., bacteria and algae, respectively) from marine surface water, with some terrestrial higher plant components. The ratio of chloroform bitumen “A”: OC was high in the CRC, between 10.51 and 20.66, showing significant migration characteristics of n-alkanes. Four mineralization categories of organic matter were recognized based on GC chromatograms of n-alkane molecules: (1) primitive type (bacteria and algae), which is characterized by moderately mature of n-alkanes preserving the original characteristics of the organic matter from microorganisms and lower plankton; (2) microbial degradation type, which is characterized by low contents of n-alkanes and rising baseline in the chromatogram, with the “bulge” being the products of organic matter by biodegradation; (3) organic matter migration type, which is characterized by low carbon number of n-alkanes with nC18 as the main peak carbon, without odd even predominance, and low concentrations of isoprenoids and hydrocarbons with high carbon number; and (4) organic matter hydrothermal type, which is characterized by relatively low concentration of small molecular weight n-alkanes, pristane, and phytane, accompanied by higher concentration of n-alkanes with carbon number greater than nC18. This study shows that biomarkers can record controlling factors of mineralization and their variation.