Biomarker and organic carbon isotope geochemistry of cobalt-rich crust in the Mid-Pacific seamounts and its palaeoenvironmental significance

LI Xue-fu; WU Guang-hai; LIU Jie-hong; ZHANG Hong

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Journal of Marine Sciences ›› 2012, Vol. 30 ›› Issue (4) : 29-36.

Biomarker and organic carbon isotope geochemistry of cobalt-rich crust in the Mid-Pacific seamounts and its palaeoenvironmental significance

LI Xue-fu, WU Guang-hai, LIU Jie-hong, ZHANG Hong

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Abstract

The cobalt-rich (Co-rich) crust sample KXD28 was obtained from Mid-Pacific seamounts during DY105-12,14 cruise. The contents of soluble organic matter ( chloroform bitumen “A”) and its group compositions ( saturated hydrocarbon, aromatic hydrocarbons and resin),n-alkanes and isoprenoid hydrocarbon were analyzed quantitatively using gas chromatography (GC) internal standard method. Total organic carbon ( TOC) and organic carbon isotope ( δ¹³C) in the cobalt-rich crusts were also detected. Compared with the analyzed results from the sediments nearby metallogenic zone， and using the abundance data of organic matter composition recorded in the crust layer and in the seafloor sediments, the ecological environment of seamounts in the past， as well as the Co-rich crust forming environment and evolution characteristics were studied. Finally， the relationship between the composition of the crust and the environment of growth was investigated. And our principal conclusions show that: (1) The biomarkers of n-alkanes and isoprenoid hydrocarbons were detected in Co-rich crusts. According to the molecular characteristics index，main peak of carbon, EnC23^- /EnC24⁺，CPI, Pr/Ph，Pr/nC17 and Ph/nC18， it is indicated that crust sample KXD28 was originated mainly from the sinking or floating hydrophily macrophytes and homonemeaes with less terrigenous materials. The organic carbon isotopes also matched with the carbon isotope characteristics of marine aquatic organisms; (2) The changes of the organic carbon isotopes of crust were consistent with the changes of marine environment and climate during the crusts’growth. Before crusts formed, and in the early period of crusts’ growth, δC changed from positive to negative, corresponding to global cooling and Antarctic Bottom Water flowing into Pacific. During the late period of crusts’ growth, δ¹³C turned positive gradually, related to global warming and Antarctic Bottom Water receding.

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Pacific seamounts / cobalt rich ferromanganese crust / biomarker / carbon isotope / paleoceanography significance

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LI Xue-fu, WU Guang-hai, LIU Jie-hong, ZHANG Hong. Biomarker and organic carbon isotope geochemistry of cobalt-rich crust in the Mid-Pacific seamounts and its palaeoenvironmental significance[J]. Journal of Marine Sciences. 2012, 30(4): 29-36

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