Mild hydrothermal alteration of immature organic matter in sediments from the Bransfield Strait, Antarctica

Abstract

Surface sediments in the eastern Bransfield Strait, Antarctica were piston ccored to reconfirm previously observed hydrothermal alteration at depth (3–6 m subbottom). The Bransfield Basin is still actively rifting, with concomittant slow advection of fluids through the sediments covering the the back-arc spreading axis, and mild thermal alteration near the sea bed of the immature marine organic matter. Bitumen products from hydrothermally enhanced alteration are superimposed over the endogenous lipids which had undergone normal diagenesis. For example, the total hydrocarbbon yields are enhanced in the gray facies and had to a lesser degree in the red beds. The CPI for the n-alkanenes (C 25–C 33) is about 2 for all samples, with no major n-alkane generation observed due to hydrothermal activity. In altered zones the biomarker signature changes, thus confirming some thermal stress; however, the steroid distributions are controlled more by iputs from source organisms. Nevertheless, chemical reactions have occurred and include dehydration of sterols to sterenes, isomerization of triterpenes and sterenes, rearrangement of sterenes to diasterenes, and reductive processes generating phytane from phytol via phytenes. Polynuclear aromatic hydrocarbons comprise the parent PAH with significant amounts of alkylated homologs and aromatic terpenoid hydrocarbons. These compounds are probably derived from disseminated coal in the sediments and not from hydrothermal activity. Such a source would also explain the n-alkane distribution. Thus, hydrothermal activity induced by the intrusion of a volcanic plug/dike in the study are caused accelerated diagenesis of immature organic matter with simulataneous migration of volatile products in the sediments.