T23 Mud, Mud, Glorious Mud: Advances in Stratigraphic, Sedimentologic, Geochemical, and Geomechanical Analyses of Fine-Grained Lithologies

Sunday, 25 September 2016
8:00 AM - 12:00 PM
Room 401

EFFECTS OF EARLY DIAGENESIS ON MAGNETIC SUSCEPTIBILITY IN FINE-GRAINED, METHANE-BEARING MARINE SEDIMENTS

Paper number: T23-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

Magnetic susceptibility (MS) of fine-grained sediments and sedimentary rocks can provide high resolution records of both depositional and diagenetic processes; however, unraveling these processes can be difficult. We present an integrated rock magnetic and geochemical analysis of Quaternary marine sediments from the upper ~100 m of five scientific drilling sites along the eastern Indian and Cascadia margins (India National Gas Hydrate Program Sites 10 and 16; Ocean Drilling Program Sites 1249 and 1252; Integrated Ocean Drilling Program Site U1325) in an effort to disentangle depositional and diagenetic influences on MS. These sediments are dominantly clay-rich, contain variable concentrations of methane hydrate, and porewater sulfate is depleted within the upper 0 to 20 m below seafloor. Isothermal remanent magnetization and thermal demagnetization techniques indicate a magnetic mineral assemblage dominated by titanomagnetite in each site with the exception of some intervals dominated by magnetic iron sulfides at two sites.

We utilize a Zr-based heavy mineral proxy from X-ray fluorescence to establish a predictive relationship with detrital MS. Through this approach we identify intervals several m to tens of m thick in which (1) MS is decreased compared to predicted detrital MS and (2) total sulfur is increased compared to background levels. These results suggest that magnetite is dissolved in these intervals and this loss is balanced by sulfur gains via pyrite formation. We observe repeated intervals of low MS and high sulfur that correspond to increases in total organic carbon, and in methane-venting sites MS is completely altered.

Organoclastic sulfate reduction and anaerobic oxidation of methane likely produced hydrogen sulfide in these low MS intervals, stimulating the pyritization of magnetite that leaves clearly identifiable drawdowns in MS preserved in the fine-grained marine sediment record. These records suggest that diagenetic changes driven by sedimentation rate, marine biological productivity, and fluid migration can be recorded in MS patterns in fine-grained sediments. We hypothesize that this early magnetic overprint is likely stable in anoxic sedimentary rocks over geologic time, until additional contact with sulfide/sulfate-bearing fluids or uplift into oxic conditions occurs.

Geological Society of America Abstracts with Programs. Vol. 48, No. 7

doi: 10.1130/abs/2016AM-284026

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