Mineralogy and geochemistry of secondary carbonates from NanTroSEIZE reference sites C0011 and C0012

Author(s): Fisher, A. J.; Sample, James C.; Tripati, A.; Torres, M. E.
Author Affiliation(s): Primary:
Northern Arizona University, School of Earth Sciences and Environmental Sustainability, Geology, Flagstaff, AZ, United States
University of California, Los Angeles, United States
Oregen State University, United States
Volume Title: AGU 2013 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2013; American Geophysical Union 2013 fall meeting, San Francisco, CA, Dec. 9-13, 2013. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Two reference sites were drilled seaward of the Nankai Trough subduction system to characterize properties of materials before entering the zone of deformation. Secondary carbonates can increase sediment strength and record variations in pore water characteristics over time. For this study, we analyzed up to 86 carbonates for mineralogy, major element geochemistry, and stable isotope geochemistry. Carbonate cement abundances range from 2% to 79%. In order from most to least abundant, the carbonate mineralogy is calcite, Mn-Fe bearing calcites, magnesium calcite, and dolomite. Total carbonate abundances show no trend with depth, and Mn-Fe calcites with abundances of 2-5%, are common. High-Mg calcite and dolomite are scarce and are most abundant in the upper 500 m at site C0011. Calcite abundance maxima (14.49% and 13.36%) occur at two depths at site C0012 corresponding with sandier volcanic units in the Shikoku basin facies (Unit II 151-220 mbsf) and Unit V (416-529 mbsf). Calcite abundance maxima (14% and 10%) occur at two depths at site C0011 (Unit I (0-340 mbsf) and Unit IV (529-538 mbsf)), but do not appear related to sedimentary lithology. General textural relationships obtained from scanning electron microscopy display primarily calcite and Mn-Fe calcite pore-filling cements with less abundant secondary Fe- and Mg-rich clay cements. Carbon isotope delta values (vs. VPDB) range from -23.36 ppm to 0.26 ppm at Site C0011 and from -22.48 ppm to 0.09ppm at Site C0012. The values are consistent with mixtures of seawater dissolved inorganic carbon and contributions from oxidized methane. Carbon isotope values become depleted starting at ∼400 mbsf at site C0011 and ∼180 mbsf at site C0012. Oxygen isotope values (vs. VPDB) range from -11.88 ppm to 1.65 ppm at Site C0011 and from -15.96 ppm to 5.70 ppm at Site C0012. The strongest 18O depletions occur below a depth of 420 mbsf at C0011 and 330 mbsf at C0012. Carbonate clumped isotope thermometry of samples from C0012 indicate that four carbonates have formed at temperatures in equilibrium with current downhole temperatures. These carbonates may have formed very recently, and their 18O depletions can be explained strictly by temperature. Low-temperature water-rock interactions likely also contribute to 18O depletion in some of the carbonate cements. Carbonate cements are present throughout the sediments, although carbonate cementation is not the dominant process in altering sediment properties at Sites C0011 and C0012. Formation of carbonates at shallow depths in both Sites indicates secondary carbonate formation is an important process early in the sediment burial history. Carbonate clumped isotope data also demonstrate that carbonate formation is still an important process later in the burial history.
Year of Publication: 2013
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 07 Marine Geology and Oceanography; Chemical composition; Geochemistry; IODP Site C0011; IODP Site C0012; Integrated Ocean Drilling Program; Marine sediments; Mineral composition; NanTroSEIZE; North Pacific; Northwest Pacific; Pacific Ocean; Sediments; Shikoku Basin; West Pacific
Coordinates: N324944 N324945 E1365256 E1365254
Record ID: 2015031165
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