ORGANIC
GEOCHEMISTRY
At Site 1118, the
shipboard organic geochemistry program consisted of determinations of TOC,
inorganic carbon, total carbon, total nitrogen, and total sulfur in sediments,
in addition to the routine hydrocarbon gas safety monitoring procedure. The
analytical techniques used are outlined in "Organic
Geochemistry" in the "Explanatory
Notes" chapter.
Volatile
Hydrocarbons
Headspace methane
concentrations for Site 1118 display a typical profile for deep-sea sediments
(Fig. F61; Table T10).
Methane concentrations were found to be low (
3.30
ppmv) between 214 and 243 mbsf. Below this depth, the C1
concentration increased rapidly to ~40,000 ppmv by 304 mbsf. Methane
concentrations remained between 16,000 and 98,000 ppmv until 660 mbsf where they
decreased slowly to 9
ppmv by 734 mbsf. Methane remained below this concentration to the bottom of the
hole. Ethane was the only other hydrocarbon detected. This was found at
concentrations 55 ppmv
between 264 and 686 mbsf, which coincided with the broad methane maximum (Fig.
F61; Table T10).
The C1/C2 ratio decreased from ~5000 at ~300 mbsf to ~1600
at ~600 mbsf. The broad interval of elevated methane and ethane concentrations
between ~300 and ~700 mbsf coincides with the absence of sulfate in the
sedimentary interstitial water (Fig. F59A)
and indicates that the methane and ethane at this site are most likely of
biogenic origin.
Sedimentary CaCO3
concentrations at Site 1118 are shown in Table T11
and Figure F62.
Between 200 and ~600 mbsf, CaCO3 contents are generally <20 wt%.
The peak concentration (72.5 wt%) found at 279 mbsf represents a layer rich in
foraminifers (see "Lithostratigraphic
Unit I"). Between ~600 and ~850 mbsf, corresponding to the
lower half of lithostratigraphic Unit III through Unit V (Fig. F1),
CaCO3 concentrations are more variable and range between 0 and 36
wt%. The data scatter is caused by the presence of thin ash layers throughout
these units, which are characterized by low CaCO3 levels. The peak in
CaCO3 between 854 and 869 mbsf represents lithostratigraphic Unit
VII, a layer of reworked shallow-water carbonate deposits (Fig. F1).
Organic carbon and sulfur
concentrations in sediments at Site 1118 were low, averaging 0.28 and 0.19 wt%,
respectively (Table T11;
Fig. F62). The peaks
in both organic carbon (0.73 wt%) and sulfur (1.18 wt%) at 256 mbsf correspond
to an interval of bioturbated volcaniclastic siltstone and claystone that is
probably partially pyritized (see "Lithostratigraphic
Unit I"). Organic carbon and sulfur display a trend of
generally increasing concentrations downhole between 300 and 800 mbsf. The data
between ~500 and ~800 mbsf, however, display considerable scatter, thought to be
caused by the presence of thin ash layers that have exceptionally low organic
carbon and sulfur concentrations. The available C/N ratios show that the organic
carbon has a mixed marine and terrigenous source with one exception (650 mbsf),
which shows a marine origin for the organic material (Table T11).
