Volatile OSCs in Cariaco Basin sediments have sulfur isotopic
composition and depth profiles similar to coexisting pyrite
(Fig. 3). A relatively large but variable proportion of sedimentary
pyrite is formed in the water column (Lyons et al., 2003); this ‘‘syngenetic’’
pyrite forms in an effectively open system with respect to
sulfide and its d34S value is affected by processes at the O2–S2
interface. A smaller component of pyrite in Cariaco Basin sediments
is diagenetic, forming in a diffusion-controlled regime
within sediments in which sulfide d34S values increase with depth
(Werne et al., 2003). Like pyrite, Cariaco OSCs are likely produced
via both syngenetic and diagenetic processes. Different individual
compounds may, however, preferentially form in one environment
or the other. For example, Werne et al. (2000) demonstrated diagenetic
production of the triterpenoid thiane based on the extent of
precursor compound conversion. Consistent with the conclusions
of that study, we find that the triterpenoid thiane increases in concentration
from 5.7 to 69 nmol/g OC as its d34S value increases
from 43.6‰ to 32.3‰. Both concentration and sulfur isotope
data thus support predominantly diagenetic formation of this compound.
C20 isoprenoid thiophenes in F3 and compound U-14 also
have similar concentration profiles with only slightly smaller shifts
in their d34S values between 70 and 535 cm (Fig. 1). Thus they are
also probably products of sedimentary diagenesis, forming on a
timescale of thousands of years.
In contrast, other OSCs exhibit no concentration change with