In contrast, other OSCs exhibit no concentration change with
depth, thereby providing no conclusive evidence for accumulation
during diagenesis. Nevertheless, the d34S values of HBI thiolanes
from F3, C20 isoprenoid thiophene from F1, and compound U-1 also
increase by 7–10‰ over the 6 m core, comparable with the pattern
observed for diagenetic OSCs and implying ongoing formation.
Concentration and d34S profiles for these compounds can be reconciled
in one of two ways. First, they may have a significant syngenetic
source, with their concentration profiles reflecting variable
production in the water column over time. Isotopic variability on
the scale of a few permil would then reflect dynamic sulfur cycling
near the chemocline, where sulfide d34S can vary due to changes in
local redox state (Li et al., 2010, 2011). Alternatively, changes in the
d34S values of non-accumulating OSCs with depth could result from
slow equilibration between the OSCs and (poly)sulfides. Given the
scale of depositional variability evident in the core, either scenario
seems viable.