Our results indicate that high sedimentation rate correspond to heavy isotopic signals, e.g. d34S can research 20‰, while low SD leads to light pyrite with d34S around -20‰. Could anyone tell me the mechanism for it?
Dear Dr. Liu,
the fractionation of S isotopes is not a monocausal process and not governed by a single process alone through time. How good is your correlation between the rate of sedimentation and the variation of S isotopes ? How did you determine your rate of sedimentation and what is the scale? Is on a microscale (laminae) or believed to be controlled by the variation of the bed thickness ? There are a great deal of questions which need to be answered prior to directing your thoughts to the sedimentation rate only. Pyrite is a jack of all trades and even the morphology of this Fe sulfide which may provide you an insight into the physical-chemical regime too, is often correlated with the S isotope variation. Pyrite is not as monotonous as it might look like from the chemical point of view. It can accommodate a lot of trace elements into its lattice such as Ni, Co and As paving the way into different groups of minerals. This is to make you a bit more cautious in branding sedimentation rate as the only "culprit".
Best regards
H.G.Dill
Xiting:
We do not know for sure as to how sedimentation rate alone can cause fluctuations in Sulphur isotopes. But have a look at this link:
http://www.sciencedirect.com/science/article/pii/S0031018297000679
Best
Syed
Dear Liu, here is my thought regarding your question:
Even though the diagenetic regime of pyrite in marine sediments in oxic and euxinic levels is different where oxic condition is enriched in 34S by ~10‰ relative to that in euxinic condition, but these two environmental levels are reversed in d34S values of pyrite saturated sediments, which may be resulted by either the variations in rate of bacterial reduction in the surface layers "cause depletion in d34S " , or may be due to an increase in d34S as the organic matter increased with depth or in other word increase in rate of sedimentation.
All my best,
Omar
Thanks for all your answers and suggestions. I really agree that the sulfur signals of pyrite are controlled by multi-processes. Here I would like to attach our fresh results from the inner-shelf of the East China Sea (36 m water depth).
In spite of consistent contents of the organic carbon (TOC), we find that the sulfur contents (TS) and isotopic values (δ34S) show significant variability from the core bottom to top. In general, when sedimentation rates are high (i.e., U2, U4 U6), the TS increase combined with high δ34S values. In detail, the U2 with the extremely high δ34S values (as high as 75‰) is different from the U4 (near the sulfate values in present seawater 20‰). Could these both unites can be explained by high rates of bacterial sulfate reduction (BSR, related to sedimentation rates) in a closed system (BSR)? In contrast, do these lighter pyrites in the U3 and U5 relate to BSR in the open system?
In spite of all of your mentioned processes, also some attention needs to be taken while you are dealing with near sediment water interface as may be oxidation of partially dissolved sulfide which may contribute an increase in isotopic fractionation between the sulfide/sulfate phases.
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