The consequences of early diagenesis

Reconstructing past Earth’s surface environment relies on the measurement and interpretation of the chemical and/or isotopic composition of sedimentary archives, which we hope to be passive recorders of marine chemistry. Although there is increasing evidence that local processes within the sediment modify those geochemical signals and are in part responsible for the preserved isotopic composition in the rock record.

Quaternary shelf sediments offer a means to disentangle the roles of global vs. local processes in controlling the proxy of interest. For example, some of my work has shown glacial-interglacial δ34SPYR swings. As the timescale of glacially driven sea-level fluctuations (~100 kyr) is much shorter than the residence time of seawater sulfate (13 million years), such variation challenges the widely held view that the sedimentary archives preserve a record of the global sulfur cycle. It becomes unclear, then, whether stratigraphic variation in pyrite δ34S values observed in marine sediments and sedimentary rocks reflects changes in the global sulfur cycle or whether this variation might reflect changes in the local depositional environment. To understand the information encoded in the sedimentary record of pyrite abundance and isotopic composition, we must decipher the extent to which local processes may affect the sedimentary sulfur cycle, the mechanisms behind the effects (e.g., microbial vs. sedimentological processes), and the rate at which these effects take place.

Examples:

Iron speciation in marine sediments also reflects diagenetic processes ! And rarely the chemistry and oxidation state of the water column, as commonly assumed.

Using the New Zealand sedimentary system, we identified a clear role for sea level and the local sedimentation rate (rather than changes in the global sulfur cycle fluxes) in controlling the sulfur isotope composition of pyrite.

In the Peruvian OMZ, the organic carbon delivery rate, another local parameter of the depositional environment, was the main determinant of pyrite sulfur isotope composition.

In the Gulf of Lion, we related remarkable Glacial-Interglacial swing in pyrite sulfur isotope and changes in the depositional conditions, rather than changes in the global sulfur cycle fluxes, rather than changes in the global sulfur cycle fluxes