NEW APPROACHES TO ACCESSING HIGH-RESOLUTION BIOGEOCHEMICAL SIGNALS FROM ANCIENT ORGANIC MATERIALS

When assessing the geological record, the dynamics of the sedimentary environment blur our analytical resolution and limit our ability to understand processes that occur over decadal to subannual scales. Even with mm-scale sampling in marine rocks, the resulting organic material may comprise hundreds if not thousands of years. Yet within the geologic record, there are materials that reveal information about paleoecology, ontogeny, and environmental dynamics at extraordinary temporal resolution. Skeletal carbonates such as bivalves and corals grow over the course of years to decades, and contain trace quantities of organic matter within their shells that can be accessed and utilized for carbon and nitrogen isotope analyses. With nanoEA, which is a cryo-trapping, capillary focusing technique for nitrogen and carbon isotopes, we can begin to analyze recalcitrant organic material preserved in ancient skeletal carbonates as well as extractable organic materials such as individual organic microfossils.

We will highlight a suite of new organic nitrogen and carbon isotope data from Middle Devonian rugose corals and modern bivalves as well as carbon isotope measurements from individual Devonian organic microfossils and Eocene fossil wood. For example, the nitrogen isotopic composition of organic matter extracted from rugose corals is, on average, enriched by 2-3‰ relative to the bulk rock nitrogen in the host rock. Assuming that the bulk nitrogen is largely representative of the long-term primary production background, the modest enrichment is consistent with a trophic effect, and that rugose corals are planktivores. The organic content of the corals is extremely low, but nanoEA allows for serial sampling of 5-10 samples per coral. In an individual coral, δ¹⁵N ranges by 3-4‰ over the length of an individual coral, and when adjusted for trophic enrichment varies around the average δ¹⁵N of bulk sedimentary organic matter (2.0‰). The variability in the δ¹⁵N of the coral organic matter likely reflects short-term variability in basinal conditions or changes in coral food supply.