Laboratory Assessment of the Impact of Chemical Oxidation, Mineral
Dissolution, and Heating on the Nitrogen Isotopic Composition of
Fossil-bound Organic Matter
Abstract
Fossil-bound organic material holds great potential for the
reconstruction of past changes in nitrogen (N) cycling. Here, with a
series of laboratory experiments, we assess the potential effect of
oxidative degradation, fossil dissolution, and thermal alteration on the
fossil-bound N isotopic composition of a range of fossil types,
including deep and shallow water scleractinian corals, foraminifera,
diatoms and tooth enamel. Our experiments show that exposure to
different strongly oxidizing reagents does not significantly affect the
N isotopic composition or N content of any of the fossil types analyzed,
demonstrating that organic matter is well protected from changes in the
surrounding environment by the mineral matrix. In addition, we show that
partial dissolution (up to 70-90%) of fossil aragonite, calcite, opal,
or enamel matrixes has a negligible effect on the N isotopic composition
or N content of the fossils. These results suggest that the isotopic
composition of fossil-bound organic material is relatively uniform, and
also that N exposed during dissolution is lost without significant
isotopic discrimination. Finally, our heating experiments show
negligible changes in the N isotopic composition and N content of all
fossil types at 100 ºC. At 200 ºC and hotter, the N loss and associated
nitrogen isotope changes appear to be directly linked to the sensitivity
of the mineral matrix to thermal stress. These results suggest that, so
long as high temperature does not compromise the mineral structure, the
biomineral matrix acts as a closed system with respect to N, and the N
isotopic composition of the fossil remains unchanged