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 different fossil types, including
deep and shallow water scleractinian corals, foraminifera, diatoms and
tooth enamel. Our experiments show that exposure to different 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 (of 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
οC. At 200 οC 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.