Interpreting Nitrogen Isotope Excursions in the Sedimentary Record
Open Access
- Author:
- Fulton, James Mark
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 09, 2009
- Committee Members:
- Michael Allan Arthur, Dissertation Advisor/Co-Advisor
Michael Allan Arthur, Committee Chair/Co-Chair
Katherine Haines Freeman, Committee Chair/Co-Chair
Lee Kump, Committee Member
John Michael Regan, Committee Member - Keywords:
- purple sulfur bacteria
cyanobacteria
nitrogen cycle
Black Sea
Green Lake - Abstract:
- This dissertation presents an examination of the biological and chemical processes that determine the nitrogen stable-isotope compositions of organic matter (OM)-rich sediments. Using the Black Sea and Fayetteville Green Lake (FGL), NY as natural laboratories, I evaluate processes that lead to low d15N values of phytoplankton and sediments in anoxic basins. I synthesize sedimentary profiles of bulk N and C stable isotope values with down-core pigment distributions and pigment-specific d15N and d13C values to infer ecological changes responsible for variability in sedimentary d15N values. The primary goal of this research is to test the hypothesis that cyanobacterial N2 fixation is responsible for black shale d15N values that are near and below 0‰. Oceanic anoxia has been prevalent during specific times in geological history. These intervals often are associated with broadly distributed deposits of OM-rich black shales. Widespread anaerobic ammonium oxidation (anammox) and denitrification in anoxic waters near the chemocline (interfaces between oxygenated and anoxic waters) are expected to have led to fixed-N deficiencies that favor cyanobacterial N2 fixation. Such conditions have also been identified in modern meromictic water bodies, most notably the Black Sea. I present sedimentary scytonemin data from the Black Sea that indicate that cyanobacterial growth in the Holocene Black Sea was variable and responded to climate-induced changes in salinity stratification. The sediment intervals with scytonemin also have low d15Ntot values, indicative of cyanobacteria using N2 as a substrate. Green sulfur bacterial (GSB) pigments in Black Sea sediments correlate inversely with scytonemin, leading to a phytoplankton productivity model whereby strong density stratification inhibits mixing of high phosphate, low N:P waters to the sea surface. This model may be applied to ancient marine settings, such as the Paleotethys Ocean during the Permian-Triassic oceanic anoxic event. FGL has a shallow (~20 m) chemocline that allows purple sulfur bacteria (PSB) to thrive. This compares with a Black Sea chemocline at 80-100 m, which allows GSB growth but inhibits PSB. A similar shallow chemocline may have been present in the Mesoproterozoic ocean, as PSB biomarkers have been identified in the ancient sediments. Pigments from PSB and GSB are abundant in the chemocline and sediments of FGL. Sedimentary intervals with high PSB pigment concentrations correlate with low bulk d15N and d13Corg values, supporting my assertion that an increased proportion of PSB biomass causes the low stable-isotope values. Low water-column particulate d15N and d13C values at the chemocline support this model. I also analyzed d13C and d15N values of specific pigments to examine responses of chemocline productivity to elevated mixolimnion productivity. These data demonstrate that sedimentary PSB and GSB pigments were derived from the chemocline, and they provide a ~500-year record of d13CDIC surface-deep gradients and deep water ammonium d15N values.