HR: 1340h
AN: PP33B-0949    [Abstracts]
TI: Sedimentary Nitrogen Stable Isotopes and Variations in Nutrient Cycling in the Holocene Black Sea
AU: * Fulton, J M
EM: jfulton@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences , University Park, PA 16802 United States
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences , University Park, PA 16802 United States
AB: Interpreting the evolution of organic matter production and preservation in anoxic basins such as the Holocene Black Sea depends on developing an understanding of changes in nutrient cycling within the water column with time. The organic fraction of sediments may preserve evidence of such changes in nutrient utilization. One model proposes changes in phosphorus availability as a driver for changes in algal productivity in the Black Sea. Nitrogen, the other macronutrient commonly considered to limit algal growth, is the focus of this study as we examine the nitrogen content and stable isotope variations of Black Sea sediments to determine what role it may have played in temporal changes in productivity and organic matter accumulation. High-resolution samples from five gravity cores collected by the RV Knorr 1988 expedition were analyzed for $\delta$$^{15}$N-total and $\delta$$^{13}$C-organic as well as their percent composition of organic carbon and total nitrogen. One core, GC71, was subjected to sequential extractions with KCl and hydrogen peroxide to remove exchangeable ammonia and labile organic matter respectively. The KCl extraction did not remove a statistically significant amount of ammonia, having no measurable effect on the percent nitrogen or $\delta$$^{15}$N of the solid samples. The hydrogen peroxide extraction removed ca. 95% of the organic carbon and 85% of the nitrogen, leaving a relatively nitrogen-enriched residual material, probably due to ammonium fixed within the clay lattice. The fixed nitrogen has a minimal effect on the bulk nitrogen isotope values, suggesting the bulk nitrogen values are similar to the organic nitrogen signal. All cores examined were from below the modern Black Sea chemocline and are thought to have remained under anoxic bottom water continuously since soon after the incursion of saline Mediterranean water ca. 7800 years ago. Water depths for these cores range from 411 meters along the south margin of the sea to 2088 meters in the eastern Black Sea basin, and the samples analyzed span the past 10,000 years. This study incorporates new nitrogen isotopic data, higher resolution carbon isotopic data, and C/N ratios with previously published bulk organic carbon, Rock-Eval pyrolysis, regional climate, and molecular organic geochemical data from other sources. The results of this study reveal three intervals in sedimentary units I, IIa, and IIb, preserved in multiple cores, with less enriched $\delta$$^{15}$N values from +0.3 to +1.5 permil, separated by positive excursions with $\delta$$^{15}$N values between +3 and +4.5 permil. These intervals are the result of basin-wide processes as they occurred synchronously across the Black Sea. $\delta$$^{13}$C values suggest a primarily marine source for organic matter throughout units I and II, and fluctuations of $\delta$$^{15}$N do not seem to correlate with regional vegetation or precipitation changes; thus, assuming a relatively constant $\delta$$^{15}$N for inorganic and organic nitrogen riverine influx, changes in $\delta$$^{15}$N are due primarily to marine activity. We believe the lower $\delta$$^{15}$N values in units I and IIa were caused by decreases in water column denitrification due to a deeper chemocline and greater oxygen availability for respiration. The lower $\delta$$^{15}$N values in unit IIb may be due to nitrogen fixation caused by the release of excess phosphorus from the sediments due to the bottom water anoxia.
DE: 4805 Biogeochemical cycles (1615)
DE: 4267 Paleoceanography
DE: 1040 Isotopic composition/chemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting