HR: 09:30h
AN: PP51D-07    [Abstracts]
TI: A compound specific nitrogen isotope record utilizing multiple geoporphyrins and chlorins for Oceanic Anoxic Event 2 at Demerara Rise
AU: * Junium, C K
EM: cjunium@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
AU: Keely, B J
EM: bjk1@york.ac.uk
AF: University of York, Department of Chemistry, Heslington, York, YO10 5DD, United Kingdom
AU: Freeman, K H
EM: kate@essc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16802, United States
AB: Compound specific nitrogen isotope analyses on geoporphyrins and chlorins have the unique potential in that they can link nitrogen isotope values directly to chlorophyll and primary producers. Nitrogen isotope values from bulk sediments have proven useful for numerous recent studies investigating the state of the nitrogen cycle in the past, however questions remain as to the fidelity of such records. Bulk sedimentary nitrogen can be composed of both organic and inorganic fractions and organic nitrogen compounds such as proteins are very labile and degraded rapidly in the early diagenetic environment. It is therefore advantageous to utilize, where possible, compounds that resist such degradation, are easily extracted from sedimentary organic matter and recalcitrant on geologic time scales. Several nitrogen isotope records of bulk sediments from Oceanic Anoxic Event 2 have values that are typically less than 0 permil and in some cases show a 2-3 permil decrease through the OAE. These values are interpreted to be the result of primary producers utilizing nitrogen substrates supplied by nitrogen-fixing cyanobacteria that reflect the isotopic composition of their atmospheric nitrogen source. Nitrogen isotope data from geoporphyrins and chlorins from sediments recovered during Ocean Drilling Program Leg 207 to Demerara Rise deposited during OAE 2 will allow us to assess this hypothesis. The geoporphyrin and chlorin moiety is dominated by high concentrations of metallo and free-base C33 bicycloalkanoporphyins (BiCAPs) but also contains small amounts of other porphyrins, the most notable being the C34 and 35 porphyrins that are derived from bacteriochlorophylls. The high concentration of geoporphyrins and chlorins has allowed for analyses of the nitrogen isotopic composition of up to 4 compounds from each sample but only the BiCAP chlorins have been isolated in sufficient concentrations from each sample thus far. Initial nitrogen isotope data through OAE 2 for the BiCAP chlorins and vanadyl BiCAPs reflect the negative excursion in bulk N-isotopic values, however, the range of values is greatly expanded and the negative excursion is much larger. Immediately prior to OAE 2 N-isotopic values for the C33 BiCAP chlorins are +7.1 permil and drop to -3.9 permil during the event and return to 2.7 permil. The vanadyl C33 BiCAPs follow a similar trend with values starting at +1.6 permil and decrease to -6.0 permil. These data support earlier hypotheses suggesting large changes in oceanic nitrogen cycling during OAE 2 on the basis of bulk nitrogen isotope values. However the large range and anomalously positive N-isotopic values outside of the OAE suggest that we have either greatly underestimated the change in nitrogen cycling during OAE II or that diagenetic processes have resulted in fractionation between the geoporphyrin and chlorin fractions.
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting