HR: 0800h
AN: OS51A-0554    [Abstracts]
TI: Diversity and Variability of Geoporphyrins and Chlorins During Cretaceous Oceanic Anoxic Event II.
AU: * Junium, C K
EM: cjunium@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16801 United States
AU: Mawson, D H
EM: dhm102@york.ac.uk
AF: University of York, Department of Chemistry, Heslington,York, YO10 5DD United Kingdom
AU: Arthur, M A
EM: arthur@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16801 United States
AU: Keely, B J
EM: bjk1@york.ac.uk
AF: University of York, Department of Chemistry, Heslington,York, YO10 5DD United Kingdom
AB: Geoporphyrins and chlorins are biomarkers that result from the transformation of tetrapyrroles including chlorophylls, bacteriochlorophylls and haems. The transformation reactions are initiated in the water column and sediments during early diagenesis and are dependent on a range of variables including, but not limited to water column redox state, burial conditions, and time. Geoporphyrins and chlorins can retain structural characteristics that allow unambiguous assignment of precursor structures and source organisms making their utility in paleoenvironmental studies extraordinary where such information is preserved. Black shales from Oceanic Anoxic Event II (OAE II, Cenomanian-Turonian Boundary) of ODP Leg 207 present a unique opportunity for investigating the variations in the tetrapyrrole record in very well preserved sediments across a globally significant biogeochemcal event. Identification and structural assignment of tetrapyrroles in this study were achieved by a combination of high-performance-liquid-chromatography (HPLC)/diode-array-detection (DAD) and liquid chromatography-mass spectrometry (LC-MSⁿ) on acetone extracts. Stratigraphic variations in geoporphyrin compounds occur through OAE II. The relative proportions of metallated vs. free-base (metal free) porphyrins vary throughout the sequence, favoring free-base porphyrins during the height of the anoxic event. The greater proportion of free-base porphyrins associated with more extensive reducing conditions is consistent with metal ion limitation during euxinia. For example, vanadyl porphyrins become much less abundant during the peak of the event suggesting that the oceanic inventory of V was sequestered in black shales and unavailable. Preliminary characterization of the tetrapyrroles through OAE II of ODP Leg 207, Demerara Rise, reveals a wide range of geoporphyrins and chlorins. Notably, positive identification of chlorins, the geologically unstable intermediates between highly reactive chlorophylls and the more stable geoporphyrins, predates the previous oldest described sedimentary occurrence by more than 70 Ma (Miocene, Vena del Gesso). Presence of chlorins implies exceptional preservation associated with strongly reducing conditions that existed during OAE II at Demerara Rise and the excellent condition of the recovered core material. Chlorins attributed to green sulfur bacteria, on the basis of specific mass-spectral characteristics 43 and 57, confirm the presence of anaerobic photoautotrophs and periods of photic-zone euxinia at Demerara Rise during the mid-Cretaceous. Unusually high abundances of the C₃₃ bicyclo-alkanoporphyrin were also observed. The C₃₃ bicyclo-alkanoporphyrin is possibly derived from rearrangement of chlorophyll a and suggests that a high proportion of the extractable geoporphyrins were derived from cyanobacteria and calcareous nannoplankton.
DE: 1055 Organic and biogenic geochemistry
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4855 Phytoplankton
DE: 4900 PALEOCEANOGRAPHY (0473, 3344)
DE: 9610 Cretaceous
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005