HR: 0800h
AN: PP51B-0593    [Abstracts]
TI: High-Resolution C and N isotopic Records from the Aptian-Albian of Northeastern Mexico: Insight into OAE 1a and 1b Environmental Dynamics
AU: * Montanez, I P
EM: montanez@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology, Davis, CA 95616-5270
AU: Osleger, D A
EM: osleger@geology.ucdavis.edu
AF: University of California, Davis, Dept. of Geology, Davis, CA 95616-5270
AU: Bralower, T J
EM: bralower@geosc.psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802
AU: Li, Y
EM: yul5@psu.edu
AF: Penn State University, Department of Geosciences, University Park, PA 16802
AB: Globally distributed, mid-Cretaceous organic-rich sediments are the expression of significant environmental perturbations that resulted in widespread, and geologically ephemeral (~0.5 to 1 m.y.) oceanic anoxic events (OAEs). Characteristic δ¹³C_carb and δ¹³C_org patterns record significant changes in global carbon budgets during these OAEs. The nature of the forcing mechanism and the mechanistic link to organic-rich deposits, however, remains elusive reflecting in large part the limited temporal resolution afforded by stratigraphically condensed deep-sea records. Here we present continuous dm-scale % carbonate and TOC, and C and N isotopic records for significantly expanded outer shelf and slope deposits from the Sierra Madre Oriental of northeast Mexico that contain early Aptian OAE1a and latest Aptian-early Albian OAE1b. Magnetic susceptibility and anhysteretic remanent magnetization measurements, at a dm-scale, integrated with nannofossil biostratigraphy and Sr isotopes are used for constraining the rates of these perturbations. C isotope variations previously defined in Tethyan and Atlantic deposits are recorded by northeastern Mexican δ¹³C_carb and δ¹³C_org. However, these expanded records show significantly more structure including an out-of-phase relationship and decoupling of C and N isotopes through the main OAE events. These data define an initial prominent negative excursion followed by repeated and short-lived (10³ to 10⁴ yr) fluctuations and a final interval of near invariant positive δ¹³C_carb and δ¹³C_org values. Rapid shifts to low δ¹⁵N values (-1 to -4‰) coincident with positive shifts in δ¹³C_org throughout the interval of repeated isotopic fluctuations may represent episodic blooms of marine N₂-fixing cyanobacteria under nitrate-poor, low-O₂ levels in response to the initial environmental perturbation. Elevated TOC and a significant increase in C/N ratios from background marine values through this interval of decoupled C and N isotopes suggests that cyanobacterial N₂-fixation may have been a significant contributor to the nutrient N pool during these OAEs. Notably, the interval of most stable and in-phase C isotope values is characterized by a return to background TOC and C/N ratios suggesting that the positive C isotope excursion mostly postdates the period of sustained anoxia and enhanced organic C burial in the paleo-Gulf of Mexico. These initial results indicate that Mexican Aptian-Albian deposits hold promise for refining phase relationships between geochemical and isotopic proxies throughout OAE1a and OAE1b and in turn the interactions and feedbacks of their inferred environmental changes.
DE: 0330 Geochemical cycles (1030)
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 0470 Nutrients and nutrient cycling (4845, 4850)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005