| Title: | Orbital Forcing of the Early Cretaceous Ocean - A Perspective from Mediterranean Tethys |
| Author: | Alfred G Fischer |
| Date Submitted: | 05/10/2002 |
| Address: | Department of Earth Sciences
University of Southern California
Los Angeles
CA
USA
90089-0740
|
| Phone: | 213.740-8802 |
| Email: | grippo@earth.usc.edu |
| Co-Authors: | |
| Affiliation: | University of Southern California |
| | |
| Abstract URL: | http://cis.whoi.edu/science/GG/ccod/viewAbstracts.cfm?RefNumber=19725644 |
| Keywords: | Orbital Cycles, Albian, Tethys, Selli, Bonarelli, OAE |
| Abstract: | Work on pelagic Cretaceous deposits in the Italian Apennines with Premoli Silva, Erba, Herbert, Grippo and others recently culminated in a coherent and spectrally documented cyclostratigraphy for the pelagic Albian, and will provide a basis for oceanographic assessment. Normal coccolith-globigerinid limestones and marls are interbedded with red marls which despite their oxidation state lack ichnofauna and represent advection and sinking of warm, saline, and nutrient-depleted oceanic surface waters. Both facies record responses to the orbital variations: the precession (damaged by bioturbation), the obliquity (at intervals), and the 95-ka and 406-ka eccentricity rhythms and yield an for an Albian duration of 11.9+- 0.5 Ma. The precessional alternation of a marl bed with a limestone, in a ca. 8-cm couplet, reflects oscillations in coccolith productivity and redox conditions on the sea floor (evidenced in ichnofauna). These differences are modulated over the 95-ka scale by the short eccentricity rhythm, which drove redox oscillation to anoxia, recorded in cm-scale sapropels for about one fifth of the cycles. Inconstancy between correlative sections implies that half or more of the couplets initially contained sapropels, but lost many to succeeding scavengers. The 406-ka cycles are less distinct, fractal repetions of the 95-ka cycles. Similar cycles pervade the Barremian, Aptian and Cenomanian. Those preceding both OAE-1A (the Selli sapropel) and OAE2 (the Bonarelli sapropel) are radiolarian cherts, suggesting that at these times the development of anoxic sulfurets in the oxycline had reached dimensions in space and time that allowed them to trap that scarcest of nutrients, iron, increasing fertility of overlying waters. The precessional cycle must be related to the alternation of highly seasonal perihelial summers with the damped seasonality of perihelial winters; the location between supercontinents suggests that the waxing and waning of monsoonal regimes must was important in eccentricity forcing; but the phase relations remain to be clarified. Abundance of soot in the sapropels (and in many other Albian deposits) suggests an abundance of fires, and the possibility of higher atmospheric oxygen content. These regimes ended with the widening of Tethys and the high sea level stands attained in the Turonian. |
|