HR: 1340h
AN: PP13A-1033    [Abstracts]
TI: Orbital Tuning as an Inverse Problem: Dating the Aptian Selli OAE in the Cismon borehole (Southern Alps, Italy)
AU: * Malinverno, A
EM: alberto@ldeo.columbia.edu
AF: Lamont-Doherty Earth Obs., 61 Route 9W, Palisades, NY 10964, United States
AU: Erba, E
EM: elisabetta.erba@unimi.it
AF: Dept. of Earth Sciences, Univ. of Milano, Via Mangiagalli 34, Milano, 20133, Italy
AB: The Selli Level is a black shale interval that records in Italy an Aptian global oceanic anoxic event. We studied about 30 m of the Cismon APTICORE borehole, where the Selli Level is about 5 m thick and is composed of alternating limestone and black shale. A thin nodular interval (about 2 cm) immediately below the base of the Selli suggests condensed sedimentation or a hiatus. The goal of our study is to date by orbital tuning the duration of the Selli Level and of the hiatus/condensation at is base. Orbital tuning is the process of matching measurements of climate-related sediment properties (eg., CaCO₃ content) with Milankovitch periodicities. Orbital tuning can be cast as an inverse problem: find sedimentation rate models that result in measurement time series with spectral peaks at the Milankovitch frequencies. To minimize unrealistic variations in sedimentation rate, we use as a model a sequence of layers with constant sedimentation rates. This model allows for abrupt changes of sedimentation rate (eg., at the boundaries of the Selli) and for hiatuses (eg., at the base of the Selli). We quantify the final uncertainty of the sedimentation rates and account for uncertainty in the tuning frequencies with Monte Carlo modeling. For tuning, we use the orbital periods of short eccentricity (fixed at 100 kyr) and obliquity (allowed to vary in the 35-41 kyr interval). Our measurement series is a downhole log of electrical resistivity (which is well correlated to CaCO₃ content) with a vertical resolution of about 1 cm. The inversion results give sedimentation rates of 3-5 m/Myr in the Selli Level and higher rates of 5-9.5 m/Myr above and below. The most likely duration for the Selli Level is about 1.2 Myr. If ages are calibrated to the magnetic stratigraphy with a base M0 at 121 Ma, the Selli Level was deposited from about 120.1 to 119 Ma. The inversion gives a duration of about 19 kyr for the hiatus at the base of the Selli. Nannofossil abundance and composition in the nodular interval suggest a 12-14 kyr condensation, resulting from carbonate dissolution under rapidly increasing pCO₂. Finally, we determine a best-fit obliquity period of about 38.4 kyr at this time (around 120 Ma) compared to the present-day value of 41 kyr.
DE: 0915 Downhole methods
DE: 1165 Sedimentary geochronology
DE: 4802 Anoxic environments (0404, 1803, 4834, 4902)
DE: 4946 Milankovitch theory
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting