Milankovitch-Controlled Paleoclimate Signal Recorded by Rock Magnetics, Lower Cretaceous Platform Carbonates of Northern Mexico

Linda A. Hinnov¹, David Anastasio², Diana Latta², Ken Kodama², and Maya Elrick^3
1Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD
²Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA
³Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM

Study of cyclic carbonate platforms relies on field observations of repeating, often cryptic, shallowing upward facies. This can be difficult when cycles are thin or lack lithologic distinction. The cyclicity typically occurs at Milankovitch time scales, and needs to be analyzed for orbital forcing. An assessment of how the signal is encoded in the rocks is integral to assessing the validity of climate proxies. The challenge is how to render measured sections into objective time series using a facies-independent physical parameter. Here we report on anhysteretic remanent magnetism (ARM) of the lower Aptian Cupido Formation, which reveals 150 m of pervasive, non-random variations at sites from the inner and middle shelf (30 km apart). Fine-grained detrital magnetite from terrigenous sediment, possibly eolian dust, is responsible for the ARM. At both sites, ARM variations show a synchronous ~30-35 m oscillation with maxima coinciding with fourth-order sequence boundaries, superimposed by prominent high-frequency variability. Chronostratigraphy suggests that the variations reflect Milankovitch cycles. Tuning the low-frequency oscillation to a 405-kyr periodicity (long eccentricity) focuses high-frequency variability into short eccentricity, obliquity and precession bands; the precession-band signal modulates with a pronounced eccentricity signature. The ARM signal is tightly correlated between the sites, but decoupled from interpreted fifth-order depositional cycles. ARM amplitude weakens upsection as facies record deepening conditions, likely due to a warming, humid climate, changing global circulation and/or greater dispersal of magnetite grains in the water column. The high fidelity of this ARM proxy underscores its great promise in the objective retrieval of Milankovitch cycles, and in the high-resolution stratigraphic correlation of carbonate platforms.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas