Sedimentary Iron Cycling and the Origin and Preservation of Magnetization in Platform Carbonate Muds, Andros Island, Bahamas
Carbonate muds deposited on continental shelves are abundant and well-preserved throughout the geologic record because shelf strata are difficult to subduct and peritidal carbonate units often form thick, rheologically strong units that resist penetrative deformation. Much of what we know about pre-Mesozoic ocean chemistry, carbon cycling, and global change is derived from isotope and trace element geochemistry of platform carbonates. Paleomagnetic data from the same sediments would be invaluable, placing records of paleolatitude, paleogeography, and perturbations to the geomagnetic field in the context and relative chronology of chemostratigraphy. To investigate the depositional and early diagenetic processes that contribute to magneitzation in carbonates, we surveyed over 500 core and surface samples of peritidal, often microbially bound carbonate muds spanning the last not, vert, similar 1000 yr and deposited on top of Pleistocene aeolianites in the Triple Goose Creek region of northwest Andros Island, Bahamas. Sedimentological, geochemical, magnetic and ferromagnetic resonance properties divide the sediment columns into three biogeochemical zones. In the upper sediments, the dominant magnetic mineral is magnetite, produced by magnetotactic bacteria and dissimiliatory microbial iron metabolism. At lower depths, above or near mean tide level, microbial iron reduction dissolves most of the magnetic particles in the sediment. In some cores, magnetic iron sulfides precipitate in a bottom zone of sulfate reduction, likely coupled to the oxidation of decaying mangrove roots. The remanent magnetization preserved in all oriented samples appears indistinguishable from the modern local geomagnetic field, which reflects the post-depositional origin of magnetic particles in the lower zone of the parasequence. While we cannot comment on the effects of late-stage diagenesis or metamorphism on remanence in carbonates, we postulate that early-cemented, thin-laminated parasequence tops in ancient peritidal carbonates are mostly likely to preserve syn-depositional paleomagnetic directions and magnetofossil stratigraphies.
Author postprint. Published version © 2007 Elsevier B.V. Received 25 January 2007; received in revised form 10 May 2007; accepted 11 May 2007. Available online 23 May 2007. This project was supported by an Agouron Postdoctoral Fellowship to ACM, and Agouron Institute grants to JPG and JLK. REK was supported by a Moore Foundation fellowship and a grant from NASA ASTID. We appreciate keen field observations from John Abelson, Chris Guthrie, Peter Johnson, Jim Dahlberg, and Elsebet Lund and logistical support from Joan Kobori. We thank Angelo Di Bilio for support with the EPR spectrometer and David Mohrig and John Woodruff for stimulating discussions. Amy Myrbo and Anders Noren at the Limnology Research Center (University of Minnesota) provided us with coring equipment and a wealth of practical knowledge. This work would not have been possible without the genius of Roberta Bennett–Calorio, field assistance from Jay Ewing, able seamanship of Bradley Mackey, and logistical support from Agent Nixon of the Andros Island Police and Jeff Birch and the staff of Small Hope Bay Lodge.
Updated - MALepsl07.pdf