Preservation of magnetization in carbonate sediments

In carbonate platform sediments, what are the sources of magnetization and what controls their preservation?

Carbonate platforms are a common marine sedimentary environment from the late Archean onward. Carbonate platform sediments therefore form an important archive for sedimentary paleomagnetism. To interpret this archive properly, however, it is necessary to understand the sources of magnetic materials in carbonate sediments and the factors that control their preservation. Moreover, a better understanding of these sources and processes will facilitate interpretation of the paleoenvironmental information carried by the makeup of sedimentary magnetic particles. Toward that end, Adam Maloof and I are studying cores of carbonate sediments from the Three Creeks Region, Andros Island, the Bahamas. Our approach combines paleomagnetic, rock magnetic, and FMR techniques with sedimentological, geochemical, and electron microscopy analyses.

In general, possible sources of magnetic sediments include: lithogenic particles transported by wind or water, biogenic particles produced either by magnetotactic bacteria or bacteria that produce magnetic minerals extracellularly as a byproduct of iron reduction or iron oxidation, iron oxides and iron sulfides produced by diagenetic processes, and anthropogenic pollutants. On Andros Island, lithogenic input is minimal, while our preliminary FMR and rock magnetic data suggest that the microbial contribution to the magnetization of the sediments is considerable. The strong microbial role is consistent with the presence of layered microbial communities, which frequently grow in the uppermost sediments and would have been even more common in the Precambrian carbonate platforms. Our analyses of the cores will determine how diagenetic processes like redox chemistry and bioturbation disrupt paleomagnetic signals and alter the magnetic mineralogy. These studies will thereby facilitate interpretation of records in carbonate platforms in general.

Related Publications

• A. C. Maloof, R. E. Kopp, J. P. Grotzinger, D. A. Fike, T. Bosak, H. Vali, P. M. Poussart, B. P. Weiss, and J. L. Kirschvink (2007). Sedimentary iron cycling and the origin and preservation of magnetization in platform carbonate muds, Andros Island, Bahamas. Earth Planet. Sci. Lett. 259: 581-598. doi:10.1016/j.epsl.2007.05.021 [Journal link]