Search

Further X-ray diffraction (XRD) analytical data.

On the mid-Atlantic Coastal Plain of the United States, Paleocene sands and silts are replaced during the Paleocene-Eocene Thermal Maximum (PETM) by the kaolinite-rich Marlboro Clay. The clay preserves abundant magnetite produced by magnetotactic bacteria and novel, presumptively eukaryotic, iron-biomineralizing microorganisms. Using ferromagnetic resonance spectroscopy and electron microscopy, we map the magnetofossil distribution in the context of stratigraphy and carbon isotope data and identify three magnetic facies in the clay: one characterized by a mix of detrital particles and magnetofossils, a second with a higher magnetofossil-to-detrital ratio, and a third with only transient magnetofossils. The distribution of these facies suggests that suboxic conditions promoting magnetofossil production and preservation occurred throughout inner middle neritic sediments of the Salisbury Embayment but extended only transiently to outer neritic sediments and the flanks of the embayment. Such a distribution is consistent with the development of a system resembling a modern tropical river-dominated shelf.

Among Earth sciences, paleomagnetism is particularly linked to the statistics of large sample sets as a matter of historical development and logistical necessity. Because the geomagnetic field varies over timescales relevant to sedimentary deposition and igneous intrusion, while the fidelity of recorded magnetization is modulated by original properties of rock units and by alteration histories, "ideal" paleomagnetic results measure remanent magnetizations of hundreds of samples at dozens of progressive demagnetization levels, accompanied by tests of magnetic composition on representative sister specimens. We present an inexpensive, open source system for automating paleomagnetic and rock magnetic measurements. Using vacuum pick-and-place technology and a quartz-glass sample holder, the system can in one hour measure remanent magnetizations, as weak as a few pAm2, of ~30 specimens in two vertical orientations with measurement errors comparable to those of the best manual systems. The system reduces the number of manual manipulations required per specimen ~8 fold.

Previous workers identified a magnetically anomalous clay layer deposited on the northern United States Atlantic Coastal Plain during the Paleocene-Eocene Thermal Maximum (PETM). The finding inspired the highly controversial hypothesis that a cometary impact triggered the PETM. Here we present ferromagnetic resonance (FMR), isothermal and anhysteretic remanent magnetization, first order reversal curve, and transmission electron microscopy analyses of late Paleocene and early Eocene sediments in drillcore from Ancora, New Jersey. A novel paleogeographic analysis applying a recent paleomagnetic pole from the Faeroe Islands indicates that New Jersey during the initial Eocene had a ~6-9 degrees lower paleolatitude (~27.3 degrees for Ancora) and a more zonal shoreline trace than in conventional reconstructions. Our investigations of the PETM clay from Ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria. This result, the first identification of ancient biogenic magnetite using FMR, argues that the anomalous magnetic properties of the PETM sediments are not produced by an impact. They instead reflect environmental changes along the eastern margin of North America during the PETM that led to enhanced production and/or preservation of magnetofossils.

Paleomagnetic data are presented for a set of orientated cores from a talus cone at Kromdraai B, a South African cave deposit associated with early Pleistocene fauna including important hominid remains of Paranthropus Australophitecus) robustus. Polarity interpretations of calcified sediments and flowstones suggest that the cave deposits include episodes of deposition that span the Olduvai Event of the Matuyama chron. Results suggest that matrix of the kind associated with the type specimen of robustus (TM 1517) corresponds closely to the beginning of the Olduvai Event, c. 1.9 Myr ago

The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during post-glacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low pale-olatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity. Here we report the first Zn isotopic data, together with carbon and oxygen isotopic profiles from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly Zn-66-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove delta Zn-66 (Zn-66/Zn-64, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in delta Zn-66 records a vigorous increase in primary production and export from a nutrient-laden surface ocean. Refereed/Peer-reviewed