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2008-10-22 – Proliferazione di organismi produttori di magnetite durante il “massimo termico del Paleocene-Eocene” (“Magnetic Death Star” fossil)

Studi geobiologici su microcristalli “giganti” di magnetite di origine biologica consentono nuove considerazioni sul “massimo termico del Paleocene-Eocene”.

Gli autori dello studio sostengono infatti che lo sviluppo di una spessa zona subossica con un elevata disponibilità di ferro bio-assilmilabile (prodottosi in seguito ai drammatici cambiamenti climatici e di pattern sedimentari dovuti a un forte riscaldamento globale) condusse alla diversificazione di organismi (anche eucarioti) produttori di magnetite.

N.B. vedi il filmato del cluster di cristalli ripreso al microscopio elettronico (Supporting Information)

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Geobiologists Discover Unique ‘Magnetic Death Star’ Fossil

 General Science / Archaeology & Fossils

Geobiologists Discover Unique 'Magnetic Death Star' Fossil

          (PhysOrg.com) — An international team of scientists has discovered microscopic, magnetic fossils resembling spears and spindles, unlike anything previously seen, among sediment layers deposited during an ancient global-warming event along the Atlantic coastal plain of the United States.

The researchers, led by geobiologists from the California Institute of Technology and McGill University, describe the findings in a paper published online this week in the Proceedings of the National Academy of Sciences (PNAS).
Fifty-five million years ago, Earth warmed by more than 9 degrees Fahrenheit after huge amounts of carbon entered the atmosphere over a period of just a few thousand years. Although this ancient global-warming episode, known as the Paleocene-Eocene Thermal Maximum (PETM), remains incompletely explained, it might offer analogies for possible global warming in the future.

Perhaps in response to the environmental stress of the PETM, many land mammals in North America became dwarfed. Almost half of the common sea bottom-dwelling microorganisms known as foraminifera became extinct in newly warmer waters that were incapable of carrying the levels of dissolved oxygen for which they were adapted.

“Imagine our surprise to discover not only a fossil bloom of bacteria that make iron-oxide magnets within their cells, but also an entirely unknown set of organisms that grew magnetic crystals to giant sizes,” said Caltech postdoctoral scholar Timothy Raub, who collected the samples from an International Ocean Drilling Program drill-core storehouse at Rutgers University in New Jersey.

A typical “giant” spearhead-shaped crystal is only about four microns long, which means that hundreds would fit on the period at the end of this sentence. However, the crystals found recently are eight times larger than the previous world record for the largest bacterial iron-oxide crystal.

According to Dirk Schumann, a geologist and electron microscopist at McGill University and lead author of the study, “It was easy to focus on the thousands of other bacterial fossils, but these single, unusual crystals kept appearing in the background. It soon became evident that they were everywhere.”

In addition to their unusually large sizes, the magnetic crystals occur in a surprising array of shapes. For example, the spearhead-like crystals have a six-sided “stalk” at one end, a bulbous middle, and a sharp, tapered tip at the other end. Once reaching a certain size, spearhead crystals grow longer but not wider, a directed growth pattern that is characteristic of most higher biological organisms.

The spearhead magnetic crystals compose a minor fraction of all of the iron-oxide crystals in the PETM clay layer. Most of the crystals have smaller sizes and special shapes, which indicate that they are fossils of magnetotactic bacteria. This group of microorganisms, long studied at Caltech by study coauthor Joseph Kirschvink, the Nico and Marilyn Van Wingen Professor of Geobiology, use magnets to orient themselves within Earth’s magnetic field, and proliferate in oxygen-poor water.

Spearheads are not, however, the rarest fossil type in the deposit. That honor belongs to a spherical cluster of spearheads informally dubbed the “Magnetic Death Star” by the researchers. The Magnetic Death Star may have preserved the crystals as they occurred in their original biological structure.

The researchers could not find a similar-shaped organism anywhere in the paleontological annals. They hypothesize that it may have been a single-celled eukaryote that evolved for the first time during the PETM and was outcompeted once the strange climate conditions of that time diminished. Alternatively, it may still exist today in a currently undiscovered location.

“The continental shelf of the mid-Atlantic states during the PETM must have been very iron-rich, much like the Amazon shelf today,” notes study coauthor Robert Kopp of Princeton University, who first started working on the project while a graduate student at Caltech. “These fossils may be telling a story of radical environmental transformation: imagine a river like the Amazon flowing at least occasionally where the Potomac is today.”

  Click here to enlarge image

The paper, “Gigantism in unique biogenic magnetite at the Paleocene-Eocene Thermal Maximum,” will appear in the early online issue of PNAS the week of October 20. The Caltech work was supported by the NASA Exobiology program.
Provided by California Institute of Technology

souurce: http://www.physorg.com/news143818456.html

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Original article:

Gigantism in unique biogenic magnetite at the Paleocene–Eocene Thermal Maximum

Dirk Schumann et al.

PNAS doi:10.1073/pnas.0803634105

 Abstract

We report the discovery of exceptionally large biogenic magnetite crystals in clay-rich sediments spanning the Paleocene–Eocene Thermal Maximum (PETM) in a borehole at Ancora, NJ. Aside from previously described abundant bacterial magnetofossils, electron microscopy reveals novel spearhead-like and spindle-like magnetite up to 4 μm long and hexaoctahedral prisms up to 1.4 μm long. Similar to magnetite produced by magnetotactic bacteria, these single-crystal particles exhibit chemical composition, lattice perfection, and oxygen isotopes consistent with an aquatic origin. Electron holography indicates single-domain magnetization despite their large crystal size. We suggest that the development of a thick suboxic zone with high iron bioavailability—a product of dramatic changes in weathering and sedimentation patterns driven by severe global warming—drove diversification of magnetite-forming organisms, likely including eukaryotes.

Abstract  –  Full Text (PDF)  –  Supporting Information

ottobre 22, 2008 - Posted by | 6 Eocene, 7 Paleocene, G - Geobiology, Geology - Geologia, Multimedia, P - Extinctions, P - Paleoclimatologia, Paleontology / Paleontologia, X - PNAS | , , , , , , ,

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