Shih-Bin R. Chang

Ultrafine-grained magnetite in deep-sea sediments: Possible bacterial magnetofossils

A new extraction technique now permits ultrafine magnetite crystals to be separated from a variety of deep-sea sediments. Morphologic characterization of these particles with transmission electron microscopy reveals the presence of several distinct crystal types, some of which closely resemble those formed by the magnetotactic bacteria. The apparently biogenic magnetite particles are of single-domain size and dominate the population in calcareous deep-sea sediments. Bacterially precipitated magnetite may therefore be a major source of the stable magnetic remanence in some marine sediments. These objects possibly constitute the smallest mineral fossils yet recovered from the sedimentary record.

A Candidate Magnetic Sense Organ in the Yellowfin Tuna, Thunnus albacares

Single-domain magnetite crystals have been isolated and characterized from tissue located in a sinus within the dermethmoid bone of the skull of the yellowfin tuna, Thunnus albacares. Their chemical composition, narrow size distribution, and distinctive crystal morphology indicate that these crystals are biochemical precipitates. Experiments on the interaction between particles reveal the organization of the particles in situ and suggest a possible form for candidate magnetoreceptor organelles. The consistent localization of such particles with similar arrangement within the dermethmoids of this and other pelagic fishes suggests that the ethmoid region is a possible location for a vertebrate magnetic sense organ.

Magnetostratigraphic dating of shallow-water carbonates from San Salvador, Bahamas

Magnetostratigraphic results are reported here from a sequence of late Neogene-Quaternary shallow-water carbonate sediments from a continuous core drilled on the island of San Salvador, Bahamas. On the basis of the remanent magnetism of 136 samples from a 91-m measured section of core, the polarity sequence can be correlated with the magnetic polarity time scale from the Gilbert chron (early Pliocene) through the late Brunhes chron (late Pleistocene-Holocene). Magnetic polarities were determined on the basis of relative up-down direction in the unoriented core. Extraction studies of the magnetic particles reveal the presence of single-domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae. The sequence of reversals provides a minimum of six new major chronostratigraphic markers for the Pliocene-Pleistocene of the Bahamas; it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the Bahamas as upper late Pliocene (between 2.6 and 2.7 Ma). That the primary magnetic remanence is preserved in shallow-water carbonates, including replacement dolomites, suggests that this technique could be used to date similar Tertiary and possibly even older carbonate sequences. The establishment of a reliable magnetostratigraphy provides refined dating of shallow-water carbonates and regional faunal appearances or disappearances, sediment accumulation rates, subsidence, and depositional events.

Biogenic magnetite as a primary remanence carrier in limestone deposits

Abstract Studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at Sugarloaf Key, FL, U.S.A. and calcareous laminated sediments from Laguna Figueroa, Baja California, Mexico have revealed the existence of magnetotactic bacteria and ultrafine-grained single domain magnetite in both environments. Magnetotactic bacteria were identified by light and electron microscopy. The single domain magnetite was detected by coercivity spectra analysis with a SQUID magnetometer and examined under the transmission electron microscope. The similarity, in terms of size and shape, between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine-grained magnetite in these two marine environments was biologically formed. These results, combined with the common occurrences of ultrafine-grained magnetite in limestone deposits detected rock magnetically, suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks. Several Cambrian limestone samples, separately collected from Siberia, China, and Kazakhstan, were examined for the presence of bacterial magnetite. Samples from the Lower Cambrian Sinskian Formation at Siberia Platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component. Post-Cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them. Assessing the potential role of biogenic magnetite as a primary remanence carrier in other Phanerozoic limestone deposits ought to be further pursued.

Possible Biogenic Magnetite Fossils from the Late Miocene Potamida Clays of Crete

In the 23 years since Lowenstam (1962) first discovered the mineral magnetite in chiton teeth, many other organisms have been reported to be able to form this mineral as well (Blakemore, 1975; Gould et al., 1978; Frankel et al., 1979; Walcott et al., 1979; Kirschvink, 1981a; Walker and Dizon, 1981; Lins de Barros et al., 1981). Magnetite is now the fourth most common biogenic mineral after carbonate, opal, and ferrihydrite and related ferric oxide in terms of its production by different groups of organisms (Lowenstam and Weiner, 1982). A variety of magnetite-forming organisms live in aquatic environments and hence there is the question whether magnetite formed by organisms can be preserved in sediments. In particular, one group of magnetite-synthesizing organisms, the magnetotactic bacteria, are cosmopolitan in their aquatic distribution. Based on calculations considering their natural population density, sedimentation rates, and volume of magnetite per cell, the biologic contribution of magnetic remanence in sediments has been estimated by Kirschvink and Lowenstam (1979) to reach the 10−4 A/m level. Towe and Moench (1981) revised this estimate upwards to 10−3 A/m which is more compatible with the remanence generally observed in sediments.

Biogenic magnetite in stromatolites. I. Occurrence in modern sedimentary environments

Abstract In this paper we report the occurrence of biogenic ultra-fine-grained, single-domain magnetite in both marine and non-marine modern stromatolitic environments. Magnetotactic bacteria were found associated with the microbial communities involved in the deposition of laminated sediments at Laguna Figueroa, Baja California, Mexico and carbonate stromatolitic nodules at Sugarloaf Key, FL, U.S.A. These bacteria and the ultra-fine-grained magnetite they produce have a profound effect on the magnetic properties of the sediments. The presence of this single-domain magnetite was detected using rock magnetic methods, while the morphology was identified by transmission electron microscopy. Examination by rock magnetic methods of microbial mats and laminated sediments from Solar Lake, Sinai, Guerrero Negro, Baja California, Mexico and Shark Bay, Australia, as well as stromatolites from Shark Bay and Clifton Lake, Australia, Bacalar Lake and Cuatro Cienegas, Mexico, and Walker Lake, U.S.A., indicate these localities contain single-domain magnetite. Biogenic magnetites of presumed bacterial origin have been extracted from Shark Bay mats, Walker Lake stromatolites and Bahama carbonates. These findings suggest that bacterial magnetites can be recognized as the tiniest trace fossil and may be found preserved in fossil stromatolites and laminated sediments.

Magnetic stratigraphy and a test for block rotation of sedimentary rocks within the San Andreas fault zone, Mecca Hills, southeastern California

A 500-m section of the Palm Spring Formation in the southern Mecca Hills, located within the San Andreas fault zone in southeastern California, has been paleomagnetically sampled to determine possible tectonic rotation in this area and to establish time-stratigraphic control. This work was partly stimulated by the fact that 80 km farther south, previous studies demonstrated 35° of postdepositional rotation in the Palm Spring Formation of the Vallecito-Fish Creek basin east of the Elsinore fault. Several lines of evidence suggest that hematite is the main magnetic carrier of the Mecca Hills samples. Large anhedral hematite grains observed in magnetic extracts and a positive fold test imply a detrital origin of the remanence. The polarity reversal patterns, together with earlier vertebrate paleontologic studies, restrict the time span for deposition of this unit to the middle-late Matuyama chron (2.0–0.75 myr ago), thus of uppermost Pliocene and early Pleistocene age. Characteristic directions of best least-squares fit for 73 samples suggest little or no overall rotation, despite the severe late Quaternary tectonic activity demonstrated by the intense deformation of these strata.

The Effect of Magnetotactic Bacteria on the Magnetic Properties of Marine Sediments

Magnetotactic bacteria have been studied in three distinct sedimentary marine environments: a hypersaline lagoon, an intertidal CO3 marsh, and an open ocean basin. The bacteria and the ultra-fine grained, single domain magnetite (Fe3O4) they produce were extracted from the sediments and studied with transmission electron microscopy. Magnetic properties of the sediments were measured by rock magnetic techniques using a SQUID magnetometer. Our results show that magnetotactic bacteria contribute a significant fraction of the natural remanent magnetization to their sedimentary environment and in some cases may be the sole source of the stable remanence carrying mineral. The occurrence and abundance of these bacteria in a diversity of marine environments implies that they may also play a role in the microbial iron cycle.