Magnetite (Fe_3O_4) is one of the most stable carriers of natural remanent magnetization (NRM) in sedimentary rocks, and paleomagnetic studies of magnetite-bearing sediments, such as deep-sea cores and pelagic limestones, have provided a detailed calibration between the biostratigraphic and magnetic polarity time scales. Despite this important role, there is as yet a very poor understanding of how ultrafine-grained (< 0.1 µm) magnetite is formed, transported, and preserved in marine environments. A major conceptual advance in our understanding of these processes is the recent discovery that biogenic magnetite, formed by magnetotactic bacteria and/or other magnetite-precipitating organisms, is responsible for much of the stable magnetic remanence in many marine sediments and sedimentary rocks. Since these magnetite particles are of biogenic origin, they are termed properly magnetofossils (Kirschvink & Chang 1984).

In this paper we report the discovery of fossil bacterial, single-domain magnetite particles in ancient stromatolites. The biogenicity of the crystals was determined by the following criteria: (1) distinctive morphology and habit, (2) composition and (3) environment of deposition. Stromatolites ranging in age from the Middle Archean to Pleistocene, composed of both carbonate and chert, were analyzed for the presence of single-domain magnetite using rock magnetic methods. The granulometry and composition of the ultra-fine-grained magnetite crystals extracted were determined by transmission electron microscopy and electron diffraction. The oldest magnetofossils were extracted from stromatolitic chert of the Gunflint Iron Formation which is approximately 2000 Ma old. The implications of these findings and the potential uses of fossil bacterial magnetite in studies of the evolution of biomineralization and prokaryotic metabolic processes, paleomagnetism, and as an indicator of ancient oxygen levels are discussed. Bacterial magnetite represents the oldest evidence of biomineralization yet discovered in the fossil record.

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.

Research Article| January 01, 1988 Magnetostratigraphic dating of shallow-water carbonates from San Salvador, Bahamas Donald F. McNeill; Donald F. McNeill 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Fisher Island Station, Miami Beach, Florida 33139 Search for other works by this author on: GSW Google Scholar Robert N. Ginsburg; Robert N. Ginsburg 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Fisher Island Station, Miami Beach, Florida 33139 Search for other works by this author on: GSW Google Scholar Shih-Bin R. Chang; Shih-Bin R. Chang 2California Institute of Technology, Geological and Planetary Science Division 170-25, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar Joseph L. Kirschvink Joseph L. Kirschvink 2California Institute of Technology, Geological and Planetary Science Division 170-25, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar Author and Article Information Donald F. McNeill 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Fisher Island Station, Miami Beach, Florida 33139 Robert N. Ginsburg 1Comparative Sedimentology Laboratory, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Fisher Island Station, Miami Beach, Florida 33139 Shih-Bin R. Chang 2California Institute of Technology, Geological and Planetary Science Division 170-25, Pasadena, California 91125 Joseph L. Kirschvink 2California Institute of Technology, Geological and Planetary Science Division 170-25, Pasadena, California 91125 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1988) 16 (1): 8–12. https://doi.org/10.1130/0091-7613(1988)016<0008:MDOSWC>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Donald F. McNeill, Robert N. Ginsburg, Shih-Bin R. Chang, Joseph L. Kirschvink; Magnetostratigraphic dating of shallow-water carbonates from San Salvador, Bahamas. Geology 1988;; 16 (1): 8–12. doi: https://doi.org/10.1130/0091-7613(1988)016<0008:MDOSWC>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

For the first time, a detailed sequence of late Neogene/Quaternary magnetic reversals is documented in shallow-water carbonate sediments. The magnetostratigraphy of a continuous core from San Salvador, Bahamas, correlates with the established magnetic polarity time scale. The remanent magnetism of 140 samples from a 91-m limestone/dolomite core was measured with a SQUID magnetometer. All samples were demagnetized using alternating field and thermal demagnetization to isolate characteristic components of remanent magnetization. Natural remanent magnetism in the carbonates ranged from 1.4 x 10/sup -7/ to 4.5 x 10/sup -9/ A m/sup 2//kg. Magnetic polarities were determined based on relative updown direction in the unoriented core. Extraction of magnetic minerals, followed by TEM examination, revealed magnetofossils of predominantly bacterial origin. The measured section ranges from early Pliocene (Gilbert chron 3) through late Pleistocene/Holocene (Brunhes chron). The reversals provide a minimum of five new major chronostratigraphic markers for the late Neogene-Quaternary of the Bahamas. Previously established biostratigraphic data (last appearance of the coral Stylophora affinis and a molluscan fauna equivalent to the Bowden formation of Jamaica) are in concordance with the magnetostratigraphy. The timing of the major lithologic change from skeletal to nonskeletal sediments is dated as upper late Pliocene. The establishment of a more » reliable magnetostratigraphy provides much refined dating of shallow-water carbonates that contain a record of sea level fluctuations, subsidence, changes in depositional facies and fauna, and possibly climatic events. That the magnetostratigraphic record is still legible in recrystallized and dolomitized carbonates suggests that it may also be present in older strata. « less

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.

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.

Remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time-scale, study geomagnetic reversals and secular variation, and measure the rates of seafloor spreading. Although these sediments may contain different magnetic minerals, magnetite is the most commonly observed and magnetically stable phase, and its size, shape and post-depositional fate affect the magnetic remanence of the sediments. Biogenic magnetites are single-domain, with a high natural magnetic remanence (NRM), and have been suggested as a significant source of magnetic remanence in marine sediments. We have studied surface sediments from the Santa Barbara Basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra-fine-grained, single-domain magnetite. Using a novel extraction technique, transmission electron microscopy and SQUID magnetometry, we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments.

Research Article| September 01, 1984 Ultrafine-grained magnetite in deep-sea sediments: Possible bacterial magnetofossils Joseph L. Kirschvink; Joseph L. Kirschvink 1170-25, Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar Shih-Bin R. Chang Shih-Bin R. Chang 1170-25, Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar Geology (1984) 12 (9): 559–562. https://doi.org/10.1130/0091-7613(1984)12<559:UMIDSP>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Joseph L. Kirschvink, Shih-Bin R. Chang; Ultrafine-grained magnetite in deep-sea sediments: Possible bacterial magnetofossils. Geology 1984;; 12 (9): 559–562. doi: https://doi.org/10.1130/0091-7613(1984)12<559:UMIDSP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.