Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Magnetic methods and the timing of geological processes L. JOVANE1*, L. HINNOV2, B. A. HOUSEN3 & E. HERRERO-BARVERA4 1Instituto Oceanogra´fico da Universidade de Sa˜o Paulo, Fisica, Geoquimica e Geologia, Prac¸a do Oceanogra´fico 191, Sa˜o Paulo, Sa˜o Paulo 05508-120, Brazil 2John Hopkins University, Earth and Planetary Sciences, 3400 N. Charles Street, Baltimore, Maryland 21218, USA 3Department of Geology, Western Washington University, 516 High St., Bellingham, Washington 98225, USA 4University of Hawaii at Manoa, Hawaii Institute of Geophysics and Planetology, Honolulu, Hawaii 96822, USA *Corresponding author (e-mail: [email protected]) Abstract: Magnetostratigraphy is best known as a technique that employs correlation among different stratigraphic sections using the magnetic directions that define geomagnetic polarity reversals as marker-horizons. The ages of the polarity reversals provide common tie points among the sections, allowing accurate time correlation. Recently, magnetostratigraphy has acquired a broader meaning, now referring to many types of magnetic measurements within a stra- tigraphic sequence. Many of these measurements provide correlation and age control not only for the older and younger boundaries of a polarity interval, but also within intervals. Thus, magnetos- tratigraphy no longer represents a dating tool based only on the geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of all geomagnetic field parameters, environmental magnetism, rock magnetic and palaeoclimatic change recorded in sedimentary rocks, and key corrections to magnetic directions related to geodynamics, tectonics and diagenetic processes. Discovery of geomagnetic reversals motions of the ocean floor. The oceanic magnetic anomalies are related to the magnetization of the Over the past century numerous methodologies oceanic basalts that cool down while spreading have been developed to detect time variations of the from mid-oceanic ridges, and can be used in combi- geomagnetic field and environmentally significant nation with geomagnetic field polarity sequences magnetic properties in rocks. These methods com- from rocks found on land to further develop a prise measurements of natural remanence, magnetic globally extensive GPTS (Heirtzler et al. 1968). susceptibility, demagnetization and induced artifi- Opdyke (1972) first integrated magnetostratigra- cial magnetizations. Brunhes (1906) and Matuyama phy and biostratigraphy for Plio-Pleistocene marine (1926) were among the first to recognize that old sediments, and since that time biostratigraphic rocks have inclination values that are very different information has been increasingly used for corre- from today’s values, and sometimes of opposite lation of the observed polarity sequences in sedi- polarity to the present-day magnetic field. Accord- mentary rocks with the appropriate part of the ing to Matuyama (1926), these changes represent radioisotope-calibrated GPTS. Subsequently, Alva- reversals in the polarity of the ancient geomagnetic rez et al. (1977) recognized sets of magnetic polar- field. Cox et al. (1963) recognized that these polar- ity reversals within the Cenozoic Gubbio (Italy) ity reversals were global events, and that, by com- sedimentary sequence. William Lowrie studied the bining palaeomagnetic and geochronologic data, a geological and physical processes that permit sequence of geomagnetic field reversals could be pelagic sediments to keep magnetization and defined constructed. This led directly to the development the magnetostratigraphy of those Italian sections of the Geomagnetic Polarity Time Scale (GPTS). (Lowrie et al. 1982), allowing the scientific commu- Vine & Matthews (1963), using data acquired nity to build and further refine the GPTS (Cox et al. during marine cruises, recognized that magnetic 1963; Heirtzler et al. 1968; LaBreque et al. 1977; anomalies had a symmetrical pattern with respect Berggren et al. 1985; Cande & Kent 1992, 1995; to the mid-ocean ridges, and that there was a rela- Huestis & Acton 1997; Singer et al. 2002; Channell tionship between geomagnetic field reversals and et al. 1995; Malinverno et al. 2012; Ogg 2012). From:Jovane, L., Herrero-Bervera, E., Hinnov,L.A.&Housen, B. A. (eds) 2013. Magnetic Methods and the Timing of Geological Processes. Geological Society, London, Special Publications, 373, http://dx.doi.org/10.1144/SP373.17 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 L. JOVANE ET AL. TimeScale Creator chart Geomagnetic Standard Chronostratigraphy Polarity Primary Ma Period Epoch Age/Stage 0 Holocene Tarantian 50 C22 Ionian 1 C1 51 Quaternary Calabrian Pleistocene C23 2 Gelasian C2 52 Ypresian 3 Piacenzian 53 Eocene C2A 4 Pliocene 54 Zanclean C24 5 C3 55 6 56 Messinian C3A 7 57 Thanetian C3B C25 Paleogene 8 C4 58 9 59 Tortonian C4A 10 60 Selandian C26 Paleocene 11 C5 61 12 62 C5A C27 Serravallian 13 Neogene C5AA 63 C5AB Danian C5A C28 14 64 Miocene C C5A 15 Langhian 65 D C29 C5B 16 66 C5C 17 67 C30 C5D C-Sequence 18 68 Burdigalian Maastrichtian 19 C5E 69 C31 20 70 C6A 21 71 C6AA Aquitanian 22 72 C6B C32 23 73 C6C 24 74 C7 25 C7A 75 Chattian 26 C8 C-Sequence 76 27 77 Campanian C9 28 78 Oligocene C33 29 C10 79 30 C11 80 31 Rupelian 81 32 C12 82 Late Cretaceous 33 83 34 C13 84 Santonian 35 C15 85 Priabonian 36 C16 86 Paleogene 37 87 C17 Coniacian 38 88 39 Bartonian 89 C18 40 90 41 C19 91 Turonian C34 42 Eocene 92 43 93 C20 44 Lutetian 94 45 95 46 96 Cenomanian 47 TimeScale Creator chart C21 97 Cretaceous Normal Super-Chron ("Cretaceous Quiet Zone") 48 Geomagnetic 98 Standard Chronostratigraphy Polarity 49 Ypresian 99 PrimaryC22 Ma Period Epoch Age/Stage Early Albian Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 MAGNETOSTRATIGRAPHY: ONLY A DATING TOOL? Today, geomagnetic reversals are routinely demonstrates the occurrence of reversals (Glatzma- recognized along stratigraphic sections composed ier & Roberts 1995; Kuang & Bloxham 1997). The of sedimentary (marine or continental) or volcanic main theory relates reversals to internal fluid materials. To determine a polarity stratigraphy, the instability of the Earth’s outer core. In this region sediment or rock must first contain a record of the of the core, convective movements and complex geomagnetic field that is generally acquired at vortices are created within tangent cylinders, that the time of emplacement. In order to measure the is, cylinders that are pretended coaxial movements original magnetization that records the geomagnetic in relation to the Earth’s rotation axis, and tangent field polarity at the time of formation of the rock, to the inner core/outer core boundary with their sample direction must be measured in the field projection on Earth surface at 79.18 latitude. The (i.e. in situ). The capacity of a rock to maintain its thermal and compositional convection processes at own magnetic field and resist demagnetization is the core-mantle boundary influence the geodynamo related to the coercivity of its magnetic minerals. with long-term variations (.105 years) of intensity, There are different ways by which rocks can inclination and declination (Olson et al. 2010). This record a natural remanent magnetization (NRM) in organized motion evolves chaotically with the mag- the presence of an external magnetic field (e.g. the netic field produced by the electromagnetic dynamo geomagnetic field; Kodama 2012): (1) thermorema- growing, decaying and occasionally flipping in nent magnetization (TRM) is acquired when a rock the opposite direction (Gubbins & Bloxham 1985, cools down below the Curie temperature of its mag- 1987; Bloxham & Jackson 1992; Olson & Aurnou netic minerals; (2) chemical remanent magnetiza- 1999; Jackson et al. 2000; Hulot et al. 2002; tion (CRM) is acquired when a new magnetic Aurnou et al. 2003; Wardinski & Holme 2006). The mineral grows after the rock is formed and estab- geomagnetic reversals occur on the entire globe, lishes its own magnetization; (3) viscous remanent also near the tangent cylinder and polar regions magnetization (VRM) is attained in an ambient (Jovane et al. 2008). There are also other theories field for magnetic relaxation during time; (4) iso- to explain reversals, for example, one in which geo- thermal remanent magnetization (IRM) occurs in magnetic reversals are linked to extraterrestrial nature when rocks are struck by lightning and are impacts (Muller & Morris 1986). submitted to a magnetic field larger that their coer- civity; and, the most important in sediments, (5) det- rital remanent magnetization (DRM) is acquired Magnetostratigraphy and the geomagnetic when depositional magnetic grains align themselves polarity time scale with the geomagnetic field as they are settling through the water column or are in unconsolidated At its most fundamental level, magnetostratigra- sediment. Depositional magnetic grains deposited phy documents the geological record of polarity on the seafloor are then able to lock in and retain changes of the geomagnetic field. The individual the original magnetization in the direction of the normal (black) and reversed (white) (Fig. 1) polarity geomagnetic field during the initial consolidation intervals are known as chrons and typically range of sediments (Tauxe et al. 2006; Tauxe & Yamazaki in duration from 10 kyr to 10 Myr. The transition 2007). However, in some