Magnetic Methods and the Timing of Geological Processes

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Magnetic methods and the timing of geological processes

L. JOVANE1*, L. HINNOV2, B. A. HOUSEN3 & E. HERRERO-BARVERA4 1Instituto Oceanogra´fico da Universidade de Saõ Paulo, Fisica, Geoquimica e Geologia, Praça do Oceanogra´fico 191, Saõ Paulo, Saõ 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 stratigraphic 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 Thus, magnetostratigraphy no longer represents a dating tool based only on the geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of all geomagnetic ﬁeld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 polarity 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 coercivity; and, the most important in sediments, (5) detrital 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 deep-sea sediments, a from a reversed-polarity chron to a normal-polarity time delay of magnetization has been occasionally chron and vice-versa is very short (+5 kyr). This observed and is attributed to very low sedimenta- allows a numerical age to be assigned to each rock tion rates and delayed lock-in below the sedi- unit containing a polarity reversal within a strati- ment–water interface (Verosub 1977; Suganuma graphic succession. Since polarity reversals effec- et al. 2011). This magnetization acquired near tively occur simultaneously over the whole surface but not directly at the time of deposition is referred of the Earth, they can be used for global time corre- to as post-depositional remanent magnetization lation. Shorter periods of opposing polarity within (pDRM). There are also some conditions in which a chron are called, depending on duration, ‘sub- magnetic crystals produced by magnetotactic bac- chrons,’ ‘microchrons’ and ‘cryptochrons’ (Cande teria may record the geomagnetic field (Petersen & Kent 1992). Longer periods with dominant sin- et al. 1986; Housen & Moskowitz 2006; Vasiliev gle polarity are called ‘superchrons’ or ‘mega- et al. 2008; Yamazaki 2009; Roberts et al. 2011, chrons,’ also depending on duration (Opdyke & 2012; Jovane et al. 2012). Channell 1996). The cause of the geomagnetic field reversals is Chrons are conventionally labelled and named still unknown, although geodynamical modelling after the corresponding seafloor spreading magnetic

Fig. 1. Example of the GPTS for the past 100 million years, from TSCreator 5.0 (www.tscreator.org). Courtesy of J. Ogg. (2012). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

L. JOVANE ET AL. anomaly number (Cande & Kent 1992). Chron num- Such geomagnetic excursions have been ber is usually suffixed by the letter n or r, depending reported in geological recorders such as lava flows on whether the dominant magnetic polarity is of various ages in different parts of the world as normal or reversed, and is prefixed, for instance, well as from deep-sea, lake sediment and sedimen- by the letter C for Cenozoic, or M for Mesozoic tary rocks. This type of geomagnetic feature gener- (Gee & Kent 2007). The most recent reversal of ally are observed to start with a sudden and often the geomagnetic field occurred 781 kyr ago and is fairly smooth movement of the virtual geomagnetic the boundary between Brunhes (C1n) and Matu- poles (VGP) toward equatorial latitudes. The VGP yama (C1r) Chrons. Other normal and reversed may then return almost immediately, or it may chrons, Gauss (C2n) and Gilbert (C2r), contain dis- cross the equator and move through latitudes in tinctive subchrons, such as the Olduvai Subchron the opposite direction before travelling back again (C1r.2n) within the Matuyama Chron. The GPTS is to resume a near-axial position. The term ‘excur- continually updated to provide the most accurately sion’ was defined to describe a VGP movement of known ages for the chron boundaries (e.g. Fig. 1). more that 408 from the geographic pole for inter- Vector component diagrams (Zijderveld 1967) mediate pole positions that end up with a return of are used display stepwise demagnetization data. the Earth’s field to its pre-existing polarity (Barbetti When a series of rock layers shows the same sign & McElhinny 1976). During an excursion, the of inclination of the characteristic remanent magne- dynamo does not establish itself in the opposite tization (ChRM), it is called a magnetozone (or polarity. Defined in this way, excursions are dis- magnetostratigraphic unit), because within this tinguished from secular variation (when the VGP interval a single polarity is constant. Magnetostrati- colatitude is 108 , u ,408) and from short polarity graphy correlates magnetozones to the GPTS. episodes, which is a term applied when the opposite Namely, inclination, declination and intensity of polarity (u is ,408 or .1408) persists sufficiently the ChRM for each sample are examined through long for at least one oscillation in the strength of a principal component analysis (PCA) of demagne- the main dipole (about 104 years, Jacobs 1984). tization steps (Kirschvink 1980). Demagnetization Several short and almost complete changes in can be accomplished in different ways for different geomagnetic inclination have occurred within the magnetic components. Stepwise thermal (TH) and present-day Brunhes Chron. These rapid and global alternating field (AF) demagnetizations are the geomagnetic events are called ‘excursions’ or most popular methods. Chemical and pressure ‘aborted reversals’ (Lund et al. 2006; Laj & Chan- demagnetization methods can also be applied to nell 2007). Among these excursions, the Laschamp erase unwanted secondary components. (40–41 ka), Blake (c. 115–120 ka) and the Prin- The calculated PCA inclination values some- gle Falls (c. 211–218 ka) are the most impor- times do not represent the true inclination of the tant geomagnetic events because they have been geomagnetic field at that time of the formation or widely documented (e.g. Valet & Meynadier 1998; deposition of the rock unit (Butler 1992; Tauxe Guyodo & Valet 1999; Valet et al. 2008). Because 2010). This problem is related to geological excursion events have been recognized further factors that can be resolved using other magnetic back in time (e.g. Handschumacher et al. 1988; methods. These factors may be related to diagenetic Sager et al. 1998; Tivey et al. 2006), we can infer compaction or rotation of tectonic plates and tilt of that they occurred probably also during older structural blocks. chrons. These excursions, sometimes called tiny wiggles (e.g. Lanci & Lowrie 1997), cannot yet be used as time constrain. Beyond classical magnetostratigraphy Relative palaeointensity Dating with other recorded features of the geomagnetic field can also be undertaken, as follows. The Earth’s magnetic field can be simplified as a big dipole with the poles at the geographical poles, Excursions and aborted reversals which is called the geocentric axial dipole (GAD). The behaviour of the geomagnetic field is complex It is very well known from palaeomagnetic records and is related not only to the GAD but also to today that, in addition to polarity changes, the other components that change at different time- Earth’s magnetic field has experienced changes scales: years (secular variation), millennia (excur- from its regular near-axial configuration for brief sions) and millions of years (reversals) (Gee & periods of time without establishing, and perhaps Kent 2007). not even approaching, a reversed state of the palaeo- Several short and almost complete changes in field. These types of behaviour are called geomag- geomagnetic inclination have occurred within the netic excursions or ‘aborted reversals’. present-day Brunhes Chron. These rapid and Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

MAGNETOSTRATIGRAPHY: ONLY A DATING TOOL? global geomagnetic events are called ‘excursions’ palaeointensity. Measurements of palaeointensity or ‘aborted reversals’ (Lund et al. 2006; Laj & in different geological sequences (e.g. lava flows, Channell 2007). An important long-term (105 year) or marine or lake sediments) show a consistent variation of spherical parameters (inclination, decli- pattern, allowing the reconstruction of a relative nation and intensity) of the geomagnetic field is the palaeointensity curve for the past 2 myr (Fig. 2; secular variation (Valet et al. 2008), which includes Sint-2000; Valet et al. 2005). Sint-2000 is a stack geomagnetic jerks, westward drift and palaeointen- curve of independent palaeointensity records from sity. Geomagnetic jerks are abrupt changes in one various latitudes for the last 2 myr. It is possible of the geomagnetic components related to inner to date a geological sequence by correlating the core flow patterns or major earthquakes (Mandea reference Sint-2000 intensity record to the palaeoin- et al. 2000; Florindo et al. 2005). The secular vari- tensity record of the studied sequence. This is poss- ation, which is not constant and uniform on the ible only when the magnetic minerals along the Earth, is mainly related today to a westward drift section are relatively uniform (King et al. 1983; of about 0.28 per year since 1400 AD, and an east- Valet & Meynadier 1998), and when the magnetic ward drift from 1000 to 1400 AD (Dumberry & intensity has been previously normalized for the Finlay 2007). The variation in intensity of the geo- concentration and grain-size of magnetic material magnetic field through geological time is known as (thus ‘relative’), which may be climatically driven

Relative paleointensity arbitrary units arbitrary units 0 0 0 Laschamp Blake 0.1 Iceland Basin Pringle Falls 0.2 0.2 0.2 0.3

0.4 0.4 0.4

Calabrian Ridge, Tarrafal 0.5 Big Lost 0.6 0.6 0.6 0.7 Brunhes-Matuyama 0.8 0.8 0.8

Kamikatsura 0.9 Santa Rosa ODP Sites 983-984 Upper Jaramillo 1.0 1

Punaruu 1.1 Cobb Mountain 1.2 1.2 Bjorn 1.3

1.4 1.4 Gardar 1.5 Gilsa 1.6 1.6

1.7 Upper Olduvai 1.8 1.8

1.9 Lower Olduvai 2.0 2 Age (ka) Age (ka) Sint-2000

Fig. 2. Polarity column showing the position of the successive excursions and reversals with respect to the ﬂuctuations of dipole ﬁeld intensity derived from the composite Sint-2000 record and from ODP sites 983 (red line) and 984 (blue line) in the northeastern Atlantic Ocean (arbitrary units increasing toward right) (Guyodo & Valet 1999; Valet et al. 2008). Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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(e.g. Valet & Herrero-Barvera 2000; Channell et al. by geomagnetic components at the time of depo- 2002; Yamazaki 2008). It is worth noting that severe sition or solidification (Tauxe 2010). Consequently, declines in relative intensity coincide with geomag- magnetic susceptibility may not be isotropic, giv- netic reversals and excursions (Fig. 2; Valet et al. ing different values when measured in different 2008). directions. In such cases, the anisotropy of magnetic Another application is the scatter of inclination susceptibility (AMS) is calculated as a tensor by and declination on a sphere, or angular dispersion comparing the magnetic susceptibility values in (S) (e.g. Jovane et al. 2008). S is related to latitude, three perpendicular directions, which produce a with higher values near the poles, and may indicate matrix representing the ellipsoid of the magnetic the geomagnetic stability of core vortices (Olson & susceptibility (e.g. Hrouda 1982). This ellipsoid Aurnou 1999; Aurnou et al. 2003). of the magnetic fabric can be spherical, oblate (flattened) or prolate (cigar-shaped) providing in- Tectonics and deformation formation, for example, on the direction of a palaeo- current (e.g. Ellwood & Whitney 1980) or lava flow While palaeomagnetic data have been used for a (e.g. Stacey 1960; Ellwood 1978), strain patterns long time to examine plate-scale to regional/local (e.g. Goldstein 1980), fabric/structure of granites tectonic and structural problems (e.g. Irving 1963, (e.g. Ellwood & Whitney 1980) and the degree of Beck 1981; Van der Voo 1988), a number of compaction of strata studying the relation between recent studies have combined magnetostratigra- inclination shallowing and oblation of the fabric phic and palaeomagnetic analyses to examine (e.g. Tan & Kodama 2002). timing and temporal variations in structural processes. This approach is made possible by the Environmental magnetism collection of more samples per site (or single geological bed) and more sites than is typical for stan- Environmental magnetism is used to identify dard magnetostratigraphic studies (e.g. Zhao et al. and characterize magnetic mineralogy, and pro- 2001; Liu et al. 2003; Titus et al. 2011). This tech- vides constraints for interpreting palaeomagnetic nique has the potential to test velocity models of results and in understanding the factors that control plate-margin and fault-zone deformation that are environmental change. Environmental magnetic based primarily on Global Positioning System measurements are inexpensive, rapid and non- (GPS) data (e.g. McCaffrey 2005). GPS defor- destructive, and provide fundamental information mation studies utilize short (decades at most) on the size, abundance and composition of magnetic time-series of GPS displacement data and their pre- minerals (Verosub & Roberts 1995; Kodama 2012; dictions of vertical-axis rotation can be compared Liu et al. 2013). These measurements can be per- with observations obtained from palaeomagnetic formed along a sedimentary sequence at high- data (e.g. Titus et al. 2011). resolution and include (1) magnetic susceptibility, Magnetic inclination and declination track the and artificial remanences; (2) anhysteretic remanent position of palaeopoles through geological time magnetization (ARM); (3) isothermal remanent (Irving 1956). A reconstructed sequence of palaeo- magnetization (IRM) and back-field isothermal poles for a plate is known as an Apparent Polar remanent magnetization (BIRM); and (4) coerci- Wander Path (APWP). We define the APWP tive-dependent parameters (S-ratio and ‘hard’ IRM through study of ChRM, which points to the VGP referred to as HIRM). of the original position of a rock at the time it was Low-field magnetic susceptibility (x ¼ nor- formed. APWPs of the major tectonic plates have malized for weight, k ¼ normalized for volume) been reconstructed through much of Phanerozoic represents how much magnetization (m) a material time (e.g. Besse & Courtillot 2002; Kearey et al. retains when a magnetic field (H ) is applied 2009), but there are still large uncertainties in the (Table 1). Thus it is a complex parameter reflecting APWPs of minor plates and in reference poles the sum of magnetic materials, such as ferromag- (McElhinny & McFadden 2000). netic sensu lato (e.g. magnetite), antiferrimagnetic Magnetic anisotropy studies can help define (e.g. hematite) and non-magnetic materials, such shallowing effects related to diagenetic compaction, as paramagnetic (e.g. silicates or clays) and diamag- flow direction during deposition or cooling and netic (e.g. quartz or carbonate). Results from low- deformation of rocks owing to tectonic stresses. field magnetic susceptibility must be interpreted The larger datasets generated from these studies with caution since it can be related to numerous can be utilized to test and resolve other important processes. aspects of how sedimentary rocks record the geo- Anhysteretic remanent magnetization activates magnetic field. The inclination, declination and only the finest magnetic minerals, frequently intensity of the DRM of sedimentary magnetic single domain (SD) grains, which do not have a grains, and the TRM of lava flows, can be affected domain wall and are uniformly magnetized. It is, Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

MAGNETOSTRATIGRAPHY: ONLY A DATING TOOL?

Table 1. Units and conversions in the Syste`me Internationale d’unite´s (SI) and the old CGS

CGS SI

Energy 107 erg 1 joule (J) Force (F)105 dyne 1 Newton (N) Current (I) 0.1 adamp 1 ampere (A) Magentic Inducion (B)104 gauss (G) 1 Tesla (T) Magnetic Field (H )4p × 1023 oersted (Oe) 1 A m21 Magnetic Moment (M)103 emu 1 A m2 Magnetization/volume (m)1023 emu (¼Gcm23)1Am21 Magnetization/mass (J ) 1 emu g21 or G cm23 g21 1Am2 kg21 Magnetic susceptibility/volume (k) Dimensionless Dimensionless Magnetic susceptibility/mass (x)1023 cm3 g21 1m3 kg21

From Butler (1992) and Collinston (1983). consequently, a proxy for the concentration of fine sequences can be evaluated as time series and magnetite of eolian, biogenic or impactoclastic related to palaeoclimatic and palaeoenvironmental origin, or from other environmental processes. Iso- change. In particular, palaeoclimate change at thermal remanent magnetization activates all mag- 105 –106 year scales – often resolved in the sedi- netic grains up to saturation (also called SIRM or mentary record – has been strongly modulated by Mrs) that is usually at 1 T or 900 mT (Table 1). astronomically forced insolation (e.g. Berger 1988; IRM, therefore, is the main proxy for magnetic con- Laskar et al. 2004, 2011; Table 2). The record of centration. The ratio between ARM and IRM astronomical forcing frequencies in palaeomagnetic (ARM/IRM900) provides a proxy for magnetic proxies can be used as a tool to perform astronomi- grain size. Then, smaller IRMs can be applied back- cal tuning and develop continuous timescales along wards to IRM, which are called back-field IRM stratigraphic sequences (Hinnov & Hilgen 2012). (BIRM), to produce IRM ratios that supply information on magnetic composition. These are the so-called S-ratios (King & Channell 1991) and Organization of this special volume HIRMs (imparted at 300 and 100 mT, respectively). They are calculated as, for example, S-ratio300 ¼ Following three conference sessions entitled ‘Mag- BIRM300/IRM900, S-ratio100 ¼ BIRM100/IRM900, netostratigraphy: not only a dating tool’ and pre- HIRM300 ¼ (IRM900 + BIRM300)/2 or HIRM100 ¼ sented at the AGU Meeting of Americas (Foz de (IRM900 + BIRM100)/2 to indicate the different Iguaçu, Brazil) in 2010 and AGU Fall Meetings responses of high-coercivity magnetic minerals. The 2010, 2011 (San Francisco, USA), we decided to presence of hematite can be determined with the assemble the presentations into a Special Publi- proxy IRM900@AF120 mT for the concentration cation entitled Magnetic Methods and the Timing of hematite by AF demagnetizing the IRM900 of Geological Processes. In this volume, we step at 120 mT (Larrasoanã et al. 2003). Liu present research that includes innovations in classi- et al. (2007) also proposed the L-ratio (IRM900 + cal magnetostratigraphy and emerging techniques BIRM300)/(IRM900 + BIRM100) to define how the that use sequential rock magnetic measurements to hardness of hematite can affect HIRM and the define chronology in stratigraphy. We introduce S-ratio. applications that use magnetic direction and geo- Environmental magnetic records obtained by magnetic polarity reversals to infer geodynamo finely sampled measurements along sedimentary processes, tectonics, diagenesis and climate change.

Table 2. Periods and relative amplitudes of the main astronomical forcing cycles for 0–10 Ma (Laskar et al. 2004; Hinnov & Hilgen 2012). The frequencies and relative amplitudes change over different time intervals, and relative amplitudes change additionally as the result of climatic ﬁltering

Astronomical parameter Period in kiloyears (relative amplitude)

Orbital eccentricity 405 (1.0) 131 (0.43) 124 (0.56) 99 (0.48) 95 (0.74) Obliquity (tilt) 53.04 (0.34) 40.80 (1.0) 40.11 (0.33) 39.45 (0.42) 29.75 (0.16) Precession index 23.61 (1.0) 22.33 (0.87) 19.07 (0.45) 19.12 (0.69) 16.44 (0.08) Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021

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Finally, we present examples of astronomical forc- in press). Finally, VGP trajectories measured on the ing of environmental magnetism in the sedimentary Lower Cretaceous Serra Geral volcanic sequence record, and their utility in deﬁning high-resolution are interpreted to indicate anisotropy in the Earth’s timescales. interior (Caminha-Maciel & Ernesto 2012).

Part 1: integrated magnetostratigraphy Part 4: palaeoclimatic change from rock When integrated with biostratigraphy, cyclostrati- magnetic proxies graphy, chemostratigraphy and geochronology, The studies in this section resolve astronomical magnetostratigraphy provides valuable timescale timescales of palaeoclimatic change using rock information. In this section, modern integrated magnetism stratigraphy evaluated from a wide magnetostratigraphy is demonstrated with marine variety of marine environments: Plio-Pleistocene Cenozoic studies. New Oligocene–Miocene magne- marginal marine, Cretaceous carbonate platform, tostratigraphy from the equatorial Pacific pinpoints Permian lower slope and basinal carbonates, the Oligocene–Miocene transition at the base of Permo-Carboniferous glaciogenic rhythmites, and C6Cn.2n, and identifies three new excursions (Gui- Ordovician shallow shelf limestone/mudstone. dry et al. 2012). New work is presented on the The Plio-Pleistocene study astronomically tunes a Eocene climatic optimum interval (50–55 Ma) and magnetic susceptibility series from a sedimentary Late Eocene–Oligocene transition into the Ceno- section in northern Italy that otherwise exhibits no zoic icehouse (c. 33.5 Ma) from Integrated Ocean cyclicity (Gunderson et al. 2012). The Cretaceous Drilling Progam sites and Italian sections (Firth study (Hinnov et al., this volume, in press) finds et al. 2012; Savian et al. 2013; Jovane et al. 2013), that ARM cyclicity is independent from host car- and a composite integrated magnetostratigraphic bonate platform cycles in northeastern Mexico; the sequence from Italy is provided for the entire former is linked to a precession-forced eolian Palaeogene Period (Coccioni et al. 2012). dust flux and the latter to much lower frequency sea- level fluctuations. Ellwood et al. (2012a) develop Part 2: dating tectonic processes with obliquity-tuned ‘floating timescales’ for magnetic magnetic methods susceptibility variations along Middle Permian sections, Texas, to assess sedimentation rates and dur- The evolution of palaeomagnetic declination in ations of geological events. Franco & Hinnov Chinese continental stratigraphy reveals block (2012) evaluate anisotropy of magnetic suscepti- rotation, uplift and basin infilling related to the Cen- bility in Brazilian Permocarboniferous glacial ozoic India–Asia collision, as discussed by three rhythmites as a palaeoclimate indicator. Finally, papers in this section (Yan et al. 2012a, b; Fang the cyclic Ordovician Kope Formation, USA, is et al. 2012). The timing of these declination evaluated for astronomical frequencies through changes is constrained by magnetic reversal strati- analysis of a composite high-resolution magnetic graphy. A fourth paper (Zhao et al. 2013) analyses susceptibility series (Ellwood et al. 2012b). tectonic-driven sedimentation change in the Nan- kai Trough, Japan, with the assistance of integrated We gratefully acknowledge the review of G. Acton, M. magnetostratigraphy. Ernesto, A. Malinverno and X. Zhao.

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