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How Old Is It? Part 2 – Magnetostratigraphy Introduction In addition to biostratigraphy, the strati- Earth’s Magnetic Field graphic record of the episodic reversals Earth’s magnetic field is somewhat analo- of Earth’s magnetic field is another tool to gous to a bar magnet, in that the field is a establish a stratigraphy and provide age dipole with a north pole and a south pole. information for cored crustal rocks, intru- The magnetic field is induced by an elec- sions, and sedimentary sequences. This tromagnetic current created by fluid mo- is just one attribute of the magnetic sig- tion in the liquid outer core due to Earth’s nature preserved in sediments or oceanic rotation. The north and south magnetic crust (basalt) that can tell us about the poles are close to Earth’s rotational axis, changing nature of Earth’s magnetic field the latter represented by the geographic through time. North Pole and South Pole, but Magnetic The magnetic signal that geoscientists are North and Magnetic South are offset from interested in measuring is called natural true North and true South. remanent magnetization (NMR). This Earth’s magnetic field is dynamic and signal is carried by magnetic minerals complex. The magnetic poles are not such as magnetite. The NMR is preserved stationary. In 2005, the north magnetic in basalt as it cools and crystallizes past pole was at 82.7°N, 114.4°W, moving the Curie temperature (~580°C for the northwest at approximately 40 km/yr, mineral magnetite), or in sediments as while the south magnetic pole (2001) was detrital magnetic mineral grains accumu- at 64.7°S, 138.0°E (www.ngdc.noaa.gov/ late on the seafloor. In both situations, the seg/geomag/faggeom.shtml). Teaching for Teaching magnetic minerals become aligned with the Earth’s magnetic field at the time of In addition, Earth’s magnetic field episodi- crystallization and deposition, respectively. cally reverses polarity. Prior to a reversal of the magnetic field, the field randomizes Not all rocks are good carriers of a mag- and becomes weaker, and there may be Science netic signal. The basalt that makes up multiple poles for a short time. The pro- oceanic crust is a magnetite-bear- ing rock, while granite, a typical • Learning for igneous rock of continental crust, is generally poor in magnetic min- erals. Likewise, marine sediments composed of terrigenous silt and clay derived from the erosion of Life continental rocks have a much higher concentration of detrital TM | www.deepearthacademy.org magnetic mineral grains than does a pure biogenic sediment, such as calcareous or siliceous ooze. Describe the Earth’s magnetic field. What are the key features or characteristics of our Figure 1. Earth’s magnetic field under “normal polarity”. The arrows magnetic field? A sketch may depicting magnetic lines of force would be oriented in the opposite help. direction with a reversed field (reversed polarity). Figure from http:// www.geocities.com/CapeCanaveral/Lab/6488/magfield.html. How Old Is It? Part 2 – Magnetostratigraphy cess is geologically rapid, taking several thousand The Nature of the Earth’s Magnetic Field as years for the field to completely reverse polarity Recorded in Sediments and Rocks and stabilize again. Today’s field is referred to as There are two ancient field directions typically pre- “normal polarity” (Figure 1). The last reversal of served in sedimentary or igneous rocks containing the magnetic field occurred approximately 780,000 magnetic minerals. A reversal of the magnetic field years ago. Refer to http://www.ngdc.noaa.gov/seg/ is seen as an abrupt change in these directions. geomag/faqgeom.shtml#q1 for additional informa- Plate motion over time also affects the field direc- tion about Earth’s magnetic field. tions preserved in rocks. Inclination, or magnetic The ancient magnetic field (paleomagnetism) is dip, represents the angle of magnetization into or measured in rocks and sediments using a magne- out of the Earth’s surface (Figures 1 and 3). tometer. See http://www-odp.tamu.edu/sciops/labs/ The magnetic lines of force are directed into the pmag/ for a detailed discussion of how shipboard Earth in the Northern Hemisphere (positive down), paleomagnetic data are collected.SHIPBOARD SCIENTIFIC PARTYand directed out of the Earth in the Southern CHAPTER 4, SITE 1149 130 Activity Hemisphere (positive up; magnetic field is oriented Figure F66. The inclinations ofupwards). lithologic Unit In Iother (Cores words, 185-1149A-1H today’s through magnetic 13H) compared field (= to the late The figures below depict the Cenozoicmagnetic magnetic character polarity time scale (Berggren et al., 1995). of two deep-sea sediment cores. Site 1149 (Fig- 0 ure 2a) is located in the northwest Pacific (~31°N latitude) near the Marianas Trench. Site 1172 (Fig- ure 2b) is located in the 20 Chron, Chron, southwest Pacific (~44°S subchron Polarity subchron Age latitude) near Tasmania. late Both records have been C1n BRUNHES interpreted by correlating to middle the late Cenozoic Geomag- 0.780 0.78 1 r 0.990 1 netic Polarity Time Scale n 1.070 Jaramillo 1.201 1r Cobb Mountain 40 1n 1.211 (Berggren et al., 1995; C1r PLEISTOCENE 2r early Cande and Kent, 1995). 2r MATUYAMA 1.770 1.77 Teaching for Teaching Make a list of observa- C2n Olduvai 1.950 1 r 2 tions about the paleo- n 2.140 Reunion C2r 2.150 late magnetic record in these 2r two deep-sea sediment 2.581 2.58 60 n GAUSS Science sequences. How are they 1 3.040 Kaena 3 similar, and how are they (mbsf) Depth C2An r 3.110 n 2 3.220 middle different? r 3.330 Mammoth 3n 3.580 3.58 • Learning for C2Ar GILBERT 4 80 4.180 n Cochiti 1 4.290 r 4.480 2 n Nunivak PLIOCENE C3n 4.620 r Life 4.800 3 n 4.890 Sidufjall r 4.980 5 TM 4n Thvera 5.230 | www.oceanleadership.org 5.23 100 C3r 5.890 6 Figure 2a. Interpreted paleomagnetic 1 n C3An late stratigraphy from Site 185-1149 in r 2n MIOCENE the northwest Pacific (31°20.095’N, 143°21.805’E; 5817 m water C3Ar depth). http://www-odp.tamu.edu/ 120 publications/185_IR/chap_04/c4_f66. -90 -50 0 +50 +90 htm#573212. Inclination (°) 2 Life Science | www.oceanleadership.org | for Learning • for Teaching Magnetostratigraphy – 2 Part It? Is Old How TM C S HIPBOARD Figure F16. Long-core measurements from 0 to 100 mbsf for Holes 1172A, 1172B, and 1172C showing inclination, intensity, and interpreted mag- HAPTER netostratigraphy. Hole 1172A Hole 1172B Hole 1172C 7, S Inclination (°) Intensity (mA/m) Age Polarity Depth Inclination (°) Intensity (mA/m) Inclination (°) Intensity (mA/m) S -6 -4 -2 -6 -4 -2 -6 -4 -2 CIENTIFIC 10 10 10 (Ma) (mbsf) 10 10 10 10 10 10 ITE chap_07/c7_f16.htm#382526 (43°57.575’S, 149°55.701’E;2622 mwaterdepth).From inthesouthwestPacific Figure 2b.Interpretedpaleomagnetic stratigraphyfromSite189-1172 0 0 1172 C1n P 0.78 12.5 ARTY 1.77 17.1 1.75 C2n 19.55 20 20 . 2.58 30.9 C2An.1n 3.04 36.9 3.11 C2An.2n 38.6 40 3.22 40.2 40 3.33 41.7 C2An.3n 3.58 46.25 3 4.18 Depth(mbsf) 57.8 4.29 C3n.1n 60 http://www-odp.tamu.edu/publications/189_IR/ 60 60 4.48 63.4 4.62 C3n.2n 66.6 C3n.4n 5.23 78.9 80 80 5.89 94.1 C3An.1n 100 6.13 100.75 100 6.26 105.4 6.56 C3An.2n 107.4 -90 0 90 -90 0 90 -90 0 90 Normal polarity Reverse polarity No recovery 71 How Old Is It? Part 2 – Magnetostratigraphy “normal polarity”) is represented by positive inclina- Ships towing magnetometers across the ocean tion in the Northern Hemisphere, and by negative basins during the post-WWII era recorded patterns inclination in the Southern Hemisphere. of greater and lesser magnetic intensity relative to Near the equator, the magnetic lines of force are the present field. These positive and negative mag- parallel to the Earth’s surface. In this way, magnet- netic anomalies represent linear bands of alternat- ic inclination is a function of latitude; ~90° inclina- ing polarity that trend parallel to the spreading axis tion at the north and south magnetic pole, ~0° at with a mirror image pattern on the opposite side of the equator, and intermediate values in between. the ridge. The positive anomalies correspond with Inclination is used to determine the paleolatitude of a magnetic polarity as today which amplifies the igneous or sedimentary rocks and sediments at the magnetic intensity, while the negative anomalies time of crystallization or deposition. correspond with a reversed polarity, which partially dampens out the influence of the present-day field. Declination, or azimuth, is the angle between true These observations led to a test of the seafloor north and the horizontal trace of the magnetic field spreading hypothesis (Vine and Matthews, 1963; for your location (Figure 4). It represents the direc- Morley, 1963) advanced by Dietz (1961) and Hess tion of magnetization (today’s field is in a northerly (1962). Heirtzler et al. (1968) created the first direction (+/- 0°), whereas during a reversed field, GPTS by correlating a single marine magnetic the declination would be in a southerly direction anomaly profile from the South Atlantic to radio- (+/- 180°). Declination can only be determined on metrically dated terrestrial reversal sequences. sediment cores that have been oriented during cor- ing with respect to the present day magnetic field For the Late Cretaceous to present, the prominent using the Tensor tool.
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