How Old Is It?
Part 2 – Magnetostratigraphy
Introduction
In addition to biostratigraphy, the stratigraphic record of the episodic reversals
of Earth’s magnetic field is another tool to
establish a stratigraphy and provide age
information for cored crustal rocks, intrusions, and sedimentary sequences. This
is just one attribute of the magnetic signature preserved in sediments or oceanic
crust (basalt) that can tell us about the
changing nature of Earth’s magnetic field
through time.
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The magnetic signal that geoscientists are
interested in measuring is called natural
remanent magnetization (NMR). This
signal is carried by magnetic minerals
such as magnetite. The NMR is preserved
in basalt as it cools and crystallizes past
the Curie temperature (~580°C for the
mineral magnetite), or in sediments as
detrital magnetic mineral grains accumulate on the seafloor. In both situations, the
magnetic minerals become aligned with
the Earth’s magnetic field at the time of
crystallization and deposition, respectively.
Not all rocks are good carriers of a magnetic signal. The basalt that makes up
oceanic crust is a magnetite-bearing rock, while granite, a typical
igneous rock of continental crust,
is generally poor in magnetic minerals. Likewise, marine sediments
composed of terrigenous silt and
clay derived from the erosion of
continental rocks have a much
higher concentration of detrital
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
magnetic field? A sketch may
help.
Earth’s Magnetic Field
Earth’s magnetic field is somewhat analogous to a bar magnet, in that the field is a
dipole with a north pole and a south pole.
The magnetic field is induced by an electromagnetic current created by fluid motion in the liquid outer core due to Earth’s
rotation. The north and south magnetic
poles are close to Earth’s rotational axis,
the latter represented by the geographic
North Pole and South Pole, but Magnetic
North and Magnetic South are offset from
true North and true South.
Earth’s magnetic field is dynamic and
complex. The magnetic poles are not
stationary. In 2005, the north magnetic
pole was at 82.7°N, 114.4°W, moving
northwest at approximately 40 km/yr,
while the south magnetic pole (2001) was
at 64.7°S, 138.0°E (www.ngdc.noaa.gov/
seg/geomag/faggeom.shtml).
In addition, Earth’s magnetic field episodically reverses polarity. Prior to a reversal
of the magnetic field, the field randomizes
and becomes weaker, and there may be
multiple poles for a short time. The pro-
Figure 1. Earth’s magnetic field under “normal polarity”. The arrows
depicting magnetic lines of force would be oriented in the opposite
direction with a reversed field (reversed polarity). Figure from http://
www.geocities.com/CapeCanaveral/Lab/6488/magfield.html.
The Nature of the Earth’s Magnetic Field as
Recorded in Sediments and Rocks
There are two ancient field directions typically preserved in sedimentary or igneous rocks containing
magnetic minerals. A reversal of the magnetic field
is seen as an abrupt change in these directions.
Plate motion over time also affects the field directions preserved in rocks. Inclination, or magnetic
dip, represents the angle of magnetization into or
out of the Earth’s surface (Figures 1 and 3).
middle
0.780
0.78
1.770
r
n
1.77
Olduvai
2.140
Reunion
2.150
2
2r
2.581
n
GAUSS
1
C2An
2
3.040
3.110 Kaena
3.220
3.330 Mammoth
r
n
r
3n
2.58
3
middle
60
3.58
3.580
GILBERT
C2Ar
PLIOCENE
Depth (mbsf)
1
C2r
1.950
late
C2n
4
80
1
C3n
2
3
4.180
Cochiti
4.290
n
r
4.480
n
r
n
r
4.620
100
Nunivak
4.800
4.890 Sidufjall
4.980
4n
TM
5.230
Thvera
5
5.23
C3r
5.890
Figure 2a. Interpreted paleomagnetic
stratigraphy from Site 185-1149 in
the northwest Pacific (31°20.095’N,
143°21.805’E; 5817 m water
depth). http://www-odp.tamu.edu/
publications/185_IR/chap_04/c4_f66.
htm#573212.
C3An
1
r
2n
C3Ar
120
-90
-50
0
+50
Inclination (°)
2
+90
n
6
late
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Make a list of observations about the paleomagnetic record in these
two deep-sea sediment
sequences. How are they
similar, and how are they
different?
early
0.990
1.070
1.201
1.211
PLEISTOCENE
The ancient magnetic field (paleomagnetism) is
measured in rocks and sediments using a magnetometer. 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
Hemisphere (positive up; magnetic field is oriented
Activity
In Iother
today’s
magnetic
field (=to the late
Figure F66. The inclinations ofupwards).
lithologic Unit
(Coreswords,
185-1149A-1H
through
13H) compared
The figures below depict the magnetic
character
Cenozoic magnetic
polarity time scale (Berggren et al., 1995).
of two deep-sea sediment
0
cores. Site 1149 (Figure 2a) is located in the
northwest Pacific (~31°N
latitude) near the Marianas
Trench. Site 1172 (Figure 2b) is located in the
Chron,
Chron,
20
southwest Pacific (~44°S
Age
subchron
subchron
Polarity
latitude) near Tasmania.
late
Both records have been
C1n
BRUNHES
interpreted by correlating to
the late Cenozoic Geomagr
1
1
n
netic Polarity Time Scale
Jaramillo
1r
Cobb Mountain
1n
40
(Berggren et al., 1995;
C1r
2r
2r
MATUYAMA
Cande and Kent, 1995).
MIOCENE
How Old Is It? Part 2 – Magnetostratigraphy
cess is geologically rapid, taking several thousand
years for the field to completely reverse polarity
and stabilize again. Today’s field is referred to as
“normal polarity” (Figure 1). The last reversal of
the magnetic field occurred approximately 780,000
years ago. Refer to http://www.ngdc.noaa.gov/seg/
geomag/faqgeom.shtml#q1 for additional information about Earth’s magnetic field.
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Depth (mbsf)
3
-90
100
80
60
40
20
0
0
90
10-6
10-4
Normal polarity
Reverse polarity
C3An.2n
Hole 1172B
Hole 1172C
90
No recovery
0
10-6
10-4
-90
10-2
0
90
10-6
10-4
10-2
Inclination (°) Intensity (mA/m) Inclination (°) Intensity (mA/m)
-90
105.4
107.4
80
60
40
6.26
6.56
94.1
78.9
57.8
60
63.4
66.6
36.9
38.6
40.2
41.7
46.25
30.9
100.75 100
C3An.1n
C3n.4n
C3n.2n
C3n.1n
C2An.3n
C2An.2n
C2An.1n
20
0
6.13
5.89
5.23
4.18
4.29
4.48
4.62
3.04
3.11
3.22
3.33
3.58
2.58
17.1
19.55
C2n
Depth
(mbsf)
12.5
C1n
Polarity
1.77
1.75
Age
(Ma)
0.78
10-2
Inclination (°) Intensity (mA/m)
Hole 1172A
Figure F16. Long-core measurements from 0 to 100 mbsf for Holes 1172A, 1172B, and 1172C showing inclination, intensity, and interpreted magnetostratigraphy.
How Old Is It? Part 2 – Magnetostratigraphy
Figure 2b. Interpreted paleomagnetic stratigraphy from Site 189-1172 in the southwest Pacific
(43°57.575’S, 149°55.701’E; 2622 m water depth). From http://www-odp.tamu.edu/publications/189_IR/
chap_07/c7_f16.htm#382526.
How Old Is It? Part 2 – Magnetostratigraphy
“normal polarity”) is represented by positive inclination in the Northern Hemisphere, and by negative
inclination in the Southern Hemisphere.
Ships towing magnetometers across the ocean
basins during the post-WWII era recorded patterns
of greater and lesser magnetic intensity relative to
the present field. These positive and negative magnetic anomalies represent linear bands of alternating polarity that trend parallel to the spreading axis
with a mirror image pattern on the opposite side of
the ridge. The positive anomalies correspond with
a magnetic polarity as today which amplifies the
magnetic intensity, while the negative anomalies
correspond with a reversed polarity, which partially
dampens out the influence of the present-day field.
These observations led to a test of the seafloor
spreading hypothesis (Vine and Matthews, 1963;
Morley, 1963) advanced by Dietz (1961) and Hess
(1962). Heirtzler et al. (1968) created the first
GPTS by correlating a single marine magnetic
anomaly profile from the South Atlantic to radiometrically dated terrestrial reversal sequences.
Near the equator, the magnetic lines of force are
parallel to the Earth’s surface. In this way, magnetic inclination is a function of latitude; ~90° inclination at the north and south magnetic pole, ~0° at
the equator, and intermediate values in between.
Inclination is used to determine the paleolatitude of
igneous or sedimentary rocks and sediments at the
time of crystallization or deposition.
Declination, or azimuth, is the angle between true
north and the horizontal trace of the magnetic field
for your location (Figure 4). It represents the direction of magnetization (today’s field is in a northerly
direction (+/- 0°), whereas during a reversed field,
the declination would be in a southerly direction
(+/- 180°). Declination can only be determined on
sediment cores that have been oriented during coring with respect to the present day magnetic field
using the Tensor tool.
For the Late Cretaceous to present, the prominent
positive anomalies (i.e., those corresponding to
time intervals, or chrons, of normal geomagnetic
polarity) have been numbered from Chron 1n in the
central axis of the spreading centers to Chron 34n
at the younger end of the Cretaceous Long Normal
(also called the Cretaceous Quiet Zone; 83.5 Ma).
The suffix “n” after the anomaly number refers to
normal polarity, and the “r” refers to reversed polarity. Many of the younger chrons are divided into
shorter polarity intervals, or subchrons.
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The inclination and declination data collected on
rock or sediment cores can be used to determine
the ancient polarity of the earth, or paleomagnetism. Inclination data are widely used for paleomagnetic studies. However, near the equator,
paleomagnetists rely heavily on declination data
because inclination is so low. See the Integrated
Ocean Drilling Program (IODP) document describing the Paleomagnetism Laboratory aboard the
JOIDES Resolution for a discussion of magnetic
overprinting and what demagnetization techniques
are used to remove this overprint from deep-sea
cores (http://www.oceanleadership.org/learning/
classroom/lab_briefs.html).
The GPTS based on marine magnetic anomalies
extends back through the late Middle Jurassic
(Callovian stage, ~162 Ma; e.g., Kent and Gradstein, 1986), while land-based polarity records go
back through the Triassic (e.g., Gradstein et al.,
1995). A nomenclature called the “M-sequence”
is used for marine magnetic polarity chrons of the
Early Cretaceous to late Middle Jurassic extending
from Chron M0r at the base the Cretaceous Long
Normal (Chron 34n; base of the Aptian stage in the
Early Cretaceous, ~121 Ma) to Chron M39 in the
Jurassic (Callovian stage).
Development of the Geomagnetic Polarity Time
Scale (GPTS)
Sequences of magnetic reversals measured in
deep-sea cores can be correlated with the Geomagnetic Polarity Time Scale (GPTS) in order to
determine the age and sedimentation history of a
drill site (Figure 5). The Geomagnetic Polarity Time
Scale is based on the marine magnetic anomaly
sequence recorded in the South Atlantic Ocean,
which has then been compared with other ocean
basin sequences (Cande and Kent, 1992, 1995).
This is the currently accepted GPTS for the later
part of the Cretaceous Period and Cenozoic Era
(0-84 Ma; Ma = mega-annums). Nine radiometric
age tie points, plus the zero-age ridge axis, are
used to calibrate this part of the timescale (ages
for the marine magnetic anomalies between the
calibration points are interpolated by a cubic spline
function; Berggren et al., 1995).
Berggren et al. (1995) correlated Cenozoic calcareous microfossil datum events (calcareous nannofossil and planktic foraminifera) and zonations
to the GPTS, which serves as the modern standard for biochronology of the past 65 million years
(Figure 6). Astrochronologic calibration (“orbital
tuning”) of the GPTS polarity boundaries based on
Milankovitch cyclicity in continuous marine sections with excellent magneto- and biostratigraphy
has been applied to the Pliocene and Pleistocene
(Berggren et al., 1995, and references therein) and
older Neogene sequences (e.g., Gradstein, Ogg,
4
How Old Is It? Part 2 – Magnetostratigraphy
Smith et al., 2004, and references therein). More
recently, the International Commission on Stratigraphic has published the latest time scale for the
Phanerozoic Eon, which succeeds the 1989 Harland et al. timescale (Gradstein, Ogg, Smith et al.,
2004; http://www.stratigraphy.org).
reversals, and motions of the ocean floor and
continents. Journal of Geophysical Research,
73:2119-2136.
Hess, H.H., 1962. History of the ocean basins. In,
Petrologic Studies, A Volume in Honor of A.F.
Buddington, Geological Society of America, p.
599-620
Examples of paleomagnetic data collected from
two Ocean Drilling Program sites and their interpretations (correlation to the Geomagnetic Polarity
Timescale) are shown in Figures 7 and 9. These
data provide excellent age control for the two
sites. Age-depth plots based on the paleomagnetic
reversal ages are shown in Figures 8 and 10. Agedepth plots are used to quantify and graphically
depict the sedimentation rate history of the sites.
Kent, D.V., and Gradstein, F.M., 1986. A Jurassic to recent chronology. In, The Geology of
North America, Western North Atlantic Region,
Volume M, Geological Society of America, p.
45-50.
Vine, F.J., and Matthews, D.H., 1963. Magnetic anomalies over oceanic ridges. Nature,
199:947-949.
References
Activity by
Berggren, W.A., Kent, D.V., Swisher, C.C., III,
and Aubry, M.-P., 1995. A revised Cenozoic
geochronology and chronostratigraphy. In,
Geochronology, Time Scales, and Global
Stratigraphic Correlation, SEPM (Society for
Sedimentary Geology) Special Publication 54,
p. 129-212.
Kristen St. John, James Madison University
([email protected]), and
R. Mark Leckie, University of MassachusettsAmherst ([email protected]).
Adapted from School of Rock materials by Leckie
and St. John, 2005.
Cande, S.C., and Kent, D.V., 1992. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. Journal of Geophysical
Research, 97:13,917-13,951.
TM
Teaching for Science • Learning for Life | www.oceanleadership.org
Cande, S.C., and Kent, D.V., 1995. Revised calibration of the geomagnetic polarity timescale
for the late Cretaceous and Cenozoic. Journal
of Geophysical Research, 100:6093-6095.
Dietz, R.S., 1961. Continent and ocean basin
evolution by spreading of the sea floor. Nature,
190:854-857.
Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., Van Veen, P., Thierry, J., and Huang,
Z., 1995. A Triassic, Jurassic, and Cretaceous
time scale. In, Geochronology, Time Scales,
and Global Stratigraphic Correlation, SEPM
(Society for Sedimentary Geology) Special
Publication 54, p. 95-126.
Gradstein, F.M., Ogg, J.G., Smith, A.G., et al.,
2004. Geologic Time Scale 2004. International
Commission on Stratigraphy, Cambridge University Press, 500 p.
Harland, W.B., Armstrong, R.L., Cox, A.V., Craig,
L.E., Smith, A.G., and Smith, D.G., 1990. A
Geologic Time Scale 1989. Cambridge University Press, 263 p.
Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pittman, W.C., III, and LePichon, X., 1968. Marine magnetic anomalies, geomagnetic field
5
How Old Is It? Part 2 – Magnetostratigraphy
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Figure 3. Inclination in the Earth’s magnetic field. (http://www.ngdc.noaa.gov/seg/geomag/icons/wmm2000i.gif)
6
How Old Is It? Part 2 – Magnetostratigraphy
TM
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Figure 4. Declination in Earth’s magnetic field. (http://www.ngdc.noaa.gov/seg/geomag/icons/WMM-00D.gif)
7
6.935 Ma
7
C3B
8
9
late Miocene
7.432 Ma
C4
8.699 Ma
C4A
9.740 Ma
C1r
C2n
C3n
C2Ar
C3A
C5r
C5AD
14.800Ma
C5B
16
16.014 Ma
C5C
17
C5Ar C5An
C5ADn
C5AC
C5Br C5Bn
15
C5AA
C5AB
C5Cr C5Cn
14
middle Miocene
13
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11.935Ma
C5A
3.040-3.110
3.220-3.330
Cochiti
Nunivak
4.180-4.290
4.480-4.620
4.800-4.890
4.980-5.230
2.140-2.150
C3r
C5n
C5
12
Kaena
Mammoth
C3An.2n
10
11
1.770-1.950
C3An.1n
C3An
5.894 Ma
6
Olduvai C2n
Reunion
Sidufjall
Thvera
C3Bn
Gilbert
5
Age (Ma)
3.58 Ma
C4n
4
Gauss
C2r
2.581 Ma
0.990-1.070
Jaramillo
Cobb Mountain
C2An
2
3
Chron
0.78 Ma
Matuyama
Age (Ma)
C1n
Brunhes
Polarity Polarity event
C4r
1
Magnetic
Epoch
C4Ar C4An
0
Pleistocene
Epoch
Pliocene
How Old Is It? Part 2 – Magnetostratigraphy
Figure F6. Miocene to Holocene geomagnetic polarity time scale (GPTS). In the polarity colum
normal and white = reversed polarity. Absolute ages, geologic periods, and magnetic chron term
shown at left. This figure is based on the Berggren et al. (1995b) and Cande and Kent (1995) G
5.894-6.137
6.269-6.567
C3Bn
C3Br.2n
C4n.1n
C4n.2n
6.935-7.091
C4r.1n
8.225-8.257
C4An
8.699-9.025
C4Ar.1n
C4Ar.2n
C5n.1n
9.230-9.308
9.580-9.642
9.740-9.880
C5n.2n
9.920-10.949
C5r.1n
11.052-11.099
C5r.2n
11.476-11.531
C5An.1n
C5An.2n
C5Ar.1n
C5Ar.2n
C5AAn
C5ABn
11.935-12.078
12.148-12.401
12.678-12.708
12.775-12.819
12.991-13.139
13.302-13.510
C5ACn
13.703-14.076
C5ADn
14.178-14.612
C5Bn.1n
C5Bn.2n
14.800-14.888
15.034-15.155
C5Cn.1n
C5Cn.2n
C5Cn.3n
16.014-16.293
16.327-16.488
16.556-16.726
7.341-7.375
7.432-7.562
7.650-8.072
Figure 5. Geomagnetic Polarity Time Scale (GPTS) for 17-0 Ma. In the polarity column, black = normal and
white = reversed polarity. Absolute ages, geologic periods, and magnetic chron terminology are shown at left.
This figure is based on the Berggren et al. (1995b) and Cande and Kent (1995) GPTS. From ODP Leg 191
Initial Reports volume, Explanatory Notes chapter, (http://www-odp.tamu.edu/publications/191_IR/chap_02/
chap_02.htm)
8
How Old Is It? Part 2 – Magnetostratigraphy
Interpreting the Paleomagnetic Record
Preserved in Deep-Sea Sediments
Use Figure 4 to interpret the paleomagnetic stratigraphy (chrons and subchrons) recorded at Site
1208 (below) in the northwest Pacific (Shatsky
Rise). What assumptions must you make if you
simply correlate the magnetic reversal pattern preserved in Site 1208 with the Geomagnetic Polarity
SHIPBOARD
SCIENTIFIC
PARTY
Time Scale
shown
in Figure
4?
Activity
Examine Figure 4. Make a list of observations
about the character of the Earth’s magnetic field
over the last 17 million years.
CHAPTER 4, SITE 1208
53
Figure F19. Inclination after AF demagnetization at peak fields of 20 mT as measured with the shipboard
pass-through magnetometer at Hole 1208A. The column at the right of each plot shows interpreted zones
of normal (black) and reversed (white) polarity. Gray intervals indicate zones in which no polarity interpretation is possible. Polarity zones at the top of the section and certain polarity zones farther downsection
are tentatively correlated to polarity chrons.
Inclination (°)
0
-80
-40
0
40
80
Inclination (°)
Polarity
-80
-40
0
40
80
Polarity
C2Ar
C1n
C3n
50
200
50
C3r
J
C3An
C1r
C2n
100
250
C4n
C4r
C4An
100
C2r
C4Ar
C5n
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Depth (mbsf)
CM
C2An
C5An
K
150
M
C5r
150
300
9
C5Ar
boundaries are not clearly delimited. All information and references are given in Tables T1, p. 52, T2, p. 54,
and T3, p. 57. TC = top of common occurrence, BC = bottom of common occurrence. CK95 = Cande and
Kent, 1995. (Continued on next four pages.)
C1r
2
C2r
2
1
C2An
2
3
4
C2Ar
Pliocene
3
2
3
5
b
4
PT1
a
PL6
CN12d NN18
PL5
CN12b+c
PL3+4
Messinian
2
PL2
CN11
NN13-15
CN10b
NN12
M14
CN9a
M13
C4An
C4Ar
CN8a
T Amaurolithus spp. (4.56)
B Sphaeroidinella dehiscens s.l. (4.94)
T Ceratolithus acutus (5.05)
B Ceratolithus rugosus (5.10)
T Triquetrorhabdulus rugosus (5.23)
B Ceratolithus acutus (5.37)
B Nicklithus amplificus (6.84)
B Discoaster berggrenii (8.28)
B Discoaster loeblichii (8.43)
NN10
2
C5n 1
T Globorotalia plesiotumida (4.15)
T Hirsutella cibaoensis (4.16)
B Globorotalia crassaformis (4.31)
T Globoturborotalita nepenthes (4.37)
B paracme Reticulofenestra pseudoumbilicus (8.79)
B Globorotalia plesiotumida (8.91)
B Globigerinoides extremus (8.94)
a
1
CN7
3
10
B Globorotalia tosaensis
T Menardella miocenica (3.48)
B Hirsutella cibaoensis (7.91)
CN8b
Tortonian
9
2
T Discoaster surculus (2.63)
T Globigerina decoraperta (2.70)
T Discoaster tamalis (2.83)
T Menardella multicamerata (2.95)
T Dentoglobigerina altispira (3.02)
T Sphaeroidinellopsis seminulina (3.18)
B Globoconella conomiozea (7.12)
1
C4r
T Discoaster pentaradiatus (2.52)
T paracme Reticulofenestra pseudoumbilicus (7.29)
B Amaurolithus primus (7.39)
b
late
Miocene
C4n 2
T Globigerina apertura (1.68)
B medium Gephyrocapsa (1.71) [= B "G. oceanica"]
T Globigerinoides extremus (1.91)
T Discoaster brouweri (1.95)
B Globorotalia truncatulinoides (2.03)
T Pulleniatina finalis (2.05)
T Globoturborotalia woodi (2.30)
T Menardella miocenica (2.38)
CN9bB
M12
NN9
T Discoaster hamatus (9.63)
T Catinaster calyculus (9.64)
T Catinaster coalitus (9.69)
B Hirsutella juanai (9.75)
B Neogloboquadrina acostaensis (9.89)
T = Top B = Base
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1
T large Gephyrocapsa spp. (1.24)
T Globigerinoides obliquus (1.3)
B Tuborotalia humilis (5.84)
B Globigerinoides conglobatus (5.84)
B Globorotalia tumida (5.96)
B Hirsutella margaritae (5.99)
T Nicklithus amplificus (6.00)
T Fohsella lenguaensis (6.00)
CN9bA
3
B Gephyrocapsa parallela (1.05)
[= reentrance medium G.]
T Discoaster quinqueramus (5.54)
NN11
C3Br 2
T Pseudoemiliania lacunosa (0.46)
CN10c
C3Ar
1
B Emiliania huxleyi (0.26)
T Pulleniatina primalis (3.65)
T Sphenolithus spp. (3.66)
T Reticulofenestra pseudoumbilicus (3.80)
T Neogloboquadrina acostaensis (3.83)
T Hirsutella margaritae (3.88)
CN9bC
1
C3Bn
NN17
CN12aB NN16
CN10a
C3An
8
CN13b NN19
CN13a
C3r
7
NN21
NN20
Datum events
T Globorotalia tosaensis (0.61)
PL1
5.33
6
CN15
CN14
CN12aA
early
Age (Ma)
1
C3n
middle
- late
1.81
1
late
C2n
middle
2
early
1
1
Pleistocene
C1n
Age Foraminifers Nannofossils
Piacenzian Gelasian Pleistocene + Holocene
Epoch
Zanclean
How Old Is It? Part 2 – Magnetostratigraphy
Chron
0
Figure 6.(page 1 of three pages) Integration of biostratigraphy and magnetostratigraphy. Calcareous nannofossil and planktonic foraminiferal zonation used during ODP Leg 208. Use this
as a reference for the exercise that follows. References are cited in the text for each microfossil group. Shaded bands with dashed lines indicate that the zonal boundaries are not clearly
delimited. T = top, or last occurrence (LO) datum; B = base, or first occurrence (FO) datum; TC =
top of common occurrence, BC = bottom of common occurrence. CK95 = Cande and Kent, 1995.
From ODP Leg 208 Initial Reports volume, Explanatory Notes chapter, (http://www-odp.tamu.
edu/publications/208_IR/chap_02/chap_02.htm)
10
46
Figure F4 (continued).
10
Chron
Epoch
Age Foraminifers Nannofossils
C5n
11
2
1
C5r
Tortonian
M12
late
1
1
C5Ar 2
14
3
NN8
C5ABr
C5ACn
CN5b
CN5a
NN6
T Globigerina decoraperta (11.46)
TC Discoaster kugleri (11.60)
B Globoturborotalita nepenthes (11.64)
B Triquetrorhabdulus rugosus (12.81)
TC Cyclicargolithus floridanus (13.19)
B Fohsella fohsi s.l. (13.33)
T Sphenolithus heteromorphus (13.55)
M7
CN4
NN5
T Hirsutella praescitula (13.73)
B Menardella archeomenardii (13.92)
T Fohsella peripheroronda (13.92)
B Menardella praemenardii (13.95)
B Fohsella peripheroacuta (14.02)
T Praeorbulina circularis (14.58)
T Praeorbulina sicana (14.63)
B Orbulina spp. (14.71)
Langhian
C5Br
T Paragloborotalia mayeri (10.70)
B Catinaster coalitus (10.79)
B Fohsella robusta (12.85)
M6
Miocene
2
T Paragloborotalia sikaensis (10.43)
B Discoaster neohamatus (10.45)
B Discoaster hamatus (10.48)
Discoaster bellus gr. (10.48)
M9
C5ACr
C5Bn 1
T Menardella praemenardii (10.09)
BC Discoaster kugleri (11.88)
T Fohsella fohsi s.l. (11.91)
T Clavatorella bermudezi (12.02)
C5ADr
15
Datum events
T Coccolithus miopelagicus (11.02)
B Globigerina apertura (11.06)
M8
middle
C5AAn
C5AAr
C5ABn
C5ADn
Age (Ma)
CN6
M11
M10
C5An 2
13
NN9
NN7
3
12
CN7
2
Serravallian
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 2, EXPLANATORY NOTES
T Helicosphaera ampliaperta (15.03)
B Praeorbulina circularis (15.15)
b
T Acme Discoaster deflandrei (15.6)
B Discoaster signus (15.6)
M5
16
a
2
3
17
CN3
NN4
B Praeorbulina sicana (16.86)
C5Cr
M4
C5Dn
T Catapsydrax dissimilis (17.51)
B Globigerinatella insueta s.s. (17.57)
B Sphenolithus heteromorphus (17.76)
T Sphenolithus belemnos (17.89)
M3
C5Dr
C5Er
19
C6n
20
early
C5En
Burdigalian
18
TM
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B Praeorbulina glomerosa (16.16)
B Menardella archeomenardii (16.16)
C5Cn 1
CN2
NN3
B Sphenolithus belemnos (18.92)
T Globoquadrina binaiensis (19.08)
M2
CN1c
T Triquetrorhabdulus carinatus (19.6)
NN2
C6r
B Globoquadrina binaiensis (19.98)
T = Top B = Base TC = Top common BC = Base common
s.s. = senso stricto
s.l. = senso lato
Figure 6, page 2.
11
47
Figure F4 (continued).
1
2
C6AAr
3
C6Bn 12
C6Br
1
23 C6Cn 2
22
3
CN1a+b
2
C9n
28
C9r
C10n 12
C16r
C17n
39
CP16b
CP16a
C17r
1
2
3
C18n 1
T Pseudohastigerina (32.0)
T Increase Ericsonia obruta (32.2)
T Isthmolithus recurvus (32.7)
T Ericsonia formosa (32.9)
NP21
NP20
Priabonian
C16n 2
37
38
CP16c NP22
P16
C15r
T Turborotalia ampliapertura (30.12)
B Sphenolithus distentus (30.32)
B Paragloborotalia opima (30.54)
T Reticulofenestra umbilicus ≥14 µm (31.7)
33.7
C15n
T Subbotina angiporoides (29.67)
CP17
P19
C13n
C13r
T Sphenolithus pseudoradians (29.10)
NP23
P18
1
36
Rupelian
early
C12r
B Cassigerinella chipolensis (33.6)
T Hantkenina spp. (33.7)
B Increase Ericsonia obruta (33.7)
T Turborotalia cerroazulensis (33.8)
T Discoaster saipanensis (34.0)
T Discoaster barbadiensis (34.2)
T Globigerinatheka index (34.3)
T Calcidiscus protoannulus (35.4)
T Globigerinatheka semiinvoluta (35.3)
CP15
P15
Bartonian
35
P20
Switch to CK95 break in timescale
late
34
B "Globigerina" angulisuturalis (28.89)
CP18
NP19
NP18
B Isthmolithus recurvus (36.6)
B Chiasmolithus oamaruensis (37.0)
T Chiasmolithus grandis (37.1)
T Subbotina linaperta (37.7)
CP14b NP17
T Morozovella spinulosa (38.1)
B Globigerinatheka semiinvoluta (38.4)
B Dictyococcites bisectus (38.5)
T Acarinina coalingensis (39.0)
P14
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33
TC Chiloguembelina cubensis (27.92)
B Sphenolithus ciperoensis (27.55)
P21a
1
middle
32
T Acme Cyclicargolithus abisectus (24.79)
T Paragloborotalia pseudokugleri (25.15)
T Sphenolithus distentus (25.98)
P21b
Eocene
Age (Ma)
31
CP19b NP25
CP19a NP24
C11n 2
C11r
C12n
P22
T Paragloborotalia opima (26.64)
C10r
30
NN1
B Sphenolithus disbelemnos (22.67)
B Discoaster druggii (22.82)
T Paragloborotalia kugleri (22.87)
B Globigerinoides trilobus (22.87)
T Globigerina euaptertura (23.03)
T Sphenolithus delphix (23.07)
B Sphenolithus delphix (23.33)
T Tenuitella gemma (23.54)
BC Globigerinoides primordius (23.54)
T Sphenolithus ciperoensis (24.33)
C7r
C7An
C7Ar 1
C8r
T Paragloborotalia kugleri (21.03)
T Paragloborotalia pseudokugleri (21.22)
B Globoquadrina dehiscens (21.44)
T "Globigerina" angulisuturalis (21.90)
2
27
29
M1
1
C8n
26
NN2
23.03
Chattian
25
CN1c
C6Cr
C7n
Datum events
M2
late
24
Age Foraminifers Nannofossils
Aquitanian
C6An 12
21 C6Ar
C6AAn
Epoch
early
Chron
Miocene
20
Oligocene
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 2, EXPLANATORY NOTES
40
2
B = Base
Figure 6, page 3.
12
T = Top TC = Top common BC = Base common
Declination (°)
0
180
Inclination (°)
-90
0
90
Brunhes
0
Chron
A
Polarity
4
Matuyama
How Old Is It? Part 2 – Magnetostratigraphy
Figure F12. Composite magnetic stratigraphy at Site 1218. Virtual geomagnetic pole (VGP) latitudes were
obtained after partial AF demagnetization of continuous measurements at a peak field of 20 mT. Polarity
column shows interpreted zones of normal (black) and reversed (white) magnetization, and gray intervals
indicate zones with an uncertain polarity interpretation. A. Top 24 m of Hole 1218B, including cores not
oriented with the Tensor tool. (Continued on next two pages.)
12
Gauss
Depth (mcd)
8
16
TM
Gilbert
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20
24
Figure 7. (page 1 of 3 pages) An example of geomagnetic data and interpretation. Composite magnetic
stratigraphy at ODP Site 1218. Virtual geomagnetic pole (VGP) latitudes were obtained after partial AF
demagnetization of continuous measurements at a peak field of 20 mT. Polarity column shows interpreted
zones of normal (black) and reversed (white) magnetization, and gray intervals indicate zones with an
uncertain polarity interpretation. A. Top 24 m of Hole 1218B, including cores not oriented with the Tensor
tool. Note that both declination and inclination data are provided. B. Early Miocene-Pliocene section.
Note the use of VGP (Virtual Geomagnetic Pole). VGP shows the latitude of the magnetic pole based
on a single location rather than an average of locations; it is calculated from the declination, inclination,
and the drillsite location. C. Early Oligocene-early Miocene section. From ODP Leg 199 Initial Reports
volume, Site 1218 chapter, (http://www-odp.tamu.edu/publications/199_IR/chap_11/chap_11.htm)
13
54
-90
10
-60
-30
0
30
60
90
VGP latitude (°)
-90
35
-60
-30
0
30
60
90
Chron/
Subchron
VGP latitude (°)
Polarity
B
Chron/
Subchron
Figure F12 (continued). B. Early Miocene–Pliocene section.
Polarity
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 11, SITE 1218
C5n
Hole 1218A
Hole 1218B
40
15
C5AAn
C2Ar
C5ABn
C3n.1n
20
C3n.2n
C3n.3n
C3n.4n
Depth (mcd)
C5An.1n
C5An.2n
45
C5ACn
C5ADn
C5Bn
C3An.1n
C3An.2n 50
25
TM
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C5Cn
C3Bn
55
30
C5Dn
C4n
C4An
C5En
C5n
35
60
Figure 7 (page 2).
14
C6n
55
-100
60
-50
0
50
100
VGP latitude (°)
-100
140
C6n
0
50
100
C9n
C6An.1n
70
-50
Chron/
Subchron
VGP latitude (°)
Polarity
C
Chron/
Subchron
Figure F12 (continued). C. Early Oligocene–early Miocene section.
Polarity
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 11, SITE 1218
C6An.2n
150
C6AAn
C6AAr.1n
80
C6AAr.2n
C6Bn.1n
C6Bn.2n
90
160
C10n.1n
C10n.2n
Depth (mcd)
C6Cn.1n
C6Cn.2n
170
C6Cn.3n
100
C7n.1n
110
180
C11n.1n
C7n.2n
TM
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C7An
120
190
C11n.2n
C8n.1n
C8n.2n
200
130
C12n
C9n
140
210
Hole 1218A
Hole 1218B
Hole 1218C
Figure 7 (page 3).
15
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 11, SITE 1218
61
Figure F18. LSRs and chronostratigraphic markers.
0
4.44 m/m.y.
2.69 m/m.y.
Slump?
1.1 m/m.y.
2.63 m/m.y.
50
5.92 m/m.y.
8.64 m/m.y.
Depth (mcd)
100
150
18.11 m/m.y.
200
10.32 m/m.y.
Nannofossils
TM
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250
Radiolarians
4.84 m/m.y.
Foraminifers
Paleomagnetic reversal
300
0
10
11.1 m/m.y.
20
30
40
50
Age (Ma)
Figure 8. Age-depth plot of Site 1218 using paleomagnetic chron boundaries and biostratigraphic
datums. LSRs (linear sedimentation rates) and chronostratigraphic markers. Data tables for the
depths and ages of the paleomagnetic chrons and biostratigraphic datums are also available
online at the following URL. From ODP Leg 199 Initial Reports volume, Site 1218 chapter. (http://
www-odp.tamu.edu/publications/199_IR/chap_11/chap_11.htm)
16
How Old Is It? Part 2 – Magnetostratigraphy
Figure F19. Inclination after AF demagnetization at peak fields of 20 mT as measured with the shipboard
pass-through magnetometer at Hole 1208A. The column at the right of each plot shows interpreted zones
of normal (black) and reversed (white) polarity. Gray intervals indicate zones in which no polarity interpretation is possible. Polarity zones at the top of the section and certain polarity zones farther downsection
are tentatively correlated to polarity chrons.
Inclination (°)
0
-80
-40
0
40
80
Inclination (°)
Polarity
-80
-40
0
40
80
Polarity
C2Ar
C1n
C3n
50
50
C3r
J
Depth (mbsf)
CM
C3An
C1r
C2n
100
C4n
C4r
C4An
100
C4Ar
C5n
C2An
C5An
K
150
M
C5r
150
C5Ar
TM
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C2r
Figure 9. A second example of geomagnetic data and interpretation. Inclination after AF demagnetization at peak
fields of 20 mT as measured with the shipboard pass-through magnetometer at Hole 1208A. The column at the right
of each plot shows interpreted zones of normal (black) and reversed (white) polarity. Gray intervals indicate zones in
which no polarity interpretation is possible. Polarity zones at the top of the section and certain polarity zones farther
downsection are tentatively correlated to polarity chrons. From ODP Leg 198 Initial Reports volume, Site 1208 chapter.
(http://www-odp.tamu.edu/publications/198_IR/chap_04/chap_04.htm)
17
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 4, SITE 1208
54
How Old Is It? Part 2 – Magnetostratigraphy
Figure F20. Age-depth curve for Site 1208 derived from the magnetic stratigraphy shown in Figure F19,
p. 53, using the geomagnetic polarity timescale of Cande and Kent (1995). Average sedimentation rates in
meters per million years are also plotted.
Geomagnetic polarity timescale
C1n
C2n
C2An
C3n
C3An
C4n
C4An
C5n
C5An
0
50
40 m/m.y.
Depth (mbsf)
100
150
24 m/m.y.
200
TM
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250
10 m/m.y.
300
0
2
4
6
8
10
12
Age (Ma)
Figure 10. Age-depth curve for Site 1208 derived from the magnetic stratigraphy shown in Figure 9 using
the geomagnetic polarity timescale of Cande and Kent (1995). Average sedimentation rates in meters per
million years are also plotted. From ODP Leg 198 Initial Reports volume, Site 1208 chapter. (http://wwwodp.tamu.edu/publications/198_IR/chap_04/chap_04.htm)
18
How Old Is It? Part 2 – Magnetostratigraphy
Paleomagnetic Stratigraphy Exercise
In this exercise, you will interpret the polarity
chrons (geomagnetic reversal stratigraphy) for
ODP Site 1237 in the southeast Pacific, located
on the continental margin of Peru (16°0.421’S,
76°22.685’W; Figures 11 and 12). First, notice the
latitude of this site. In the Northern Hemisphere,
normal polarity is represented by positive inclination. Since this site is in the Southern Hemisphere,
normal polarity will be represented by negative
inclination (positive upwards, out of the Earth).
Second, well-preserved magnetic data are either
normal polarity or reversed polarity, and there have
been many reversals during the past 30 million
years represented by the sequence cored at Site
1237. You will need some independent means of
age control in order to interpret the black and white
stripes of normal and reversed polarity, respectively. Biostratigraphy based on the established
sequence of first and last occurrences of microfossil species provides that first-order age control.
1. Using the Site 1237 biostratigraphic data
provided (Table 1), interpret the polarity chrons
for ODP Site 1237 (Holes 1237B, 1237C, and
1237D). The cored sequence is shown in 4
separate figures: Figure 13 (upper 100 meters
composite depth, mcd); Figure 14 (100-200
mcd); Figure 15 (200-300 mcd); and Figure 16
(300-360 mcd). Construct your paleomagnetic
stratigraphy to the right of the data presented in
Figures 13-16. Color in segments representing
normal polarity, and leave segments of reverse
polarity blank (white).
2. Plot an age versus depth profile based on your
paleomagnetic interpretations (you can plot
it on the same graph as your biostratigraphic
datums from the biostrat exercise). Use
the graph paper provided. How well do the
calcareous nannofossil datums compare with
the polarity chron boundaries?
TM
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19
A
Water depth (m)
Pisco
<1000
1000-2000
2000-3000
3000-4000
4000-5000
5000-7000
>7000
14°
S
16°
Peru
Site 1237
18°
ca
ge
Pe
ru
id
R
az
N
e
tur
20°
ca
az
ch
en
Tr
How Old Is It? Part 2 – Magnetostratigraphy
Figure F1. A. Locations of Sites 1236 and 1237 and bathymetry. B. Site locations and oceanograph
tures off Peru and northern Chile (CC = Coastal Current, PCCC = Peru-Chile Countercurrent, PCC =
Chile Current), after Strub et al. (1998). Modern mean annual sea-surface temperatures (SSTs) (conto
in degrees Celsius, after Ocean Climate Laboratory, 1999.
ne
Zo
c
Fra
N
Site 1236
22°
24°
B
22
Pisco
14°
S
Peru
1237
16°
C
20°
22°
CC
CC
PC
TM
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PC
18°
20
20
1236
24°
18
84°W
82°
80°
78°
76°
74°
72°
70°
Figure 11. A. Locations of Sites 1236 and 1237 and bathymetry. B. Site locations
and oceanographic features off Peru and northern Chile (CC = Coastal Current,
PCCC = Peru-Chile Countercurrent, PCC = Peru-Chile Current), after Strub et
al. (1998). Modern mean annual sea-surface temperatures (SSTs) (contours
are in degrees Celsius, after Ocean Climate Laboratory, 1999. From ODP
Leg 202 Initial Reports volume, Site 1237 chapter. (http://www-odp.tamu.edu/
publications/202_IR/chap_08/chap_08.htm)
20
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
32
Annual mean sea-surface temperature (°C)
0°
S
SEC
24
24
10°
25 Ma
20°
5 Ma
22
How Old Is It? Part 2 – Magnetostratigraphy
Figure F4. Tectonic backtrack of Site 1237, relative to a fixed South America. Poles of rotation are from Duncan and Hargraves (1984) and Pisias et al. (1995). The dotted path represents positions in million-year increments. Numbers note ages (in millions of years) of changes in rate or direction of drift. Contours of modern mean annual sea-surface temperature are superimposed. PCC = Peru-Chile Current, SEC = South Equatorial Current).
Site 1237
5 Ma
25 Ma
Site 1236
38 Ma
32 Ma
20
22
20
PCC
30°
20
18
18
14
100°
90°
80°
70°
Figure 12. Tectonic backtrack of Site 1237, relative to a fixed South America. Poles of rotation are
from Duncan and Hargraves (1984) and Pisias et al. (1995). The dotted path represents positions
in million-year increments. Numbers note ages (in millions of years) of changes in rate or direction of drift. Contours of modern mean annual sea-surface temperature are superimposed. PCC =
Peru-Chile Current, SEC = South Equatorial Current). From ODP Leg 202 Initial Reports volume,
Site 1237 chapter. (http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm)
TM
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40°
110°W
14
16
16
21
60°
62
Figure F29. Inclination after demagnetization at peak alternating fields of 25 mT for the upper 100 mcd of
(A) Hole 1237B, (B) Hole 1237C, and (C) Hole 1237D with the accompanying polarity interpretations.
-90
0
-30
30
B
90 -90
C
Hole 1237C
Inclination (°)
-30
30
90 -90
Hole 1237D
Inclination (°)
-30
30
90
Polarity Polarity zone
interpretation
Interpretation
C1n
1n
20
1r
1r.1n
2n
2r.1n
2r
Depth (mcd)
Matuyama
1r
40
C1r
C2n
C2r
C2An
Jaramillo
Hole 1237B
Inclination (°)
2Ar
3n.3n
3n.3n
3r
100
Gilbert
3n.2n
Mammoth
Kaena
C3r
C3An
C3Ar/C3Bn/
C3Br
C4n
C4Ar/C4An/
C4Ar
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
C4ACn/C5ACr/
C5ADn
Figure 13. Inclination after demagnetization at peak alternating fields of 25 mT for the upper 100 mcd
of (A) Hole 1237B, (B) Hole 1237C, and (C) Hole 1237D with the accompanying polarity interpretations. From ODP Leg 202 Initial Reports volume, Site 1237 chapter. (http://www-odp.tamu.edu/
publications/202_IR/chap_08/chap_08.htm)
TM
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3n.1n
80
Nunivak
Sifurfall
Cochiti
3n
Thvera
2n
Gauss
2An.1n
60
C2Ar
C3n
Reunion
Oldavai
A
Brunhes
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
22
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
63
Figure F30. Inclination after demagnetization at peak alternating fields of 25 mT for the 100- to 200-mcd
interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the accompanying polarity interpretations.
A
Hole 1237B
B
Inclination (°)
-90 -60 -30 0 30 60 90
100
-90
Hole 1237C
Inclination (°)
-30
30
C
90
MCD stack
Inclination (°)
-90
-30
30
90
Polarity zone
Polarity
interpretation
Interpretation
C1n
C1r
C2n
C2r
3r
C2An
3An.1n ?
120
C2Ar
C3n
3An.2n?
3Ar
C3r
3Bn/3Br/4n
Depth (mcd)
140
160
4r
4An
C3An
C3Ar/C3Bn/
C3Br
C4n
C4Ar/C4An/
C4Ar
4Ar
180
5r
3n.3n
5An.1n
5An.2n
200
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
C4ACn/C5ACr/
C5ADn
Figure 14. Inclination after demagnetization at peak alternating fields of 25 mT for the 100- to 200mcd interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the
accompanying polarity interpretations. From ODP Leg 202 Initial Reports volume, Site 1237 chapter.
(http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm)
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5n
23
A
Hole 1237B
Inclination (°)
-90 -60 -30 0 30 60 90
200
B
Hole 1237C
Inclination (°)
-90 -60 -30 0 30 60 90
C
Hole 1237D
Inclination (°)
-90 -60 -30 0 30 60 90
D
Stack
Inclination (°)
-90 -60 -30 0 30 60 90
PolarityPolarity zone
interpretation
Interpretation
C1n
5Ar
AAn
to 5ADn
How Old Is It? Part 2 – Magnetostratigraphy
Figure F31. Inclination after demagnetization at peak alternating fields of 25 mT for the 200- to 300-mcd interval of
1237C, (C) Hole 1237D, and (D) the stacked and smoothed record with the accompanying polarity interpretations.
5Br
C2r
C2An
5Cn
220
C1r
C2n
C2Ar
5Cr
5Dn
5Dr
C3r
5En
C3An
6n
260
C3Ar/C3Bn/
C3Br
C4n
280
C4Ar/C4An/
C4Ar
C5n
C5r
C5An
6Bn
C5Ar
6Br
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
6Cn
300
C4ACn/C5ACr/
C5ADn
Figure 15. Inclination after demagnetization at peak alternating fields of 25 mT for the 200- to 300-mcd interval of (A) Hole 1237B,
(B) Hole 1237C, (C) Hole 1237D, and (D) the stacked and smoothed record with the accompanying polarity interpretations. From
ODP Leg 202 Initial Reports volume, Site 1237 chapter. (http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm)
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Not interpreted
Depth (mcd)
240
C3n
24
How Old Is It? Part 2 – Magnetostratigraphy
SHIPBOARD SCIENTIFIC PARTY
CHAPTER 8, SITE 1237
65
Figure F32. Inclination after demagnetization at peak alternating fields of 25 mT for the 300- to 360-mcd
interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the accompanying polarity interpretations.
A
Hole 1237B
Inclination (°)
-90 -60 -30 0 30 60 90
300
B
-90
Hole 1237C
Inclination (°)
-30
30
C
90
MCD Stack
Inclination (°)
-90
-30
30
Polarity zone
Polarity
interpretation
Interpretation
90
C1n
6Cn.3n
6Cr
C2An
310
7N
Depth (mcd)
C1r
C2n
C2r
8N
320
C2Ar
C3n
C3r
9N
C3An
330
C3Ar/C3Bn/
C3Br
C4n
350
C4Ar/C4An/
C4Ar
C5n
C5r
C5An
C5Ar
C5AAn/C5AAr/
C5ABn/C5ABr/
C5ACn
360
C4ACn/C5ACr/
C5ADn
Figure 16. Inclination after demagnetization at peak alternating fields of 25 mT for the 300- to 360mcd interval of (A) Hole 1237B, (B) Hole 1237C, and (C) the stacked and smoothed record with the
accompanying polarity interpretations. From ODP Leg 202 Initial Reports volume, Site 1237 chapter.
(http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm)
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Not interpreted
340
25
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FO Emiliania huxleyi
FO Globorotalia hirsuta
LO Pseudoemiliania lacunosa
LO Nitzschia reinholdii
LO Globorotalia tosaensis
LO Nitzschia fossilis
LO Reticulofenestra asanoi
LO Rhizosolenia matuyamai
FO Reticulofenestra asanoi
FO Rhizosolenia matuyamai
LO Gephyrocapsa (large)
FO Gephyrocapsa (large)
LO Calcidiscus macintyrei
LO Globigerinoides extremus
LO Discoaster brouweri
FO Fragilariopsis doliolus
LO Globorotalia puncticulata
LO Thalassiosira convexa s.l.
LO Discoaster pentaradiatus
LO Discoaster surculus
LO Discoaster tamalis
LO Nitzschia jouseae
LO Dentoglobigerina altispira
LO Sphaeroidinellopsis seminula
FO Rhizosolenia praebergonii s.l.
FO Sphaeroidinella dehiscens
LO Globorotalia plesiotumida
LO Reticulofenestra pseudoumbilicus
FO Pseudoemiliania lacunosa
LO Globoturborotalia nepenthes
LO Sphaeroidinellopsis kochi
LO Nitzschia cylindrica
FO Nitzschia jouseae
LO Discoaster quinqueramus
FO Thalassiosira oestrupii
FO Globorotalia tumida
LO Nitzschia miocenica s.s.
FO Globorotalia margaritae
FO Globigerinoides conglobarus
FO Pulletianita primalis
FO Thalassiosira convexa v. aspinosa
LO Absence interval Reticulofenestra pseudoumbilicus >7 µm
FO Globorotalia conomiozea
FO Amaurolithus primus
FO Nitzschia miocenica s.l.
FO Nitzschia reinholdii
Datum
CN
PF
CN
D
PF
D
CN
D
CN
D
CN
CN
CN
PF
CN
D
PF
D
CN
CN
CN
D
PF
PF
D
PF
PF
CN
CN
PF
PF
D
D
CN
D
PF
D
PF
PF
PF
D
CN
PF
CN
D
D
0.26
0.45
0.46
0.62
0.65
0.70
0.88
1.05
1.08
1.18
1.24
1.45
1.59
1.77
1.96
2.00
2.41
2.41
2.44
2.61
2.76
2.77
3.09
3.12
3.17
3.25
3.77
3.80
4.00
4.20
4.53
4.88
5.12
5.56
5.64
5.82
6.08
6.09
6.20
6.40
6.57
6.80
7.12
7.24
7.30
7.64
0.26
0.45
0.46
0.62
0.65
0.70
0.88
1.05
1.08
1.18
1.24
1.45
1.59
1.77
1.96
2.00
2.41
2.41
2.44
2.61
2.76
2.77
3.09
3.12
3.17
3.25
3.77
3.80
4.00
4.20
4.53
4.88
5.12
5.56
5.64
5.82
6.08
6.09
6.20
6.40
6.57
6.80
7.12
7.24
7.30
7.64
Source Minimum Maximum
Age (Ma)
202-1237B1H-4, 75
1H-CC, 9
2H-5, 75
2H-CC, 24
1H-CC, 9
2H-CC, 24
3H-4, 60
3H-3, 75
3H-CC, 19
3H-CC, 19
4H-1, 75
4H-2, 75
4H-3, 75
2H-CC, 24
4H-CC, 20
4H-3, 40
3H-CC, 19
4H-6, 40
5H-6, 75
5H-6, 75
6H-1, 75
6H-1, 75
5H-CC, 22
6H-CC, 30
7H-1, 110
6H-CC, 30
7H-CC, 19
8H-1, 75
9H-3, 78
8H-CC, 39
8H-CC, 39
8H-CC, 39
9H-CC, 20
11H-4, 75
11H-CC, 25
10H-CC, 10
12H-4, 38
11H-CC, 25
12H-CC, 18
12H-CC, 18
13H-CC, 1
14H-CC, 26
13H-CC, 1
15H-3, 75
13H-CC, 1
14H-CC, 26
Core, section,
interval (cm)
5.26
5.51
12.28
15.47
5.51
15.47
20.13
18.77
25.00
25.00
25.25
26.76
28.27
15.47
34.55
27.92
25.00
32.46
42.30
42.30
44.25
44.25
44.02
53.51
54.10
53.51
62.80
63.25
75.78
72.66
72.66
72.66
81.86
96.28
101.10
91.29
105.40
101.10
109.81
109.81
119.75
129.40
119.75
132.77
119.75
129.40
Depth
(mbsf)
Top sample
(FO presence/LO absence)
202-1237B1H-CC, 9
2H-CC, 24
2H-7, 40
3H-5, 75
2H-CC, 24
3H-CC, 19
3H-5, 60
3H-5, 75
4H-1, 75
4H-1, 40
4H-2, 75
4H-3, 75
4H-6, 75
3H-CC, 19
5H-1, 75
4H-6, 40
4H-CC, 20
4H-CC, 20
5H-CC, 22
5H-CC, 22
6H-2, 75
6H-2, 22
6H-CC, 30
7H-CC, 19
7H-3, 73
7H-CC, 19
8H-CC, 39
8H-2, 78
9H-4, 75
9H-CC, 20
9H-CC, 20
9H-6, 30
10H-CC, 10
11H-5, 75
12H-1, 22
11H-CC, 25
13H-CC, 1
12H-CC, 18
13H-CC, 1
13H-CC, 1
14H-3, 34
15H-1, 75
14H-CC, 26
15H-5, 75
14H-3, 34
15H-CC, 1
Core, section,
interval (cm)
5.51
15.47
14.94
21.79
15.47
25.00
21.64
21.79
25.25
24.90
26.76
28.27
32.81
25.00
34.75
32.46
34.55
34.55
44.02
44.02
45.76
45.23
53.51
62.80
56.74
62.80
72.66
64.79
77.27
81.86
81.86
79.83
91.29
97.79
100.72
101.10
119.75
109.81
119.75
119.75
122.86
129.75
129.40
135.79
122.86
138.66
Depth
(mbsf)
Bottom sample
(LO presence/FO absence)
Table T10. Age-depth control points, Hole 1237B. (See table notes. Continued on next page).
0.26
0.45
0.46
0.62
0.65
0.70
0.88
1.05
1.08
1.18
1.24
1.45
1.59
1.77
1.96
2.00
2.41
2.41
2.44
2.61
2.76
2.77
3.09
3.12
3.17
3.25
3.77
3.80
4.00
4.20
4.53
4.88
5.12
5.56
5.64
5.82
6.08
6.09
6.20
6.40
6.57
6.80
7.12
7.24
7.30
7.64
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Uncertainty
Average
(±)
Age (Ma)
5.39
10.49
13.61
18.63
10.49
20.24
20.89
20.28
25.13
24.95
26.01
27.52
30.54
20.24
34.65
30.19
29.78
33.51
43.16
43.16
45.01
44.74
48.77
58.16
55.42
58.16
67.73
64.02
76.53
77.26
77.26
76.25
86.58
97.04
100.91
96.20
112.58
105.46
114.78
114.78
121.31
129.58
124.58
134.28
121.31
134.03
5.39
10.07
12.76
18.76
10.07
20.36
21.99
21.38
27.83
27.65
30.31
31.82
34.84
20.36
38.44
34.49
32.48
37.81
46.43
46.43
50.73
50.46
53.26
63.80
60.99
63.80
73.38
69.74
83.91
83.81
83.81
82.79
94.87
106.71
111.13
105.64
123.49
115.68
125.69
125.69
132.71
141.73
135.98
146.83
132.71
146.18
0.13
4.98
1.33
3.16
4.98
4.77
0.75
1.51
0.13
–0.05
0.76
0.75
2.27
4.77
0.10
2.27
4.78
1.04
0.86
0.86
0.75
0.49
4.75
4.65
1.32
4.65
4.93
0.77
0.75
4.60
4.60
3.59
4.72
0.75
–0.19
4.90
7.18
4.36
4.97
4.97
1.56
0.17
4.83
1.51
1.56
4.63
Average Average Uncertainty
(mbsf)
(mcd)
(±m)
Depth
How Old Is It? Part 2 – Magnetostratigraphy
Table 1. (two pages) Age-Depth control points for ODP Hole 1237B. These are the biostratigraphic datums (microfossil
first and last occurrence datums; FOs and LOs) used to calibrate the paleomagnetic record of Site 1237. This image is
scanned from the ODP Leg 202 Initial Reports volume, Site 1237 site chapter (choose pdf version rather than online html
version), http://www-odp.tamu.edu/publications/202_IR/chap_08/chap_08.htm.
26
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Table 1, page two.
27
PF
CN
CN
PF
CN
PF
CN
CN
PF
CN
PF
CN
PF
PF
PF
PF
PF
PF
PF
PF
PF
CN
CN
CN
PF
CN
PF
PF
PF
PF
PF
CN
PF
CN
CN
PF
CN
CN
CN
PF
CN
PF
PF
8.58
8.85
9.40
9.82
10.40
11.19
12.43
11.6
11.68
12.4
13.42
13.57
14.2
14.6
14.80
14.9
15.9
16.1
16.7
17.30
17.30
17.40
18.20
18.50
19.10
19.20
21.50
21.60
22.10
23.20
23.40
23.90
24.30
24.50
24.75
26.70
27.5
27.5
29.1
29.40
29.9
30.00
30.30
8.58
8.85
9.40
9.82
10.40
11.19
12.43
13.19
11.68
12.4
13.42
13.57
14.2
14.6
14.80
14.9
15.9
16.1
16.7
17.30
17.30
17.40
18.20
18.50
19.10
19.20
21.50
21.60
22.10
23.20
23.40
23.90
24.30
24.50
24.75
26.70
27.5
27.5
29.1
29.40
29.9
30.00
30.30
Source Minimum Maximum
15H-CC, 1
17H-1, 75
17H-5, 75
17H-1, 98
18H-4, 75
18H-CC, 1
19H-3, 75
19H-5, 75
18H-CC, 1
19H-6, 75
19H-CC, 13
20H-7, 40
19H-CC, 13
20H-CC, 24
21H-CC, 14
21H-CC, 14
21H-CC, 14
22H-CC, 1
23H-CC, 16
23H-CC, 16
23H-CC, 16
23H-3, 75
23H-CC, 16
23H-CC, 16
24H-CC, 10
24H-6, 75
25H-CC, 10
25H-CC, 10
27H-CC, 14
27H-CC, 14
28H-CC, 16
29H-4, 75
29H-CC, 15
29H-5, 75
29H-5, 75
30H-CC, 0
31H-7, 40
31H-CC, 16
33H-5, 75
31H-CC, 16
33H-5, 75
32H-CC, 25
32H-CC, 25
Core, section,
interval (cm)
138.66
148.75
154.78
148.98
162.43
166.89
170.78
173.80
166.89
175.30
176.81
185.95
176.81
186.41
195.79
195.79
195.79
205.44
214.67
214.67
214.67
208.77
214.67
214.67
224.17
222.81
233.68
233.68
252.80
252.80
262.48
267.27
271.95
268.78
268.78
281.24
290.47
291.07
306.78
291.07
306.78
300.54
300.54
Depth
(mbsf)
16H-CC, 12
17H-1, 86
17H-6, 75
17H-CC, 21
18H-5, 75
19H-CC, 13
19H-4, 75
19H-6, 75
19H-CC, 13
19H-7, 40
20H-CC, 24
20H-CC, 24
20H-CC, 24
21H-CC, 14
22H-CC, 1
22H-CC, 1
22H-CC, 1
23H-CC, 16
24H-CC, 10
24H-CC, 10
24H-CC, 10
23H-4, 75
24H-1, 75
24H-1, 75
25H-CC, 10
24H-7, 40
26H-CC, 9
26H-CC, 9
28H-CC, 16
28H-CC, 16
29H-CC, 15
29H-5, 75
30H-CC, 0
29H-5, 75
29H-5, 75
31H-CC, 16
31H-CC, 16
32H-1, 75
33H-6, 75
32H-CC, 25
33H-6, 75
33H-6, 75
33H-6, 75
Core, section,
interval (cm)
148.21
148.86
156.28
158.00
163.94
176.81
172.29
175.30
176.81
176.46
186.41
186.41
186.41
195.79
205.44
205.44
205.44
214.67
224.17
224.17
224.17
210.28
215.25
215.25
233.68
223.96
243.48
243.48
262.48
262.48
271.95
268.78
281.24
268.78
268.78
291.07
291.07
291.25
308.29
300.54
308.29
308.29
308.29
Depth
(mbsf)
Bottom sample
(LO presence/FO absence)
Notes: FO = first occurrence, LO = last occurrence. CN = calcareous nannofossils, PF = planktonic foraminifers, D = diatoms.
FO Globorotalia plesiotumida
FO absence interval Reticulofenestra pseudoumbilicus >7µm
LO Discoaster hamatus
FO Neogloboquadrina acostaensis
LO Coccolithus miopelagicus
FO Globoturborotalia nepenthes
LO Coronocyclus nitescens
LO Cyclicargolithus floridanus
LO Globorotalia fohsi s.l.
LO Calcidiscus premacintyrei
FO Globorotalia fohsi s.l.
LO Sphenolithus heteromorphus
LO Globorotalia archeomenardii
LO Globorotalia peripheroronda
FO Globorotalia peripheroacuta
FO Globorotalia praemenardii
LO Globorotalia miozea
FO Globigerinoides diminitus
FO Globorotalia birnageae
LO Globorotalia semivera
LO Catapsydrax dissimilis
FO Calcidiscus premacintyrei
FO Sphenolithus heteromorphus
LO Sphenolithus belemnos
LO Globoquadrina binaiensis
FO Sphenolithus belemnos
LO Paragloborotalia kugleri
LO Globoturborotalia angulisuturalis
FO Globoquadrina binaiensis
FO Globoquadrina dehiscens
FO Globigerinoides trilobus
LO Reticulofenestra bisecta
FO Globigerinoides primordius common
LO Zygrhabdolithus bijugatus
LO Sphenolithus ciperoensis
FO Globigerinoides primordius
LO Sphenolithus distentus
LO Sphenolithus predistentus
LO Sphenolithus pseudoradians
FO Globoturborotalia angulisuturalis
FO Sphenolithus ciperoensis
LO Subbotina angiporoides
LO Turborotalia ampliapertura
Datum
Age (Ma)
Top sample
(FO presence/LO absence)
8.58
8.85
9.40
9.82
10.40
11.19
12.43
12.40
11.68
12.40
13.42
13.57
14.20
14.60
14.80
14.90
15.90
16.10
16.70
17.30
17.30
17.40
18.20
18.50
19.10
19.20
21.50
21.60
22.10
23.20
23.40
23.90
24.30
24.50
24.75
26.70
27.50
27.50
29.10
29.40
29.90
30.00
30.30
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.80
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Uncertainty
Average
(±)
Age (Ma)
143.44
148.81
155.53
153.49
163.19
171.85
171.54
174.55
171.85
175.88
181.61
186.18
181.61
191.10
200.62
200.62
200.62
210.06
219.42
219.42
219.42
209.53
214.96
214.96
228.93
223.39
238.58
238.58
257.64
257.64
267.22
268.03
276.60
268.78
268.78
286.16
290.77
291.16
307.54
295.81
307.54
304.42
304.42
156.06
163.61
170.33
168.29
178.92
188.39
188.89
191.90
188.39
193.23
199.36
204.33
199.36
209.78
220.39
220.39
220.39
231.31
242.17
242.17
242.17
231.68
237.71
237.71
253.50
246.74
264.31
264.31
286.40
286.40
297.55
299.64
309.21
300.39
300.39
320.67
326.18
327.77
347.40
332.42
347.40
355.28
355.16
4.78
0.06
0.75
4.51
0.75
4.96
0.75
0.75
4.96
0.58
4.80
0.23
4.80
4.69
4.83
4.83
4.83
4.62
4.75
4.75
4.75
0.75
0.29
0.29
4.76
0.57
4.90
4.90
4.84
4.84
4.74
0.75
4.65
0.00
0.00
4.91
0.30
0.09
0.75
4.74
0.75
3.88
3.88
Average Average Uncertainty
(mbsf)
(mcd)
(±m)
Depth
How Old Is It? Part 2 – Magnetostratigraphy
Table T10 (continued).
CHAPTER 8, SITE 1237
86
How Old Is It? Part 2 – Magnetostratigraphy
TM
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