238 Deep-Sea Sediments - Patterns, Processes, and Stratigraphic Methods
8.7.3 Magnetostratigraphy. Reversals of the Earth's magnetic field (already encountered when discussing sea floor spreading and age-mapping of the sea floor;
Sect 1.8) impart to deep-sea sediments a sequence of normal (N) and reversed (R)
magnetization, which is preserved under favorable conditions and can be measured.
The strength of the signal depends on the concentration of iron-rich minerals, mostly.
Results - a sequence of Nand R tags, one for each sample - can be matched to the
established record, as first shown by C. O. A. Harrison and B. Funnell in 1964. For
example, on going downward into the sediment from the sea floor, the first clear
switch from N-samples to R-samples would be expected to correlate to the BrunhesMatuyama boundary. This reversal event is dated at 790 000 years, based on radioactive decay of 40K and on counting astronomical cycles within the 8 18 0 record (cyclostratigraphy, Sect. 8.7.5).
When faced with "floating" sections, a match cannot be made uniquely, unless
other information is available (fixpoints from biostratigraphy, for example). The more
confirmation from other methods, the greater is the confidence that a particular match
Z
IQ.
W
C
~
o
SITE 310
North Pacif ic
~
8"C (%. POB, G. sIJbgloboso)
g + 0·8 .0.4 0 -0.4 - 0.8
cD 150
;:)
a"c ("10. B-1, G. sIJbgloboso) 8'3C(%. B- 1, Uvigerina spp.)
-0.6 -to -1.4
+0 .1 - 0.3 -0.1 -1-1
90
!
!
!
!
!
!
!
143
110
130
150
170
190
163
183
203
SITE 208
Southwest Pacific
8 U C ("to.POB, G. sIJbgloboso) 8 '3 C(%. POB, Oridorsolis sPP)
+to -0-6 -0.2 -0.2
+0.5 +0.1 -0-3
!
1
,
1
I I
!
!
21
1
!
!
1
1
VI 110 FAD T proteon,uo
110
9°l
fAD Amouro/i /hus 19P·
190
130
210
150
SITE 238
West Indian Ocean
Fig. 8.18. Correlation by chemo- and biostratigraphy. Global ocean shift in 1)13C values as seen in
benthic foraminifers, about 6 million years ago, identified by association with biostratigraphic
markers. FAD First appearance; T. praeconl'exa: a centric diatom: Amaurolithus sPf': a horseshoeshaped nannofossIl. Note consistent relatIonship of FAD Amaurolithus to shift In I) 3C . [B. U. Haq
et aI., 1980, Geology 8: 427; redrawn]
8.7.3 Magnetostratigraphy. Reversals of the Earth's magnetic field (already encountered when discussing sea floor spreading and age-mapping of the sea floor;
Sect 1.8) impart to deep-sea sediments a sequence of normal (N) and reversed (R)
magnetization, which is preserved under favorable conditions and can be measured.
The strength of the signal depends on the concentration of iron-rich minerals, mostly.
Results - a sequence of Nand R tags, one for each sample - can be matched to the
established record, as first shown by C. O. A. Harrison and B. Funnell in 1964. For
example, on going downward into the sediment from the sea floor, the first clear
switch from N-samples to R-samples would be expected to correlate to the BrunhesMatuyama boundary. This reversal event is dated at 790 000 years, based on radioactive decay of 40K and on counting astronomical cycles within the 8 18 0 record (cyclostratigraphy, Sect. 8.7.5).
When faced with "floating" sections, a match cannot be made uniquely, unless
other information is available (fixpoints from biostratigraphy, for example). The more
confirmation from other methods, the greater is the confidence that a particular match
Z
IQ.
W
C
~
o
SITE 310
North Pacif ic
~
8"C (%. POB, G. sIJbgloboso)
g + 0·8 .0.4 0 -0.4 - 0.8
cD 150
;:)
a"c ("10. B-1, G. sIJbgloboso) 8'3C(%. B- 1, Uvigerina spp.)
-0.6 -to -1.4
+0 .1 - 0.3 -0.1 -1-1
90
!
!
!
!
!
!
!
143
110
130
150
170
190
163
183
203
SITE 208
Southwest Pacific
8 U C ("to.POB, G. sIJbgloboso) 8 '3 C(%. POB, Oridorsolis sPP)
+to -0-6 -0.2 -0.2
+0.5 +0.1 -0-3
!
1
,
1
I I
!
!
21
1
!
!
1
1
VI 110 FAD T proteon,uo
110
9°l
fAD Amouro/i /hus 19P·
190
130
210
150
SITE 238
West Indian Ocean
Fig. 8.18. Correlation by chemo- and biostratigraphy. Global ocean shift in 1)13C values as seen in
benthic foraminifers, about 6 million years ago, identified by association with biostratigraphic
markers. FAD First appearance; T. praeconl'exa: a centric diatom: Amaurolithus sPf': a horseshoeshaped nannofossIl. Note consistent relatIonship of FAD Amaurolithus to shift In I) 3C . [B. U. Haq
et aI., 1980, Geology 8: 427; redrawn]