Tertiary Oceans: the Mid-Miocene Cooling Step
263
SILICA DEPOSITION
g
...
«
2! !l! ~
~
:l! '" &! '" R~~!£ .,
~
~
,...
'" "
'II;f'lt-oo;f
,...
'"
C;;
DSDP SITE ~ .. g ' "
_
C7I
'"
LATIT UDE ~ l:l ~ ~ 10 ~ ~
~
h 1->
' "
h
~"' . '" h
... N
'" ... '" '" ... "''''5
5
17
21
NORTH ATLANTIC
NORTH PACIFIC
Fig. 9.14. The middle Miocene "silica switch" from the North Atlantic to the North Pacific, suggesting the turning on of the Nordic heat pump about 15 million years ago. Silica-rich sequences
marked black. (F. Woodruff and S. M. Savin, 1989, Paleoceanog. 4: 87, based on a compilation by
G . Keller and J. A. Barron, 1983, Geol. Soc. Am. Bull. 94: 590]
record, the crucial transition appears as a drastic increase in oxygen isotope values in
benthic foraminifera, between 15 and 13 million years ago (Fig. 9.11). This increase
reflects both growth of ice on Antarctica, and a world wide cooling of abyssal waters,
which in tum reflects a fundamental change in deep circulation. A strong increase in
the diversity of planktonic foraminifers at the time suggests the development of a
strong thermocline below warm surface waters, providing for diversification of
plankton habitats which resulted in divcersification of plankton species.
The cooling step is preceded by a substantial carbon isotope excursion toward
heavy values. This carbon isotope anomaly is seen in all oceans, at all depths. The
excursion is synchronous with the onset of abundant sedimentation of organic-rich
sediments in the margins of the Pacific. Presumably, the increase in organic deposition preferentially extracts 12C from the ocean, so that the carbon isotope values of
dissolved inor~anic carbon in the ocean become enriched in the isotope 13C, which
increases the I C/ 12 C ratio as observed. In this manner, a link is established between
margin sedimentation, and the isotope record of deep-sea sediments. There is a possibility that the increased burial of organic carbon was in part responsible for the
mid-Miocene cooling that followed: the additional extraction of carbon from the
263
SILICA DEPOSITION
g
...
«
2! !l! ~
~
:l! '" &! '" R~~!£ .,
~
~
,...
'" "
'II;f'lt-oo;f
,...
'"
C;;
DSDP SITE ~ .. g ' "
_
C7I
'"
LATIT UDE ~ l:l ~ ~ 10 ~ ~
~
h 1->
' "
h
~"' . '" h
... N
'" ... '" '" ... "''''5
5
17
21
NORTH ATLANTIC
NORTH PACIFIC
Fig. 9.14. The middle Miocene "silica switch" from the North Atlantic to the North Pacific, suggesting the turning on of the Nordic heat pump about 15 million years ago. Silica-rich sequences
marked black. (F. Woodruff and S. M. Savin, 1989, Paleoceanog. 4: 87, based on a compilation by
G . Keller and J. A. Barron, 1983, Geol. Soc. Am. Bull. 94: 590]
record, the crucial transition appears as a drastic increase in oxygen isotope values in
benthic foraminifera, between 15 and 13 million years ago (Fig. 9.11). This increase
reflects both growth of ice on Antarctica, and a world wide cooling of abyssal waters,
which in tum reflects a fundamental change in deep circulation. A strong increase in
the diversity of planktonic foraminifers at the time suggests the development of a
strong thermocline below warm surface waters, providing for diversification of
plankton habitats which resulted in divcersification of plankton species.
The cooling step is preceded by a substantial carbon isotope excursion toward
heavy values. This carbon isotope anomaly is seen in all oceans, at all depths. The
excursion is synchronous with the onset of abundant sedimentation of organic-rich
sediments in the margins of the Pacific. Presumably, the increase in organic deposition preferentially extracts 12C from the ocean, so that the carbon isotope values of
dissolved inor~anic carbon in the ocean become enriched in the isotope 13C, which
increases the I C/ 12 C ratio as observed. In this manner, a link is established between
margin sedimentation, and the isotope record of deep-sea sediments. There is a possibility that the increased burial of organic carbon was in part responsible for the
mid-Miocene cooling that followed: the additional extraction of carbon from the