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170
Fifth North American Paleontological Convention
ISOTOPIC EVIDENCE FOR PALEOCLIMATIC
AND
PALEOATMOSPHERIC
VARIATIONS FROM THE PALEOGENE BIGHORN BASIN SEQUENCE
KOCH*, Paul
L.,
Carnegie Inst. of Washington, Geophysical. Lab.,
5251
Broad Branch
Rd.,
NW, Washington,
DC
20015 U.S.A.; DETTMAN, David
L.
and ZACHOS, James
C.,
Dept. of Geol. ScL, Univ. of Michigan, Ann Arbor,
MI
48109 U.S.A.
Land mammal faunas changed dramatically from the late Paleocene to the Eocene.
This interval was marked by substantial paleoceanographic changes, including marine
warming, and mass extinction of benthic foraminifera. Yet study of the impact of
marine events on continental climates and faunas
is
problematic, due to imprecision
in
marine/continental correlation.
Oxygen isotope measurements of paleosol carbonates, mammals, and bivalves from
the Paleogene Bighorn Basin can
be
used
to
estimate mean annual temperature (MAT).
The 0180 of paleosol carbonate is controlled by two factors: the 0180 of the groundwater
from which it crystallizes, and the temperature during crystallization. Because
mammals and bivalves secrete mineral
in
abiologically-restricted range of
temperatures, their isotopic compositions serve as proxy indicators
of
surface water
0180. The difference
in
0180 between these proxy indicators of water and paleosol
carbonate is used to calculate MAT through application of standard oxygen isotope
thermometry relationships. Calculated temperatures vary significantly throughout
the interval, with high values (>20°C)
in
the late Paleocene, lower values
in
the
earliest Eocene (10-20°C), and renewed warmth later
in
the Early Eocene.
In
modern temperate regions, MAT and the 0180 of precipitation are closely
correlated. Estimates for Paleocene/Eocene surface waters from tooth apatite and
bivalves range from
-8
to
-12%0
(SMOW). Today, precipitation with values this low
is usually found
in
cold regions (0-10°C). These temperature estimates are too low
in
light of other isotopic and paleobotanical indicators, which suggest very warm
temperatures
in
the Bighorn Basin during the Paleogene. We hypothesize that vapor
transport to the region was substantially different than at present. Apotential model
may
be
Amazonia, where wet season air masses lose
up
to
800/0
of their water vapor
as
they move across the basin, resulting
in
180-depleted rainfall despite awarm climate.
Variations
in
013C provide atool for marine/continental correlation. Carbon
in
tooth apatite and soil carbonate
is
derived from plants, which fix atmospheric
C02.
Atmospheric
C02
is,
in
turn,
in
isotopic equilibrium with marine carbonate. Because
of these links, marine carbonate, land plants, land mammals, and soil carbonate should
exhibit coupled carbon isotope variations.
In
the Paleogene, when C3 plants dominated
floras, isotopic variations due to shifting floral composition, such as those documented
in
the Miocene Siwalik Sequence, could not occur. Paleogene continental and marine
carbon isotope records are tightly coupled.
In
particular, there is
an
unusual marine
carbon isotope excursion immediately preceding the Paleocene/Eocene boundary, which
corresponds precisely to the major paleoceanographic changes and the extinction of
benthic foraminifera. Tooth apatite and soil carbonate from the Paleogene Bighorn
Basin also record this excursion, which occurs at the base of the Wasatchian, where
perrisodactyls, artiodactyls, and modern primates first appear. Carbon isotopes allow
tight marine/continental correlation of the Paleocene/Eocene boundary, and
demonstrate that major biologic and climatic events at this time were globally
synchronous.