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Highlights
Paleoceanography
Larry C.
Peterson and Lisa C. Sloan
The interrelated fields of paleoceanography and
paleoclimatology seek to understand the causes and mechanisms of past climate
states and climate variability. The following sampling of research from
the past year highlights the unique role that the geologic record can play
in understanding climate change.
Tropical temperatures and
orbital pacemakers
Controversy over ice age temperatures in the tropical
ocean has persisted for more than 30 years, with some geochemical data
and models yielding estimates cooler than those inferred from microfossil-based
studies such as CLIMAP. Using a new faunal calibration strategy, Alan Mix
(Oregon State University) and colleagues reassessed planktonic foraminiferal
estimates of sea surface temperature in the Atlantic and eastern Pacific
and found that classical transfer function methods underestimate temperature
changes in some regions of the tropical ocean (Paleoceanography,
v. 14, p. 350, 1999). Glacial cooling of 5 to 6 degrees Celsius is indicated
for the equatorial current systems of these regions, while little change
from CLIMAP’s inference of temperature stability for the subtropical gyre
centers was found. Using an atmospheric general circulation model to examine
climatic implications of the revised estimates, Steven Hostetler (U.S.
Geological Survey) and Mix (Nature, v. 399, p. 673, 1999) found
that the cooler equatorial ocean temperatures modified seasonal air temperatures
by 1 to 2 degrees Celsius or more across parts of South America, Africa
and southeast Asia, leading to modified regional moisture patterns. These
observations may help to resolve some discrepancies between continental
and marine proxy climate data.
One shortcoming of the astronomical or Milankovitch
theory has been a satisfactory explanation for how changes in orbital eccentricity
are transformed into the 100,000-year quasiperiodic record of ice volume
changes preserved in late Pleistocene oxygen isotope records. José
Rial of the University of North Carolina (Chapel Hill) showed that a frequency
modulation of the 100,000 year eccentricity-related changes in insolation
by the longer 413,000 year eccentricity component can explain the variable
duration of glacial periods, as well as the notorious absence of significant
spectral peaks at the 413,000-year period (Science, v. 285, p. 564,
1999). Although a specific physical mechanism has yet to be identified,
Rial’s hypothesis strengthens the view that orbital forcing is the ultimate
pacemaker of the ice ages.
Ice core paleoclimatology
| J.R. Petit (CNRS, France) and others (Nature,
v. 399, p. 429, 1999) extended the Antarctic Vostok ice-core record of
atmospheric composition and climate back through four complete glacial-interglacial
cycles. In a pattern similar to that observed during the past two climate
cycles, atmospheric records of CO2 and methane were found to
correlate well with Antarctic air temperature over the extended record,
strengthening the idea that greenhouse gases contribute significantly to
glacial-interglacial change. Present-day levels of these two key greenhouse
gases were never approached within the natural range of variability throughout
the past 400,000 years.
Jacqueline Flückiger (University of Bern)
and others describe variations in atmospheric N2O, another important greenhouse
gas, from the Summit ice core site in central Greenland (Science,
v. 285, p. 227, 1999). Data from the last glacial-interglacial transition
and an abrupt climatic event during the last glacial (Dansgaard-Oeschger
Event 8) show that N2O concentrations paralleled the rapid temperature
variations recorded in the northern hemisphere. Likely sources for the
changing N2O values are the oceans and terrestrial soils; ongoing efforts
to constrain the origins of this signal should increase our understanding
of the response of the environment to climate change. |
In preparation for drilling ice cores, a research team in
Pucahirca in the Cordillera Blanca of the Purvian Andes
assembles a satellite-linked weather station in 1991.
Lonnie G. Thompson, Bryd Polar Research Center,
Columbus Ohio. |
Ancient methane burps
Compelling new evidence adds support to the hypothesis
that massive methane release from the dissociation of frozen sea floor
gas hydrates contributed to the exceptional warmth of the Late Paleocene
Thermal Maximum. Miriam Katz of Rutgers University and others reported
discovery (Science, v. 286, p. 1531, 1999) of a buried sediment
sequence on the Blake Nose off northeastern Florida that contains a complete
record of deep sea changes predicted by the "methane burp" hypothesis.
In addition to finding the distinctive carbon isotope signature and benthic
foraminiferal extinctions associated with this event, Katz and her colleagues
found a mud-clast interval in cores recovered by the Ocean Drilling Program
and seismic evidence for a large-scale submarine landslide. These observations
are consistent with slope failure triggered by hydrate dissociation.
Although Blake Nose now seems a likely site for
methane release during the Late Paleocene Thermal Maximum, this area alone
cannot explain the magnitude of global perturbations at this time. Santo
Bains (Oxford University) and others (Science, v. 285, p. 724, 1999)
describe evidence for multiple injections of 12C-rich methane
into the ocean, suggesting that methane dissociation probably occurred
at more than one time and place. Studying the same Blake Nose cores, Richard
Norris (Woods Hole Oceanographic Institution) and Ursula Röhl (University
of Bremen) used sedimentary iron cycles, orbitally tuned to precession,
to better constrain the timing of the thermal maximum events (Nature,
v. 401, p. 775, 1999). They conclude that roughly two-thirds of the estimated
trillion tons of methane required to produce the Late Paleocene Thermal
Maximum carbon-isotope anomaly was released in 10,000 years or less, a
rate comparable to that at which burning of fossil fuels is presently adding
carbon dioxide to the atmosphere.
Cenozoic atmospheric carbon
dioxide levels
Two investigations of ancient atmospheric carbon
dioxide have provoked intriguing questions about Cenozoic changes in pCO2.
and about our understanding of climate sensitivity. Mark Pagani (University
of California, Santa Cruz) and colleagues Mike Arthur and Kate Freeman
(Pennsylvania State University) reconstructed late Oligocene to late Miocene
pCO2.
concentrations based on carbon isotope analyses of diunsaturated alkenones
and planktonic foraminifera from three sites around the globe (Paleoceanography,
v. 14, p. 273, 1999). They found that atmospheric pCO2.
during the late Oligocene was near present values (~350 parts per million
by volume), while pCO2. during the Miocene was only
260-190 parts per million by volume. Surprisingly,
pCO2.
changed little at times in the Miocene when it has been hypothesized to
play a major role in climate change. Paul Pearson (University of Bristol)
and Martin Palmer (Royal School of Mines) used boron isotopic compositions
of planktonic foraminifera to reconstruct seawater pH-depth profiles and
atmospheric pCO2. for the middle Eocene (Science,
v. 284, p. 1824, 1999). Their best estimate of middle Eocene pCO2
was 370-400 parts per million by volume, not much greater than the present-day
value. Both studies revive questions about the importance of factors other
than atmospheric pCO2. for explaining past climate
changes. Clearly, much remains to be discovered about the sensitivity of
Earth's climate to small changes in atmospheric
pCO2.
Peterson is an associate professor
of marine geology and geophysics at the Rosenstiel School of Marine and
Atmospheric Science in Miami. He focuses on Neogene paleoceanography and
paleoclimatology. E-mail: lpeterson@rsmas.miami.edu
Sloan is an associate professor
at the University of California, Santa Cruz, and specializes in paleoclimate
modeling. E-mail: lsloan@earthsci.ucsc.edu
