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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 July 2000


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 CLIMAPís 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, Rialís 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



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