PerspectivePALEOCLIMATE

Tropical Paradise at the Cretaceous Poles?

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2199-2200
DOI: 10.1126/science.282.5397.2199

One can imagine the shock that went through the scientific community a century ago when Captain Larsen and his whaling crew discovered fossil wood in the northern Antarctic Peninsula [HN1]. This barren landscape is the most formidable place on the planet, inhabited only by moss and lichens. But Captain Larsen's surprising discovery meant that the polar climate had been warmer in the past, and it triggered the quest for an explanation. In the years that followed, leaf fossils, mosasaurs, pleisiosaurs, dinosaurs, marsupials, and diverse assemblages of mollusks [HN2] added to a growing body of evidence that polar temperatures in the deep past were warm and the equator-to-pole thermal gradient was low. As reported on page 2241 of this issue, Tarduno et al. (1) have now added champsosaurs [HN3] (see figure below) to the list of organisms once thought to be restricted to lower latitudes.

Champsosaur, freshwater fish eater.CREDIT: MARY PARRISH/NATIONAL MUSEUM OF NATURAL HISTORY, SMITHSONIAN INSTITUTION

Previous discoveries of terrestrial vertebrates at high latitudes included only dinosaurs and turtles. But their paleoclimatic importance is limited because it is not known whether dinosaurs were cold-blooded (ectotherms) or warm-blooded (endotherms) (2) [HN4] and turtles are known to survive subfreezing conditions by hibernating in well-protected burrows. However, the discovery of champsosaurs is important, as their occurrence on Axel Heiberg Island (72°N) [HN5] is in stark contrast to the tropical to subtropical distribution of their nearest living relatives, the crocodiles [HN6]. Although uncertainties remain regarding the strict relationship between crocodiles and champsosaurs, it is known that champsosaurs were ectotherms, their distribution in the fossil record and their body size were similar to those of crocodiles, and they were adapted for a mostly aquatic life. It is therefore likely that champsosaurs could not have tolerated prolonged exposure to subfreezing conditions. Their survival required that the temperature of the water in which they lived never fell below freezing so that air holes would remain available for breathing. Further, the critical minimum body temperature below which modern ectotherms of their size die is known to be 5°C (3).

The high-latitude paleobotanical record also provides convincing evidence of polar warmth during the Cretaceous. The occurrence of deciduous trees as far north as 82°N during the middle Cretaceous indicates that permafrost was absent, and the abrupt cessation of cell growth in their tree rings [HN7] reveals that winter darkness was the seasonal growth-limiting factor rather than cold temperatures (4). A more quantitative measure of terrestrial climate stems from the temperature-controlled size and shape relationships among modern leaf assemblages. This “leaf physiognomic” approach [HN8] to paleotemperature reconstruction has been applied mostly to latest Cretaceous and Tertiary floras with internally and externally consistent results. Its reliability is less certain, however, when used for mid-Cretaceous plant assemblages, because this was a time of evolutionary innovation and radiation among the angiosperms. Using the leaf physiognomy method, Herman and Spicer (5) estimate that the mean temperature of the warmest summer month in the Arctic during the Turonian and Coniacian ranged between 18° and 20°C, whereas the coldest winter month ranged from −4° to 0°C during the Turonian and 0° to 4°C during the Coniacian (see figure below). Mean annual temperatures estimated from the Alaskan North Slope with this method yield similarly mild temperatures.

Polar heat wave.

Arctic (left) and Southern Ocean (right) temperatures over the past 112 million years based on leaf physiognomy analyses of Arctic plant assemblages and oxygen isotope analysis of planktic and benthic foraminifera from Southern Ocean deep- sea sites. Isotopic paleotemperatures were calculated with the paleotemperatures equation of Anderson and Arthur (12), assuming a mean oceanic δ18O value of −1.2 per mil. Standard mean ocean water for an ice-free Earth (13). Surface-water paleotemperature estimates incorporate the latitudinal salinity correction of Zachos et al. (13). Lower and upper (? = uncertain) paleotemperature implications of champsosaur discovery by Tarduno et al. (1) are also shown.

The case for extreme high-latitude warmth during the middle Cretaceous has recently been strengthened by oxygen isotope paleotemperature estimates from extraordinarily well-preserved foraminifera [HN9] from the circum-Antarctic region (see figure on next page). An Aptian-Maastrichtian record from a deep-sea site in the southern South Atlantic [Deep Sea Drilling Project (DSDP) Site 511, Falkland Plateau] [HN10] reveals that the entire water column warmed abruptly during the early Turonian, with deep waters (∼1000-m paleodepth) reaching 18°C and surface waters reaching over 30°C at a site located at 59°S paleolatitude (6). The high-latitude ocean remained very warm from the Turonian through earliest Campanian, with surface waters varying between 20° and 27°C and deep waters varying between 14° and 16°C. This period of sustained warmth was followed by long-term cooling through the Maastrichtian, which yields the lowest temperatures of the Cretaceous (7).

Although tropical surface-water temperatures near the Antarctic Circle seem hard to believe for any period of Earth history, there are many reasons to trust the Site 511 data. First, the exquisite preservation of the shells analyzed indicates that there have been no secondary changes in their original isotopic values. Second, the site was located in the open ocean away from the influence of continental runoff, so riverine waters enriched in the light oxygen isotope (16O) cannot explain the highly negative oxygen isotope ratios of foraminifera from Site 511 (a decrease in 18O/16O ratios indicates carbonate precipitation under warmer conditions unless the oxygen isotopic composition of seawater is changed because of an unusual amount of evaporation, precipitation, or influence by continental runoff). Third, even if the estimates of surface water temperatures are inaccurate because of uncertainties about how much latitudinal changes in salinity affected the 18O/16O ratios in the upper water column during the Cretaceous, we are still left with the problem that ocean water at 1000-m depth was over 18°C warmer than today at the same latitude. And finally, oxygen isotope measurements of planktic and benthic foraminifera from other deep-sea sites are yielding results that are consistent with those from Site 511. This has been found for a site in the southern Indian Ocean (DSDP Site 258; 57°S), which yields surface water paleotemperatures that range between 20° and 24°C and deep waters that range between 14° and 16°C (6). And, most recently, a new benthic oxygen isotope curve generated from a deep-sea site in the subtropical western North Atlantic reveals a pattern similar to Site 511, with an abrupt warming in the early Turonian (with deep waters reaching 18°C), long-term warmth in deep waters lasting from the Turonian through Santonian, and long-term cooling through the Maastrichtian (8).

So why was the Cretaceous climate so warm? The different land-sea configurations provide a partial explanation [HN11]. In the middle Cretaceous, sea level was higher than at any other time during the past 250 million years. The greater proportion of continental surface covered by seawater resulted in reduced seasonal variations in temperature because of the lower surface albedo and greater thermal capacity of water. Seaways covering the Arctic, West Antarctica, and parts of East Antarctica also provided a means for heat transport to both poles throughout the year. With Australia against Antarctica and the Drake Passage closed, ocean surface currents sourced in the tropics reached further poleward than they do today, providing an additional moderating effect on Antarctic climate.

However, computer simulations of Cretaceous climate indicate that radiative warming caused by increased greenhouse gas concentrations (principally CO2) [HN12] were more important than paleogeography in explaining Cretaceous global warmth (9). Estimates of Cretaceous pCO2 generally range from four to eight times preindustrial values (10), and some intervals, such as the Turonian-Coniacian (1), may have exceeded this amount severalfold (perhaps explaining the warming spike observed for that time). Climate models have revealed, however, that although CO2-induced warming can approximate globally averaged temperatures for the Cretaceous, the models predict steeper latitudinal temperature gradients (both warmer tropics and colder poles) than geologic data seem to allow. This has led some to suggest that the oceans played a greater role in transporting heat from the tropics to the poles than they do today, particularly through sinking of dense, saline waters formed in restricted low-latitude basins (9). However, Sloan et al. (11) [HN13] calculated that doubling the ocean heat transport to balance the energy budget for the warm climate of the early Eocene would require a mechanistically prohibitive poleward flow of warm, saline water masses. These authors concluded that either the oceanic processes of a greenhouse world were very different from those of the present or some other mechanisms must be used to explain the low equator-to-pole temperature differences.

The new lines of evidence for extreme warmth at polar latitudes during the middle Cretaceous reveal that some basic processes of atmospheric and oceanic circulation are not adequately simulated in computer climate models [HN14]. Increasing sophistication of climate models by coupling atmospheric and oceanic simulations and incorporating such features as cloud and vegetation cover will help to narrow this gap. The most important clues to how the Earth System operated in a greenhouse world are recorded in an imperfect geologic record, but discoveries like those of Tarduno et al. (1) force refinements in hypotheses of greenhouse climate dynamics.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

The National Geographic Society presents maps of Antarctica and the Arctic Region.

Polar Web, managed by the Arctic Centre at the University of Lapland, Rovaniemi, Finland, is a guide to Internet resources dealing with the lands and waters surrounding the North Pole and the South Pole, and with other cold regions of Earth.

Polar Pointers from the Byrd Polar Research Center, Ohio State University, provides links to Internet resources about the Arctic and Antarctic.

The Encarta Web encyclopedia includes a brief article about paleoclimatology.

The Paleoclimatology Program of the National Oceanic and Atmospheric Administration is a central resource for paleoclimate data, research, and education. A primer on paleoclimatology is provided.

The divisions of the Mesozoic Age are outlined in this chart from T. M. Keesey's Dinosauricon Web site.

The University of California Museum of Paleontology (UCMP) provides a brief introduction to the Cretaceous and lists the subdivisions of the period.

P. Gore, Georgia Perimeter College, Clarkston, provides an overview of the Cretaceous period in lecture notes for a course on historical geology.

S. Baum, Texas Center for Climate Studies and Department of Oceanography, Texas A&M University, provides Web links to climatology and paleoclimatology and oceanography resources and a Glossary of Oceanography and the Related Geosciences with References.

The PaleoNet Pages, maintained by N. MacLeod, Department of Palaeontology, Natural History Museum, London, is a collection of Internet resources designed to enhance electronic communication among paleontologists. A mirror site at the Natural History Museum is available.

Links for Paleobotanists, compiled and maintained by K.-P. Kelber, Mineralogisches Institut, Universität Würzburg, Germany, is an extensive collection of annotated links to Web resources in paleobotany and paleontology.

The Virtual Earth, maintained by P. Ingram, School of Earth Sciences, Macquarie University, Sydney, Australia, is an annotated tour of Web resources for earth scientists.

The Delphi Project at the Godwin Laboratory at the University of Cambridge, UK, is an attempt to develop data storage facilities for marine geological paleoclimate research.

The Ocean Drilling Program Web site at Texas A&M University provides a description of its research activities. A press release and a preliminary scientific report about recent studies on Antarctic and Southern Ocean paleooceanography are available.

The Climate Puzzle: Climates of the Earth is a unit in the online hypertextbook Planet Earth and the New Geosciences from the University of Pittsburgh. The various methods used to determine past climates are discussed.

“Paleoclimatology and climate system dynamics” by J. Overpeck is a chapter in the U.S. National Report to IUGG, 1991-1994.

This Dynamic Earth, available online from the U.S. Geological Survey, is an introduction to plate tectonics and continental drift.

Numbered Hypernotes

1. A timeline of Antarctica exploration on the Antarctic Philatelic Home Page lists the 1892 fossil discoveries of Captain Carl Larsen; the explorers section includes an account of O. Nordenskjöld's 1901-1902 expedition, which was under Larsen's command. The Mining Company offers a feature about Antarctica and links to information about the continent.

2. The On-Line Medical Dictionary has an entry for Mosasaurus. The Hypertext Webster Gateway has an entry for mosasauria. The Augustana College Geology Department reports on dinosaur and other fossil discoveries by W. Hammer in Antarctica. Information about the Antarctic dinosaur Cryolophosaurus is provided on the Dinosauricon Web site. An illustration of a pleisiosaur is available in the What Is a Dinosaur? tutorial. Pleisiosaurs, mosasaurs, and molluscs are described in a presentation on the Mesozoic Seas available from the Department of Palaeontology, University of Bristol, UK.

3. The UCMP introduction to diapsids is illustrated with a champsosaur skull. The Royal Tyrrell Museum, Drumheller, Alberta, Canada, provides a brief description of a champsosaur and a photograph of its skeleton. J. Tarduno is in the Department of Earth and Environmental Sciences, University of Rochester; there is a Web page about the 1996 Arctic expedition to Axel Heiberg Island. In illustrated excerpts from his field journal, R. Cottrell, a graduate student in the Paleomagnetic Research Group, University of Rochester, describes finding champsosaur fossils on Axel Heiberg Island during the 1996 expedition.

4. Ectotherm and endotherm are defined in the On-Line Medical Dictionary. Dinobase, a Web site from the University of Bristol devoted to dinosaur information, provides definitions of endothermy and ectothermy in a discussion of whether dinosaurs were cold- or warm-blooded. The question “Hot-Blooded or Cold-Blooded?” in regard to dinosaurs is discussed on the UCMP Web site.

5. The location of Axel Heiberg Island is shown and photos of the island are provided by A. MacRae, Department of Geology and Geophysics, University of Calgary.

6. The Crocodile Specialist Group maintains a Web page devoted to the natural history and conservation of crocodilians.

7. Dendroclimatology and the other subdisciplines of dendrochronology are defined in S. Baum's Glossary of Oceanography and the Related Geosciences with References. A conference paper titled “Modelling tree-ring climatic relationships” by P. Horàéek is available from the International Union of Forestry Research Organizations Web site. H. Grissino-Mayer, Department of Physics, Astronomy and Geosciences, Valdosta State University, GA, maintains the Ultimate Tree-Ring Web Pages, which includes an introduction to the principles of dendrochronology. Tree Rings: A Study of Climate Change is a available from Athena, a project that develops instructional materials for students and teachers. The Alaska Science Forum, from the Geophysical Institute, University of Alaska Fairbanks, offers an article by C. Helfferich titled “Skinny trees and paleoforests.”

8. R. Spicer of the Palaeoenvironmental Research Group, Open University, UK, discusses plants as climatic indicators on his home page and in a presentation about CLAMP (Climate - Leaf Analysis Multivariate Program), a tool for interpreting past climates from fossil leaves.

9. Isotope is defined in the Glossary of Geological Terms from Iowa State University. An entry about oxygen isotope analysis is included in S. Baum's Glossary of Oceanography and the Related Geosciences with References. A definition of oxygen isotope record is included in an article on paleoclimatology by M. Maslin titled “Sultry last interglacial gets a sudden chill,” which is available from the Science for Everyone Web page of the American Geophysical Union. The Stable Isotope Laboratory of the School of Environmental Sciences, University of East Anglia, UK, provides a research report on an oxygen isotope study of the Princess Elizabeth Trough, Antarctica. S. Baum's Glossary of Oceanography defines foraminifera. UCMP provides an introduction to the foraminifera. The Department of Paleobiology of the National Museum of Natural History, Smithsonian Institution, provides an introduction to foraminifera and its foraminifer collection. A brief introduction to paleooceanography and the use of foraminifera to study changes is available from the Delphi Project.

10. A brief history of the Deep Sea Drilling Project is available from the Web page of the Ocean Drilling Program, its successor. The Lamont-Doherty Earth Observatory provides an introduction to deep sea coring. An image of the Falkland Plateau was the February 1997 IPT Image of the Month from the Image Processing for Teaching Project at the University of Arizona.

11. An illustrated presentation titled “Gondwana reconstruction and dispersion,” based on the Geological Map of Sectors of Gondwana by Maarten de Wit et al. (American Association of Petroleum Geologists,1988), is available from Datapages, Inc. A map of the late Cretaceous is available on the Paleogeographic Atlas of the World presented by the Institute of Geology and Palaeontology, University of Stuttgart, Germany. Global Earth History, presented by R. Blakey, Department of Geology, Northern Arizona University, Flagstaff, includes a collection of tectonic and paleogeographic maps of the late and early Cretaceous.

12. The Information Unit on Climate Change of the United Nations Environment Programme (UNEP) provides a Climate Change Information Kit that includes fact sheets about the greenhouse effect and the evidence for climate change from past climates and climate models; a more extensive series of UNEP fact sheets from 1993 includes a more detailed discussion of how records from past climates support the case for global warming. Information about climate change, the greenhouse effect, and heat distribution by the atmosphere and oceans are topics included in a presentation on Global Warming and Climate Change from the US Global Change Research Information Office. An article titled “Remembrance of things past: Greenhouse lessons from the geologic record” by T. Crowley appeared in the Winter 1996 issue of Consequences, an online publication of Saginaw Valley State University, MI.

13. L. Sloan's Web page at the Department of Earth Sciences, University of California, Santa Cruz, provides information about her research in paleoclimatology.

14. An introduction to global climate change including a discussion of climate modeling is presented by Environment Canada. P. Warwick, of the Palaeoclimates Group, Department of Meteorology, University of Reading, UK, discusses his research interest in linking the geological record to paleoclimate models. S. Baum, Department of Oceanography, Texas A&M University, provides annotated links to climate modeling groups. The Center for the Study of Carbon Dioxide and Climate Change offers a fact sheet on climate models. A presentation about the Parallel Climate Model is provided by the Climate Change Research Section of the Climate and Global Dynamics Division, National Center for Atmospheric Research.

15. B. T. Huber is in the Department of Paleobiology, National Museum of Natural History, Smithsonian Institution.

References

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