Evolving ideas about the Cretaceous climate and ocean circulation
Introduction
The Cretaceous has long been recognized as a special episode in the history of the Earth and several of the most important ideas in geology derive from the study of Cretaceous rocks. Among the major stratigraphic units into which Earth history is divided, only the Cretaceous and the Carboniferous are named for unique sedimentary deposits. For the Cretaceous, the special type of rock is chalk (fr. craie for the rock, Crétacé for the time interval). Chalk was immortalized in Huxley's famous public lecture to the working men of Norwich, England and published in Macmillans Magazine (Huxley, 1868). When deep sea deposits were recovered from the North Atlantic in 1853 it became apparent that such deposits were not restricted to the late Mesozoic, but that somehow the oceanic plankton producing the modern Globigerina ooze had thrived in the seas covering the continents during the Late Cretaceous. The question as to how open-ocean plankton penetrated into shallow seas and even to the shore awaits a definitive answer.
A second peculiarity of the Cretaceous is evidence for warm conditions extending into the polar regions, at first conjectured from the presence of limestone (chalk) deposits in Denmark and Sweden (Lyell, 1837). During the latter part of the 19th century there was debate over whether the Earth had a meridional temperature gradient and climate zones prior to the Tertiary. After Lord Kelvin's calculations of the loss of heat by the Earth (Lord Kelvin, 1863, Lord Kelvin, 1864) it was thought that the internal heat flux from the cooling Earth exceeded the solar energy flux until the Cenozoic. Hence, most scientists believed that the climate of the entire planet had been initially hot, with gradual cooling during the Precambrian, Paleozoic and Mesozoic. The Cretaceous was the last epoch of geologic time during which the Earth was uniformly warmed by the heat flux from the interior of the Earth. With the beginning of the Cenozoic the radiation from the sun came to exceed the flux of energy from the interior of the Earth and permitted a meridional temperature gradient to develop. Today we know that the energy flux from the sun, measured in hundreds of watts per square meter, has dominated the heat flux from the interior of the Earth, measured in hundredths of a watt per square meter, since the early Archaean. However in the late 19th century cooling of the Earth was assumed to be not only responsible for pre-Tertiary climates, but to drive many geologic processes.
Neumayr (1883) countered the idea of homogenous climates prior to the Cenozoic by demonstrating that the biogeographic distribution of ammonites and other marine molluscs showed that meridional climate zones had existed at least since the beginning of the Jurassic. It was in this context that he applied the terms boreal, temperate, and equatorial to the latitudinally distinct fossil assemblages. He also recognized a number of distinct biogeographic regions and believed that the boundaries between the biogeographic provinces were determined by ocean currents. His ideas were challenged by Ortmann (1896) who argued that the distributions of fossils only reflected paleobiogeographic regions, not climatic differences, and that their boundaries had nothing to do with ocean circulation. However, Haug (1910), in his global account of stratigraphy, accepted Neumayr's ideas and recognized the importance of rudists as indicators of equatorial conditions. Haug used the terms “boréale, équatoriale, australe, and tempérée nord et sud” in describing climatic zones of the Mesozoic. Much work has been accomplished in using proxies to document Cretaceous temperatures and using numerical models to understand the climate of the Cretaceous, but uncertainties remain.
Shortly after the arguments for climate zones on a warm Earth had been developed, two revolutionary ideas were introduced into geology. Chamberlin (1899) proposed that changes in the amount of a greenhouse gas, CO2 might be responsible for the alternation of ice ages and interglacials and account for globally warm climates. Subsequently, he suggested that a reversal of the thermohaline circulation of the ocean might also be responsible (Chamberlin, 1906).
Another major idea arose as the concept of climate-related biogeographic provinces in the Jurassic and Cretaceous was developed further by Uhlig (1911). Although overthrusts in the Alps had been proposed earlier by Bertrand and others, Uhlig recognized that the apparent sharpness of the boundaries between the Alpine-Carpathian faunas and those of northern Europe were a result of large-scale horizontal movement of parts of the Earth's surface. His ideas of mobility of the Earth's surface anticipated the more general work of Alfred Wegener and modern plate tectonics. The most important tools in making plate tectonic reconstructions of the continents and oceans are seafloor magnetic lineations. Unfortunately, these are unambiguous only for the Atlantic. They are incomplete guides for reconstruction of the Indian and Pacific Oceans and their surrounding areas, and offer little if any information on the past locations of terranes. Because much of the Cretaceous seafloor has been subducted (57% of the latest Cretaceous, 80% of the mid-Cretaceous, and 95% of the earliest Cretaceous!), there are a variety of reconstructions differing particularly in treatment of the Caribbean, Tethys, Indian and Pacific Oceans (for further information see the detailed discussion in Hay et al., 1999).
Finally, rare cores of Cretaceous deep sea sediment were recovered in expeditions after WWII. Some of these contained black shales. On the first leg of the Deep Sea Drilling Project more extensive recovery of Cretaceous black shales was made in the deep western Atlantic off the Bahamas. This initiated an ongoing discussion about whether and how the ocean could become anoxic.
Because the Cretaceous was so different from the modern world, because its sediments are widespread and often visible in outcrops, and because it is the oldest period for which plate tectonic reconstructions can still be constrained by seafloor magnetic lineations, it will always be the best example of a radically different state of the Earth. Although studies of the Cretaceous climate are not often cited in works dealing with future climate change, background knowledge of the Cretaceous has shaped many of the arguments. The Cretaceous is a laboratory for testing ideas about causes and effects of global climate change.
Section snippets
The chalk problem
Early Cretaceous strata resemble those of most other geologic periods, but starting in the Cenomanian and lasting in many parts of the world until the end of the Maastrichtian, conditions became unique in Earth history. Chalk is a porous, earthy, limestone made up of the calcitic remains of calcareous nannofossils and planktonic foraminifera. As a deposit it is analogous to modern deep-sea Globigerina/nannoplankton ooze. However, it was deposited over broad areas of the continental platforms.
The problem of the warm, equable climate
As Neumayr (1883) and Uhlig (1911) had proposed, there was clearly a meridional climatic zonation, reflected in the modern use of the terms “Boreal” and “Tethyan” to describe the two major climate zones, even though the equator-pole temperature gradients were less than today. However, their “boreal realm” was in northern Europe, and on modern plate tectonic reconstructions it plots at a latitude of about 30° N (Voigt et al., 1999, Mutterlose et al., 2003). Hay (1995a) suggested that the Boreal
The anoxic ocean problem
Broecker (1969) published an informative abstract entitled “Why the deep sea remains aerobic,” observing that today the supply of oxygen to the deep ocean overwhelms any possible supply of organic carbon from the surface waters. At the same time, the Deep Sea Drilling project recovered black shales from the deep Atlantic off the Bahamas. This initiated a debate which has become the “anoxic ocean conundrum.” The problem has plagued many paleoceanographers: to achieve anoxia in the deeper waters
Summary and conclusions
The Cretaceous is a special episode in the history of the Earth. Its uniqueness was early recognized by giving the name of a unique rock type, chalk, to this interval of geologic time. Chalk turned out to be similar to modern deep-sea calcareous ooze, but the question remains; why was it deposited on the continental blocks only during the Cretaceous. In part, this may have been simply due to the evolution of the calcareous plankton. Planktonic foraminifera have their origins in the Jurassic and
Acknowledgments
This work was supported by grants from the Deutsche Forschungsgemeinschaft. The author is indebted to Sascha Flögel, Wolf-Christian Dullo, Silke Voigt, Elisabetta Erba, Erle Kauffman, Claudia Johnson, Enriqueta Barrera and many other colleagues for helpful discussions.
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