Pre Quaternary climate

Pre-Quaternary climate and environment conditions are not discussed in detail in this chapter but are included briefly to provide a general context for the subsequent sections. For longer time perspectives, including the paleoclimatic context of hominid evolution, the reader is referred to the chapter 16 by Etter (Patterns of Diversification and Extinction) of this handbook. One of the best indicators for global climatic conditions on timescales of millions of years is provided by the ratio of oxygen isotopes recorded in shells of deep-sea dwelling foraminifera preserved in deep-sea sediments. O Figure 12.1 shows one such record from a deep core drilled in the equatorial Pacific (Mix et al. 1995a, b). Such records are influenced by a number of local variables, including for example seawater temperature, but are nonetheless thought to provide an estimate of global atmospheric temperature and the volume of water stored in continental ice. Thus, O Figure 12.1 illustrates a time history of the transition from relatively warm, conditions of the late Miocene with little or no Northern Hemisphere glaciation, into the predominantly cold and glaciated, but highly variable, conditions of the late Quaternary.

Records such as the one shown in O Figure 12.1 highlight a number of intriguing questions about the nature and variability of pre-Quaternary climate dynamics. The primary driver of both the overall trend and much of the variability is thought to be the pattern of solar radiation incident on the Earth. Variations of the eccentricity of the Earth's orbit, from more circular to more elliptical, influence the degree of seasonality of climate, with opposite forcing in the northern and southern hemispheres. Over the past 5 million years, the eccentricity in earth's orbit varied quasiperiodically on timescales of 413,000 and 100,000 years. At the same time, changes in the tilt of the earth's axis, or

O Figure 12.1

A benthic foraminiferal oxygen isotope record from the eastern equatorial Pacific (ODP Sites 846 and 849) covering the past several million years. An overall trend to colder more glaciated conditions began approximately 3.5 million years ago. Superimposed on this overall trend is marked variability on 41,000-year timescales in the early Pleistocene followed by dramatically dominant 100,000 cycles of the Quaternary ice ages (Mix, et al. 1995a, b)

obliquity, varied with a fairly periodic oscillation on a timescale of 41,000 years. The effect of obliquity changes is primarily on the amount of solar radiation reaching high latitudes and has the same sign in both hemispheres. Finally, precession of the equinoxes, i.e., changes in the location of the earth within its elliptical orbit relative to the seasons varies with a primary period of about 22,000 years. Individual orbital forcing terms are not independent of one another, for example, the influence of precessional changes is coupled to eccentricity variations, the location of the earth within its orbit at a certain time of year being most important at times when the orbit is most elliptical.

Although changes in orbital parameters are the main external driver of paleoclimatic changes over the past several million years, internal dynamics of the climate system itself are far more important if one is interested in the actual temporal and spatial patterns of climatic change. For example, orbital changes do not explain why the global climate system responded primarily to the relatively weak 100,000 year forcing associated with eccentricity during the Quaternary, but to the 41,000 year forcing associated with obliquity during the late Pliocene and early Pleistocene. Nor do they explain the long-term cooling trend starting in the early Pliocene. Relevant processes internal to the Earth system that influence the global climate state on these timescales include concentrations of atmospheric greenhouse gases, the pattern and degree of reflectivity of the earth associated with changes in the biosphere and cryosphere, and orographic changes associated with tectonics.

One example of climatic variability driven primarily by a combination of external and internal system dynamics is the development and variability of the East Asian Monsoon. Conveniently, one of the most well-preserved terrestrial archives of climatic change over the Plio-Pleistocene come from loess deposits in central China. These widespread 100-300-m thick deposits are characterized by alternating layers of loess dust and interbedded paleosols. The loess was deposited during relatively cool periods dominated by northerly winter winds while the paleosols reflect pedogenesis associated with relatively warm conditions and the moisture bearing summer monsoon. The time series of monsoonal fluctuations captured in loess records has been extensively correlated with deep-sea oxygen isotope records and, not surprisingly, cool dry periods of strong loess deposition tend to correlate fairly well with periods of high global ice volume. As shown in the Lingtai loess section in O Figure 12.2 (An 2000), magnetostratigraphic measurements suggest an onset of loess deposition, and thus the inception of the East Asian monsoon, may have been as early as 7 Ma. The uplift of the Tibetan Plateau, a primary driver of continental aridity, may have already been substantial at that time. Loess records also indicate evidence for a later pulsed uplift in the Tibetan plateau around 3.5 million years ago, again consistent with the onset of strong northern hemisphere glaciation indicated by marine records.

Although intriguing, when thinking about paleoclimate records of the Plio-Pleistocene such as those shown in O Figures 12.1 and O 12.2, it is important to keep in mind several substantial caveats. Because there have been an enormous number of poorly understood processes involved in the development, preservation, and measuring of these archives, there remains enormous uncertainty in interpreting deceptively precise measurements of material properties such as oxygen isotopes and magnetic susceptibility in terms of anything beyond a qualitative impression of climatic parameters such as global temperature, ice volume, or east Asian monsoon strength. Quantitative estimates, or uncertainty ranges, provided in units such as temperature in degrees centigrade or ice volume in cubic meters, are usually, for good reason, avoided altogether. A relative paucity of climate proxies on these timescales makes it difficult to independently verify many of our interpretations of those we do have without employing circular logic. For example, the records are not absolutely dated, other than perhaps at a few critical points, and are usually tuned either to other records or orbital parameters, leading to obvious logical circularity. Although these types of problems are common to all paleoclimatic reconstructions, on all timescales, they become substantially less severe for Quaternary reconstructions due, largely, to the availability of ice cores.

O Figure 12.2

Magnetostratigraphy, lithology, magnetic susceptibility, and grain size variations of the Lingtai loess section (35°04'N, 107°39'E) covering the past several million years. A shift to stronger monsoon occurred approximately 3.5 million years ago. Marked variability on 41,000-year timescales in the early Pleistocene followed by dramatically dominant 100,000 cycles of the Quaternary ice ages is also evident (An 2000)

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