Relative Abundance Studies

The possibility of reconstructing paleoclimates by using the relative abundance of a particular species, or species assemblage, in ocean sediment cores was first proposed by Schott (1935). Schott recognized that variations in the number of Globorotalia menardii (a foraminifera characteristic of subtropical and equatorial waters) are indicative of alternating cold and warm intervals in the past. However, 30 yr were to elapse before the availability of relatively long undisturbed cores and improved dating techniques enabled others to capitalize on Schott's work, and to develop his ideas further. For example, Ruddiman (1971) derived time series of the ratios of all warm-water species to cold-water species to obtain qualitative paleotemperature estimates that showed good correlations with oxygen isotope paleoglaciation curves. Although somewhat of an improvement over earlier studies based on individual species, Ruddiman recognized that the technique was still relatively simplistic in considering all species as equally "warm" or "cold" when gradations in their individual tolerances obviously exist.

In the early 1970s, major advances in paleoclimatic and paleo-oceanographic reconstructions were made by a number of workers. Multivariate statistical analyses of modern and fossil data were used to quantify former marine conditions ( "marine climates") in an objective manner (Imbrie and Kipp, 1971; Hecht, 1973; Berger and Gardner, 1975; Williams and Johnson, 1975; Molfino et al., 1982). The general approach in all of these studies is to calibrate the species composition of modern (core-top) samples in terms of modern environmental parameters (such as sea-surface temperatures in February and August). This is achieved by developing empirical equations that relate the two data sets together. These equations (transfer functions) are then applied to down-core faunal variations to reconstruct past environmental conditions (Fig. 6.18). Mathematically, the procedure can be simply expressed as follows:

where Tm and Tp are modern and paleotemperature estimates, respectively, Fm and F are modern and fossil faunal assemblages, respectively, and Xis a transfer coefficient (or set of coefficients).

A fundamental assumption in the use of transfer functions to reconstruct marine climates is that former biological and environmental conditions are within the "experience" of the modern (calibration) data set (as illustrated in Fig. 6.19). If this is not so, a no-analog condition exists and erroneous paleoclimatic estimates may result (Hutson, 1977). Another important assumption is that the relationship that currently exists between marine climate and marine fauna has not altered over time due, for example, to evolutionary changes of the species in question. However, perhaps the major uncertainty in these studies concerns the very nature of the calibration

FIGURE 6.18 Schematic quantitative paleoclimatic model. In step I, the transfer function (X) is calculated by calibration of the modern (core-top) foraminiferal data set (Fm) with modern sea-surface temperatures (Tm). In step 2 the transfer function is applied to a down-core (fossil) data set (Fp) to yield estimates of past temperatures (Tp) (Hutson, 1977).

FIGURE 6.18 Schematic quantitative paleoclimatic model. In step I, the transfer function (X) is calculated by calibration of the modern (core-top) foraminiferal data set (Fm) with modern sea-surface temperatures (Tm). In step 2 the transfer function is applied to a down-core (fossil) data set (Fp) to yield estimates of past temperatures (Tp) (Hutson, 1977).

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