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HIGH-RESOLUTION APPROACH TO DOCUMENTING ANCIENT ENVIRONMENTAL CHANGE

Quaternary analyses of global environmental changes, and biological responses to them, are conducted at scales of years (varves) to tens of thousands of years with nearly continuously represented data (Figure 3.1), commonly sampled at the 1- to 10-cm stratigraphic scale. Excellent preservation of original sedimentological, geochemical, and paleobiological materials allows integrated environmental and ecological analyses through small increments

FIGURE 3.1 A comparison between the resolution of Quaternary data and time scales used in global change studies, and that possible from high-resolution Cretaceous paleoenvironmental studies in fine-grained marine facies. Quaternary data taken from Oeschger and Arquit (1991): Lower field of small black circles represent CO 2 values obtained from centimeter-scale (100- to 200 yr) intervals in ice cores at Byrd Station, Antarctica; the thin line graph above it connects d18O values from the same core and sample set. The triangles connected by a heavy dashed or solid line and superimposed on the Quaternary data set represent a typical high-resolution, centimeter-scale (1000-yr) sample interval for middle Cretaceous global change and mass extinction research, applied to the same data set. Note that all major climate trends shown by high-resolution Quaternary data are also shown by the data set representing Cretaceous sample intervals, and only the smallest fluctuations are lost between the 100-yr and 1000-yr scales.

of time. These analyses reveal the dynamics and mechanisms of global change at several scales (e.g., Thompson, 1991; Webb, 1991). However, claims that older stratigraphic data sets are not completely enough preserved or highly enough resolved to contribute significantly to global change research and predictive modeling, are rejected. High-resolution event stratigraphic methodology (Kauffman, 1988a; Kauffman et al., 1991) provides interdisciplinary data, at 100- to >1000-yr Quaternary scales, for Phanerozoic strata.

Rock accumulation rates (RARs) for marine strata, which may preserve the most continuous and diverse record of Phanerozoic environmental changes, range from <1 cm/1000 yr (basinal fine-grained facies) to >1 m/1000 yr (e.g., in coarse-grained turbiditic, slope fan, shoreface, foreshore, and estuarine channel facies). Whereas more rapidly deposited strata allow finer stratigraphic time divisions to be sampled easily, these facies commonly reflect episodic high-energy sedimentation events separated by erosive intervals and do not preserve a long, continuous record of environmental change. More slowly but more continuously deposited basinal marine and lacustrine sequences characterized by shales, mudstones, and biogenic pelagic or hemipelagic facies provide the best Phanerozoic record of global change at scales comparable to long-term Quaternary records.

Such data do exist and have been gathered largely through the application of methods inherent in high-resolution event stratigraphy (HIRES: Kauffman, 1988a; Kauffman et al., 1991). HIRES focuses the analysis of stratigraphic sections on the centimeter-scale in search of event and cyclic stratification (see papers in Einsele et al., 1991). These events are expressed as physically unique surfaces or thin intervals; as short-term geochemical excursions from background values; as short-term evolutionary and ecological phenomena; and as depositional cycle and hemicycle boundaries, all with regional to interregional extent. Thus, stratigraphic deviations from background patterns are emphasized, and data from various disciplines are integrated into holistic interpretations of these depositional events. Initially, these data comprise a working chronostratigraphy for regional correlations at very high levels of resolution (days to hundreds of years per event surface or thin stratigraphic interval, typically spaced at intervals hundreds to thousands of years apart; Kauffman, 1988a). The correlation potential for HIRES exceeds that of the best biozonation, geochronology, or magnetostratigraphy (Kauffman et al., 1991).

Ultimately, a diverse, high-resolution physical, chemical, biological, and cyclostratigraphic data base collected continuously over a significant interval of Phanerozoic time will enhance integrated analysis of dynamic changes in regional to global environments, and biological responses



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