Paleoclimatic indicators

MATCHING PALEOCLIMATIC INDICATORS WITH WATER AGES... 

Andrews J. N. and Lee D. J. (1979) Inert gases in groundwater from the Bunter Sandstone of England as indicators of age and paleoclimatic trends. J. Hydrol. 41, 233 —252. 

The first uses of radiocarbon in deep-sea core dating were based on few data points and depended on extrapolation assuming the constant rate of titanium deposition (Arrhenius et al., 1951) or interpolation (Suess, 1956) for determination of rates of accumulation and chronology. The first systematic study of radiocarbon incorporating possible changes in accumulation rates with depth in a core was performed by Broecker et al. (1958). They showed that accumulation rates of both the carbonate fraction and the detrital fraction varied with time in the equatorial Atlantic and those variations were linked to paleoclimatic indicators inferred from paleontologic data (Figure 3). 

Witzke B. J. and Heckel P. H. (1988) Paleoclimatic indicators and inferred Devonian paleolatitudes of Euramerica. In Devonian of the World Proceedings of the Second... 

Figure 5. Records of 5 0ct variation (given relative to the PBD standard) for speleothems from six different karst regions between 22° and 62° north latitude in North America and Bermuda after Harmon et al. (1978a). Vertical lines indicate suggested correlation between isotopic maxima and minima that are interpreted to denote paleoclimatic events. Small upward pointing arrowheads indicate U-series age determinations. Horizontal arrows labeled modern indicate the 5 0ct value of calcite presently forming a particular speleothem locality.

Hendy, C.H., 1971, The isotopic geochemistry of speleothems I. The calculation of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as paleoclimatic indicators, Geochimica et Cosmochimica Acta, 35 801-824. 

Whitney, J.W. Harrington, C.D. (1993) Relict colluvial boulder deposits as paleoclimatic indicators in the Yucca Mountain region, southern Nevada. Geological Society of America Bulletin 105, 1008-1018. 

Because of the implications for possible human causation of global warming, this is not just an academic issue. Rather, the ability of paleoclimatic indicators to allow quantitative reconstruction of paleotemperatures and other paleoclimatic variables influences public perceptions and perhaps public policy. It is much easier to believe that an event was human-caused if it is unique over the last millennium than if the recent change duplicates earlier, natural fluctuations. 

Our discussions with colleagues indicate that there is a wide range of opinions in the community on these issues. On one end of the spectrum is the desire to use our best understanding of proxies calibrated against modern or recent conditions to produce the quantitative estimates and uncertainties of paleoclimatic conditions desired by modelers and policy-makers. On the other end is the worry that we cannot assess the uncertainties because the uniformitarian hypothesis of time-invariance of the calibration is inherently untestable instead, we might focus on producing consistent interpretations, multiparameter reconstructions of likely conditions, or some other less-precise or less-quantitative though perhaps still-useful interpretation of the paleoclimatic data. 

And model skill allows evaluation of such questions as whether past changes in vegetation were forced by changes in water availability or in some other factor such as atmospheric carbon-dioxide concentration. Thus, and 5D of old water are among the most valuable of paleoclimatic indicators. 

Despite these promising early indications of the potential of excess air as a tool for hydrologic and even paleoclimatic investigations, later noble gas paleoclimate studies mostly treated excess air as a disturbance for which the measured data had to be corrected. A reason for this lack of interest may be that systematic variations of excess air have not been observed in aquifers from temperate climate zones (Stute and Sonntag 1992 Stute and Schlosser 2000 Aeschbach-Hertig et al. 2001). Only in recent years, the interest in the excess air signal has been revived. 

The paleoclimatic expression by clay mineral successions is direct or indirect, i.e., it either indicates the climate that actually prevailed at a given period, or reflects other events depending on climate migration of lithospheric plates across successive climatic zones, varying extension of ice caps controlling the surficial erosion, variations in the marine circulation regime due to changing latitudinal and... 

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