Palaeogeography, Palaeoclimatology, Palaeoecology
“Too low” δ18O of paleo-meteoric, low latitude, water; do paleo-tropical cyclones explain it?
Introduction
The isotopic composition of hydrogen and oxygen in water is an efficient tool for tracing their origin and evolution. The hydrological cycle is a site of strong fractionation between these isotopes (Epstein and Mayeda, 1953, Dansgaard, 1954). Isotopic fractionation occurs during evaporation and condensation of water. At equilibrium the liquid is enriched in the heavy isotopes (18O and 2H(D)) with respect to the vapor phase. Thus, the condensation of vapor to rain drops removes preferentially the heavy oxygen and hydrogen isotopes and leaves in the vapor the light isotopes (16O and 1H). The resultant major feature of the hydrologic cycle is the depletion of meteoric water in the heavy isotopes of both elements. The main reason for this depletion is the Rayleigh distillation rainout effect of a limited water (vapor) reservoir in the atmosphere. Thus, both δD and δ18O of rainwater are negative compared to the mean ocean water (SMOW). As a result of a prolonged global sampling program (GNIP — Global Network of Isotopes in Precipitation; IAEA, 2000, Fig. 1) several regularities in their distribution have been established (Dansgaard, 1964, Mook, 2001). Phenomenologically δD and δ18O in meteoric water have been related to geographic parameters such as latitude, altitude, and distance from the coast (all of which are correlated with temperature), and to meteorological characteristics — the fraction of rainout from a cloud. The nature of the global changes has been summarized by Dansgaard (1964) as reflecting: a. the latitude effect — both oxygen and hydrogen becoming lighter from the equator poleward, δ18O reaching the extreme value of −62‰ in Antarctic ice (Epstein et al., 1965), b. the altitude effect — both elements being depleted in their heavy isotopes as the altitude of rain increases, c. the continentality effect — lowering δD and δ18O values from coastlines, the source of water evaporation, inland, and d. the amount effect — lowering of delta values with increased amount of rainfall at a given locality. Overall, Dansgaard (1964) stressed the good correlation between the isotopic composition of meteoric water and the surface temperature.
Whereas the present day distribution of δ18O in meteoric water has been studied with great detail, much less is known about that distribution in the geological past. Our major sources of information here are the analyses of dated solid phases that formed in equilibrium with meteoric water or its derivates: cave deposits (speleothems), lake deposits, fossils (fish, snails etc.) and soil. In most cases these deposits are carbonates; phosphates (mostly apatite) and silica (mostly quartz) have also been analyzed. The fractionation factors of oxygen isotopes between these solids and water, as well as their dependence on temperature, are known (Friedman and O'Neil, 1977) so that the isotopic composition of the water from which they formed can be reconstructed, within the limits of error on the assumed paleotemperature.
After such reconstruction one expects that the above outlined regularity, which has been observed for the present, should hold for the past as well. Thus, one expects fresh-water deposits that were formed when a location was at low latitude or at the coastline to have heavier isotopic values than a similar mineral formed when the locality has moved towards the pole or was uplifted. We present here three cases of an analysis of this kind that lead to significantly lower than expected values of δ18O in paleo-meteoric waters in Israel over a time range of about 60 million years (Upper Cretaceous to Miocene).
Present δ18O in rainwater in the southern parts of Israel (Fig. 1) varies between ~ −6‰ and −4‰ (Gat et al., 1994, Ayalon et al., 1998, IAEA, 2000). Throughout about 60 Ma (Cretaceous to Neogene), Israel was located at much lower latitudes, moving poleward from about 8°N in the Campanian to about 25°N in the Miocene (Scotese, 1994, Scotese et al., 1999, Besse and Courtillot, 2002). The isotopic composition of foraminifera from deep sea sediments (Savin, 1977), indicates that the earth surface has been cooling (though not homogeneously) for the past 100 Ma. The cooling trend is also indicated by presently accepted models (Barron, 1985, Tajika, 1998, Crowley and Berner, 2001). Hence in all three cases presented here (the Upper Cretaceous, the Eocene and the Miocene) one should expect higher than present temperatures. Indeed, the estimated average sea surface temperatures (SST) in that area of the Tethys were 26–30 °C for the Late Cretaceous (Campanian–Maastrichtian, Kolodny and Raab, 1988, Maestas et al., 2003, Puceat et al., 2007), 27–28 °C for the Eocene (Kolodny and Raab, 1988), and about 29 °C for the Miocene (Savin et al., 1985). Models that take into account latitudinal changes in δ18O of seawater (Zachos et al., 1994, Roche et al., 2006) suggest that these estimates are minimum values; the real low-latitude temperatures having been by several degrees higher. The present-day SST at the equivalent sites (in the equatorial Indian Ocean) is about 26 °C. If past temperatures were higher (as it was in most cases), fractionation between water and vapor was smaller resulting in somewhat heavier rain. Hence one should expect rainwater to be heavier, 18O-enriched, in comparison to present day rain. In fact, much lighter (as light as δ18O ~ −14‰) rainwater was deduced from analyzed solid phases. This has to be viewed in the perspective that the temperature effect on making the vapor heavier is secondary in its importance with respect to the amount effect that makes the rain lighter. The subject of this paper is presenting this apparent paradox and offering a possible explanation for it.
Section snippets
Sampling and results
Both siliceous and carbonate rocks were analyzed. The oxygen isotopic composition of all silicate samples was determined in the laboratories of The Institute of Earth Science, The Hebrew University of Jerusalem and all carbonate samples at the Geological Survey of Israel. Silica was analyzed by reacting the sample with fluorine or BrF5 (Taylor and Epstein, 1962, Clayton and Mayeda, 1963). Carbonates were analyzed by reaction with phosphoric acid.
The samples can be divided into the following
Discussion
The relationship between the oxygen isotopic composition of an aqueous phase, a solid in isotopic equilibrium with that phase, and temperature is described by what is termed as “paleotemperature equations”. When paleo-temperatures are calculated, an isotopic composition of the aqueous phase is assumed. Usually one has to choose between several available “paleotemperature equations”, some which are linear approximations, of the form: δS–δW ≈ A + B106/T2 (Clayton and Epstein, 1961) where δS
Conclusions
We propose a possible explanation to the occurrence of 18O-depleted meteoric waters in Upper Cretaceous to Miocene rocks in Israel. We propose that a possible explanation for this occurrence is a predominance of tropical cyclones in that area, in this time range. Paleogeography was favorable for TCs throughout the Middle Cretaceous to the Early Miocene, when the Tethys Sea extended many thousands of kilometers to the east and south of Israel, which was in a transition zone between the sea and
Acknowledgements
The collection of data for this study stretched over almost thirty years. Hence many people deserve our thanks. We shall limit ourselves to a very partial list. We are grateful to (by now Drs.) Albert Tarablous and Uri Frieslander (for the Mishash samples), Zvi (Kul) Karcz (the Matred samples) and Avner Ayalon (the Hazeva samples). We benefited much from discussions with Zvi Garfunkel, Hagai Ron and Benjamin Buchbinder on regional paleogeography. Boaz Luz read an earlier version of this
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