Regular intra-tooth variations in the delta(18)O value of mammalian tooth enamel phosphate (delta(18)O(p)) have been considered a potential measure of seasonal changes in continental climate variables since they were first observed. In order to investigate this possibility in more detail, analyses were made of teeth from a number of mammalian herbivores (sheep, cattle, elk, and pigs) that lived over a wide range of geographic locations, ecological settings, and climatic conditions (Iowa, Florida, Wyoming, Iceland,. England, Croatia, and the Philippines). The lack of intra-tooth delta(18)O(p) variations in teeth of cattle that were given tap water to drink provides strong evidence that the underlying cause of observed intra-tooth variations is primarily a change in the isotopic composition of ingested water. In concert with this interpretation, the range of intra-tooth delta(18)O(p) values and their absolute values from each locality mirror observed differences in the range and absolute delta(18)O values of local precipitation (delta(18)O(pt)) and in climate variables. Thus intra-tooth delta(18)O(p) values can indeed be considered a qualitative measure of seasonal climate change in continental settings. Quantitative use of intra-tooth delta(18)O(p) values as a climate proxy is possible, but is hindered by lack of detailed information on aspects of mammalian physiology, behavior, and perhaps local hydrology that may also play a role in influencing delta(18)O(p). This problem is exemplified by the different range in delta(18)O(p) values measured for sheep and cattle from the same locality around York, UK (3.4 vs. 2.6 parts per thousand, respectively). The observed difference most likely reflects a difference in the relative amount of leaf water ingested by the two species. Future studies of well-constrained samples are required to test physiological models and to develop empirical relations that accurately relate delta(18)O(p) to delta(18)O(pt). In addition to their use as indicators of seasonality, intra-tooth variations in delta(18)O(p) values provide valuable information for longer-term climate change and paleobiological investigations. Copyright (C) 1998 Elsevier Science Ltd.

Oxygen isotope records of Cenozoic sea water temperatures indicate that a rapid warming event known as the Latest Paleocene Thermal Maximum (LPTM) occurred during the otherwise gradual increase in world temperatures during the Late Paleocene and Early Eocene. Oxygen isotope analysis of the carbonate and phosphate components of hydroxyapatite found in mammalian tooth enamel and body scales of river-dwelling fish from the Bighorn Basin in Wyoming were made to investigate corresponding changes in the terrestrial climate. A comparison of carbonate and phosphate isotope data from modern and fossil material indicates that some diagenetic alteration of the fossil material has occurred, although systematically larger intra-tooth ranges in the oxygen isotope composition of carbonate indicate that it is more likely to have been affected than phosphate. Carbonate and phosphate from the ecologically diverse mammals and fishes both record a shift to higher oxygen isotope ratios at the same time and of the same duration as the LPTM. These shifts reflect a change in the isotopic composition of regional precipitation, which in turn provides the first evidence for continental climate change during the LPTM. Assuming the present-day relation between the oxygen isotope composition of precipitation and temperature applies to conditions in the past, and that animal physiology and behavior is relatively invariant over time, the isotopic shift is equivalent to an increase of surface temperature in western North America of several degrees. This result is consistent with the magnitude of high-latitude ocean warming, and provides a basis for relating marine and terrestrial oxygen isotope records to records of terrestrial biotic change. (C) 1998 Elsevier Science B.V. All rights reserved.

Correlations between mean annual temperature (MAT) and the weighted average oxygen isotope composition of yearly precipitation (delta(18)O(pt)) are well-known, but the utility of modern relations to make reliable estimates of temperature change over geological time is uncertain. This question has been addressed by using seasonal subsets of the global data base of temperature and isotopic measurements to represent two different climate modes. A comparison of middle- to high-latitude delta(18)O(pt)/temperature relations for each climate mode reveals (1) a significant offset between them, and (2) a difference in the strength of their correlations. The offset in relations is due to differences in temperature and water vapor budget in the tropics, and can lead to serious underestimates of temperature change. Differences in the strength of correlations arise from the influence of climate mode-specific, non-temperature factors on delta(18)O(pt). The overall result is that no single relation can be used in all cases to make unambiguous temperature estimates using a temporal record of delta(18)O(pt) values. One way to overcome these problems is to reconstruct delta(18)O(pt)/temperature relations for the time periods being investigated. If an appropriate proxy for delta(18)O(pt) is available, it may also be possible to estimate temperature without relying on delta(18)O(pt)/temperature relations. A promising alternative to these options is to use records of delta(18)O(pt) to test predictions of global climate models, an approach that may allow a reliable and more complete reconstruction to be made of climate change over geologic time. (C) 1999 Elsevier Science B.V. All rights reserved.

Neodymium-142 data for rocks from the Nuvvuagittuq greenstone belt in northern Quebec, Canada, show that some rock types have lower Nd-142/Nd-144 ratios than the terrestrial standard (epsilon Nd-142 = - 0.07 to - 0.15). Within a mafic amphibolite unit, 142Nd/144Nd ratios correlate positively with Sm/Nd ratios and produce a Sm-146-Nd-142 isochron with an age of 4280(-81)(vertical bar) (53) million years. These rocks thus sample incompatible- element- enriched material formed shortly after Earth formation and may represent the oldest preserved crustal section on Earth.

Andreasen and Sharma raise concerns about the neodymium-142 data and age that we reported for rocks from the Nuvvuagittuq greenstone belt in Quebec, Canada. We agree that the issue of accurate mass fractionation correction is important, but stand by our discussion of this issue in our original report and our conclusion that the variation in Nd-142/Nd-144 ratios reflects the decay of Sm-146 caused by Sm-Nd fractionation within 300 million years of Earth's formation.

U-Pb geochronology for the Nuvvuagittuq greenstone belt put a minimum age constraint of similar to 3.8 Ga for the supracrustal lithologies. Recent Nd-142 work raised the possibility that the dominant lithology of the belt formed at similar to 4.28 Ga, which would make it the only known remnant of Hadean crust preserved on Earth. The dominant lithology of the belt has a mafic composition that consists of gneisses ranging from cummingtonite amphibolite to garnet-biotite schist composed of variable proportions of cummingtonite + biotite + quartz, +/- plagioclase +/- garnet +/- anthophyllite +/- cordierite. The composition of this unit ranges from basalt to andesite and it is divided into two distinct geochemical groups that are stratigraphically separated by a banded iron formation (BIF). At the base of the sequence, the mafic unit is mainly basaltic in composition and generally has relatively low Al2O3 and high TiO2 contents, whereas above the BIF, the unit is characterized by high Al2O3 and low TiO2 contents and exhibits a wider range of compositions from basaltic to andesitic. The low-Ti unit can be further subdivided into a trace element depleted and a trace element enriched subgroup. The high-Ti unit is characterized by relatively flat REE patterns as opposed to the low-Ti gneisses, which display light REE-enriched profiles with flat heavy REE slopes. The incompatible element depleted low-Ti rocks have U-shaped REE profiles. The geochemical groups have compositional analogues in three types of ultramafic sills that exhibit the same stratigraphic succession. Generally, the mafic gneisses have low Ca, Na and Sr contents, with many samples having CaO contents < 1wt %. Such low Ca contents are unlikely to represent the original composition of their igneous precursors and are interpreted to reflect intensive alteration of plagioclase. These compositional characteristics along with the presence of cordierite + anthophyllite suggest that the protoliths of the mafic gneisses were mafic volcanic rocks exhibiting variable degrees of hydrothermal alteration. The high-Ti compositional type shares geochemical characteristics with tholeiitic volcanic suites with low Al2O3 and high TiO2 contents and is consistent with crystal fractionation at low pressures under dry conditions. In contrast, the low-Ti compositional group is geochemically similar to boninitic and calc-alkaline volcanic suites. The high Al2O3 and low TiO2 contents in the andesitic compositions suggest the early crystallization of Fe-Ti oxides and late appearance of plagioclase, and are more consistent with fractionation at elevated water pressures. The succession from 'tholeitic' to 'calc-alkaline' magmatism seen in the Nuvvuagittuq greenstone belt is typical of the volcanic successions of many younger Archean greenstone belts. Regardless of the exact tectonic setting, this volcanic succession suggests that the geological processes responsible for the formation and evolution of Archean greenstone belts were active at 3.8 Ga and perhaps as early as 4.3 Ga.

To investigate formation of the Earth's earliest continental crust, partial-melting experiments were conducted (at 900-1100 degrees C and 0.5-3.0 GPa) on two greenstones from the 4.3 Ga Nuvvuagittuq complex of Quebec, Canada. For comparison, experiments were also conducted on a compositionally similar but modern arc volcanic (a Tongan boninite). At 1.5-3.0 GPa and 950-1100 degrees C, the experimentally produced melts are compositionally similar to the tonalite-trondhjemite-granodiorite (TTG) granitoids that compose most of Earth's early continental crust, including a 3.66 Ga tonalite that encloses the Nuvvuagittuq Complex. Because the degree of melting needed to produce the TTG-like melts is comparatively high (> 30%), the relative concentrations of most incompatible elements in the melts are similar to those in their greenstone parent rocks. These greenstones have compositional affinities with modern subduction zone magmas and do not resemble mid-oceanic ridge basalts. That arc-like mafic rocks could have been selectively involved in TTG formation (in spite of their volumetrically subordinate status in most greenstone terrains) must reflect tectonic circumstances that were specific to their generation. These must have enabled accumulations sufficiently deep to melt at the 1.5-3.0 GPa needed to generate TTG magmas from eclogitic sources. They are also likely to have been related to some form of crustal recycling whereby mafic crust and water were returned to the mantle and arc-like mafic magmas generated as a consequence. To what degree these circumstances replicated modern plate tectonics is difficult to say, but it seems likely that, as in the modern Earth, the Hadean crust was organized into different tectonic environments and that one of these gave rise to the first continental crust.