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Marine and continental records (1) show an abrupt negative shift in carbon isotope values at ~55.8 Myr ago. This carbon isotope excursion (CIE) is consistent with the release of a massive amount of isotopically light carbon into the atmosphere and was associated with a dramatic rise in global temperatures termed the Palaeocene-Eocene thermal maximum (PETM). Greenhouse gases released during the CIE, probably including methane, have often been considered the main cause of PETM warming. However, some evidence from the marine record suggests that warming directly preceded the CIE (2-4), raising the possibility that the CIE and PETM may have been linked to earlier warming with different origins. Yet pre-CIE warming is still uncertain. Disentangling the sequence of events before and during the CIE and PETM is important for understanding the causes of, and Earth system responses to, abrupt climate change. Here we show that continental warming of about 5°C preceded the CIE in the Bighorn Basin, Wyoming. Our evidence, based on oxygen isotopes in mammal teeth (which reflect temperature-sensitive fractionation processes) and other proxies, reveals a marked temperature increase directly below the CIE, and again in the CIE. Pre-CIE warming is also supported by a negative amplification of [δ.sup.13]C values in soil carbonates below the CIE. Our results suggest that at least two sources of warming--the earlier of which is unlikely to have been methane--contributed to the PETM., Abrupt warming during the PETM coincided with a profound reorganization of mammalian fauna as the first representatives of several modern groups dispersed among the northern continents (5). In North America, [...]

A high-resolution record of Eocene paleotemperature variation is preserved within the high southern latitude, marine shelf succession of the La Meseta Formation on Seymour Island, off the NE side of the Antarctic Peninsula. [sup.87] Sr[sup.86] Sr ratios of bivalve shell carbonate indicate that the La Meseta Formation spans virtually the entire Eocene, and suggest the presence of an early middle Eocene unconformity. Paleoclimatic and paleoceanographic inferences are based on the stable oxygen and carbon isotope values of two genera of bivalves collected with a high degree of stratigraphic resolution within the formation, and with multiple replicate samples from each horizon. [[delta]sup.18] O data indicate roughly 10[degrees] C of cooling from the early Eocene climatic optimum (~15[degrees] C) through the end of the Eocene (minimum ~5 [degrees] C), much of which took place in two comparatively short intervals at ca. 52 and ca. 41 Ma. Many features of the isotope curves generated from this Eocene shelf section are apparent in [[delta]sup.18] O and [[delta]sup.13] C data from the Southern and global oceans, including warm intervals that likely correspond to the early and middle Eocene climatic optima (EECO and MECO). A rapid middle Eocene shift to much more positive values is the most significant in the section and reflects a drop to universally cooler temperatures in the late middle and late Eocene that might also be associated with a short-lived glacial advance. However, even using a somewhat depleted value for [[delta]sup.18] O of seawater in the Antarctic peninsular region, average Seymour Island shelf-water paleotemperatures did not reach freezing before the end of the Eocene. [[delta]sup.13] C data similarly reflect the documented middle Eocene surface ocean enrichment followed by more negative values, but depletion is much more pronounced on Seymour Island and persists for the remainder of the Eocene, suggesting a combination of upwelling, metabolic effects, and/or atypical carbon cycling on the shelf in this region. Isotope data capture information about changes in the paleoenvironment that also had consequences for the biota, as published paleontological records document marked change in the nature of terrestrial and marine biota at this time. The fact that middle Eocene cooling and biotic turnover in the Peninsular region correspond well in time to the proposed initial opening of Drake Passage suggests that the formation of gateways, in addition to changes in [pCO.sub.2], had significant consequences for the Earth's climate system during the Paleogene. Keywords: stable isotope, Eocene, climate, bivalve, Antarctica, strontium.

Author(s): Linda C. Ivany (corresponding author) [1, 2]; William P. Patterson [2, 3]; Kyger C. Lohmann [1] The Eocene/Oligocene boundary, at about 33.7 Myr ago, marks one of the largest [...]

A large, global positive excursion in [Delta]13C (from -0.5 to 4.5[per thousand]) during the Late Cambrian Pterocephaliid biomere/Steptoean Stage (Aphelaspis-Elvinia zones) is documented at high stratigraphic resolution in three sections in the eastern Great Basin. The excursion, which we refer to as the Steptoean Positive Carbon Isotope Excursion, or SPICE excursion, began coincident with a world-wide extinction event. The [Delta]13C data from the Great Basin reflect global seawater values in a wide range of lithologies, including oolitic grainstones, wackestones, thrombolitic boundstones, and flat-pebble conglomerates. We use a section at Shingle Pass in the southern Egan Range to divide the Pterocephaliid biomere into eight isotope steps that represent equal divisions of the [Delta]13C rise and fall ([+ or -]4[per thousand]). This provides a basis for recognition of a revised chronostratigraphic framework for the Pterocephaliid biomere/Steptoean Stage. Strata deposited during the beginning of the SPICE excursion record a major change in the pattern of sedimentation in the eastern Great Basin. This is reflected in a siliciclastic-carbonate transition at Shingle Pass, Nevada, and a carbonate-siliciclastic transition at the House Range and Lawson Cove sections in Utah. A regional siliciclastic influx recognized throughout the Great Basin occurs near the peak of the SPICE excursion. Carbon isotope analyses from cratonal sections in Wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the SPICE excursion. The correlated changes in [Delta]13C, relative sea level, and the marine biota during the SPICE excursion provide remarkably detailed records of a major paleoceanographic event. We speculate that changes in sea level, climate, or tectonics may have triggered the SPICE excursion and coeval extinction event. Subsequent burial of organic carbon caused the increase in [Delta]13C and may have led to an interval of global cooling. The results of this study lend confidence to carbon-isotopic studies of pre-Mesozoic rocks.

Were the first metazoan reefs ecologically similar to modern tropical reefs, enabling them to persist under oligotrophic conditions? We tested the hypothesis of ecological similarity by employing a geochemical approach. Petrography, cathodoluminescence, trace elements, and stable isotope analyses of primary precipitates of the Lower Cambrian Ajax Limestone, South Australia, indicate preservation of original C isotopic composition. All primary carbonate components exhibit C isotopic values similar to the composition of inorganically precipitated fibrous marine cements, suggesting that archaeocyaths and the calcimicrobe Epiphyton precipitated skeletal carbonate in equilibrium with ambient seawater in the absence of vital effects. Such data do not support the contention that archaeocyaths possessed photosymbionts. However, a +0.5[per thousands] shift in [Delta]13C occurs in reefs developed under shallower-water conditions relative to deeper reefs. This shift suggests the stratification of primary production in Early Cambrian oceans. The pattern is similar to that seen in the modern ocean, whereby significant photosynthesis modulates the C isotopic composition of the photic zone.

Isotope ([Delta]18O) and minor element (Mg/Ca, Sr/Ca) analyses of a fossil shell of the marine mussel Mytilus trossulus (Gould) from the Whisky Run terrace at Coquille Point, Bandon, Oregon, enable reconstruction of a paleoclimatic profile for the temperate northeastern Pacific ca. 80-85 ka. The Whisky Run terrace is correlative with marine oxygen isotope ([Delta]18O) substage 5a, the final sea-level highstand of the last interglacial complex. Herein we provide sea-surface temperature estimates and report on a new technique that allows estimates to be made of [Delta]18O of seawater and riverine discharge from the skeletal chemistry of a single mussel shell. Estimates of mean annual temperature from Mg/Ca ratios are lower than those suggested by the accompanying fossil fauna, and significantly lower than present-day conditions. The data also suggest a decrease in the range of seasonal temperature extremes at 80 ka compared to today. Estimates of the fresh-water and seawater [Delta]18O end members, -4.5[per thousand] and +0.5[per thousand], respectively, are slightly greater than modern values.

Paleotemperatures have been widely deduced from skeletal 18O/16O ratios, but these are also dependent on salinity. Without an independent measure of salinity, 18O/16O ratios cannot provide accurate data on past temperature and climate. We grew marine mollusks (bivalves) in the field while real-time data on local seawater temperature and chemistry (Mg, Ca, [Delta]18O, and salinity) were gathered. Here we show that for Mytilus trossulus, skeletal Mg/Ca ratios provide an accurate measure of temperature and that weekly sea-surface temperatures may be estimated with an apparent accuracy of approximately [+ or -] 1.5 [degrees] C. Thus, with analyses of both Mg/Ca and [Delta]18O from the same specimen, it is possible to determine seawater temperature and [Delta]18O.

Paleomagnetic data from the interglacial Nyborg Formation (653 [+ or -] 7 Ma), northern Norway, compare with data from the Egersund dikes (ca. 650 Ma), southwest Norway, and demonstrate that tillite formations associated with the Varanger Ice Age in Baltica accumulated at latitudes greater than 30 [degrees] S. Vendian glaciations are associated with reorganization of the Rodinia supercontinent, but they are controlled by latitude. The breakup of Rodinia and opening of the proto-Pacific imposed clockwise rotation and southward movement of Laurentia-Baltica. On their combined approach toward the south pole, Baltica and northeast Laurentia were first glaciated during the Varanger Ice Age (ca. 650 Ma) and then by the Ice Brook glacial events (625-580 Ma) when Laurentia had reached intermediate southern latitudes. During the 625-580 Ma period Laurentia and Baltica drifted apart, resulting in the opening of the Iapetus ocean, but both continents remained in high southern latitudes during most of the Vendian.

Statistical models of hydrogeological heterogeneity are often used in aquifer and reservoir characterization. The number of data required to estimate objectively the spatial correlation structure of permeability, however, is often prohibitive. The objective of this study was to develop a better understanding of how information about depositional processes can be used to characterize hydrogeological heterogeneity. An outcrop of the fluvial/interfluvial Sierra Ladrones Formation of New Mexico was studied for this purpose. On the basis of previous studies of paleogeography and our own field observations, deposits of the Sierra Ladrones Formation are interpreted as marginal ancestral Rio Grande flood-plain and tributary deposits. Architectural elements were mapped over a 0.16-km squared peninsular outcrop of Pliocene-Pleistocene deposits of the central Albuquerque Basin. Geostatistical analysis of the architectural-element map data indicates non-orthogonal anisotropy in the horizontal direction. The orientations of the strongest (N30 degrees W) and weakest (N90 degrees E) correlation correspond to the orientation of the tributary system and the ancestral Rio Grande flood plain, respectively. In the vertical direction, the correlation structure exhibits exponential behavior corresponding to the average-element thicknesses. The results demonstrate that information about depositional environment can be used to help to quantify statistically sub-surface heterogeneity.