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Neodymium isotopes of fish debris from two sites on Demerara Rise, spanning ~4.5 m.y. of deposition from the early Cenomanian to just before ocean anoxic event 2 (OAE2) (Cenomanian-Turonian transition), suggest a circulation-controlled nutrient trap in intermediate waters of the western tropical North Atlantic that could explain continuous deposition of organic-rich black shales for as many as ~15 m.y. (Cenomanian--early Santonian). Unusually low Nd isotopic data ([[epsilon].sub.Nd(t)] ~-11 to ~-16) on Demerara Rise during the Cenomanian are confirmed, but the shallower site generally exhibits higher and more variable values. A scenario in which southwest-flowing Tethyan and/or North Atlantic waters overrode warm, saline Demerara bottom water explains the isotopic differences between sites and could create a dynamic nutrient trap controlled by circulation patterns in the absence of topographic barriers. Nutrient trapping, in turn, would explain the ~15 m.y. deposition of black shales through positive feedbacks between low oxygen and nutrient-rich bottom waters, efficient phosphate recycling, transport of nutrients to the surface, high productivity, and organic carbon export to the seafloor. This nutrient trap and the correlation seen previously between high Nd and organic carbon isotopic values during OAE2 on Demerara Rise suggest that physical oceanographic changes could be components of OAE2, one of the largest perturbations to the global carbon cycle in the past 150 m.y. doi: 10.1130/G31195.1

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, [...]

Detailed multitaxon stable isotope ([[delta].sup.18]O and [[delta].sup.13]C) data from Blake Nose (western North Atlantic) argue against a mid-Cenomanian glaciation event during the mid-Cretaceous greenhouse. Results generated are precisely correlated to sea-level changes inferred from European sequence stratigraphy using the twin [[delta].sup.13]C excursions mid-Cenomanian event (MCE) Ia and MCE Ib. Microfossils analyzed (surface-dwelling to deep-dwelling planktonic foraminifera, benthic foraminifera, coccoliths) show remarkably consistent intertaxon [[delta].sup.18]O and [[delta].sup.13]C offsets; comparative scanning electron microscope and Sr/Ca analyses allow some [[delta].sup.18]O data to be eliminated because of selective diagenesis. Across MCE Ia, the proposed interval of major glacioeustatic regression, the planktonic [[delta].sup.18]O values are constant for each taxon. The absence of a mean seawater [[delta].sup.18]O shift contradicts predictions for the mid-Cenomanian glaciation episode. The benthic [[delta].sup.18]O records show significant fluctuations during MCE I, implying short-term variability in North Atlantic intermediate-water and deep-water circulation patterns and/or sources at that time.

Late Cretaceous fish debris from Demerara Rise exhibits a dramatic positive excursion of 8 [[epsilon].sub.Nd] units during ocean anoxic event 2 (OAE2) that is superimposed on extremely low [[epsilon].sub.Nd(t)] values (-14 to -16.5) observed throughout the rest of the studied interval. The OAE2 [[epsilon].sub.Nd] excursion is the largest yet documented in marine sediments, and the majority of the shift is estimated to have occurred over Keywords: ocean anoxic event 2, black shale, Demerara Rise, Nd isotopes, Cenomanian-Turonian.

Differences in regional responses to climate fluctuations are well documented on short time scales (e.g., El Nino-Southern Oscillation), but with the exception of latitudinal temperature gradients, regional patterns are seldom considered in discussions of ancient greenhouse climates. Contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years, this paper shows that the North Atlantic warmed by as much as 6[degrees]C (1.5 [per thousand] decrease in [delta][sup.18]O values of planktic foraminifera) during the Maastrichtian global cooling interval. We suggest that warming was the result of the importation of heat from the South Atlantic. Decreasing North Atlantic [delta][sup.18]O values are also associated with increasing gradients in planktic [delta][sup.13]C values, suggesting increasing surface-water stratification and a correlated strengthening of the North Atlantic Polar Front. If correct, this conclusion predicts arctic cooling during the late Maastrichtian. Beyond implications for the Maastrichtian, these data demonstrate that climate does not behave as if there is a simple global thermostat, even on geologic time scales. Keywords: Maastrichtian, foraminifera, stable isotopes, paleoclimate, Ocean Drilling Program, greenhouse climate.

Analyses of [sup.87]Sr/[sup.86]Sr in foraminifera and sedimentological observations suggest that the Chicxulub impact was not the trigger for slumps or a hiatus within the Maastrichtian section recovered at Ocean Drilling Program Sites 1049, 1050, and 1052 (subtropical western North Atlantic). The slumps and hiatus occur within a sequence dominated by pelagic chalk. The [sup.87]Sr/[sup.86]Sr measurements show a general increase through the Maastrichtian at each site, but suggest chronostratigraphic gaps on the order of [10.sup.6] yr across the slumps. Some of the slumps have burrowed upper surfaces and are composed of material that is older and coarser grained than the bounding chalks. Pelagic deposition punctuated by gravity flows easily explains the Maastrichtian record on Blake Nose, whereas an impact-based explanation for slumping is difficult to reconcile with sedimentological and geochemical observations. Keywords: Cretaceous-Tertiary boundary, strontium isotopes, slump, Maastrichtian, Ocean Drilling Program.

Oxygen isotope analyses of well-preserved foraminifera from Blake Nose (30[degrees]N paleolatitude, North Atlantic) and globally distributed deep-sea sites provide a long-term paleotemperature record for the late Albian-Maastrichtian interval that is difficult to reconcile with the existence of significant Cretaceous ice sheets. Given reasonable assumptions about the isotopic composition of Cretaceous seawater, our results suggest that middle bathyal water temperatures at Blake Nose increased from ~12[degrees]C in the late Albian through middle Cenomanian to a maximum of 20[degrees]C during the latest Cenomanian and earliest Turonian. Bottom waters were again ~12[degrees]C during the middle Campanian and cooled to a minimum of 9[degrees]C during the Maastrichtian. Correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from Blake Nose. Comparison of global bottom-water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late Albian through late Cenomanian to a hot greenhouse phase during the latest Cenomanian through early Campanian, then to cool greenhouse conditions during the mid-Campanian through Maastrichtian. Keywords: Cretaceous climate, greenhouse effect, oxygen isotopes, ocean circulation, polar regions.

The rate, timing, and pattern of change in different regions and paleoenvironments are critical for distinguishing among potential causes for the Permian-Triassic (P-T) extinction. Carbon isotopic stratigraphy can provide global chronostratigraphic control. We report a large (delta)(super 13)C excursion at the P-T boundary and no long-term Permian (delta)(super 13)C trends for samples from the interior of Pangea. Stratigraphic gaps between available samples limit the resolution of our (delta)(super 13)C curve, but the excursion is within a 15-m-thick zone of overlap between Permian and Triassic taxa. Sedimentological and taphonomic observations demonstrate that this 15 m interval does not represent geologically instantaneous deposition. Together these data support a rapid and globally synchronous P-T event, but suggest that it occurred over a geologically resolvable interval of time.

The Maastrichtian and Danian intervals of Ocean Drilling Program (ODP) Hole 738C contain numerous microfossils above the level of their putative extinction, suggesting either (1) persistence of local communities long after species turnover occurred across the rest of the globe or (2) large-scale reworking. These interpretations have very different paleoenvironmental implications, but discriminating between them has proved difficult. To test the competing hypotheses, we measured the 87Sr/86Sr ratios of taxon-specific separates from a number of samples and compared these values both to each other and to expected seawater values at the time of deposition. Our results indicate extensive and pervasive reworking throughout Maastrichtian and lower Danian strata in ODP Hole 738C. We estimate that up to 30% of the mass of foraminifers in any sample can be contributed by individuals that have been reworked.

Radio isotopic data of ocean temperature and salinity predicts the biotic and environmental changes in the Late Cretaceous oceans of the Southern hemisphere. During biotic changes in the Late Cretaceous age, the ocean waters at the bottom cooled and became less saline, and the reorganization in the ocean circulation induced climatic changes. Inoceramid extinction is related to the palaeoceanographic change in the Late Cretaceous, and changes in the bottom-water source from low to high latitudes.

Notable among the biological changes of the middle part of the Maastrichtian Age was a major pulse of extinction among inoceramid bivalves. Analysis of the distribution of inoceramid shell fragments suggests that the population of burrowing organisms increased across the same stratigraphic interval that the bivalve population decreased. A global reorganization of ocean circulation that resulted in cooler, better oxygenated bottom waters is proposed as an explanation for the observed changes in the deep sea. Other observations regarding the mid-Maastrichtian are consistent with this hypothesis.

Previous studies of [sup.87]Sr/[sup.86]Sr patterns across the Cretaceous-Tertiary (K-T) boundary have generated inconsistent results. Analyses of samples from Ocean Drilling Program Hole 1049C provide better taphonomic and diagenetic control than has been previously achieved and indicate (1) that there was a rapid increase of ~0.000 030 in seawater [sup.87]Sr/ [sup.86]Sr ratios across the K-T boundary, (2) that post--K-T Cretaceous foraminifera at this site are reworked, and (3) that subtle diagenetic overprinting affects the basal ~15 cm of the Danian ooze. These conclusions are consistent with the asteroid impact hypothesis. Reworking rather than survivorship confirms nearly complete extinction of Cretaceous Tethyan planktic foraminifera; the [sup.87]Sr/[sup.86]Sr excursion can be explained by enhanced continental weathering, perhaps related to acid rain in the aftermath of the K-T impact. Keywords: Cretaceous-Tertiary boundary, reworking, foraminifera, extinction, strontium, chemostratigraphy.

Postel Nunatak in the Patuxent Range has been previously mapped as Nelson Limestone but there was no biostratigraphic support for that interpretation until now. We confirm that limestone exposures at Postel Nunatak are at least partly correlated with the Nelson Limestone of the Neptune Range, 160 km north-east, and are not correlative with the lower Cambrian Schneider Hills Limestone of the Argentina Range. Upper beds have yielded the trilobites Suludella? davnii Palmer & Gatehouse, 1972 and Solenopleura pruina Palmer & Gatehouse, 1972, which provide a basis for assignment to Cambrian Series 3 (late middle Cambrian), within the Drumian or lower Guzhangian stages. Limestone beds were deposited in a shallow marine setting, ranging from supratidal to lagoonal facies with rare subtidal intervals. These settings contrast with deeper water facies of the Neptune Range. Despite limitations in sampling density, isotopic analysis indicates that a greater than +2.5‰ shift in δ13C is consistent with δ13C trends documented for the Drumian Stage. Because the upper and lower contacts at Postel Nunatak are covered by snow and ice, the relationship with rocks mapped as the Patuxent Formation in the Patuxent Range remains uncertain, but part of it may belong to the Precambrian Hannah Ridge Formation. [ABSTRACT FROM AUTHOR]