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Lithified marine sediments are not equitably distributed through time, raising the possibility that lithification masks biological signals when data from unlithified and lithified sediments are compared or combined. Using mollusk-dominated assemblages from the early Cenozoic of the Gulf Coastal Plain, we find that lithification conceals small taxa, decreases taxonomic resolution, and exacerbates the undersampling of rare taxa. Lithified assemblages appear less diverse and have less even abundance distributions than coeval unlithified samples. These limitations cannot be overcome by standardization procedures, nor are they likely to be circumvented by collecting larger samples. The effects of this bias, however, can be mitigated by restricting analyses to a single lithification state or to specific size classes. Since lithification selectively obscures small taxa, the magnitude of this bias will be most severe when organisms are particularly small, such as in the aftermath of mass extinctions. In the study area, lithification artificially protracts the recovery period following the Cretaceons-Paleogene mass extinction by ~7 m.y.

Abrupt global warming and profound perturbation of the carbon cycle during the Paleocene-Eocene Thermal Maximum (PETM, ca. 55 Ma) have been linked to a massive release of carbon into the ocean-atmosphere system. Increased phytoplankton productivity has been invoked to cause subsequent C[O.sub.2] drawdown, cooling, and environmental recovery. However, interpretations of geochemical and biotic data differ on when and where this increased productivity occurred. Here we present high-resolution nannofossil assemblage data from a shelf section (the U.S. Geological Survey [USGS] drill hole at Wilson Lake, New Jersey) and an open-ocean location (Ocean Drilling Program [ODP] Site 1209, paleoequatorial Pacific). These data combined with published biotic records indicate a transient steepening of shelf-offshelf trophic gradients across the PETM onset and peak, with a decrease in openocean productivity coeval with increased nutrient availability in shelf areas. Productivity levels recovered in the open ocean during the later stages of the event, which, coupled with intensified continental weathering rates, may have played an important role in carbon sequestration and C[O.sub.2] drawdown. Keywords: plankton, Paleocene, Eocene, paleoproductivity.

The strontium isotope ratios ([sup.87]SF/[sup.86]Sr) of marine barite microcrystals separated from Cretaceous sedimentary deposits from Ocean Drilling Program and Deep Sea Drilling Project sites from the Pacific and Indian Oceans have been compared to the composite Sr isotope curve of McArthur et al. The barite in these cores accurately recorded the seawater [sup.87]Sr/[sup.86]Sr ratio, thereby reaffirming the composite Cretaceous strontium curve. Moreover, marine barite is a more reliable recorder of [sup.87]Sr/[sup.86]Sr than is carbonate in sedimentary deposits with high clay content, thereby providing an opportunity for Sr isotope stratigraphy and dating in carbonate-poor or diagenetically altered sections. We have used the barite-derived Sr isotope record to refine the biostratigraphic age models of the sites investigated. Keywords: barite, strontium isotopes, stratigraphy, Cretaceous.

A distinctive mixture of reworked microfossils, impact-derived materials, and lithic fragments occurs in sediments at the Cretaceous-Tertiary boundary in the basinal Gulf of Mexico and Caribbean. We have named this mixture the Cretaceous-Tertiary boundary 'cocktail.' Lithologic and paleontologic evidence suggests that the cocktail was deposited by giant sediment gravity flows, apparently triggered by the collapse of continental margins around the Gulf of Mexico as a result of the Chicxulub impact. As most microfossils in the gravity-flow units are reworked, biostratigraphy provides only maximum ages. Recognition of the cocktail is a reliable way to identify Cretaceous-Tertiary boundary deposits in the basinal Gulf of Mexico and Caribbean.

The planktonic foraminiferal genera Morozovella and Acarinina rapidly (in [approximately]10 k.y.) diversified during the late Paleocene thermal maximum (LPTM), giving rise to such morphotypes as M. allisonensis (new species), M. africana, and A. sibaiyaensis. Single-specimen isotopic analysis confirms that M. allisonensis and A. sibaiyaensis are restricted to the LPTM carbon isotope excursion recorded at Ocean Drilling Program Site 865 (equatorial Pacific Ocean). The short-lived (50 to several 100 k.y.) 'excursion' taxa attest to the ephemeral effects of the LPTM on the calcareous plankton. Single-specimen oxygen isotope data show that evolution of M. allisonensis and A. sibaiyaensis was accompanied by migration to deeper water depths. Ancestral M. velascoensis and A. soldadoensis were extremely rare or absent during the early stages of the LPTM, but immigrated back into the study area to coexist with their descendants in later LPTM horizons. Photosymbiosis may have facilitated the morozovellid and acarininid diversifications during the oligotrophic conditions of the LPTM.

The allegation by Gerald R Dickens that the association of the late Paleocene thermal maximum (LPTM) with abrupt changes in the global carbon cycle was baseless is untrue. The assertion was based on a study by Dickens et al. in 1995. In the study, Dickens inadvertently contradicted himself when he claimed that modern global changes were influenced by the abrupt change in global carbon cycle during the LPTM. Moreover, the analysis of methane hydrate dissociation is more useful in understanding the LPTM than the massive carbon dioxide input proposed by Dickens.

Presents a reply relating to the article `High-Resolutions Records of the Late Paleocene Thermal Maximum and Circum-Caribbean volcanism: Is There a Causal Link?' Discussion on the late Paleocene thermal maximum in the Caribbean; Reference to the carbon isotope excursion; Decrease of CaCO3 in deep sea.