Calcareous nannofossil assemblage changes across the Paleocene–Eocene Thermal Maximum: Evidence from a shelf setting

Biotic response of calcareous nannoplankton to abrupt warming across the Paleocene/Eocene boundary reflects a primary response to climatically induced parameters including increased continental runoff of freshwater, global acidification of seawater, high sedimentation rates, and calcareous nannoplankton assemblage turnover. We identify ecophenotypic nannofossil species adapted to low pH conditions (Discoaster anartios, D. araneus, Rhomboaster spp.), excursion taxa adapted to the extremely warm climatic conditions (Bomolithus supremus and Coccolithus bownii), three species of the genus Toweius (T. serotinus, T. callosus, T. occultatus) adapted to warm, rather than cool, water conditions, opportunists adapted to high productivity conditions (Coronocyclus bramlettei, Neochiastozygus junctus), and species adapted to oligotropic and/or cool‐water conditions that went into refugium during the PETM (Zygrablithus bijugatus, Calcidiscus? parvicrucis and Chiasmolithus bidens). Discoaster anartios was adapted to meso- to eutrophic, rather than oligotrophic, conditions. Comparison of these data to previous work on sediments deposited on shelf settings suggests that local conditions such as high precipitation rates and possible increase in major storms such as hurricanes resulted in increased continental runoff and high sedimentation rates that affected assemblage response to the PETM

Calcareous Nannofossil Evidence for the Existence of the Gulf Stream during the Late Maastrichtian

Upper Maastrichtian calcareous nannofossil assemblages, from eight cores on the South Carolina Coastal Plain (onshore set) and three deep sea drilling sites from the continental slope and abyssal hills (offshore set), were analyzed by correlation and principal component analysis to examine the ancient surface water thermal structure. In addition, a temperature index derived from independently published paleobiogeographic information was applied to the sample data. All three methods indicate a strong separation of the samples into onshore and offshore sets, with the offshore data set exhibiting significantly warmer paleotemperatures. The great disparity between these two sample sets indicates that there was a strong thermal contrast between the onshore and offshore surface water masses that persisted throughout the late Maastrichtian despite evident short-term changes in fertility, productivity, and community structure. This suggests the Gulf Stream was present as a major oceanographic feature during the late Maastrichtian

Upper Cretaceous calcareous nannoplankton paleoecology and its biostratigraphic consequences: Western central Atlantic Ocean

First and last occurrences of calcareous nannofossil taxa are generally thought to be synchronous across latitude and paleodepth, especially in temperate to tropical latitudes. This belief is particularly prevalent of regional studies, where stratigraphic sections examined for calcareous nannofossil content are derived from the same paleolatitude. However, even in these local studies, paleoceanographic and paleobiogeographic parameters can influence the fossil assemblage greatly, and models based on the assumption that first and last occurrences are synchronous may be incorrect. Two cores from the Blake Nose, Western Atlantic Ocean, and nine cores from the Atlantic Coastal Plain of South Carolina were examined for calcareous nannofossil content in an effort to better understand the role that paleoecology played in fossil distribution patterns. One new calcareous nannofossil genus and four new calcareous nannofossil species are described from Upper Cretaceous sediments. The first occurrence of Micula murus, a marker species for low- to mid-latitude sites, is shown to be diachronous across the study area and its usefulness as a biostratigraphic marker for neritic sediments is questioned. The evolutionary radiation and resulting biostratigraphic utility of species of Ceratolithoides, Lithraphidites and Micula is discussed in detail and their first and last occurrences are tied to the geochronologic timescale where possible. Differences in population abundance and species richness between deep ocean and nearshore areas are shown to have been controlled by paleoceanographic factors. The investigated Maastrichtian sections are representative of similar paleolatitudes and vary significantly only in their environment of deposition. This study illustrates the difficulties of creating age models based on floral first and last occurrences calibrated from different regions and the significant role that environment can play in determining calcareous nannofossil assemblages

Abundance of Discoasters during the PETM at Bass River, Wilson Lake and South Dover Bridge

The Paleocene-Eocene Thermal Maximum (PETM) is characterized by a transient group of nannoplankton, belonging to the genus Discoaster. Our investigation of expanded shelf sections provides unprecedented detail of the morphology and phylogeny of the transient Discoaster during the PETM and their relationship with environmental change. We observe a much larger range of morphological variation than previously documented suggesting that the taxa belonged to a plexus of highly gradational morphotypes rather than individual species. We propose that the plexus represents malformed ecophenotypes of a single species that migrated to a deep photic zone refuge during the height of PETM warming and eutrophication. Anomalously, high rates of organic matter remineralization characterized these depths during the event and led to lower saturation levels, which caused malformation. The proposed mechanism explains the co-occurrence of malformed Discoaster with pristine species that grew in the upper photic zone; moreover, it illuminates why malformation is a rare phenomenon in the paleontological record