Stable carbon and oxygen isotope ratios across the Cretaceous/Tertiary boundray on the Walvish Ridge

The isotopic composition and diversity of nannofossils were studied in cores from the Deep Sea Drilling Project (DSDP) Sites 525A, 527, 528, and 529 from the Walvis Ridge, South Atlantic to better understand the changes which occurred across the Cretaceous/Tertiary boundary (K/T boundary). The stratigraphic range of the samples is from the Arkhangelskiella cymbiformis Zone in the Maastrichtian to the Heliolithus kleinpelli Zone in the Danian. Nannofossil diversity was high (Shannon-Weaver diversity index, 'H= 2.5-3) in the late Cretaceous, but decreased sharply (H c. 1 ) across the K/T boundary. The delta13C values also decrease across the K/T boundary at the four sites, suggesting a reduction in surface productivity in the South Atlantic concomitant with the reduction in diversity. During the Danian, nannofossil diversity and delta13C show some recovery approximately 500-700 k.y. after the boundary event. However, not until 2.5 Ma after the boundary event did diversity become constant. Diversity values similar to those for the late Cretaceous were not attained again in the early Paleocene interval studied. Carbon isotopic compositions similar to those from the Cretaceous were not attained until 4.5 Ma after the K/T event.

Maastrichtian calcareous nannofossils of the Indian Ocean and Shatsky Rise

The biotic effects of volcanism have long been the unknown factors in creating biotic stress, and the contribution of the Deccan volcanism to the K-T mass extinction remains largely unknown. Detailed studies of the volcanic-rich sediments of Indian Ocean Ninetyeast Ridge Sites 216 and 217 and Wharton Basin Site 212 reveal that the biotic effects of late Maastrichtian volcanism on planktic foraminifera and calcareous nannofossils are locally as severe as those of the K-T mass extinction. The biotic expressions of these high stress environments are characterized by the Lilliput effect, which includes reduced diversity by eliminating most K-strategy species, and reduction in specimen size (dwarfing), frequently to less than half their normal adult size of both r-strategy and surviving K-strategy species. In planktic foraminifera, the most extreme biotic stress results are nearly monospecific assemblages dominated by the disaster opportunist Guembelitria, similar to the aftermath of the K-T mass extinction. The first stage of improving environmental conditions results in dominance of dwarfed low oxygen tolerant Heterohelix species and the presence of a few small r-strategy species (Hedbergella, Globigerinelloides). Calcareous nannofossil assemblages show similar biotic stress signals with the dominance of Micula decussata, the disaster opportunist, and size reduction in the mean length of subordinate r-strategy species particularly in Arkhangelskiella cymbiformis and Watznaueria barnesiae. These impoverished and dwarfed late Maastrichtian assemblages appear to be the direct consequences of mantle plume volcanism and associated environmental changes, including high nutrient influx leading to eutrophic and mesotrophic waters, low oxygen in the water column and decreased watermass stratification.

Paleocene-Eocene calcareous nannofossils of onshore wells from the Coastal Plain of New Jersey and Maryland, USA

Paleogene calcareous nannofossils from split spoon cores recovered from five wells along the Coastal Plain of New Jersey and Maryland have been analyzed in order to provide onshore information complementary to that derived from the offshore DSDP Site 605 (upper continental rise off New Jersey). Hiatuses are more numerous and of greater extent in the onshore sections, but the major ones correlate well with those noted in the offshore section. At one site at least (Leggett Well), sedimentation may well have been continuous across the Cretaceous/Tertiary boundary, as it is believed to have been at DSDP Site 605. These various correlations are discussed elsewhere in a companion paper (Olsson and Wise, this volume). Important differences in nannofossil assemblages are noted between the onshore (shelf paleoenvironment) and offshore (slope-rise paleoenvironment) sections. Lithostromation simplex, not present offshore, is consistently present onshore and seems to be confined to the Eocene shelf sediments of this region. The same relationship holds for the zonal marker, Rhabdosphaera gladius Locker. The Rhomboaster-Tribrachiatus plexus is more diverse and better preserved in the onshore sections, where the lowermost Eocene Zone CP9 is well represented. Differential preservation is postulated to account for two morphotypes of Tribrachiatus bramlettei (Brönnimann and Stradner). Type A is represented at DSDP Site 605 by individuals with short, stubby arms, but these forms are not present in the equivalent onshore sections. There they are replaced by the Type B morphotypes, which exhibit a similar basic construction but possess much longer, more delicate arms.

Cretaceous calcareous nannofossils from Exmouth and Wombat Plateau, Northwest Australia

Three sites drilled during Leg 122, Site 761 on the Wombat Plateau and Sites 762 and 763 on the Exmouth Plateau, provide a composite Cretaceous section ranging in age from Berriasian to Maestrichtian. Together, these sites contain an apparently complete, expanded Aptian-Maestrichtian record. Consistently occurring and moderately well-preserved nannofossil assemblages allow reasonably high biostratigraphic resolution. Our data indicate that traditional middle and Upper Cretaceous nannofossil biozonations are not entirely applicable in this region. In this investigation, we compare in detail the relative ranges of key Cretaceous nannofossil markers in the eastern Indian Ocean and in sections from Europe and North Africa. We have determined which previously used events are applicable, and which additional markers have biostratigraphic utility in this region. Significant differences in Campanian-Maestrichtian assemblages exist between the more northern Site 761 and Sites 762 and 763. Such differences are surprising, considering that these sites are only separated by 3° of latitude. We interpret them as marking a strong thermal gradient over the Exmouth Plateau region. Other results include the recovery of an expanded Albian-Cenomanian sequence containing a mixture of Austral and Tethyan floras, which will enable correlation of biozonations established for these two realms; the recovery of two condensed but apparently complete Cenomanian-Turonian boundary sections; correlation of Upper Cretaceous calcareous nannofossil biostratigraphy with magneto- and foraminifer stratigraphy; and correlation of portions of the Barrow Group equivalents to the Berriasian and Valanginian stages.

Calcareous nannofossil biostratigraphy of ODP Leg 108 sediments

Calcareous nannofossils were examined from the 400 cores recovered at 12 sites during Ocean Drilling Program Leg 108 in the eastern equatorial Atlantic Ocean and along the northwest African margin, representing a transect spanning 24° of latitude. Thirty calcareous nannofossil biohorizons were recognized in the Neogene and Quaternary sequences; only Site 661, located in water depths of 3500 m, contains a fossiliferous record older than the Oligocene. At Site 661, a 200-m-thick sequence of Upper Cretaceous sediments yielded Maestrichtian and uppermost Campanian nannofossils, yet a continuous Cretaceous/Tertiary boundary was not recovered. Widespread sediment slumps and turbidites deposited at many sites interrupted the pelagic sedimentation. A careful study of calcareous nannofossil and foraminifer assemblages correlated to paleomagnetic records suggests that "slumped" units at most sites were added as extra sediments to rapidly deposited pelagic sediments, with minor disturbance of the surrounding layers. Nannofossils are generally common to abundant and moderately preserved at all sites except for those located in two upwelling areas, where placoliths are etched and discoasters overgrown. Typical low-latitudinal zonal markers were used during this study, yet some of them were considered to be of little biostratigraphic value because of their inconsistent stratigraphic ranges and low abundances. This is especially apparent for the intervals representing the Miocene/Pliocene and Oligocene/Miocene boundaries. Characteristic nannofossils of cool-water conditions and low discoaster abundances occur at the coastal African upwelling and along the south equatorial divergence sites, signifying a stronger advection of cold waters toward the equator within the Canary and Benguela eastern boundary currents.

Calcareous nannofossils of ODP Leg 101 holes

Leg 101 of the Ocean Drilling Program drilled 19 holes at 11 sites to investigate the geology of the Straits of Florida and the northern Bahamas. Drilling at Site 626 indicated that the Gulf Stream has had significant flow through the Straits of Florida for at least the last 24 million years. Winnowed, foraminiferal grainstones and packstones with sparse nannofossil assemblages and the reworking of older nannofossils suggest strong bottom-current activity throughout this interval. Drilling north of Little Bahama Bank and in Exuma Sound documents the growth of platform slopes during the late Cenozoic. Nannofossil biostratigraphy of the upper Cenozoic sediments from the Little Bahama Bank and Exuma Sound slope transects indicates relatively continuous deposition, with only short breaks in the periplatform ooze and/or calciturbidite accumulation during the late Pliocene. These unconformities may be linked to sea-level lowstands. Nannofossil assemblages are generally poorly preserved owing to accelerated diagenesis caused by high aragonite and high magnesium calcite contents of bank-derived material. High rates of influx of bank-derived materials appear to coincide with highstands of sea level. Periplatform sediments are largely limited to the upper Cenozoic at Little Bahama Bank. Pelagic and/or hemipelagic conditions existed during the Late Cretaceous and Paleogene. A relatively complete, continuous section of Oligocene is present in the Little Bahama Bank area, although the rest of the Paleogene is thin. Paleogene material is also present in Northeast Providence Channel, although its thickness is uncertain. A thick upper Campanian chalk sequence with abundant, moderately to well-preserved nannofossils occurs in the Little Bahama Bank area. Hemipelagic nannofossil marls and marly chalks at Little Bahama Bank contain an excellent nannofossil record, which indicates a continuous lowermost to middle Cenomanian sequence overlying the upper Albian drowned platform. These hemipelagic sediments are significantly younger than the organic-rich, middle Albian limestones in Northeast Providence Channel. The latter indicate that a deep-water channel was already well established by the middle Albian.

Maastrichtian calcareous nannofossils of Kronsmoor-Section

The latest Campanian-earliest Maastrichtian interval is well known as a period of intense climate cooling. This cooling caused a distinctive bipolar biogeographic distribution of calcareous nannofossil assemblages: High latitude settings were dominated by newly evolving endemic taxa, former cosmopolitan species disappeared at the same time and equatorial communities experienced an invasion of cool water taxa. The impact of this cooling on northern mid-latitude assemblages is, however, less well known. In order to overcome this gap we studied the Kronsmoor section (northwest Germany). This section provides a continuous upper Campanian - lower Maastrichtian succession with moderately to well preserved nannofossils. Uppermost Campanian assemblages are dominated by Prediscosphaera cretacea; other common taxa include Prediscosphaera stoveri, Watznaueria barnesiae and Micula staurophora. The lower Maastrichtian is characterized by lower numbers of P. cretacea and frequent Kamptnerius magnificus, Arkhangelskiella cymbiformis and Cribrosphaerella ehrenbergii. These changes reflect, in part, the Campanian-Maastrichtian boundary cooling since some successful taxa (e.g. K. magnificus) are related to cool surface waters. Other shifts in the nannofossil communities were perhaps the result of a changing nutrient regime. Stronger latitudinal gradients may have increased wind velocities and thus the eolian input of ferruginous dust required by N-fixing bacteria. The enhanced high latitude deep-water formation probably changed the bottom-water environment in disfavor of denitrificating organisms. A decline of chemical weathering and fluviatile transport may have reduced the amount of bioavailable phosphate. These processes led to an increased nitrate and a decreased phosphate content shifting the nutrient regime from nitrate towards phosphate limitation.

Occurrence of nannoplankton observed in Early Pliocene samples from Zanclean stratotype of Capo Rossello, Sicily (Table I)

Calcareous nannoplankton biostratigraphy has been worked out in the eastern Mediterranean utilizing deep-sea sediments recovered from DSDP Leg 42A Sites 375 and 376. These two drill sites were located approximately 55 km west of Cyprus on the Florence Rise. Sediments, ranging in age from early Miocene (Helicosphaera ampliaperta Zone) through Holocene, contain sufficient age-diagnostic species to recognize essentially all of the lowlatitude nannoplankton zones described by Bukry, although regional, secondary marker species are needed to define some zonal boundaries. Reworked Cretaceous and Paleogene nannoplankton occur throughout the stratigraphic interval studied, but not in quantities large enough to mask indigenous species. Sedimentation rates at Sites 375 and 376 were highest in the late Miocene and late Pleistocene. Open-marine, warm-water species of discoasters are present in significant numbers throughout the Miocene and Pliocene. Earliest Pliocene assemblages contain numerous specimens of ceratoliths. Nannoplankton in post-Messinian sediments at the drill sites and the Zanclean stratotype at Capo Rossello, Sicily, indicate that the base of the Amaurolithus tricorniculatus Zone (base of Triquetrorhabdulus rugosus Subzone) corresponds with the Miocene-Pliocene boundary.