Maestrichtian calcareous nannofossils of MAud Rise, Weddell Sea

Recovery of an essentially complete upper Maestrichtian/lower Paleocene interval on Maud Rise at 65 °S latitude in the Weddell Sea during Ocean Drilling Program Leg 113 marks the first time that this interval has been cored at these high latitudes. The entire interval was missing at all Falkland Plateau sites drilled during DSDP Legs 36 and 71. Maestrichtian nannofossil assemblages in sediments from Sites 689 and 690, therefore, provide the basis for a needed revision of Maestrichtian coccolith zonation schemes for high southern latitudes. Three zones and two new subzones are described: the uppermost Maestrichtian Nephrolithus frequens Zone, which is subdivided into the Cribrosphaerella daniae Subzone and the underlying N. corystus Subzone, and the Biscutum magnum and B. coronum Zones. A complete calcareous nannofossil biostratigraphy based on the proposed scheme is given including a description of individual species abundance, preservation, and stratigraphic distribution. At this site, the southernmost carbonate site yet drilled by DSDP/ODP, it is evident that the Falkland Plateau Nannofossil Biogeographic Province can be extended to the margins of Antarctica. In addition, the biogeographic ranges of many calcareous nannofossils can likewise be extended. Last, we hypothesize that Nephrolithus frequens evolved from N. corystus prior to its dispersal to the lower latitudes where it is an important zonal marker. Three new taxa, Neocrepidolithus watkinsii n. sp., Nephrolithus frequens miniporus emend, n. comb, and Psyktosphaera firthii n. gen., n. sp. are described.

Abundance of calcareous nannofossils in ODP Hole 121-752B (Table 1)

A biostratigraphically complete Cretaceous/Tertiary boundary was recovered during Ocean Drilling Program Leg 121. The boundary, cored in ODP Hole 752B on Broken Ridge, is the most expanded deep-sea section yet recovered by ODP/DSDP. The initial Danian subzone, CP la, spans nearly 5 m and the underlying uppermost Maestrichtian Nephrolithus frequens Zone extends 50 m below the boundary. The paleolatitude of Broken Ridge at Cretaceous/Tertiary time is estimated at 50°-55°S which includes this site among the latest in a series of complete or near complete high southern latitude Cretaceous/Tertiary boundary sections recovered by ODP (Leg 113 Site 690 and Leg 119 Site 738). The boundary at Site 752 lies at the base of a thick (6-6.5 m) volcanic ash unit composed of multiple ash layers which overlies indurated Maestrichtian chalks. Magnetostratigraphy indicates that the boundary lies within Subchron 29R, which is the case for all other known complete sections for which the polarity has been determined. Anomalous abundances of the trace element iridium are present at the boundary. A second iridium peak, 80 cm above the boundary, corresponds to an increase in redeposited Cretaceous nannofossils. The nannofossil succession is similar to that found at previously studied austral high-latitude ODP drill sites with few differences due to the more northerly location of this site. Individual nannofossil species were counted and placed into three categories. A plot of the percent abundance of Cretaceous, Tertiary, and 'survivor' groups illustrates the rapid replacement of the Cretaceous nannoflora by 'survivor' forms beginning at the boundary and the dominance of this latter group through the initial Danian biozone. This 'survivor' or opportunistic assemblage is then rapidly replaced by newly evolved Tertiary taxa. The assemblage of the uppermost Maestrichtian is biased toward dissolution-resistant forms such as Micula decussata. In those few intervals where preservation is good, the dissolution susceptible species, Prediscosphaera stoveri, is more prevalent and overall diversity of the assemblage is higher. The 'survivor' assemblage is dominated by Zygodiscus sigmoides and Thoracosphaera. The Tertiary assemblage consists of rare Biantholithus sparsus, the first of this group to appear. It is followed several meters upsection by Cruciplacolithus primus. Cruciplacolithus tenuis and small Prinsius spp. dominate the assemblage beginning at about 5 m above the boundary.

Calcareous nannofossil species richness across the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM, ~5 million years ago) was an interval of global warming and ocean acidification attributed to rapid release and oxidation of buried carbon. We show that the onset of the PETM coincided with a prominent increase in the origination and extinction of calcareous phytoplankton. Yet major perturbation of the surface-water saturation state across the PETM was not detrimental to the survival of most calcareous nannoplankton taxa and did not impart a calcification or ecological bias to the pattern of evolutionary turnover. Instead, the rate of environmental change appears to have driven turnover, preferentially affecting rare taxa living close to their viable limits.

Calcareous nannofossil during the Paleocene Eocene Thermal Maximum in sediments of Walvish Ridge

Compositions and abundances of calcareous nannofossil taxa have been determined in a ca 170 kyrs long time interval across the Paleocene/Eocene boundary at 1-cm to 10-cm resolution from two ODP Sites (1262, 1263) drilled along the flank of the Walvis Ridge in the South Atlantic. The results are compared to published data from ODP Site 690 in the Weddell Sea. The assemblages underwent rapid evolution over a 74 kyrs period, indicating stressed, unstable and/or extreme photic zone environments during the PETM hyperthermal. This rapid evolution, which created 5 distinct stratigraphic horizons, is consistent with the restricted brief occurrences of malformed and/or weakly calcified morphotypes. The production of these aberrant morphotypes is possibly caused by major global scale changes in carbon cycling in the ocean-atmosphere system, affecting also photic zone environments. No marked paleoecologically induced changes are observed in abundances of the genera Discoaster, Fasciculithus and Sphenolithus at the Walvis Ridge sites. Surprisingly, there is no significant correlation in abundance between these three genera, presumed to have had a similar paleoecological preference for warm and oligotrophic conditions.