Middle Eocene-lower Miocene calcareous nannofossil magnetobiochronology of ODP Holes 699A and 703A in the subantarctic South Atlantic

The distribution of calcareous nannofossils is documented for the middle Eocene through lowermost Miocene cores from Ocean Drilling Program Holes 699A and 703A in the subantarctic South Atlantic. The detailed nannofossil biostratigraphies established, in combination with published magnetostratigraphic data, have provided a fairly detailed age model for each hole. This study suggests that the middle Eocene through lowermost Miocene section from Hole 699A is virtually complete. A major hiatus has been identified in Hole 703A in the earliest Oligocene, coincident with n abrupt cooling in the Southern Ocean. Comparison of the nannofossil datum ages calibrated with magnetostratigraphy in the two holes with those from mid and southern high latitudes demonstrates synchroneity or diachroneity for the following nannofossil datums: (1) The last occurrence (LO) of Reticulofenestra bisecta is a consistent and reliable biostratigraphic marker for the Oligocene/Miocene boundary from mid- to high latitudes but not in extreme high latitudes; (2) similarly, the LO of Chiasmolithus altus has a consistent age of about 26.8 Ma in the Southern Ocean except in the extreme high latitudes where the datum appears to be substantially younger; (3) the LO of Reticulofenestra umbilica is about 32.9 Ma in the Southern Ocean; (4) the LO of Isthmolithus recurvus is reliable and consistent from mid through high latitudes and correlates with the lower part of Subchron C12R (~34.4 Ma); (5) the LO of Reticulofenestra oamaruensis has a consistent age of 36.0 Ma at all four Southern Ocean sites that have yielded a lower Oligocene magnetostratigraphy; (6) the first occurrence (FO) of R. oamaruensis is at 38.4 Ma in the Southern Ocean; and (7) the FO of I. recurvus shows some age variations from mid to high latitudes and the age range is 38.5-39.0 Ma at the five Southern Ocean sites

Distribution of calcareous nannofossils in the lower sequence of the CIROS-1 sediment core (Table 2)

Semiquantitative data of calcareous nannofossil species abundance were collected for 31 samples from 380.00 to 696.61 m below sea floor (mbsf) in the CIROS-1 core from the western McMurdo Sound (Ross Sea). This core has provided the most complete record of Paleogene glacial history of Antarctica. Only half of the samples yielded calcareous nannofossils and species diversity is generally low. However, several samples contain diverse assemblages and nannofossil zonal markers. Isthmolithus recurvus was found between 406.57 and 681.16 mbsf. This species has an age range of ~35-39 Ma in the mid to high latitudes, as calibrated previously by magnetostratigraphy at a number of DSDP/ODP sites. The interval from 406.57 to 681.16 mbsf can be assigned to the late Eocene-earliest Oligocene (~35-39 Ma), and the sample at 391.85 mbsf, which still contains Reticulofenestra hillae and Reticulofenestra umbilica but does not contain I. recurvus, is identified as early Oligocene age (~33-34 Ma). This represents an age refinement of the previous nannofossil biostratigraphy, where the interval from 385.77 to 690.40 mbsf was dated as middle Eocene-early Oligocene. Comparison of the nannofossil assemblages in the CIROS-1 core with those in a similar glaciomarine sequence recovered in Prydz Bay (East Antarctica) and those at deep-sea ODP Sites 738 and 744, where lowermost Oligocene ice-rafted debris (IRD) were found, suggests that the nannofossils in the CIROS-1 core samples examined are in situ. The semiquantitative nannofossil data also suggest that Isthmolithus recurvus and Reticulofenestra davies¸ are most tolerant of cold waters within the late Eocene-early Oligocene nannofloras

Nannofossil datums and occurrence of microspherules in Upper Eocene sediments of the lower latitudes (Table 1)

Upper Eocene microspherules (microtektites and microkrystites) have been found in deep-sea sediments from the western North Atlantic, Caribbean, Gulf of Mexico, equatorial Pacific, and eastern equatorial Indian Ocean. Recent studies proposed that there are three or four microspherule layers: the North American microtektite and microkrystite couplet in chron C15, and one (or two) significantly older layer(s) in chron C16 in DSDP Sites 216, 292 and 612. Based on calcareous nannofossil biochronologic and morphometric data, I suggest that the microspherules in the latter sites correlate with the North American microtektite-microkrystite couplet. My results as well as previously published data are all compatible with the model of one couplet of microtektites-microkrystites. The inference of three or four microspherule layers in the upper Eocene is shown to be an artifact of interpretation

Calcareous nannofossil abundance of ODP Sites 174A-1071, 174A-1072 and 174A-1073

Calcareous nannofossil range charts for Leg 174A sites on the New Jersey continental margin are presented in this report, and nannofossil biostratigraphy is established. Nannofossil biostratigraphic resolution is low in shallow-water Sites 1071 and 1072, where nannofossils are generally rare or frequently absent. Site 1073 yields generally common to abundant nannofossils, which allows a fairly detailed nannofossil biostratigraphy for the entire Pleistocene through upper Eocene sequence. Quantitative and semiquantitative nannofossil data for the upper Pleistocene section from Site 1073 reveal an average sedimentation rate of about 80 cm/k.y. The unusually high sedimentation rate makes this calcareous section very valuable for high-resolution studies