Southern Ocean Pleistocene calcareous nannofossil events: calibration with isotope and geomagnetic stratigraphies

Abstract

Several cores recovered from the northern belt of the Southern Ocean were analysed to study the Pleistocene calcareous nannofossil records. Calcareous nannofossil events previously described in medium and low latitudes were identified and calibrated with the oxygen isotope and geomagnetic time scales. Although sedimentation rates, hiatuses and degree of calcareous nannofossil preservation sometimes prevent the identification and/or accurate calibration of some of these events, a useful stratigraphic framework was obtained. The possibility of using these calibrated events from high to low latitudes facilitates correlations and should facilitate isotope event identification in a region with low temperature, where calcareous plankton stratigraphies are in general restricted. In general, Pleistocene southern high latitude calcareous nannofossil events show synchronism with those observed in warm and temperate surficial waters. Small discrepancies in the assigned ages are sometimes related to low sampling resolution due to low sedimentation rates. The first occurrence (FO) of Emiliania huxleyi and the last occurrence (LO) of Pseudoemiliania lacunosa are observed in Marine Isotope Stages (MIS) 8 and 12, respectively. A reversal in abundance between Gephyrocapsa muellerae and E. huxleyi is observed close to the MIS 4/5 boundary. MIS 6 is characterised by an increase in G. muellerae and MIS 7 features a dramatic decrease in the proportion of Gephyrocapsa caribbeanica. This latter species began to increase its proportions from the MIS 13/14 boundary to MIS13, showing diachronism between the different sites. The LO of Reticulofenestra asanoi is observed at MIS 22, confirming this event as a global synchronous reference datum. By contrast, the FO of R. asanoi occurs at MIS 35 and is diachronous with the existing data from other oceanic regions. A re-entry of medium sized Gephyrocapsa (3–5 μm maximum diameters) can be identified in some cores close to MIS 25; although the low abundance of this taxon prevents an accurate calibration, it may be concluded that this event is diachronous as compared with the existing low-latitude data. The LO of large morphotypes of Gephyrocapsa is well correlated with MIS 37, showing synchronism with other oceanic regions, whereas the FO of this species is not well calibrated due to the absence of age-control points.

Introduction

Over the last decades calcareous nannofossils have shown excellent potential in Pleistocene biostratigraphy. Improved isotope and magnetostratigraphic data have permitted the calibration of the calcareous nannofossil data, allowing a more precise correlation between different regions, and consequently, the dating of paleoceanographic events. Accurate age determination and the evaluation of synchrony of micropaleontological data are essential in paleoceanographic analyses to provide age-control points, which then help to infer ages or to interpret isotope stratigraphy. Such analyses are also important in paleoecological studies on the Pleistocene, a time interval of special relevance in the understanding of climate evolution and cyclicity. Examples of these biochronological data come from Thierstein et al., 1977, Gartner, 1977, Pujos-Lamy, 1977, Matsuoka and Okada, 1990, Giraudeau and Pujos, 1990, Wei, 1993, Raffi et al., 1993, Weaver and Thomson, 1993, Pujos and Giraudeau, 1993, Wells and Okada, 1997, Hine and Weaver, 1998, Bollmann et al., 1998 among others. However, most of the above studies focus on middle- and low-latitude sections and only few of them are based on material recovered in the Southern Ocean, which plays an important role in climatic and oceanic evolution (Kennet and Barron, 1992, Shipboard et al., 1999). The present study focuses on the identification and calibration of calcareous nannofossil biostratigraphic data in order to improve the stratigraphic framework for southern high-latitude paleoceanographic studies.

The zonations of Martini, 1971, Okada and Bukry, 1980 for Quaternary nannofossil Pleistocene biostratigraphy were improved when quantitative data and new deep-sea material became available. Several authors, cited previously in this section, generated new higher-resolution schemes, in most cases calibrated chronologically. Worldwide events were observed, mainly in tropical and in temperate water masses, due to the cosmopolitan characteristics of most of the species involved (Raffi et al., 1993, Wei, 1993).

As mentioned above, stratigraphic studies carried out on Pleistocene material from the Southern Ocean in general disclose low sedimentation rates and/or hiatuses of different stratigraphic extent (Wise and Wind, 1977, Wise, 1983, Crux, 1991, Wei and Wise, 1992). In other cases, at these latitudes the calcareous plankton is in general diluted by siliceous organisms or is affected by dissolution. Consequently, the number of sections available is restricted. To these factors should be added restrictions in ecological calcareous nannofossil marker species. Alternative quantitative events to the “standard” can improve stratigraphic resolution in this region.