ALBERT

Description: Carbon isotope measurements were made on bulk sediments from the well preserved calcareous sequences recovered at ODP Sites 689 and 690 on the Maud Rise, Weddell Sea, Antarctica. The very positive delta13C values that characterize the late Paleocene and the rapid trend toward lighter values in the early Eocene established in other sites are clearly recorded here and may be of value for long-distance stratigraphic correlation. However, values in the late Eocene are significantly more positive than have been reported from other areas. The general pattern of the records from Sites 689 and 690 is sufficiently unlike those previously reported from lower latitudes that we suggest that carbon isotope data should be used only with considerable caution for correlating sequences from such high latitudes with lower latitude records.

Description: ODP Leg 113 drilled the first nearly continuous pre-Neogene calcareous biogenic sequence from the Antarctic Ocean at Sites 689 and 690. At 65∞S, these are probably the highest latitude calcareous sequences available in the Southern Hemisphere deep oceans. Together these two sites provide a nearly complete planktonic foraminifer history for the Late Cretaceous through late Oligocene. Planktonic foraminifers are abundant and generally well preserved from the Upper Cretaceous to the Eocene. Abundances and the quality of preservation are reduced during the Oligocene as calcareous microfossil groups are progressively replaced by siliceous groups. The Neogene is marked by only rare, isolated occurrences of planktonic foraminifers, the most conspicuous of which are of Quaternary age. The diversity of planktonic foraminifers was low following the mass extinction event at the Cretaceous/Paleogene boundary. The lowermost Paleogene fossil assemblages following the mass extinction event closely resemble those of lower latitudes. During the middle Paleocene, a "high latitude" assemblage developed that lacked species found at lower latitudes, particularly the early morozovellids. The diversity increased during the late Paleocene through evolutionary radiation in conjunction with warm conditions at high latitudes. Diversity remained high in the Antarctic throughout most of the early and early middle Eocene. Subbotinids and acarininids dominated the assemblages along with various Planorotalites species. At no time, however, did any of the large-keeled morozovellids, the hantkeninids, or the globigerapsids characteristic of the low latitudes inhabit the Antarctic Ocean. Beginning in the late middle Eocene, planktonic foraminifer diversity was reduced due to the extinction of the acarininids and several Subbotina and Planorotalites species. In the upper Eocene the fossil assemblages exhibit even lower diversity and are dominated by three to four species. These changes correspond to the beginning of a long-term climatic cooling trend that is also recorded in the stable isotopic records. Further reduction in diversity occurred across the Eocene/Oligocene boundary in response to continued cooling and increased CaCO3 dissolution. At that time siliceous microfossils began to appear in increasing abundance. A new planktonic foraminifer biostratigraphy has been developed for the Weddell Sea area, Antarctica. Fourteen biozones are defined on the basis of distinct biohorizons used to mark the top and bottom of each zone, and are intercalibrated with magnetostratigraphy. This stratigraphy has been correlated with the well-established low-latitude zonations.