Pliocene foraminiferal biostratigraphy and paleoceanography of ODP site 1125, eastern New Zealand

Abstract

A planktonic foraminiferal census study has been made on the Pliocene (upper Opoitian to Nukumaruan Stages) of ODP site 1125, north side of Chatham Rise, SW Pacific Ocean. This study recognised of 29 species and 1 subspecies of planktonic foraminifera and a total of 14 planktonic foraminiferal datums (bioevents). Some of these bioevents are important for local and regional correlation. Calibration of the studied interval with paleomagnetic polarity revealed that part of the uppermost Pliocene is missing. Correlation of the studied interval with other sequences around New Zealand's onshore and offshore record (DSDP sites 593, 284, 594, and Mangaopari Stream) has been made based on the estimated ages inferred from the magnetostratigraphic positions of planktonic foraminiferal datums. This correlation provides important information about the distribution of planktonic foraminiferal species in different water masses. Modern analogue technique (MAT) has been applied to the planktonic foraminiferal census data in this study to estimate the sea surface temperature (SST) record. The MAT SST warm records range between 8°C and 20.6°C. The MAT SST cold records display a similar pattern with a minimum value 5.1°C and maximum value 15.2°C. These SST records show a general shift to warmer climate upward the studied interval. There were 3 major cold excursions at about 3.35 to 3.22 Ma, 2.96 to 2.88 Ma, and 2.75 to 2.72 Ma. In addition, 2 minor cold excursions occurred at about 2.65 Ma and 1.9? Ma. The last minor cold excursion occurs around the hiatus, which is recognised between the LAD of dextral Globorotalia crassaformis and the FAD of Globorotalia crassacarina. Comparison between the late Pliocene SST records of this study and modern isotherms show that although the modern position of site 1125 is just north of Subtropical Front, it has experienced temperatures typical of the modern Subantarctic Zone during late Pliocene. Comparison between SST results of late Pliocene and Last Glacial Maximum shows that some intervals during late Pliocene were colder than Last Glacial Maximum. Comparison between this study and other paleoceanographic studies on the Pliocene of the South and North Atlantic Ocean and equatorial Pacific Ocean reveal that the climatic and oceanographic changes appear to have been synchronous between the world's oceans. Correlation coefficients between selected species, SST, fragmentation index, carbonate reflectance, number of planktonic foraminifera/gram of sediment and /gram of sand shows that most of the relationships are more or less as expected but the relationship between reflectance and other variables is not. The coiling ratio of Globorotalia crassaformis show a general increase in dextral form with the shift to warmer climate, while the coiling ratio of Neogloboquadrina pachyderma show a general increase in sinistral form during colder periods. The fragmentation of planktonic foraminifera increased with the increasing sea surface temperature, which is opposite of what is generally found in deeper water sediments. This may be due to increased carbon flux during warm periods. Both species diversity and number of fauna found in sediment show general increase with the shift to warmer climate upward the studied interval.