INTRODUCTION (continued)
Modern Hydrography
The two northernmost sites drilled during Leg 189 are located in temperate (cool subtropical)
waters north of the present-day position of the Subtropical Front or Subtropical Convergence
(Fig.
11). The southern sites are located in subantarctic waters between the Subtropical Front and the
Subantarctic Front. The area drilled during Leg 189 therefore lies north of and straddling the
Subtropical Front and south to the region near the Subantarctic Front. The Polar Front lies farther
to the south of our southernmost site. Rintoul and Bullister (1999) showed that the Subtropical
Front is centered on 46°S and lies just south of the saddle between Tasmania and the STR. The
Subantarctic Front is centered on 51°S and lies ~200 km south of the STR. The Polar Front is
centered on 53°S, ~200 km south of the Subantarctic Front. The Subtropical Front is marked by a
zone of rapid north-south decrease in temperature and salinity and an increase in dissolved
nutrients (Barrows et al., 2000) and is approximated by the 34.8-35.1 isohalines and the ~10°C
winter and the ~15°C summer isotherms in the southern Tasman Sea (Garner, 1959). During
summer, sea surface temperatures are >15°C north of the Subtropical Front, ~10°C between the
Subtropical Front and the Subantarctic Front, ~8°C between the Subantarctic Front and the Polar
Front, and <6°C south of the Polar Front. During midwinter, sea surface temperatures are several
degrees lower. The sedimentary sequences drilled during Leg 189 should record migrations of
these fronts as a result of climatic change. Furthermore, as a result of plate tectonic motion, the
Tasmanian continental block migrated northward in relation to these fronts during the Cenozoic,
leaving records in the marine sediments.
Subantarctic surface water south of the Subtropical Front is driven eastward across the STR by the prevailing westerly winds as the northern part of the Antarctic Circumpolar Current. These surface currents extend to great depths, sweeping the seabed as deep as 2000 m in places. The East Australian Current is a western boundary current that flows southward along the east coast of Tasmania to the vicinity of the ETP. Here subtropical surface water converges with cooler, less saline subantarctic surface water at the Subtropical Front (Orsi et al., 1995).