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History of Oceanic Front Development in the New Zealand Sector of the Southern Ocean During the Cenozoic—A Synthesis

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History of Oceanic Front Development in the New Zealand Sector of the Southern Ocean During the Cenozoic—A Synthesis New Zealand Journal of Geology & Geophysics, 2001, Vol. 44: 535–553 535 0024–8306/01/4404–0535 $7.00/0 © The Royal Society of New Zealand 2001 History of oceanic front development in the New Zealand sector of the Southern Ocean during the Cenozoic—a synthesis CAMPBELL S. NELSON particularly with restriction of the Indonesian gateway and PENELOPE J. COOKE redirection of intensified warm surface flows southwards into the Tasman Sea, as well as complete opening of the Department of Earth Sciences Drake gateway by 23 Ma allowing more complete University of Waikato decoupling of cool circum-Antarctic flow from the Private Bag 3105 subtropical waters. During the late Middle–Late Miocene Hamilton, New Zealand (15–5 Ma), both the STF and SAF proper were established email: [email protected] in their present relative positions across and about the Campbell Plateau, respectively, accompanying renewed ice buildup on East Antarctica and formation of a permanent Abstract The New Zealand sector of the Southern Ocean ice sheet on West Antarctica, as well as generally more (NZSSO) has opened about the Indian-Pacific spreading expansive and intensified circum-Antarctic flow. ridge throughout the Cenozoic. Today the NZSSO is The ultimate control on the history of oceanic front characterised by broad zonal belts of antarctic (cold), development in the NZSSO has been plate tectonics through subantarctic (cool), and subtropical (warm) surface-water its influence on the paleogeographic changes of the masses separated by prominent oceanic fronts: the Australian-New Zealand-Antarctic continents and their Subtropical Front (STF) c. 43°S, Subantarctic Front (SAF) intervening oceanic basins, the timing of opening and closing c. 50°S, and Antarctic Polar Front (AAPF) c. 60°S. Despite of critical seaways, the potential for submarine ridges and a meagre database, the broad pattern of Cenozoic evolution plateaus to exert some bathymetric control on the location of these fronts is reviewed from the results of Deep Sea of fronts, and the evolving ice budget on the Antarctic Drilling Project-based studies of sediment facies, microfossil continent. The broad trends of the Cenozoic climate curve assemblages and diversity, and stable isotope records, as well for New Zealand deduced from fossil evidence in the uplifted as from evidence in onland New Zealand Cenozoic marine sedimentary record correspond well to the principal sequences. Results are depicted schematically on seven paleoceanographic events controlling the evolution and paleogeographic maps covering the NZSSO at 10 m.y. migration of the oceanic fronts in the NZSSO. intervals through the Cenozoic. During the Paleocene and most of the Eocene (65–35 Keywords New Zealand; Southwest Pacific; Antarctica; Ma), the entire NZSSO was under the influence of warm to Southern Ocean; Cenozoic; paleoclimate; paleoceanography; cool subtropical waters, with no detectable oceanic fronts. oceanic fronts; DSDP cores; ice sheets In the latest Eocene (c. 35 Ma), a proto-STF is shown separating subantarctic and subtropical waters offshore from Antarctica, near 65°S paleolatitude. During the earliest INTRODUCTION Oligocene, this front was displaced northwards by development of an AAPF following major global cooling The New Zealand subcontinent lies in the global oceanic and biotic turnover associated with ice sheet expansion to hemisphere at the interface of the Southwest Pacific Ocean sea level on East Antarctica. Early Oligocene full opening and the Southern Ocean. It is proximal to both the frigid (c. 31 Ma) of the Tasmanian gateway initiated vigorous Antarctic continent to the south, and the Western Pacific proto-circum-Antarctic flow of cold/cool waters, possibly Warm Pool to the north (Tomczak & Godfrey 1994). through a West Antarctic seaway linking the southern Pacific Consequently, seafloor deposits in the vicinity of New and Atlantic Oceans, including detached northwards Zealand have the potential for being a sensitive monitor of “jetting” onto the New Zealand plateau where condensation present and past oceanographic changes driven by any and unconformity development was widespread in cool- changes in these cold and warm source regions. The most water carbonate facies. Since this time, a broad tripartite prominent feature evident on satellite imagery of the modern division of antarctic, subantarctic, and subtropical waters Southern Ocean is the conspicuous bands of circumpolar has existed in the NZSSO, including possible development surface-water masses separated by oceanic fronts (Fig. 1), of a proto-SAF within the subantarctic belt. In the Early– each marked by sharp changes in vertical water structure, early Middle Miocene (25–15 Ma), warm subtropical waters temperature, salinity, and nutrients. This paper overviews expanded southwards into the northern NZSSO, possibly some of the geological evidence for the timing and associated with reduced ice volume on East Antarctica but development of these oceanic fronts in the New Zealand sector of the Southern Ocean (NZSSO) during the Cenozoic, an evolution that is closely linked to the changing configuration of the southern continents in relation to their surrounding ocean basins as a consequence of seafloor G00052 spreading, and the resultant dramatic changes in the ice Received 28 August 2000; accepted 30 July 2001 budget of Antarctica over this period. 536 New Zealand Journal of Geology and Geophysics, 2001, Vol. 44 Fig. 1 Modern surface-water 090W 40S 50 60 70 80 80 70 60 50 40S masses and their bounding oceanic fronts in the New Zealand 090E AAPF sector of the Southern Ocean (adapted from Belkin & Gordon 1996). A SAF nt arc ) 120 tic S W urface Water (AAS AAPF SAF ) 120 C face W W ir ur a S STF cumpolar S ter (C ) STF 30S SW S (SA uba ace Water ntar rf ctic Su Oceanic Fronts 150 C o W) Antarctic Polar Front (AAPF) ol ST Sub ter (C 20S Subantarctic Front (SAF) tropical Wa Subtropical Front (STF) Modern sea ice limit Continental margin 2000 m 180 150 SOUTHERN OCEAN FRONTS there are presently three main oceanic fronts separated by four main surface water masses (Fig. 1; Table 1). Although Despite various definitions of the Southern Ocean (e.g., other fronts lie to the north (Tasman Front at c. 30–36°S Lazarus & Caulet 1993; Belkin & Gordon 1996), it is and Southern Tropical Convergence at c. 20°S) and in the conveniently treated here as including all antarctic and far south (Antarctic Divergence at c. 65°S) of this region, it subantarctic water masses south of the Subtropical Front is the history of development of the three main NZSSO which, in the New Zealand region, presently lies near 43– fronts—the Subtropical Front (STF), Subantarctic Front 45°S latitude (Fig. 1). The Southern Ocean is a key (SAF), and Antarctic Polar Front (AAPF) (Table 1)—that component of the Earth’s ocean-climate system through the are the principal topic of this review. major role it plays in the meridional transfer of heat. Gordon (1988) estimated that presently at least 60% of the oceanic volume is cooled around Antarctica before being transferred ESTABLISHING PALEOPOSITIONS OF WATER north. In the deep ocean, the transfer of cold water occurs MASSES mainly via deep western boundary currents as portrayed in the “thermohaline conveyor belt” model of Broecker & The evidence for determining past positions of oceanic fronts Denton (1990). For surface waters, the heat transfer relates is essentially limited by our ability to distinguish between to a combination of wind drift and eddies associated with cold, cool, and warm surface-water provinces in the oceanic fronts. geological record of marine sedimentary deposits. Proxy Oceanic fronts are characterised by marked changes in estimations of past sea-water temperatures can be made from vertical structures from one side to the other. Typically these various lithological, paleontological, and geochemical are manifested at various levels by enhanced horizontal properties of sedimentary deposits (e.g., Boggs 1987). gradients in temperature and/or salinity, and by concentrated Lithological features include the type of deep-sea sediment geostrophic flow. A comprehensive review of the position, facies, their textural properties, and compositional data such definition, and structure of the oceanic fronts in the circum- as the occurrence of ice-rafted debris or particular clay Antarctic Southern Ocean as a whole is provided by Belkin mineral assemblages. Paleontological criteria relate & Gordon (1996). Between New Zealand and Antarctica especially to the relative abundances of calcareous and Table 1 Major oceanic front characteristics and their intervening surface- water masses in the modern NZSSO (see also Fig. 1). Water mass or Latitude Temperature Salinity oceanic front Symbol (°S) (°C) (‰) Subtropical Water STW Subtropical Front STF c. 43–45 15 summer 10 winter 34.8 Subantarctic Surface Water* SASW Subantarctic Front SAF c. 52 8 summer 34.5 Circumpolar Surface Water CSW Antarctic Polar Front AAPF c. 56 2 34.2 Antarctic Surface Water AASW * In the New Zealand region previously named Australasian Subantarctic Water (ASW). Nelson & Cooke—Paleoceanography of Southern Ocean 537 Fig. 2 Composition of core-top 281 207 sediment samples in relation to DSDP site 271 274 266 278 280 284 206 208 209 modern surface-water masses in a 100 north–south transect of DSDP Foraminifera sites from well north of New Zealand to offshore Antarctic 80 Nannofossils continent (based on data from Burns 1977). Abbreviations Radiolarians defined in Table 1, plus CSTW, 60
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