SCIENTIFIC OBJECTIVES
Leg 181 drill sites are mostly located in sediment drift sites across a depth range of 315-4460 m, and will provide a moderate resolution record (2-5 cm/k.y.) of climatic and paleohydrographic changes since the early Miocene. We aim to recover material that will allow the following scientific problems to be studied.
1.
Delineate the Cenozoic development of zonal water masses and the
ACC system.
Current understanding of paleoclimate suggests that the earliest
major meltwater events from Antarctic glaciation occurred at ~38
Ma (Eocene/Oligocene boundary; Shackleton and Kennett, 1975;
Miller et al., 1990). At ~32 Ma (mid-Oligocene), the South Tasman
Gateway opened, including the Balleny Fracture Zone (Lonsdale,
1988), thereby connecting the Indian and Pacific Oceans for the
first time (Kennett, Burns, et al., 1972). Finally, it was not
until ~20 Ma (earliest Miocene) that the opening of Drake Passage
(Boltovskoy, 1980) allowed the establishment of the full pattern
of circum-Antarctic ocean circulation. The evolution of this
system, including periods when the boundary current component may
have extended into shallow depths or reversed (Mikolajewicz et
al., 1993), is the target of Sites SWPAC-5B and 6B. These sites
may also penetrate to the regionally widespread 29 Ma
mid-Oligocene unconformity (Marshall Paraconformity; Carter,
1985), the genesis of which may relate to the inception of
ACC-DWBC activity as much as to global sea-level change (cf. the
postulated large 29 Ma lowstand of Haq et al., 1987). Site
SWPAC-1C is targeted on large Miocene-Pliocene platform drifts
that grew from a paleowater depth of 1000 m in the head of the
Bounty Trough (Fulthorpe and Carter, 1991).
2. Infer
the changing paleohydrography of the CDW supply to the Pacific
Ocean; in particular, to trace the history of mixing of Weddell
Sea Deep Water (WSDW) and NADW components.
Measurement of d18O, d13C, and Cd/Ca from
microfossil tests will be used to distinguish the relative
contribution of nutrient-enriched NADW and d13C
depleted components (Boyle and Keigwin, 1987; Oppo and Fairbanks,
1987; Charles and Fairbanks, 1992; Bertram et al., 1995). Sites
SWPAC-2B and 5B were selected to maximize the chance of the
high-quality carbonate-rich records required for such
measurements. For the deeper water sites, should carbonate
percentages be low, the bulk carbonate technique of Shackleton et
al. (1993) may yield a satisfactory isotope stratigraphy.
However, we have obtained monospecific benthic and planktonic
oxygen isotope records for mid-Holocene to Stage 3 from a core at
4802 m near Site SWPAC-16A, and McCave and Carter (1997) estimate
the carbonate compensation depth (CCD) to lie at ~4750 m. The
late Neogene stratigraphy may be strongly supported by a
tephrochronology derived from the numerous widespread Cenozoic
ash beds deposited east of New Zealand. Global understanding of
the history of these water masses will require the comparison of
gradients of water composition between sites in the North
Atlantic, South Atlantic, North Pacific, and Southern Oceans.
3.
Determine the relative paleoflow speeds of deep and intermediate
waters, and thereby estimate the changing flux of CDW into the
Pacific through time.
The noncohesive sortable silt (10-63 mm) fraction has been shown
at widely separated locations to yield coherent indications of
flow speed and hence water mass movement (McCave et al., 1995a,
b; Manighetti and McCave, 1995; Robinson and McCave, 1994;
Haskell et al., 1991). Evaluation of such grain-size signals in
the North Chatham Drift (Site SWPAC-5B) and Campbell Drift (Site
SWPAC-7B) will permit estimation of the velocity behavior of the
DWBC. Sites SWPAC-1C, 2B, and 6B will yield indications of the
behavior of low salinity AAIW. Site SWPAC-7B, located a little to
the south of the Bounty Trough, will be important for assessing
the extent to which the ACC acts as a driving force for CDW
inflow, because the site is at the latitude of the mouth of the
trough at which the modern ACC veers east into the Pacific, at
that point becoming decoupled from the deep boundary flow (cf.
Semtner and Chervin, 1992).
4.
Establish the history and depth ranges of AAIW across the New
Zealand Plateau.
In the North Atlantic, an intermediate water (possibly
paleo-Labrador sea water) has been shown to increase both in
depth range and speed during glaciations, concomitant with a
decrease in NADW production in the Norwegian-Greenland Sea (Boyle
and Keigwin, 1987; Manighetti and McCave, 1993). As this change
is associated with suppression of NADW, there is little reason to
expect the same glacial/interglacial changes to AAIW in the
Southern Ocean. However, Pudsey et al. (1988) have argued that
AABW production also diminished during glacials as a result of
the grounding of ice sheets, in which case the thickness of AAIW
may well have increased concomitantly. If the vigor of global
deep circulation was decreased by these North Atlantic and
Antarctic events, then during glacial times the Indian/Pacific
upper CDW should have become even more nutrient enriched and
oxygen depleted than it is today. Material from Site SWPAC-5B
(depth 3308 m) will be used for d13C and trace element
analysis (e.g., Cd/Ca in calcite and opal) to allow ocean
paleochemistry to be used to determine whether during glaciations
the site lay under severely depleted AAIW or enriched CDW.
5.
Determine the history of productivity and surface water mass
fluctuations in the vicinity of the Subtropical Convergence.
Near zones of upwelling, such as the Subtropical Convergence
(STC), it is usually difficult to distinguish between
climatically controlled temporal and spatial changes in
productivity because the convergence moves. Unusually, however,
for at least the last full glacial/interglacial cycle the STC has
apparently been topographically trapped over the Chatham Rise
(Fenner et al., 1992; Nelson et al., 1993; Weaver et al., in
press). This raises the prospect of being able to obtain temporal
records of productivity change from faunal, isotopic, and
chemical data without the aliassing usually produced by shifts in
the position of such convergences. DSDP Site 594, in 1200 m of
water just south of the present STC, is a valuable control
because it shows that cold water reached there in the last
glaciation (Nelson et al., 1993), probably representing waters
wind-drifted from the Subantarctic Front (SAF), which itself
remained bounded by the Campbell Plateau. Site 594 provides a
high-quality record extending to the middle Miocene, and we aim
to match it by similar records from beneath the STC and farther
north (Site SWPAC-5B) and south (Sites SWPAC-6B, 7B).
6.
Examine the shifting positions of the Subantarctic Front and
Antarctic Convergence.
The zone of cold water between the STC and the Antarctic
Convergence (AAC) at 60°S is divided by the SAF at about 51°S.
Nelson et al. (1993) found a sharp cooling of waters at glacial
levels in Site 594 (latitude near 45°S), suggesting that the SAF
lay nearby. Our most southerly drift sites (Sites SWPAC-6B, 7B)
and Site SWPAC-8A on the levee of the Bounty Channel will allow
us to assess the shift in position of these climatically
important fronts, using faunal and coarse fraction analysis,
stable isotopes, and magnetic susceptibility measurements to
trace ice-rafted detritus.
7. Test
the record of circum-Antarctic flow against the Milankovitch
orbital model, including estimates of simultaneity with Northern
Hemisphere records.
Achieving this objective requires the retrieval of high-quality,
long-term faunal and isotopic records from Southern Ocean sites
to assess changes in temperature, salinity, and CO2 as
components of the climate system. The relative timing of events
between the Northern and Southern Hemisphere will be evaluated
(cf. Nelson et al., 1985). We anticipate the best long-term
records will come from Sites SWPAC-2B and 5B on the north flank
of Chatham Rise, where the North Chatham Drift is up to 1000 m
thick and probably extends back to the late Oligocene. The
earlier period of drift formation will be principally examined
farther north (Site SWPAC-9B), where the thickness is reduced,
although obviously with less stratigraphic resolution.
8. Study
the effect of an oscillating sediment source controlled by
Pleistocene sea-level cyclicity on fan overbank turbidite
deposition and sediment supply to the DWBC.
The Bounty Channel and Fan are fed with terrigenous sediment
through a number of submarine canyons, which cut the eastern
South Island shelf edge. Located about 30-60 km offshore, these
canyons were alternately supplied directly with sediment during
Pliocene-Pleistocene glacial lowstands, and cut off during
interglacial highstands when sediment was moved directly north
along the inner shelf (Carter and Carter, 1993). Consequently,
the Bounty Fan was supplied with terrigenous sediment mostly
during glacial periods, and the levees of the Bounty Channel
comprise a regular sequence of 5- to 8-m-thick packets of glacial
silt-mud turbidites alternating with interglacial biopelagic,
calcareous ooze (Carter et al., 1990). The regularity of these
cycles is such that they can be matched prima facie with
the oxygen isotopic record back to Stage 100, and viewed as the
deep-sea record of continental shelf stratigraphic sequences
(Carter and Carter, 1992). Site SWPAC-8A will penetrate about 50
of the cycles identified on seismic records, providing (1) a
high-quality record of turbidity current activity through the
Bounty Channel since the early Pleistocene; (2) a test of the
correlation between the observed seismic cyclothems and the
oxygen isotopic stages; and (3) a quantitative model of rates of
sediment supply to the DWBC and deep sea during a time of
glacio-eustatic oscillation of sea level.
9.
Estimate the quantities of sediment fed into the DWBC from all
sources, including terrigenous sediment delivered through the
Solander, Bounty, and Hikurangi Channels; to understand the
relative importance of tectonic, climatic, sea-level, and
water-mass controls and to derive a sedimentary budget for the
ENZOSS.
Since the Pliocene, the three largest sources of sediment for the
DWBC drifts have been turbidity currents travelling the length of
the Solander, Bounty, and Hikurangi channel systems (Carter et
al., 1990; 1994; 1996; Lewis, 1994). Other major sediment sources
are direct transport into the region by the DWBC, and pelagic,
hemipelagic, and volcanic fallout. In addition to the differing
primary targets at each site, all SWPAC sites will contribute
data towards the development of a quantitative sedimentary model
and budget for the ENZOSS.
10.
Examine the history of large volcanic eruptions from the Taupo
Volcanic Zone.
Some of the drift sequences targeted for drilling, particularly
those nearer North Island, will contain Pliocene-Pleistocene
volcanic ash layers of potential value for correlation. Several
very large explosive eruptions have occurred in the Taupo
Volcanic Zone (TVZ) of New Zealand in the last few million years
(Shane et al., 1996). The largest in the last 50 k.y. have
exceeded 100 km2 in volume. These ashes are generally
distributed to the east of New Zealand (Stewart and Neall, 1984)
and occur widely in marine cores (Ninkovitch, 1968; Lewis and
Kohn, 1973; Watkins and Huang, 1977; Kyle and Seward, 1984;
Nelson, 1988; Carter et al., 1995). Volcanic ash horizons may
also provide useful stratigraphic markers at horizons as old as
late Miocene (van der Lingen, 1968), and perhaps earlier. We will
investigate possible climatic links between ash eruptions and sea
surface temperature (SST), through oxygen isotope analysis and
transfer function paleoecology.
The application of combined isothermal plateau fission-track (ITPFT) for dating and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for fingerprinting has revolutionized the ash chronostratigraphy of the Pliocene-Pleistocene Wanganui Basin (e.g., Naish et al., 1996), and holds great promise for application to any ash layers that are recovered from Leg 181 sites.