50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

The MCC is bounded by deep-seated transform
faults separating domains in which extension has
been strongly localized on weak, individual faults
(MCC’s) vs. where it is distributed.
ORAL
MAPPING THE DEEP SEA: APPLICATION
OF MULTIBEAM DATA TO DEEP SEA
GEOLOGICAL SAMPLING
K. Mackay, Malcolm Clark, Ian Wright,
& Miles Dunkin
NIWA, Private Bag 14 901, Kilbirnie, Wellington,
New Zealand.
(k.mackay*niwa.co.nz)
Multibeam echosounders are becoming
increasingly and routinely used for detailed seafloor
mapping up to several kilometres either side of the
survey vessel. Such multibeam data have been used
to determine seafloor bathymetry and substrate
composition for geological research by accurately
map submarine topography, such as seamounts or
canyons. In addition, the multibeam seafloor
acoustic backscatter data can differentiate between
different substrate types, including sediment and
exposed bedrock, and hence guide successful
geological sampling operations. Multibeam
mapping of the ocean bottom is markedly more
efficient than conventional single-beam
echosounders, and provides greater resolution and
accuracy of both seafloor morphology and
composition.
Since 2001, the Simrad EM300 multibeam system
on NIWA’s research vessel Tangaroa has been used
extensively in support of ocean geology sampling
operations, especially around the east coast of the
North Island, Cook Strait, Kermadec Ridge,
Norfolk Ridge, and Lord Howe Rise.
The existing charts of these regions have generally
proved to be adequate on a large scale, indicating
the approximate position of ridges or seamounts.
However, on the scale of an individual feature, the
depth, size, and shape of a seamount are often
poorly charted. The width of seafloor that can be
mappedisupto4timesthewaterdepth,andsoat
1000 m depth the bathymetry was determined 2 km
either side of the vessel. This enables rapid
evaluation of the shape of the seabed, which was
often very different from the chart.
The seafloor bathymetry is often rugged and
complex, with the multibeam proving essential to
the success of sampling several sites. The acoustic
backscatter intensity gives an indication of
substrate composition, which was used to plan rock
dredging and/or sediment sampling operations. It
also gives us data on the type of sediment and
habitat which may be used later in analyses such as
defining a unique benthic community structure.
POSTER
THE WILD WEST: IMAGING WEST COAST
SEDIMENT CONDUITS
K. Mackay, H.L.Neil&J.Mitchell
NIWA, Private Bag 14 901, Kilbirnie, Wellington,
New Zealand.
(k.mackay*niwa.co.nz)
New Zealand is in a critical location for
reconstructing changes in westerly circulation and
evaluating its role in controlling global climate
change. Surrounded by vast areas of ocean, the
main axial ranges intersect the prevailing
westerlies. The Southern Alps of New Zealand are
an ideal location to understand the history of the
westerly wind system through time as the
interaction between orographic effects and the
westerlies produce significant precipitation on the
West Coast of the South Island.
The first in a probable series of research voyages
projected for the West Coast Canyon region off
South Island, New Zealand was completed in
October 2005. The aim of this research programme
is to define a history of glaciation during the late
Quaternary in New Zealand, in particular the
relationship with the Westerly Wind system and
correlation with abrupt climate change globally and
to determine the sources and fluxes of sediment
influencing the southern New Zealand region.
The West Coast is supplied by 3 of the 5 largest
rivers in New Zealand with respect to sediment
load. Presently fifty times more sediment is
injected into this system than that into the Bounty
Trough as a consequence of the closer proximity of
the Southern Alps source area and the lack of large
natural lakes that trap up to 15% of the sediment
yield to the east of the Alps. Glacial-Interglacial
changes recorded in sediment sequences in
downslope basins should reflect a switch on with
large sediment flux in peak glacial times and a
switch off during interglacials. At the last glacial
maximum, major valley glaciers reached the open
coast, probably as grounded tidewater glacial fronts
providing voluminous mud, sand and gravel via a
range of processes directly to the continental slope
(Barnes et al., 2001).
Seafloor topography was surveyed using the
Simrad EM300 multibeam sonar, which uses a fan
of 135 sonar beams to simultaneously ensonify and
map a swath of the seafloor up to a width of up to
5-times the water depth. These bathymetric data are
processed within a GIS to form a digital terrain
model (DTM). Using spatial analysis functions of
GIS, the DTM is interrogated to identify, and
50 th Kaikoura05 -47- Kaikoura 2005