Thursday, January 22, 2009

hot off the press

Hi everyone!

I got permission form the chief scientist on board today to relay a
weekly update he prepares for interested parties. It's a nice summary
of the past weeks activity and the cruise as a whole and it relieves
me of some writing if I post it. Also there is often humor scattered
in. So here it is, only slightly edited. Feel free to ask ?'s if it's

A primary science objective of NBP09-01 is to investigate why ice
shelves fringing the Antarctic coastline in the Amundsen Sea are
rapidly melting. Ice shelves form when glaciers flow off the land and
float in the sea, and range from ~100-2000 meters thick. Most
Amundsen ice shelves are small but deep at their grounding lines in
glacially cut troughs. Melting at their undersides can average 100
times faster than beneath the Texas-sized ice shelves in the Ross and
Weddell Seas. Those giants melt slowly and grow basal ice in some
areas, in part because they float in seawater that is cooled in
winter and made saltier (denser) by sea ice formation and export.
Amundsen ice shelves float in 3-4 degrees (C) warmer seawater, which
has upwelled from the Circumpolar Deep Water and flowed into the
continental shelf depressions. Their high basal melt rates may also
be increasing, thinning the shelf ice and allowing its incoming
glaciers to move more rapidly into the sea. That will contribute to
an increase in the rate of sea level rise, and to ocean freshening
when the ice melts.

In pursuit of that objectve during cruise week #2, we have profiled,
sampled and deployed instruments along a traverse extending from the
continental slope in the NE Amundsen to the front of the Pine Island
Glacier ice shelf (PIG), with additional transects along the ice
front and around its southern corner. The work included 31 CTD/
rosette casts, 4 moorings anchored to the sea floor and a full-depth
test run of the UK Autosub. Mild weather and a relatively low sea ice
cover for the region at this time of year facilitated progress, and
our first access to the PIG calving front since 1994. Preliminary
results revealed deep water properties in the main trough within the
ranges previously measured, and concentrated areas of outflow along
the PIG front. Retreat at the front of the northern lobe has left a
former pinning point exposed as an ice-domed feature, tentatively
named Capt Mike Is.

Swath mapping of the sea floor covered ~1220 km during the week and
focused on filling gaps between existing tracks. One of those tracks
may set the southern limit of the deepest depression on the outer
shelf, directly west of Thurston Island and the Abbot Ice Shelf. The
atypical lack of any sea ice in Pine Island Bay provided the
opportunity to better map the PIG trough seaward of its snout.

Chris Little (woo-hoo!) has been running numerical simulations of
ocean circulation within the PIG ice shelf cavity, using a Hallberg/
GFDL model modified for ice shelf-ocean interactions. Updated with
0901 CTD and multibeam bathymetry, preliminary results show areas of
inflow and outflow in general agreement with our CTD observations,
while providing some guidance for sampling strategy. In turn, the
ongoing observations should help to constrain model mixing
parameterizations. Model output depends on assumptions about cavity
shape, e.g., but suggests that the southern side lobe of the PIG is
actively involved in the circulation and upwelling.

Sea ice studies began with hourly underway sea ice observations
(ASPeCT protocol) on 11 Jan near 68 37 S, 99 35 W and ended on 16 Jan
at 73 36S, 106 45W near the southern ice edge. The transit work
included an automated camera system, and sampling of 6 floes ~30 nm
apart, 5 of which coincided with CTD casts. Snow and ice properties
were examined at the coring sites, and ten cores sectioned on the ice
for salinity, d18O and chl-a, with an additional 15 for structural
analysis. Predominant ice types were thick multiyear (>2m), thick
first year (1-2m) and thinner first year ice (0.5-1m). Snow cover was
generally heavy, sea surface and sea ice temperatures were above
freezing and flooding was pervasive, leading to highly porous ice
with multiple gap layers that appeared favorable for ice algal

Anne Alderkamp's group sampled at 9 stations for the DynaLiFe
project, on 2 of which the Trace Metal Clean (TMC) frame obtained 80L
of surface waters to start two different ligand experiments. Surface
water profiles down to 300m were obtained on the other 7 casts, with
TMC Goflo bottles on the TMC cable, to determine dissolved iron (Fe)
concentrations and biological parameters. On two of these stations,
samples of different depths were filtered in different size fractions
to obtain characteristics of the Fe-binding ligands that are
naturally present. Surface water was obtained from the CTD/rosette on
6 stations to determine carbon uptake kinetics, CO2/HCO3
disequilibrium uptake and the activity of carbonic anhydrase.
Preliminary results indicate low Fe concentrations in the partially
sea ice covered waters north of the Pine Island Polynya, and in
highly productive areas within the polynya, where signs of Fe
limitation of biological production were observed in the biological
experiments. Waters near the PIG contained higher Fe values, and no
signs of Fe limitation of the phytoplankton.

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