excerpts from weekly report 3

Hi all, another edition of the NBP0901 weekly science report is
available below:

Before that, an answer for D. Tortise.
I'm going to have to ask the chief scientist on board if it's OK to
use the CTD profiles. I'll get back to you.

And thanks to those (two) who participated in the "name the sea
iceketeers" contest. It's a tie between the core convicts and the
ice ice babies.

As promised, the weekly report
The Antarctic ice sheet, which appears to have been losing mass at an
accelerating rate over recent decades, is potentially the largest
contributor to future sea level rise. Current ice loss is focused in key
drainage basins where dynamical changes in outlet glaciers have led to
increased discharge, most significantly along the Amundsen Sea coast.
The
apparently synchronous response of independent glaciers that thin most
rapidly over their ice shelf extensions is generally assumed to be
ocean-driven change, but the actual mechanisms are speculative.
Flooding of
the deeper parts of the continental shelf by 'warm' deep water (CDW)
causes
rapid melting of floating ice, but we need to understand better the
processes that control CDW inflow variability and interactions with that
ice in order to quantify past, present and possible future oceanic
forcing
of the glaciers. CTD profiles and moored instruments on the continental
shelf provide pictures of the spatial and temporal variability of CDW
inflows, while an AUV can do the same in the ocean cavity beneath an ice
shelf. Although the technology is relatively new and its use beneath
ice is
rare, the UK/NERC Autosub-III was specifically designed with a deep
under-ice capability. On this second Autosub expedition into the
Amundsen
Sea, unusually light ice conditions have allowed much of the past
week to
be devoted to a study of the 'black box' under the Pine Island Glacier
(PIG) ice shelf.

Autosub is 7 meters long, powered by 5000 D cells, and can range to
400 km
and to depths of 1600 m. On 0901 it is fitted with a CTD system that
includes a dissolved oxygen sensor and transmissometer, upward- and
downward-looking Acoustic Doppler Current Profilers and a multi-beam
echo-sounder that can be pointed up or down. An extendable gantry
mounted
on the main deck lifts Autosub from its protective container and
lowers it
over the stern of the ship. Once sent on its way, the sub runs for
10-15 km in open water before passing under the ice
front, returning 15 to 30 hours later. Four successful missions this
week, including one that penetrated nearly 60
km to the region of the grounding line, have yielded a wealth of data on
the cavity shape and its seawater properties.

The DynaLiFe project sampled 6 trace metal clean stations near
the PIG, four profiling surface waters down to 300m for Fe, ligands and
other parameters, and two catching large volumes of surface water to
start
new experiments. High Fe concentrations and low biological activity
occurred at sites characterized by melt-driven upwelling of CDW from
beneath the PIG, and biological activity was patchy at the other sites.
Dissolved Fe concentrations in the upper 100m were low, yet no
indications
of Fe limitation of phytoplankton were observed, in contrast to
conditions
seen at earlier stations in the middle of the polynya. Experiments
examined
the accessibility of organically bound iron to phytoplankton and
successfully measured the effects of Fe limitation on its
susceptibility to
photoinhibition. In some cases Fe bound to ligands appears accessible to
the phytoplankton. The experiments clearly showed that Fe limitation
increases susceptibility of phytoplankton to photodamage by high light
conditions experienced near the surface. These findings will help to
explain the observed distribution of phytoplankton primary
productivity in
the Amundsen Sea and other Antarctic polynyas.

Two bottom-moored instrument arrays measuring deep temperature,
salinity,
pressure and ocean currents were deployed in the PIG/Thwaites Trough for
later (2011) recovery. Fifty-four CTD/rosette/LADCP profiles were
made of
temperature, salinity, oxygen, ocean current, light transmission, PAR
and
fluorescence, many at a time-series station for tidal analyses. Water
samples were drawn and processed aboard or stored for
later analyses. The water column profiling reveals such features as
layers
high in meltwater, surface and bottom boundary layers, levels of
stronger
and weaker currents, and the top of the CDW reservoir. Along with active
upwelling near an ice front, we have observed remarkable melt-generated
thermohaline staircases, both previously reported on smaller scales in
laboratory experiments.