Methodologies
Sampling
Trace-metal clean samples:
Samples for trace element determinations, and for culture work were obtained
using a Go-Flo bottles mounted on a polyurethane-coated rosette frame
containing a SeaBird SBE 911 ctd, an SBE 43 DO dissolved oxygen sensor and
a Wet Labs ECO-AFL Flourometer. The package was lowered using a
polyurethane coated 4-conductor Kevlar cable and individual bottles were fired
using the SBE seasoft computer program. Water samples were obtained during
the upcast and bottles were tripped while the package was still moving slowly (510 m/min) upwards into clean water. As soon as the package was recovered the
Go-Flo bottles were removed from the rosette frame and brought into the TM
clean van within which a Mac-10 HEPA unit and a ENVIRCO class 100 laminar
flow hood were operated constantly to remove particulates from the air.
Unfiltered samples were collected for salinity, nutrients etc. to establish the
integrity of the samples. For filtered samples the Go-Flo bottles were
pressurised using filtered compressed air to <10 psi and the water passed
though 0.45 um Supor membrane polysulfone filters which had previously been
acid washed and rinsed in DI water. After sub-sampling the filters were washed
in DI water, dried and returned to shore for determination of particulate loads.
Shipboard trace metal determinations
Total Dissolved Iron Analysis:
Dissolved Fe was determined using the Flow Injection Analysis (FIA) scheme
(Measures et al., 1995). The pre-concentration column used for this
determination was found to bleed an unacceptably high level of Fe into the
system and was replaced after station ?? with a ~ 1m injection loop. While the
substitution eliminated the blank problem the sensitivity of the method was
reduced by ~ 10-fold. The minimal detectable quantity using the injection loop
was approximatley 60-150 pM. Precision was typically 1.8% at 1 nM
Shore-based Total Dissolved Fe Analysis:
The accuracy of the shipboard dissolved Fe analyses will be assessed by the
analysis of replicate subsamples (stored acidified to 0.024M Q-HCl) from
selected depths in each profile using the Fe-57 isotope dilution method of Wu
and Boyle(1998). These determinations will be carried out by W.M. Landing at
FSU using high-resolution (HR-ICPMS) analysis with a Finnegan Element I
high-resolution magnetic sector ICPMS. Landing has a precision of 3-6% for
replicate determinations using this techniques, and is also participating in the
SCOR-sponsored dissolved Fe inter-comparison study. It is expected that this
phase of the work will be accomplished by December.
Total Dissolved Aluminium Analysis:
The Flow Injection Analysis (FIA) method of Resing and Measures (1994) was
used to determine Al concentrations. This fluorometric method (based on Hydes
and Liss, 1976), using a 1-minute pre-concentration had a detection limit of ~0.4
nM and a precision of 2.7% at 3 nM.
Total Dissolved Manganese Analysis:
The FIA method (Resing and Mottl, 1992) was used to determine total dissolved
Mn. The method, was used with a 1-minute pre-concentration and had a
detection limit of 50-100 pM and a precision of 8% at 0.5 nM.
Particulate material:
Total particulate Fe, Al and Mn will be determined by analysis of material
collected onto preweighed 0.2µm filters. The filters will be digested using HNO3
in Teflon bombs using a microwave digestion system. The digest will be
analysed using the FIA systems outlined above. Due to equipment deployment
requirements for other NSF projects, the results of this work are not expected
until late Spring 2005.
Shipboard results and interpretation
A total of 450 samples were collected at 36 stations. Shipboard dissolved Fe, Al
and Mn are reported. Each sample is reported with a quality flag to indicate the
reliability of the data. At this stage the data set the only quality flag assigned to
samples is 8 to indicate samples that were obtained from Go-Flo bottles that
were suspected of pre or post-tripping. These samples were identified by a
mismatch between the in situ salinity recorded by the SBE 911 ctd and the
discrete salinity sample collected from the Go-Flo rosette during the subsampling operation. At this stage comparison between discrete salinity data and
ctd records has only reached Station 48. Thus samples with higher station
numbers than this may also contain erroneous data from pre or post-tripped
bottles.
Run with pre-concentration column
Station 3
Station 16
Station 19
–currently not blank corrected
0.69 nM Fe blank correction applied
0.55 nM Fe blank correction applied
Run with injection loop
All others
Interpretation
Drake Passage
Dissolved Fe values ranged by two orders of magnitude from 0.050 to 5.46 nM,
dissolved Al varied 15 fold from 0.48 to 7.6 nM and Mn varied by more than 200fold from 0.04 to 9 nM.
The systematics of the trace metal distributions can be envisaged as being the
result of the addition of large amounts of these trace metals to very low
background source water followed by the biological modification of that water.
Addition of the trace metals appears to result from the impingement of the ACC
water onto the shelf at the region of the Shackleton fracture zone.
Lowest concentrations for all three dissolved trace elements were found
associated with the "blue water" regions as represented by stations upstream, or
close to the Shackleton FZ e.g.16b, 29b, 55a/g, 70c. In addition low
concentrations for all metals were observed at offshore stations in the NE
quadrant of the station grid, e.g. 42e, 64g and 68a.
In contrast, the highest values were found for all three elements in the coastal
shelf stations e.g. 3a, 19a, 57e, 58c,
Or in the water advected off the shelf into the NE quadrant of the sampling grid,
e.g. 31a, 33a, 66c.
Intermediate values, i.e. high concentrations of Mn, a tracer of shelf addition, but
low values of Fe were found at stations 36a,39a, 46a,48b,54c, 61a, 63c, 71b,
73a, 75a, 76b. These stations are presumed to represent the downstream
biogeochemical modification of the enriched shelf waters where Fe has been
removed from the dissolved phase into the particulate phytoplankton phase.
The shape of the Mn profile appears to vary between regions of the sampling
grid. For example stations 19a, 73a,75a,76b,33a and 36a all display a mid depth
Mn maxima around 60-80 m, with lower values above and below. In contrast,
several stations clustered together (48b,46a,54c,53a) show high Mn surface
waters (1.5-4 nM) but with much lower values (<0.5nM) below 60m. The profile
shapes may indicate advective outflow from different shelf regions.
A third kind of profile shows uniformly high Mn in the upper water column,
examples of this kind of station are 57e,58c, 31a, 66c, 39a. In these cases the
water column appears to composed of a large mass of vertically mixed shelf
water that has advected offshore. The inner shelf stations in this group (57e, 58c
have uniformly high Fe concentrations (1-2 nM) which erode somewhat in the
more offshore station 31a to 1- 1.5 nM, to the most offshore station 66c where
upper water Fe has dropped to <0.5 nM despite persistently high Mn.
At a larger scale it can be seen that in the Drake Passage stations Fe
concentrations correlate well with both Al (figure 1; R= 0.73) and Mn (figure 2; R=
0.63)
Bransfield Strait and Deception Island
A total of four stations were occupied in the Bransfield strait, 2 in an offshore
transect and 2 in an along strait transect , and a single station was occupied in
the caldera of Deception Island.
In the offshore transect the most elevated trace metals were found in the inshore
station (86a), where Mn levels were 8-10 nM from 60 m to 120m, Fe values
reached ~ 5nM over the same depth range and Al values were also enhanced
over this depth range at~ 3 nM. The attenuation of this signal in the offshore
station (89d) indicated a shelf origin for the trace elements in the Strait. Little is
seen in the way of trace metal gradients along the strait (stations 94a and 95a)
although a deep water (900m) maximum was seen in all metals at stn 94a,
immediately downstream of a potential hydrothermal source.
Within Deception Island caldera, trace metal values were extremely enriched.
Mn which was uniform in the upper 80m showed a maximum of almost 200nM in
the bottom waters below 110m. Fe showed a layer of uniform values (~20 nM) in
the upper 50m (approximately equal to the sill depth of the caldera), declining to
~ 5 nM in deep waters. Al was fairly uniflrm at ~ 3 nM throughout the water
column. While the bottom water Mn is likely a result of hydrothermal input to the
bottom of the caldera, the relatively low levels of Fe at the same depth are
puzzling.