Polarstern
|
1. 1 Itinerary and summary
The "Polarstern"-cruise ARK XIV/2 covered the Nordic
Seas from Fram Strait to Denmark Strait (Fig. 1). Physical and
chemical oceanography investigations were carried out as part
of a climate research programme. The mechanisms of heat exchange
between ocean and atmosphere and the cycles of organic matter
were investigated. Biogeochemical investigations concentrated
on the composition, concentration and distribution of dissolved
organic matter (DOM) and the production of methane in ocean.
During the cruise, measurements were carried out on 282 stations
by use of a CTD (Conductivity, Temperature Depth) probe combined
with a water sampler and a Lowered Acoustic Doppler Current Meter
(LADCP). The water samples will be used to measure the concentrations
of oxygen, nutrients (including nitrate, nitrite, phosphate and
silicate), CFCs, tritium, helium, stable isotopes 16O/18O,
barium and sulfur hexafluoride (SF6).
During part of the cruise samples were taken for the determination
of methane and DOM. In addition, 15 oceanographic moorings were
recovered and 17 were redeployed.
Exchanges between the North Atlantic and the Arctic Ocean result
in the most dramatic water mass conversions in the World Ocean:
warm and saline Atlantic waters, flowing through the Nordic Seas
into the Arctic Ocean, are modified by cooling and freezing into
shallow fresh waters (and ice) and saline deep waters. The outflow
from the Nordic Seas to the south provides the initial driving
of the global thermohaline circulation cell; the outflow to the
north has a major impact on the large scale circulation of the
Arctic Ocean. Measurement of these fluxes is a major prerequisite
for the quantification of the rate of overturning within the large
circulation cells of the Arctic and the Atlantic Oceans, and is
also a basic requirement for understanding the role of these ocean
areas in climate variability on interannual to decadal scales.
Fram Strait represents the only deep connection between the Arctic
Ocean and the Nordic Seas. Just as the freshwater transport from
the Arctic Ocean is thought to be of major influence on water
mass formation in the Nordic Seas, the transport of warm and saline
Atlantic water significantly affects the water mass characteristics
in the Arctic Ocean. The inflow from the Arctic Ocean into the
Nordic Seas determines to a large extent the formation of water
masses which are advected through Denmark Strait to the south
and participate in the formation of the North Atlantic Deep Water.
The obtained data will be used, in combination with a regional
model, to investigate the nature and origin of the transport fluctuations
as well as the modification of signals during their propagation
through the strait.
Whereas in the Nordic Seas the ventilation of deeper layers is
dominated by open ocean convection, in the Arctic Ocean the sinking
of shelf water plumes is the major ventilation process. For example,
water masses from the Storfjord (Spitsbergen) and the Barents
Sea sink along the continental slope off Spitsbergen into the
deep ocean. The plumes of newly formed water can be detected by
the measurement of temperature, salinity, tracers and, possibly,
also suspended sediment (via light attenuation), the latter since
it is hypothesised that the suspended matter can help create the
density gain required for a sinking plume. As the plumes are subject
of significant variability, time series are needed to understand
the dynamics of the sinking plumes and their injection from the
shelf into the open ocean.
The measurements indicated that during the last winter (1997/1998)
the convection in the Greenland Sea reached only to 700 m depth.
The deep layers of bottom water were subject to further warming
by about 0.01 K per year as found with a short interruption during
the last years. Both observations suggest that the phase of little
water mass renewal of the last years still continues. Additionally,
there is further evidence of the previously reported downwelling
in the deeper layers of the central Greenland Sea. The low salinity
and the high temperatures in the uppermost 1000 m suggest that
there will be no deep convection in the next winter neither. The
deep outflow from the Nordic Seas across Fram Strait seems to
be warmer than in the last year indicating that the changes in
the Greenland Sea are spreading into the Arctic Ocean. The shallow
inflow from the Arctic Ocean was relatively fresh which could
explain the low salinities in the Greenland Sea. The strong inflow
of low salinity water can affect the potential for deep convection.
The spreading of the water from the Greenland Sea into the Denmark
Strait overflow was studied with a series of sections across the
East Greenland Current up to the Irminger Sea. Of particular importance
were the measurements of the tracer sulfur hexafluoride (SF6)
which was deployed in 1996 in a 300 m deep layer over 400 km2
in the Greenland Sea. The measurements showed that the layer sank
by several hundreds of meters and had spreaded out of the Greenland
Sea. However the strong drop in concentration over deep sea ridges
indicates that they affect the spreading significantly, even if
they do not reach the SF6-layer.
However there must be gaps, because SF6-enrichment
was observed in Fram Strait. In the south, the SF6-patch
had reached the Denmark Strait, but it had not yet crossed the
sill as the SF6-rich layer was
deeper than the sill depth.
The investigations represent a contribution to a long term programme
in the framework of the îArctic Climate System Studyî
(ACSYS) of the îWorld Climate Research Programmeî
(WCRP). The work in Fram Strait is partly funded by the European
Union project "VEINS" (Variability of Exchanges in Northern
Seas). In this context, the Norwegian vessel "Lance"
operated simultaneously in Fram Strait. Four of the moorings in
Fram Strait are maintained by the Norsk Polarinstitutt. The tracer
observations and the moorings at the continental slope of Spitsbergen
are a contribution to the Deep Sea Research programme ARKTIEF
of the German Ministry of Education, Science and Technology (BMBF).
The SF6-measurements took place
in the framework of the EU MAST-III programme ESOP-2 (European
Sub-Polar Oceans Programme phase 2). Besides AWI, there were groups
from the universities of Hamburg, Heidelberg, Kiel und Rostock
and from England, Finland, Italy, Norway and the USA involved
in the programme.
The cruise started on 27 August in Tromsø (Fig. 1). The
first operations took place on the southern shelf of Spitsbergen
where the outflow from the Storfjord was surveyed. One oceanographic
mooring was exchanged and a hydrographic section was carried out.
Along a section across the western continental slope of Spitsbergen,
two moorings were recovered and a second section was completed.
The observations continued along a zonal section across Fram Strait
at approximately 79°N. On the section across Fram Strait,
10 oceanographic moorings were recovered, and 14 were redeployed.
To determine the water mass properties CTD measurements with water
sampler profiles were taken. At the Greenwich Meridian a meridional
section started to the north, turned to the northeast and ended
at the slope of the Yermak Plateau. After the end of the section
we returned to 79°10íN, 2°W where we met the
Norwegian RV îLanceî. After finishing the zonal section
at 16°W, we continued the meridional section to the south
up to 77°30íN, to measure the recirculation in the
southern Fram Strait. Finally, we returned to the western slope
of Spitsbergen and repeated the inshore part of the two sections.
Another effort to dredge one of the moorings was with no success.
As there was intensive fishery in the area the moorings were not
redeployed to avoid further losses. The first part of the cruise
ended on 16 September in Longyearbyen, where 24 participants left
and 20 new ones came on board. Furthermore, material from the
Koldewey Station was collected by the helicopter and instruments
recovered during the cruise were deposited at the airport.
The second part of the cruise started with the continuation of the southward meridional section across the Greenland Sea (Fig. 1). In the central Greenland Sea, two moorings were recovered and redeployed. Then, a zonal transect at 75°N with high horizontal resolution to determine the convective state of the Greenland Sea, started off the Greenland coast and ended in the Barents Sea at 18°E. The meridional transect was continued at 3°W towards the Mohns Ridge into the Norwegian Sea. Mohns Ridge was crossed with another section along a gap north of the Jan-Mayen-Fracture-Zone. At Jan Mayen the 2277-m-high Beerenberg was in sight. A further section across the East Greenland Current along 71°N ended at the Greenlandic coast. On our way to the next section which started at 69°22.7íN, 23°43.2íW, we stopped in the Scoresbysund for a short visit of the Inuit village Ittoqqoortoormit. The following section headed southward to the Islandic coast where we turned west into Denmark Strait. The section across the sill was continued along 66°30íN across the Storfjord Deep to the coast. The next section was directed from the coast into the Irminger Sea. It was interrupted for a small section across the southern sill of the Storfjord Deep, to find out if the shelfwater is spilling over the sill. But there was no indication of it. The last section went back from the deep Irminger Basin to the Greenland Coast. It ended on 10 October. From there, "Polarstern" returned to Bremerhaven, where the cruise ended on 15 October.
| ARK XIV-2 part 1 + 2 | ||
| Participants | ||
| Address | Part 1 | Part 2 |
| Finland | ||
| IMR Finnish Institute of Marine Research | 1 | |
P.O.B. 33 Lyypekinkuja 3 a
FIN-00931 Helsinki
| ||
| France
| ||
| IFREMER IFREMER/DITI/NOE | 1 | |
| Technopolis 40
155 rue J. J. Rousseau
F-92138 Issy les Moulineaux cedex
| ||
| Germany AWI Alfred-Wegener-Institut | 12 | 13 |
|
für Polar- und Meeresforschung Columbusstraße
D-27568 Bremerhaven DWD Deutscher Wetterdienst | 3 | 3 |
|
- Seewetteramt - Bernhard-Nocht-Str. 76
D-20359 Hamburg HSW Helicopter-Service | 3 | |
|
Wasserthal GmbH Kätnerweg 43
D-22393 Hamburg IfMHH Institut für Meereskunde | 2 | 2 |
|
der Universität Hamburg Troplowitzstr. 7 D-22529 Hamburg
IfMK Institut für Meereskunde | 5 | 9 |
|
der Universität Kiel Düsternbrooker Weg 20
D-24105 Kiel IOW Institut für Ostseeforschung | 2 | 1 |
|
an der Universität Rostock Seestraße 14 D-18112 Rostock
IUPH Institut für Umweltphysik | 2 | 2 |
|
der Universität Heidelberg Im Neuenheimer Feld 366
D-69120 Heidelberg IUPT IUP-Institut für Umweltphysik | 2 | |
|
Abt. Tracer-Ozeanographie Universität Bremen, FB 1 Postfach 33 04 40
D-28334 Bremen
| ||
| Italy CNR Istituto Sperimentale Talassografico | 2 | |
|
Viale R. Gessi, 2
34123 Trieste
| ||
| Norway NPI Norsk Polarinstitutt Storgata 25A Box 399
N-9001 Tromsoe | ||
| UNIS The University Courses on Svalbard | 1 | |
|
P. O. Box 156 N-9170 Longyearbyen
| ||
| UK UEA School of Environmental Sciences | 2 | 1 |
|
University of East Anglia
NORWICH NR4 7TJ
| ||
| USA WHOI Woodshole Oceanographic Institution | 1 | 1 |
|
Woodshole, Massachusettes 02543-1053 UT University of Texas at Austin | 1 | |
|
Marine Science Institute 750 Channel View Drive Port Aransas, TX 78373
CTD and water sampling system (AWI)
The CTD system (Conductivity, Temperature, Depth) used during
part the first part of the cruise (stations 1 to 113) consisted
of a SBE 911+ and an SBE32 rosette water sampler. CTD S/N was
09P16392-0485, S/N of the used sensors are 1642 for the SBE3 temperature
sensor and 1493 for the SBE4 conductivity sensor. A Seatech transmissiometer
with 25 cm beam length was attached to the CTD as an additional
sensor. The used water sampler is a 24 bottle type for 12 Liter
Ocean-Test-Equipment bottles. Places 10, 11, and 12 were occupied
by an ADCP (Acoustic Doppler Current Profiler). Bottles 1, 5,
13, 17, and 21 were equipped with electronic reversing instruments
(SIS) for temperature and pressure measurements. During the second part of the cruise (Stations No. 114 to 282), the instrument configuration from the first part was used up to station No. 125. After this station, the following sensors were used: - Temperature sensors T0, T1: S/N 1491, 1642 - Conductivity sensors C0, C1: S/N 1198, 1493 - Transmissiometer (same as during leg 2a) - Gelbstoff-fluorescence-sensor Dr. Haardt
The reversing instruments were the same as during the first part.
In addition, a SBE35 reference thermometer (S/N 003) was used,
to check calibrations of the CTD sensors at selected locations
with sufficiently small temperature fluctuations.
The conductivity was corrected using salinity measurements from
water samples. IAPSO Standard Seawater from the P-series P 133
and P 131 was used. A total of 2742 water samples were measured
using a Guildline Autosal 8400A. On the basis of the water sample
correction salinity is measured to an accuracy of more than 0.003.
Halocarbons (IFMK, IOW)
The chlorofluorocarbon components CFC-11 and CFC-12 are analysed
on board with a GC-ECD (gaschromatograph-electron capture detection)
technique as described by Bullister and Weiss (1988). The analysis
of carbontetrachloride (CCl4) is
done with a similar system, but with different material for the
cooling trap and gaschromatographic column. A capillary column
is used to separate CCl4 from other
seawater components instead of a packed column applied for the
CFC analysis.
Shortly after leaving Tromsoe, the GC-ECD system dedicated to
the analysis of CFC-11 and CFC-12 failed and could not be repaired
on board. Thus our second GC-ECD system which was prepared to
analyse carbon tetrachloride (CCl4)
had to be used as a backup for CFC. Due to the different connection
between column and ECD for packed columns (CFCs) and capillary
columns (CCl4), the CFC analysis
suffered a 10 - fold decrease in the strength of the chromatographic
signals compared to the usual performance. The smaller signal/noise
ratio led to an accuracy of +-2% or +-0.012pmol/kg (whichever
is greater) for both, CFC-11 and CFC-12 instead of the usual +-1%.
The accuracy of the data was determined by analysing about 10%
of the samples twice and by closing all bottles in one depth (2300m,
CTD station 87).
The unfavorable signal to noise ratio might also affect the precision
of the tracer data, especially the relatively low concentrations
in deep water. The small signals also forced us to prolongate
the time between two measurements from 11 minutes to about 15
minutes, which decreased the numbers of samples which could be
measured between two CTD casts.
The blanks for the CFCs and CCl4
were negligible. Calibration of the water samples (CFCs) was done
with a gas standard kindly provided by D.Wallace, IFM Kiel. The
concentrations are reported on the SIO93 scale.
The first reliable CFC analysis could be carried out at CTD station
30, thus only few tracer observations were obtained off Spitsbergen
to study the Storfjord outflow. During the rest of the cruise,
the CFC system worked continuously and 1750 water samples have
been analysed on 166 CTD stations.
A new GC-ECD system (for capillary columns) was sent to Longyearbyen
to allow the analysis of CCl4 on
the second part of the leg. The system was operating from 26 September
so that profiles from the Greenland Sea, the Norwegian Sea and
Irminger Sea were taken. In total, 270 water samples from 28 CTD
stations were analysed, the accuracy was checked by analysing
20 samples twice and was higher than 1 0/00.
One of the gas standards used for calibration was lacking its
CCl4 signal. After the cruise,
the second standard, which maintained a CCl4
peak and was used for calibration of the CCl4
measurements, will be calibrated again to check the precision,
thus the presented concentrations are preliminary.
Oxygen and nutrients (AWI, CNR, IFMK)
At each station discrete bottle samples were collected for the
analysis of dissolved oxygen and inorganic nutrients (silicate,
nitrate, nitrite and phosphate) which were measured within a few
hours after collection. Dissolved oxygen was determined according
to Winkler method (Strickland and Parson, 1972) using potentiometric
titration, and inorganic nutrients were determined with a Technicon
Autoanalyzer system. The determination of nitrate and nitrite
is based on the method described by Armstrong et al. (1967), silicate
was measured according to Grasshoff et al. (1983) and phosphate
according to Eberlein and Kattner (1987). In some stations along
the 79°N section samples have been collected for the analysis
of total dissolved nitrogen (TDN) and phosphorus (TDP), which
will be carried out at Istituto Talassografico di Trieste after
UV photo-oxidation according to the procedures described by Armstrong
et al. (1966) and Walsh (1989). During the cruise, 3100 samples
were taken for the analysis of dissolved oxygen, 3670 for nutrients
and 182 samples for the determination of TDN and TDP.
Helium-, tritium-, sulfur hexafluoride- and oxygenisotopes
(IUPH)
During the cruise, 780 helium, 780 tritium/18O,
365 additonal 18O samples and 75
sulfur hexafluoride (SF6) samples
were taken from the water bottles. Additionally, 45 samples of
helium were taken with an alternative in-situ sampling method.
These samples will serve as a first test of the in situ sampling
device. The tracer sampling (helium, tritium, 18O,
SF6) was done over the full water
column along all of the sections. The vertical sampling resolution
was adjusted in such a way, that the core water masses and transitions
between them were sampled. The resolution of 18O
sampling was increased in the East Greenland Current as well as
in the sections at the Barents Shelf near to Svalbard in the Storfjord
(Spitsbergen) outflow region. SF6
sampling was restricted to some stations in the Fram Strait, the
Boreas Basin, the Greenland Basin, the Lofoten Basin, the Jan
Mayen Fracture Zone, the Denmark Strait and the Irminger Basin.
All of the samples will be analysed in the Heidelberg tracer laboratory.
Oxygen isotopes sampling (AWI, UEA)
To measure the oxygen isotope 18O
content 1590 samples were taken, around 700 from Tromsø
to Longyearbyen. The first Storfjord (Spitsbergen) section was
sampled completely, to enable interlaboratory comparison of results
with the group from IUPH who also sampled this location for isotopes.
The Fram Strait section was of prime importance for this leg due
to its relevance to the VEINS program. For this section, the upper
layers of each cast were sampled, with most casts also being sampled
to the bottom. This strategy was also adopted for the northern
meridional section, whilst the southern meridional section was
much more sparsely covered to save bottles for sampling on the
second leg. The bottles were shipped back to the U.K. for sample
analysis in the Stable Isotope Laboratory of the University of
East Anglia.
In addition to the water samples, four sets of ice samples were
also collected. Three of these samplings were performed using
the helicopters (78°57.9íN, 0°34.4íW;
79°3.8íN, 03°1.3íW; 78°59.0íN,
11°00.7íW), the other when the ice thickness permitted
direct access from the ship (78°55.9íN, 16°12.9íW,
adjacent to Greenland). Both surface snow and ice were collected,
and allowed to melt slowly in sealed bottles in Polarsternís
cold rooms to minimise sample equilibration with the atmosphere.
Again, sample analysis will be performed at UEA. Results from
these analyses will provide better determinations of the isotopic
characteristics of the freshwater inputs to waters in the region,
and enable their more accurate quantification.
The remaining bottles were used during the second part of the
cruise, with priority being given to the two southernmost sections
(Denmark Strait) due to the requirements of VEINS. 20 stations
were also sampled to full depth on the 75°N section, with
a few other stations from the other sections additionally being
sampled.
Barium (AWI)
To identify different fresh water sources 1800 barium samples
were taken for Dr. K. Falkner of Oregon State University, (USA),
on 123 stations from the rosette water sampler.
Measurements with Acoustic Doppler Current Profilers (AWI,
IFMK, IOW)
a) Vessel Mounted Acoustic Doppler Current Profiler (VM-ADCP)
A 150kHz ADCP is mounted in the ship's hull and monitors continuously
the velocity distribution in the upper water column. Navigation
is provided by DGPS. The VM-ADCP data were processed with the
CODAS 3.1 software from Eric Firing et al. Processing steps were
done according to that program: -estimating the time drift of the PC-clock and correcting the profile times -loading the data into a codas database -verification of the transducer temperature and determination of thresholds -viewing of all profiles for flagging bottom and hydrographic wire interference and other glitches -calculation of misalignment angle between gyro compass and data acquisition unit with water track method -rotation of the velocities by estimated angle -calculation of reference layer velocities -comparison of smoothed reference layer velocities with raw reference layer velocities in order to determine bad satellite fixes and Schuler oscillations
-calculation of misalignment angle with watertrack method, this
time with edited satellite fixes
The data were almost finalized on board, the incorporation of
the 3DGPS heading instead of the gyro compass heading will be
done in the home lab. The velocity profiles reached down to 200
m depth during the first part and to 350 m depth during the second
part of the cruise. The currents averaged between 50 and 100 m
depth are presented in Fig. 7. Despite the tidal noise (about
5 cm/s) the main features of the circulation are evident in the
data: the West Spitsbergen Current near Fram Strait heading north,
the recirculation of the Atlantic Water towards the east and the
East Greenland Current flowing south along the Greenland coast.
b) Lowered Acoustic Doppler Current Profilers (L-ADCP)
The measurements were done with two RDI 150 kHz NB - ADCPs, one
from the IfM Kiel, the other from the AWI Bremerhaven. During
the first part of the cruise, the AWI instrument was attached
to the CTD rosette, while the Kiel instrument was lowered solitary
on a wire. About 25 simultaneous casts of the two instruments,
were taken. This allows for an intercomparison between the data
quality from the two instruments.
On the second part of the cruise, due to malfunction of the AWI
instrument, the Kiel LADCP was attached to the rosette. In total,
216 LADCP profiles were taken, from which 44 had to be rejected
due to bad data quality. On most of these profiles the determination
of the vertical velocity failed causing a wrong assignment of
depth. One part of the rejected profiles were taken on the shelf,
where the waterdepth was shallow compared to the range of the
ADCP. The other part consists of profiles taken by the AWI instrument,
whose range was reduced by one third compared to the Kiel instrument.
The weaker performance and the unusually high consumption of energy
of the AWI instrument was most likely due to moisture in the instrument's
housing. At 9 September, at station 114, seawater penetrated into
the instrument causing a fatal damage.
The data set include numerous profiles from different topographic
regimes, e.g. from abyssal plains, continental slopes, oceanic
ridges and fracture zones. This offers the opportunity to study
the influence of topography on vertical mixing.
The mean currents averaged over the 300 m above the bottom obtained
from the LADCP measurements, are presented in Fig. 8.
On the eastern side of the West Spitsbergen Current flows north
and splits into two branches, one continuing to the north along
Yermak Plateau, the other one recirculating in the Fram Strait
and and joining the southward flowing East Greenland Current.
The velocities in these boundary currents are of the order of
20-40 cm/s. Apart from the strong boundary current, the flow field
is less clear. In the Denmark Strait, the velocities are generally
higher. South of the strait, the high velocities in the overflow
plume are confined to the bottom layer and reach up to 80 cm/s.
XBT measurements (AWI)
24 XBT probes manufactured by Sparton from Canada were launched
during the crossing of the Arctic Front. For location see station
list (Part 4 of Annex)
Moorings (AWI, IFMHH, NPI)
To quantify the current fields of the East Greenland and the West
Spitsbergen Currents by direct measurements, moored instruments
were used. The current field was measured with 14 moorings, deployed
across Fram Strait at latitudes between approximately 78 and 79°N,
in water depths of between 200 m and 2600 m water depth (Part
6 of the annex, Fig. 9). For a sufficient vertical resolution,
3 to 4 instruments per mooring are required. Temperatures and
salinities are measured together with the currents, to allow derivation
of the heat and salt transports. Mooring VFS 1 to 10 were recovered
from "Polarstern", VFS 11 could not be recovered and
VFS 12 to VFS 14 were recovered from "Lance". All 14
moorings were redeployed from "Polarstern".
Another set of moorings (SF1 to 5) was deployed on the southwestern
continental slope of Spitsbergen. However, SF3 and SF4 could not
be recovered in spite of extensive dredging. It is most likely
that they were damaged by the intensive fishing which could be
observed during the work in the area. Inbetween, the ADCP of mooring
SF3 was found in Denmark Strait. Because of the high risk of further
losses the programme was strongly restricted and only SF1 was
redeployed.
Two moorings with a profiling CTDs were recovered and redeployed
in the central Greenland. | ||
