V. Shevchenko
The Secchi disc was used for qualitative estimates of the concentration of sus-pended particulate matter (turbidity). The Secchi disc is a flat circular plate, 30 cm in diameter, all white. It is lowered through the water column in a horizontal attitude until it is observed just to disappear. The depth, at which this happens, is called the Secchi depth, and it depends on the turbidity of the water. The Secchi disc is both cheap and easily made, and it has been used by oceanog-raphers for over a century as a rapid means of assessing water clarity (Sea-water, 1995).
In our expedition nine measurements of the Secchi depth were carried out (Ta-ble 6.1). It varied from 13 to 18 m with an average of 14.7 m. It shows that the concentration of suspended particulate matter (SPM) in water is very low. The SPM concentration in the Yermak Plateau area in September 2004 was esti-mated using Secchi depth data and fitting power equation calculated from data of simultaneous SPM and Secchi disk measurements in the White Sea in April 2003 (V. Shevchenko and A. Filippov, unpublished data). Accordingly to these qualitative estimates SPM concentration in the upper 5 m of water column in the studied area varied from 0.23 to 0.37 mg/l (0.31 mg/l in average). For compari-son, in the Kara Sea the 5-m transparency isoline shows the border of the main area of influence of the fresh turbid waters from Ob, Yenisey and other rivers (Lisitzin et al., 2000). Only in the outer part of the Kara Sea where the concen-tration of suspended matter is <0.5 mg/l the Secchi depth increases up to 15 m.
In the Gakkel Ridge area in August–September 2001, the Secchi depth varied from 16 to 30 m with an average of 21 m (Shevchenko, 2002).
Table 6.1 Time, locations and results of Secchi depth measurements.
7. Bathymetric Investigations B. Platten, R. Rathlau, A. Winkler
The main task of the bathymetric working group on this cruise was the support of the geological and geophysical working groups by creating bathymetric charts of their research areas by means of the Hydrosweep system developed by STN Atlas.
The deep-water sounding system Hydrosweep D2 (Fig. 7.1) on RV “Polarstern”
DS2 stands out through a 90°/120° coverage angle in which the seafloor is re-corded with 59 specific values for water depths (hard beams) perpendicular to the ship's long axis. The sonar beam has an accuracy of approximately 1% de-viation from the real waterdepth after the correction of an automated crossfan calibration. The mean sound velocity over the vertical water column is deter-mined by regular transmission and measurement of a sweep profile in the ship’s longitudinal direction and comparison of the slant beams with the vertical beam.
These data are used for the depth computation
Fig. 7.1: Multibeam sonar system
The operation of the sonar system and the data processing made up the main part of the work. 12 days of the 27 days lasting survey were done under thick ice conditions. In order to prevent abnormal functioning and false measure-ments due to the thick ice conditions the mostly automatically working multi-beam sonar system had to be observed continuously. The changing of the ship’s speed and direction due to ice breaking as well as hydroacoustic distur-bances caused faulty measurements. Even post-processing became difficult because of the ice conditions, so it was mandatory to check the positions, depths and the ship's attitude data continuously for outlier- or blunder-values.
The corrections of the data as well as the general post-processing tasks were done with the software CARIS HIPS and SIPS v. 5.3.
Despite these difficulties the bathymetry working group managed to process all data so far that they now can be used for further examination directly at the AWI.
Beside the sonar operation and the data editing the analysis of multibeam data, the preparation of quick-look track plots and the creation of preliminary bathy-metric charts were made. Further presentations of the sea bottom topography grids were calculated out of the edited data with the Generic Mapping Tool (GMT) software.
Based on the grids, contour line maps with a contour interval of 100 m and
maps allowed a vivid display of the oceanic ridge topography. The IBCAO Vol. 1 (grid spacing 2500 m) data set was used as background bathymetric data in the contour maps. But since the IBCAO in that region is based on sporadic single beam sonar tracks and bathymetry estimated from satellite data, there were substantial differences between the systematically surveyed maps and the IB-CAO data. By the combination of previously recorded data with the new data from this cruise the working group bathymetry was able to close some major gaps in the existing data as well as to discover new examination areas.
Bathymetric issues have been very important for the geological investigations on this cruise as shown in three examples.
(1) The eastern slope Yermak Plateau and the North East Land shelf of Sval-bard was of special interest due to a suspected megaslide in this area (See Chapter 9). The shelf and its eastern and western slopes (Fig. 7.2) could be surveyed by using the multibeam sonar system Hydrosweep. Altogether the sea-ice conditions were good, so that reasonable profile measurements could be done on the Yermak Plateau.
Fig. 7.2: Hydroweep profiles in the Sophia Basin, southeast of the Yermak Plateau, and at the northern Svalbard continental margin
(2) On top of the central Yermak Plateau, probably out-cropped older sedimen-tary rocks should be sampled by means of the sediment dredge. The localiza-tion of petrologic sampling stalocaliza-tions was accomplished based on the bathymetric maps (Fig. 7.3).
Fig. 7.3: Station PS 66/332 on the central Yermak Plateau, selected based on a bathymetric chart
(3) With systematic seismic profiles it was possible to determine the depth and course of iceberg plough marks on the Yermak plateau from the recorded bathymetric depth data (Fig. 7.4). On the western slope of the plateau the tran-sition from the plateau area in Fram Strait-direction to the deep sea could be documented successfully.
Fig. 7.4: Iceberg plough marks on Yermak Plateau