Cruise Report

CRUISE REPORT MARINE GEOLOGICAL CRUISE TO OFOTFJORDEN AND VESTFJORDEN, NORTHERN NORWAY RV Johan Ruud 24. -28. 5. 2004 by Jan Sverre Laberg DEPARTMENT OF GEOLOGY UNIVERSITY OF TROMSØ N-9037 TROMSØ, NORWAY 1. Introduction and scientific objectives During the RV Johan Ruud cruise from the 24th to the 28th of May 2004 high-resolution seismic data, gravity and box core samples were acquired from Ofotfjorden and Vestfjorden in Northern Norway (Figure 1a and b, Tables 1 and 2). This is a following up cruise after the 2003 cruise (Laberg and Forwick, 2003) in order to expand on the seismic and seabed core data base. The data will be studied as part of the Norwegian Research Council-funded SPONCOM project (http://www.ig.uit.no/sponcom/index.htm). SPONCOM (Sedimentary Processes and Palaeo-environment on Northern Continental Margins) is a strategic University project led by Prof. Tore O. Vorren at the University of Tromsø. In more detail our activities focus on three main aspects: 1) The chronology and dynamics of the last glaciation-deglaciation in the Troms- Lofoten area. 2) Processes and fluxes of fjord, continental shelf and –slope sedimentation. 3) Rapid paleoceanographic and palaeoclimatic changes, during the last glacial maximum, the last deglaciation and Holocene. The aims of the new seismic data is to elucidate the chronology and dynamics of the last glaciation-deglaciation in the Vestfjorden – Ofotfjorden/Tysfjorden area by identifying possible submarine glacigenic deposits that can be correlated with recessional moraines, in particular the Skarpnes (Older Dryas) and Tromsø – Lyngen (Younger Dryas) events. So far, mainly on land studies have been published from this area, partly with a conflicting view on the position of the Older Dryas and Younger Dryas moraines (see Andersen, 1968, 1975; Olsen 2002; Vorren and Plassen, 2002 and references therein). Identification and dating of submarine moraines will hopefully contribute to solve this discrepancy. 2 The seismic data will also form the basis for identifying the deglaciation and Holocene sediments in the fjords, their flux and processes of deposition and optimal core sites for paleoceanographic and paleoclimatic studies. These results will be correlated and compared with previous studies from the SPINOF-program (http://www.ig.uit.no/spinof/index.html) (Plassen and Vorren, 2002) as well as other relevant studies. 2. Cruise participants In addition to the regular crew of RV Johan Ruud (http://www.nfh.uit.no/hmenyvis.aspx) under Captain Anfinn Utheim the cruise participants were: Jan Sverre Laberg, Research Scientist, University of Tromsø (cruise leader) Trine Dahl, Lab. engineer, University of Tromsø Kai Fløystad, Master student, University of Tromsø Steinar Iversen, Science engineer, University of Tromsø Gaute Salomonsen, Research Scientist, University of Tromsø 3. Cruise narrative Monday 24.5. Departure from Tromsø in sunshine and light northerly breeze at about 10:00 (local time = GMT + 2:00 hours). The ship was stopped in Tromsøsundet for a life boat exercise. Then we sailed for a box corer station in Malangen where we arrived at 13:00. At the position we collected CTD-data and a bottom water sample, and then we used a small box corer for sea floor sediment sampling. We got a good quality sample. Then we sailed for the start of our first seismic profile in Ofotfjorden (Figure 1a). 3 Seismic line 04JR312 started in Ofotfjorden at about 23:00 and was the first of a series of transects across Ofotfjorden using the Boomer and 3.5 kHz systems. Tuesday 25.5. A cloudy day with light northerly breeze. We continued profiling during the night and data of good quality were recorded. We had some problems with the EPC printer but the problems were solved by resetting the printer. During the morning we were profiling across Ofotfjorden in the Balangen area. We continued profiling across the inner part of Ofotfjorden during the afternoon and early evening and the profiles were of very good quality. Then we started coring in the inner part of Ofotfjorden using the gravity corer. The first core, 04JR-331 was recovered from the innermost sedimentary basin identified in Ofotfjorden and here we acquired a 3.16 m long sample. The next two cores were recovered from a slide scar identified from seismic and bathymetric data in the inner part of the fjord. The cores were of 3.09 m (04JR-332) and 2.45 m (04JR-333) length (Figure 1b). Wednesday 26.5. The weather was cloudy with almost no wind and calm sea. We continued coring during the night and one core (04JR-334) from the middle and one from the outer part of the fjord (04JR-335) were recovered. The cores were 2.52 m and 1.7 m long, respectively (Figure 1b). A very preliminary analyse of the foraminifera from core catcher samples indicated that only Holocene sediments were sampled (Salomonsen, pers. com.). Then we sailed for the inner part of Vestfjorden where we had a box corer station and then we started profiling across the fjord using the Boomer and 3.5 kHz system. Again, data of good quality were recovered. Thursday 27.5. A rainy day with south westerly breeze and 1 – 2 m wave height. During the night and morning we continued collecting seismic profiles across the inner part of Vestfjorden. The outermost profile, 04JR-436 was located west of Skrova. No gravity core samples were acquired from this area because of little or no coverage of marine and/or 4 glacimarine sediments and because of the wave height, the ship was moving too much for coring to be done safely. However, one box corer station was done successfully close to Skrova. The next profile was oriented towards the northeast. Then we continued with one more seismic profile across Vestfjorden before we finished in this area and sailed for Tjeldsundet where we planned two profiles. Friday 28.5. A rainy day with almost no wind. We ended the two profiles across a possible morainal bank across the southern part of Tjeldsundet at about 00.30 (Figure 1a). Then we sailed for Tromsø where we docked at about 11.00 and started unloading the ship. 4. Geophysical and geological equipment 4.1. Seismic equipment 4.1.1 The Boomer (Figure 2a and c) The Boomer is a seismic system that generates sound waves through electrically induced movement of metal sheets. The Boomer was operated with an energy of 300 J at a shotrate of 0.5, 1 or 1.5 s depending of the water depth and sediment thickness. When printed the signal was filtered at 300 – 2000 Hz. The boomer operates to a water depth of up to 700 m and was towed at a speed of 4 knots. 4.1.2 The Streamer (Figure 2b and d) A Benthos MESH 25/50P streamer with a total length of 50 m was used. The active part of the streamer (containing hydrophones) is 6 m long. The sound wave is recorded by the hydrophones, the signal from each hydrophone is summed and continues to the receiver. 4.2 3.5 kHz Echo sounder 3,5 kHz Echosounder records have been acquired simultanously to Boomer profiling. The 3.5 kHz system operated an energy level of 10kW using 2 pulse cycles and filtered at 3 – 5 kHz 5 when plotted. The principal aims are: (1) to image the morphology of the ocean floor and its shallow subbottom sedimentary layers and structures and (2) to select sediment core stations. 4.3 Onboard analogue and digital recording (see Table 1 for more details) Onboard the data were plotted analogue on an EPC 9800 Recorder. In addition, the raw data was stored on hard disk using a Delph2 recording/processing unit on a Windows-based PC. 4.4 Upset of the seismic equipment (Figure 3) When surveying the Boomer was towed 21 m behind the vessel. The streamer was 40 m behind the vessel implying that the point of reflection of seismic waves (the Common Depth Point) is 30.5 m behind the ship, and the Boomer signal is 50.5 m delayed compared with the 3.5 kHz signal. 4.5 Geological equipment (Figure 4) A gravity corer (6 meter steel barrel) and small box corer (25 x 25 x 60 cm) was used for sediment sampling. 5. Preliminary results 5.1 Ofotfjorden The fjord width in the outer part of Ofotfjorden is 4 to 5 kilometre and the water depth reaches 550 meter. In cross-section the fjord has a U-form characterised by steep sidewalls and a flat bottom. Close to land there is a relatively narrow shallow water zone. Our seismic data from this area shows that below the flat and deepest part of the sea bottom there are up to 0.2 seconds (measured in two-way travel time) of sediments overlying the bedrock corresponding to about 160 meter of sediments (assuming a P-wave velocity of about 1600 m/s, as will be used for the rest of the report). The sediments are acoustically laminated. Reflections of medium to high amplitude separates low amplitude intervals (Figure 5). 6 The inner fjord width decreases from about 6 to 3 kilometre and the water depth reaches 330 meter. The northern sidewall is steep; the deepest part is located slightly north of the middle part of the fjord and is characterized by a flat sea floor while the southern part has an irregular sea floor and a steep sidewall. The thickest sediment succession is found beneath the flat and deepest part of the sea floor. Here the thickness reaches 40 milliseconds (two way travel time) or about 32 meter of sediments. The sediments are acoustically laminated (Figure 6).

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