The Gotland Basin (Figure 1) is the largest basin of the Baltic Sea. Because of its size, its location between northern and southern Baltic Sea and its properties it was chosen as a region for integrated studies of the recent oceanographic, biological, chemical and geological environment as well as of the geological history. The water column (with a thickness of about 240m in the central part) is usually well stratified causing anoxic conditions at the sea bottom. Because of this the sedimentary history is well preserved in the laminated sequences (no bioturbation). The present study focuses on the physical (mainly acoustic) properties of the sediments in order to reconstruct the postglacial history of the basin.
Figure 1 GOBEX working area
R. Endler, K.-C. Emeis and T. Förster 135
Material and Methods
In the frame of the GOBEX activity one expedition with R/V “A. v. Humboldt” (1994) and two cruises with R/V
“Poseidon” (1995, 1996) were performed to collect acoustic profiling data and sediment samples (Figure 1). A GeoChirp subbottom profiling system (GeoAcoustics) consisting of the tow fish (transmitter, receiver), the chirp-processor, a Sonar Enhancement System (SES) and a thermo-recorder (Wideline 200/138, Ultra Electronics) were used for acoustic surveys. The bandwidth of the transmitted acoustic pulses was 2–8kHz with a duration of 32ms. Reflected acoustic signals were received by a ministreamer (which was attached to the end of the tow fish), amplified and sent to the chirp-processor for matched filtering (signal compression). Further onboard processing, on-line printing and digital storing (SEGY format, 8mm ExaByte) were performed with the Sonar Enhancement System. Navigation data were collected from a GPS receiver (Sercel NR51, WGS84). During profiling the tow fish was towed at a depth of about 20m (due to limited cable length) and with a speed of about 5 knots. Two pulses per second were transmitted giving a firing distance of about 1.3m (at 5 knots). Depending on the sediment type a penetration down to 40m was achieved with a vertical resolution of about 0.3–0.5m. Later on, postprocessing using an extended Seismic Unix-package (Cohen & Stockwell, 1994) was performed on an IBM R6000 workstation. The general structure of the postglacial sediments is depicted by two GeoChirp profiles in Figure 2 and Figure 3.
Figure 2 Acoustic image of Gotland Basin sediments—transversal section a) Geochirp line 940823 western part
b) GeoChirp line 940823 eastern part
Major acoustic reflectors are named by letters (sound velocity for time to depth conversion: 1450ms–1)
Figure 3 Acoustic image of Gotland Basin sediments—longitudinal section a) GeoChirp line 940830 northern part
b) GeoChirp line 940830 southern part
Major acoustic reflectors are named by letters (sound velocity for time to depth conversion 1450ms–1)
a) b)
a) b)
Acoustic images of Gotland Basin sediments 136
Based on the acoustic records a variety of sediment sampling locations were selected. Gravity corers (6m, 12m) with plastic liners were used for sampling the postglacial sediment layers. After recovery the liners were cut into 1m sections, capped and stored (4°C). The physical properties of the split sediment cores were measured with a Multi Sensor Core Logger (MSCL, GEOTEK): optical scan of sediment surface along the core (by gray scale line scanner), wet bulk density (by gamma ray attenuation), sound velocity (by 500kHz pulse transmission) and magnetic susceptibility (Bartington loop and point sensors). Based on the logging data subsamples were taken for further analyses. First results of the measure-ments on core 20048 are presented in Figure 4.
Figure 4 Physical properties of Gotland Basin sediments—results of MSCL logging core 20048 SL
Results and Discussion
In general the central Gotland Basin shows an asymmetric structure. Along the longitudinal basin axis (Figure 3) the water depth slowly increases from about 140m in the north to about 240m in the central part. Further to the south the central basin is bordered by a small elevation with water depth of 230m. The same asymmetric morphology is displayed in the cross section of Figure 2.
The acoustic images of Figure 2 and Figure 3 reflect the structure of the postglacial basin fill which is well penetrated by the acoustic signals. In general the sediments are well layered. Based on the acoustic characteristics a variety of major reflectors were chosen, marked by letters and traced by lines. Because a definitive link from all the acoustic reflectors to the several sedimentological sequences is not yet established, the choice of the reflector names is not based on sedimen-tological units (e.g. Andren & Sohlenius, 1995).
The lower light line I marks the very rough surface of the glacial deposits (sand, gravel, till). The next sequence up to reflector D contains the varved clays of the Baltic Ice Lake. The thickness of this sequence is approximately 20m and is nearly constant over the entire central basin. During that time sedimentation was rapid without lateral transport (no near bottom currents). From the acoustic image, the varved clays of the Baltic Ice Lake can be subdivided into several units separated by the reflectors H, G and E. This set of reflectors in the varved clays suggests that the sedimentation during the Baltic Ice Lake was not only influenced by seasonal variations (varves) but also by depositional, possibly climatic changes of longer periods. The sediments below reflector H show an acoustically turbulent image in the very central basin whereas a band of reflections appear in the more distal regions. A very prominent reflector is G (from the signal strength) which is the lower boundary of a strong reflection sequence. This reflector can be traced over the entire basin.
The reflection band is bounded at the upper end by the reflector F (not indicated in the figures). A rather transparent region follows up to reflector E indicating continuous sedimentation during this period. In the interval between reflectors
R. Endler, K.-C. Emeis and T. Förster 137
G and E, black and irregularly distributed dots appear over the entire basin. It is unclear whether these spots are caused by diagenetic processes or local anomalies during sedimentation.
In Figure 4 an enlarged part of the acoustic record at station 20048 is compared with the core log depth profiles. The subbottom depth of the selected reflectors is calculated with a sound velocity of 1400ms–1 and marked in the logging profiles. The acoustic reflectors correspond to gradients in the acoustic impedance profile. Most prominent changes in the physical properties occur in the subbottom depth interval between 6m (reflector D) and 4.5m (reflector C) indicating the transition from typical Baltic Ice Lake sedimentation (varved clays) to the younger Littorina environment (transition clays of Ancylus and Yoldia stages according to Winterhalter, 1992). The reflector D also marks a change in the sedimentation regime. As a result of strong lateral sediment transport (start of the near bottom circulation) the thickness of the sediments overlying reflector D continuously increases from west towards the steep eastern slope of the basin (Figure 2). A maximum thickness of about 12m is reached just at the lower edge of the eastern slope. The uppermost band of reflections bounded by reflectors B and A (surface of the recent mud) shows the transition from well laminated gyttja clays of the Litorina stage to the recent very soft, black sulphitic mud. Because of the high water content the uppermost mud is easily transported by lateral currents.
Conclusions
It was demonstrated that the physical properties of the sediment column clearly reflect the variations in the sedimentary environment during the postglacial history of the Baltic Sea. They give important additional information for paleoenvi-ronmental reconstructions which can not be obtained by other methods.
References
Andren, T. & Sohlenius, G. (1995). Late Quaternary development of the north western Baltic Proper—results from the clay-varve investigation. Quaternary International, Vol. 27, pp. 5–10
Cohen, J. K. & Stockwell, J. (1994). Seismic Unix, V. 22 ff. Center for Wave Phenomena, Colorado School of Mines Golden, CO 80401; john@dix.mines.edu
Winterhalter, B. (1992). Late-Quaternary Stratigraphy of Baltic Sea Basins—a Review. Bull. Geol. Soc. Finland 64, Part 2, 189–194
Water exchange, nutrients, hydrography, and database of the Gulf of Riga 138
ICES Cooperative Research Report, No. 257 Baltic Marine Science Conference, Rønne, Denmark, 22–26 October 1996