A GeoChirp Subbottom Profiler was used for acoustic surveys during the expedition (1994) with R/V “A. v. Humboldt”.
The Chirp profiler (transmitting signals 2–8kHz or 1.5–11.5kHz, depending on operating mode) consisted of a deck unit and a tow fish. The penetration depth of the unit was up to 40m below the seafloor and typically had high resolution (0.3–0.5 m). Based on results of the acoustic surveys, sediment stations were targeted to areas where echograms indicated either high sediment accumulation and a sufficiently resolved recent sediment cover, or where acoustic units were within reach of the gravity coring techniques used. Different tools were used to obtain sediment samples.
1. A multi-corer provided up to eight sediment cores from an area of 1.5m2 to a depth of 45cm and recovered bottom water and an intact sediment/water interface. Several of these subcores were sliced in 1cm thick discs on board and either stored frozen for shore-based analyses or were left intact for radiometric dating.
2. For longer cores a gravity corer with an inner plastic liner was used. The corer had a top weight of 1200kg. A maximum length of 10m sediment was recovered with this weight, which was found to be sufficient for penetrating the soft sediment of the Gotland Basin to the icelake stage. The liners were cut into sections of 1m length, capped, and stored for shore-based logging of p-wave velocity, magnetic susceptibility, bulk density with a gamma-ray atten-uation porosity evaluator, and structures with a 600 dpi grey-scale scanner in a GEOTEC Multisensor Track.
3. Finally, a Kastenlot of 15x15cm diameter and variable length was used, weighted by 1200–2200kg lead. The longest core recovered with this tool was 970cm. These cores were opened on board, slabs and u-channels were taken over the entire length for x-ray photography and magnetic measurements, and the cores were described after visual exami-nation.
Recent net sedimentation rates were determined from dating of multicores using low-level Gamma-spectrometric measurements. Measurements of 210Pb, 137Cs, and 226Ra activities were carried out using a reverse-electrode coaxial Ge-detector (10 percent rel. efficiency) with energy resolution values of 640eV (at 5.9keV) and 1.7keV (at 1332keV).
Subtracting 210Pb supported, i.e. the amount equivalent to the 226Ra activity, the unsupported activity 210Pbunsup is used to estimate linear sedimentation rates for the cores using the constant initial concentration (CIC) model of interpretation.
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The historical profiles were constructed using petrophysical core data (density and porosity) and the constant rate of supply (CRS) model for 210Pb.
Under stationary conditions in the sea bottom layer there is a sedimentation-diffusion equilibrium, or a vertical distri-bution of suspension concentration, at which the gravity flow of sedimentary particles is balanced by the process of verti-cally oriented diffusion. A concentration gradient is caused by gravity, which results in downward flux of suspensions PS:
(1) where U = settling velocity of suspended particles and C = concentration of particles.
According to Fick’s first law, the gradient of concentration gives rise to a diffusive flow Pd, directed upwardly:
(2) where D = coefficient of turbulent diffusion, C = concentration of suspension, z = depth.
Thus, the sedimentation-diffusion equilibrium can be expressed as follows:
(3) By integrating equation (3), using separation of variables, we will obtain the exponential (logarithmic) law of vertical distribution of suspension. If suspension concentration in close proximity to the bottom C0 is considered as a boundary condition, the following expression is available as a result of integration:
(4) where and Cz = concentration of suspended matter at a distance z from the bottom.
To calculate α, and further U, we will use expression (4).
Suspension concentrations, found by Stryuk (V.L. Stryuk, personal communication) in the course of expeditions carried out in the central part of Gotland Deep by the Atlantic Branch of the Russian Academy of Science during the last decade, were used as original data for computations. A total of 170 concentration values, obtained by means of nuclear filters (ultra-filtration method), having pore diameter of 0.45µm have been used to build up an averaged vertical profile of suspension concentration.
Results
Long term sedimentation rates
Examining the high-resolution seismic record of the mud distribution, a clear asymmetry in the thickness of the most recent basinal sediment is discerned (Figure 2). The asymmetry must be a reflection of a deep circulation system in the Gotland Basin, which induces sediment winnowing and accumulation and which results in spatially heterogeneous sediment records at the deep basin floor. Based on the reflector geometry, the asymmetric deposition of sediment in the deep basin began immediately after brackish conditions of the current Littorina stage of Baltic Sea development were established some 7650 years ago. From the reflector geometry, sedimentation rates generally vary from 0.2mma–1 in the north-western part of the deep basin over 0.4mma–1 in the central part of the basin to 0.8mma–1 in the south-eastern part.
PS = UC
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Figure 2 NW-SE seismic profile across the Gotland Basin. The stratigraphy is based on core correlations. For location of the line see Figure 1.
Recent sedimentation rates from concentrations of suspended matter
The vertical concentration profile of suspended matter, in the region with depths ranging from 120 to 220m, is not exponential (Figure 3). This suggests an occurrence of sedimentation-diffusion inbalance due to action of additional sources of suspension. At these depths two such sources occur:
1. crystallisation of Fe, Mn and other elements from solution at the geochemical barrier H2S–O2
2. resuspension of bottom deposits by bottom currents, especially over slopes of the basin, where decreased stability of sediments occurs
As the water column in Gotland Deep is strongly stratified, the gravity sedimentation of suspension material, supplied from the above mentioned sources, is hindered, resulting in accumulation of suspension at a depth of the sources. At a depth below the level of 220m the Gotland Deep bottom flattens, reducing conditions predominate in the water column, while the above mentioned sources are inessential. Thus, the assumption of sedimentation-diffusion balance may generally be valid in the deepest layer of Gotland Deep (220–240m). Therefore, the computations were carried out using data from this layer only.
Using C0 = 1.1mg l–1, z = 20m, Cz = 0.60mg l–1, according to means of observations (Figure 3), the following expression is derived:
α = 0.03 (m–1)=3·10–4 (cm–1)
Taking the coefficient of vertical turbulent diffusion D to be equal to 1cm2 s–1 (Dyer, 1986), the settling velocity will be as follows:
U=αD= 3·10–4 (cm s–1)
Since the vertical flow of suspension Pz is determined as Pz = UCz, then for the bottom level (where Cz = C0) it will amount to:
P0=(3·10–4) · (1.1·10–3) = 3.3·10–7 (mgcm2 s–1) = 106 (gm–2year–1)
This value of P0 characterises the minimal possible rate of sedimentation, because besides gravitational sedimentation there are other mechanisms of sediment accumulation (saltation, sliding over slope, redeposition, authigenous sedimen-tation).
Sedimentation rate variabilities in the eastern Gotland Basin 130
Figure 3 Vertical profile of suspension concentration in the Gotland Deep. Dotted linesshow the geochemical “O2– H2S” barrier.
Recent sedimentation rates from dating
The accumulation rates calculated from 210Pb profiles and determination of physical properties are highly variable and reflect very different sedimentation rates (Table 1). However, we are confident that they represent true trends and rates, because the 137Cs profiles resulting from the Chernobyl accident corroborate the 210Pb datings in all cores and none had irregular features that indicate hiatuses. In the basinal stations, the accumulation rates range from 119 (upper portion of core 20004) to 322gm–2a–1 at the site of core 20001. The calculated sedimentation rates are between 2.1 and 2.5mma–1, respectively. The slope station 20000 displays a disturbed upper layer of 8cm thickness; fitting the 210Pb profile below this interval suggests an accumulation rate of 340gm–2a–1 and an average sedimentation rate of 2.5mma–1. The station 20008 in a small intraslope basin (68m water depth) has uncharacteristically high sedimentation and accumulation rates:
The data suggest that the sediment accumulates at a rate of 6100gm–2a–1 and has an average sedimentation rate of 30mma–1. Using wave prediction formula, storm with winds from the west in the area of this intraslope basin may reach the following characteristics: height ~5m, length ~100m and period ~8s. Near-bottom maximum orbital velocities for such waves in areas with a depth of 40–50m can reach 20–30cms–1. Such orbital velocities exceed the threshold velocity for grains with sizes of up to 60µm (Christiansen & Emelyanov, 1995). Therefore, the very high sedimentation rate in the basin most probably reflects intensive shallow water erosion along the rim of the basin.
Table 1 Dating results for the cores
Stations on the steep eastern slope of Gotland Basin have disturbances in the depth profiles of both 210Pb and 137Cs activity in the upper centimetres of the sediment (Figure 4). There is no decrease in 210Pb activity, which suggests
Core Position Average sediment
4.3 ± 0.6 238 ± 19 Disturbed surface
20000 57o 15.17 N 20o 33.64 E
2.5 ± 0.5 340 ± 30 Disturbed surface
20038 3.5 ± 0.3 399 ± 32 Disturbed surface
20008 57o 27.60 N 21o 09.60 E
30 ± 6 6099 ± 1860 Doubtful core
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sediment mixing and the 137Cs activity increases towards the top which indicate that the mixed layer is settled from suspension. Further, grainsize analysis from station 798 (Figure 5) corroborate such observations in that a general fining-upward sequence in the upper layer is twice abruptly interrupted by sediment coarsening. These observations together indicate that the sediment disturbance is not due to bioturbation. The Chernobyl peak in 137Cs is displaced with depth indicating that the disturbance has taken place in recent years.
Figure 4 Radiometric profiles of sediment cores from the Gotland Basin. For location of sampling position see Figure 1.
A) Depth profiles of supported 210Pb activities.
B) Depth profiles of 137Cs activities.
Figure 5 Down-core variations in grain-size and organic matter content of the slope core 20000.
Discussion
The areal asymmetrical distribution of long term sedimentation rates with high rates in the southeastern basinal part may be an indication of winnowing of the steep southeastern slope and lateral transport, as well as for possible contourite deposition in a depth interval characterised by anticlockwise currents around the rim of the Gotland Basin and at deeper levels. Observational data supporting this view of a dynamic deep-basin sedimentary environment have been collected during GOBEX (Mittelstaedt, 1994). Even though detailed measurements of near-bottom currents are scant in the Gotland Basin, initial observations suggest that current speeds at the sediment/water interface are of the order of 1–
3cms–1 over periods of days, reaching maxima of up to 20cms–1 during events of hourly duration (E. Hagen, pers.
comm. 1995). The image of a stagnant, quiescent and anoxic sedimentary environment in the Gotland Basin thus may be erroneous.
Such a conclusion is corroborated by additional evidence. Estimated sediment accumulation rates from the averaged suspended matter concentration profile were generally 3 times smaller in the central basin than rates from core datings, indicating more sources of sedimentation than primary sedimentation. There is a strong variation in the vertical concen-tration of suspended matter on Figure 3. However, the estimated sediment accumulation rates (106gm–2y–1) are in
Sedimentation rate variabilities in the eastern Gotland Basin 132
reasonable agreement with observations from sediment trap studies. Saarso (1995) observed that fluxes in traps ranged from 33–107gm–2y–1. One additional sediment source may be sliding of sediment on slopes. This is evidenced by obser-vations that cores taken on the slope to the central basin had disturbances in their radiometric profiles and their downcore grain-size distributions showed signs of redeposition. Further, geochemical data from the present cores show differences between the deep basin and the slope in enrichment patterns of organic matter (and associated trace elements) and of sulphides (and associated trace elements), suggesting lateral transport (Emeis et al., 1996).
Recent net sedimentation rates from the present study seem to be 1.5–2 times higher than observed from 210Pb datings 20–25 years ago (Ignatius et al., 1971; Niemestö & Voipio, 1974). The present study thereby corroborates the findings by Jonsson et al. (1990) estimated from varve counting in recent laminated sediments. A number of factors may explain this apparent increase in sedimentation rate:
1. It may be accidental in that only a few number of datings are involved and the regional variability seems to be high. In a much smaller area in the southwestern Kattegat Christiansen et al. (1996a) observed a high variability in that sedimentation rates on accumulation bottoms ranged from 469 to 4290 gm–2a–1.
2. An increase in organic matter is observed in the upper parts of the present cores (Figure 5, see also Neuman et al., 1996). Such observations corroborate the estimations made by Jonsson & Carman (1994) that in general, a more than 1.7-fold increase in sediment organic matter content has taken place in the Baltic proper between the late 1920s and the late 1980s. This may be explained by higher primary production in connection with the present eutrophication.
3. A 16 year long period of bottom water anoxia in recent years (Neumann et al., 1997) may have contributed to better preservation of organic matter and authigenetic mineral production.
4. Erosion of shallow water sediments seems to be the major source of sedimentation in the Gotland Basin. In their nutrient budget for the Baltic proper Jonsson et al. (1990) found that as much as 85% of the organic matter and nutrients sequestered in the laminated sediments in the Baltic deeps may have originated from shallow water erosion.
Such observations have been corroborated by resuspension studies (Christiansen & Emelyanov, 1995). Shallow water erosion may have been enhanced in recent years. This is apparently the case in the southern Kattegat where a shift in the wind regime has caused increased coastal and shallow water erosion (Christiansen et al. 1993) and induced higher sedimentation rates since the beginning of the 1970s (Christiansen et al. 1996b). Also, the number of storm surges on the German Baltic coast has increased in the last decades (Baerens & Hupfer, 1995).
Acknowledgements
C. Christiansen and V. Sivkov gratefully acknowledges the invitation from the Institute of Baltic Sea Research to take part in one of their R/V “Alexander von Humboldt” expeditions to the Gotland Basin. This research was performed under the Gotland Basin Experiment (GOBEX) as part of the ECOPS Baltic Sea Initiative.
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