165 Seismoacoustic research of Lake Szurpiły bottom sediments (north-eastern Poland)

165 Seismoacoustic research of Lake Szurpiły bottom sediments (north-eastern Poland)

research of Lake Szurpiły bottom sediments (north-eastern Poland)
LimnologicalSeismoacoustic
Review 8, 4: 165-170
165
Seismoacoustic research of Lake Szurpiły bottom sediments
(north-eastern Poland)
Janusz Dworniczak1, Andrzej Osadczuk2, Wojciech Tylmann1
University of Gdańsk, Institute of Geography, Department of Geomorphology & Quaternary Geology, Dmowskiego 16A, Gdańsk, Poland;
e-mail: [email protected]
2
University of Szczecin, Institute of Marine Sciences, Department of Marine Geology, A. Mickiewicza 18, Szczecin, Poland
1
Abstract: This paper concerns the application of seismoacoustic surveying in the study of the bottom sediments of Lake Szurpiły, one
of the lakes investigated as part of the NORPOLAR project. High-resolution seismoacoustic profiling allows remote characterization of
lake sediments stratigraphy thanks to the recording of acoustic waves reflected from lake bottom. In the presented study, relatively clear
seismoacoustic records were obtained only for shallower parts of the lake bottom. The records from deeper parts were homogenous and
it was not possible to distinguish the depth of primary bottom built of minerogenic sediments. Distinct changes in the lake sediment’s
lithology were not possible to determine as well. This could be a result of a greater thickness of sediments containing gas bubbles, in the
deep areas of the lake bottom. The presence of a considerable quantity of free gas in the sediment column causes acoustic turbidity and
makes the obtained seismoacoustic record very difficult to interpret.
Key words: lake sediments, seismoacoustic profiling, north-eastern Poland
Introduction
High-resolution seismoacoustic methods have
been widely used, especially in research on sea bottom topography and geology (Rudowski and Gajewski
1998; Przezdziecki 2005). As a result of measurement
systems miniaturization, the method has been applied
to investigate smaller water bodies, including lakes
(Rudowski et al. 2001; Rutkowski et al. 2002; Rudowski
2005; Rutkowski, Pietsch et al. 2005). Seismoacoustic
profiling enables detailed investigating of lake bottom
topography and, in favorable conditions, obtaining
information concerning the thickness and structure
of lake sediments, which makes the method ideal for
the determination of the primary lake basin configuration and stratigraphy of overlying lake sediments (e.g.:
Dobson et al. 1995; Gilbert 2003). This information is of
crucial significance for optimal location of coring sites
with a complete stratigraphic sequence of lake sediments. Therefore, continuous seismoacoustic profiling
should be routinely used prior to coring lake sediments
for paleoenvironmental reconstruction (Scholz 2001).
In Poland, seismoacoustic methods have been
more frequently used in lake sediments investigations
during the last several years (Rutkowski et al. 2002;
Rutkowski, Pietsch et al, 2005; Giżejewski 2002; Kowalewski 2006; Osadczuk et al. 2006; Osadczuk and Osadczuk 2007; Osadczuk 2007b; Dworniczak and Rudowski 2005; Dworniczak 2006). The research accomplished so far proved that the results depend strongly
on the type of sediments. Particularly good results
were obtained in the research on Lake Drawsko, where
the relief of primary lake basin was exceptionally clear
(Osadczuk et al. 2006). During the research on Lake
Wigry it was possible to determine a few seismoacoustic facies within the lake sediments (Rutkowski,
Król et al. 2005). However, an accurate interpretation
of seismoacoustic profiles and subsequent delimitation of seismofacies is not always possible.
Lake Szurpiły is one of the key sites intended
for multidisciplinary investigations as part of the
NORPOLAR project (Tylmann et al. 2008). In this
project, four lakes were selected for high-resolution
climate and environmental reconstruction along a W-
166
Janusz Dworniczak, Andrzej Osadczuk, Wojciech Tylmann
E transect in northern Poland to identify the spatiotemporal modes of climate variability that were active
during the past millennia.
Owing to the complex topography of the Lake
Szurpiły bottom, it was assumed that significant differences of sediment thickness occur, depending on
erosion, transportation and accumulation processes
intensity within the lake. A seismoacoustic survey
was done prior to coring in order to determine spatial changeability of the sediment cover. Our first and
foremost goal was to locate optimal sites for coring,
where retrieving a complete sedimentary sequence
would be most probable.
Study site
Lake Szurpiły is situated in the north-eastern
part of Poland, within the Suwałki Landscape Park, a
region of well developed post-glacial relief. The catchment area of the lake belongs to the river Niemen basin and has 11.14 km2 (Bajkiewicz-Grabowska 1994),
covered mainly by sands, gravels and glacial tills.
There are morainic hills in the southern part of the
catchment, located alongside the lakeshore from NW
to SE. Additionally, kames and dead-ice moraines were
formed behind the morainic hills (Ber 1968).
Lake Szurpiły is an example of morainedammed lake created among post-glacial marginal
forms as a result of dead-ice melting (Smolska 1996).
This is a flow-through lake which receives water from
several streams and only one stream flows out. The
surface area of the lake is 80.9 ha. As a result of welldeveloped shoreline, the lake can be divided into four
morphologically distinct water basins: deep northeastern part, central part and two shallow bays – Targowisko in the west and Jodel in the north-west. The
shores are relatively steep, littoral zone rather narrow
and the bottom in the deeper parts diversified with
maximum depth of 46.5 m (Fig. 1). The lake is classified as holomictic and eutrophic, with strong oxygen
depletion in the hypolimnion which causes anoxic
conditions during the summer and winter stagnation
(Górniak et al. 2007).
Methods
In the present research continuous seismoacoustic profiling was applied, which enables remote
investigation of the lake bottom structure owing to
the recording of acoustic waves reflected from the
Fig. 1. Location and bathymetric chart of Lake Szurpiły and location of seismoacoustic profiles
lake bottom. On this basis, it is possible to establish
borders between sediment layers of different physical
properties (Osadczuk 2007a; Przeździecki 2005). The
seismoacoustic profiling was carried out in 2007 using
a Seabed Oretech 3010 subbottom profiler (Institute
of Marine Sciences, University of Szczecin) with 5
kHz frequency. The data were digitally recorded with
CODA OCTOPUS DA 50 data acquisition system and
positioned using DGPS. The measurements were done
from a boat adapted for seismoacoustic research. The
profile lines crossed lake bottom of the most diversified topography, particularly the deepest areas.
Altogether 14 seismoacoustic profiles of a total
length of 5.6 km were made, which is 6.96 km of profile per 1 km2. This ratio is relatively high because of
Seismoacoustic research of Lake Szurpiły bottom sediments (north-eastern Poland)
the small surface area of the investigated lake. For instance, it was 3.4 km km-2 in Lake Wigry (Rutkowski,
Król et al. 2005) and 5.3 km km-2 in Lake Upper
Raduńskie (Dworniczak 2008).
During the next stage of research, complete
sediment cores were taken from a coring platform anchored at the deepest point of the lake (54°13′45.2″N,
22°53′51.5″E). Several types of coring systems were
used, both gravity (ETH gravity corer, Kajak corer)
and piston (Uwitec Niederreiter, modified Livingstone
Piston Corer). The complete sediment sequence from
this part of the lake was obtained on the basis of visual correlation of the taken cores, which was possible
thanks to lithological changes and distinct lamination
of almost the entire profile (Kinder et al. 2008). The
coring results were then compared to seismoacoustic
profiles covering this part of the lake bottom.
Results
The analysis of the obtained seismoacoustic
profiles reveals that the records are not easy to decipher (Fig. 2). In the shallow parts of the bottom,
multiplying (secondary reflections) occurs, which
usually makes interpretation of the seismoacoustic
record very difficult or impossible (Osadczuk 2007a).
Reflections in the uppermost layer of sediments are
Fig. 2. Seismoacoustic cross-section SUR11
167
generally unclear and variable. This could be a result
of high water content in surface sediments, which
makes the sediment-water interface undecipherable
in seismoacoustic records.
In the case of Lake Szurpiły, a reliable determination of the primary lake basin configuration
was possible only for the parts of the lake bottom in
a relatively shallow zone (Fig. 2). However, these are
rather small sections and extrapolation to other parts
of the lake bottom would not be reasonable. The seismoacoustic record from deep areas is homogenous. It
shows a lack of characteristic reflections marking a
significant difference in sediments lithology. An evident example is given by the comparison of seismoacoustic record with cored sediment sequence from
the deepest point of the lake (Fig. 3). The thickness
of lake sediments (mainly laminated calcareous and
organic-calcareous gyttja, clays and silts in the basal
part of the core) is about 12 m and fine-grained sand
occurs below the lake sediments, whereas the seismoacoustic record shows acoustic turbidity masking
the reflections which could be interpreted as the base
of the lake sediments. It is also impossible to recognize
a medium-grained sandy layer of a thickness of more
than 1 m which interrupts the calcareous gyttja series
at a depth of 815-935 cm.
168
Janusz Dworniczak, Andrzej Osadczuk, Wojciech Tylmann
Fig. 3. Comparing seismoacoustic record with the lithology of
bottom sediments
Discussion
Numerous examples of seismoacoustic profiling proved its usefulness in research on both deep
lakes of tectonic origin with great sediments thickness
and postglacial lakes of moderate depth and sediment
thickness (Scholz 2001). Potential precision of the
method is high enough to determine not only a general sediment thickness, but also to resolve sedimentary
horizons of different lithological features or turbidite
deposits (e.g. Bertrand et al. 2008; Fanetti et al. 2008;
Król et al. 2005; Moernaut et al. 2007). Such excellent
results are generally achieved in lakes with clastic
sedimentation i.e. clays and silts, which covers glacial
sediments e.g. morainic tills. The significant difference
in physical properties between such sediments causes
clear echo in the seismoacoustic record, which marks
the border between glacial base and overlying lake
sediments. Moreover, clastic sediments do not contain
free gas causing acoustic turbidity. Sediments with
higher organic matter content, like organic or calcareous gyttja, usually contain a considerable amount of
gas bubbles originated from organic matter decomposition in the sediment column. This fact results in
problems with seismoacoustic record interpretation.
In the case of Lake Szurpiły, the analyzed sediment sequence consists of fine-grained sands overlaid
by lake sediments, namely clays and silts, calcareous
and organic-calcareous gyttja with a sandy layer of a
thickness of more than one meter. Despite such diverse
lithology, the seismoacoustic record in this part of the
lake bottom is homogenous (Fig. 3). The most probable reason for this fact is the presence of gas bubbles
in the sediment, which was also observed in the field
when cutting the recovered cores. It occurs especially
in deeper parts of lake bottom, where more organic
sediments are the thickest and contain more gas.
A similar layout of reflections has been described by Rutkowski, Król et al. (2005) as facies C –
acoustic turbidity impossible to interpret. Analysis of
the seismoacoustic records from Lake Szurpiły reveals
that this type of reflections dominates in the deeper
parts of the lake. Unfortunately, this makes it impossible to estimate the thickness of lake sediments within
these most interesting parts of the lake. Therefore, the
information necessary for an optimal location of coring sites is not available.
Acknowledgements
The research was supported by the Ministry
of Science and Higher Education grant NORPOLAR
(DFG/46/2007). The authors would like to express
their gratitude to Prof. Jacek Rutkowski for the hospitality at the Wigry National Park and helpful comments on the manusctipt. The technical help from
Bartosz Płóciennik is also acknowledged.
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