Results

Sapropel and sapropel–like mud is common not only
in the Bornholm Basin but also in many other Baltic
Sea basins. In the Bornholm Basin this mud contains
up to 7.87% Corg, 7.08% Fe, 0.86% Mn (Table 2). Two
mud samples taken near the shipwreck (stations 26S
and 47) contained 17.7 and 41.5% Fe respectively
(Table 3). Apparently these sediments might have
some metal scrap corrosion products. Ignoring these
outliers, Fe concentration varies in a much narrower
range–between 0.05-7.08% (Table 2). Mud containing
Fe (above 5%) and Mn (above 0.10%) can normally
be found at a depth exceeding 80-90 m, i.e. under
reducing conditions. As opposed to sapropel–like mud
in Gdansk Basin that is less affected by stagnation
(Emelyanov 1986). Bornholm mud shows higher
contents of Corg, Mn, P, Zn, Cu (Fig. 3) and in some
cases of Pb and Cd.

Corg occurs predominantly in biogenic components,
i.e. darnels of cereals, spores and pollen and fragments
of algae. All these particles are commonly 0.05-0.001
mm in size. Hence the elevated Corg is not as comparable
to the finest pelitic mud, but rather for aleuro–pelitic
mud enriched with this fraction. Organic carbon may
also be on clayey particles.

Table 2. Minimal, maximal and average contents of fraction <0.01 mm and chemical elements in the surface
layer of sediments (0-5 cm) of the Bornholm Basin* (contents of fraction <0.01 mm, Сorg, Fe and Mn – in %;
from Cu to Cd – in 10-4% or mg/kg).


* The data are presented for all studied samples (375 samples) taken in the RVs Professor Shtokman, Centaurus and Fritz Reuter
cruises in 1997–2008, except three samples from the stations 47-S-1, 26-S-1 and PSh-4030 (see Table 3).
83
Table 3. Contents of the chemical elements in aleuro–pelitic (AP) and pelitic (P) mud (layer 0-5 cm) of the
Bornholm Deep with the highest contents of Fe, Mn and As; contents from Сorg to P – in %, from Cu to Cd –
in 10-4% (mg/kg).

 


* The stations located near the wrecks see Figure 1.
Content of heavy metals (HM) in sediments varies
depending on their grain–size composition (Fig. 3):
contents of all components and elements are increasing
from sand towards coarse aleurite—fine aleuritic
mud—aleuro–pelitic mud—pelitic mud in case of Fe,
Zn, Cu, Ni and K.
Based on their average content in the sediments
(Fig. 3) the examined elements may be divided into two groups. Elements in the first group
(Corg, Cr, Co, As) are mainly confined
to the 0.05-0.01 mm and 0.01-0.001
mm fractions, in particular to aleuro–
pelitic mud. This means that elements
in this group are predominantly concentrated
in sediments in central areas
of the basin and over its periphery.
Elements in the second group (Fe,
Cu, Ni, Pb) are closely related to pelite
(mainly clayey and sub–colloidal particles).
These elements are distributed
in sediments according to the so called
“Pelitic Fraction Rule” (Emelyanov
1986, 2002, 2005): the higher the
content of the <0.01 mm fraction in
the sediments, the higher the content
of the second group elements (Fig. 3).
The highest values of these elements
are confined to central areas of the
basin covered with the finest (pelitic)
mud (Figs 4, 5, 6, 7, 8).

In some cases, increased concentrations
of As and HM in the more shallow
bottom sediments of Bornholm
Basin peripheral area are explained
by the presence of oxyhydroxides of
Fe and Mn (micro–nodules, fragments
of crusts) there. Such oxyhydroxides
actively concentrate certain metals.
Crusts and nodules of hydro–oxides
of Fe and Mn may contain up to 653
mg/kg As, up to 301 mg/kg Ni and increased
amounts of other trace metals
(Baturin et al. 1995). In the mud of the
central area of Bornholm Basin under
reducing condition ferric sulphides
are formed. Due to their sorption ability
such sulphides may contain up to
440 mg/kg As (Emelyanov, Kravtsov
2007).

Marine Holocene mud is sometimes
enriched in Pb (up to 101 mg/kg,
Table 4). This element is concentrated
in the mud from central part of the
basin (Fig. 8). Similarly to Mn and Cu
(Figs 5, 6) and also Ni, its values are increasing from
periphery towards the centre of the basin.
In the Gdańsk and Northern Baltic Basins, where
there is also periodically stagnation of near–bottom
waters but more weak, than in the Bornholm Deep,
distribution of Mn is strictly follows according to
the “Pelitic Fraction Rule” (Emelyanov 1986). At
the same time its distribution in the Bornholm Basin
sediments is different: the average content of Mn in
the aleuro–pelitic and pelitic mud is five to ten times
higher than in the more coarse sediments (see Table 2,
Fig. 3). This may be explained by the fact that major
part of Mn in the sediments of the Bornholm Deep is
not related to terrigenous pelitic material as was in
the case of previously mentioned basins, but rather
to authigenous minerals–finest crystals of manganese
Fig. 3. Average distribution of the grain size fractions (0.05-0.01 and <0.01
mm) and selected chemical components and elements in the sediment (0-3; 0-5
cm) types in the Bornholm (solid line) and Gdansk (dashed line) basins. After
E. M. Emelyanov 2002.

carbonate–rhodochrosite (Emelyanov et al. 1995),
which is forming under the influence of the periodical
stagnation of the near–bottom waters (Shpaer, Larionov
1983; Stryuk et al. 1995).

Based on their pair correlations Zn, Cu, Cr and Ni
fall into the same group – their correlation coefficients
(r) are generally above 0.50. All of them however correlate
differently with the <0.01mm fraction, Corg, and
Fe. Zn and Ni are better correlated with Fe (r=0.63),
thus making their spatial distribution pattern similar
to one of Fe: Cu and Cr have weak (r=0.50-0.30) or
bad correlation (r<0.30) with Fe and Corg. Ni is largely
associated with Corg, Cu, Mn and Fe. Co is better correlated
with Ni and Corg. As only correlates with Corg



Fig. 4. Isoline plot of Fe content (in %) in the surface (0-3 cm) sediments of the Bornholm Basin. Compiled by E.M.
Emelyanov, V.A. Kravtsov, Y.I. Savin, V.T. Paka, I.S. Khalikov.
CW – area, where chemical weapons were sunken. The numbers (5.10; 17.67; others) on the maps of the polygons (A,
B, others) mean maximum contents of Fe, points – investigated samples, W.1, W.2 – wrecks, a – distribution of Fe in
the different sediment types (from left to right: sand; coarse aleurite (silt); fine aleuritic (silty) mud; aleuro–pelitic mud
(or mud); pelitic (clayey) mud; Ancylus+Yoldia clay (see Fig. 3). Circle in the rectangle CW see Fig.1. Number 41.52 –
maximal Fe content, %.

Table 4. Contents of toxic elements in the mud of “hot spots”, in 10-4% (mg/kg); Fe and Mn in %.

 

(r=0.58) and shows weak correlation with Ni, whereas
Cd correlates with Zn and weakly (r=0.45) with Corg.
Manganese is distributed in sediments very specific.
None of the examined elements correlate with this element.
Increased concentration of Mn is found in the
very centre of the basin. The area of mud containing
0.10-0.20% Mn covers 550 km2, whereas areas containing
>0.20% are only about 2 sq. km in size. Compared
to Fe the highest Mn content is more clearly confined
to the centre of the basin (Fig. 4, 5). This is apparently
largely due to stronger stagnation condition in the nearbottom
layer of the central part of the basin. It results in
more intensive accumulation of Mn, which in heavily
reducing conditions is diagenetically accumulated in
the form of composite manganese carbonates or rhodochrosite
(Emelyanov 2005).



Fig. 5. Isoline plot of Mn content (in %) in the surface (0-3 cm) sediments of the Bornholm Basin. Compiled by E.M.
Emelyanov.


Fig. 6. Isoline plot of Cu content (in 10-4% or mg/kg) in the surface (0-3 cm) sediments of the Bornholm Basin. Compiled
by E.M. Emelyanov, V.A. Kravtsov, Y.I. Savin, V.T. Paka, I.S. Khalikov.


Fig. 7. Isoline plot of Cd content (in 10-4% or mg/kg) in the surface (0-3 cm) sediments of the Bornholm Basin. Compiled
by E.M. Emelyanov, V.A. Kravtsov.


Fig. 8. Isoline plot of As content (in 10-4% or mg/kg) in the surface (0-3 cm) sediments of the Bornholm Basin; a – distribution
of As in the separate sediment types (for the legend see Fig. 3). Compiled by E.M. Emelyanov, V.A. Kravtsov, Y.I.
Savin, V.T. Paka, I.S. Khalikov.