BIODEEP (EVK3-2000-00042) First year Scientific Report
WP2 – ANNEX
Contribution of Partner 9: UP
SEDIMENT GEOCHEMISTRY
Sediment Core BCO2 obtained from Discovery Basin was analyzed for Ca, Al, Fe,
Mn, Ni, Cu, Pb, Li, Co and Ba. The bulk and the HCl – soluble concentrations of these
elements were determined.
Methodology
Sample preparation and analysis
The sediment samples were ground with a pestle and mortar prior to analysis. For
the bulk analysis the samples were completely digested in a mixture of HF, HNO 3 and
HClO4. Final solutions in 1M HCl, were analyzed on a Perkin – Elmer 2100 Atomic
Absorption Spectrophotometer. Accuracy was checked using international standard
reference materials and was better than ±5%. Analytical precision based on replicate
analyses was generally better than ±5%.
For the HCl – soluble fraction analysis 0.5 g of sediment powder was placed in
beakers and 6 N HCl was added (5 ml). Samples were then covered with a watch glass and
put on a hot plate at 90°C for 3.5 h, until a yellow-orange colour was obtained.
The HCl-leaching method was applied in order to isolate the silicate-aluminosilicate
fraction of the sediments. HCl dissolves the carbonates, the interstitial water evaporates
together with some colloidal iron and also removes elements from ion-exchange position.
Also, the HCl dissolves manganese and iron crystalline oxides and partially attacks clay
minerals including iron clay nontronite.
The residue from the HCl attack consists of resistant silicates and aluminosilicates.
On cooling the samples were filtered with Millipore membrane filters (0.45 μm) and were
made up to 25 ml. All reagents were of analytical grade commercially obtained from
Merck. Samples were analyzed applying flame atomic absorption techniques, using the
Perkin Elmer 2001 atomic absorption spectrophotometer and the working conditions were
those suggested by the manufacture.
Multipoint calibration was applied in all cases using working standards prepared
daily coming from the dilution of the ready to use Merck stock standards for atomic
absorption spectroscopy. On the basis of replicate analyses the analytical precision was
better than ±5%.
Results and Discussion
Summary statistics on analytical data are given in Table 1, while Table 2 shows the
correlation coefficients among the elements bulk concentrations. Figures 1 and 3 to 9 show
the vertical variability of the element concentrations, while figure 2 shows the vertical
variability of HCl – soluble Mn.
BIODEEP (EVK3-2000-00042) First year Scientific Report
Table 1
Summary Statistics on analytical data of sediment core
BCO2 - Discovery Basin
Variable
FE
AL
CA
CO
PB
LI
CU
NI
BA
MN
Mean
Std Dev
Minimum
Maximum
3.65
6.93
12.53
22.43
30.11
64.89
85.25
94.91
378.27
526.16
.50
1.80
5.85
7.99
21.21
13.54
22.07
15.70
271.79
85.26
2.55
4.53
3.35
11
4
41
49
63
111
339
5.13
13.90
25.03
56
81
93
142
146
1323
663
N Units
44
44
44
44
44
44
44
44
44
44
%
ppm
%
%
ppm
ppm
ppm
ppm
ppm
ppm
Table 2
Correlation Coefficients among the element concentrations in sediment
core
BCO2 - Discovery Basin
AL
AL
BA
CA
CO
FE
CU
LI
MN
PB
NI
1,0000
,3888**
-,5407**
,1585
,6193**
,5536**
,4752**
-,0664
,4494**
,2662
* - Signif. LE ,05
LI
MN
PB
NI
LI
1,0000
,1341
,6221**
,6321**
* - Signif. LE ,05
BA
1,0000
-,6855**
,0843
,5055**
,5741**
,3665*
-,3507*
,5976**
,1106
CA
1,0000
-,3702*
-,8152**
-,7984**
-,7738**
,1815
-,7961**
-,4542**
** - Signif. LE ,01
MN
1,0000 *
-,2640
,4779**
PB
1,0000
,5434**
** - Signif. LE ,01
CO
1,0000
,4267**
,4524**
,3465*
,1293
,6405**
,7243**
FE
1,0000
,6140**
,7668**
,2118
,6869**
,6065**
CU
1,0000
,4226**
-,2893
,6962**
,3113*
(2-tailed)
NI
1,0000
(2-tailed)
On the basis of the vertical variability of the element concentrations the following
distinct geochemical units are recognized in Discovery Basin: the upper sedimentary layer
with high concentrations of Fe, Cu, Ba and Cr and low concentrations of Mn, Ca and Mg;
the lower part of the core with lower concentrations of Fe, Cu, Ba and Cr and higher
concentrations of Mn, Ca and Mg. At 19-21 cm depth a distinct thin sediment horizon
occurs which is characterized by Fe, Mn, Co, Ni Cr and Al enrichments. At 23-24 cm depth
there is a major Ca peak, which coincides with minima in Cu, Mn Ni, Li and Cr. The HCl –
soluble Mn tends to decrease from the deeper to the shallower sediments suggesting that the
process leading to the anoxic conditions become more intense towards the present.
However this process was interrupted by a number of important geochemical events (i.e. at
19-21 cm, 23-24 cm). Partition geochemichal analysis and the application of statistical
analysis are used to deduce the prevailing environmental conditions during these events.
BIODEEP (EVK3-2000-00042) First year Scientific Report
Iron
The mean Fe value is lower compared with the mean Fe value reported for eastern
Mediterranean deep sea sediments. Fe increases in the upper sediments although its highest
concentration was found at 20 cm depth.
Most of Fe is soluble in HCl. It is noted that the HCl-soluble Fe at 20 cm is
markedly lower compared with the remaining horizons indicating that Fe there is present in
a more stable phase. Also Fe/Al ratio there shows a low.
BULK
Fe (%)
0,00
0
10
Depth (%)
20
30
40
50
60
70
1,00
2,00
3,00
4,00
5,00
6,00
Figure 1:Vertical
variability of bulk
Fe
BIODEEP (EVK3-2000-00042) First year Scientific Report
Manganese
The mean value of Mn is lower compared with its mean value given for eastern
Mediterranean sediments. Mn shows significant variability in its vertical distribution.
However, it is clear that overall it tends to decrease towards the upper sediments. Mn shows
a distinct peak at 20 cm depth.
Most of Mn is suluble in HCl. However at 20 cm depth a significant portion of Mn
(20%) is present in the HCl-insoluble fraction of the sediments Mn/Al ratio there is also
low.
HCl - SOLUBLE
Figure 2: Vertical
variability of HCl
– soluble Mn
Mn (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
100
200
300
400
500
600
700
BIODEEP (EVK3-2000-00042) First year Scientific Report
Aluminium
Aluminium concentration levels are within the range reported for eastern
Mediterranean sediments, except for certain horizons, such as 6 cm, 14 cm, 20 cm where Al
is markedly enriched. Weal Al peaks are also observed at 30 cm and 50 cm depth.
Generally most Al (> 60% and in certain layers >90%) is present in the HCl-soluble
fraction. By contrast, at 14 cm and 20 cm depth 67.48% and 53.87% of Al respectively are
held in the HCl - insoluble fraction. Similarly elevated amounts of Al are present in the
HCl-insoluble fraction of the other Al-rich horizons.
BULK
Al (%)
0,00
0
10
Depth (cm)
20
30
40
50
60
70
2,00
4,00
6,00
8,00
10,00
12,00
14,00
16,00
Figure 3: Vertical
variability of bulk
Al
BIODEEP (EVK3-2000-00042) First year Scientific Report
Lithium
In the lower part of the core Li is within the range reported for eastern
Mediterranean sediments, while at certain horizons (i.e. 5-9 cm, 15 cm, 20 cm, 24 cm, 2731 cm) Li exceeds this range. The vertical variability of Li shows clearly its tendency to
increase towards the recent sediments.
Most of Li is present in the HCl-soluble fraction.
BULK
Li (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
20
40
60
80
100
Figure 4: Vertical
variability of bulk
Li
BIODEEP (EVK3-2000-00042) First year Scientific Report
Copper and Lead
Copper and Pb exhibit similar vertical variability. Below 40 cm their values are low
remaining almost constant, whereas above this depth both metals have higher values
exhibiting significatn variability. Their geochemical variability is also similar with that of
Fe.
The majority of Pb is held in the HCl-soluble fraction, whereas in the case of Cu at
certain layers significant proportion of Cu is present in the HCl-insoluble fraction (i.e. at 14
cm 65.96%).
BULK
Cu (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
20
40
60
80
100
120
140
160
Figure 5: Vertical
variability of bulk
Cu
BIODEEP (EVK3-2000-00042) First year Scientific Report
BULK
Pb (ppm)
0
0
10
Depth (cm0
20
30
40
50
60
70
20
40
60
80
100
Figure 6: Vertical
variability of bulk
Pb
BIODEEP (EVK3-2000-00042) First year Scientific Report
Cobalt
The concentration levels of Co are generally lower compared with its concentration
in eastern Mediterranean sediments.
A characteristic peak of Co is observed at 21 cm depth. Overall the majority of Co
is held in the HCl-soluble fraction.
BULK
Co (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
10
20
30
40
50
60
Figure 7: Vertical
variability of bulk
Co
BIODEEP (EVK3-2000-00042) First year Scientific Report
Nickel
The geochemical behaviour of Ni shows similarities with that of Co with a distinct
peak at 20 cm depth.
As in the case of Co it is noted that between 17 cm and 22 cm a symmetric pattern is
obtained. The majority of Ni is present in the HCl-soluble fraction. However at 19 cm and
23 cm a significatn portion of Ni is held in the HCl - insoluble fraction (30,19% and
35,13% respectively).
BULK
Ni (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
20
40
60
80
100
120
140
160
Figure 8: Vertical
variability of bulk
Ni
BIODEEP (EVK3-2000-00042) First year Scientific Report
Barium
Barium exhibits very characteristic enrhicments at certain horizons in the upper part
of the sediment core (i.e. at 7 cm, 16 cm depth). The majority of Ba at these horizons is
present in the HCl-insoluble fraction suggesting its presence in a resistant form, in contrast
to the case of many other elements (i.e. Fe, Mn, Al).
Below 25 cm depth Ba values are much lower and less variable compared with the
upper part of the core (0-25 cm). In the later part, Ba exhibits significant variability.
BULK
Figure
9:
Vertical
variability of bulk Ba
Ba (ppm)
0
0
10
Depth (cm)
20
30
40
50
60
70
200
400
600
800
1000
1200
1400
BIODEEP (EVK3-2000-00042) First year Scientific Report
The Fe, Mn, Ni and Co enrichments encountered at 18-22 cm dept would suggest an
important change of the environmental conditions during the period represented by this
sediment horizon. The distinct gradual increase of the above metal concentrations from the
outer part to the central part of the layer would suggest that there has been a gradual change
of the environmental conditions: first becoming more favorable for the incorporation of the
metals and secondly becoming less favorable but with the same rate of changing. This is
evinced by the symmetric shape of the geochemical anomaly observed at 20 cm depth.
Below 42 cm no significant variability of Mn is observed suggesting no significant
changes in the Eh-pH conditions.
By contrast the clear tendency observed for the HCl soluble Mn to decrease above
this depth would suggest that Eh conditions tend to decrease towards the present, allowing
only limited amounts of Mn to be incorporated in the sediments. On the other hand such
conditions allow the formation of sulphides in the presence of H2S. The increase of bulk Fe
in the upper 15 cm of the core relative to its lower part (below 42 cm) suggests that Fe may
precipitate as Fe – sulphide. This will be investigated further by SEM studies. However,
this similar vertical variability of metals usually associated with Fe-sulphides, such as Cu
and Pb, with marked enrichments in the upper than in the lower part of the core support the
formation of Fe-sulphides.
Lithium known as a characteristic lithogenic element usually expresses along with
Al the variability of aluminosilicates. However, it has been reported that Li is leached out
of sediments after their compression in the subduction zones. The increase of bulk Li
towards the present in the Bannock sediments indicates the increase in the rate of its release
and in turn the relative rate of formation of the brines. In addition, the strong variability of
Li above 40 cm with distinct peaks at 26-30 cm and 21 cm, indicate the strong variability in
the amoutn of leachates released from the sediments.
The above is consistent with the Li-Al associations in the bulk sediments and their
HCl - soluble fraction.
Both bulk and HCl-soluble Li shows only a weak positive correlation with Al, ink
contrast to the strong positive correlation observed in normal sediments.
CONCLUSIONS
1.
The vertical variability of the elements studied in a Discovery Basin sediment Core
suggests that during the time represented by the sediment layers occurring below 40cm no
significant geochemical variability took place.
2.
Above 40 cm there have been important geochemical changes, which become more
intense towards the present. These geochemical changes are related to the rate of the
dynamic physical evolution of the basin.
3.
The decrease of Mn towards the more recent sediments would suggest a parallel
decrease of Eh values towards the present.
4.
During the time represented by 18 – 22 cm layer there has been an important physical
event, which led to the enrichment of Al, Fe, Li, Mn, Co, Ni.
5.
Although the metal enrichments at 18 – 22 cm occur in a very thin sediment horizon
the symmetric element geochemical anomalies observed at 20cm suggest a gradual
changing in the environmental conditions including the rate of incorporation of the metals
in the sediments.
6.
Similar results of another significant physical event are recognized at 28-32 cm depth
(enrichments in Al, Fe, Li, Mn, Co, Ni).
7.
At 23 cm depth the geochemical results of the physical event which gave rice to the
formation of this horizon are different from those observed at 18-22 cm and 28-32 cm. In
BIODEEP (EVK3-2000-00042) First year Scientific Report
contrast to the above cases, here Ca exhibits marked enrichments, whereas Cu, Mn, Ni and
Li show depletions.
8.
Barium, a known biogenic element, in contrast to most of the other elements studied,
is mostly held in the more resistant fraction). It increases in the more recent sediments
exhibiting characteristic peaks at 16 cm and 7 cm depth.
Interestingly at both depths Ba anomalies have similar asymmetric shape, with gradual
increase from the older to the younger sediments just below 7 and 16 cm, and a sudden
decrease just above these depths.
FURTHER WORK
Scanning Electron Microscope work (SEM) will be carried out for determining the
geochemical phases present in the metal rich horizons (i.e. 20cm, 18 – 22cm, 28 – 32cm).
On the basis of the above data an abstract was submitted for presentation at the Second
International Conference on "Oceanography of the Eastern Mediterranean and Black Sea:
Similarities and Differences of Two Interconnected Basins":
S.P. Varnavas, C. Corselli, D. Panagiotaras, E. Malinverno, G.J. de Lange, J. Thomson,
T.Tselepidis - EVOLUTION OF ENVIRONMENTAL CONDITIONS IN EASTERN
MEDITERRANEAN ANOXIC BASINS