BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
Annex to WP2
Comparisons of BIODEEP brine basin sediments with “normal” eastern
Mediterranean sediments deposited under oxic seawater.
G.J. de Lange, Utrecht University
J. Thomson, Southampton Oceanography Centre and
S.P. Varnavas, University of Patras.
Abstract
Introduction.
The sedimentation flux from the water column that forms sediments on the sea floor
comprises organic matter (Corg) with associated biogenic silica (opal) and calcium
carbonate, and detrital material especially aluminosilicates (“clay”). Under the oxic
conditions that normally exist in bottom waters and the surficial sediments of the eastern
Mediterranean, early diagenesis produces a marked change in the composition of the
sediments deposited compared with that of the sedimentation flux. Corg itself remineralises
rapidly to low levels under oxic conditions in the sediments, and opal usually dissolves
quantitatively after deposition. As a result the sediments contain mainly calcium carbonate
and clay with low contents of Corg and negligible opal. In the case of sediments deposited
in the anoxic brines with high ionic strength, an improved or differential C org, opal and
calcium carbonate preservation might be expected.
The large mass of salt residue from the pore waters in dried unwashed sediments from the
brine basins acts so as to dilute the concentrations of elements present in other phases in
these sediments. This is the “closed sum effect” that affects all sediment analysis, and is a
consequence of referring elemental concentrations to a fixed mass in sediments of
variable composition. The closed sum effect complicates interpretation of the significance
of measured elemental concentrations. One objective of the initial analytical work on
BIODEEP was therefore to find a procedure through which make a first-order assessment
of the differences observed in the elemental concentrations of the sediments deposited in
the brine basins compared with those deposited in the same area but outside the basins.
The high ionic strength of the brines also contributes large concentrations of particular
elements (Na, K, Ca, Mg, Cl, S) to dried brine basin sediment that are measured in
analysis of dried unwashed sediments. Since the major ion contents have been measured
for the different eastern Mediterranean brines, it would be convenient if the brine
contribution of these elements could be subtracted from whole sediment analyses. This is
generally straightforward for sediments where the pore waters are derived from seawater.
A second objective was therefore to evaluate the validity of this procedure by comparing
analyses of washes of brine pool sediments with the known brine contents.
Results and discussion
Construction of “background sediment composites”.
The observation that the concentrations of many elements in fine-grained sediments are
proportional to the concentration of aluminium is the basis for the most common procedure
employed in sediment analysis to account for the closed sum effect. For comparative
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
purposes, data are often normalized to aluminium, i.e. the measured element content is
divided by the corresponding Al datum. Fine-grained detrital rocks derived from sediments
produced by continental weathering are termed shales, and the detrital fraction of marine
sediments laid down remote from land (referred to above as clay) is often considered to
resemble shale. For those several elements whose concentrations are related to that of
aluminium, the element/Al ratio in shale is often used to judge whether the measured
elemental concentration is likely to be solely detrital, or whether another process
contributing to the observed elemental concentration needs to be sought. Various shale
composites are in common use, but for the purposes here an equivalent but improved
alternative was adopted. Al-normalized data from the brine basin sediments were
compared with similar data collected from sediments from the surrounding local area
where bottom waters are oxic and the pore waters are derived from sea water (“a
background sediment composite”).
Samples from 0-10 cm in three different cores were analysed to derive background
sediment composites, cores UM15 and UM35 from the area of Discovery, L’Atalante and
Urania basins and core SL139 from the Bannock area. The Discovery, L’Atalante and
Urania basins are located within a sufficiently small area that the same background
sediment composite is appropriate for all three basins, but Bannock Basin is located some
distance further south. A major difference was noted in the comparison of background
sediment composites derived from cores UM15 and 35 on the one hand and from core
SL139 on the other. Cores UM 15 and 35 both had higher Ni/Al and Cr/Al values that were
caused by an enhanced contribution of ultrabasic detrital material over that received in the
area of core SL139.
Discovery, L’Atalante and Urania basin sediments.
This comparison was made between unwashed samples from cores UM 15 and 35 and
dried unwashed sediment from brine basin cores. Samples were fused with a lithium
metaborate flux that ensures dissolution of all minerals for analysis by simultaneous multielement ICP-AES. This flux method also retains Si for analysis, unlike dissolution methods
that volatilise Si. First the mean element/Al ratios from all samples from each basin and
from samples of the two background cores were calculated, and then the mean element/Al
ratios of the brine sediments were divided by the mean element/Al ratio of the two
background cores (Table 2). This procedure will not discriminate subtle concentration
differences, but it is effective in revealing gross compositional differences. Note that the
individual element/Al ratios for the two background cores are similar (the mean of the
UM15/UM35 element/Al ratios is 0.97±0.10) whereas much larger differences are seen
between the background cores and the brine sediments and between the sediments from
the different brine basins themselves.
This comparison of brine basin and background sediment data revealed (Table 1):
(i)
background sediments which contain ~50% CaCO3 whereas brine basin
sediments have much lower CaCO3 contents (and Ca/Al ratios) than. This suggests that
CaCO3 is likely to be dissolving in the brines or in their underlying sediments.
(ii)
the Sr/Ca ratio in eastern Mediterranean sediments is usually controlled by
biogenic CaCO3 and falls in the range 0.004-0.005. In the brine sediments the Sr/Ca ratio
is variable, very high in Urania (0.0103) and somewhat lower in Discovery (0.0035). This is
mainly due to the different Sr/Ca ratios contributed by the brines.
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
(iii)
Si/Al ratios in the brine basin sediments are all >3.00, whereas the background
sediment Si/Al ratios are <3.00. This suggests that opal is preferentially preserved in the
brine sediments.
(iv)
the pore water contribution to the brine basin sediments is clear in the high
Mg/Al ratio (particularly in Discovery Basin) and Na/Al and K/Al ratios (particularly in
L’Atalante Basin).
(v)
detrital material in the eastern Mediterranean basin has a Ba/Al ratio of ~0.0035,
as seen in the background sediments. Higher Ba/Al ratios are measured in Discovery and
L’Atalante basin sediments, probably caused by an improved preservation of biogenic
BaSO4 along with Corg in the brines. The single sample analysed from Urania has no
enhancement of the detrital Ba/Al ratio.
(vi)
Mo/Al ratios are consistently high in all the brine sediments, especially in the
Urania sample, caused by uptake of Mo from seawater during formation of sulfides. The
slight increases in Zn/, Cu/, Co/ and Ni/Al seen in the brine sediments relative to the
background sediments may also be associated with sulfides.
(vii)
Detrital elements (Zr, Y, La, Ce, and Ti) are present in the brine sediments at
slightly lower levels relative to Al than in the background sediments. The reasons for this
are not clear, but a particle size effect might be involved.
(viii)
The redox-sensitive elements Mn and P are generally present at lower
concentrations in the brine sediments relative to Al than in the background sediments.
These elements are expected to be mobile under reducing conditions such as are found in
the brines.
Bannock Basin sediments.
The comparison of Bannock Basin sediments was made with core MT6 retrieved from
inside the brine basin and core SL139 retrieved on the surrounding topography outside the
pool. The comparison is between the mean of 10 analyses from 0-5 cm in core MT6 with
the mean of 5 analyses from the upper 0-2.5 cm in core SL139 (Table 2). In this case the
brine pool sediments were washed free of salts before analysis, whereas the core SL139
sediment were analysed with pore water seawater (not brine) salts present. All these
analyses were made after sample dissolution with HF and hence no Si data were
obtained. The effect of washing is evident in the fact that the Na/Al ratio in core SL139
(0.32) is much higher than that in the brine sediments (0.064).
This comparison of brine basin and background sediment data revealed (Table 2):
(i)
more Ba in the Bannock basin sediments (Ba/Al 0.0065 compared with 0.0040),
which is explicable by an improved preservation of Ba along with Corg in this brine.
(ii)
less Ca (Ca/Al 2.95 compared with 4.17) which is explicable by dissolution of
CaCO3 in the Bannock brine.
(iii)
greatly increased S (S/Al 0.21 compared with 0.023) which is explicable by
formation of sulfides in the brine sediments.
(iv)
slightly more Cu (Cu/Al 0.0014 compared with 0.0011) and Zn (Zn/Al 0.0018
compared with 0.0013) which may also be related to sulfide formation.
Sediment washes
The soluble component of dried brine basin sediments was extracted by suspending 0.2 g
of dried brine basin sediment 3 times in 33 mls of demineralised water. The resultant
solutions were separated from residual solids by filtration (0.45 μm filter) to avoid
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
contamination of the solution phase with solids. All three filtrates from each sample were
combined and analysed by ICP-AES for the elements Li, Na, K, Mg, Ca, Sr, S and B.
The most revealing comparison between these wash data and the values measured in the
basin brines is through inter-element ratios (Table 3). It is immediately apparent from the
large differences between the brine and sediment wash data that other components
(authigenic minerals?) besides brine salts must be dissolving in the sediment washes. For
all three basins, but especially Discovery Basin with its MgCl2 brine, there is a consistent
increase in Ca and S in the sediment washes compared with the corresponding brine. This
is likely to be due to dissolution of gypsum (CaSO4) from the sediments; two different
crystals each several mm in size separated on sampling a Urania basin core both
analysed 1:1 molar for Ca and S.
These dissolution effects make it necessary to know the pore water brine volume
associated with the sediment mass in order to subtract a brine contribution directly from
total dried sediment analyses. The pore water contribution will be over-estimated by a
comparison of washed and unwashed dried sediment.
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
Table 1. Comparison of analyses of dried sediments from Discovery, L’Atalante and
Urania basins with local background cores UM15 and 35 (numbers of samples in italics).
Discovery
L'Atalante
Urania
UM35
UM15
UM15
Discovery
L'Atalante
Urania
mean (3)
mean (3)
mean (1)
mean (4)
mean (4)
mean/
mean/
mean/
mean/
UM35
UM core
UM core
UM core
mean
mean
mean
mean
Si ppm
73174
91290
203696
120781
141908
Al ppm
23661
26441
67318
41202
49672
*
21
3
55
48
0.0035
0.0052
0.0103
0.0040
0.0045
Ca as CaCO3%
Sr/Ca
Si/Al
3.37
3.46
3.03
2.93
2.86
0.97
1.15
1.18
1.04
Ti/Al
0.0561
0.0577
0.0555
0.0587
0.0561
0.96
0.95
0.98
0.94
Fe/Al
0.5951
0.5772
0.8313
0.5725
0.5690
0.99
1.06
1.02
1.47
Mn/Al
0.0078
0.0194
0.0045
0.0215
0.0246
1.14
0.34
0.84
0.20
Mg/Al
5.1735
1.0994
0.3293
0.4908
0.4129
0.84
10.72
2.28
0.68
Ca/Al
2.7907
3.1575
0.1543
5.3319
3.8265
0.72
0.54
0.61
0.03
K/Al
0.2160
0.6801
0.2901
0.2453
0.2430
0.99
0.88
2.78
1.19
Na/Al
0.1433
4.4896
0.8878
0.2764
0.2884
1.04
0.50
15.55
3.07
P/Al
0.0077
0.0094
0.0065
0.0103
0.0088
0.86
0.75
0.91
0.64
Ba/Al
0.0051
0.0068
0.0032
0.0036
0.0035
0.98
1.40
1.85
0.86
Ce/Al
0.0012
0.0012
0.0010
0.0012
0.0012
0.98
0.96
0.96
0.78
Co/Al
0.00059
0.00035
0.00048
0.00045
0.00049
1.10
1.27
0.77
1.05
Cr/Al
0.0021
0.0018
0.0027
0.0016
0.0017
1.05
1.37
1.17
1.74
Cu/Al
0.0018
0.0021
0.0015
0.0014
0.0013
0.93
1.38
1.57
1.17
La/Al
0.0004
0.0004
0.0004
0.0005
0.0005
0.93
0.84
0.88
0.75
Mo/Al
0.00014
0.00015
0.00113
0.00007
0.00005
0.81
2.49
2.63
19.69
Nb/Al
0.00029
0.00019
0.00019
0.00017
0.00017
0.97
1.64
1.08
1.10
Ni/Al
0.0017
0.0014
0.0019
0.0014
0.0015
1.10
1.36
1.07
1.52
Rb/Al
0.00070
0.00089
0.00119
0.00095
0.00099
1.04
0.74
0.95
1.26
Sc/Al
0.00018
0.00017
0.00021
0.00017
0.00017
1.01
1.04
1.01
1.22
V/Al
0.0012
0.0013
0.0027
0.0015
0.0015
1.04
0.77
0.87
1.88
Y/Al
0.00032
0.00033
0.00028
0.00037
0.00035
0.95
0.83
0.86
0.74
Zn/Al
0.0015
0.0012
0.0012
0.0012
0.0012
0.98
1.22
1.01
1.01
Zr/Al
0.0020
0.0022
0.0020
0.0024
0.0022
0.92
0.81
0.90
0.81
* Ca is mainly present as CaSO4 rather than CaCO3 in Discovery and Urania Basin
samples.
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
Table 2: Comparison of analyses of dried sediments from Bannock Brine core MT6 with
local background core SL139 (numbers of samples in italics). These analyses were made
by HF dissolution of dried washed sediments from MT6 (mean of 10 analyses from 0-5
cm) and of dried sediments including seawater salts from SL139 (mean of 5 analyses from
0-2.5 cm).
MT6
MT6
SL139
mean (10) Std. dev./ mean (5)
mean %
Al ppm
Ca as CaCO3%
Sr/Ca
49642
37
0.0037
2
1.6
Ba/Al
Be/Al
Ca/Al
Ce/Al
Co/Al
Cr/Al
Cu/Al
Fe/Al
K/Al
Li/Al
Mg/Al
Mn/Al
Na/Al
Ni/Al
P/Al
S/Al
Sc/Al
Ti/Al
V/Al
Y/Al
Zn/Al
Zr/Al
0.0065
0.00003
2.950
0.0011
0.00035
0.0013
0.0014
0.548
0.274
0.00084
0.535
0.0218
0.064
0.0009
0.0096
0.206
0.00017
0.0629
0.0016
0.00040
0.0018
0.0020
35.2
1.7
4.8
1.4
4.7
1.5
1.4
0.9
1.9
1.7
5.3
3.8
9.1
3.1
4.4
7.8
1.3
1.5
1.2
1.4
12.0
2.9
MT6 mean/
SL139 mean
47295
49
0.0050
0.0040
1.64
4.168
0.71
0.00042
0.0012
0.0011
0.544
0.258
0.00070
0.399
0.0228
0.326
0.0008
0.0096
0.023
0.84
1.12
1.28
1.01
1.06
1.21
1.64
0.96
0.20
1.07
1.00
9.11
0.0015
0.00034
0.0013
0.0017
1.05
1.17
1.39
1.15
BIODEEP (EVK3-2000-00042) Second year Scientific Report
(April 1st 2002 – March 31st 2003)
Table 3: Comparison of inter-element ratios measured in brines and in washes of dried
sediments from Discovery, L’Atalante and Urania brine basins. The number of samples
analysed from which the means were derived is in italics. It is evident that other
components besides brine salts must be dissolving in the sediment washes, e.g. for all
three basins but especially Discovery Basin there is a consistent increase in Ca and S in
the sediment washes compared with the corresponding brine, which may be due to
dissolution of gypsum (CaSO4).
Brine
Mg/S wt
Sr/Ca wt
Mg/K wt Mg/Na wt Mg/Ca wt
L'Atalante
Brine
Wash mean(10)
Wash minimum
Wash maximum
2.34
0.88
0.62
0.95
0.0126
0.0087
0.0063
0.0098
0.69
0.97
0.93
1.06
0.09
0.13
0.12
0.14
34.23
4.60
2.34
5.27
Urania
Brine-1
Wash mean (3)
Wash minimum
Wash maximum
6.70
0.53
0.44
0.70
0.0256
0.0354
0.0094
0.0835
1.61
0.92
0.71
1.16
0.10
0.07
0.07
0.08
4.85
1.49
0.94
1.99
Discovery
Brine
Wash mean (12)
Wash minimum
Wash maximum
118.40
5.23
2.53
8.46
0.0023
0.0067
0.0038
0.0235
153.36
97.72
51.18
131.85
78.25
47.30
21.82
68.15
1181.15
4.35
3.04
7.62