Geology Reference
In-Depth Information
Table 1.
Mineralogical and geochemical data for Brejo do Espinho sediment and pore water.
13
C
V-PDB
(‰)
18
O
V-PDB
(‰)
18
O
V-SMOW
(‰)
Depth (cm)
High-Mg calcite (%)
Dolomite (%)
TOC (%)
0
84
16
12.50
3.40
0.59
1.62
2
90
10
11.49
2.76
0.99
1.78
3
79
21
11.55
2.23
1.68
2.00
4
80
20
11.35
2.35
1.64
2.4
5
79
21
9.75
2.14
1.97
2.63
6
69
31
9.80
1.99
1.95
2.70
7
75
25
7.75
2.16
1.86
2.85
8
70
30
7.05
2.30
2.06
2.85
9
62
38
5.35
2.71
2.16
2.40
10
58
42
5.30
2.66
2.06
2.24
11
55
45
4.71
2.78
2.04
2.55
12
63
37
5.72
2.70
2.08
2.63
13
67
33
5.73
2.15
2.04
2.70
14
71
29
4.82
2.33
2.00
2.89
15
67
33
4.70
2.09
1.96
3.17
16
69
31
4.15
2.42
2.06
3.11
17
65
35
3.69
2.22
2.08
2.60
18
59
41
3.76
2.67
2.25
2.20
19
44
56
3.43
4.04
2.67
2.10
20
31
69
2.65
5.74
2.97
1.78
21
0
100
2.55
10.16
3.49
1.30
22
0
100
2.45
9.71
3.62
1.50
23
0
100
2.35
9.10
3.79
1.62
(c)
Stable isotope value (‰)
(a)
(b)
Carbonate mineral (%)
2
0
40
60
0
80
100
−
1
0
−
6
−
2
2
6
0
Fig. 3.
(a) Photograph of Brejo do Espinho
short core. A layer of 100% dolomite is
present between 20 and 23 cm, based on
the XRD carbonate mineral analysis of
samples as shown in (b), which indicates
a marked change from predominantly
high-Mg calcite above 20 cm to predomi-
nantly dolomite below. (c) Stable isotope
compositions of bulk carbonate samples
and associated pore-water show that the
5
10
15
20
18
O value of the 100% dolomite samples
below 20 cm becomes more positive, while
the
13
C
V-PDB
x
δ
dolomite
high-Mg calcite
25
18
O
V-PDB
o
δ
13
C trends to relatively negative values,
indicative of microbial activity.
18
O
V-SMOW
δ
laminations below 10 cm with less distinct
laminations above (Fig. 3a). The carbonate min-
eral composition of the sediments includes some
calcite but predominantly comprises a mixture
of high-Mg calcite and dolomite, in varying pro-
portions (Fig. 3b). There is a down-core trend in
the relative ratio of high-Mg calcite to dolomite
with the latter increasing to 100% at 21 cm. The
XRD analysis of the sediment samples from 5 and
21 cm indicates that the dolomite is nearly stoi-
chiometric with approximately 47-52 mol% Mg,
respectively, whereas the high-Mg calcite from
5 cm contains 25 mol% Mg (Figs 4a and b).
The increase in percentage dolomite with depth
is probably related to early diagenetic processes
(Vasconcelos & McKenzie, 1997). The primary
carbonate precipitate in the uppermost sediments
of Brejo do Espinho contains high concentrations
of high-Mg calcite, which are susceptible to early
diagenetic alteration to dolomite with burial. The
sediments are rich in organic carbon, but show a
steadily decreasing trend in percentage TOC from
12.5% at the surface to 2.5% in the level of 100%
dolomite (Table 1).
