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Authigenic carbonates in sediments from the Gulf of Mexico
Earth and Planetary Science Letters, 88 (1988))63-272 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
263
[1]
Authigenic carbonates in sediments from the Gulf of Mexico R e i n e r Botz 1, E c k h a r d F a b e r
2 Michael
J. W h i t i c a r 2 a n d J a m e s M. B r o o k s 3
1 Geologisch-Paliiontologisches Institut und Museum der Universitiit, Kiel (F.R.G.) 2 Bundesanstalt flit Geowissenschaften und Rohstoffe, Hannover (F.R.G.) 3 Department of Oceanography, Texas A & M University, College Station, T X 77843 (U.S.A.) Received October 12, 1987; revised version accepted February 2, 1988 The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between "normal-marine" (813C ca. 0%0) and -14.6%o which suggests that biogenic CO 2 contributed to the carbonate formation. The dJÀ80 values of dolomite and coexisting calcite are very similar but variable down-core. Dolomite and calcite precipitated early from pore water where SO 2 - was not reduced. However, during (and after?) SO2 - reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.
1. Introduction
The formation of authigenic carbonates is well documented in anoxic organic-rich sediments where the organic material is degraded by microbial activity [1-6]. The formation of sedimentary dolomite is still an unknown process. However, diagenetic dolomite is frequently formed early within the upper part of the sediment colunm where the Mg 2+ and Ca 2+ concentrations have their maximum [5]. Baker and Kastner [7] and Kastner [8] showed that dolomite formation requires a low SO42- concentration in the pore water. Isotope analyses have proved to be of great value in clarifying the formation processes of authigenic carbonates. Fig. I shows the general scheme of diagenetic zones within anoxic sediments and hypothetic trends in the isotopic composition of diagenetic carbonates which formed in various sediment depths [1,2,9-11]. Four zones are distinguishable within the sediment column. These are from top to bottom: bacterial oxidation zone, (anaerobic) bacterial sulfate reduction zone, CO 2 reduction zone, and zone of thermocatalytic decarboxylation (not shown in Fig. 1). Authigenic carbonates show a characteristic isotopic composition depending on the diagenetic zone in which they form. Aerobic oxidation and anaerobic sulfate reduction processes result in 8a3C values of authi0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
genic carbonates as low as -21%o [2]. However, positive 813C carbonate values are characteristic for the methane generation zone because of 12C depletion in the carbon dioxide remaining during methanogenesis. Methane is only produced in large quantities at depths where the sulfate in the pore water is low due to exhaustion by sulfate-reducing bacteria. At greater sediment depth thermocatalytic decarboxylation reactions dominate and again 12C-rich carbonates may form.
Depth
Diogenetic
Zones
Geochemical
Effect
D i o g e n e t i c Corbonotes Isotopic T r e n d
Generol
13 C
E ~soluhon
18 0
i
-0-0
01
" 0 01 - 10
Bacteriol O x i d a h o n Bocter~a~ Sulfote Reducllon
............"// / ' ' "
_-2
g~
('" -21 ~
."
Methone Oxidahon
CO2 Reduction - I 0 - I0 O
Methonogenesls
i
HeaW ¢~bon ." re~voi, CO2 ." ."
.......... . ~
..... .15 i
-50-250
Fig. 1. Schematics of diagenetic zones and hypothetic trends of isotopic composition of diagenetic carbonates (after Claypool and Kaplan [9]; Irwin et at. [10]; Irwin [11]; Pisciotto and Mahoney [1]; and Kelts and McKenzie [2]).
264
The 180 content of dolomite in equilibrium with calcite should be higher by 4-7%0 at sedimentary temperatures [12-14]. Land [15] suggested that the equilibrium value of A]80 dolomite-calcite is probably 3 + 1%o at 25 o C. McKenzie [16] reported for dolomite-calcite mixtures from the Abu Dhabi region an equilibrium value of 3.2%0. However, the oxygen isotopic compositions of sedimentary dolomites and coexisting calcites were often found to be similar. Degens and Epstein [17] explained this fact as dolomitization of a calcite precursor without alteration of the CO~- unit (metasomatic exchange). Other possibilities are that calcite and dolomite did not form in isotopic equilibrium [13,14] or that first poorly-ordered dolomite was precipitated whose oxygen isotopic composition was controlled by a lower dolomitewater fractionation [18,19]. The carbonate minerals low- and high-magnesian calcite (Mg calcite), aragonite and dolomite were identified in the sediments from the Gulf of Mexico. There is clear evidence that aragonite derives from shell hash. Although it is not clear whether the small amount ( < 10% of total carbonate) of high-Mg calcite is the result of carbonate production by organisms or inorganic precipitation from the surface water, it is probably not a diagenetic product. In contrast, low-Mg calcite may have formed during early diagenesis although it also may be of primary origin (coccoliths). Dolomite, which occurs in the sediments,
could be detrital in origin or, alternatively, could have formed during early stages of diagenesis.
2. Geological setting The cores were sampled in the Green Canyon Block 183 lease area in the Gulf of Mexico. The geology of the area is described by Brooks et al. [20]. This area is characterised by salt diapirs, faults, and collapsed structures. The faults may result from the upward movement of the diapirs while the collapsed structures may be generated by dissolution due to seawater entering the faults. The faults provide conduits for the upward migration of hydrocarbons from depth. Biodegraded, extractable oil in shallow cores [20] and thermogenic gas hydrates [21] have been found near the core 23 site.
3. Sampling and methods applied Fig. 2 shows the area investigated and the core locations. Table 1 gives the water depths and the lengths of the cores. The sediments are dark grey to black in color and of fine grain size (mud). Seventy-two samples were taken from four cores (cores 3, 20, 23, 24). The sediment samples were dried at 60 ° C and homogenized by slight grinding. The mineralogical composition was determined by X R D (Cu-K~
MISS
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..I TEXAS
HOUSTI N 1-, ~
..,, •
" :S~
' "~..
!..~:..~.(t~
•
....
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!:i' ~~i:
e23
•3
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24
................
INDEX TO CORE LOCATIONS 0
50
MILES
Fig. 2. Area investigated and core locations.
100
265 TABLE 1 Positions, water depths and lengths of the cores analysed Core No.
Location
3 20 23 24
27 ° 34.1'N, 96 o 29.8'W 28°40.6'N, 90 ° 00.7'W 27 °46.3'N, 91° 35.2'W 26 ° 58.4'N, 91°17.3'W
Water depth (m) 85 90 550 2200
Core length (cm) 960 880 840 1150
radiation). The relative amounts of the carbonate minerals were estimated using their major peak heights [22]. The carbonate content was measured gasometrically [23]. Organic carbon was measured using a Leco system. Although the concentrations of Fe, Mn, Mg, Ca and Sr were determined by atomic absorption spectroscopy, only the results for Sr are given in Table 2 (the Fe, Mn, Ca, Mg concentrations were not found to give further information on the genesis of the carbonate minerals). Sulfate in the pore water was analysed by an Autoanalyser II by the turbimetric measurement of barium sulfate. Prior to isotopic analysis the samples were treated with a 6% sodium hypochlorite solution to remove organic matter. After that, the samples were washed with distilled water and dried at 60 ° C. The CO 2 for isotopic analysis was released by reaction of the carbonate with 100% H3PO4 at 25 o C [24]. Reaction times were one day for calcite and three days for dolomite. Samples which contained a mixture of calcite and dolomite were analysed after various reaction times [12,25-27]. The gas fractions collected after 10-20 minutes reaction time were believed to represent mainly the calcite, whereas the CO 2 collected after reaction times of three hours, one day and three days is characteristic for the dolomite component. The isotopic results are given in per rail deviations from the PDB standard. 4. Results Table 2 shows the mineralogical and geochemical data. Cores 23 and 24 contain significant amounts of dolomite, up to 57% of the carbonate content (estimates based on X R D data). This is the reason for their detailed discussion, whereas cores 3 and 20 are used for comparison only. The
mineralogical data show that clay minerals (smectite, muscovite/illite, mixed-layer minerals, kaolinite and chlorite), quartz, feldspar, calcite, dolomite and sometimes a little pyrite are present. Dolomite is well-ordered with a general composition of Ca51Mg49(CO3) 2. The carbonate content of the sediments varies between 2 and 23%. The organic carbon content ranges from < 0.1 to 6.6%. The Sr content of the sediments ranges from 130 to 520 ppm. The SO4z- concentration ranges from 1.6 to 42.7 mM. Table 3 shows the carbon and oxygen isotope data. The most negative 813C value of bulk sampies is -9.7%0, which was found for a sample at the depth of 840 cm in core 23. Calcite in this sample has even a more negative 81SC value of -14.6%o. The carbon in coexisting dolomite is isotopically much heavier, with a 8a3C value of -6.6%0. The most positive 813C value of the bulk samples is + 1%o. Dolomite has always less negative 8a3C values than the coexisting calcite. Fig. 3 and Table 3 show that the 8a3C values of core 23 bulk-carbonate start at 560 cm to decrease systematically with depth. In contrast Fig. 4 shows that core 24, although the sediments contain calcite and dolomite similar to core 23, has homogeneous 813C values near 0%0. However, the 8180 values of the bulk samples vary between - 4 . 7 and + 1.4%o. Calcite and coexisting dolomite usually have very similar 8180 values (Table 3; Figs. 3 and 4). 5. Discussion The dark-grey to black color of the sediments in cores 23 and 24 indicates a reducing environment. The systematic decrease of 13C with depth in the lower part of core 23 is shown in Fig. 3. As also can be seen in Fig. 3 the 813C values of the calcites in core 23 are always more negative than those of the coexisting dolomites (in particular in the lowermost section). The in-situ formation of the dolomites and coexisting calcites (see below) can be inferred from the covariant depth trends in both their 813C and 8a80 values. Most 813C values of bulk carbonates from cores 3 and 24 (Table 3) are in the range of - 1 to + 1%o which is typical for marine carbonates, which precipitated near isotopic equihbrium with atmospheric CO 2 (813C-CO2 a t m = - 7 % o [28]). The fractionation between solid calcium carbonate and
266
TABLE 2 Mineralogical a n d g e o c h e m i c a l d a t a of cores 3, 23 a n d 24 Depth
Carbonate
Calcite
Dolomite
(cm)
(%)
(%)
(%)
Core 3 40
Co~g" (%)
0.66 0.61
3
60 100 120
5 5 12
160 180 200 220 280
5 6 2 2 2
340 38O 400 440 520 560
10 10 23 5 6 10
640 680 760
14 20 12
920 960
7 16
Sr (ppm)
SO~ (mM)
270 -
12.7 13.2
-
0.34 0.45
220
0.46 < 0.10 0.42
190
-
-
0.42 -
-
0.48
300 -
0.36
290
13.7 13.4 13.2 12.9 13.4 14.2 13.2 11.8 12.2 12.2 12.5 13.2
280
13.2 12.7 13.2
0.37
-
12.2 13.2 13.7
0.86 0.89 0.94 0.59 0.74
260 220 290 170 240
30.2 29.7 30.2 31.1 31.1
0.72 0.61 0.63 0.68 0.62 0.62 0.56 0.57 0.59 0.61 0.61
220 160 170 150 190 150 130 150 130 130 170
30.7 28.4 27.9 31.6 25.7 25.7 18.3 15.7 12.2 6.0 2.1
-
0.26 0.45
Core 23 84
16
43
57
60
40
8 12 12 12 18 15 12 18
60
40
47
53
48
52
0.62
190
3.8
4
91
9
0.76
260
32.9
80 -
20 -
0.97 1.46 6.64
520 -
32.9 33.5 35.4
87 63 84 82 51 58
13 -
0.63 0.96 1.21 1.33 1.20 1.15 0.91 1.17 1.30 1.49 0.92 1.11 1.32
190 280 140 370 -
41.3 42.2 42.7 36.7 1.6 37.6 38.5 37.2 35.7 36.2 36.2 36.7 36.7
2O 6O 80 120 140
8 8 11 20 12
160 240 260 340
12 6 8 8
360 380 520 560 600 640 760 840
Core 24 25 100 125 200 300 425 450 575 600 675 750 900 925 1025 1050 1125 1150 -- ~n.d.
4
16 15 4 10 13 15 15 12 15 16 4 13 4
37 16 18
49 42
130 370 150
TABLE
3
Carbon
and oxygen isotope ratios of carbonates
267
Depth
Bulk
(cm)
813C (~)
f r o m c o r e s 3, 2 0 , 2 3 a n d 2 4 D o l o m i t e (3 d a y s )
Calcite (20 minutes)
8180 (~)
813C (~)
8180 (%°)
813C (%)
8180 (~)
-0,2
-1.1
Core 3 40
0.4
0.2
60
- 0.4
- 1.7
100
-0.6
-1.5
120
- 1.1
- 2.5
160
- 0.1
- 0.2
180
- 0.3
- 1.1
200
- 0.4
- 0.2
220
- 0.6
- 0.4
280
- 1.3
1.1
340
- 0.3
- 0.2
380
- 0.4
- 1.3
400
0.5
440
-0.1
1.0 0.2
520
0.6
560
0.1
0.6 1.4
640
- 0.6
- 1.8
680
- 0.8
- 2.9
760
- 0.3
- 1.6
920
- 0.2
- 1.4
960
- 0.6
- 2.4
20
- 0.1
- 0.2
60
- 2.4
- 1.7
80
- 0.5
- 1.1
-0.6
-0.6
120
- 0.6
- 4.1
-2.1
-4.3
140
- 1.4
- 2.0
-2.0
160
- 0.9
- 2.1
-1.5
240
- 0.4
- 4.0
260
- 0.6
- 3.3
340
- 1.1
- 4.3
Core 23
0,1
-4.0
-0,9
-2.3
-1.3
-0,4
-2.2
-0.4
-4.5
-0.3
-3.5
-0.9
-3.2
-0,4
-4.6
-0,4
-3.9
-1.6
-0.4
-2.6
33
-0.4
-3.4
-
2.4
1.0
-
-
3.0
360
- 1.0
- 2.2
-2.0
380
- 1.0
- 3.4
-2.1
520
- 1.2
- 4.6
-2.6
-5.3
-0,4
-4.1
560
- 1.0
- 4.3
-3.2
-5.0
-0.3
-4.1
600
- 1.8
- 4.7
-4.4
-4.8
-0.7
-4.3
640
- 2.5
- 4.6
-5.5
-0.8
-4.3
760
- 5.6
- 3.2
-
-2.6
-3.6
-3.6
840
- 9.7
- 2.9
-14.6
-2.6
-6.6
-3.1
0.4
-1.0
-2.2
-0.3
- 1.0
11.4
-
-
5.0
Core 24 25
0.6
100
0.0
0.4
125
0.7
- 0.2
200
0.6
- 0.3
300
- 0.3
- 1.0
425
0,2
- 0.4
450
0.3
0.1
575
0,7
0.1
600
- 0,6
- 2.5
675
0.3
- 0.4
750
0.1
-0.3
900
- 0,3
- 1.7
925
0.1
0.0 1.0
1025
1.0
0.2
1050
- 0.5
- 3.0
-0.7
1125
0.2
-0.1
-0.2
-3.7
-1.4
1150
-0.7
0.3 -2.8 0.0 -4.3
0.0
-0.2 1.1 -0.2
- 2.6
-2.9 -0.1 -3.1
0.3
-0.3
--0.4
-3.5
268
TABLE
3
(continued)
Depth
Bulk
Calcite (20 minutes)
Dolomite
(cm)
813C
3180
613C
3180
813C
3180
(%0)
(%)
(%°)
(%°)
(%0)
(%0)
(3 days)
Core 20 20
- 2.0
- 2.7
60
- 2.5
- 3.0
80
- 2.0
- 3.5
100
- 2.2
-
140
- 2.0
- 1.6
200
- 2.2
- 2.5
240
- 2.4
- 2.9
260
- 3.0
- 1.9
380
-2.0
-1.7
480
- 2.4
- 2.2
560
- 1.9
- 2.2
600
- 1.7
- 2.9
640
- 1.8
- 2.5
680
- 2.2
- 3.0
720
- 2.8
- 3.0
760
- 1.4
- 0.9
800
- 2.3
- 3.0
880
-2.9
-3.2
3.6
- = not detected.
gaseous C O 2 is 10%o at 20 ° C [29] which results in a 6~3C value of +3%0 for carbonate in isotopic equilibrium with atmospheric CO2. It is likely that the carbonates of cores 3 and 24 formed in the sediment where small amounts of 12C-rich bio-
.S/"
100
\ 300
Tu
~u
LN
genic CO2 were mixed to the marine bicarbonate dissolved in the pore water. This may be the reason for the small shift towards more negative 813C values relative to the equilibrium value of + 3%o. Core 20 shows an even larger shift in the
I
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700
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~
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d~
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"-~.
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+'d
/
/ I
?
./" book----~---C./
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7t
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:cllcite /
\
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///2 0o, 4! I
900
i
1
I
20 40 60 80 C a l c / D o l (%) F i g . 3.
L
5
l
J
I
10 15 20
Ca CO 3 (%1
I
I
10 20
I
L
l
I
30 40
-I0
-5
0
I
SOL--mM
813 C 101001
l
I
-5
I
-3
I
I
8180(*/,.I
Carbonate mineralogy and carbonate content, pore water sulfate and isotopic c o m p o s i t i o n of carbonates in core
23.
269
7
i
i /
\
\
300
o calcite 1 J dolomite / ;
\
/
i""
•-~..
/
u
I
£~_ 600
=
/ 900
-
o/ cl
/
\
i
°
[ I
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1200
2O 40 60 80 Calcl Dol (%}
5
10 15 20
C a C O 3 1%1
10 20 30
40
-10
-5
0
Ot3c (%0)
S04-- mM
-5
-3
-1
5180 (%,)
Fig. 4. Carbonate mineralogy and carbonate content, pore water sulfate and isotopic composition of carbonates in core 24.
~13C values to more negative values (to -3%o)
which most likely is attributable to a contribution of isotopically light CO 2 derived from the aerobic or anaerobic oxidation of organic matter during early stages of diagenesis. No observations were made of biogenic carbonates such as foraminifera or molluscs in the four cores under consideration (in other cores from the Gulf of Mexico, however, we observed those biogenic carbonates). Thus, "vital effects" which result in lower 813C values (around 0%0) are not a likely explanation for the 813C values found for carbonates from cores 3, 20 and 24. Further support of a diagenetic origin of dolomite and coexisting calcite is also given by their usually very similar 8180 values (see below)• In the lower part of core 23 the ~13C values of the bulk carbonate samples decrease systematically with increasing core depth (Table 4). Fig. 3 shows that in the lower part of core 23 the sulfate content in the pore water is reduced• Hence it is likely that the increasing 12C content of the carbonates with increasing core depth is the result of the anaerobic degradation of organic matter during sulfate reduction. Fig. 1 shows that bacterial sulfate reduction processes occur during early stages of diagenesis in organic-rich marine sediments• Here the organic matter is oxidized by anaerobic sulfate-reducing bacteria, resulting in contributions of 12C-rich bio-
genie CO 2 with ~13C values ranging between about -25%o to zero. The latter is typical for "normal" marine bicarbonate in pore water (~13C = 0%0). Carbonate 813C values [9] were observed to be as negative as -21%o (Fig. 1) within the sulfate reduction zone depending on the relative proportions of 12C-rich biogenic CO 2 and normal marine H C O 3. Although it is evident that sulfate reduction occurs in core 23 and, hence, isotopically hght CO 2 is produced by the oxidation of the organic matter, isotopically light CO 2 may also derive from the bacterial oxidation of methane. As bacterial methane is enriched in 12C ( ~ 1 3 C - C H 4 = - 5 0 to -90%o [30]) CO 2 produced by the oxidation of methane will also be 12C-rich early in the oxidation process [31,32]. The carbonates formed from that CO 2 are rich in 12C. Although extremely negative ~13C values have not been measured for TABLE 4 Core 23: carbon and oxygen isotope ratios of bulk carbonates for different reaction times Depth
10 minutes
3 hours
(cm)
813C
8180
813C
8180
813C 8180
(%o)
(%o)
(~oo)
(%o)
(%o)
(~)
-7.3 -12.2 -15.6
-3.6 -1.5 -2.2
-1.8 -4.6 -10.8
-4.8 -3.8 -2.9
-0.5 -2.3 -3.7
-4.1 -3.2 -3.2
640 760 840
3 days
270
carbonates from the cores investigated, it cannot be excluded that small quantities of 12C-rich CO2 from methane oxidation may have been mixed with the pore water bicarbonate, especially because high concentrations of biogenic methane [33] were present in cores 23 and 24. Fig. 3 and Table 3 show increasing differences in 613C of calcite (10-20 minutes reaction time) and dolomite (1-3 days reaction time) with increasing sediment depth. The 613C values of dolomite are virtually constant between 0 and - 1%o down to 640 cm in core 23. In contrast, the 813C values of the calcite are distinctly more negative at that depth (-7.3%o for the 10-minute fraction and -5.5%o for the 20-minute fraction). This shows that dolomite and calcite, although coexisting in the core, did not form cogenetically from the same dissolved bicarbonate. It is likely that dolomite formed earlier than calcite at depths where the dissolved bicarbonate still was similar to the normal marine bicarbonate. This is somewhat different in the lower part of core 23 where also 12C-rich dolomite (13-day gas fraction; 613C = -6.6%o at 840 cm) has been found. However, similar to the situation in the upper part of the core, dolomite is distinctly enriched in 13C relative to the coexisting calcite. In summary, it appears that 12C-rich biogenic C O 2 (•13C-C02 < -25%0) from anaerobic sulfate reduction or C H 4 oxidation was added to the " n o r m a l " marine bicarbonate dissolved in pore water. The pore water bicarbonate therefore was enriched in 12C and first formed dolomite and later, at greater depths,
Sr ppm
500
400
3O0
© o •
200
•
calcite. The difference in isotopic composition of calcite and dolomite also suggests that dolomite did not form by solid state cation exchange from a precursor calcite (at least not from the now coexisting calcite). Calcite and dolomite in core 24 have 613C values near 0%o and the sulfate concentration is uniformly high (except the sample at 600 cm; Fig. 4). As no primary biogenic carbonate was observed (see above) and the 6a80 values of calcite and coexisting dolomite are very similar (but variable down-core) a diagenetic rather than detrital origin of the carbonate of core 24 is indicated. This is a very similar situation to that in the upper part of core 23 where also calcite and dolomite precipitated (using " n o r m a l " marine bicarbonate with only minor contributions of biogenic CO2) with 613C values near 0%o. The strong covariance of the 6180 values of calcite and dolomite indicates that each " p a i r " dolomite and coexisting calcite precipitated from an isotopically characteristic pore water. Although the isotopic compositions of the pore waters have not been determined the large variation in 6180 of the carbonates probably is due to fluctuations in salinity rather than temperature changes (a difference in 6180 by 4-5%o as found in cores 23 and 24 would imply a temperature change of about 15-20 °C). Furthermore the Sr concentration of cores 23 and 24 seem to be correlated with the 6180 values (Fig. 5). This also suggests that salinity plays possibly a significant role (even though the Sr concentrations are rather low as is the carbonate content). However, only a detailed 180 stratigraphy using primary foraminifera would allow to draw final conclusions about primary fluctuations in temperature and salinity of the seawater. Dolomite is not enriched in 180 by around 3%o relative to calcite as was found by Land [15] and McKenzie [16]. However, no final decision can be made whether or not both minerals are in isotopic equilibrium with surrounding pore fluids.
O o• m
°° O
•1
•
O
6. Conclusions
C:
100
-2
-~
-:~
-;
+
6'~ oo,'o°
Fig. 5. Cores 23 and 24: correlation of the Sr concentrations of sediments with the 61~O values of carbonates.
In some organic-rich sediments from the Gulf of Mexico diagenetic dolomite and calcite are formed. Both minerals form in situ from pore water-HCO 3 which derives from two sources: (1)
271
"normal-marine" HCO 3 with ~13C values around 0%0, and (2) biogenic CO 2 (~13C < -25%0) probably from oxidized bacterial methane or from oxidized organic matter during sulfate reduction. The first carbonate generation (dolomite and calcite) formed early from "normal-marine" HCO3- and the second generation (or perhaps more likely further generations) is enriched in 12C which indicates contributions of biogenic C02 during later dolomite and calcite formation. Dolomite formation started when contributions of biogenic CO~ still were negligible and the SO2content of the pore water was not reduced. When S042- reduction o c c u r s l Z c - r i c h biogenic CO 2 is mixed to the normal marine HCO 3 causing a shift towards negative 813C-HC03 values. Diagenetically formed calcite reflects this 12C-rich pore water bicarbonate.
Acknowledgements The quality of this paper was considerably improved by the critical review of the early manuscript by P. Stoffers and W. Stahl which is kindly acknowledged. Helpful remarks were provided by the reviewers. This work was supported by the BMFT grant No. 03E 6237 A at BGR, Hannover, F.R.G.
References 1 K.A. Pisciotto and J.J. Mahoney, Isotopic survey of diagenetic carbonates, Deep Sea Drilling Project Leg 63, in: R.S. Yeats, B.U. Haq et al., Init. Rep. DSDP 63, 595-609, 1981. 2 K.R. Kelts and J.R. McKenzie, Diagenetic dolomite formation in Quaternary anoxic diatomaceous muds of Deep Sea Drilling Project Leg 64, Gulf of California, in: J.R. Curray, D.G. Moore et al., Init. Rep. DSDP 64, 553-569, 1982. 3 D.L. Gautier, Siderite concretions: indicators of early diagenesis in the Gammon Shale (Cretaceous), J. Sediment. Petrol. 52, 859-871, 1982. 4 H. Wada, N. Niitsuma, K. Nagasawa and H. Okada, Deep sea carbonate nodules from the Middle American Trench area off Mexico, Deep Sea Drilling Project Leg 66, in: J.S. Watkins, J.C. Moore et al., Init. Rep. DSDP 66, 453-474, 1982. 5 N. Tasse and R. Hesse, Origin and significance of complex authigenic carbonates in Cretaceous black shales of the western Alps, J. Sediment. Petrol. 54, 1012-1027, 1984. 6 J. Hennessy and L.P. Knauth, Isotopic variations in dolomite concretions from the Monterey formation, California, J. Sediment. Petrol. 55, 120-130, 1985.
7 P.A. Baker and M. Kastner, Constraints on the formation of sedimentary dolomite, Science 213, 214-216, 1981. 8 M. Kastner, Control of dolomite formation, Nature 311, 410-411, 1984. 9 G.E. Claypool and I.R. Kaplan, The origin and distribution of methane in marine sediments, in: Natural Gases in Marine Sediments, I.R. Kaplan, ed., pp. 99-139, Plenum Press, New York, N.Y., 1974. 10 H. Irwin, C. Curtis and M. Coleman, Isotopic evidence for source of diagenetic carbonates formed during burial of organic-rich sediments, Nature 269, 209-213, 1977. 11 H. Irwin, Early diagenetic carbonate precipitation and pore fluid migration in the Kimmeridge clay of Dorset, England, Sedimentology 27, 577-591, 1980. 12 S. Epstein, D.L. Graf and E.T. Degens, Oxygen isotope studies on the origin of dolomites, in: Isotopic and Cosmic Chemistry, H. Craig, S.L. Miller and G.J. Wasserburg, eds., pp. 169-180, North-Holland, Amsterdam, 1964. 13 J.R. O'Neil and S. Epstein, Oxygen isotopic fractionation in the system dolomite-calcite-carbon dioxide, Science 152, 198-201, 1986. 14 D.A. Northrop and R.N. Clayton, Oxygen-isotopic fractionation in systems containing dolomite, J. Geol. 74, 174-196, 1966. 15 L.S. Land, The isotopic and trace element geochemistry of dolomite: the state of the art, Spec. Publ. Soc. Econ. Paleontol. Mineral. 28, 87-110, 1980. 16 J.A. McKenzie, Holocene dolomitization of calcium carbonate sediments from the coastal sabkas of Abu Dhabi, U.A.E.: a stable isotope study, J. Geol. 89, 185-198, 1981. 17 E.T. Degens and S. Epstein, Oxygen and carbon isotope ratios in coexisting calcites and dolomites from recent and ancient sediments, Geochim. Cosmochim. Acta 28, 23-44, 1964. 18 P. Fritz and D.G.W. Smith, The isotopic composition of secondary dolomites, Geochim. Cosmochim. Acta 43, 1161-1173, 1970. 19 A. Matthews and A. Katz, Oxygen isotope fractionations during the dolomitization of calcium carbonate, Geochim. Cosmochim. Acta 41, 1431-1438, 1977. 20 J.M. Brooks, H.B. Cox, W.R. Bryant, M.C. Kennicutt, R.G. Mann and T.J. McDonald, Association of gas hydrates and oil seepage in the Gulf of Mexico, in: Advances in Organic Geochemistry 1985, Petroleum Geochemistry, D. Leyth~iuser and J. Riillkoetter, eds., Pergamon, Oxford, 1986. 21 J.M. Brooks, M.V. Kennicutt, R.A. Fay, T.J. McDonald and R. Sassen, Thermogenic gas hydrates in the Gulf of Mexico, Science 225, 409-411, 1984. 22 E. Gavish and C.M. Friedman, Quantitative analysis of calcite and Mg-calcite by X-ray diffraction: effect of grinding on peak height and peak area, Sedimentology 20, 437-444, 1973. 23 G. Miiller and M. Gastner, The "Karbonatbombe" a simple device for the determination of the carbonate content in sediments, soils and other materials, Neues Jahrb. Miner. Mh. 10, 466-469, 1971. 24 J.M. McCrea, The isotope chemistry of carbonates and a paleotemperature scale, J. Chem. Phys. 18, 849-857, 1950. 25 R.N. Clayton, B.F. Jones and R.A. Berner, Isotope studies
272
26
27
28
29
of dolomite formation under sedimentary conditions, Geochim. Cosmochim. Acta 32, 415-432, 1968. L.J. Waiters, G.E. Claypool and P.W. Choquette, Reaction rates and 81SO variation for the carbonate-phosphoric acid preparation method, Geochim. Cosmochim. Acta 36, 129-140, 1972. J.A. McKenzie, Isotope study of the hydrology and the coexisting carbonate phases from site of recent dolomitization, the coastal sabka of Abu Dhabi, Persian Gulf, Ph.D. Dissertation, Swiss Federal Institute of Technology, Ziirich, 1976 (unpublished). C.D. Keeling, The concentration and isotopic abundance of carbon dioxide in rural areas, Geochim. Cosmochim. Acta 13, 322-334, 1958. K. Emrich, D.H. Ehhalt and J.C. Vogel, Carbon isotope
30
31 32
33
fractionation during the precipitation of calcium carbonate, Earth Planet. Sci. Lett. 8, 363-371, 1970. M. Schoell, The hydrogen and carbon isotopic composition of methane from natural gases of various origins, Geochim. Cosmochim. Acta 44, 649-662, 1980. J.C. Hathaway and E.T. Degens, Methane-derived marine carbonates of Pleistocene age, Science 165, 690, 1969. W.G. Deuser, Extreme 13C/12C variations in Quaternary dolomites from the continental shelf, Earth Planet. Sci. Lett. 8, 118-124, 1970. E. Faber, M.J. Whiticar, J.M. Brooks, M.C. Kennicutt II and E.J. Schiener, Differentiation of thermogenic and biogenic gases in a shallow Gulf Coast Core: sorbed vs. free sediment gases, in preparation.
263
[1]
Authigenic carbonates in sediments from the Gulf of Mexico R e i n e r Botz 1, E c k h a r d F a b e r
2 Michael
J. W h i t i c a r 2 a n d J a m e s M. B r o o k s 3
1 Geologisch-Paliiontologisches Institut und Museum der Universitiit, Kiel (F.R.G.) 2 Bundesanstalt flit Geowissenschaften und Rohstoffe, Hannover (F.R.G.) 3 Department of Oceanography, Texas A & M University, College Station, T X 77843 (U.S.A.) Received October 12, 1987; revised version accepted February 2, 1988 The carbon isotopic composition of diagenetic dolomite and calcite in some sediments of the Gulf of Mexico varies between "normal-marine" (813C ca. 0%0) and -14.6%o which suggests that biogenic CO 2 contributed to the carbonate formation. The dJÀ80 values of dolomite and coexisting calcite are very similar but variable down-core. Dolomite and calcite precipitated early from pore water where SO 2 - was not reduced. However, during (and after?) SO2 - reduction dolomite and calcite still formed and there are at least two generations of carbonate minerals present.
1. Introduction
The formation of authigenic carbonates is well documented in anoxic organic-rich sediments where the organic material is degraded by microbial activity [1-6]. The formation of sedimentary dolomite is still an unknown process. However, diagenetic dolomite is frequently formed early within the upper part of the sediment colunm where the Mg 2+ and Ca 2+ concentrations have their maximum [5]. Baker and Kastner [7] and Kastner [8] showed that dolomite formation requires a low SO42- concentration in the pore water. Isotope analyses have proved to be of great value in clarifying the formation processes of authigenic carbonates. Fig. I shows the general scheme of diagenetic zones within anoxic sediments and hypothetic trends in the isotopic composition of diagenetic carbonates which formed in various sediment depths [1,2,9-11]. Four zones are distinguishable within the sediment column. These are from top to bottom: bacterial oxidation zone, (anaerobic) bacterial sulfate reduction zone, CO 2 reduction zone, and zone of thermocatalytic decarboxylation (not shown in Fig. 1). Authigenic carbonates show a characteristic isotopic composition depending on the diagenetic zone in which they form. Aerobic oxidation and anaerobic sulfate reduction processes result in 8a3C values of authi0012-821X/88/$03.50
© 1988 Elsevier Science Publishers B.V.
genic carbonates as low as -21%o [2]. However, positive 813C carbonate values are characteristic for the methane generation zone because of 12C depletion in the carbon dioxide remaining during methanogenesis. Methane is only produced in large quantities at depths where the sulfate in the pore water is low due to exhaustion by sulfate-reducing bacteria. At greater sediment depth thermocatalytic decarboxylation reactions dominate and again 12C-rich carbonates may form.
Depth
Diogenetic
Zones
Geochemical
Effect
D i o g e n e t i c Corbonotes Isotopic T r e n d
Generol
13 C
E ~soluhon
18 0
i
-0-0
01
" 0 01 - 10
Bacteriol O x i d a h o n Bocter~a~ Sulfote Reducllon
............"// / ' ' "
_-2
g~
('" -21 ~
."
Methone Oxidahon
CO2 Reduction - I 0 - I0 O
Methonogenesls
i
HeaW ¢~bon ." re~voi, CO2 ." ."
.......... . ~
..... .15 i
-50-250
Fig. 1. Schematics of diagenetic zones and hypothetic trends of isotopic composition of diagenetic carbonates (after Claypool and Kaplan [9]; Irwin et at. [10]; Irwin [11]; Pisciotto and Mahoney [1]; and Kelts and McKenzie [2]).
264
The 180 content of dolomite in equilibrium with calcite should be higher by 4-7%0 at sedimentary temperatures [12-14]. Land [15] suggested that the equilibrium value of A]80 dolomite-calcite is probably 3 + 1%o at 25 o C. McKenzie [16] reported for dolomite-calcite mixtures from the Abu Dhabi region an equilibrium value of 3.2%0. However, the oxygen isotopic compositions of sedimentary dolomites and coexisting calcites were often found to be similar. Degens and Epstein [17] explained this fact as dolomitization of a calcite precursor without alteration of the CO~- unit (metasomatic exchange). Other possibilities are that calcite and dolomite did not form in isotopic equilibrium [13,14] or that first poorly-ordered dolomite was precipitated whose oxygen isotopic composition was controlled by a lower dolomitewater fractionation [18,19]. The carbonate minerals low- and high-magnesian calcite (Mg calcite), aragonite and dolomite were identified in the sediments from the Gulf of Mexico. There is clear evidence that aragonite derives from shell hash. Although it is not clear whether the small amount ( < 10% of total carbonate) of high-Mg calcite is the result of carbonate production by organisms or inorganic precipitation from the surface water, it is probably not a diagenetic product. In contrast, low-Mg calcite may have formed during early diagenesis although it also may be of primary origin (coccoliths). Dolomite, which occurs in the sediments,
could be detrital in origin or, alternatively, could have formed during early stages of diagenesis.
2. Geological setting The cores were sampled in the Green Canyon Block 183 lease area in the Gulf of Mexico. The geology of the area is described by Brooks et al. [20]. This area is characterised by salt diapirs, faults, and collapsed structures. The faults may result from the upward movement of the diapirs while the collapsed structures may be generated by dissolution due to seawater entering the faults. The faults provide conduits for the upward migration of hydrocarbons from depth. Biodegraded, extractable oil in shallow cores [20] and thermogenic gas hydrates [21] have been found near the core 23 site.
3. Sampling and methods applied Fig. 2 shows the area investigated and the core locations. Table 1 gives the water depths and the lengths of the cores. The sediments are dark grey to black in color and of fine grain size (mud). Seventy-two samples were taken from four cores (cores 3, 20, 23, 24). The sediment samples were dried at 60 ° C and homogenized by slight grinding. The mineralogical composition was determined by X R D (Cu-K~
MISS
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..,, •
" :S~
' "~..
!..~:..~.(t~
•
....
J ~
!:i' ~~i:
e23
•3
•
24
................
INDEX TO CORE LOCATIONS 0
50
MILES
Fig. 2. Area investigated and core locations.
100
265 TABLE 1 Positions, water depths and lengths of the cores analysed Core No.
Location
3 20 23 24
27 ° 34.1'N, 96 o 29.8'W 28°40.6'N, 90 ° 00.7'W 27 °46.3'N, 91° 35.2'W 26 ° 58.4'N, 91°17.3'W
Water depth (m) 85 90 550 2200
Core length (cm) 960 880 840 1150
radiation). The relative amounts of the carbonate minerals were estimated using their major peak heights [22]. The carbonate content was measured gasometrically [23]. Organic carbon was measured using a Leco system. Although the concentrations of Fe, Mn, Mg, Ca and Sr were determined by atomic absorption spectroscopy, only the results for Sr are given in Table 2 (the Fe, Mn, Ca, Mg concentrations were not found to give further information on the genesis of the carbonate minerals). Sulfate in the pore water was analysed by an Autoanalyser II by the turbimetric measurement of barium sulfate. Prior to isotopic analysis the samples were treated with a 6% sodium hypochlorite solution to remove organic matter. After that, the samples were washed with distilled water and dried at 60 ° C. The CO 2 for isotopic analysis was released by reaction of the carbonate with 100% H3PO4 at 25 o C [24]. Reaction times were one day for calcite and three days for dolomite. Samples which contained a mixture of calcite and dolomite were analysed after various reaction times [12,25-27]. The gas fractions collected after 10-20 minutes reaction time were believed to represent mainly the calcite, whereas the CO 2 collected after reaction times of three hours, one day and three days is characteristic for the dolomite component. The isotopic results are given in per rail deviations from the PDB standard. 4. Results Table 2 shows the mineralogical and geochemical data. Cores 23 and 24 contain significant amounts of dolomite, up to 57% of the carbonate content (estimates based on X R D data). This is the reason for their detailed discussion, whereas cores 3 and 20 are used for comparison only. The
mineralogical data show that clay minerals (smectite, muscovite/illite, mixed-layer minerals, kaolinite and chlorite), quartz, feldspar, calcite, dolomite and sometimes a little pyrite are present. Dolomite is well-ordered with a general composition of Ca51Mg49(CO3) 2. The carbonate content of the sediments varies between 2 and 23%. The organic carbon content ranges from < 0.1 to 6.6%. The Sr content of the sediments ranges from 130 to 520 ppm. The SO4z- concentration ranges from 1.6 to 42.7 mM. Table 3 shows the carbon and oxygen isotope data. The most negative 813C value of bulk sampies is -9.7%0, which was found for a sample at the depth of 840 cm in core 23. Calcite in this sample has even a more negative 81SC value of -14.6%o. The carbon in coexisting dolomite is isotopically much heavier, with a 8a3C value of -6.6%0. The most positive 813C value of the bulk samples is + 1%o. Dolomite has always less negative 8a3C values than the coexisting calcite. Fig. 3 and Table 3 show that the 8a3C values of core 23 bulk-carbonate start at 560 cm to decrease systematically with depth. In contrast Fig. 4 shows that core 24, although the sediments contain calcite and dolomite similar to core 23, has homogeneous 813C values near 0%0. However, the 8180 values of the bulk samples vary between - 4 . 7 and + 1.4%o. Calcite and coexisting dolomite usually have very similar 8180 values (Table 3; Figs. 3 and 4). 5. Discussion The dark-grey to black color of the sediments in cores 23 and 24 indicates a reducing environment. The systematic decrease of 13C with depth in the lower part of core 23 is shown in Fig. 3. As also can be seen in Fig. 3 the 813C values of the calcites in core 23 are always more negative than those of the coexisting dolomites (in particular in the lowermost section). The in-situ formation of the dolomites and coexisting calcites (see below) can be inferred from the covariant depth trends in both their 813C and 8a80 values. Most 813C values of bulk carbonates from cores 3 and 24 (Table 3) are in the range of - 1 to + 1%o which is typical for marine carbonates, which precipitated near isotopic equihbrium with atmospheric CO 2 (813C-CO2 a t m = - 7 % o [28]). The fractionation between solid calcium carbonate and
266
TABLE 2 Mineralogical a n d g e o c h e m i c a l d a t a of cores 3, 23 a n d 24 Depth
Carbonate
Calcite
Dolomite
(cm)
(%)
(%)
(%)
Core 3 40
Co~g" (%)
0.66 0.61
3
60 100 120
5 5 12
160 180 200 220 280
5 6 2 2 2
340 38O 400 440 520 560
10 10 23 5 6 10
640 680 760
14 20 12
920 960
7 16
Sr (ppm)
SO~ (mM)
270 -
12.7 13.2
-
0.34 0.45
220
0.46 < 0.10 0.42
190
-
-
0.42 -
-
0.48
300 -
0.36
290
13.7 13.4 13.2 12.9 13.4 14.2 13.2 11.8 12.2 12.2 12.5 13.2
280
13.2 12.7 13.2
0.37
-
12.2 13.2 13.7
0.86 0.89 0.94 0.59 0.74
260 220 290 170 240
30.2 29.7 30.2 31.1 31.1
0.72 0.61 0.63 0.68 0.62 0.62 0.56 0.57 0.59 0.61 0.61
220 160 170 150 190 150 130 150 130 130 170
30.7 28.4 27.9 31.6 25.7 25.7 18.3 15.7 12.2 6.0 2.1
-
0.26 0.45
Core 23 84
16
43
57
60
40
8 12 12 12 18 15 12 18
60
40
47
53
48
52
0.62
190
3.8
4
91
9
0.76
260
32.9
80 -
20 -
0.97 1.46 6.64
520 -
32.9 33.5 35.4
87 63 84 82 51 58
13 -
0.63 0.96 1.21 1.33 1.20 1.15 0.91 1.17 1.30 1.49 0.92 1.11 1.32
190 280 140 370 -
41.3 42.2 42.7 36.7 1.6 37.6 38.5 37.2 35.7 36.2 36.2 36.7 36.7
2O 6O 80 120 140
8 8 11 20 12
160 240 260 340
12 6 8 8
360 380 520 560 600 640 760 840
Core 24 25 100 125 200 300 425 450 575 600 675 750 900 925 1025 1050 1125 1150 -- ~n.d.
4
16 15 4 10 13 15 15 12 15 16 4 13 4
37 16 18
49 42
130 370 150
TABLE
3
Carbon
and oxygen isotope ratios of carbonates
267
Depth
Bulk
(cm)
813C (~)
f r o m c o r e s 3, 2 0 , 2 3 a n d 2 4 D o l o m i t e (3 d a y s )
Calcite (20 minutes)
8180 (~)
813C (~)
8180 (%°)
813C (%)
8180 (~)
-0,2
-1.1
Core 3 40
0.4
0.2
60
- 0.4
- 1.7
100
-0.6
-1.5
120
- 1.1
- 2.5
160
- 0.1
- 0.2
180
- 0.3
- 1.1
200
- 0.4
- 0.2
220
- 0.6
- 0.4
280
- 1.3
1.1
340
- 0.3
- 0.2
380
- 0.4
- 1.3
400
0.5
440
-0.1
1.0 0.2
520
0.6
560
0.1
0.6 1.4
640
- 0.6
- 1.8
680
- 0.8
- 2.9
760
- 0.3
- 1.6
920
- 0.2
- 1.4
960
- 0.6
- 2.4
20
- 0.1
- 0.2
60
- 2.4
- 1.7
80
- 0.5
- 1.1
-0.6
-0.6
120
- 0.6
- 4.1
-2.1
-4.3
140
- 1.4
- 2.0
-2.0
160
- 0.9
- 2.1
-1.5
240
- 0.4
- 4.0
260
- 0.6
- 3.3
340
- 1.1
- 4.3
Core 23
0,1
-4.0
-0,9
-2.3
-1.3
-0,4
-2.2
-0.4
-4.5
-0.3
-3.5
-0.9
-3.2
-0,4
-4.6
-0,4
-3.9
-1.6
-0.4
-2.6
33
-0.4
-3.4
-
2.4
1.0
-
-
3.0
360
- 1.0
- 2.2
-2.0
380
- 1.0
- 3.4
-2.1
520
- 1.2
- 4.6
-2.6
-5.3
-0,4
-4.1
560
- 1.0
- 4.3
-3.2
-5.0
-0.3
-4.1
600
- 1.8
- 4.7
-4.4
-4.8
-0.7
-4.3
640
- 2.5
- 4.6
-5.5
-0.8
-4.3
760
- 5.6
- 3.2
-
-2.6
-3.6
-3.6
840
- 9.7
- 2.9
-14.6
-2.6
-6.6
-3.1
0.4
-1.0
-2.2
-0.3
- 1.0
11.4
-
-
5.0
Core 24 25
0.6
100
0.0
0.4
125
0.7
- 0.2
200
0.6
- 0.3
300
- 0.3
- 1.0
425
0,2
- 0.4
450
0.3
0.1
575
0,7
0.1
600
- 0,6
- 2.5
675
0.3
- 0.4
750
0.1
-0.3
900
- 0,3
- 1.7
925
0.1
0.0 1.0
1025
1.0
0.2
1050
- 0.5
- 3.0
-0.7
1125
0.2
-0.1
-0.2
-3.7
-1.4
1150
-0.7
0.3 -2.8 0.0 -4.3
0.0
-0.2 1.1 -0.2
- 2.6
-2.9 -0.1 -3.1
0.3
-0.3
--0.4
-3.5
268
TABLE
3
(continued)
Depth
Bulk
Calcite (20 minutes)
Dolomite
(cm)
813C
3180
613C
3180
813C
3180
(%0)
(%)
(%°)
(%°)
(%0)
(%0)
(3 days)
Core 20 20
- 2.0
- 2.7
60
- 2.5
- 3.0
80
- 2.0
- 3.5
100
- 2.2
-
140
- 2.0
- 1.6
200
- 2.2
- 2.5
240
- 2.4
- 2.9
260
- 3.0
- 1.9
380
-2.0
-1.7
480
- 2.4
- 2.2
560
- 1.9
- 2.2
600
- 1.7
- 2.9
640
- 1.8
- 2.5
680
- 2.2
- 3.0
720
- 2.8
- 3.0
760
- 1.4
- 0.9
800
- 2.3
- 3.0
880
-2.9
-3.2
3.6
- = not detected.
gaseous C O 2 is 10%o at 20 ° C [29] which results in a 6~3C value of +3%0 for carbonate in isotopic equilibrium with atmospheric CO2. It is likely that the carbonates of cores 3 and 24 formed in the sediment where small amounts of 12C-rich bio-
.S/"
100
\ 300
Tu
~u
LN
genic CO2 were mixed to the marine bicarbonate dissolved in the pore water. This may be the reason for the small shift towards more negative 813C values relative to the equilibrium value of + 3%o. Core 20 shows an even larger shift in the
I
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u
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,
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900
i
1
I
20 40 60 80 C a l c / D o l (%) F i g . 3.
L
5
l
J
I
10 15 20
Ca CO 3 (%1
I
I
10 20
I
L
l
I
30 40
-I0
-5
0
I
SOL--mM
813 C 101001
l
I
-5
I
-3
I
I
8180(*/,.I
Carbonate mineralogy and carbonate content, pore water sulfate and isotopic c o m p o s i t i o n of carbonates in core
23.
269
7
i
i /
\
\
300
o calcite 1 J dolomite / ;
\
/
i""
•-~..
/
u
I
£~_ 600
=
/ 900
-
o/ cl
/
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i
°
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1200
2O 40 60 80 Calcl Dol (%}
5
10 15 20
C a C O 3 1%1
10 20 30
40
-10
-5
0
Ot3c (%0)
S04-- mM
-5
-3
-1
5180 (%,)
Fig. 4. Carbonate mineralogy and carbonate content, pore water sulfate and isotopic composition of carbonates in core 24.
~13C values to more negative values (to -3%o)
which most likely is attributable to a contribution of isotopically light CO 2 derived from the aerobic or anaerobic oxidation of organic matter during early stages of diagenesis. No observations were made of biogenic carbonates such as foraminifera or molluscs in the four cores under consideration (in other cores from the Gulf of Mexico, however, we observed those biogenic carbonates). Thus, "vital effects" which result in lower 813C values (around 0%0) are not a likely explanation for the 813C values found for carbonates from cores 3, 20 and 24. Further support of a diagenetic origin of dolomite and coexisting calcite is also given by their usually very similar 8180 values (see below)• In the lower part of core 23 the ~13C values of the bulk carbonate samples decrease systematically with increasing core depth (Table 4). Fig. 3 shows that in the lower part of core 23 the sulfate content in the pore water is reduced• Hence it is likely that the increasing 12C content of the carbonates with increasing core depth is the result of the anaerobic degradation of organic matter during sulfate reduction. Fig. 1 shows that bacterial sulfate reduction processes occur during early stages of diagenesis in organic-rich marine sediments• Here the organic matter is oxidized by anaerobic sulfate-reducing bacteria, resulting in contributions of 12C-rich bio-
genie CO 2 with ~13C values ranging between about -25%o to zero. The latter is typical for "normal" marine bicarbonate in pore water (~13C = 0%0). Carbonate 813C values [9] were observed to be as negative as -21%o (Fig. 1) within the sulfate reduction zone depending on the relative proportions of 12C-rich biogenic CO 2 and normal marine H C O 3. Although it is evident that sulfate reduction occurs in core 23 and, hence, isotopically hght CO 2 is produced by the oxidation of the organic matter, isotopically light CO 2 may also derive from the bacterial oxidation of methane. As bacterial methane is enriched in 12C ( ~ 1 3 C - C H 4 = - 5 0 to -90%o [30]) CO 2 produced by the oxidation of methane will also be 12C-rich early in the oxidation process [31,32]. The carbonates formed from that CO 2 are rich in 12C. Although extremely negative ~13C values have not been measured for TABLE 4 Core 23: carbon and oxygen isotope ratios of bulk carbonates for different reaction times Depth
10 minutes
3 hours
(cm)
813C
8180
813C
8180
813C 8180
(%o)
(%o)
(~oo)
(%o)
(%o)
(~)
-7.3 -12.2 -15.6
-3.6 -1.5 -2.2
-1.8 -4.6 -10.8
-4.8 -3.8 -2.9
-0.5 -2.3 -3.7
-4.1 -3.2 -3.2
640 760 840
3 days
270
carbonates from the cores investigated, it cannot be excluded that small quantities of 12C-rich CO2 from methane oxidation may have been mixed with the pore water bicarbonate, especially because high concentrations of biogenic methane [33] were present in cores 23 and 24. Fig. 3 and Table 3 show increasing differences in 613C of calcite (10-20 minutes reaction time) and dolomite (1-3 days reaction time) with increasing sediment depth. The 613C values of dolomite are virtually constant between 0 and - 1%o down to 640 cm in core 23. In contrast, the 813C values of the calcite are distinctly more negative at that depth (-7.3%o for the 10-minute fraction and -5.5%o for the 20-minute fraction). This shows that dolomite and calcite, although coexisting in the core, did not form cogenetically from the same dissolved bicarbonate. It is likely that dolomite formed earlier than calcite at depths where the dissolved bicarbonate still was similar to the normal marine bicarbonate. This is somewhat different in the lower part of core 23 where also 12C-rich dolomite (13-day gas fraction; 613C = -6.6%o at 840 cm) has been found. However, similar to the situation in the upper part of the core, dolomite is distinctly enriched in 13C relative to the coexisting calcite. In summary, it appears that 12C-rich biogenic C O 2 (•13C-C02 < -25%0) from anaerobic sulfate reduction or C H 4 oxidation was added to the " n o r m a l " marine bicarbonate dissolved in pore water. The pore water bicarbonate therefore was enriched in 12C and first formed dolomite and later, at greater depths,
Sr ppm
500
400
3O0
© o •
200
•
calcite. The difference in isotopic composition of calcite and dolomite also suggests that dolomite did not form by solid state cation exchange from a precursor calcite (at least not from the now coexisting calcite). Calcite and dolomite in core 24 have 613C values near 0%o and the sulfate concentration is uniformly high (except the sample at 600 cm; Fig. 4). As no primary biogenic carbonate was observed (see above) and the 6a80 values of calcite and coexisting dolomite are very similar (but variable down-core) a diagenetic rather than detrital origin of the carbonate of core 24 is indicated. This is a very similar situation to that in the upper part of core 23 where also calcite and dolomite precipitated (using " n o r m a l " marine bicarbonate with only minor contributions of biogenic CO2) with 613C values near 0%o. The strong covariance of the 6180 values of calcite and dolomite indicates that each " p a i r " dolomite and coexisting calcite precipitated from an isotopically characteristic pore water. Although the isotopic compositions of the pore waters have not been determined the large variation in 6180 of the carbonates probably is due to fluctuations in salinity rather than temperature changes (a difference in 6180 by 4-5%o as found in cores 23 and 24 would imply a temperature change of about 15-20 °C). Furthermore the Sr concentration of cores 23 and 24 seem to be correlated with the 6180 values (Fig. 5). This also suggests that salinity plays possibly a significant role (even though the Sr concentrations are rather low as is the carbonate content). However, only a detailed 180 stratigraphy using primary foraminifera would allow to draw final conclusions about primary fluctuations in temperature and salinity of the seawater. Dolomite is not enriched in 180 by around 3%o relative to calcite as was found by Land [15] and McKenzie [16]. However, no final decision can be made whether or not both minerals are in isotopic equilibrium with surrounding pore fluids.
O o• m
°° O
•1
•
O
6. Conclusions
C:
100
-2
-~
-:~
-;
+
6'~ oo,'o°
Fig. 5. Cores 23 and 24: correlation of the Sr concentrations of sediments with the 61~O values of carbonates.
In some organic-rich sediments from the Gulf of Mexico diagenetic dolomite and calcite are formed. Both minerals form in situ from pore water-HCO 3 which derives from two sources: (1)
271
"normal-marine" HCO 3 with ~13C values around 0%0, and (2) biogenic CO 2 (~13C < -25%0) probably from oxidized bacterial methane or from oxidized organic matter during sulfate reduction. The first carbonate generation (dolomite and calcite) formed early from "normal-marine" HCO3- and the second generation (or perhaps more likely further generations) is enriched in 12C which indicates contributions of biogenic C02 during later dolomite and calcite formation. Dolomite formation started when contributions of biogenic CO~ still were negligible and the SO2content of the pore water was not reduced. When S042- reduction o c c u r s l Z c - r i c h biogenic CO 2 is mixed to the normal marine HCO 3 causing a shift towards negative 813C-HC03 values. Diagenetically formed calcite reflects this 12C-rich pore water bicarbonate.
Acknowledgements The quality of this paper was considerably improved by the critical review of the early manuscript by P. Stoffers and W. Stahl which is kindly acknowledged. Helpful remarks were provided by the reviewers. This work was supported by the BMFT grant No. 03E 6237 A at BGR, Hannover, F.R.G.
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