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Gypsum formation in Recent submarine sediments from Kattegat, Denmark
Chemical Geology, 28 (1980) 349--353 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
349
GYPSUM FORMATION IN RECENT SUBMARINE SEDIMENTS FROM KATTEGAT, DENMARK
NIELS OLUF JORGENSEN Institute of Historical Geology and Paleontology, University of Copenhagen, DK-1350 Copenhagen K (Denmark)
(Received July 19, 1979; revised and accepted October 16, 1979)
ABSTRACT J~brgensen, N.O., 1980. Gypsum formation in Recent submarine sediments from Kattegat, Denmark. Chem. Geol., 28: 349--353. Large lenticulate bodies of pure, coarse crystalline gypsum were collected in Recent submarine environment in close association with methane-derived carbonate cements. Sulphur isotope analyses indicate that the gypsum formation is caused by the admission of 34S-enriched formation water from Quaternary methane reservoirs to the site of precipitation. The present example of submarine mineralizations demonstrates that unusual geochemical conditions can arise due to interaction between interstitial solutions and inflowing gaseous hydrocarbons.
INTRODUCTION Large b o d i e s o f g y p s u m w e r e c o l l e c t e d a n d e x a m i n e d in c o n n e c t i o n w i t h studies o f R e c e n t c a r b o n a t e - c e m e n t e d s u b m a r i n e s e d i m e n t s f r o m K a t t e g a t , D e n m a r k (N.O. J~brgensen, 1976). T h e y w e r e e x c a v a t e d during h a r b o u r cons t r u c t i o n s at F r e d e r i k s h a v n in close a s s o c i a t i o n w i t h t h e c a r b o n a t e - c e m e n t e d s e d i m e n t s at t h e s a m e l o c a l i t y , i.e. f r o m 1 - - 3 m b e l o w t h e sea floor. T h e p r e c i p i t a t i o n of c a r b o n a t e c e m e n t s a n d g y p s u m is c o n t r a r y to e x p e c t a t i o n f o r this e n v i r o n m e n t and reflects u n u s u a l g e o c h e m i c a l c o n d i t i o n s . D u e to e x t r e m e negative 5~3C values, it was suggested t h a t t h e c a r b o n a t e c e m e n t s - - a r a g o n i t e a n d m a g n e s i a n calcite - - o r i g i n a t e d f r o m o x i d i z e d m e t h a n e f r o m Q u a t e r n a r y d e p o s i t s (N.O. J~brgensen, 1976). H o w e v e r , t h e p r e c i p i t a t i o n o f s u l p h a t e s n e e d s c o n c e n t r a t i o n s in t h e b r i n e c o n s i d e r a b l y higher t h a n in case o f c a r b o n a t e s . T h e p u r p o s e o f t h e p r e s e n t s t u d y is t o yield s o m e insight i n t o t h e g y p s u m f o r m a t i o n a n d t h e role o f gaseous h y d r o c a r b o n s o n g e o c h e m i s t r y in s e d i m e n t a r y d e p o s i t s . GEOLOGICAL SETTING T h e s e d i m e n t a r y s e q u e n c e at t h e l o c a l i t y includes m a r i n e q u a r t z sands
350
interbedded with subordinate marine silts and clays. The gypsum collected consists of large lenticulate bodies, 12--30 cm in thickness and up to 75 cm in diameter. The lenticulate shape and outline indicate a subparallel orientation to the bedding plane as defined by the adjoining unconsolidated sediments. Mineralogical examination b y use of the petrographic microscope, scanning electron microscope and X-ray diffraction reveals that the slabs almost entirely consist of pure and rather coarsely crystalline gypsum with subordinate intercalations of detrital quartz and clays in the silt/clay fraction (Fig. la--d).
Fig. 1. Scanning electron micrographs of polished and etched gypsum from Frederikshavn Kattegat. a. Laminar gypsum with interlaminations of detrital material (x 300). b. Detail of (a) showing gypsum crystals enveloped by detrital matter (x 2000). c. Coarse-granular interlocking gypsum crystals (x 200). d. Palisade-like gypsum crystals. Note the sharp contact to the laminar gypsum below (x 75).
351
Three distinguishable textures occur in the material studied: (1) a subordinate texture of laminar gypsum crystals with diameters ranging from 25 to 50 #m, intermixed with a considerable amount of discrete detrital matter (Fig. l a , b); (2) a coarse-granular mosaic of interlocking gypsum crystals with diameters ranging from 100 to 300 #m (Fig. lc); and (3) a coarse palisadelike texture where the longest axes of the crystals range from approximately 300 to 800 #m (Fig. ld). The orientation of the palisade-like crystals is perpendicular to the lamination of the gypsum bodies. The contacts between the different textures are generally sharp and they are frequently separated b y thin detrital interlaminations. In addition to the large bodies, gypsum has been traced, using X-ray diffraction, in very small amounts in samples of carbonate-cemented sediments from several submarine localities in the Kattegat. However, it has not been possible visually to observe these discrete gypsum occurrences. SULPHUR ISOTOPE ANALYSES
In order to elucidate the formation conditions of the gypsum an investigation of the 34S/32S isotope composition was carried out. The values obtained are quoted as 634S vs. the Canyon Diablo Meteorite Troilite and the results are shown in Fig. 2. The data reveal that the sulphate is heavily enriched in ~S, ranging within +27 to +33%0 63~S. The values are significantly above +20%0 5 ~S, the characteristic value for sulphate precipitated from normal seawater. PRESENT SEA WATER 10. Z III
. . . . .
0
I.Ll
~ililii~iii!iiiiiiiiiiii~i~!iJ ~iiiiiiiiiiiiii!i!i!!iiiiiiil i i i i i i i i i!i i i i i i i i i i i i !~!il iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
5_
1_
~! iiiiiiiiiiiiiiiiiiii~iiiiiiiiiii [i!i!!ii!%iiiiiiiiiiiiiii!iiiiiii!! : = = : : :
Ili~?i~i~iiiii~iiiiiiiiiii~!iii~i!ii!iiiiii~~i!~iiiiill riIi~~;iiii!ii~:iii~iiii~iii~ii~ii!i~ii~iiiiiiii!liiiiiiiii!t ! liii~:i~i~i~i!i~ii~'"iiiii!t2'O ~
J7
2
1 3~-]33 "b34S°/o0
Fig. 2. Diagram showing the a348 values obtained from the studied gypsum bodies collected at Frederikshavn, Denmark. I N T E R P R E T A T I O N AND DISCUSSION
The gypsum bodies have an appearance like concretions rather than interstitial cement. The presence of discrete detrital material within the lamina gypsum indicates the primary origin of this fabric. Thus the coarse-crystalline
352 textures and the detrital interlaminations most likely reflect subsequent diagenetic alterations, which have probably also generated the concretionary habit of the occurrences (cf. Schreiber, 1978). The precipitation of sulphates needs considerable increase of concentrations in the brine, ca. 5 × that of normal seawater. A supersaturation with respect to gypsum b y increasing salinity in the brine due to evaporation is certainly inconceivable in the present case. Furthermore, the isotope data make it evident that the gypsum formation cannot be explained by direct precipitation from seawater, since it is generally accepted that the isotopic fractionation between evaporites and the marine sulphate reservoir is almost negligible (Holser and Kaplan, 1966; Schidlowski et al., 1977). Alternatively, an extraordinary conveyance of SO4 ~- to the site of precipitation from an outside source is required. A number of different environments are known to possess sulphates enriched in 34S. Metabolic processes, predominantly by sulphate reducing bacteria, cause severe fractionation of the end products of reduced sulphur and are depleted with respect to 34S (McCready, 1975). Hereby the remaining reactant becomes enriched in 34S, which generally implies an increasing content of 34S with depth in the subsurface sediments (Hartmann and Nielsen, 1969; Claypool and Kaplan, 1974). However, it is very unlikely that this residuum of heavy SO42- will gain a concentration necessary for gypsum precipi tation. A vast a m o u n t of heavy sulphur is held in Palaeozoic evaporitic sulphate (Holser, 1977). But the geographical location of the present site, close to the Fennoscandian borderzone, is far from the known salt dome structures in the axial part of the Danish basin and therefore excludes the possibility of any conveyance of dissolved heavy sulphate from this source. Another important source of isotopically heavy SO42- is found in the formation waters of shallow oil and gas reservoirs (Krouse, 1977). At low temperatures, i.e. below 60 ° C, the SO42- in the formation water is enriched in 34S by as much as 20%0 when compared to that of the postulated original seawater. The geochemistry in the studied sequence is definitely affected by methane originating from shallow reservoirs, i.e. Quaternary deposits. An example of reaction between methane and the interstitial solution is the formation of a considerable amount of carbonate cement (N.O. J~rgensen, 1976). It is therefore reasonable to suggest that discharge of methane at the sea floor (which is known from several locations in Kattegat) (see K.D. J~rgensen, 1945) is accompanied b y an outflow of formation waters rich in SO42- and will thus increase the sulphate concentration considerably in local parts of the sediment column. The sedimentological record of permeable sands intermixed with less permeable silts and clays is probably responsible for the relatively closed system which would be a precondition to gypsum precipitation. The exact chemical composition of the interstitial solution in the subsurface sediments under these circumstances is largely speculative,
353 b u t it is evident t h a t s u p e r s a t u r a t i o n w i t h r e s p e c t t o g y p s u m t o o k place. The present example of submarine sediments containing mineralization o f g y p s u m and c a r b o n a t e s clearly d e m o n s t r a t e s t h a t u n u s u a l g e o c h e m i c a l c o n d i t i o n s can arise d u e t o i n t e r a c t i o n b e t w e e n an interstitial s o l u t i o n and inflowing gaseous h y d r o c a r b o n s . ACKNOWLEDGEMENT T h e a u t h o r is i n d e p t e d t o H. E g e l u n d w h o p r e p a r e d t h e graph and t o J. Bailey w h o k i n d l y i m p r o v e d t h e English text. This research was s u p p o r t e d by t h e Danish Natural Science Council. REFERENCES Claypool, G.E. and Kaplan, I.R., 1974. The origin and distribution of methane in marine sediments. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments, Plenum, New York, N.Y., pp. 99--139. Hartmann, M. and Nielsen, H., 1969. 634S-Werte in rezenten Meeressedimenten and ihre Deutung am Beispiel einiger Sedimentprofile aus der Westlichen Ostsee. Geol. Rundsch., 58: 621--655. Holser, W.T., 1977. Catastrophic chemical events in the history of the ocean. Nature (London), 267 : 403--408. Holser, W.T. and Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulphates. Chem. Geol. 1: 93--135. J~$rgensen, K.D., 1945. Frederikshavn--Strandby gasomr~dets geologi. Nat. Verden, 29: 11--32. J~rgensen, N.O., 1976. Recent high magnesian calcite/aragonite cementation of beach and submarine sediments from Denmark. J. Sediment. Petrol., 46: 940--951. Krouse, H.R., 1977. Sulfur isotope studies and their role in petroleum exploration. J. Geochem. Explor., 7: 189--211. McCready, R.G.L., 1975. Sulfur isotope fractionation by Desulfovibrio and Desulfotomaculum species. Geochim. Cosmochim. Acta, 39: 1395--1401. Schidlowski, M., Junge, C.E. and Pietrek, H., 1977. Sulfur isotope variations in marine sulfate evaporites and the Phanerozoic oxygen budget. J. Geophys. Res., 82: 2557-2565. Schreiber, B.C., 1978. Environments of subaqueous gypsum deposition. In: N.E. Dean and B.C. Schreiber (Editors) Marine Evaporites, Soc. Econ. Paleontol. Mineral., Short Course No. 4, Oklahoma City, Okla., 1978, pp. 43--73.
349
GYPSUM FORMATION IN RECENT SUBMARINE SEDIMENTS FROM KATTEGAT, DENMARK
NIELS OLUF JORGENSEN Institute of Historical Geology and Paleontology, University of Copenhagen, DK-1350 Copenhagen K (Denmark)
(Received July 19, 1979; revised and accepted October 16, 1979)
ABSTRACT J~brgensen, N.O., 1980. Gypsum formation in Recent submarine sediments from Kattegat, Denmark. Chem. Geol., 28: 349--353. Large lenticulate bodies of pure, coarse crystalline gypsum were collected in Recent submarine environment in close association with methane-derived carbonate cements. Sulphur isotope analyses indicate that the gypsum formation is caused by the admission of 34S-enriched formation water from Quaternary methane reservoirs to the site of precipitation. The present example of submarine mineralizations demonstrates that unusual geochemical conditions can arise due to interaction between interstitial solutions and inflowing gaseous hydrocarbons.
INTRODUCTION Large b o d i e s o f g y p s u m w e r e c o l l e c t e d a n d e x a m i n e d in c o n n e c t i o n w i t h studies o f R e c e n t c a r b o n a t e - c e m e n t e d s u b m a r i n e s e d i m e n t s f r o m K a t t e g a t , D e n m a r k (N.O. J~brgensen, 1976). T h e y w e r e e x c a v a t e d during h a r b o u r cons t r u c t i o n s at F r e d e r i k s h a v n in close a s s o c i a t i o n w i t h t h e c a r b o n a t e - c e m e n t e d s e d i m e n t s at t h e s a m e l o c a l i t y , i.e. f r o m 1 - - 3 m b e l o w t h e sea floor. T h e p r e c i p i t a t i o n of c a r b o n a t e c e m e n t s a n d g y p s u m is c o n t r a r y to e x p e c t a t i o n f o r this e n v i r o n m e n t and reflects u n u s u a l g e o c h e m i c a l c o n d i t i o n s . D u e to e x t r e m e negative 5~3C values, it was suggested t h a t t h e c a r b o n a t e c e m e n t s - - a r a g o n i t e a n d m a g n e s i a n calcite - - o r i g i n a t e d f r o m o x i d i z e d m e t h a n e f r o m Q u a t e r n a r y d e p o s i t s (N.O. J~brgensen, 1976). H o w e v e r , t h e p r e c i p i t a t i o n o f s u l p h a t e s n e e d s c o n c e n t r a t i o n s in t h e b r i n e c o n s i d e r a b l y higher t h a n in case o f c a r b o n a t e s . T h e p u r p o s e o f t h e p r e s e n t s t u d y is t o yield s o m e insight i n t o t h e g y p s u m f o r m a t i o n a n d t h e role o f gaseous h y d r o c a r b o n s o n g e o c h e m i s t r y in s e d i m e n t a r y d e p o s i t s . GEOLOGICAL SETTING T h e s e d i m e n t a r y s e q u e n c e at t h e l o c a l i t y includes m a r i n e q u a r t z sands
350
interbedded with subordinate marine silts and clays. The gypsum collected consists of large lenticulate bodies, 12--30 cm in thickness and up to 75 cm in diameter. The lenticulate shape and outline indicate a subparallel orientation to the bedding plane as defined by the adjoining unconsolidated sediments. Mineralogical examination b y use of the petrographic microscope, scanning electron microscope and X-ray diffraction reveals that the slabs almost entirely consist of pure and rather coarsely crystalline gypsum with subordinate intercalations of detrital quartz and clays in the silt/clay fraction (Fig. la--d).
Fig. 1. Scanning electron micrographs of polished and etched gypsum from Frederikshavn Kattegat. a. Laminar gypsum with interlaminations of detrital material (x 300). b. Detail of (a) showing gypsum crystals enveloped by detrital matter (x 2000). c. Coarse-granular interlocking gypsum crystals (x 200). d. Palisade-like gypsum crystals. Note the sharp contact to the laminar gypsum below (x 75).
351
Three distinguishable textures occur in the material studied: (1) a subordinate texture of laminar gypsum crystals with diameters ranging from 25 to 50 #m, intermixed with a considerable amount of discrete detrital matter (Fig. l a , b); (2) a coarse-granular mosaic of interlocking gypsum crystals with diameters ranging from 100 to 300 #m (Fig. lc); and (3) a coarse palisadelike texture where the longest axes of the crystals range from approximately 300 to 800 #m (Fig. ld). The orientation of the palisade-like crystals is perpendicular to the lamination of the gypsum bodies. The contacts between the different textures are generally sharp and they are frequently separated b y thin detrital interlaminations. In addition to the large bodies, gypsum has been traced, using X-ray diffraction, in very small amounts in samples of carbonate-cemented sediments from several submarine localities in the Kattegat. However, it has not been possible visually to observe these discrete gypsum occurrences. SULPHUR ISOTOPE ANALYSES
In order to elucidate the formation conditions of the gypsum an investigation of the 34S/32S isotope composition was carried out. The values obtained are quoted as 634S vs. the Canyon Diablo Meteorite Troilite and the results are shown in Fig. 2. The data reveal that the sulphate is heavily enriched in ~S, ranging within +27 to +33%0 63~S. The values are significantly above +20%0 5 ~S, the characteristic value for sulphate precipitated from normal seawater. PRESENT SEA WATER 10. Z III
. . . . .
0
I.Ll
~ililii~iii!iiiiiiiiiiii~i~!iJ ~iiiiiiiiiiiiii!i!i!!iiiiiiil i i i i i i i i i!i i i i i i i i i i i i !~!il iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
5_
1_
~! iiiiiiiiiiiiiiiiiiii~iiiiiiiiiii [i!i!!ii!%iiiiiiiiiiiiiii!iiiiiii!! : = = : : :
Ili~?i~i~iiiii~iiiiiiiiiii~!iii~i!ii!iiiiii~~i!~iiiiill riIi~~;iiii!ii~:iii~iiii~iii~ii~ii!i~ii~iiiiiiii!liiiiiiiii!t ! liii~:i~i~i~i!i~ii~'"iiiii!t2'O ~
J7
2
1 3~-]33 "b34S°/o0
Fig. 2. Diagram showing the a348 values obtained from the studied gypsum bodies collected at Frederikshavn, Denmark. I N T E R P R E T A T I O N AND DISCUSSION
The gypsum bodies have an appearance like concretions rather than interstitial cement. The presence of discrete detrital material within the lamina gypsum indicates the primary origin of this fabric. Thus the coarse-crystalline
352 textures and the detrital interlaminations most likely reflect subsequent diagenetic alterations, which have probably also generated the concretionary habit of the occurrences (cf. Schreiber, 1978). The precipitation of sulphates needs considerable increase of concentrations in the brine, ca. 5 × that of normal seawater. A supersaturation with respect to gypsum b y increasing salinity in the brine due to evaporation is certainly inconceivable in the present case. Furthermore, the isotope data make it evident that the gypsum formation cannot be explained by direct precipitation from seawater, since it is generally accepted that the isotopic fractionation between evaporites and the marine sulphate reservoir is almost negligible (Holser and Kaplan, 1966; Schidlowski et al., 1977). Alternatively, an extraordinary conveyance of SO4 ~- to the site of precipitation from an outside source is required. A number of different environments are known to possess sulphates enriched in 34S. Metabolic processes, predominantly by sulphate reducing bacteria, cause severe fractionation of the end products of reduced sulphur and are depleted with respect to 34S (McCready, 1975). Hereby the remaining reactant becomes enriched in 34S, which generally implies an increasing content of 34S with depth in the subsurface sediments (Hartmann and Nielsen, 1969; Claypool and Kaplan, 1974). However, it is very unlikely that this residuum of heavy SO42- will gain a concentration necessary for gypsum precipi tation. A vast a m o u n t of heavy sulphur is held in Palaeozoic evaporitic sulphate (Holser, 1977). But the geographical location of the present site, close to the Fennoscandian borderzone, is far from the known salt dome structures in the axial part of the Danish basin and therefore excludes the possibility of any conveyance of dissolved heavy sulphate from this source. Another important source of isotopically heavy SO42- is found in the formation waters of shallow oil and gas reservoirs (Krouse, 1977). At low temperatures, i.e. below 60 ° C, the SO42- in the formation water is enriched in 34S by as much as 20%0 when compared to that of the postulated original seawater. The geochemistry in the studied sequence is definitely affected by methane originating from shallow reservoirs, i.e. Quaternary deposits. An example of reaction between methane and the interstitial solution is the formation of a considerable amount of carbonate cement (N.O. J~rgensen, 1976). It is therefore reasonable to suggest that discharge of methane at the sea floor (which is known from several locations in Kattegat) (see K.D. J~rgensen, 1945) is accompanied b y an outflow of formation waters rich in SO42- and will thus increase the sulphate concentration considerably in local parts of the sediment column. The sedimentological record of permeable sands intermixed with less permeable silts and clays is probably responsible for the relatively closed system which would be a precondition to gypsum precipitation. The exact chemical composition of the interstitial solution in the subsurface sediments under these circumstances is largely speculative,
353 b u t it is evident t h a t s u p e r s a t u r a t i o n w i t h r e s p e c t t o g y p s u m t o o k place. The present example of submarine sediments containing mineralization o f g y p s u m and c a r b o n a t e s clearly d e m o n s t r a t e s t h a t u n u s u a l g e o c h e m i c a l c o n d i t i o n s can arise d u e t o i n t e r a c t i o n b e t w e e n an interstitial s o l u t i o n and inflowing gaseous h y d r o c a r b o n s . ACKNOWLEDGEMENT T h e a u t h o r is i n d e p t e d t o H. E g e l u n d w h o p r e p a r e d t h e graph and t o J. Bailey w h o k i n d l y i m p r o v e d t h e English text. This research was s u p p o r t e d by t h e Danish Natural Science Council. REFERENCES Claypool, G.E. and Kaplan, I.R., 1974. The origin and distribution of methane in marine sediments. In: I.R. Kaplan (Editor), Natural Gases in Marine Sediments, Plenum, New York, N.Y., pp. 99--139. Hartmann, M. and Nielsen, H., 1969. 634S-Werte in rezenten Meeressedimenten and ihre Deutung am Beispiel einiger Sedimentprofile aus der Westlichen Ostsee. Geol. Rundsch., 58: 621--655. Holser, W.T., 1977. Catastrophic chemical events in the history of the ocean. Nature (London), 267 : 403--408. Holser, W.T. and Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulphates. Chem. Geol. 1: 93--135. J~$rgensen, K.D., 1945. Frederikshavn--Strandby gasomr~dets geologi. Nat. Verden, 29: 11--32. J~rgensen, N.O., 1976. Recent high magnesian calcite/aragonite cementation of beach and submarine sediments from Denmark. J. Sediment. Petrol., 46: 940--951. Krouse, H.R., 1977. Sulfur isotope studies and their role in petroleum exploration. J. Geochem. Explor., 7: 189--211. McCready, R.G.L., 1975. Sulfur isotope fractionation by Desulfovibrio and Desulfotomaculum species. Geochim. Cosmochim. Acta, 39: 1395--1401. Schidlowski, M., Junge, C.E. and Pietrek, H., 1977. Sulfur isotope variations in marine sulfate evaporites and the Phanerozoic oxygen budget. J. Geophys. Res., 82: 2557-2565. Schreiber, B.C., 1978. Environments of subaqueous gypsum deposition. In: N.E. Dean and B.C. Schreiber (Editors) Marine Evaporites, Soc. Econ. Paleontol. Mineral., Short Course No. 4, Oklahoma City, Okla., 1978, pp. 43--73.