Loss of calcareous microfossils from sediments through gypsum formation

Marine Geology, 36 (1980) M35--M44

M35

Elsevier Scientific Publishing Company, Amsterdam - - P r i n t e d in The Netherlands

Letter Section LOSS OF CALCAREOUS MICROFOSSILS FROM SEDIMENTS THROUGH GYPSUM FORMATION D. SCHNITKER', L.M. MAYER', and S. NORTON 2

'Department of Oceanography, University of Maine at Orono, Walpole, Me. 04573

(U.S.A.) ~Department of Geological Sciences, University of Maine at Orono, Orono, Me. 04469 (U.S.A.) (Accepted April 2, 1980)

ABSTRACT

Schnitker, D., Mayer, L.M. and Norton, S., 1980. Loss of calcareous microfossils from sediments through gypsum formation. Mar. Geol., 36: M35--M44. Laboratory experiments with fresh marine sediments demonstrate the dissolution of calcareous microfossils (foraminifers) and the production of "authigenic" gypsum. Slow oxidation of iron sulfide in the presence of oxygen drives the following reactions: (1) 4FeS + 902 + 6H20 = 4FeO(OH) + 4SO:`- + 8H + or 2FeS~ + 7'/202 + 5H~O = 2FeO(OH) + 4SO:`- + 8tC (2) H + + C a C O 3 = Ca ~÷ + H C O ~ (3) Ca 2÷ + SO:,- + 2 H 2 0 = C a S O , - 2 H 2 0 Three-month experiments resulted in minor to total dissolution of different species from estuarine and open ocean sediments. Elphidium ustilatum was the most resistant to dissolution whereas Elphidium subarcticum was the firstspecies to disappear completely. Agglutinated foraminifers are nearly completely destroyed largely because of bacterial decay of the test binding agent. Loss or partial loss of species m a y occur either in conditions of storage after collection or because of changing environmental conditions (primarily oxygen levels)at the sediment-water interface.

INTRODUCTION

Care is usually taken to sample sediment cores for ephemeral substances (e.g. CH4) or properties (e.g. color) immediately after recovery or upon their arrival in the laboratory. However, the manner in which a core is treated and stored after recovery is of great significance for properties that are not normally considered to be ephemeral. For example, Geyh et al. (1974) noted that 14C dating of cores became unreliable after extended cool and moist storage; bacterial activity introduced " m o d e r n " carbon into the cores. We have found that under certain conditions calcareous microfossils, which usually are n o t considered an ephemeral portion of a sediment sample, m a y 0025-3227/80/0000-0000/$02.25 © Elsevier Scientific Publishing Company

M36 suffer heavy losses or disappear completely. If this process is not recognized by the paleontologist, such selectively destroyed fauna or the total lack of a fauna will lead to erroneous interpretations. In a recent study of nearshore and estuarine benthic foraminifers, some sediment samples collected in the field were stored in closed plastic containers or screw-capped glass vials. Analysis of the microfossils of these samples t o o k place two or three years after collection, by which time many samples had dried because of poor seals of container lids or vial caps. The resulting data were inexplicable. Absolute abundances of individuals and relative abundance of species did n o t present coherent patterns; barren areas occurred next to areas of rich faunal contents; species distribution appeared to be arbitrary. Resampling of supposedly barren areas provided rich and diverse calcareous foraminiferal faunas, arousing suspicion concerning the fate of foraminifers during sample storage. Examination of the dried samples revealed gypsum crystals, whereas no gypsum could be found in samples that had not dried prior to washing on a 125-~m mesh sieve. Consequently, we postulated that the partial or total dissolution of the calcareous foraminifers occurred in the following manner: Many coastal sediments contain reduced sulfur in the form of free or iron sulfides. Oxidation of these sulfides released dissolved sulfate and caused a lowering of pH. (1) 4FeS + 902 + 6HzO = 4 FeO(OH) + 4SO~- + 8H ÷ Calcareous microfossils dissolved in the more acidic interstitial waters, releasing calcium and bicarbonate. (2) H÷+ CaCO3 = Ca2+ + HCO~ The elevatedlevels of both calcium and sulfate led to a supersaturation and precipitation of gypsum. (3) Ca:+ + SO~- + 2H:O = CaSO4"2H:O The desiccation of the samples promoted the supersaturation of gypsum. The fact that the undried samples did not develop gypsumis probably caused by the unavailabilityof O2 for sulfide oxidation. Desiccation by itself would not produce significantamounts of gypsum. This mechanism for gypsum formation in cores of deep-sea sediments was suggested by Arrhenius (1963). EXPERIMENTAL OBSERVATIONS This hypothesis was tested on material from two short (%0.5 m) sediment cores, one from Johns Bay, an open Gulf of Maine environment, the other from Clark's Cove, Maine, an estuarine setting. Triplicate samples of roughly equal volume were taken from the top, middle and b o t t o m of each core for chemical and microfaunal analysis. One set of samples was stored overnight under an inert atmosphere, prior to analysis of the acid-volatile sulfide contents. This analysis was performed by purging the HCl-acidified sediment with N2 into an AgNO3 solution. The resulting Ag2S precipitate was then collected and weighed. The core splitting was n o t carried o u t under anaerobic conditions; thus the sulfide contents

M37 TABLE I

Acid-volatile sulfur contents o f G u l f o f Maine and estuarine sediment samples F o r a m i n i f e r a l individuals lost f r o m o x i d i z e d samples, r e f e r e n c e d t o a n equally sized unoxidized sample Sample

Individuals ppm S

lost

Individuals %loss

Gulf of Maine T o p o f core Middle o f core B o t t o m o f core Average

131 21 123

271 573 460

92

ppm S

lost

% loss

31 114 110

49 35 48

Estuary 51 69 63

159 171 14.5

6~1

125

44

determined represent minimum values only of either free dissolved sulfide or labile iron monosulfides (JCrgensen and Fenchel, 1974}. The second set of samples was washed immediately with tap water on a 125-gm mesh sieve; the coarse fraction was air dried. The third set of samples was placed in plastic containers and allowed to dry slowly for three months at room temperature. The principal results of these experiments are presented in Table I. The estuarine sediments contained, on the average, more acid-volatile sulfur than the Gulf of Maine sediments. This result was expected, because the estuarine muds were nearly black in color and exuded a much stronger H2S odor than did the olive-grey colored offshore muds. However, within~core variability of sulfide content is high. Assuming that the immediately washed samples did not suffer significant dissolution, their faunal contents were taken as representative of the true faunal assemblage; the difference between them and the faunal contents of the slowly dried samples (after compensating for sample size differences) was ascribed to dissolution. Lack of relationship between the amount of dissolution and the amount of acid-volatile sulfur within the samples may have been caused by unequal drying histories, or the proportionally greater loss of foraminifers from the offshore samples is due to the greater abundance there of species that are very susceptible to dissolution. None of the samples that were washed immediately and air dried contained gypsum crystals, b u t all of the slowly dried samples did. Quantitative assessment of gypsum crystals proved impractical. Most crystals were very small and did not separate well from the sediment matrix; maximum length was 1.6 mm (Plate I). DIFFERENTIAL DISSOLUTION

A partially dissolved foraminiferal fauna may still be of use for ecological-paleoecological interpretations but only if dissolution has affected all species

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equally and has thus not altered the proportional composition of the fauna. Ruddiman and Heezen (1967) and Berger (1968) have shown that species of planktonic foraminifers display a wide range of susceptibility and that a fauna of planktonic foraminifers changes its proportional composition as dissolution progresses. Such faunas are difficult, if not impossible, to interpret for their paleoecologic meaning. Our experiment tests the dissolution susceptibility of several common, cold shallow-water species of benthic foraminifers. Fig. 1 shows the frequency distribution of foraminiferal species in a Gulf of Maine sample. Superimposed upon the specimen counts from the unaffected sample are the counts from the oxidized and partially dissolved sample. All species suffered dissolution, but some species were affected more severely than others. In Fig. 2 the species are ranked according to dissolution susceptibility (= % loss) with an indication of the number of specimens involved. Table II shows that the dissolution loss of individual species has not been uniform in all three samples. The species are here arranged in increasing order of their average dissolution susceptibility. 140 130 120 I10 I00 90 ¢n 80 z '~E " 70 60 LU a. ¢n 50

--A

40 30 20 I0

I

2

3

4

5

6

7

8

9

I0

II

12 13 14

15

Fig. 1. Frequency distribution o f f o r a m i n i f e r a l species in a sample f r o m the G u l f o f Maine. undissolved sample; f r o n t row - - species c o u n t s o f o x i d i z e d sample. Asterisk d e n o t e s agglutinated species:

B a c k r o w - - species c o u n t s o f

1 = G l o b o b u l i m i n a auriculata; 2 = Cibicides lobatulus; 3 = Islandiella islandica; 4 = E l p h i d i u m u s t i l a t u m ; 5 = E l p h i d i u m a l b i u m b i l i c a t u m ; 6 = *EggereUa s c a b r a ; 7 = * R e o p h a x a t l a n tica; 8 = * T r o c h a m m i n a o c h r a c e a ; 9 = B u c c e l l a f r i g i d a ; 1 0 = E l p h i d i u m c l a v a t u m ; 11 = C r y p t o e l p h i d i e U a itriaensis; 12 = * A d e r c o t r y m a g l o m e r a t u m ; 1 3 = C a s s i d u l i n a crassa; 14 = N o n i o n l a b r a d o r i c u m ; 1 5 -- F u r s e n k o i n a f u s i f o r m i s .

M39

I00 r %

9° f8o g To

*

*

6° r o

30~-

,o

I .... I

2

3

4

5

6

7

8

I 9

I0

Fig. 2. D i s s o l u t i o n loss o f t h e t e n m o s t a b u n d a n t f o r a m i n i f e r a l species f r o m t h e G u l f o f Maine s a m p l e d u r i n g slow o x i d a t i o n in t h e l a b o r a t o r y . A s t e r i s k d e n o t e s a g g l u t i n a t e d species: 1 = Islandiella islandica; 2 = Globobulimina auriculata; 3 = Cibicides lobatulus; 4 = E l p h i d i u m ustilatum; 5 = E l p h i d i u m clavatum; 6 = Buccella frigida; 7 = E l p h i d i u m albiumbilicatum; 8 - * R e o p h a x atlantica; 9 = *Eggerella scabra; 10 = *Trochammina ochracea. T A B L E II D i s s o l u t i o n loss o f t h e n i n e m o s t c o m m o n species o f calcareous f o r a m i n i f e r s , in p e r c e n t o f t h e originally p r e s e n t individuals - - G u l f o f Maine core samples Species

Top

Middle

Bottom

Mean

Islandiella islandica Buccella frigida G l o b o b u l i m i n a auriculata E l p h i d i u m ustilatum E l p h i d i u m albiumbilicatum Cibicides lobatulus EIphidium clavatum E l p h i d i u m subarcticum N o n i o n labradoricum

7 22 26 -0 73 79 86 77

35 74 39 59 80 54 64 100 88

59 14 68 32 77 62 48 25 88

34 37 44 45 52 63 64 70 84

The estuarine sediments, as shown in Fig. 3, contain fewer individuals and species, but here t o o dissolution is severe. Comparing the dissolution susceptibility of these species in the estuarine samples {Table III) with that of the Gulf of Maine samples {Table II), it is notable that the relative position of the various species of Elphidium has remained unchanged. Elphidium ustilatum, a thick-walled, robust species with a heavy umbilical plug is the most resistant to dissolution, whereas Elphidium subarcticum, a thin-walled, fragile species with a finely papillate surface is the first species to disappear completely. Buccelta frigida, fairly resistant to dissolution in the offshore samples, apparently is nearly twice as vulnerable in the estuarine sediments. However, the low ranking of Buccella frigida in the estuarine samples may in part be an artifact of its rare occurrence in estuaries.

M40 IO0

80 70 60 50

i

40 30 20 I0

3

4

5

6

?

8

Fig. 3. Frequency distribution of foraminiferal species in a sample from Clark's Cove, Damariscotta River estuary, Maine. Back r o w - - species counts of undissolved sample, f r o n t r o w - - s p e c i e s counts of oxidized sample. Asterisk denotes agglutinated species: 1 = Elphidiurn u s t i l a t u m ; 2 = *Eggerella scabra; 3 = E l p h i d i u m c l a v a t u m ; 4 = E l p h i d i u m albiurnbilicatum; 5 = Cibicides lobatulus; 6 = Buccella frigida; 7 = * T r o c h a m m i n a ochracea; 8 = Elphidium subarcticum.

TABLE III Dissolution loss of the six most common species of calcareous foraminifers, in percent of the originally present individuals -- Estuarine core samples Species

Top

Middle

Bottom

Average

Elphidium ustilatum Elphidium albiumbilicatum Elphidium clavatum Buccella frigida Cibicides lobatulu$ Elphidium subarcticum

41 88 56 -100 100

31 44 41 70 85 --

39 0 52 54 60 100

37 44 50 62 82 100

T h e greater loss i n c u r r e d b y t h e o f f s h o r e s a m p l e s in c o m p a r i s o n t o the e s t u a r i n e s a m p l e s is d u e p r i m a r i l y t o t h e f a c t t h a t o f f s h o r e t h e n u m e r i c a l l y d o m i n a n t species such as G l o b o b u l i m i n a a u r i c u l a t a a n d C i b i c i d e s l o b a t u l u s are relatively d i s s o l u t i o n p r o n e . In t h e e s t u a r i n e s a m p l e s t h e m o s t resistant species, E l p h i d i u m u s t i l a t u m , is invariably t h e d o m i n a n t species. EFFECTS OF DISSOLUTION M o s t u n a l t e r e d c a l c a r e o u s f o r a m i n i f e r s e n c o u n t e r e d in this s t u d y h a v e a s m o o t h or even glossy s u r f a c e . T h e tests m a y b e finely or c o a r s e l y p e r f o r a t e d a n d , as is t h e case f o r several species o f E l p h i d i u m a n d f o r B u c c e l l a f r i g i d a , be c o v e r e d in c e r t a i n areas b y pustules. Nearly all s p e c i m e n s in u n a f f e c t e d s a m p l e s are i n t a c t . T h e first sign o f d i s s o l u t i o n is t h e d i s a p p e a r a n c e o f t h e

M41

glossy shine. At first, surfaces become dull, then white. Hyaline calcite walls become opaque. The next stage of dissolution results in enlarged pores, a frosted appearance of test surfaces and for several species, notably Nonion labradoricum, breakage and/or detachment of terminal chambers. Further dissolution causes much thinning of the test walls, perhaps largely by peeling off the layers of multilamellar chamber walls. At this stage most species become very susceptible to breakage by biological or other mechanical processes. Loosely constructed and thin-walled species, such as Globobulimina auriculata or Elphidium subarcticum rarely persist b e y o n d this stage. Thick-walled, robust or compactly constructed species, such as Islandiella islandica and Elphidium ustilatum also lose their outer chambers, but can still be recognized, often as veritable hulks, in strongly dissolved samples. Many of these dissolution effects are illustrated on Plate I. Some syndepositional dissolution apparently occurred on the seafloor. All of the quickly washed samples had a few foraminifers that displayed various stages of dissolution. AGGLUTINATED FORAMINIFERS

An unexpected result of this experiment is the almost total disappearance of agglutinated foraminifers from the oxidized samples (Figs. 1 and 3). Because these species are resistant to weak acids, a second process to which these specimens succumb must operate in the slowly oxidizing samples. This process may be bacterial decay of the organic binding agent of the agglutinated foraminifers. A test on a sample which had been preserved in an unbuffered formaldehyde solution showed that after an oxidation period of a b o u t t w o months the agglutinated faunas had suffered a loss of only 32% compared to the (biologically) unpreserved samples where the loss was nearly complete. DISCUSSION

Gypsum has been noted by several workers as an apparently authigenic mineral in a variety of non-evaporitic sediments, c o m m o n l y in association with reducing conditions (Briskin and Schreiber, 1978; Siesser and Rogers, 1976; Criddle, 1974). These writers have postulated gypsum formation to occur via the reaction of calcium dissolved from calcareous skeletons (with CaCO3 undersaturation induced by either intrusion of colder b o t t o m waters or low pH resulting from bacterial anaerobiosis) with sulfate derived by diffusion from overlying waters. On the basis of our core storage studies we suggest a modification to these models. Gypsum is undersaturated in seawater by a factor of a b o u t 4 to 5. Increase in both calcium and sulfate is necessary to achieve saturation in situ, as calcium concentrations in seawater only triple at pH = 6.2 (a pH lower than is likely to be encountered in carbonate sediments) and they increase even less with temperature drops of 5 ° to 10°C (see Edmonds and Gieskes, 1970). The

M43

only plausible source for increased sulfate is oxidation of accumulated iron sulfides, which will also serve as a source of acidity to dissolve further CaCO3, particularly in localized microenvironments. The resultant enhanced levels of both calcium and sulfate will lead to supersaturation and precipitation of gypsum. Thus, we propose that gypsum may form in deep-sea sediments as a result of iron sulfide accumulation in carbonate-containing sediments followed by an intrusion of oxygen into the sediments, perhaps as a result of bioturbation or replacement of stagnant b o t t o m waters by oxygenated waters. That pyrite is often found in association with gypsum (Criddle, 1974; Siesser and Rogers, 1976) supports this model; pyrite formation requires an intermediate oxidation of sulfide to native sulfur (Berner, 1970) and is thus suggestive of an oxygen intrusion. An analogous mechanism may well operate in shallow sediments as a result of either bioturbation or seasonality of redox cycles. Diagenetic gypsum formation may therefore be a fairly c o m m o n occurrence, albeit ephemeral. CONCLUSIONS

Considerable dissolution of calcareous microfossils in sulfide-containing sediments occurs when these are exposed to oxygen. This observation is of importance to micropaleontologists and curators of core collections. (1) The practice of storing sediment cores at low temperatures and high h u m i d i t y may be very detrimental to the preservation of calcareous microfossils. (2) Sediment samples should be exposed to oxygen as briefly as possible and/or be processed as quickly as possible.

PLATE I 1. E u h e d r a l g y p s u m crystal f r o m o x i d i z e d s a m p l e ; m a x . l e n g t h 0.9 m m . 2. Detail o f g y p s u m crystal f r o m o x i d i z e d s a m p l e ; h o r i z o n t a l field o f view 60 urn. 3. Elphidium clavatum f r o m freshly w a s h e d s a m p l e , w i t h o u t n o t i c e a b l e signs o f dissolut i o n ; m a x . l e n g t h 0.6 m m . 4. Elphidium clavatum f r o m o x i d i z e d s a m p l e ; d i s s o l u t i o n a n d b r e a k a g e has r e m o v e d a b o u t o n e w h o r l o f c h a m b e r s ; m a x . l e n g t h 0.5 ram. 5. Elphidium clavatum f r o m o x i d i z e d s a m p l e , d i s s o l u t i o n a n d b r e a k a g e h a s r e m o v e d a b o u t t w o w h o r l s o f c h a m b e r s , massive calcite o f u m b i l i c a l plugs has resisted b r e a k a g e ; max. l e n g t h 0.3 ram. 6. D e l a m i n a t i o n o f wall o f Globobulimina auriculata f r o m o x i d i z e d s a m p l e ; h o r i z o n t a l field o f view 0.2 ram. 7. Deeply e t c h e d wall o f Globobulimina auriculata f r o m o x i d i z e d s a m p l e ; h o r i z o n t a l field of view 20 ~ m . 8. S m o o t h wall o f Elphidium clavaturn f r o m freshly w a s h e d s a m p l e ; h o r i z o n t a l field o f view 50 # m . 9. Deeply e t c h e d wall o f Elphidium clavatum f r o m o x i d i z e d s a m p l e ; h o r i z o n t a l field o f view 6 0 urn.

M44

(3) Paucity or absence of calcareous microfossils is not necessarily due to (paleo-) ecological conditions. (4) Dissolution phenomena on foraminiferal tests may not necessarily be caused by "corrosive" b o t t o m waters. ACKNOWLEDGEMENTS

Financial support for this study was provided by the National Science Foundation under Grant Number OCE 7825726.

REFERENCES Arrhenius, G., 1963. Pelagic sediments. In: M.N. Hill (Editor), The Sea, 3. Wiley, New York, N.Y., pp. 655--727. Berger, W.H., 1968. Planktonic foraminifera: selective solution and paleoclimatic interpretation. Deep-Sea Res., 15 : 31--43. Berner, R.A., 1970. Sedimentary pyrite formation. Am. J. Sci., 268: 1--23. Briskin, M. and Schreiber, B.C., 1978. Authigenic gypsum in marine sediments. Mar. Geol., 28: 37--49. Criddle, A.J., 1974. A preliminary description of microcrystalline pyrite from the nanoplankton ooze at Site 251, Southwest Indian Ocean. In: Initial Reports of the Deep Sea Drilling Project, 26. U.S. Govt. Printing Office, Washington, D.C., pp. 603--607. Edmonds, J.M. and Gieskes, J.M.T.M., 1970. On the calculation of the degree of saturation of sea water with respect to calcium carbonate under in situ conditions. Geochim. Cosmochim. Acta, 34: 1261--1291. Geyh, M.A., Krumbein, W.H. and Kudrass, H.R., 1974. Unreliable 14C dating of long-stored deep-sea sediments due to bacterial activity. Mar. Geol., 17: 45--50. J~brgensen, B.B. and Fenchel, T., 1974. The sulfur cycle of a marine sediment model system. Mar. Biol., 24: 189--201. Ruddiman, W.F. and Heezen, B.C., 1967. Differential solution of planktonic foraminifera. Deep-Sea Res., 14: 801--808. Siesser, W.G. and Rogers, J., 1976. Authigenic pyrite and gypsum in South West African continental slope sediments. Sedimentology, 23: 567--577.