Signature of Non-steady-state Diagenesis in Continental Shelf Sediments

Estuarine, Coastal and Shelf Science (1996) 42, 361–369

Signature of Non-steady-state Diagenesis in Continental Shelf Sediments

B. R. Manjunatha and R. Shankar Department of Marine Geology, Mangalore University, Mangalagangothri (D.K.), Karnataka 574 199, India Received 9 February 1994 and in revised form 19 December 1994

Keywords: non-steady-state diagenesis; base-metal partitioning; estuarine disequilibrium/equilibrium; late-Pleistocene Holocene sedimentation rates; western continental shelf of India The phenomenon of non-steady-state diagenesis is well recognized in open ocean sediments and occurs either due to the deposition of turbidites on pelagic sediments or to changes in ocean productivity. However, the occurrence of oxidized sediment below reducing sediments in many near-shore and continental shelves indicates that non-steady-state diagenesis takes place even in shallow-water regimes. This paper shows that oxic conditions on the continental shelf result from low sedimentation rates induced by the rapid rise in sea level during the early Holocene to the beginning of late Holocene time, whereas a comparatively high rate of sedimentation (with increased burial of organic matter, generating reducing conditions) due to a relatively stable sea level during the late Holocene, is responsible for the initiation of non-steady-state diagenesis. The lower oxidized sediments have not yet experienced a sufficient flux of reductants to mobilize base metals. In contrast, base metals in the upper reducing sediments have undergone mobilization and diffused out of the ? 1996 Academic Press Limited sedimentary column to the overlying water.

Introduction Two types of early diagenesis in marine sediments are characterized by: (1) a steady-state redox boundary that moves upwards in relation to sedimentation rate (Lynn & Bonatti, 1965; Froelich et al., 1979); and (2) a non-steady-state redox boundary that moves downwards because of the trapping of oxidants resulting from turbidite deposition on pelagic sediments (Colley et al., 1984; Wilson et al., 1986) or from changes in oceanic fertility (Berger et al., 1983; Thomson et al., 1984). The latter results in the degradation of organic matter leading to the immobility of diagenetically sensitive metals in the redox front (Colley et al., 1984; Buckley & Cranston, 1988). The non-steady-state redox boundary movement is documented in deep-sea sediments, and termed ‘ downward moving oxidation front ’ (Wilson et al., 1986). Such a relatively catastrophic change in the mode of sedimentation is linked to glacial–interglacial climatic changes coupled with eustatic sea-level fluctuations (Weaver & Kuijpers, 1983). In contrast, reports about non-steady-state diagenesis in shallow-water domains are sparse. Mucci and Edenborn 0272–7714/96/030361+09 $12.00/0

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(1992) have inferred that a catastrophic landslide displacement and transport of marine clay into the Saguenay Fjord have been responsible for non-steady-state diagenesis. In other areas, the occurrence in near-shore–continental shelf environments of oxidized sediment below reducing late Quaternary deposits (Wilkinson & Byrne, 1977; Long et al., 1986; Svendsen et al., 1992) indicates that non-steady-state diagenesis takes place in shallow-water regimes as well. This study reports a signature of non-steady-state diagenesis on the south-west continental shelf of India. A sediment core collected from the shelf off Mangalore has been radiometrically and geochemically examined to understand changes in terrigenous influx to the shelf as influenced by sea-level changes and estuarine equilibrium/disequilibrium since the late Pleistocene, and to study the factors responsible for base metal enrichment in the sub-surface sediments as compared to the surficial deposits. Partitioning of base metals between the lithogenic and non-lithogenic fractions has been studied to evaluate the importance of continental shelf sediments as sources or sinks for these metals. The western continental shelf of India, particularly off Mangalore, has two main sediment types: Holocene clayey silt on the inner shelf (<45 m water depth) and late Pleistocene sand on the outer shelf (>50 m water depth) separated by silty sand in the transition zone (Nair & Hashimi, 1980; Majunatha, 1990; Manjunatha & Shankar, 1992). The Netravati and Gurpur rivers discharge 14 and 1#105 tonnes of sediment and 12 015 and 2822#106 m3 of water annually into the study area. They deliver only c. 8% of the total water discharge of all west-flowing rivers of the Indian Peninsula (Manjunatha & Shankar, 1992).

Materials and methods A gravity core SS/MG/04 (12)47*19+N, 74)40*30+E, 35 m water depth) was collected from the inner shelf off Mangalore during a special cruise of R V Samudra Saudhikama of the Geological Survey of India on 6 March 1987. Nineteen sub-sections of the core were dried at 110 )C, powdered, digested with HF, HNO3 and HClO4, and analysed for Cu, Zn, Ni, Co, Mn, Fe and Al by AAS (model Perkin-Elmer 403), Ca by EDTA titrimetry (Vogel, 1978), and loss-on-ignition (LOI) by heating the sample at 450 )C for 4 h and expressing the weight loss as loss-on-ignition. This includes structural water (from clays), organic matter and other volatiles released during ignition. Therefore, LOI estimated in this study is about 2·5 times higher than organic carbon values reported for shelf sediments off Mangalore (Paropkari, 1983). Although the absolute values are on the high side, the relative values among samples and the pattern of variation remain essentially the same. To study the partitioning of metals among five phases, samples were sequentially leached with 25% v/v acetic acid (Chester & Hughes, 1967), 0·1 M hydroxylamine hydrochloride (Chao, 1972), 30% hydrogen peroxide extracted with 1 M ammonium acetate (Gupta & Chen, 1975), and hot 50% hydrochloric acid (Cronan, 1976). These treatments were followed by total dissolution of the residue with HF–HNO3–HClO4. Elemental concentrations were determined by AAS. Procedural blanks, replicate samples, and USGS (SGR-1, MAG-1 and SCo-1) and National Research Council, Canada (BCSS-1 and MESS-1) standards were also analysed in a similar way to check the accuracy of analysis. The analytical results showed that the overall accuracy for Cu, Co, Mn, Fe and Al is <6%; and for Ni, Pb, Zn is 6–13%.

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T 1. Average elemental concentrations, loss-on-ignition (LOI) and metal/Al ratios for lower brownish gray (LBG), upper dark gray (UDG) and inner shelf surficial sediments off Mangalore Lower brownish gray (LBG) sediment Cu (ppm) Zn (ppm) Ni (ppm) Co (ppm) Mn (ppm) Fe (%) Al (%) Ca (%) LOI (%) Cu/Al (#10 "3) Zn/Al (#10 "3) Ni/Al (#10 "3) Co/Al (#10 "3) Mn/Al (#10 "3) Fe/Al

39·0 86·0 117·0 39·0 813·0 5·93 9·09 3·15 7·79 0·43 0·94 1·25 0·44 9·00 0·63

Upper dark gray (UDG) sediment 23·0 62·0 74·0 20·0 280·0 4·42 6·30 7·40 9·55 0·36 0·99 1·11 0·32 4·52 0·70

Inner shelfa surficial sediment off Mangalore 32·0 44·0 88·0 25·0 301·0 4·80 7·90 6·44 11·10 0·41 0·56 1·11 0·32 3·80 0·64

a

From Manjunatha (1990).

Results and discussion The gravity core used in this study was from the shelf off the Netravati–Gurpur river mouth, and consists of: (1) silty sand (330–302 cm) deposited in a beach environment during the late Pleistocene low stand (Nair & Hashimi, 1980; Shankar & Karbassi, 1992) and; (2) lower brownish gray (LBG; 302–150 cm) and upper dark gray (UDG; 150–0 cm) mud sequences deposited in the marine environment since the onset of Holocene. Knowing the sedimentation rate for this core by Pb-210 geochronology (0·72 mm year "1; Manjunatha & Shankar, 1992), the junction between UDG and LBG sediments at 150 cm depth is dated at 2100 year . Such an extrapolation of 210Pb chronology to a long time-scale may involve some errors, but the age deduced from this technique correlates with the age (2000–2800 year ) of maximum sea-level rise in the Holocene along the west coast of India (Bruckner, 1989). Thereafter, the sea level reached the current level and the UDG mud was deposited. The sedimentation rate for LBG sediment (0·17–0·22 mm year "1) is deduced from the age of the underlying silty sands of late Pleistocene age (9000–11 000 year ; Nair & Hashimi, 1980) and the thickness of LBG sediments (i.e. 152 cm/11 000–2100 year=0·017 cm year "1; 152 cm/9000–2100 year cm=0·022 cm year "1). The LBG and UDG sediments were deposited under different environmental conditions as suggested by their colour, bulk and partition geochemical data. The lower brownish gray mud has high metal contents and metal/Al ratios, but low concentrations of biogenic components like CaCO3 (i.e. Ca) and LOI (Table 1; Figure 1). A significant proportion of the diagenetically sensitive Mn, besides Cu, Zn and Fe, is present in the acetic acid—and hydroxylamine hydrochloride—soluble fractions (Figure 2). In LBG, acetic acid-soluble metals, particularly Mn, are considerably higher than those soluble in hydroxylamine hydrochloride indicating that the physical process of adsorption is faster in shallow-water regions than the chemical process of oxyhydroxide precipitation

ppm

Ni

Co

Mean

Mean Mean Late Pleistocene silty sand

600 1000 2

Mn 4

6

Fe

Mean Mean Mean Early Holocene–beginning stage of Late Holocene brownish gray mud

15 30 45 40 60 80 100 40 60 80100 120 140 10 20 30 40 200

Zn 8 4

Mean

2

4

% 6

Ca 8 10 6

8

Oxic

anoxic

10 12 14

Loss-onignition (LOI)

Mean Late Holocene dark gray mud

6 8 10 0

Al Age (year BP)

2083

330

270

240

210

Figure 1. Down-core variations of lithology, elemental concentrations and loss-on-ignition for core SS/MG/04. Concentrations higher than the mean are stippled. , before present.

Mean

9000– 11 000

Depth (cm)

300

Lithology

180

150

120

90

60

30

0

Cu

Sedimentation rate –1 = 0.72 mm year Sedimentation rate = 0.17–0.22 mm year–1

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LBG layer

Ni

Mn (ppm)

100 Zn 80

7

600

6

500

5

400

4

300

60

3

Co

Cu

200

2

20

100

1

0

0

0

40

Inner shelf, Mangalore

(b) (a)

800

140

700

7

120

600

6

500

5

400

4

100

Mn (ppm)

Ni

80

300

60

Fe (%)

Cu, Zn, Ni, Co (ppm)

120

700

Fe (%)

140

Cu, Zn, Ni, Co (ppm)

(a) (b)

800

3

Zn 200

2

20

100

1

0

0

0

40

Cu

Co

Non-lithogenic fraction (NLF) I

HOAc (Carbonate phase and loosely sorbed ions)

II

NH2 OH.HCl (easily reducible phase)

III

H2O2–NH4OAc (organic fraction) Lithogenic fraction (LF)

IV

Hot 50% HCl (resistant/ detrital phase)

V

HF–HNO3–HClO4 (acid insoluble/ highly resistant phase)

Figure 2. Variations of base metal contents in different accumulative phases to illustrate the immobility of metals in lower brownish gray (LBG) sediment (a) relative to inner shelf surficial sediments off Mangalore. (b) Manjunatha, 1990.

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(Duinker, 1980). This view has been further supported by base metal partitioning data for resuspended sediments from the oxygenated estuarine–coastal water near the area of study (Manjunatha, 1990). For instance, Mn is 806 ppm in the HOAc leach in comparison with 113 ppm in the NH2OH · HCl leach. All these characteristics point to an oxic condition of deposition for the LBG sediment. In such environments, oxic diagenesis is initiated due to the degradation of organic matter, releasing metals which would be added to the particulate phase either as adsorbed ions or as diagenetic/ authigenic component of Mn–Fe oxides/hydroxides (Calvert, 1976; Chester, 1990). The existence of oxic conditions in the LBG sequence is, therefore, suggested to be due either to lack of decomposable organic matter or to sediments not yet having experienced a sufficient flux of reductants to mobilize the base metals. As a result, such sediments can retain the indication of oxic depositional conditions even after their burial (Berner, 1981). Froelich et al. (1979) have shown that as dissolved oxygen decreases, MnO2 is reduced and Mn2+ comes into porewater. However, ferric oxide reduction takes place under even more anoxic conditions prevalent at greater sediment depths. In contrast to LBG sediments, the UDG sediments have a dark gray colour, lower metal contents and metal/Al ratios, and higher Ca content and LOI (Table 1). Mean partition geochemical data for the reducing surficial sediments of the study area (Manjunatha, 1990) show that metals in the acetic acid-soluble fraction of UDG are low (Figure 2). All these features show that the UDG sediments were deposited in a reducing environment. Noteworthy is the absence of metals in the easily reducible phase because of suboxic diagenesis which reduces and releases the insoluble Mn–Fe oxides/hydroxides and adsorbed metals into the soluble phase (Sundby et al., 1981; Trefry & Presley, 1982; Chester, 1990; Silverberg & Sundby, 1990). The divalent cations thus released eventually diffuse out of the sedimentary column to the overlying water, indicating that the UDG sediment is a source rather than a sink for base metals. The prevalence of reducing conditions is evidenced by the dark gray colour of sediments and the occurrence of H2S and authigenic pyrite in sediments (Mallik, 1972). Normally, near-shore–continental shelf sediments have a very thin (<1 mm) surficial oxidized layer (Calvert, 1976; Chester, 1990). However, the evidence from this study for oxic conditions in deeper layers (LBG; 302–150 cm) and reducing conditions in shallow layers (UDG; 150–0 cm) is puzzling. A possible reason for this could be variation in sedimentation rate (see discussion below): 0·72 mm year "1 for UDG sediment (Manjunatha & Shankar, 1992) and an estimated three-fold lower value of 0·17– 0·22 mm year "1 (see Figure 1) for LBG sediment. The slowly deposited LBG sediment may not reflect low river discharge because the south-west monsoon was intense over the Indian subcontinent, particularly at the beginning of the Holocene (Van Campo, 1986). The oxic conditions of the LBG mud sequence seem to be due to low sedimentation rate when the core site was close to the shoreline during early Holocene to the beginning of late Holocene (11 000–9000 year to 2000 year ). A rapid rise in sea level (18 mm year "1) and a faster rate of transgression has, in fact, been proposed for the west coast of India during early–middle Holocene (Kale & Rajguru, 1985, 1987). During rapid transgression, the coast retreats, drowning the river channels and displacing the estuaries landward. These disequilibrium estuaries serve as effective filters for riverine detritus (Schubel & Kennedy, 1984; Fracer, 1989) and allow only a small quantity of sediments to escape to the continental shelf (Fracer, 1989). The sea level was close to the present level around 6000 year , but has fluctuated thereafter (Kale & Rajguru, 1985). High strand lines are represented by raised beaches along the west coast of India (Bruckner,

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1989; Subrahmanya & Bhat, 1992). Radiometric dating of these beaches suggests that the Holocene transgression reached its maximum around 2000–2800 year  (Bruckner, 1989). This corroborates the ages of the transition from LBG to UDG sediment (Figure 1) in the study area. Reducing conditions during the deposition of UDG sediments must have been produced because of rapid burial of organic matter resulting from high sedimentation rates on the inner shelf during the late Holocene (<2000 year ) when the sea level was relatively stable (sea-level rise=0·3–0·8 mm year "1; Kale & Rajguru, 1985). Under stable sea-level conditions, the estuarine tidal prism attains equilibrium and more riverine sediments are transported beyond the estuarine zone (Fracer, 1989). There are other instances of occurrence of oxic sediment below shallow-water anoxic sediment during the late Quaternary. Long et al. (1986) report soft brown gray mud (>13 000 year ) and soft brown mud (9800–13 000 year ) beneath the Holocene olive gray silty sand from the Witch Basin, central North Sea. Down-core increases of Fe/Al and Mn/Al in these sediments indicate not only a reduced detrital input, but also oxic conditions. Sediment cores from the inner shelf off western Spitsbergen, Greenland, show evidence for the prevalence of aerobic conditions during the early Holocene (7000–10 000 year ) rather than late Holocene (Svendsen et al., 1992). Wilkinson and Byrne (1977) report a mud sequence (from bottom to top) of grayish brown, grayish green, hematitic brown and organic-rich gray mud that formed during the Holocene transgression in the central Texas estuary, Lavaca Bay. The subsurface brown units are reflective of oxidizing conditions. These additional lines of evidence indicate that non-steady-state diagenesis takes place in shallow-water regions as well. The occurrence of metal-rich layers is more pronounced in deep-sea, rather than near-shore–continental shelf, sediments because of their long time span record of glacial/interglacial climatic changes. There are many turbidite sequences intercalated with pelagic sediments (Buckley & Cranston, 1988). However, during the course of burial (>50 000 year) and after the exhaustion of all oxidants in subsurface layers, the oxidized sediment gradually becomes anoxic which, in turn, favours the reduction of oxides and oxyhydrates of Fe and Mn, thereby liberating base metals to porewater (Buckley & Cranston, 1988). In near-shore–continental shelf such a change would also be expected during the course of burial. Early diagenesis in marine sediments, particularly in near-shore–continental margin sediments, transports the redox-sensitive metals such as Mn, Co, Cu, etc. in the non-lithogenic fraction of reducing sediments to the overlying water (Duinker et al., 1974; Sundby et al., 1981; Trefry & Presley, 1982; Martin & Whitfield, 1983; Chester, 1990; Silverberg & Sundby, 1990). This is an important source of metals for pelagic sediments and associated polymetallic nodules. Most of these findings (Sundby et al., 1981; Trefry & Presley 1982; Silverberg & Sundby, 1990) are based on studies of short cores (<1 m). However, the evidence of oxidizing conditions in subsurface sedimentary column of near-shore–continental shelf areas (Wilkinson & Byrne, 1977; Long et al., 1986; Svendsen et al., 1992) including the LBG sediment of the present study, indicates lack of sufficient oxidants to mobilize base metals. Further studies on longer cores (off river mouths) that penetrate the Pleistocene–Holocene transition and cover a wider geographic area of shallow marine environments the world over, may throw more light on the importance of non-steady-state diagenesis in understanding sediment–water interactions of base metals, which is driven by global climatic changes and the associated sea-level fluctuations during the late Quaternary.

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Acknowledgements We thank Drs B. R. J. Rao and K. Venkaji, Geological Survey of India for making available the samples and AAS facility, and Drs T. R. S. Wilson, J. Thomson, T. F. Pedersen and an anonymous referee for reviewing an earlier draft of the manuscript and offering valuable comments. The Department of Ocean Development, Government of India, financed this work through a research project to RS. References Berger, W. H., Finkel, R. C., Killingley, J. S. & Marchig, V. 1983 Glacial–Holocene transition in deep-sea sediments: Manganese spike in the east-equatorial Pacific. Nature 303, 213–233. Berner, R. A. 1981 A new geochemical classification of sedimentary environments. Journal of Sedimentary Petrology 51, 359–365. Bruckner, H. 1989 Late Quaternary shorelines in India. In Late-Quaternary Sea-level Correlation and Applications (Scott, D. B., Pirazzoli, P. A. & Honing, C. A., eds.). Kluwer Academic Publishers, Dordrecht/Boston/London, pp. 169–194. Buckley, D. E., & Cranston, R. E. 1988 Early diagenesis in deep sea turbidites: The imprint of paleo-oxidation zones. Geochimica et Cosmochimica Acta 52, 2925–2939. Calvert, S. E. 1976 The mineralogy and geochemistry of nearshore sediments. In Chemical Oceanography (Riley, J. P. & Chester, R., eds). Academic Press, London, Vol. 6, pp. 187–220. Chao, T. 1972 Selective dissolution of manganese oxides from soils and sediments with acidified hydroxylamine hydrochloride. Proceedings of the Soil Science Society of America 36, 746–768. Chester, R. 1990 Marine geochemistry. Unwin Hyman, London, 698 pp. Chester, R. & Hughes, M. J. 1967 A chemical technique for the separation of ferromanganese minerals, carbonate minerals and adsorbed trace elements from pelagic sediments. Chemical Geology 2, 249–262. Colley, S., Thomson, J., Wilson, T. R. S. & Higgs, N. C. 1984 Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the north east Atlantic. Geochimica et Cosmochimica Acta 48, 1223–1235. Cronan, D. S. 1976 Basal metalliferous sediments from the eastern Pacific. Geological Society of America Bulletin 87, 928–934. Duinker, J. C. 1980 Suspended matter in estuaries. In Chemistry and Biogeochemistry of Estuaries (Olausson, E. & Cato, I., eds). John Wiley & Sons, New York, pp. 121–151. Duinker, J. C., Van Eck, G. T. M. & Nolting, R. F. 1974 On the behaviour of Cu, Zn, Fe and Mn and evidence for mobilization processes in the Dutch Wadden Sea. The Netherlands Journal of Sea Research 5, 214–239. Fracer, G. S. 1989 Clastic Depositional Sequences: Processes of Evolution and Principles of Interpretation. Prentice Hall, Englewood Cliff, New Jersey, 459 pp. Froelich, P. N., Klinkhammer, G. P., Bender, M. L., Luedtke, G. R., Heath, G. R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. & Maynard, V. 1979 Early oxidation of organic matter in pelagic sediments of eastern equatorial Atlantic: Suboxic diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1090. Gupta, S. K. & Chen, K. Y. 1975 Partitioning of trace metals in selective chemical fractions of nearhsore sediments. Environmental Technology Letters 10, 129–158. Kale, V. S. & Rajguru, S. N. 1985 Neogene and Quaternary transgressional and regressional history of India—An overview. Bulletin Deccan College Research Institute, Pune 44, 153–167. Kale, V. S. & Rajguru, S. N. 1987 Late Quaternary alluvial history of the northwestern Deccan upland region. Nature 325, 612–614. Long, D., Bent, A., Harland, R., Gregory, D. M., Grahma, D. K. & Morton, M. 1986 Late Quaternary paleontology, sedimentology and geochemistry of a vibro core from the Witch Ground basin, central North Sea. Marine Geology 73, 109–123. Lynn, D. C. & Bonatti, E. 1965 Mobility of manganese in diagenesis of deep-sea sediments. Marine Geology 3, 457–474. Mallik, T. K. 1972 Opaque minerals from the shelf sediments off Mangalore, western coast of India. Marine Geology 12, 207–222. Manjunatha, B. R. 1990 Geochemistry and magnetic susceptibility of riverine, estuarine and marine environments around Mangalore, west coast of India. Ph.D. Thesis, Mangalagangothri, Mangalore University. Manjunatha, B. R. & Shankar, R. 1992 A note on the factors controlling the sedimentation rate along the western continental shelf of India. Marine Geology 104, 219–224.

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