Complex Cementation Textures and Authigenic Mineral Assemblages in Recent Concretions from the Lincolnshire Wash (East Coast, UK) Driven by Fe(0) to Fe(I1) Oxidation

Journal ofthe Geological Society, London, Vol. 152, 1995, pp. 157-171, 10 figs. 1 table. Printed in Northern Ireland

Complex cementation textures and authigenic mineral assemblages in Recent concretions from the Lincolnshire Wash (east coast, UK) driven by Fe(0) to Fe(I1) oxidation

M. R. Al-AGHA’.’, S. D. BURLEY’,C. D. CURTIS’ & J. ESSON’ ’Diagenesis Research Group, Department of Geology, University of Manchester, Manchester M13 9PL, UK 2 Department of Geology, Islamic University of Gaza, PO Box 108, Gaza, Gaza Strip

Abstract: Concretions are common in some of the modern intertidal sediments on the Lincolnshire coast of the Wash. The mineralogy and geochemistry of numerous examples of theseconcretions have been studied in detail. The majority have metallic nucIei and those that do not, exhibit textures which suggest that they originally did so. Petrographic observations indicate that the cements within these concretions precipitated in a distinct sequence that is spatially developed around the metallic nucleus and that are arranged from core to periphery: (a) a ferrous hydroxy chloride mineral (similar in bulk composition to akaganeite) together with iron monosulphide (amorphous FeS and mackinawite), pyrite and possibly elemental sulphur; (b) ferroan carbonate cements (including siderite, ankerite and calcite); (c) mixed ferrous and ferric minerals (“green rust” together with magnetite and possibly greigite); (d) fully oxidized minerals (including akaganeite, goethite, hematite, gypsum and a complex basic sulphate of Fe, Ca, Mg, Si and AI). This latter material has not been possible to characterize fully but is probably an amorphous mixture. The initiating reaction for the precipitation of these cements is anaerobic corrosion of iron at zero valence state with sulphateas an oxidant.Metal, hydroxide and monosulphide are found in close spatial association, indicating the reaction: 4Fe0 + SO:- + 4H20+ 3Fe(OH), + 20H- + FeS Further away from the metallic nucleus, carbonates cement the host clastic sediment. These are the products of reaction between the first-formed hydroxides and pore water solutes (principally HCO;, Mg’+ and Ca”). When oxygen gains access to the growing concretion, Fe(1I) minerals are replaced very rapidly by akaganeite and either goethite or hematite. Gypsum and other sulphate minerals also precipitate as new cements around the periphery of the concretions. The cementation process in these concretions is driven by the extreme instability of metallic iron (here, relic military armaments and shrapnel fragments) in contact with saline, anaerobic water and is indicative more of cathodic corrosion than the growth of ancient carbonate-sulphide concretions.

Keywords: concretions, modern, diagenesis, intertidal environment, Lincolnshire England.

Concretions have stimulatedwidespread interest for many brackish water environments (Ho & Coleman 1969; Postma years. Originally they were sought for the exquisite fossils 1977, 1982; Stoops 1983; Moore et al. 1992) whereas pyrite, notuncommonly preserved within them.More recently, magnesian calcite and aragonite arethe earlydiagenetic sedimentologists and geochemists have obtained numerous minerals most frequently encountered in temperate shallow chemical and isotope data from the various cement minerals marinesediments (VanStraaten, 1957; Coleman & present within concretions in order to learn more about the Gagliano, 1965; Wiedemann 1972; Jorgensen 1976; post-depositional history of ancient sedimentary rocks (e.g. Al-Hashimi 1977; Adams & Schofield 1983). In terms of Gautier 1982; Scotchman 1991). limitationA of this their mineralogy, therefore, these Recent concretions from approach, however,results fromthe common difficulty of the coast of eastern England exhibit a remarkable diversity relatingancient concretion growth tothe geochemical and appear to be the result of formation from mixed, rather environment of mineral authigenesis. The use of modern complex geochemical environments. analogues to overcome this problem, where concretions seen Pye et al. (1990) also pointout thatthe Warham to be forming now can be studied in situ directly within their concretions are remarkable forthe rapidity of their ambient environment of formation, has beenimpeded by formation. Model calculations (e.g. Berner 1968; Wilkinson the relative rarity of such concretions. This is especially the & Dampier 1990) suggest that whilst on a geological time case for marine muds. scale carbonate concretions form rapidly, they typically take Concretions are formingtoday in intertidal marsh severalthousands of years to develop. Along thenorth sediments of the Wash near Warham, Norfolk, on the east Norfolk coast, however, concretions up to 0.4 m in diameter coast of the UK, and have been suggested torepresent can be shown to have grown in marsh sediments accreted analogues of ancient siderite concretions (Pye 1982; Pye et since the Second World War. al. 1990). These concretions are iron-rich and reported to The work described here was undertakenon Recent be variously cemented by hydratediron oxides, iron concretions collected from intertidal marsh sediments of the sulphides,siderite, ankerite, calcite and gypsum. Siderite Lincolnshire Wash, similar to those described by Pye et al. cements aremore frequentlydescribed from fresh and (1990) fromthe north Norfolk coast. Acomprehensive 157

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 158 M. R. AL-AGHA ET AL.

description of the petrography, mineralogy and chemistry of inner Arenicola sandflat, lower mudflat and lower sandflat. the variousauthigenic cements within thesepresent-day Evans (1965, 1975) described the sedimentology and biology concretions is used to provide a better basis for discussion of of these zones in detail, and estimated the thickness of the the concretiongrowth processes thanhas hitherto been tidal flat sequence to be in excess of 7 m, although the base possible. These data are then used to consider the relevance of the tidal sandsequence at Freiston was not recorded. of theRecent concretionsfrom the Wash as possible Processes of deposition on theintertidal zone were analogues for ancient concretions. documented by Evans & Collins (1975) and Collins et al. (1981) who showed that each tidal flood brings vast Study area and its modern sedimentology quantities of suspended andbed loadsediment over the intertidalzone. Much of this sediment is deposited in The Wash is a large macrotidal embayment situated on the response to the decrease in energy of the advancing tidal easternseaboard of England, off the Lincolnshire and wave and results in the grain size differentiation of Norfolk coasts (Fig. 1).The embayment is fringed by a sedimentsacross the intertidalzone. Suttill et al. (1982) variable belt of fenlands,saltmarshes and intertidal calculated an overall sedimentation rate for the tidal flat of sediments, thatare arranged roughly parallel to the 1.5 mm per year over the last 1000 years which is directly coastline. Low tideexposes a broad intertidalzone of comparable to estimates of the combined rate of rise of sea readily accessible active mud and sand sedimentation that is level and subsidence in the Wash over the same time period typically 2 km wide and locally exceeds 10 km width. (Coles & Funnel1 1981). The intertidal zone of the Wash in south Lincolnshire, For the purpose of this study we briefly describe the from Freiston in the southtowards Gibralter Pointin the salient features of the main depositional zones in transects north, comprisesa series of well defined sedimentological across the intertidalzone at Freiston and Leverton (lines zones,starting onthe landward edge with vegetated A-A' and B-B' respectively in Fig. 1). We simplify the saltmarsh and passing through mudflats and sandflats sedimentary facies designations described by Evans (1965) seawards (Fig. 1). In detail,these zones were subdivided and recognize saltmarsh, higher mudflats, sandflats and an further by Evans (1965) into saltmarsh, higher mudflat,

Fig. 1. Sketch map of the coastal belt between Freiston and Leverton showing the distribution of the main intertidal sediment facies and the historical development of the sea defence embankments. Sample collection was made along lines A-A' and B-B'. Access points at Freiston Shore and Lodge Farm, Leverton are shown. Insets indicate the setting of the field area in relation to the Wash embayment and the UK.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE THEFROM WASH 159

irregular belt of sporadically developed muds close to the equivalent mudflats of the Solway Firth. The mudflats are low tidemark thatare broadlyequivalent tothe lower covered with a film of algal mat andare extensively mudflats of Evans (1965). burrowed, although not as pervasively as the sand flats. The saltmarsh sediments consist of silty sands and clayey Shallow inter-creek depressions, referred to as pans, are silts which are intensely vegetated and well stabilized as a common on both the lower edge of the saltmarsh and across result of algal matand higherplant colonization. the higher mudflats (Fig. 2a & b). Pans are crudely oriented Lamination is weakly preserved but root channels penetrate SW-NE, parallel to the dominant wind direction across the throughout the sediments. Interbedded coarsesand layers intertidal flats. In winter months they are usually filled with are present,being most readily seen close tocreeks and sea water (Fig. 2a). They probably originated as oxbows of probably represent either levee deposits or the channel lag abandoned tidal creekmeanders, but their present shape deposits of older tidalcreeks. Thedegree of plant andorientation resultsfrom the wind driven, panwater colonizationvaries seasonally (Fig. 2a & b)but decreases erosion of the mudflats. During the summer, the mudflats progressively seawards across the saltmarsh. and pans dry out between high tides, and are covered with The higher mudflats are dissected by numerous tidal desiccation cracks (Fig. 2b). The cracks and burrows are creeks (Fig. 2a). These actively meander across the mudflats typically bound by algal coatings and cemented by brown during theebb flow and rework the tidaldeposits. The colourediron hydroxides such that they now standabove edges of creeks that erodedown through these mudflats, and the surface of the mudflats. the walls of trenches dug in them, display low-angled The lower edge of the higher mudflats hasundergone cross-stratification as aresult of tidalchannel migration considerable erosion. Much of this is known from aerial comparable tothat described by de Mowbray (1983) for photographs and local knowledge to have taken place since

+-,?"W--

Fig. 2. Thc Wash intertidal zone and field appearance of the concretions. (a) View across the intertidal zone taken at the beginning of winter looking towards the 1982 embankment at Freiston Shore. Note the vegetation and the water-filled pans betwecn the tidal creeks. (b) Similar view taken during high summer showing the typical development of desiccated upper mudflats illustrating the seasonal variation in the character of the upper marsh. (c) A collection of concretions from immediately below the sandflat surface. The small sperical concretions all contained metal fragments whilst the elongate concretions containedrelic metal rods. Drink container is 30 cm high. (a) A large concretion developed around an iron tube dug outof the sandflats at Freiston. The concretion was found a few centimetres below the rippled sandsurface. The black interior of the concretion is visible. Width of spade is 20 cm.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 160 M. R. AL-AGHA ET AL.

the building of the latest sea embankment in 1982, although On the upper marsh, the majority of the sediments are earlier erosion of the mudflats is documented by Collins et oxygenated to a depth of about 2 m and are light brown or al. (1981). Erosion has exposed the underlying tidal sands grey incolour. Below this depth,the sediments are which have been extensively modified by tidal currents into permanently waterlogged andare black and anoxic. sandwaves anddunes, and ebb currents have variously Adjacent to majorcreeks and in the marsh sediments reworked these sandflats resulting in a complex surface. beneath the pans the zone of oxygenation is greatly reduced The sandflatshave also accreted landwards as a result of in thickness and typically penetrates only a few millimetres new sand additionintroduced during each tidal cycle beneath the surface. (Collins et al. 1981). The largest area of sand flats is Onthe intertidalsand flats, the zone of oxygenation developed onthe Leverton Flats (Fig. 1). Herethe tidal reaches a maximum of about 50cm but the thickness of this creeks are much larger and more stabilized than on the sand oxidized layer thins dramatically towards the low tide mark. flats at Freiston although they are still observed to actively The actual thickness of the oxygenated layer depends meander during the ebb flow. significantly on the host sediment lithology, and this effect is At Freiston a lower area of very recent mud deposition, seen clearly on the lower intertidal flats. Here, in the sands probably equivalent to the lower mudflats of Evans (1965), 2-3 cm of the upper sediment may remain oxygenated whilst is present close to the low water mark. These muds are very in the muds it is restricted to the top 2-3 mm. soft and unconsolidated and have been deposited in the last Love (1967) first documentedthe vertical change in few years, possibly in response to construction of the latest diagenetic mineralogy through these sediments and laterally sea embankment. across the intertidalzone. The upper oxidized zone is present everywhere, although its thickness varies across the The vertical sedimentary sequence, diagenetic flats. The mineralogy of this oxidized zone is dominated by zonation and location of the concretions acid soluble ferric hydroxides, which Love (1967) considered as being introducedas detrital material adsorbed on, for A schematic cross section through the intertidal zone shows example, clay minerals. The thickness of the hydroxide zone the idealized vertical distribution of sediments (Fig. 3). is related to the tidally-controlled water table, certainly in Beneath the laminatedmarsh muds and mudflat sediments is the sand lithologies, although the higher mudflat and a laterally persistant tidal marine sand that has been proved saltmarshsediments appearto be essentially impermeable to a depth of 12m at Freiston (Suttill et al. 1982; A. Boult andthe water table is suppressed in these sediments. pers.comm. 1992), but which is known from the coastal Beneath this zone the sediments are black and anoxic, and section at ChapelPoint, some 15 km north of Gibralter containiron monosulphide minerals (Fig. 3). Oxygen Point(see Fig. 1 inset map),to be underlain by a brought into this zone along mudcracks andthrough the pre-Roman period of brackish water marsh sequence, Iron activities of benthic organisms oxidizes the immediately Age peat deposits and glacial clay (Swinnerton & Kent adjacent muds. 1976).

a Vegetated Tidal Creek Salt Pan Levee Tidal Creek LeveeCreek Tidal Levee Pan Salt Meander Zone ,Upper Arenicola Sand Hat LowerMudflats 3

Sea-blite Marsh Samohire 2 Sea Purslane esiccatton cracks

Fig. 3. Schematic cross-section through the intertidal zone at Freiston (along transect A-A' in Fig. 1) illustrating the vertical and lateral extent of the sedimentary features and the distribution of diagenetic zones, based partly on Love (1967). Vertical scale greatly exaggerated and size of tidal creeks not in proportion to width of intertidal zone.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE THEFROM WASH 161

Approximately 50-70 cm below the monosulphide layer, embankments wereconstructed from 1935 with the latest and usually coinciding with the occurrence of the tidal sands, embankment at Freiston being completed in 1982 (Fig. 1). the sediments turn grey coloured and Love (1967) records The construction of embankments and reclamation result the presence of pyriteframboids in them. Love (1967) in an increase of sedimentation on the upper marshes and a argues thatthe transformation of monosulphide to pyrite gradual seaward advance of the saltmarsh and the adjacent takes place in situ, andthat the pyriteinherits boththe mudflats, but it appears that the low water mark does not morphology and size distribution of its monosulphide advance progressively seawards(Doody 1987). As a result precursor. there is a progressive narrowing of the intertidal zone with Concretions are exclusively seen in the tidal sands of the successive landreclamations and, in some areas of the pyrite zone, and appear in all cases to be concentrated in the intertidalzone, theupper mudflats merge with the lower upperpart of these sediments. No concretions have been mudflats and the intervening sand flats are covered in mud. recovered from the equivalent anoxic zone on the flanks of Thiscan be seen aroundthe Witham outfall and may be tidal creeks or beneath pans,although reworked, partially responsible for the mud accretion on the Freiston flats and oxidized concretions are found as lag concentrates in some the associated erosion of the sand flats andthe higher of the tidalcreeks. Most of the concretions fromthe mudflats. Lincolnshire Wash have been recovered from the sand flat The intertidal zone has been used as a bombing range surfaces, especially in the upper sand flats at Freiston where and site for military exercises (in common with the intertidal erosion is currentlyremoving the lower mudflat sequence flats around much of the Wash) since the establishment of (alongtransect A-A’ of Fig. 1). After winter or spring an artillery range from Freiston Shore to Wainfleet in 1891 storms fresh concretions are commonly exposed on the sand by the First Lincolnshire Artillery Regiment (Curtis 1987). flat surface, and digging in the vicinity of exposed Intermittent use of the rangecontinued through the First concretions usually reveals many more in thetop few World War, when Freiston and Wrangle flats were used for centimetres of the sand flat sediments (Fig. 2c & d). The aerial bombing trials. The area was defended as a potential majority of the concretions are c. 10-20cm in diameter, but invasion siteduring the Second World War, and has theirshape and size varies greatly and in most cases is remained in the flight path of RAF bombing training governed by theshape of the nucleus they have grown exercises at Wainfleet ever since. The intertidal flats are around. It is obvious that many have grown around iron or now littered with the debris of these activities, including steel nuclei and theycan exceed 1 metre in diameter munitions and the remains of several aircraft crashes. (determined by the size and shape of the original metallic nucleus; see Fig. 2c & d). Pebbles and calcareous shell fragments are frequently incorporated in the concretions by Investigation methods cementation. The concretions are black when fresh,but Some 80 concretions were collected over a period of three years and brown ororange oxidation rims develop very rapidly on several sampling excursions to the Wash. Of these concretions, 35 exposure to air. have been investigated in detail. In the field, after removal from the intertidalsediments, the concretions were immediately placed in plastic containers which were filled with seawater to minimize Land reclamation and anthropogenic activities aerial oxidation and the containers were then sealed. Once in the laboratory, the concretions were stored under water Inthe Holocene, the Fenland of eastern England was a in sealedjars. All the concretionswere subsequently cut with a low-lying swamp which was gradually inundated by the diamond saw and thecut surface polished and examined.A post-glaciation rise in sea level (Coles & Funnel1 1981; representative selection of the cut concretions were photographed Shennan 1982, 1987) and became the inner part of a wide and impregnated with epoxy. Polished thin sections were prepared coastal lagoon and brackish watermarshland complex in paraffin from 35 of the concretions and then carbon coated and stored in a vacuum desiccator to minimize oxidation effects. protected by an outer sand barrier. As the sea continued to The petrography and textures of the concretions were studied in rise, thebarrier bar complex migratedlandwards and thin section employing both transmitted and reflected light optical prograded across the deposits of the brackish swamp, microscopy as well as in the SEM using back-scatteredelectron depositing the so-called fen ’silt-lands’ in a wide crescent (BSE) imagery. Freshly cut or fractured material was sub-sampled aroundthe margins of the Wash. Themodern tidal flat with a microdrill for XRD analysis of specific areas of the environment in the Wash is the result of the Romano-British concretions to determine their mineralogy. Particularcare was taken to sample and analysethe different textural zones which marinetransgression of ~OBC (Godwin & Clifford 1938; develop around the metallic nuclei and that are clearly discernable Shennan 1986a, b). in handspecimen. The chemical composition of theauthigenic Since Saxon times, man has interfered with the natural cements was determined quantitatively with EDS analysis using a sedimentary processes of thefen wetlandsbordering the Jeol 6400 SEMand with WDS/EDS using a CAMECA electron present day Wash to the extent that thepresent shape of the microprobe. Wash is a result of successive periods of reclamation. Around most of theperimeter of the Wash an artificial Mineralogy and composition of concretion cements embankment serves toprevent marine flooding of the reclaimed marshland. Most of the concretions collected proved to containa Systematic reclamation of the Wash fenlands began with metallic nucleus (Fig. 4a). Theseare presumed to be the the construction of the ‘Roman Bank’ in the thirteenth and remains of fragments of military shells and other hardware fourteenth centuriesand continues tothe present.Some from either wartime or subsequent training exercises. Those 1500 km2 of land has been reclaimed since the seventeenth concretions that do not now contain a metal nucleus exhibit century when reclamation began in ernest in an attempt to textures suggestive of the former presence of one. drain the fenlands through a programme of land drainage The mineralogy of the samplesuite as a whole was and river engineering. In recent times aseries of sea establishedfrom many chemical analyses (using both

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 162 M. R. AL-AGHA ET AL.

a W

Fig. 4. Aspects of the concretions and their mineralogy. (a) Polished section cut through a typical concretion showing the presence of metal iron nuclei (m). Note replacive alteration zone (arrowed a) around the nucleus and the crudely concentric structure tothe concretion. Width of concretion = 6.75 cm. (b) BSE-SEM micrograph showing the contact between the metal nucleus (m) and ferrous hydroxide chloride (fhc, now akaganeite where oxidized)and goethite (g) corrosion products together with the adjacent, enclosing host cemented detrital sediment.Width of micrograph = 6 mm. (c) Details of the crystal growth texture commonly developed in ferrous hydroxide chloride (fhc) in cavities on the surface of the metal nucleus (m). BSE-SEM micrograph. Width of micrograph = 250 pm. (a) FHC microsphere (example arrowed, ms) and microgranular (mg) replacement textures developed in the metal nucleus. BSE-SEM micrograph. Width of micrograph = 20 pm.

microprobe andSEM-EDS) coupled with XRD crystal Petrography and formation mechanisms of individual structuraldata. A full characterization of the authigenic cements mineralogy of the concretions actually proved to be very difficult to achieve because few of the minerals gave good X-ray reflections and some areapparently amorphous. Ferrous hydroxide-chloride (FHC) RepeatXRD traces were takento establishwhether Examination of metallic cores in optical thin section showed unknown minerals were oxidation product artifacts, as some themto be surrounded by a dark green, transparent of the primary authigenic minerals are unstable in air and lath-shapedmineral together with lesser amounts of an rapidly decomposewhereas oxidation product XRD opaque, amorphous-appearingsulphide. In concretions reflections strengthen with time. exposed toair theseminerals aresurrounded by and Minerals (ortheir amorphous analogues) identified intimately mixed with goethite (Fig. 4b). Theamount of include a ferrous hydroxide-chloride, an acid volatile goethiteincreases with thedegree of oxidation. SEM monosulphide(probably mackinawite), pyrite, elemental micrographs reveal that microcracks in the metal. nucleus are sulphur, ankerite, siderite, Fe-calcite, Mg-calcite, ‘green invariably filled with a fibrous mineral that has precipitated rust’, magnetite, possibly greigite, akaganeite, goethite, from the microfracture walls and exhibitsa growth habit hematite, gypsum and a complex, amorphoussulphate comparable with that of thetransparent greencoloured mineral which has notbeen possible to characterize fully. mineral. Other textural details associated with the metallic Chemical composition data expressed as averages for some cores are illustrated in Fig. 4c & d. Aggregates of of these minerals are given in Table 1. microgranular,lath and filament shaped crystals coat the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE THEFROM WASH 163

Table 1. Chemical composition (wt%) of authigenic minerals in the Wash concretions

FHC SD Akagan. SD Goeth. SD Mackin. SD Pyrite SD Siderite SD Calcite SD Ankerite SD 17 14 11 7 11 13 22 9

Si02 0.40 0.10 0.38 0.12 0.40 0.1 1 b.d. b.d. 0.34 0.01 0.36 0.12 0.31 0.1 1 0.41 0.17 Ti02 0.04 0.04 0.35 0.04 0.02 0.04 b.d. b.d. b.d. b.d. 0.02 0.03 0.05 0.06 0.02 0.03 A1203 0.05 0.06 0.06 0.07 0.04 0.04 b.d. b.d. 0.03 0.06 0.04 0.04 0.04 0.04 0.05 0.08 Fe0 70.93 0.86 69.69 3.19 80.38 6.29 65.76* 0.05 47.45* 0.28 54.88 2.27 0.23 0.36 17.13 8.27 MnO 0.11 0.1 1 0.35 0.28 0.99 0.82 b.d. b.d. b.d. b.d. 0.55 0.07 0.29 0.15 0.50 0.18 MgO 0.14 0.09 0.13 0.16 0.16 0.09 b.d. b.d. 0.03 0.06 0.34 0.37 2.45 1.11 4.69 1.13 CaO 0.22 0.18 0.06 0.08 0.05 0.06 b.d. b.d. 0.08 0.06 1.75 1.60 52.97 1.36 31.40 5.66 Na20 0.22 0.16 0.31 0.31 0.39 0.18 b.d. b.d. 0.29 0.03 0.25 0.15 0.19 0.22 0.48 0.27 K20 0.03 0.03 0.06 0.06 0.02 0.02 b.d. b.d. b.d. b.d. 0.02 0.03 0.03 0.04 0.03 0.02 so4 0.10 0.10 0.12 0.10 0.10 0.10 34.22*0.61 52.42* 0.39 0.35 0.24 0.44 0.31 1.25 2.29 p205 0.04 0.01 b.d. b.d. 0.03 0.05 b.d. b.d. b.d. b.d. 0.24 0.13 0.23 0.20 0.23 0.15 c1 6.68 8.55 6.06 1.19 0.69 0.37 b.d. b.d. b.d. b.d. 0.17 0.21 0.06 0.05 0.20 0.07 Total 78.92 77.19 83.26 99.98 99.87 58.95 57.24 56.48

~~ ~~ ~~ ~~ All the analyses determined by electron microprobe except sulphides (by EDS analysis). b.d., below detection. FHC, ferrous hydroxide chloride. SD, standard deviation. *These values reported as elemental Fe and S. The number below the mineral name is the number of analyses.

surface of the metal and occupy cavities within it. Small akaganeite (a crystalline Fe(II1) oxy-hydroxide with sphericalbodies (typically <5 pm in diameter)are also significant substitution of Cl- for OH-) but XRD reflections common, and are usually associated with microcracks and of microdrill separates of materialfrom adjacent tothe fractures. Microspheres such as these have previously been metallic nucleus do not correspond to this mineral at all. A interpreted as being of microbial origin but we have no positive identification of this material could not be obtained direct evidence of this in these samples. from American Society for TestingMaterials (ASTM) Chemicalanalyses of thesemicrogranular aggregates, reference data, but a few reflections are somewhat similar to laths, filaments and microspheres (Table 1) suggest they all those of various Fe(I1) hydroxides and chlorides and have a similar composition which is close tothat of elementalsulphur. In anattempt to characterize this material better, XRD traceswere obtained from smear preparations of fresh samples and repeated after exposure to air for periods of up to 20 days. Initially black, the samples 0 Fresh FerrousHydroxideFresh Chloride (FHC) I changed to brown in air over a short period of time. Fig. 5 0 I documents structural changes that took place. An unstable, crystalline mineralcomponent with a chemical composition very similar to that of akaganeite is replaced over a period of some 20 days by akaganeite. A

One Day One 0 second unknown mineralcomponent is formedduring I exposure to air and decays within the same period and is possibly reaction a intermediate. These observations suggest that the unstable transparent mineral seen intimately associated with metallic iron is probablya Cl--substituted Fe(I1) hydroxide analogous to akaganeiteand to which it Ten Days 0 readily converts on oxidation: FHC: Feo.99(OH),.82C&.,8(compare with Fe(OH),)

Twenty Days Twenty 0 The zonal sequence typically seen in freshly cut Akaganeite 0 concretions (Fig. 4a & b) is strongly suggestive of a corrosionalteration process that results in replacement of the metallic nuclei from the periphery inwards. Within the 56 52 40 44 40 3632 20 2420 16 12 8 anoxic sediments of the Wash the reaction process must be Two Theta (Degrees) CUK anaerobic: Fig. 5. X-ray diffractograms of corrosion product separated from a Feo+Fe2+ + 2e-anodic reaction (1) concretion core by microdrilling showing the progressive oxidation of ferrous hydroxide chloride (peaks annotated 0) to akaganeite 2H' + H2 - 2e-cathodic reaction (2) (peaks annotated U) via some undetermined, intermediate Overall, these electrochemical reactions give: crystalline material (peaks annotated ?) over a period of 20 days in air at room temperature. Fe" + 2H204 Fe(OH), + H2 (3)

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 164 M. R. AL-AGHA ET AL.

Hydrogen would be oxidized rapidly on diffusion away ranging between FeS,,,, and FeSo.948when analysed with from the vicinity of the metal nucleus (FHC represented by EDS (Table1). These correspond closely with published Fe(OH),, chloride not shown). This is exactly the reaction mackinawite analyses, although diagnostic X-ray reflections that is commonly reported for the alteration of archaeologi- as reported by Takeno et al. (1970) and Takeno & Clark cal ironartifacts recovered from marineenvironments (1967) could notbe obtainedfrom XRD analysis of (Robinson 1982). microdrill separates sub-sampledfrom adjacent tothe Bearingin mind the ubiquitouspresence of iron metallic cores. sulphides in these concretions (see below), anaerobic Iron monosulphides are well known to precipitate in sulphate corrosion is a likely alternative (or additional) anoxic marine muds as a consequence of microbial sulphate corrosion mechanism: and Fe(1II) reduction (Love 1967; Berner 1970; Morse et al. 1992). The overall reaction can be approximated as follows: 4FeO-3 4Fe2' + 8e- (4) SO:- + 4H,O + 8e- + 80H- + S'- (5) 2Fe203+ 4SO:- + 9CH20-+ 4FeS + 9HCO; + H' + 4H,O Overall, these reactions can be written: (7) 4Fe0 + SO:- + 4H,O = 3Fe(OH), + 20H- + FeS (6) Any HS- producedindependently of this reaction would also react with FHC By analogy with Fe(OH),, the Fe(I1) hydroxide-chloride (FHC) would beexpected tobe extremelyreactive with Fe(OH), + HS- + FeS + HZO+ OH- (8) oxygen and also with anionic solutes (see below).

Mackinawite (FeS) Pyrite Monosulphides could not be conclusively identified optically Pyriteframboids of upto about 50pm in diameterare in thinsection. However, SEM-BSE examination shows commonin the concretions (Fig. 6a & b) and can be that ironsulphides are preferentiallylocated close tothe observed optically in reflected light and in the SEM in BSE. metal nuclei in the concretions and only occasionally cement They comprise around 100 or so individual pyrite crystals, detrital material in the adjacent sediment. Gel-like textures each typically 5 pm in diameter. These framboids are most observedin theSEM inBSE with a high back-scatter common close tothe concretioncores, but are also coefficient are considered tobe monosulphides andare disseminatedthroughout the adjacenthost sediment. EDS sometimes spatially associated with pyriteframboids. analyses of framboidal pyrite were close to the theoretical Additionally, very small crystals of a high back scatter formulae and are consistent with published data (average for coefficient mineral (

Fig. 6. SEM-BSE photomicrographs of framboidal pyrite cements associated with the metal nucleus. (a) Pyrite framboid enclosed in calcite cement (annotated c; overgrown by siderite). Note small dissimented crystals of iron sulphide (arrowed, S). Black areas are porosity. BSE-SEM micrograph. Width of photomicrograph = 300 pm. (b) Detail of pyrite framboid (individual pyrite crystal annotated P) and small undefined iron sulphide crystals (arrowed, S). BSE-SEM micrograph. Width of photomicrograph = SO pm.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 solutions. Progressively more sulphur-rich Fe-S minerals In this context it is interesting to recall that the presence are successively formed along the reaction path: amorphous of elemental sulphur is suggested in some of the microdrill FeSto mackinawite(FeS) to greigite (Fe&) to pyrite separatesfrom the textural zone directly adjacent to the (FeS,). For greigite to form in this series requires very concretion cores according to XRD data. reducing conditions. Conversion of amorphous FeS to FeS, is favoured by relatively more oxidising environments, and pyrite, once nucleated, will grow slowly from solution. Siderite Sweeney & Kaplan (1973) demonstrated experimentally that Siderite is seen readily in thin section (with both optical and pyriteframboids are formedin the presence of limited BSEM techniques), and forms a common, often pervasive oxygen activity (greigite spheres were also produced in these cement (Fig. 7). The sideritecement is most extensively experiments). However, neither greigite nor pyrite formed developed within the host sediments in a zone close to and when oxygen was excluded from their experiments. enclosing the concretion cores and occurs in two distinctive Severalworkers have argued that elemental sulphur is morphologies. The first of these consists of relatively involved in the pyritization process: coarse, zoned rhombic crystals that infill the pore space (Fig. 7a & b). More striking is the spheroidal form which is H,S + [O]+ H,O + S" typically pore-lining (Fig. 7c & d). Individual radial crystals in theseaggregates tend to be between 20 and 30pm in FeS + S" > FeS, (10) length. Texturalrelationships within any given pore

Fig. 7. SEM-BSE photomicrographs of authigenic siderite cements in the concretions. (a) General view of showing distribution of zoned rhombic siderite cements that form a pervasive pore filling. Large intergranular pore annotated P. BSE-SEM micrograph. Width of photomicrograph = 450 pm. (b) Detail of zoned rhombic siderite cement. BSE-SEM micrograph. Width of photomicrograph = 180 pm. (c) Spheroidal siderite coating detrital quartz grains (arrowed gn) and forming radial aggregates (annotated S) from the host sand flat sediment. The uniform, structureless pore filling material is an undefined oxidation product (annotated op), probably of green rust or monosulphide. Pore space annotated P. BSE-SEM micrograph. Width of photomicrograph = 300 pm. (a) Detail of siderite cements. Concentric zoned ankerite-siderite coats detrital grains (example arrowed) and spheroidal siderite (S) nucleates on this coating. Amorphous pore filling material, probably an oxidation product of green rust or monosulphide annotated op. Pore space annotated P. BSE-SEM micrograph. Width of photomicrograph = 180 pm.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 166 M. R. AL-AGHA ET AL.

indicate thatthe rhombic sideritecement is an early Ankerite precipitate which nucleates directly on detrital sand grains, Ankerite is found as a relatively minor pore filling cement whilst the spheroidal siderite commonly post-dates sulphide within the host sands. Texturally, the ankerite is often seen cements butpre-dates calcite, akaganeiteand gypsum in thin section tobe associated with siderite and calcite cements (see Figs 7 and 10). Chemical microprobe data for cements which it commonly overgrows andis readily varioussiderites are given in Table 1 andthese analyses distinguished using BSE and EDS analysis in the SEM. The include, as expected, minor Ca, Mn and Mg substitution. composition of this carbonate was determined by electron Conditions for siderite formation are thought to be quite microprobe analysis (as listed in Table 1) and corresponds to restricted. Oxygen must be totallyexcluded, HS- activity a typical, Ca-rich formula (mean of nine analyses): extremely low, HCO; activity high and iron present in abundance (Curtis & Spears 1968). Where corrosion is C~.~zFe".z,M~.~lM~.l,CO, involved, however, it is not difficult tosee how these It seems likely that these ankerite cements developed partly conditions can be attained.Anaerobic corrosion, either from dissolution of unstable (or destabilized) biogenic simple (equations 1, 2 & 3) or involving sulphate reduction calcite or aragonite: (equations 4, 5 & 6) generates Fe2+ in excess of HS-. Overall, saturation with respect to Fe" and OH- is 2CaC0, + Fe2' + Mg2++ 2HCO; + 20H- achieved,any HS- being strippedout by excess Fe" (to -2Ca(MgFe)(CO,), + 2H20 (12) form FeS). Under theseconditions, any HCO; present (eitherfrom the marine reservoir,from unstable biogenic Fe,Mg-calcites carbonates or from microbial oxidation of organic matter) will cause siderite to precipitate: Authigenic calcite is most commonly developed away from thecentres of concretionswhere it typically formsa Fe" + OH- + HCO; +FeCO, + H20 (11) pervasive pore filling cement (Fig. S). Two distinct

Fig. 8. Optical and BSE-SEM photomicrographs of calcite cements. (a) Pervasive pore filling, blocky high-Mg calcite cement. Note the detrital grain rimming, radial calcite growth fabric. Transmitted light photomicrograph, crossed polars. Width of photomicrograph = 400 pm. (b) Detail of grain rimming, acicular ferroan calcite cement. Transmitted light photomicrograph, crossed polars. Width of photomicrograph = 220 pm. (c) Zoned ferroan calcite cements fringing detrital quartz grains and foraminifera bioclast (annotated F). BSE-SEM micrograph. Width of photomicrograph = 180pm. (a) Zoned ferroan calcite cements (annotated FeC) enclosed in ankerite (arrowed, A), lining pore in host sediment. BSE-SEM micrograph. Width of photomicrograph = 180 pm. Pore space annotated P in all photomicrographs.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE FROM THE WASH 167

morphologies of calcite are present, comprising blocky, enclosed in carbonate cements. EDS analysis provides tabular crystals (Fig. 8a)and a more acicular, lozenge inconsistant iron-rich chemical compositions thatare shaped crystal habit (Fig. 8b) which correspond to frequentlycontaminated with silicon and aluminium. magnesian and ferroan compositions respectively (Table 1) . However, the combined lack of resolvable structure, its Both magnesian and ferroan varieties are found together in X-ray amorphous nature and variable chemical composition individual examples and may beintimately associated. make this materialextremely difficult to confirm as an Calcite cements also occuras coatings to bioclastic shell oxidation product of green rust. material (Fig. 8c) butthe presence of such grains does Bernal et al. (1959) and McGill (1967) describe green appear to enhance the degree of cementation. In BSE-SEM rusts, confirm their mixed Fe(I1)-Fe(II1) valence state and most of the calcite cements are seen to be compositionally notethat thesecompounds, together with magnetite, are zoned, and may be overgrown by either ankerite or siderite common corrosion products of metallic iron in water. More cements (Fig. 8d). detailedstudies of green rust formation,alteration and As with the ankerite cement, it seems likely that these environmental controls have been reported by Schwertmann calcites developedpartly from dissolution of unstable (or & Taylor (1977), Taylor & Schwertmann (1974) and Mohr destabilized) biogenic calcite or aragonite material, but may et al. (1972). alsobe derived fromthe microbial oxidation of organic matter. Ferroan calcites would be expected tobe stable Magnetite under reducingconditions similar to thosefavouring both siderite andankerite precipitation whilst the non-ferroan Magnetite was identified only by XRD, the more clearly so magnesian calcites shouldprecipitate when iron activity is after magnetic concentration of microdrill sub-samples from low, either under oxidising conditions (Fe stripped to form adjacent tothe concretion cores. This is a very minor goethite, akaganeite, etc.) or when there is excess HS- authigenic reaction product in the concretions, and has not available (Fe stripped as FeS under conditions of intensive been satisfactorily identified in thin section. As mentioned sulphate reduction). In the concretionsstudied here,the above, it is one of the predictedcorrosion products of former reaction seems more likely. metallic iron.

Green rust (ZZ) (mixed valence oxide-hydroxide) A kaganeite Freshconcretions containing a metallic nucleus contain in Akaganeite is the most abundant authigenic cement in the handspecimen a dark greenish black materialdeveloped oxidized concretions, but is also present in many of the fresh adjacent to the nucleus. On exposure to airthis material concretionswhere it occurs towards their outer parts as a alters very rapidly (over only a few tens of minutes) to a cement in the host sands. In oxidized concretions brown coloured material. As a consequence, it is extremely akaganeite is presentthroughout the concretion and even difficult to characterize this materialand effectively occurs in the metallic cores. It is readily identified by XRD, impossible to observe it in thin section. XRD analysis of the and is shown to beformed from the breakdown of FHC freshmaterial shows diffraction peakscharacteristic of during exposure to air of concretions in the laboratory (see ‘green rust 11’. During exposure to air overa four-day Fig. 5). In thin section akaganeiteappears as elongate, period, thestrong diffraction peaksfrom ‘green rust 11’ bladed opaque crystals that attain lengths of up to 100pm disappear altogether, with only X-ray amorphous products (Fig. 10a). In the SEM in BSE the bladed morphology of remaining (Fig. 9). In this respect, ‘green rust 11’ is as akaganeite is confirmed (Fig. lob), and akaganeite is seen to unstable as FHC and the iron monosulphides. post-date all carbonate cementsalthough itis enclosed in In thin sectiona rather diffuse, amorphous-appearing gypsum cement. The average of 14 microprobe analyses opaque material is present in many of the samples and may (listed in Table1) gave the following structuralformula represent the oxidation product of green rust. This material (assuming appropriate hydrogen): is optically structureless and when viewed in the SEM with F~O.YD(OH)O.~WC~LI,(compare FeO.OH) BSE is also devoid of resolvable crystalline structure (see Fig. 7c). Varioustextural relationships with other Akaganeite was first reported as a weathering product by M. authigeniccements are developedbut it is commonly Nambu(cited by McKay 1962) andnamed afterthe

10.78 Green Rust Fresh I1

Fig. 9. X-ray diffractograms of corrosion product separated from a concretion core by Green Rust 4 Days TwoTheta (Degrees) CUK microdrilling showing the reflections of freshly separated green rust and its total decomposi- tion over a period of 4 days at room temperature in air. Numbers above diagnostic peaks are d spacings.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 168 M. R. AL-AGHA ET AL.

Fig. 10. Optical and BSE-SEM photomicrographs of oxidised cements in the host sand flat sediment. (a) Large, opaque bladed crystals of akaganeite (annotated A) enclosing grain rimming rhombic siderite cements (arrowed, S). Transmitted light optical photomicrograph. Width of photomicrograph = 220 pm. (b) Corresponding BSE-SEM micrograph showing zoned siderite cement (arrowed, S) enclosed in akaganeite (annotated A). Width of photomicrograph = 180 pm. (c) Grain-coating opaque oxidation product (annotated Ox, after monosulphide?), opaque oxidized spheroidal siderite (OS, now probably hematite) together with later pore filling gypsum (annotated G) occupying the centre of pores. Transmitted light optical phothicrograph. Width of photomicrograph = 340 pm. (a) Comparable BSE-SEM micrograph of oxidized grain coating material (arrowed Ox), oxidized spheroidal siderite (annotated S) enclosed in poikilotopic gypsum (annotated G). Width of photomicrograph = 180 pm.

Akagane mine in Japan. It has subsequently been noted by process. Oxidation would beexpected away from the several authors (Logan et al. 1976; Johnston & Glasby 1978; extreme reducing conditions adjacent to the metallic nuclei. Rozenson et al. 1980; Holm et al. 1983: Holm 1984). Pye The sequence Fe metal+ FHC+ akaganeite is one of (1982, 1988) recognized and describedakaganeite in increasing Fe oxidation state: Fe(0) > Fe(I1) > Fe(II1). The concretions at Warham. As mentionedabove, significant precipitation of akaganeitein the concretions within the chlorine is found in akaganeite but its structural location is Wash sedimentsthus reflects interaction of the anaerobic uncertain. metal corrosionenvironment with the more oxidizing Based on the laboratoryobservations of FHC conversion conditions prevailing in the host sediments. to akaganeite (andthe compositional similarity between FHC and akaganeite), alteration of one mineral to the other could be direct (simplified formulae): Goethite Although not detected by XRD, microprobe and SEM-EDS

Fes(oH)yC1+ Fes0s(0H)4C1 + 5H' i-5e- (l3) analyses corresponding very closely toFeO.OH were Texturally,however, FHCand akaganeite arequite regularly obtained from oxidized concretion zones (average dissimilar in their appearance and occurrence in the natural formulae for 11 analyses Fe,.,,O.OH). They are chemically concretions. It is not therefore clear whether the conversion quite distinguishable from FHC andakaganeite by the of FHCakaganeiteto proceeds via an in situ absence of chlorine in the analysis. recrystallization process or dissolution-reprecipitationa If this material is goethite, then it is absent from fresh

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE THEFROM WASH 169

concretionsbut is very abundant inconcretions that have was almost a kilometre further inland, and sketches of the beennaturally oxidized through erosion or artificially Freiston Shore Sand Fair of 1844 show a broad expanse of oxidized in the laboratory. In thin section goethite is dark sand flats extendingout over what is now saltmarsh and red coloured to opaque andis similar in appearance to FHC, intertidal flats (Robinson 1987). It is this sand surface that and is spatiallyassociated with the nucleus of the was used for military exercises from 1891 and upon which concretions.BSE-SEM examination of polished thin relics of munitionsaccumulated. Many, perhaps most, sections with the back scatter contrast amplifier set to became buried by anoxic, muddy sediments as the saltmarsh maximium gain enables a subtle difference between FHC and higher mudflats prograded across the tidal sandsas a andgoethite to be resolved which shows that thesetwo result of successive reclamationschemes over the period minerals are intimatelyintergrown in oxidized concretions 1935-1982, and were bathed in anoxic saline pore waters. (see Fig. 4a & b). Anaerobic corrosion atthe metal/pore waterinterface resulted in the precipitation of a ferrous hydroxide-chloride (FHC) andiron monosulphide (FeS) under extremely Hematite reducing conditions in which Fe metal at valence state zero Hematite is the most stable ironmineral at the Earth’s was present. surface and so is expected toform wherever oxygen is Asthe FHC corrosioncoating developed, its outer readily available. It was identified by XRD in 5 of the 35 surfacereacted with the anoxic pore watersolutes, HS- concretionsstudied here, specifically in those recorded generating more monosulphide, and HCO;, Ca2+ andMg”, during collection of the samples in field as being exposed by resulting in the precipitation of siderite (FeCO,), ankerite erosion. In polished thin section, hematite usually occurs as (Ca(Fe.Mg)(CO,),) and ferroan calcite (CaCO,). uniform, equidimensionalhexagonal crystals although Aroundthe growing concretions, therefore, aredox spheroidal forms are also observed, possibly having formed gradient was established and was maintained so longas from oxidation of siderite (Fig. 1Oc). somemetal remained in the concretioncore. Along this redoxgradient the gradualdevelopment of very slightly more oxidising conditions variously stabilized elemental Gvusurn sulphur, greigite (Fe,&) and/or pyrite (FeS,). Green rust (Fe,O,.nH,O) and magnetite (Fe,O,) were stabilized Gypsum is overall a quantitatively quite minor component relative to FHC. The presence of these minerals is further but it is commonly observed around the periphery of the evidence for the action of anaerobic corrosion processes in concretions in most of the samples and itspresence is the concretions. confirmed by XRD and chemical analysis. It formslarge With time,each corrosion zone expanded andthe (millimetre-sized),colourless, poikilotopic crystals that concretion grew in size. FHC and FeS effectively replaced encloseseveral detritalsand grains. Texturally, gypsum is the metal nucleus inwards by corrosion processes (in this the youngest authigenic mineral that can be petrographically case no sediment was involved) whilst the carbonate cement documentedto have formed in the concretions and is zone expanded outwards(in this case cementingand observed to enclose all carbonates, akaganeite, goethite and incorporating detrital sediment intothe concretion). Such hematite cements (Fig. 10d). reactions would havepersisted until all the metal was Free access of molecular oxygen to most of the sulphide replaced. Thereafter,the FHC and monosulphides in the and carbonate minerals so far described would result in total centralcore would presumablybe ultimately replaced by oxidation of iron to Fe(II1) and sulphide to SO:-. This greenrust and/or magnetite. Theformer presence of a reaction is classically seen in the oxidation of pyrite, metallic nucleus in concretionswhere such reactions resulting in dramatic acidification: progressed to completion can be recognized as the green 4FeS, + 150, + 8H20+ 2Fe203+ 16H’ + 8SO:- (14) rust or its oxidation products now pseudomorph the former metal. Gypsum is then precipitated if calcareous shell material Exposure to oxygen when concretionswere exhumed or diagenetic calcite is available: (either by marine erosion or by geochemists) resulted in the systematic (from periphery to core) replacement of all the CaCO, + H20+ 2H’ + SO:-+ CaS04.2H,0 + CO, (15) iron minerals by akaganeite, goethite (FeO.OH) or hematite (Fe,O,). Sulphides were oxidized in part to sulphuric acid Gypsum, goethite and hematite represent an extremely which then reacted with either detrital constituents in the common assemblage wherever iron sulphides are exposed to hostsediment or authigeniccomponents to precipitate air. This happens most notably, of course, in metalliferous amorphous Fe-Al-Ca basic sulphates (which have not yet mining spoil tips (Vear & Curtis 1981). been characterized) and gypsum (CaS0,.2H,O). Total oxidation of the metallic nucleus would leave an akaganeite-goethite-hematite core tothe concretion. The Discussion only evidence of the former presence of the metallic nucleus being the absence of incorporated detrital sediment within the hydroxide-oxide core, andthe presence of ‘ghost’ Growth of the concretions textures preserved during the replacement process. The majority of the substantial number of concretions As the age of the surface on which the concretions are collected in this studycontain metallic nuclei. Theseare found is well constrained to the nineteenthcentuary, and the fragments of iron and steel left in the sediment by military presence of metallic nuclei supports a rather younger age, training exercises that date from the endof the last century. the rate of concretion growth can be defined. Concretions At this time the high tide level along the Lincolnshire coast of 20 cm diameter are common indicating a growth rate of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 170 AL-AGHA R. M. ET AL.

around 100 pm per year. This is considerably faster than is A.and M. Boult atthe FreistonField Centre, Lincolnshire, are calculated for ancient carbonate concretions (Berner 1968; thanked for their welcoming hospitality during field seasons. Alan’s Wilkinson & Dampier 1990). helpand guidance in the field made our understanding of the intertidal flats easier and much more comprehensive than otherwise would have been. Hiscareful reading of anearly draft of the Wash concretionsas analogues for ancient concretions? manuscriptcorrected some of our historical misunderstandings of The driving force forthe growth of siderite-sulphide anthropogenic activities around the Wash. J. E. Andrews and T. R. concretions in the Recent intertidalsediments of the Wash is Astin provided constructive reviews. the presence of metallic ironat valence statezero within The University of Manchester is gratefully acknowledged for a anaerobic, saline pore waters of marine origin. This is University ResearchScholarship toMRA-A, who has alsobeen supported by thePalestinian Students Fund, Arab Students‘Aid nothing morethan cathodiccorrosion that has been well International and the British-Arab Charitable Foundation. British documentedfor metallic ironartifacts from many marine Gas Exploration & Production, Reading (SDB) and the Petroleum archaeologicalsites worldwide (Robinson 1982). Wherever Science and Technology Institute, Edinburgh (CDC/SDB) are also metallic iron is brought into contact with saline, anaerobic thanked for supportingresearch into geochemical corrosion waters, cathodic corrosion must take place. In this reaction, mechanisms. anaerobic corrosion generates unstable Fe(1I) hydroxides and sulphides which react further with the detrital sediment constituents and marine pore water solutes to precipitate the References common-place diagenetic cements (i.e. siderite, ankerite and ADAMS,A. E. & SCHOFIELD,K. 1983. Recent submarine aragonite, magnesian calcite)as ‘secondary’ corrosionproducts. Similar calcite and hematite cements in a gravel from Islay, Scotland. Journal of concretionsfrom equivalent intertidal sediments of the Sedimentary Petrology, 53, 417-421. north Norfolk coast described by Pye et al. (1990) may thus AL-HASHEMI,W. S. 1977. Recent carbonate cementation from sea water in have a comparable origin. someweathered dolostones, Northumberland, England. Journal of Sedimentary Petrology, 47,1375-1391. It is the presence of the ‘secondary’ corrosion products BERNAL,J. D.: DASGUITA,D. R. & MACKAY,A. L. 1959. Theoxides and which make the concretions from the Wash similar in terms hydroxides of iron and their structural interrelationships. Clay Minerals of their mineralogy and fabric to some ancient concretions. Bulletin, 4, 15-30. However, the siderite-sulphideconcretions in Recent BERNER, R. A.1968. Rate of concretion growth. Geochimica et Cosmochimica intertidal sediments of the Lincolnshire Wash are the result Acta, 32,477-483 - 1970. Sedimentary pyrite formation. American Journal of Science, 268, of anthropogeniccontamination andare rather poor 1-23. analogues for ancientconcretions in terms of reaction - 1984. Sedimentarypyrite formation: anupdate. Geochimica et mechanisms. Theyhave formed by cathodiccorrosion of Cosmochimica Acta, 48,605-615. metallic iron and their rapid rate of growth results from the COLEMAN,J. M. & GACLAINO,S. M. 1965. Sedimentary structures: Mississibi Riverdeltiac plan. In: MIDDLETON,G. V. (ed.) Primary sedimentary high instability of metallic iron in an anoxic, saline solution structures and their hydrodynamic interpretation. Special Publications of andthe efficiency of the electrochemicalreaction Society of Economic Paleontologists and Mineralogists, 12, 133-148. mechanism. COLES,B. P. L. & FUNNELLB. M. 1981. Holocenepalaeoenvironments of Broadland,England. In: NIO, S. D.: SHUTTENHELM,R. T. E. & VAN WEERINC,TJ. C. E. (eds) Holocene marine sedimenfation in the North sea Conclusions basin. InternationalAssociation of Sedimentologists, Special Publica- tions, 5, Blackwell Scientific, Oxford, 123-131. The Wash concretionsexamined here all appearto have COLLINS,M. B., AMOS, C. L. & EVANS,G. 1981. Observations of some nucleated around metal fragments. The overall mechanisms sediment-transport processes over intertidalflats, the Wash, UK. In: NIO, S. D.; SHUTTENHELM,R. T. E. & VAN WEERING, TJ.C. E. (eds) Holocene of authigenic mineral precipitation in these concretions and marine sedimentation in the North sea basin. International Association of the driving force for concretiondevelopment have been Sedimentologists, Special Publications, 5, Blackwell Scientific, Oxford, elucidated,although the likely involvement of microbial 81-98. communities has not been considered and remains a subject CURTIS, C. D. & SPEARS,D. A. 1968. Theformation of sedimentaryiron forfuture research.However, the main driving force for minerals. Parts I and 11. Economic Geology, 63,257-270. CURTIS,S. 1987. The history of military use of the Wash. In: DOODY,P. & concretion growth is the presence of metallic iron at valence BARNETT,B. (eds) The Wash and its Environment. Nature Conservancy statezero within anaerobic,saline pore waters of marine Council Research and Survey in Nature Conservation Reports, 7, NCC, origin. This is the process of cathodiccorrosion well Peterborough, 101-103. documented for many metallic artifactsfrom marine DOODY,P. 1987. The impact of reclamation on the natural environment of the Wash. In: DOODY, P. & BARNETT,B. (eds) The Wash and its archaeological sites. Spontaneousanaerobic corrosion Environment. NatureConservancy Council Researchand Survey in generates unstable Fe(I1) hydroxides and sulphides which Nature Conservation Reports, 7, NCC, Peterborough, 165-172. react further with sedimentconstituents andpore water EVANS,G. 1965. Intertidal flat sediments and their environment of deposition solutes to precipitate common-place diagenetic cements such in the Wash. Quarterly Journal of Geological Society of London, 121, 209-245. assiderite, ankeriteand calcite. It is these ‘secondary’ - 1975. Intertidal flat deposits of the Wash, western margins of the North corrosion products which make these concretions similar in Sea. In: GINSBURC,R.N. (ed.) Tidal Deposits. 13-20. New York, terms of their mineralogy and fabric to someancient Springer. concretions. Theirpetrographic fabrics, mineralogy, -& COLLINS,M. B. 1975. The transportation and deposition of suspended sediments over the intertidal flats of the Wash. In: HAILS,J. & CARR,A. geochemistry and spatial relationships within the concretion, (eds) Nearshore sediments dynamics and sedimentation, an Interdiscipli- however, aremore indicative of the long-term effects of nary Reuiew. Wiley, London, 273-304. anaerobic,cathodic metal corrosion ratherthan ancient GAUTIER,D. L. 1982. Sideriteconcretions: indicators of early diagenesis in siderite-sulphideconcretion formation. We conclude that theGammon shale Cretaceous. Journal of Sedimentary Petrology, 52, siderite-sulphideconcretions have formed in Recent 859-871. GODWIN.H. & CLI~~ORD,M. H. 1938. Studies of the post-Glacial history of intertidal sediments of the Lincolnshire Wash as a result of British vegetation. I. Originand stratigraphy of Fenlanddeposits near anthropogenic contamination. Woodwalton, Hunts. Philosophical Transactions, 229,323-406.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021 RECENTCONCRETIONS LINCOLNSHIRE THEFROM WASH 171

Ho,C. & COLEMAN,J. M. 1969. Consolidationand cementation of recent Conservancy Council Researchand Survey in NatureConservation sediments in Atchafalaya Basin. Geological Society of America Bulletin, Reports, 7, NCC, Peterborough. 23-33. 80,183-192. ROBINSON,W. S. 1982. The corrosion and preservation of ancient metals from HOLM,N. G. 1984. The structure of p-FeOOH. Cl akaganite and its uptake marine sites. InternationalJournal of Nautical Archaeology and of aminoacids from Red Sea hot brines. Originof Life, 14, 343- Underwater Exploration, 11,221-231. 350. ROZENSON,l., ZAK,I. & SPIRO,B. 1980. The distribution and behaviour of -, DOWLER,M. J., WADSTEN,T.& ARRHENIUS,G. 1983. p-FeOOH. Cl, iron in sequence of dolomite, clays and oxides. Chemical Geology, 31, akaganeite and Fe,-,O wustite. in hot brine in the Atlantis I1 Deep Red 83-%. Seaand the uptake of theamino acids by syntheticp-FeOOH. Cl,. SCHOONEN,M. A. A. & BARNES,H. L. 1991. Reactionsforming pyrite and Geochimica et Cosrnochimica Acta, 47, 1465-1470. marcasite from solution 11: via FeS precursors below 100°C. Geochimica JOHNSTON,J. H. & GLASBY,G. P. 1978. The secondary iron oxide hydroxide et Cosmochimica Acta, 55,1505-1514. mineralogy of some deep sea and fossil manganese nodules: a Mossbauer SCHWERTMANN,U. & TAYLOR,R. M. 1977. Iron Oxides. In: DIXON,J. & and X-ray study. Geochemical Journal, 12, 153-164. WOOD,S. B. (eds) Minerals in soil environments. Soil Science Society of JORGENSEN,N. 0. 1976. Recent high Mg-calcite-aragonitecementation of America, 145-180. beach and submarine sediments from Denmark. Journal of Sedimentary SCOTCHMAN,I. C. 1991. Thegeochemistry of concretionsfrom the Petrology, 46,940-951. Kimmeridge Clay Formation of southernand eastern England. LOGAN, N. E.,JOHNSTON, J. H. & CHILDS,C.W. 1976. Mossbauer Sedimentology, 38, 79-106. spectrographicevidence for the occurrence of akaganeite in New SHENNAN,I. 1982. Interpretation of Flandrian sea level data from the Fenland, Zealand soils. Australian Journal of Soil Research, 14, 217-224. England. Proceedings of Geologists’ Association, 93, 53-63. LOVE,L. G. 1967. Early diagenetic iron sulphides in Recent sediments of the - 1986a. Flandrian sea level changes in the Fenland I: The geographical Wash England. Sedimentology, 9, 327-352. setting and evidence of sea level changes. Journal of Quaternary Science, MCGILL,I. R., MCENANEY,B. & SMITH,D. C. 1976. Crystal structure of green 1,119-154. rust formed by corrosion of cast iron. Nature, 259,200-201. - 19866. Flandrian sea level changes in the Fenland 11: Tendancies of sea MCKAY,A. L. 1962. p-ferric oxyhydroxide-akaganeite. Mineralogical level movement, altitudinal changes, local and regional factors. Journal Magazine, 33,270-280. of Quaternary Science, 1, 155-179. MOHR,E. C.J., BAREN,F. A. & SCHUYLENBORCH1972. Tropical soils. 3rd -1987. Holocene sea level changes in the North Sea. In: TOOLEY,M. J. & edition. The Hague-Paris-London. SHENNAN,I. (eds) Sea level changes. Blackwell, Oxford, 119-151. MOORE,S. E., FERRELL,R. E. & AHARONE,P. 1992. Diagenetic siderites and STOOPS, G. 1983. SEM and light microscopic observations of minerals in bog other ferroan carbonates in Modem Subsiding Marsh Sediments.Journal ores of the Belgian Campine. Geoderma, 30, 179-186. of Sedimentary Petrology, 62,357-366. SUTTILL,R. G., TURNER,B. & VAUGHAN, D.J. 1982. The geochemistry of iron MORSE,J. W., CORNWELL,J. C., ARAKAKI,T., LIN, S. & HUERTA-DIAZ,M. in Recent tidal-flat sediments of the Wash area, England a mineralogical 1992. Ironsulphide and carbonate mineral diagenesis in Baffin Bay, Mossbauer and magnetic study. Geochimica el Cosmochimica Acta, 46, Texas. Journal of Sedimenfary Petrology, 62,671-680. 205-217. MOWBRAY,DE T. 1983. The genesis of lateralaccretion deposits in Recent SWEENY, R. E.& KAPLAN,I. R. 1973. Pyrite framboid formation: Laboratory intertidalmudflat channels, Solway Firth,Scotland. Sedimentology, 30, synthesis and marine sediments. Economic Geology, 68,618-634. 425-435. SWINNERTON, H. H. & KENT, P. E. 1976. The Geology of Lincolnshire, 2nd NEALSON,K. H. 1983. The microbial iron cycle. In: KRUMBEIN,W. E. (ed.) Edition. Revised by Kent, 1976. Special Publication of Lincolnshire Microbial Geochemistry. Blackwell Scientific Publications,London, Naturalists Union. 159-190. TAKENO,S., ZOKA,H. & NIIHARA1970. Metastable cubic iron sulphides with POSTMA,D. 1977. The Occurrence and chemical composition of recent Fe-rich special reference to mackinawite. American Mineralogist, 55, 1639-1649. mixed carbonates in river bog. Journal of Sedimentary Petrology, 47, - & CLARK,A.H. 1967. Observations on tetragonalFe, Ni, 1089-1098. CO,,, S, mackinawite. Journalof Science, Hiroshima University, 5, - 1982. Pyrite and siderite formation in brackish and freshwater swamp 287-293. sediments. American Journal of Science, 282, 1151-1183. TAYLOR, R.M. & SCHWERTMANN,U. 1974. Maghemite in soils and its origin. F’YE,K. 1982. Marshrock formed by iron sulphide and siderite cementation in Parts I & I1 Clay Minerals, 10, 289-298, 299-310. saltmarsh sediments. Nature, 294,650-652. VAN SrRAATEN, L. M. J. U. 1957. Recentsandstone in thecoasts of the - 1988. Anoccurrence of Akaganeite p-Fe00H.Cl in Recent oxidized Netherlands and Rhone delta. Geologie en Mijnbouw, 19, 196-213. carbonateconcretions, Norfolk, England. Mineralogical Magazine, 52, VEAR, A.& CURTIS,C. D. 1981. Quantitative evaluation of pyrite weathering. 125-126. Journal of Earth Surface Processes, 6,191-198. -, DICKSON,J. A. D., SCHIAVON,N., COLEMAN,M. L. & COX, M. 1990. WEIDMANN,H. U. 1972. Shell deposits and shell preservation of in Quaternary Formation of siderite-Mg-calcite-iron sulphideconcretions in intertidal andTertiary estuarine sediments in Georgia,USA. Sedimentary marshand sandflat sediments, north Norfolk, England. Sedimentology, Geology, 7,103-125. 37,325-343. WILKINSON,M. & DAMPIER,M. D. 1990. Therate of growth of sandstone ROBINSON, D.1987. The Wash: Geographical and historical perspectives. In: hosted calcite concretions. Geochimica et Cosmochimica Acta, 54, DCODY, P. & BARNEIT,B. (eds) The Wash and its Environment. Nature 3391-3399.

Received 9 January 1993; revised typescript accepted 14 January 1994.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/1/157/4889851/gsjgs.152.1.0157.pdf by guest on 28 September 2021