Can modern day molluscs help us model decalcified Red Crag back to Shelly Red Crag? a discussion

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CAN MODERN DAY MOLLUSCS HELP US MODEL DECALCIFIED RED CRAG BACK TO SHELLY RED CRAG? - A DISCUSSION HOWARD MOTTRAM Introduction The Red Crag outcrops in south-eastern Suffolk from Butley Creek to the River Stour and then in north-eastern Essex around Little Oakley and Walton-on-the Naze. The sediments are characterised by iron-stained, medium to coarse-grained sands that are sometimes topped off with a thin layer of fine sands, Boatman, 1978; Dixon, 1979.

The medium to coarse grained sands are well known for their shells, but the shells are often restricted to the lower beds and it has long been believed that the shell-less sands were originally shelly but have suffered from decalcification. This is most apparent where the boundary between shelly and shell-less sands cuts across master bedding planes, Markham, 1973, and bedding foreset planes, Kendall & Clegg, 2000.

Moreover, the moulds of shells can sometimes be found in the shell-less sands within thin clay layers that have been cemented by iron oxides and which are known as ferricretes. For the present discussion, all of the shell material in the Red Crag will be taken as having come from two groups of molluscs, the bivalves and the gastropods, as the quantity of shell material contributed by other fauna, such as foraminifers, corals, brachiopods and barnacles, is negligible.

Modern-day marine molluscs are an important source of food in many countries and research that has been stimulated by exploitation of this source of meat has provided useful information on the lifestyles of the molluscs and the constructions of their shells. Exploitation of modern-day molluscs also leads to significant quantities of waste shells, and when investigating the suitability of waste shells as aggregates in concrete, conditioners for heavy soils and as industrial filters/absorbents, further useful physical and chemical analyses have been made of the shells. The information from these two fields of research contributes to our understanding of the Red Crag molluscs and the sediments in, or on, which they lived.

The composition of modern-day marine mollusc shells The external surface of the shells of bivalve and gastropod molluscs is covered a thin, often brownish coloured, “skin� (the periostacum). This is composed of organic material, particularly a protein called conchiolin. Greater quantities of this organic material occur within the shell walls where it acts as a binder that is important to the development and integrity of the shell structure. In the Atlantic Awning Clam (Solemya velum), the shell organic content is as high as 21% by weight, Price et al., 1976, but this is an exception as there is a multitude of documented evidence that shows that the total weight of the organic content in the shells of bivalves and gastropods usually only contributes 0.1 - 6% to the weight of the shell; 3% being an average value.

At around 97% of the weight of fresh shells, the inorganic fraction is clearly extremely significant. Analyses of the inorganic fraction by x-ray fluorescence have shown that there are minor proportions of several cations including magnesium,

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silicon, aluminium, iron, and strontium but, overwhelmingly, the main cation is calcium. The latter occurs as calcium carbonate. In bivalves, some of the calcium carbonate may be amorphous during the larval and juvenile stages but in adults it occurs in two, sometimes three or even four layers that have different crystalline structures. Most commonly these crystalline calcium carbonate layers in bivalves occur as aragonite, Böggild, 1930 1, but there are exceptions. Apart from the eductor muscle scars, the shells of Oysters (Ostreoidea) are entirely composed of calcite, e.g.

Stenzel, 1963; Lee et al., 2011, while Mussels (Mytilidae) and a few other families have a transitional composition whereby the calcium carbonate occurs as calcite in its outer layers and as aragonite in its inner layers. In these mixed composition shells there can be variations in the relative developments of the calcite and aragonite in response to the prevailing environmental conditions. Lowenstam, 1954, demonstrated that calcite development is greater in the shells of the Blue Mussel (Mytilus edulis) that live in colder waters. The Mediterranean Mussel (Mytilus galloprovincialis) is a very close relative of the Blue Mussel and sea-water acidity has been demonstrated to affect its shell development, Hahn et al., 2012.

Marine gastropods are also mainly composed of aragonite, but calcite occurs more commonly than it does in bivalves. The Ormers/Abalones (Haliotidae) are somewhat unpredictable, calcite may be present either in part or in full. In other gastropods where mixed composition occurs, the relative development of the calcite and aragonite can be highly variable and may be dependent not only on the environment but the age of the individual, Dauphin et al., 1989; Gray & Smith, 2004.

Just how shelly is shelly Red Crag?

Trace levels of amino acids may be found in fossil shells by using specialised techniques and the ratios of optically left-handed (original) to right-handed (new) forms of an amino acid can help in determining the geological age of the shells and their host sediments. For the purposes of this discussion, any original organic matter will be assumed to have all decayed away and to have left behind microscopic holes in the structure of the fossil shells. As it only needs a hint of iron or manganese oxyhydroxides to give a rusty to black colouration, the chemical constituency of Red Crag sediment can be thought of as entirely composed of two inorganic compounds, silica (normal sand particles) and calcium carbonate. Zalasiewicz & Mathers, 1985, sampled a 10m shelly section from a borehole at Wantisden (TM36015215) and found that the calcium carbonate content varied from 27% to 36% by weight with an average of 33% (values obtained by my scaling off from their Fig. 6). Mottram, 1993, gave rounded average figures of 45% by weight for five samples from four boreholes at Foxhall (TM24244375) and 20% by weight for two samples from one borehole at Little Bealings (TM23334636); the respective ranges for these sites were 35% to 53% and 17% to 19%. The sieve analyses for Foxhall and Little Bealings also showed that there were virtually no “fines” (silt or clay sized particles), the average was 2% and the range was 0% to 7%. The “fines” were not further analysed but their paucity precluded the possibility of them harbouring significant quantities of comminuted calcium carbonate derived from other sources such as the Chalk or the Coralline Crag.

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Similarly, it also precluded the possibility of the “fines” harbouring significant quantities of silica. As a result of the foregoing, determining the weight of calcium carbonate in samples of Red Crag is the same as determining the weight of shell material. The shell content may be up to half of the weight of the sediment but, more commonly, it is around a third.

Evaluating the proportion of shells by weight and volume Calcite is slightly denser than silica, but aragonite is nearly 9% denser than silica. At first sight this implies that if we try to convert the proportion of shells in the sediment mix from a weight basis to a volume basis, the proportion of aragonitic shells in the Red Crag could influence the calculation: this detail will be evaluated further.

Modern day bivalves are suspension feeders and, when the environment suites a species, they often occur in huge populations. Among the gastropods, the Turret Shells (Turritellidae) are also suspension feeders that can occur in huge populations.

Most of the other gastropods are predators, or at least scavenging carnivores, and, although they can occur in large numbers, they do not occur in such huge populations: i.e. their numbers are in keeping with the ‘predator-prey ratio’. This was surely also true for the Red Crag but gastropod shells, particularly the larger types, are relatively robust and collectors have often been drawn to spectacular and whole specimens so that collections and traditional lists, e.g. Boswell & Double, 1922, can mislead us into thinking that some fossil gastropod shells are more common than they actually are. Fortunately, there have been counts of the number of individuals of each species2 e.g. Dixon, 1977a; 1977b; 1979; 2000; 2001: Zalasiewicz et al., 1988 3.

From these factorisations it is clear that the number of individuals in the Red Crag with aragonitic shells was dominant over the number of individuals with calcitic shells, see Table 1. This corresponds with what we noted earlier for modern day molluscs and we have to conclude that the supply of aragonitic shell material overwhelmingly outweighed the supply of calcitic shell material into the Red Crag sediment system.

Kendall & Clegg, 2000, analysed a 1.3m vertical section of shelly Red Crag at Walton-on-the-Naze and showed that the proportion of calcium carbonate in that small section varied between about 20% and 50% by volume over short vertical distances. (Scaling off from their Fig. 6 gives an average of 32% for their section). The proportion of calcite that they found hardly wavered from 17% (again, my scaling off from their Fig. 6) and, although the proportion of aragonite was more variable, Kendall & Clegg concluded that the calcium carbonate was composed of almost equal proportions of aragonite and calcite. However, Kendall & Clegg’s results are inconsistent with the preponderance of aragonitic shells over calcitic shells discussed above. In fact, a typical mollusc assemblage at Walton-on-the-Naze is dominated by Glycimeris with high proportions of Turritella and Venerupis and among the other molluscs, Corbula and Dosinia are of note. None of these are calcitic and, following other earlier discussions on “fines”, nor is it likely that significant quantities of calcite came from the Chalk or the Coralline Crag. Accordingly, Kendall & Clegg’s assertion in regard to the proportion of calcite will be disregarded.

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TABLE 1 COMMON NAME

TYPES OF CALCIUM CARBONATE CONTRIBUTED BY DIFFERENT MOLLUSCS IN THE RED CRAG SCIENTIFIC NAME TYPE OF CaCO3

BIVALVES THAT ARE MOST LIKELY TO BE FOUND Dog Cockle Glycimeris glycimeris Blue Mussel Mytilus edulis Soft-shell Clam Mya arenaria Carpet Shell Venerupis spp.

Basket Shell Corbula gibba Rayed Artemis Dosinia exoleta Tellin Macoma obliquax, M. praetenuisx Trough Shell Spisula arcuatax, S. obtruncatax, S. ovalis, S. constrictax Cockle Cerastoderma angustatumx, C. interuptumx,C. parkinsonix

Aragonite Calcite > Aragonite Aragonite Aragonite Aragonite Aragonite Aragonite Aragonite Aragonite

GASTROPODS THAT ARE MOST LIKELY TO BE FOUND Turret Shell Turritella triplicatax, T. incrassatax

Aragonite

BIVALVES THAT ARE ALSO COMMONLY FOUND Dogs Foot Cockle Coripia (Cardita) corbis, C. scalarisx -----------Astarte obliquatax, A. digitaria Small Scallop Aequipecten (Chlamys) opercularis Oval Venus Timoclea (Venus) ovata

Aragonite Aragonite Calcite > Aragonite Aragonite

GASTROPODS THAT ARE ALSO COMMONLY FOUND Dog Whelk Tritia (Nassa) granula, T. reticulata, T. propinquax Moon Snail Natica hemiclausa, N. catena, N. multipunctata Red incl “Left Handed” Neptunea antiqua, N. despecta, Whelks N. contraria Dog Winkle Nucella lapillus, N. tetragonax Common Whelk Buccinum undatum

Aragonite Aragonite Calcite > Aragonite Calcite Aragonite

most of the species listed above are still living: extinct species are marked x

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Consequently, in order to convert the proportion of shells in the Red Crag from a weight basis to a volume basis, the appropriate density for the calcium carbonate is that of aragonite at 2.93 Mg/m3: for the remaining sediment in the mix it is that of silica at 2.67 Mg/m3.

Earlier it was mentioned that organic material accounted for an average of 3% of the weight of fresh shells. A further refinement can be made to allow for this.

Through comparison with proteins of similar molecular weight, Fischer et al., 2004, the density of conchiolin can be taken as between 1.30 and 1.40 Mg/m3. Hence, the space taken up by organic material prior to decay and now taken up by the microscopic holes in the fossil shells equates to around 6.5% of the volume in aragonitic shells (around 6.0% in calcitic shells). This effective reduction in shell density to some extent counters the differential between the densities of aragonite and silica in samples of mixed sediment. As a result of this, when the weight to volume conversion calculations were performed, it was found that there was only a slight improvement in accuracy by making these refinements, see Table 2.

TABLE 2

COMPARISON OF THE PROPORTIONS OF SHELL MATERIAL IN SEDIMENT MIXES EXPRESSED BY WEIGHTS AND VOLUMES

% BY WEIGHT

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

% BY VOLUME*

IF SHELLS WERE COMPOSED ENTIRELY OF ARAGONITE 0.0 9.2 18.6 28.1 37.8 47.7 57.8 68.0 78.5 89.1 100.0

IF SHELLS WERE COMPOSED OF ARAGONITE BUT ALLOWING FOR MICROSCOPIC HOLES 0.0 9.3 18.8 28.4 38.2 48.1 58.2 68.4 78.8 89.3 100.0

* relative to the total volume of solids (sand grains, shell fragments, coprolites, ferricretes, etc.) i.e. excluding the intergranular pore spaces which, although weightless, may occupy some 20% to 30% of the volume of a block of the sediment

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The consequences of mollusc shells having dissolved away As shell material commonly constitutes a third and sometimes half of the volume of Red Crag sediment then, in beds where the shells have been dissolved out, the beds could have been significantly affected through the residual sand grains settling downwards: that is to say, all of the reduction in volume would have been accommodated in the vertical plane. For example, if a bed was originally 2.0m thick and if the shell content had been 35% by volume then the bed may have lost 0.7m thickness so that it is 1.3m thick today. If this change was manifested in sudden or spasmodic collapsing then we would expect to see that the bedding had been disrupted, maybe even having become chaotic. By close examination of the features at sites it has been possible to eliminate cases where the bedding has been distorted by the much later periglacial action and to recognise that the dissolution of shell material did not cause disruption and chaos. This therefore indicates that the dissolution was slow and relatively uniform over a wide area. The latter doesn’t preclude occasional pockets of undissolved shells and rare instances where only the less resistant aragonite has been dissolved leaving the calcite to preserve a Neptunea, Markham, pers. comm. Slow and relatively uniform dissolution would have been manifested through gentle settlement of the beds. In this scenario we would expect to see retention of the features of the bedding, though with modification to the angles of any inclined surfaces, as exemplified in Table 3.

TABLE 3

ORIGINAL DIP ANGLES (SHOWN WITH A GREY SURROUND) CALCULATED BACK FROM PRESENT DAY DIP ANGLES AT VARIOUS SHELL CONTENTS PRESENT DAY DIP ANGLES ° IN DECALCIFIED SEDIMENT

LOST SHELL CONTENT OF SEDIMENT 10 20 30 40 50 60

10 11 12 13 14 15 16

15 16 18 19 20 22 23

20 22 23 25 27 28 30

25 27 29 31 33 35 36

30 32 34 37 39 41 42

35 37 40 42 44 46 48

NB This is a mathematical matrix and some calculated original angles may exceed the maximum angle of repose for the sediment By way of illustration; if the foresets of a decalcified bed have a dip of 20° today and if the shell content had been 30% by volume, then by reference to Table 3, it can be expected that the original dip was 25°. If this calculated original dip angle is tested against the observations of Kendall & Clegg, 2000, it appears to be too small. For the face that they investigated, they stated that the dips of foresets were 15° in the decalcified sediment and 29° in the underlying shelly material; the latter had an

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average shell content of 32% by volume (see earlier discussion). However, if the dips in their Fig.7 are checked, it can be seen that the dips indicated for their decalcified sediment were not 15° but were approximately 20° and thus more in keeping with the magnitude of the calculated values shown above in Table 3.

As the slopes of the foresets within each bed were largely controlled by how energetic the prevailing currents were, the slopes vary from one bed to another and so we shouldn’t compare foreset slopes from different beds on either side of a decalcification boundary. Kendall & Clegg were extremely fortunate to find an exposure where the decalcification boundary cut through foresets in the same bed because decalcification boundaries, more often than not, coincide with master bedding planes or occur within horizontally bedded sands. These last two scenarios were possibly caused by muddy or indurated laminae at the bases or tops of some beds reducing the ability of acidified groundwater to percolate downwards.

The supply of shells would have fluctuated depending upon their availability and the energy levels of the currents moving them. Furthermore, shells were not necessarily buried evenly throughout a bed. This would have been especially so where the foreset slopes were steep and shells slid, “avalanched”, down the foresets so that there was a greater concentration in the lower parts of beds. Therefore, the distribution of shell material in the medium to coarse grained sands would have been variable. For example, see on-line BGS GeoScenic photos of Buckanay Farm Pit at Alderton (TM356424). Photo P211457 shows shelly sands sandwiched between nonshelly sands and P211458 shows shelly sands over-riding non-shelly sands.

Conclusions The Red Crag is, perhaps, only a little over 21/2 million years old and many species of bivalve and gastropod molluscs that lived then have continued through to the modern day, while those species that have become extinct have close relatives that live on.

Firstly, this gives us confidence that the fossil mollusc shells that we find in the Red Crag are genuinely representative of the communities that inhabited the range of shallow marine environments in which the Red Crag was deposited. Secondly, this gives us confidence that, when the Red Crag was deposited, not only was the shell material dominated by calcium carbonate, but the type of calcium carbonate was aragonite.

Where the boundary of decalcification traverses inclined surfaces, notably foresets within beds forming sandwaves (“megaripples”), it has been possible to compare the features ‘before and after’ decalcification. The settlement of the sediment that occurred after decalcification has flattened the angles and when the slope angles have been compared to calculations of the theoretical effect, it has been found that they are reasonably closely matched. On this basis it is unlikely that, following dissolution of shell material, the residual sand grains somehow repacked in an even tighter configuration (cf Kendall & Clegg, 2000).

More field data would be useful to show how reliable the theoretical calculations are for reconstructing the slopes of features in decalcified Red Crag. Unfortunately, the number of exposures has vastly reduced in recent times and exposures soon

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become degraded. Those exposures that are currently available are worthful although they haven’t allowed further observations and measurements to be made that help with the discussion here. Exposures change with time and GeoSuffolk and The Coast and Heaths Project are continuing to clean up pit faces with the invaluable permissions of the sites’ landowners. So, fingers crossed for the future of the Suffolk Red Crag sites ……………

Acknowledgements I thank my gurus on Red Crag, Bob Markham and Roger Dixon. They have always willingly shared their vast knowledge generously and constructively.

Notes 1 Böggild, 1930, gave a comprehensive account of the different layers in the shell walls of many living and fossil molluscs. This reference is still quoted but it is difficult to acquire. The Natural History Museum’s own descriptions are in part accessible on-line and cover several bivalve families, Taylor et al., 1973. Other than these two sources of information, it was necessary to use many other published articles where each research project has investigated a limited number of species. There are too many of these articles to list under References.

Böggild carried out his determinations of the calcite v aragonite composition by examination under a polarising microscope. Later researchers using microscopic examination often applied Feigl’s solution to blacken the surface of aragonite, Kato et al., 2003: Feigl’s solution would also blacken other orthorhombic carbonates if they were present, but not trigonal calcite. Modern researchers tend to use X-ray diffraction which more easily identifies the form of calcium carbonate when the crystals are very small and of low concentration.

2 For bivalves, half an individual is either an entire single valve or a fragment with so much of the hinge that the remainder would be unidentifiable.

For gastropods, one individual is either a shell with an apex or a shell lacking so little of the apex that the remainder would be unidentifiable.

3 Only the lower beds that Zalasiewicz et al., grouped into the Sizewell Member are Red Crag, the upper beds that they grouped into the Thorpeness Member should still be regarded as Norwich Crag.

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Dauphin, Y., Cuif, J. P., Mutvei, H. & Denis, A. (1989). Mineralogy, chemistry and ultrastructure of the external shell-layer in species of Haliotis with reference to Haliotis tuberculata (Mollusca: Archaeogastropoda). Bulletin of the Geological Institutions of the University of Uppsala, N. S., 15: 7-38.

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Markham, R. (1973). Itinerary I - Suffolk and east Essex, pp2-11. In Greensmith., J. T., Blezard, R. G., Bristow, C. R, Markham, R. & Tucker, E. V., No. 12: The Estuarine region of Suffolk and Essex. Geologists’ Association Guides.

Mottram, H. B. (1993). Geological details of two pits near Kesgrave. Trans. Suffolk Nat. Soc. 29: 62-69. https://issuu.com/suffolknaturalistssociety/docs/tsns29t Price, T. J., Thayer, G. W., LaCroix, M.W. & Montgomery, G.P. (1976). The organic content of shells and soft tissues of selected estuarine gastropods and pelecypods.

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Zalasiewicz, J. A., Mathers, S. J., Hughes, M. J., Gibbard, P. L., Peglar, S. M., Harland, R., Nicholson, R. A., Boulton, G. S., Cambridge, P. & Wealthall, G. P. (1988).

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