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Ostracoda and dysaerobia in the Lower Jurassic of Wales: the reconstruction of past oxygen levels
Palaeogeography, Palaeoclimatology,Palaeoecology,99 (1992): 373-379
373
Elsevier Science Publishers B.V., Amsterdam
Ostracoda and dysaerobia in the Lower Jurassic of Wales: the reconstruction of past oxygen levels Ian Boomer a and Robin Whatley b
aSchool of Environmental Sciences, University of East Anglia, Norwich, UK bMicropalaeontology Research Group, Institute of Earth Studies, University College of Wales, Aberystwyth, UK (Received December 5, 1991; revised and accepted September 18, 1992)
ABSTRACT Boomer, 1. and Whatley, R., 1992. Ostracoda and dysaerobia in the LowerJurassic of Wales: the reconstruction of past oxygen levels. Palaeogeogr., Palaeoclimatol., Palaeoecol., 99:373 379. The Ostracoda are a subclass of small Crustacea which inhabit most aquatic environments; they have been recorded from the Cambrian through to the Recent. The biology of one group of marine Ostracoda, the Platycopina (Triassic-Recent), enables them to better withstand decreased levels of dissolved oxygen, in their immediate environment, than other ostracods. Study of a number of geological sections has already shown that this suborder often dominates stratigraphical intervals which are considered to be representative of dysaerobic conditions. In the light of this work the changingfaunal composition of the LowerJurassic of the Mochras Borehole, Wales is interpreted as a series of environmental changes which include fluctuating oxygenlevels. The degree of dysaerobia is assessed and its effect on the rate of faunal turnover is discussed.
Introduction The palaeoenvironmental use of Ostracoda has been illustrated in a number of review papers (e.g. Cronin, 1988; Hazel, 1988; Whatley, 1983, 1988). Recent evidence has shown that relative changes in the abundance of certain suprageneric groups in a number of phyla can provide qualitative evidence regarding the degree to which oxygen levels have fluctuated within a sedimentary sequence (Jarvis et al., 1988; Horne et al., 1990; Babinot and Crumiere-Airaud, 1990). U p o n the realisation that during periods of kenoxia or reduced oxygen levels, platycopid genera such as Cytherella and Cytherelloidea were better able to survive than most other genera, it was suggested that biological differences between these groups may provide one with an advantage over the other (Whatley, 1990, 1992). In
Correspondence to: R.C. Whatley, Micropalaeontology Research Group, Institute of Earth Studies, University College of Wales, Aberystwyth, UK. 0031-0182/92/$05.00
this research we are considering only benthonic dwelling organisms. Soft part morphology of modern Platycopina is adapted to their filter feeding way of life whereby specialised "branchial plates" on many of the appendages circulate currents of water from which specialised tufts of setae sieve out the suspended organic particulate matter upon which the organisms feed. The ambient water containing their food supply is also circulated across the ventral surface of their bodies, an area which acts as the respiratory surface for these and most other ostracods. A more comprehensive description of this process is given in Whatley (1992). The implication is that compared to the detrital feeding ostracods (i.e. the majority of benthonic taxa) the Platycopina gain an advantage during times of kenoxia in that they remain capable of obtaining sufficient oxygen to survive. Furthermore, the reproductive strategy of the Platycopina which involves internal brood care for the earliest instars ensures that their juveniles also survive dur-
© 1992 Elsevier Science Publishers B.V. All rights reserved.
1. BOOMER AND R. WHATLEY
374
ing dysaerobic episodes. It is also suggested that platycopids may further benefit from the greater availability of particulate organic matter during kenoxic periods (Whatley, 1990, 1992; Whatley and Arias, in press). Examples of platycopid success in the fossil record have come from the Jurassic and Cretaceous of Europe. The late Cenomanian interval in Britain (Jarvis et al., 1988; Horne et al., 1990) and France (Babinot and Crumiere-Airaud, 1990) is known to be a period of decreased oxygen levels when platycopids dominate often to the exclusion of other ostracod groups. Similarly the Toarcian sequences in Spain (Whatley and Arias, in press) and Britain (Boomer, 1989, 1991; this work) indicate that the Platycopina dominate during periods which may well represent periods of high sea-level stand and reduced oxygen levels. The British sequence is discussed in more detail below. The Oxygen Minimum Zone, which occurs in all modern oceans, is a layer of oxygen depletion within which one would expect the Platycopina to dominate. Few suitable studies exist to investigate modern platycopid distribution in the Oxygen Minimum Zone, however, two papers on Atlantic faunas (Cronin, 1983; Dingle et al., 1989) demonstrate that this is the case, as does an unpublished study by Barkham (1985) (Fig. 1). THE MOCHRAS
BOREHOLE TILL
DEPTH in metres 0 80
TERTIARY
601
LOWER
iHetl.-'~r
TRIA881C
1905
1940tobase
Fig. 1. Location of Mochras borehole and lithological succession.
The Mochrasborehole
The Llanbedr or Mochras Farm Borehole was drilled in the late 1960's by the then I.G.S. (now B.G.S.) in conjunction with the University College of Wales, Aberystwyth (Woodland, 1971). The borehole, which was totally cored, was sited approximately three kilometres west of Llanbedr village, North Wales (Grid Ref. SH 5533 2594). The sequence comprises an uppermost 80 m of till which overlies 500 m of Tertiary lacustrine clays. Below this are 1300 m of sediments representing most of the Lower Jurassic, which are underlain by 30 m of Triassic brecciated carbonates. The Lower Jurassic sequence is developed as a series of grey mudstones, siltstones and occasional limestones, all of marine origin (Fig. 1). The Toarcian sediments are predominantly light to dark grey mudstones. Evidence of shallowing such as limestones and/or arenaceous sediments are not common and it is thought that during this interval mid- to outer shelf conditions persisted. Benthonic microfossil assemblages are generally abundant throughout the Toarcian suggesting that fully anoxic events did not occur during this interval. The Platycopina are only rarely recorded below the Lower Toarcian in the Mochras section (Boomer, 1989, 1991). However, during the Middle and Upper Toarcian they represent three distinct peaks in terms of their abundance within each sample when compared with the other major ostracod groups. It is important to note that their rise in numerical importance follows swiftly after the extinction of the suborder Metacopina. Figure 2a shows the percentage of Metacopina (as a percentage of the total fauna of a sample) through the Toarcian section at Mochras. The suborder, represented by 5-7 species of healdiids, first peaks in the lower part of the Tenuicostatum zone, Lower Toarcian. This initial peak, of 60%, is followed by a trough of 30%, in turn followed by a peak of 85%. The subsequent decline to zero representation is very dramatic and, at Mochras, no members of the suborder appear again in the succession. This is, in fact, merely the local manifestation of the global extinction of the Metacopina; claims that the recent genus Saipanetta McKenzie
375
OSTRACODA AND DYSAEROBIA IN LOWER JURASSIC OF WALES: PAST OXYGEN LEVELS
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
0 20 40 60 80 100
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PERCENTAGE OF METACOPINA
PERCENTAGE OF PLATYCOPINA
SPECIES OTHER THAN METACOPINA AND PLATYCOPINA (%1
PERCENTAGE OF CYTHERACEA
Fig. 2a
Fig. 2b
Fig. 2c
Fig. 2d
Fig. 2a. Percentage of Metacopina in each sample, b. Percentage of Platycopina in each sample, c. Percentage constituted by all taxa other than Platycopina and Metacopina in each sample, d. Percentage of Cytheracea in each sample.
is a metacopid is erroneous, it is a member of the Podocopina, Bairdiacea. During the preceding Middle and Lower Liassic, there seems to have been an almost mutually exclusive relationship between the Metacopina and the Platycopina. Only when the metacopids were absent or very poorly represented did the platycopids appear in the succession. This is clearly shown in the latter part of the Lower Sinemurian and rather less clearly at other intervals in the same section (Boomer, 1991; Fig. 2). Since the metacopids were almost certainly, like their platycopid cousins, filter feeders, it is difficult to see why the two groups should have had this mutually exclusive relationship. It may be the product of the dominant position of the metacopids or some other subtle biological factor of which we are unaware. Suffice to say that where the metacopids flourished, the platycopids did not. The extent to which this mutual exclusion in the Pliensbachian may have been facies determined is difficult to judge. One may hypothesise that the reason for this exclusion is that one group, possibly the metacopids, preferred shallower water than the platycopids. There is, however, little sedimentary or macrofaunal evidence to support this and, in any case, mutual incompatibility could have the probable effect of causing the apparent depth differentiation. Indeed, this is difficult to substantiate since the
work of Whatley and Arias (in press) described peak abundances of platycopids and metacopids in Spain coinciding and local factors may, therefore, have been important. Indeed, contemporary British sections in the East Midlands (Bate and Coleman, 1975) and South West Britain (Boomer, 1992) have relatively low numbers of platycopids. Both of these latter sequences were probably deposited in much shallower water. Today Cytherella and Cytherelloidea are commonly abundant and diverse in warm shallow water throughout the tropics. There is evidence that this was the case through much of post-Triassic to Lower Jurassic time. However, probably when in competition with the metacopids they were largely denied access to shallow water and only spread there following the extinction of the metacopids. Figure 2b gives the percentage of platycopids by sample through the Toarcian of Mochras, It is interesting to note that there is a distinct time lag between the extinction of the metacopids and the first major peak of the platycopids. This, it might be argued, was brought about by the onset of conditions completely inimical to filter feeders causing the global extinction of the metacopids and the local disappearance (from a very low base level in the Tenuicostatum zone) of the platycopids. If such conditions existed, and there is a concomitant decline in the Cytheracea and Bairdiacea, it is
376
difficult to rationalise this with the rise of the Cypridacea which peak at this level (Boomer, 1991). However, this peak is made up of poorly preserved cyprids including Liasina lanceolata (Apostolescu) a species which Whatley and Arias (in press) have suggested may represent deeper water. It is possible, therefore, that the conditions which wiped out the metacopids worldwide and the platycopids, bairdiids and cytherids locally, were occasioned by a transgression. The cyprids, represented by deeper water species, would have been favoured by such an event. These dysaerobic events have also been reflected in other Lower Jurassic invertebrate groups (Hallam, 1987). As Fig. 2 shows, in the subsequent Toarcian samples there are three major peaks of platycopids. The first, of shortest duration, begins in the late Falciferum zone and, with two steps followed by reversals finally peaks in the Bifrons zone with almost 70% of the total fauna being made up by platycopids. There is a very steep decline in the late Bifrons zone, which also sees a multiple set of minor peaks and troughs within the major peak which occurs in the middle (and again near the end) of the zone at some 70%. The third peak, of longest duration, commences in the Thouarsense zone and has a double peak in the high 50's% in the succeeding Levesquei zone before declining to 20% at the end of the stage. These three peaks are interpreted as representing periods of dysaerobia; times when the Platycopina were able to benefit from their edge in survival in such environments over other groups. That this phenomenon is related to the proportions of other ostracod groups is clearly illustrated by Boomer (1991, Fig. 2). For example the Cytheracea, and the Cytherellidae of the Platycopina, peak and trough oppositely through the Toarcian as do, to a less clear extent, the Cypridacea and the Platycopina. The relationship between the filter feeders and those ostracods with other feeding strategies, expressed as a percentage of the total ostracod fauna, is given in Fig. 2c where metacopids and platycopids are plotted together. This diagram clearly shows the four peaks in the incidence of filter feeders, with the metacopid peak in the Tenuicostatum zone followed by the three peaks formed
1. B O O M E R A N D R. W H A T L E Y
by the platycopids. The construction of Fig. 2c causes these peaks of Fig. 2a, b to appear as troughs. Figure 2d plots the Cytheracea as a percentage of the total Ostracoda. Given that this superfamily is overall the most abundant, its relationship to the filter feeders is a direct guide to the fortunes of the latter. To the extent that this is true, and except for the above mentioned example of a near monotypic assemblage of the cyprid L. lanceolata in the lower part of the Falciferum zone it is virtually so, by inverting the diagram it is possible to see the metacopid spike of the Tenuicostatum zone and the three subsequent platycopid peaks. The four Toarcian peaks of filter feeders are, in our opinion, brought about by dysaerobia (kenoxic events, possibly caused by changes in sea level) allowing first the metacopids and then the platycopids to dominate due to the inimical consequences of low oxygen levels in other groups. Another indicator of palaeoceanographical change in the Mochras Toarcian is provided by considering the specific diversity of the Ostracoda. This is done in two ways: In Fig. 3a the simple species diversity is plotted for the entire Toarcian with species extinctions. The marked diversity crash at the end of the Tenuicostatum zone is to be expected after such a major extinction as that of the metacopids but this is also significant in that the L. lanceolata dominated fauna occurs at this level. This may be a flood phenomenon of an opportunistic species and/or signify major environmental change. The overall diversity increase which takes place from the lower part of the Falciferum zone. to the Levesquei zone is beset by many reversions with diversity falling, at times quite considerably. While the correlation is not a consistent one, in many cases the diversity decline is associated with high % levels of platycopids. This is notably the case in the Bifrons and Levesquei zones, but less so in the latter. The second indicator of change is that derived from Fig. 3b which displays the percentage dominance of the most abundant species in each Toarcian sample. Thus, those samples with monotypic or low diversity assemblages have the highest % dominance figure. It can be seen that the peaks of the two groups of filter feeders correspond with these peaks in dominance.
377
O S T R A C O D A A N D D Y S A E R O B I A IN L O W E R J U R A S S I C O F WALES: PAST O X Y G E N LEVELS
SPECIES 10 20 3040 50 60
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SPECIES 0 20 40 60 80 100 L
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SPECIES 40 80 120 160
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/
DIVERSITY AND EXTINCTIONS
MOST ABUNDANT SPECIES (%)
CUMULATIVE SPECIES EXTINCTIONS
CUMULATIVESPECIES ORIGINATIONS
Fig. 3a
Fig. 3b
Fig. 3c
Fig, 3d
Fig. 3a. Species diversity (solid line) and species extinctions (broken line) in each sample, b. Percentage constituted by the most abundant species in each sample, c. Cumulative number of species extinctions, d. Cumulative number of species originations. All text figures record faunal change within the Toarcian sequence of the Mochras Borehole (the Spinatum zone, Upper Pliensbachian, is marked on the graph but no faunal details are given)• Toarcian ammonite zones are abbreviated as follows: L = Dumortieria levesquei; Th = Grammoceras thouarsense; V = Haughia variabilis; B = Hildoceras bifrons; F = Harpoceras falciferum; T= Dactylioceras tenuicostatum. Ammonite zonation is based upon Woodland (1971).
As with the examples of Dover (Jarvis et al., 1988; Horne et al., 1990) and Cassis (Babinot and Crumiere-Airaud, 1990) in the late Cenomanian the increases in the dominance of the filter feeders is accompanied by a concomitant decrease in overall ostracod species diversity. This happens when, with increasing dysaerobia, more and more species, unable to obtain adequate oxygen supply to survive, become locally extinct. Some measure of the relative severity of the Mochras Toarcian kenoxic events may possibly be arrived at by comparing the percentage dominance and the actual number of species of filter feeders versus other strategists within the Spanish Liassic (Whatley and Arias, in press) and with the Cenomanian event as recorded in Dover in southern England and Cassis in Provence. Arguably not only can the platycopid/metacopid dominance be used as a signal of kenoxia but also a measure of its relative severity. For example in the Spanish Riccla and Sierra Palomera sections (Whatley and Arias, in press) also in the Dover section (Jarvis et al., 1989) the less than 5% of podocopids remaining at the height of the effect of the kenoxic event were represented by a single cytheracean species, the remainder of the fauna being made up
of five platycopid species of which one alone made up over 70%. At Cassis (Babinot and CrumiereAiraud, 1990) at the same stratigraphical level, platycopids dominated with Cytherella ovata constituting 70-80% of the ostracod fauna. On this basis, it would seem that the Mochras Upper Lias kenoxic events were dysaerobic, except perhaps for the Tenuicostatum zone metacopid spike, at almost 85% of the fauna. The subsequent platycopid species were never more than 70% and, furthermore, a greater number of podocopids (in some cases 20-30 species) survived. The extent to which the Mochras kenoxic events affected the Ostracoda in an evolutionary sense is shown in a number of figures. Figure 3c is a graph of cumulative local extinction throughout the section, to a grand total of almost 160 species extinctions. Any steepening of the curve reflects enhanced levels of extinction. The metacopid peak in the Tenuicostatum zone is associated with a very steep, initially stepped, profile and witnesses a major extinction event. The almost vertical component in the late Tenuicostatum zone being related to the terminal global extinction of the metacopes. The three platycopid spikes are all marked by a stepped, steepening of the curve but this is not an entirely
378
reliable correlation although the negative correlation in late Levesquei zone times is due more to the overall regional/global decline in faunas than to oxygen levels. Another marker of evolutionary activity, cumulative appearances, is given in Fig. 3d. Again it is the steepening of the curve which is the key to enhanced levels of species originations. The metacopid peak is associated with an initial steep slope, representing the appearance of some new metacopids (metacopes constitute 80% of the fauna at this time) and the continuation of species known to be present in the preceding Pliensbachian. The high level of new taxa in the upper part of the Tenuicostatum zone is possibly related to an influx of podocopids into the area consequent upon the amelioration of oxygen levels. However, this origination event coincides with a major extinction event resulting in a high rate of faunal turnover at this time. It would appear, therefore, that the major faunal change which results in the marked diversity increase through the Toarcian sequence at Mochras is initiated prior to the interval of low diversity assemblages dominated by cyprids in the early Falciferum zone.
Conclusions It is known that Ostracoda are highly sensitive indicators of environmental change. Changes in the ostracod population structure through a stratigraphical sequence may be used to determine qualitative changes in the dissolved oxygen levels of the ambient water. Such reconstructions are built on the recognition that the suborder Platycopina are better equipped to deal with reduced oxygen conditions than cytherids, bairdiids and cyprids due to biological differences developed for their filter feeding habit. The Toarcian sequence within the Mochras Borehole shows three distinct phases alternating between cytheracean and platycopid dominance. These are interpreted as kenoxic events, periods of low dissolved oxygen, possibly caused by changes in sea level. It is also suggested that the Metacopina, which became extinct within the lower Toarcian, also possessed this filter feeding mode of life and were thus able to withstand such changes better than their contemporaries.
1. BOOMERAND R. WHATLEY
Acknowledgments I,D. Boomer wishes to acknowledge the financial support of the Department of Education for Northern Ireland and University College London during the research on the Mochras Fauna and the award of a Leverhulme Fellowship during which time the present study was undertaken. P. Judge (U.E.A.) prepared the text figures.
References Babinot, J.-F. and Crumiere-Airaud, C., 1990. The effect of global events on the evolution of Cenomanian and Turonian marginal Tethyan faunas in the Mediterranean region. In: R.C. Whatley and C. Maybury (Editors), Ostracoda and Global Events. Chapman and Hall, London, pp. 25-39. Barkham, S., 1985. Recent Ostracoda from off the N.W. African Coast. Thesis. Univ. Wales, Aberystwyth (unpublished). Bate, R.H. and Coleman, B.E., 1975. Upper Lias Ostracoda from Rutland and Huntingdonshire. Bull. Geol. Surv. G.B., 55: 1-42. Boomer, I.D., 1989. The temporal and spatial distribution of Lower Jurassic Ostracoda from North-West Europe. Thesis. Univ. London, 382 pp. (unpublished) Boomer, I.D., 1991. Lower Jurassic Ostracod Biozonation of the Mochras Borehole. J. Micropalaeontol., 9(2): 205-218. Boomer, I.D., 1992. Lower Jurassic Ostracoda from Ilminster. Somerset. J. Micropalaeontol., 11(1): 47-57. Cronin, T., 1983. Bathyal ostracodes from the Florida-Hatteras Slope, the Straits of Florida and the Blake Plateau. Mar. Micropaleontol., 8:89-119. Cronin, T.M., 1988. Ostracods and sea-level change: case studies from the Quaternary of North and South Carolina, U.S. Atlantic coast. In: P. De Deckker, J.-P. Colin and J.-P. Peypouquet (Editors), Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 77-88. Dingle, R.V., Lord, A.R. and Boomer, I.D., 1989. Ostracod faunas and water masses across the continental margin off southwestern Africa. Mar. Geol., 87: 323-328. Hallam, A., 1987. Radiations and extinctions in relation to environmental change in the marine Lower Jurassic of northwest Europe. Palaeobiology, 13(2): 152-168. Hazel, J.E., 1988. Determining late Neogene and Quaternary palaeoclimates and palaeotemperature regimes using ostracods. In: P. De Deckker, J.-P. Colin and J.-P. Peypouquet (Editors), Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 89-102. Horne, D.J., Jarvis, I. and Rosenfeld, A., 1990. Recovering from the effects of an Oceanic Anoxic Event: Turonian Ostracoda from S.E. England. In: R.C. Whatley and C. Maybury (Editors), Ostracoda and Global Events. Chapman and Hall, London, pp. 122-138. Jarvis, I., Carson, G.A., Cooper, M.K.E., Hart, M.B., Leary, P.N., Tocher, B.A., Horne, D. and Rosenfeld, A., 1988. Microfossil assemblages and the Cenomanian/Turonian (late
OSTRACODA AND DYSAEROBIA IN LOWER JURASSIC OF WALES: PAST OXYGEN LEVELS
Cretaceous) Oceanic Anoxic Event. Cretaceous Res., 9: 3-103. Whatley, R.C., 1983. The application of Ostracoda to palaeoenvironmental analysis. In: R.F. Maddocks (Editor), Applications of Ostracoda. Univ. 'Houston, Geosciences, pp. 51 77. Whatley, R.C., 1988. Population structure of ostracods: some general principles for the recognition of palaeoenvironments. In: P., De Deckker, J.-P. Colin and J-P. Peypouquet (Editors), Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 245-256.
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Whatley, R.C., 1990. Ostracoda and global events. In: R.C. Whatley and C. Maybury (Editors), Ostracoda and Global Events. Chapman and Hall, London, pp. 3-24. Whatley, R.C., 1992. The platycopid signal: a means of detecting kenoxic events using Ostracoda. J. Micropalaeontol., 10(2): 181-185. Whatley, R.C. and Arias, C.F. in press. The use of Ostracoda to detect kenoxic events: A case history from the Spanish Toarcian. Geobios. Woodland, A.W. (Editor), 1971. The Llanbedr (Mochras Farm) Borehole. Inst. Geol. Sci. Rep., 71/81: l-115.
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Elsevier Science Publishers B.V., Amsterdam
Ostracoda and dysaerobia in the Lower Jurassic of Wales: the reconstruction of past oxygen levels Ian Boomer a and Robin Whatley b
aSchool of Environmental Sciences, University of East Anglia, Norwich, UK bMicropalaeontology Research Group, Institute of Earth Studies, University College of Wales, Aberystwyth, UK (Received December 5, 1991; revised and accepted September 18, 1992)
ABSTRACT Boomer, 1. and Whatley, R., 1992. Ostracoda and dysaerobia in the LowerJurassic of Wales: the reconstruction of past oxygen levels. Palaeogeogr., Palaeoclimatol., Palaeoecol., 99:373 379. The Ostracoda are a subclass of small Crustacea which inhabit most aquatic environments; they have been recorded from the Cambrian through to the Recent. The biology of one group of marine Ostracoda, the Platycopina (Triassic-Recent), enables them to better withstand decreased levels of dissolved oxygen, in their immediate environment, than other ostracods. Study of a number of geological sections has already shown that this suborder often dominates stratigraphical intervals which are considered to be representative of dysaerobic conditions. In the light of this work the changingfaunal composition of the LowerJurassic of the Mochras Borehole, Wales is interpreted as a series of environmental changes which include fluctuating oxygenlevels. The degree of dysaerobia is assessed and its effect on the rate of faunal turnover is discussed.
Introduction The palaeoenvironmental use of Ostracoda has been illustrated in a number of review papers (e.g. Cronin, 1988; Hazel, 1988; Whatley, 1983, 1988). Recent evidence has shown that relative changes in the abundance of certain suprageneric groups in a number of phyla can provide qualitative evidence regarding the degree to which oxygen levels have fluctuated within a sedimentary sequence (Jarvis et al., 1988; Horne et al., 1990; Babinot and Crumiere-Airaud, 1990). U p o n the realisation that during periods of kenoxia or reduced oxygen levels, platycopid genera such as Cytherella and Cytherelloidea were better able to survive than most other genera, it was suggested that biological differences between these groups may provide one with an advantage over the other (Whatley, 1990, 1992). In
Correspondence to: R.C. Whatley, Micropalaeontology Research Group, Institute of Earth Studies, University College of Wales, Aberystwyth, UK. 0031-0182/92/$05.00
this research we are considering only benthonic dwelling organisms. Soft part morphology of modern Platycopina is adapted to their filter feeding way of life whereby specialised "branchial plates" on many of the appendages circulate currents of water from which specialised tufts of setae sieve out the suspended organic particulate matter upon which the organisms feed. The ambient water containing their food supply is also circulated across the ventral surface of their bodies, an area which acts as the respiratory surface for these and most other ostracods. A more comprehensive description of this process is given in Whatley (1992). The implication is that compared to the detrital feeding ostracods (i.e. the majority of benthonic taxa) the Platycopina gain an advantage during times of kenoxia in that they remain capable of obtaining sufficient oxygen to survive. Furthermore, the reproductive strategy of the Platycopina which involves internal brood care for the earliest instars ensures that their juveniles also survive dur-
© 1992 Elsevier Science Publishers B.V. All rights reserved.
1. BOOMER AND R. WHATLEY
374
ing dysaerobic episodes. It is also suggested that platycopids may further benefit from the greater availability of particulate organic matter during kenoxic periods (Whatley, 1990, 1992; Whatley and Arias, in press). Examples of platycopid success in the fossil record have come from the Jurassic and Cretaceous of Europe. The late Cenomanian interval in Britain (Jarvis et al., 1988; Horne et al., 1990) and France (Babinot and Crumiere-Airaud, 1990) is known to be a period of decreased oxygen levels when platycopids dominate often to the exclusion of other ostracod groups. Similarly the Toarcian sequences in Spain (Whatley and Arias, in press) and Britain (Boomer, 1989, 1991; this work) indicate that the Platycopina dominate during periods which may well represent periods of high sea-level stand and reduced oxygen levels. The British sequence is discussed in more detail below. The Oxygen Minimum Zone, which occurs in all modern oceans, is a layer of oxygen depletion within which one would expect the Platycopina to dominate. Few suitable studies exist to investigate modern platycopid distribution in the Oxygen Minimum Zone, however, two papers on Atlantic faunas (Cronin, 1983; Dingle et al., 1989) demonstrate that this is the case, as does an unpublished study by Barkham (1985) (Fig. 1). THE MOCHRAS
BOREHOLE TILL
DEPTH in metres 0 80
TERTIARY
601
LOWER
iHetl.-'~r
TRIA881C
1905
1940tobase
Fig. 1. Location of Mochras borehole and lithological succession.
The Mochrasborehole
The Llanbedr or Mochras Farm Borehole was drilled in the late 1960's by the then I.G.S. (now B.G.S.) in conjunction with the University College of Wales, Aberystwyth (Woodland, 1971). The borehole, which was totally cored, was sited approximately three kilometres west of Llanbedr village, North Wales (Grid Ref. SH 5533 2594). The sequence comprises an uppermost 80 m of till which overlies 500 m of Tertiary lacustrine clays. Below this are 1300 m of sediments representing most of the Lower Jurassic, which are underlain by 30 m of Triassic brecciated carbonates. The Lower Jurassic sequence is developed as a series of grey mudstones, siltstones and occasional limestones, all of marine origin (Fig. 1). The Toarcian sediments are predominantly light to dark grey mudstones. Evidence of shallowing such as limestones and/or arenaceous sediments are not common and it is thought that during this interval mid- to outer shelf conditions persisted. Benthonic microfossil assemblages are generally abundant throughout the Toarcian suggesting that fully anoxic events did not occur during this interval. The Platycopina are only rarely recorded below the Lower Toarcian in the Mochras section (Boomer, 1989, 1991). However, during the Middle and Upper Toarcian they represent three distinct peaks in terms of their abundance within each sample when compared with the other major ostracod groups. It is important to note that their rise in numerical importance follows swiftly after the extinction of the suborder Metacopina. Figure 2a shows the percentage of Metacopina (as a percentage of the total fauna of a sample) through the Toarcian section at Mochras. The suborder, represented by 5-7 species of healdiids, first peaks in the lower part of the Tenuicostatum zone, Lower Toarcian. This initial peak, of 60%, is followed by a trough of 30%, in turn followed by a peak of 85%. The subsequent decline to zero representation is very dramatic and, at Mochras, no members of the suborder appear again in the succession. This is, in fact, merely the local manifestation of the global extinction of the Metacopina; claims that the recent genus Saipanetta McKenzie
375
OSTRACODA AND DYSAEROBIA IN LOWER JURASSIC OF WALES: PAST OXYGEN LEVELS
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Fig. 2a. Percentage of Metacopina in each sample, b. Percentage of Platycopina in each sample, c. Percentage constituted by all taxa other than Platycopina and Metacopina in each sample, d. Percentage of Cytheracea in each sample.
is a metacopid is erroneous, it is a member of the Podocopina, Bairdiacea. During the preceding Middle and Lower Liassic, there seems to have been an almost mutually exclusive relationship between the Metacopina and the Platycopina. Only when the metacopids were absent or very poorly represented did the platycopids appear in the succession. This is clearly shown in the latter part of the Lower Sinemurian and rather less clearly at other intervals in the same section (Boomer, 1991; Fig. 2). Since the metacopids were almost certainly, like their platycopid cousins, filter feeders, it is difficult to see why the two groups should have had this mutually exclusive relationship. It may be the product of the dominant position of the metacopids or some other subtle biological factor of which we are unaware. Suffice to say that where the metacopids flourished, the platycopids did not. The extent to which this mutual exclusion in the Pliensbachian may have been facies determined is difficult to judge. One may hypothesise that the reason for this exclusion is that one group, possibly the metacopids, preferred shallower water than the platycopids. There is, however, little sedimentary or macrofaunal evidence to support this and, in any case, mutual incompatibility could have the probable effect of causing the apparent depth differentiation. Indeed, this is difficult to substantiate since the
work of Whatley and Arias (in press) described peak abundances of platycopids and metacopids in Spain coinciding and local factors may, therefore, have been important. Indeed, contemporary British sections in the East Midlands (Bate and Coleman, 1975) and South West Britain (Boomer, 1992) have relatively low numbers of platycopids. Both of these latter sequences were probably deposited in much shallower water. Today Cytherella and Cytherelloidea are commonly abundant and diverse in warm shallow water throughout the tropics. There is evidence that this was the case through much of post-Triassic to Lower Jurassic time. However, probably when in competition with the metacopids they were largely denied access to shallow water and only spread there following the extinction of the metacopids. Figure 2b gives the percentage of platycopids by sample through the Toarcian of Mochras, It is interesting to note that there is a distinct time lag between the extinction of the metacopids and the first major peak of the platycopids. This, it might be argued, was brought about by the onset of conditions completely inimical to filter feeders causing the global extinction of the metacopids and the local disappearance (from a very low base level in the Tenuicostatum zone) of the platycopids. If such conditions existed, and there is a concomitant decline in the Cytheracea and Bairdiacea, it is
376
difficult to rationalise this with the rise of the Cypridacea which peak at this level (Boomer, 1991). However, this peak is made up of poorly preserved cyprids including Liasina lanceolata (Apostolescu) a species which Whatley and Arias (in press) have suggested may represent deeper water. It is possible, therefore, that the conditions which wiped out the metacopids worldwide and the platycopids, bairdiids and cytherids locally, were occasioned by a transgression. The cyprids, represented by deeper water species, would have been favoured by such an event. These dysaerobic events have also been reflected in other Lower Jurassic invertebrate groups (Hallam, 1987). As Fig. 2 shows, in the subsequent Toarcian samples there are three major peaks of platycopids. The first, of shortest duration, begins in the late Falciferum zone and, with two steps followed by reversals finally peaks in the Bifrons zone with almost 70% of the total fauna being made up by platycopids. There is a very steep decline in the late Bifrons zone, which also sees a multiple set of minor peaks and troughs within the major peak which occurs in the middle (and again near the end) of the zone at some 70%. The third peak, of longest duration, commences in the Thouarsense zone and has a double peak in the high 50's% in the succeeding Levesquei zone before declining to 20% at the end of the stage. These three peaks are interpreted as representing periods of dysaerobia; times when the Platycopina were able to benefit from their edge in survival in such environments over other groups. That this phenomenon is related to the proportions of other ostracod groups is clearly illustrated by Boomer (1991, Fig. 2). For example the Cytheracea, and the Cytherellidae of the Platycopina, peak and trough oppositely through the Toarcian as do, to a less clear extent, the Cypridacea and the Platycopina. The relationship between the filter feeders and those ostracods with other feeding strategies, expressed as a percentage of the total ostracod fauna, is given in Fig. 2c where metacopids and platycopids are plotted together. This diagram clearly shows the four peaks in the incidence of filter feeders, with the metacopid peak in the Tenuicostatum zone followed by the three peaks formed
1. B O O M E R A N D R. W H A T L E Y
by the platycopids. The construction of Fig. 2c causes these peaks of Fig. 2a, b to appear as troughs. Figure 2d plots the Cytheracea as a percentage of the total Ostracoda. Given that this superfamily is overall the most abundant, its relationship to the filter feeders is a direct guide to the fortunes of the latter. To the extent that this is true, and except for the above mentioned example of a near monotypic assemblage of the cyprid L. lanceolata in the lower part of the Falciferum zone it is virtually so, by inverting the diagram it is possible to see the metacopid spike of the Tenuicostatum zone and the three subsequent platycopid peaks. The four Toarcian peaks of filter feeders are, in our opinion, brought about by dysaerobia (kenoxic events, possibly caused by changes in sea level) allowing first the metacopids and then the platycopids to dominate due to the inimical consequences of low oxygen levels in other groups. Another indicator of palaeoceanographical change in the Mochras Toarcian is provided by considering the specific diversity of the Ostracoda. This is done in two ways: In Fig. 3a the simple species diversity is plotted for the entire Toarcian with species extinctions. The marked diversity crash at the end of the Tenuicostatum zone is to be expected after such a major extinction as that of the metacopids but this is also significant in that the L. lanceolata dominated fauna occurs at this level. This may be a flood phenomenon of an opportunistic species and/or signify major environmental change. The overall diversity increase which takes place from the lower part of the Falciferum zone. to the Levesquei zone is beset by many reversions with diversity falling, at times quite considerably. While the correlation is not a consistent one, in many cases the diversity decline is associated with high % levels of platycopids. This is notably the case in the Bifrons and Levesquei zones, but less so in the latter. The second indicator of change is that derived from Fig. 3b which displays the percentage dominance of the most abundant species in each Toarcian sample. Thus, those samples with monotypic or low diversity assemblages have the highest % dominance figure. It can be seen that the peaks of the two groups of filter feeders correspond with these peaks in dominance.
377
O S T R A C O D A A N D D Y S A E R O B I A IN L O W E R J U R A S S I C O F WALES: PAST O X Y G E N LEVELS
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CUMULATIVESPECIES ORIGINATIONS
Fig. 3a
Fig. 3b
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Fig, 3d
Fig. 3a. Species diversity (solid line) and species extinctions (broken line) in each sample, b. Percentage constituted by the most abundant species in each sample, c. Cumulative number of species extinctions, d. Cumulative number of species originations. All text figures record faunal change within the Toarcian sequence of the Mochras Borehole (the Spinatum zone, Upper Pliensbachian, is marked on the graph but no faunal details are given)• Toarcian ammonite zones are abbreviated as follows: L = Dumortieria levesquei; Th = Grammoceras thouarsense; V = Haughia variabilis; B = Hildoceras bifrons; F = Harpoceras falciferum; T= Dactylioceras tenuicostatum. Ammonite zonation is based upon Woodland (1971).
As with the examples of Dover (Jarvis et al., 1988; Horne et al., 1990) and Cassis (Babinot and Crumiere-Airaud, 1990) in the late Cenomanian the increases in the dominance of the filter feeders is accompanied by a concomitant decrease in overall ostracod species diversity. This happens when, with increasing dysaerobia, more and more species, unable to obtain adequate oxygen supply to survive, become locally extinct. Some measure of the relative severity of the Mochras Toarcian kenoxic events may possibly be arrived at by comparing the percentage dominance and the actual number of species of filter feeders versus other strategists within the Spanish Liassic (Whatley and Arias, in press) and with the Cenomanian event as recorded in Dover in southern England and Cassis in Provence. Arguably not only can the platycopid/metacopid dominance be used as a signal of kenoxia but also a measure of its relative severity. For example in the Spanish Riccla and Sierra Palomera sections (Whatley and Arias, in press) also in the Dover section (Jarvis et al., 1989) the less than 5% of podocopids remaining at the height of the effect of the kenoxic event were represented by a single cytheracean species, the remainder of the fauna being made up
of five platycopid species of which one alone made up over 70%. At Cassis (Babinot and CrumiereAiraud, 1990) at the same stratigraphical level, platycopids dominated with Cytherella ovata constituting 70-80% of the ostracod fauna. On this basis, it would seem that the Mochras Upper Lias kenoxic events were dysaerobic, except perhaps for the Tenuicostatum zone metacopid spike, at almost 85% of the fauna. The subsequent platycopid species were never more than 70% and, furthermore, a greater number of podocopids (in some cases 20-30 species) survived. The extent to which the Mochras kenoxic events affected the Ostracoda in an evolutionary sense is shown in a number of figures. Figure 3c is a graph of cumulative local extinction throughout the section, to a grand total of almost 160 species extinctions. Any steepening of the curve reflects enhanced levels of extinction. The metacopid peak in the Tenuicostatum zone is associated with a very steep, initially stepped, profile and witnesses a major extinction event. The almost vertical component in the late Tenuicostatum zone being related to the terminal global extinction of the metacopes. The three platycopid spikes are all marked by a stepped, steepening of the curve but this is not an entirely
378
reliable correlation although the negative correlation in late Levesquei zone times is due more to the overall regional/global decline in faunas than to oxygen levels. Another marker of evolutionary activity, cumulative appearances, is given in Fig. 3d. Again it is the steepening of the curve which is the key to enhanced levels of species originations. The metacopid peak is associated with an initial steep slope, representing the appearance of some new metacopids (metacopes constitute 80% of the fauna at this time) and the continuation of species known to be present in the preceding Pliensbachian. The high level of new taxa in the upper part of the Tenuicostatum zone is possibly related to an influx of podocopids into the area consequent upon the amelioration of oxygen levels. However, this origination event coincides with a major extinction event resulting in a high rate of faunal turnover at this time. It would appear, therefore, that the major faunal change which results in the marked diversity increase through the Toarcian sequence at Mochras is initiated prior to the interval of low diversity assemblages dominated by cyprids in the early Falciferum zone.
Conclusions It is known that Ostracoda are highly sensitive indicators of environmental change. Changes in the ostracod population structure through a stratigraphical sequence may be used to determine qualitative changes in the dissolved oxygen levels of the ambient water. Such reconstructions are built on the recognition that the suborder Platycopina are better equipped to deal with reduced oxygen conditions than cytherids, bairdiids and cyprids due to biological differences developed for their filter feeding habit. The Toarcian sequence within the Mochras Borehole shows three distinct phases alternating between cytheracean and platycopid dominance. These are interpreted as kenoxic events, periods of low dissolved oxygen, possibly caused by changes in sea level. It is also suggested that the Metacopina, which became extinct within the lower Toarcian, also possessed this filter feeding mode of life and were thus able to withstand such changes better than their contemporaries.
1. BOOMERAND R. WHATLEY
Acknowledgments I,D. Boomer wishes to acknowledge the financial support of the Department of Education for Northern Ireland and University College London during the research on the Mochras Fauna and the award of a Leverhulme Fellowship during which time the present study was undertaken. P. Judge (U.E.A.) prepared the text figures.
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OSTRACODA AND DYSAEROBIA IN LOWER JURASSIC OF WALES: PAST OXYGEN LEVELS
Cretaceous) Oceanic Anoxic Event. Cretaceous Res., 9: 3-103. Whatley, R.C., 1983. The application of Ostracoda to palaeoenvironmental analysis. In: R.F. Maddocks (Editor), Applications of Ostracoda. Univ. 'Houston, Geosciences, pp. 51 77. Whatley, R.C., 1988. Population structure of ostracods: some general principles for the recognition of palaeoenvironments. In: P., De Deckker, J.-P. Colin and J-P. Peypouquet (Editors), Ostracoda in the Earth Sciences. Elsevier, Amsterdam, pp. 245-256.
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Whatley, R.C., 1990. Ostracoda and global events. In: R.C. Whatley and C. Maybury (Editors), Ostracoda and Global Events. Chapman and Hall, London, pp. 3-24. Whatley, R.C., 1992. The platycopid signal: a means of detecting kenoxic events using Ostracoda. J. Micropalaeontol., 10(2): 181-185. Whatley, R.C. and Arias, C.F. in press. The use of Ostracoda to detect kenoxic events: A case history from the Spanish Toarcian. Geobios. Woodland, A.W. (Editor), 1971. The Llanbedr (Mochras Farm) Borehole. Inst. Geol. Sci. Rep., 71/81: l-115.