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Implications of isotopic and other geochemical data from a Cretaceous-Tertiary transition in southern Africa
319
Chemical Geology (Isotope GeoscienceSection), 80 ( 1990) 3 19-325 Elsevier Science Publishers
B.V., Amsterdam
Implications of isotopic and other geochemical data from a Cretaceous-Tertiary transition in southern Africa BalthazarTh. Verhagena,Marian TredouxaTol, Nicholas M. Lindsay@,JacquesP.F. Sellschop”, A. Katharina von Salis Perch-Nielsenband Christian Koeberl” “SchonlandResearchCentre, Universityof the Witwatersrand,Johannesburg(SouthAfrica) bGeologicalInstitute, ETH-Z, Ziirich (Switzerland) CZnstituteforGeochemistry,Universityof Vienna, Vienna (Austria) (Received November 28, 1989; revised and accepted May 2, 1990)
ABSTRACT Verhagen, B.Th., Tredoux, M., Lindsay, N.M., Sellschop, J.P.F., von Salk Perch-Nielsen and Koeberl, Chr., 1990. Implications of isotopic and other geochemical data from a Cretaceous-Tertiary transition in southern Africa. Chem. Geol. (Isot. Geosci. Sect.), 80: 319-325. Isotopic, palaeontological and geochemical details of a drill-core section at Richard’s Bay, South Africa, are reported. Foraminiferal evidence shows that the section encompasses a transition from the Maastrichtian to the Danian, one of the very few sites in the Southern Hemisphere on land. The carbon isotope profile of the carbonate fraction shows a large negative excursion at the transition. The corresponding oxygen isotope excursion is largely parallel but less pronounced. It is argued that this Cretaceous-Tertiary (K/T) boundary is characterised by strong and well-identified local influences, seen in the mineralogy, in trace-element data, and in partially parallel carbon and oxygen isotopic signals. A possible scenario is presented in which marine regression is accompanied by increased influence of terrestrial material in this lagoonal environment. This could point towards similar. if often less pronounced, influences in some other welldocumented K/T sites.
1. Introduction Several cored boreholes were drilled for engineering geological purposes at Richard’s Bay harbour (Fig. 1). Initial microfossil analysis indicated that in one of thesecores (3F66) the K/T boundary was intersected at a depth of N 22 m, establishing the site as one of the very few in the Southern Hemisphere on land. A section of this core between - 22.5 and - 2 1.6 m (Fig. 2 ) was re-examined in detail for calacreous nannofossils. Major- and trace-elePresent addresses: “Department of Geology, University of Cape Town, Rondebosch 7700, South Africa. Pedro de Valdivia 295, Clasiflcador, 31 Correo 9, Santiago, Chile.
0168-9622/90/$03.50
ment analysis was performed in the same interval and carbon and oxygen isotope analysis over a depth range several metres above and below the boundary. A concretionary section of calcareoushardground occurs within this section between - 22.1 and - 2 1.9 m. Several such concretionary layers have been observed (Kennedy and Klinger, 1972;Orr and Chapman, 1974) to extend laterally for > 500 m in a deep trench excavated at the site aswell as in other boreholes, down to depthsof 45 m (Maud and Orr, 1975) . These concretionary hardground sections at several depths below, above and at the faunal and floral K/T transition, can be interpreted (Kennedy and Klinger, 1972; Orr and Chapman, 1974) as having formed during periods
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Fig. 1. Various reported Southern Hemisphere K/T section locations. Inset: Richard’s Bay lagoon before harbour development, showing 3F66 and two other borehole sites.
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Fig. 2. 613C (0 ), 6”O (0) and selected major and trace chemical profiles (referred to in text) for the section -23.7 to - 18.9 m of borehole 3F66, Richard’s Bay. Inset gives details of main nannofossil zones.
of marine regression.Therefore, the site seems to have alternated between regressive and transgressiveconditions over the period of its sedimentary deposition, seenin the core down to at least the mid-Cretaceous. 2. Microfossil analysis The re-examination for calcareous nannofossils confirmed the existence of a reasonably
complete record of the upper Maastrichtian (Micula prinsii, Nephrolithusfiequensand - 80 other well-preserved species) up to -22.3 m. A sample at -22.08 m within the concretionary horizon, was found to be essentially barren of nannofossils. The hardground immediately above this barren zone mostly contains the early Tertiary “disaster” planktons Thoracosphaera and Baarudosphaerabigelowii. While the Biscutum romeinii and B. parvulum sub-
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AND OTHER
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DATA
FROM
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zones are missing, the presence, between -21.88 and -21.80 m, of the Chiustozygus ultimus and Toweius petalosus subzones confirms the presenceof the upper part of the NPI of the basal Danian. Above - 2 1.63 m the appearance of Cruciplacolithus tenuis indicates the transition to the NP2 zone (see Fig. 2). Therefore, the presenceof the K/T boundary seems to be well established within this interval. 3. Methods 3. I. Isotopic analysis Carbon and oxygen isotope ratio measurements were conducted on carbon dioxide generated with 100% orthophosphoric acid from bulk samples from -23.8 to - 18.9 m of the same core. Therefore, the data reflect the isotopic content of the entire carbonate fraction, the absolute concentration of which was found to be highly variable over the sections studied; it is at its lowest in the range - 2 1.9 to 2 1.6 m, i.e. just above the hardground. The observed oxygen and carbon isotopic valuesare shown in Fig. 2. They are in the usual J-notation in parts per thousand (%o) relative deviation from the PDB limestone standard. Overall reproducibility of duplicates on all measurements is N +-O.l%o for 613C and - ? 0.2%0for alsO. 3.2. Elemental analysis XRF (X-ray fluorescence) analysis was employed for most of the major elements and/or instrumental neutron activation analysis for “chalcophile” trace elements. Selected data from these analysesare plotted in Fig. 2. 4. Results 4. I. Isotopic data The 6’*0-values in the upper Maastrichtian are compatible with marine deposition and re-
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main approximately constant over the interval -23.9 to -22.23 m. In the range of -22.1 to - 2 1.9 m the 6’ *O undergoessome excursions, the most important of which is the drop to < -4%o just above -22.1 m. It then rises abruptly to values of N -2.5%0, to return to < - 4%0above - 22.01 m. In the Danian, and up to - 18.9 m, 6 ‘*O reverts to values slightly heavier than those from below - 22.2 m. The 613C-valuesare also marine-like (0 to + l%o) up to - 22.25 m and then drop sharply between -22.1 and -21.95 m to a minimum of < - lO%o.Above this follows a more gradual rise, parallel to the rise in 6 ‘*O, although the 613C-valuesof the Danian remain somewhat more negative (at - 2 to - 1%o) than for the upper Maastrichtian. The 613Cexcursion is largely coincident with the hardground/concretionary layer. It is interpreted as representing a changefrom a marine to a terrestrial sourceof carbonate,precipitated from water of meteoric origin which was capable of perfusing and replacing the as yet unconsolidated silt when the areabecametidal to sub-tidal during marine regression. Negative values for 6’ 3C down to < - 1O%oare observed in terrestrial pedogenic carbonate (calcretes) in southern Africa (Mazor et al., 1974; Netterberg, 1980). The partially parallel negative excursion in 6 “0 can similarly be interpreted as due to the dominance of meteoric water during at least part of this period of inferred marine regression. This accords with the conclusions of Kennedy and Klinger ( 1972), who regard the concretionary hardgrounds as having formed through early diagenesis,only a little distance below the sediment-water interface, during shallower water episodes. The return to more positive 6’ *O-values between -22.07 and -22.02 m could be interpreted as the result of diagenesisor of bioturbation, or due to a temporary reestablishment of marine conditions. As this return coincides with the peak in the negative excursion of the 6 13Cand a barren section in the nannofossil
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record, a more probable interpretation would be that the freshwater inflow had declined and meteoric evaporative (lagoonal) basin conditions developed. From - 22.01 m the freshwater flow again increased, to be gradually overtaken by a renewed marine transgression. Therefore, the sudden disappearance of all nannofossils can be related to the changeaway from marine conditions, which, neatly paralleled in the isotope and nannofossil (mainly “disaster” speciessuch as Thorucosphaeraand B. bigelowii) records, show a gradual resurgenceabove - 2 1.9 m. 4.2. Chemical data The carbonate-rich nature of the concretionary horizon shows a clear chemical fingerprint in the six-fold CaO increase. SiOz and A&O3 (and also MgO, not shown) are consistently lower in this interval (see Fig. 2). Petrographic observations indicate that this is almost certainly due to dilution of the clay and silt fraction of the sediment by the carbonate (as micritic calcite) content. The terrestrial origin of the carbonate is further suggestedby the markedly lower values of Br within the horizon. No “iridium anomaly” nor any enrichment of Co, Ni, Sb and As - all usually enhancedin K/T boundary clays - could be observed. Ir concentrations (Fig. 2 ) rise monotonously from < 1 ppb in the Maastrichtian to N 3.5 ppb in the lower Danian. The same trend is seen,if at much higher concentrations, for Cr. Other elements show slightly decreasedvalues in the hardground. An anomalous zone might have beenlost in the 10cm of unrecovered core material, but the inferred trends in oxygen and carbon isotopes and in chemical variations acrossthe hiatus would argue against this. 5. Discussion The negative excursion of amplitude > 1O?ho in 613C is the largest yet reported for a K/T
B.Th. VERHAGEN
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boundary. More usual are rangesof m + 1oloofor some New Zealand sites (Wolbach et al., 1987; B.Th. Verhagen,unpublished data, 1988) and at most + 3%0for various Mediterranean and European sites (Perch-Nielsen et al., I982 ). At Caravaca, Biarritz and Lattengebirge, a strikingly parallel behaviour between6i80 and 6’ 3C is seenover the analysedsections.However, the S”O excursion rangesare greater (2-3%0) and the 613C ranges much smaller ( l-4%0) than observedat Richard’s Bay. Perch-Nielsen et al. ( 1982) found it “ ... interesting to note that there is frequently a positive correlation between the oxygen- and carbon-isotope changes...“,
but did not proffer any model which would accomodate such parallel behaviour of the two isotopes. More recently, Rampino and Volk ( 1988) proposed a mechanism, through dimethyl sulphide liberation and change in albedo, which links - albeit far from universally - decreasedmarine biological productivity and increasedocean temperature. However, in their review of carbon and oxygen isotope data at K/T transitions, theseisotopes are taken as independent indicators of biological activity and temperature of the ocean, respectively. Wolbach et al. (1987) suggestthat a terrestrial carbon source could have been responsible for a- l%o swing in their carbon isotope 6 values for a New Zealand K/T site, but do not consider the possible influence of associated meteoric water on their oxygen isotopic values. In a study of the carbon isotope shift at the Permian-Triassic boundary, Magaritz et al. ( 1988) present data which in one section shows a strong correlation between Si80 and 6i3C. However, in their discussion the authors discount the significance of this information and the 6 ’ 3C data are treated as through they behaved independently of S”O. A similar approach is taken by Holser et al. ( 1989). Gruszczynski et al. (1989) present strikingly similar excursionsof N 11%oin both S*‘0
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and 613Cin composite sections acrossthe upper Permian below the P/T transition. Having dismissed diagenetic processes,they do address the covariance of the curves by considering oxidation of marine organic matter and concomitant kinetic oxygen isotope fractionation. This in turn is postulated as having produced major global changesin the oxygen isotope composition of seawater. However, the data in the extant literature seem to discount such a possibility for the K/T boundary. If 6180-values are to be regarded as indicators of, for example, water temperature, this requires that y1oother mechanism is known to exist by which the oxygen isotopic value of the carbonate at the site could have been altered. Such a requirement is most reliably satisfied at deep ocean depositional sites, where these excursions are usually neither as pronounced nor parallel, e.g.the South Atlantic Deep SeaDrilling Project (DSDP) Site 524 (Hsii et al., 1982) or the north Pacific DSDP Site 577 (Zachos et al., 1989). Similarly, when variations in 6 13Cshow trends similar to those in 6”0, they cannot simply be treated in isolation but clearly have to be considered with referenceto the latter. Parallel behaviour of oxygen and carbon isotopes in carbonates suggestssome common driving mechanism. For the Richard’s Bay section there is independent mineralogical and chemical evidence which suggestsa marine to terrestrial transition. In cases of parallel isotopic behaviour at other K/T boundary sections, often representing shallow marine environments, the mechanism might be lessclearly discernible. This is probably the reason why, as far as we are aware, no previous analysis in terms of some unified model for concordant oxygen and carbon isotope changes at some K/T boundary sites has been attempted. 6. Concluding remarks For Richard’s Bay, we now have various indicators of the chain of events at this K/T
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boundary site. Most striking is the unprecedented carbon isotope excursion, which by its magnitude allows for practically no other interpretation than a change from marine to terrestrial carbonate sedimentation conditions and back again. The generally parallel oxygen isotope variation supports this, with the added evidence, in the trend reversal, of possible evaporative basin conditions. Therefore, taking our interpretation of a changeto a meteoric source of the water as probable, our 6’*0 data for the excursions at the transition cannot be interpreted in terms of water temperature; however, such interpretation might be possible for the isotopically more featureless (shallow marine) sections above and below the transition. The coincidence of the inferred marine regressionwith the K/T boundary needsto be discussed. According to Hallam ( 1987), increasedmantle plume activity has the “potential of causing uplift of extensive sectorsof continents and hence regression of epicontinental seas. Epeirogenic subsidence on the continents and marine transgression might be expected to follow episodes of substantial volcanic eruptions”,
which could have influenced the changes in fauna1and floral assemblagesat and following the K/T transition. Apart from the well-known El Kef section, the K/T transition was observed at two other African sites: the Majunga Basin in Malagasy and the Sokoto Basin in northern Nigeria (A.K. von Salis Perch-Nielsen, unpublished data, 1988). It was found to be barren in nannofossils in the former and non-marine in the latter. The - admittedly sparse- evidence therefore points towards a marine regression or uplift which could have involved a major part of the continent. There is little evidence for any gradual shallowing prior to the regression event inferred here. The entire section along the present coastline shows very uniform depositional conditions over extended periods. The micro-
324
fossil evidence also shows no intermediate between well-preservedvery diverse fully marine speciesand the bottom of the hardground section which is barren. The absenceof a IO-cm section of core should be noted. Any assessment of the amount of sea-level change associated with the K/T regression would be totally speculative on the basis of available data. No direct evidence of fallout is seen in the Richard’s Bay section, either from volcanic activity on the southern continent or from the Deccan traps, India, which at that stage (65 Ma) would have been in relative proximity. The clay mineral assembly both below and above the boundary is very similar and dominated by smectite, with minor illite and kaolinite. This can be interpreted as an unchanged source of weathered material from the basalts of the nearby Lebombo range. Therefore, the clay matrix of the entire core would tend to mask a diffuse volcanic contribution around the transition. Similar conclusions suggesting local conditions were were drawn by Dia et al. ( 1989) from Pb isotopic data for the marine K/T boundary sites of Stevns Klint in Denmark, Caravacain Spain and Woodside Creek in New Zealand. The section of the Richard’s Bay sequence analysed in this contribution in all probability spansan extended period and could be better explained by events associatedwith volcanism which at the end of the Maastrichtian became particularly intense (Axelrod, 1981; Courtillot et al., 1986) - also in southern Africa (Scholtz, 1985) - than by an “impact” mechanism. An indirect indication of volcanism could be the relative persistenceof isotopically “lighter” carbonate values in the siltstones above the K/T boundary. It suggeststhat this, and possibly part of the large negative excursion in the carbon isotope profile of the hardground, might have been generatedin addition by higher atmospheric concentrations of isotopically “light” CO2 of volcanic origin. The somewhat heavier 6’*0-values (by N 0.69/00)of the carbonatein the siltstone above
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the boundary concretionary layer as compared to that below could tentatively be ascribed to a cooling of the ocean. However, the relatively shallow continental shelf/lagoonal conditions which seemto have persisted at this site would tend to restrict the global validity of such a conclusion. Acknowledgements The authors wish to expresstheir appreciation to R. Maud and A. Scholtz for making the core materials available and to G. Lambert for initial microfossil information. They also gratefully acknowledgefruitful discussionswith these three colleagues,and the helpful suggestions of an anonymous referee.0. Malinga and L. de Matos are thanked respectively for assistance which the isotopic measurementsand elemental analysis and the Department of Geology, University of the Witwatersrand, for XRF and XRD analyses.For financial support the authors are indebted to the University of Witwatersrand and the South African Foundation for ResearchDevelopment. References Axelrod, D.I., 1981. Role of volcanism in climate and evolution. Geol. Sot. Am., Spec.Pap. 185, 59 pp. Courtillot, B., Besse,J., Vandamme, D., Montigny, R., Jaeger, J-J. and Cappetta, H., 1986. Deccan flood basalts at the Cretaceous/Tertiary boundary? Earth Planet. Sci. Lett., 80: 361-374. Dia, A., Manhbs, G., Dupre, B. and All&e, C.J., 1989. The Cretaceous-Tertiary boundary problem: An assessmentfrom lead isotope analysis. Chem. Geol., 75: 291-304. Gruszczynski, M., Halas, S., Hoffman, A. and Malkowski, K., 1989. A brachipod calcite record of the oceanic carbon and oxygen isotope shift at the Permian/Triassic transition. Nature (London), 337: 6468. Hallam, A., 1987. End-Cretaceous mass extinction event: argument for terrestrial causation. Science,238: 12371242. Holser, W.T., Schonlaub, H-P., Attrep, M., Boeckelmann, K., Klein, P., Magaritz, M., Orth, C.J., Fenninger, A., Jenny, C., Kralik, M., Mauritsch, H., Pak, E., Schramm, J-M., Stattegger, K. and Schmiiller, R., 1989. A unique
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AND OTHER
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DATA
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geochemical record at the Permian/Triassic boundary. Nature (London), 337: 39-44. Hsii, K.J., He, Q., McKenzie, J.A., Weissert, H., PerchNielsen, A.K., Oberhlnsli, H. et al., 1982. Mass mortality and its environmental consequences. Science, 216: 249-256. Kennedy, W.J. and Klinger, H.C., 1972. Hiatus concretions and hardground in the Cretaceous of Zululand (South Africa). Palaeontology, 15 (Part 4): 539-549. Magaritz, M., BL, R., Baud, A. and Holser, W.T., 1988. The carbon-isotope shift at the Permian/Triassic boundary in the southern Alps is gradual. Nature (London), 331: 337-339. Maud, R.R. and Orr, W.N., 1975. Aspectsof the post-Karoo geology in the Richard’s Bay area.Trans. Geol. Sot. S. Afr., 78: 101-109. Mazor, E., Verhagen, B.Th., Sellschop,J.P.F., Robins, N.S. and Hutton, L.C., 1974. Kalahari ground waters: their hydrogen, carbon and oxygen isotopes. In: Isotope Techniques in Groundwater Hydrology, 1. I.A.E.A. (Int. At. Energy Agency), Vienna, pp. 203-225. Netterberg, F., 1980. Geology of southern Africa calcretes, 1. Terminology, description, macrofeatures and
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classification. Trans. Geol. Sot. S. Afr., 83: 255-238. Orr, W.N. and Chapman, J.R., 1974. Danian (Palaeocene) marine rocks at Richard’s Bay, South Africa. S. Afr. J. Sci., 70( 8): 247-249. Perch-Nielsen, K., McKenzie, J. and He, Q., 1982. Biostratigraphy and isotope stratigraphy and the ‘catastrophic’ extinction of calcareousnannoplankton at the Cretaceous/Tertiary boundary. Geol. Sot. Am., Spec. Pap., 190: 353-371. Rampino, M.R. and Volk, T., 1988. Mass extinctions, atmospheric sulphur and climatic warming at the K/T boundary. Nature (London), 332: 63-65. Scholtz, A., 1985. The palynology of the upper lacustrine sediments of the Arnot pipe (Banke), Namaqualand. Ann. S. Afr. Mus., CapeTown, 95(Part 1): l-37. Wolbach, W.S., Gilmour, I. and Anders, E., 1987. Environmental changesacrossthe K-T boundary at Woodside Creek, New Zealand. Lunar Planet. Sci.Conf., 19: 1286-1287. Zachos, J.C., Arthur, M.A. and Dean, W.E., 1989. Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary. Nature (London), 337: 61-64.
Chemical Geology (Isotope GeoscienceSection), 80 ( 1990) 3 19-325 Elsevier Science Publishers
B.V., Amsterdam
Implications of isotopic and other geochemical data from a Cretaceous-Tertiary transition in southern Africa BalthazarTh. Verhagena,Marian TredouxaTol, Nicholas M. Lindsay@,JacquesP.F. Sellschop”, A. Katharina von Salis Perch-Nielsenband Christian Koeberl” “SchonlandResearchCentre, Universityof the Witwatersrand,Johannesburg(SouthAfrica) bGeologicalInstitute, ETH-Z, Ziirich (Switzerland) CZnstituteforGeochemistry,Universityof Vienna, Vienna (Austria) (Received November 28, 1989; revised and accepted May 2, 1990)
ABSTRACT Verhagen, B.Th., Tredoux, M., Lindsay, N.M., Sellschop, J.P.F., von Salk Perch-Nielsen and Koeberl, Chr., 1990. Implications of isotopic and other geochemical data from a Cretaceous-Tertiary transition in southern Africa. Chem. Geol. (Isot. Geosci. Sect.), 80: 319-325. Isotopic, palaeontological and geochemical details of a drill-core section at Richard’s Bay, South Africa, are reported. Foraminiferal evidence shows that the section encompasses a transition from the Maastrichtian to the Danian, one of the very few sites in the Southern Hemisphere on land. The carbon isotope profile of the carbonate fraction shows a large negative excursion at the transition. The corresponding oxygen isotope excursion is largely parallel but less pronounced. It is argued that this Cretaceous-Tertiary (K/T) boundary is characterised by strong and well-identified local influences, seen in the mineralogy, in trace-element data, and in partially parallel carbon and oxygen isotopic signals. A possible scenario is presented in which marine regression is accompanied by increased influence of terrestrial material in this lagoonal environment. This could point towards similar. if often less pronounced, influences in some other welldocumented K/T sites.
1. Introduction Several cored boreholes were drilled for engineering geological purposes at Richard’s Bay harbour (Fig. 1). Initial microfossil analysis indicated that in one of thesecores (3F66) the K/T boundary was intersected at a depth of N 22 m, establishing the site as one of the very few in the Southern Hemisphere on land. A section of this core between - 22.5 and - 2 1.6 m (Fig. 2 ) was re-examined in detail for calacreous nannofossils. Major- and trace-elePresent addresses: “Department of Geology, University of Cape Town, Rondebosch 7700, South Africa. Pedro de Valdivia 295, Clasiflcador, 31 Correo 9, Santiago, Chile.
0168-9622/90/$03.50
ment analysis was performed in the same interval and carbon and oxygen isotope analysis over a depth range several metres above and below the boundary. A concretionary section of calcareoushardground occurs within this section between - 22.1 and - 2 1.9 m. Several such concretionary layers have been observed (Kennedy and Klinger, 1972;Orr and Chapman, 1974) to extend laterally for > 500 m in a deep trench excavated at the site aswell as in other boreholes, down to depthsof 45 m (Maud and Orr, 1975) . These concretionary hardground sections at several depths below, above and at the faunal and floral K/T transition, can be interpreted (Kennedy and Klinger, 1972; Orr and Chapman, 1974) as having formed during periods
0 1990 - Elsevier Science Publishers
B.V.
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B.Th.VERHAGENETAL.
516F
Fig. 1. Various reported Southern Hemisphere K/T section locations. Inset: Richard’s Bay lagoon before harbour development, showing 3F66 and two other borehole sites.
:
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Fig. 2. 613C (0 ), 6”O (0) and selected major and trace chemical profiles (referred to in text) for the section -23.7 to - 18.9 m of borehole 3F66, Richard’s Bay. Inset gives details of main nannofossil zones.
of marine regression.Therefore, the site seems to have alternated between regressive and transgressiveconditions over the period of its sedimentary deposition, seenin the core down to at least the mid-Cretaceous. 2. Microfossil analysis The re-examination for calcareous nannofossils confirmed the existence of a reasonably
complete record of the upper Maastrichtian (Micula prinsii, Nephrolithusfiequensand - 80 other well-preserved species) up to -22.3 m. A sample at -22.08 m within the concretionary horizon, was found to be essentially barren of nannofossils. The hardground immediately above this barren zone mostly contains the early Tertiary “disaster” planktons Thoracosphaera and Baarudosphaerabigelowii. While the Biscutum romeinii and B. parvulum sub-
ISOTOPIC
AND OTHER
GEOCHEMICAL
DATA
FROM
A CRETACEOUS-TERTIARY
zones are missing, the presence, between -21.88 and -21.80 m, of the Chiustozygus ultimus and Toweius petalosus subzones confirms the presenceof the upper part of the NPI of the basal Danian. Above - 2 1.63 m the appearance of Cruciplacolithus tenuis indicates the transition to the NP2 zone (see Fig. 2). Therefore, the presenceof the K/T boundary seems to be well established within this interval. 3. Methods 3. I. Isotopic analysis Carbon and oxygen isotope ratio measurements were conducted on carbon dioxide generated with 100% orthophosphoric acid from bulk samples from -23.8 to - 18.9 m of the same core. Therefore, the data reflect the isotopic content of the entire carbonate fraction, the absolute concentration of which was found to be highly variable over the sections studied; it is at its lowest in the range - 2 1.9 to 2 1.6 m, i.e. just above the hardground. The observed oxygen and carbon isotopic valuesare shown in Fig. 2. They are in the usual J-notation in parts per thousand (%o) relative deviation from the PDB limestone standard. Overall reproducibility of duplicates on all measurements is N +-O.l%o for 613C and - ? 0.2%0for alsO. 3.2. Elemental analysis XRF (X-ray fluorescence) analysis was employed for most of the major elements and/or instrumental neutron activation analysis for “chalcophile” trace elements. Selected data from these analysesare plotted in Fig. 2. 4. Results 4. I. Isotopic data The 6’*0-values in the upper Maastrichtian are compatible with marine deposition and re-
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main approximately constant over the interval -23.9 to -22.23 m. In the range of -22.1 to - 2 1.9 m the 6’ *O undergoessome excursions, the most important of which is the drop to < -4%o just above -22.1 m. It then rises abruptly to values of N -2.5%0, to return to < - 4%0above - 22.01 m. In the Danian, and up to - 18.9 m, 6 ‘*O reverts to values slightly heavier than those from below - 22.2 m. The 613C-valuesare also marine-like (0 to + l%o) up to - 22.25 m and then drop sharply between -22.1 and -21.95 m to a minimum of < - lO%o.Above this follows a more gradual rise, parallel to the rise in 6 ‘*O, although the 613C-valuesof the Danian remain somewhat more negative (at - 2 to - 1%o) than for the upper Maastrichtian. The 613Cexcursion is largely coincident with the hardground/concretionary layer. It is interpreted as representing a changefrom a marine to a terrestrial sourceof carbonate,precipitated from water of meteoric origin which was capable of perfusing and replacing the as yet unconsolidated silt when the areabecametidal to sub-tidal during marine regression. Negative values for 6’ 3C down to < - 1O%oare observed in terrestrial pedogenic carbonate (calcretes) in southern Africa (Mazor et al., 1974; Netterberg, 1980). The partially parallel negative excursion in 6 “0 can similarly be interpreted as due to the dominance of meteoric water during at least part of this period of inferred marine regression. This accords with the conclusions of Kennedy and Klinger ( 1972), who regard the concretionary hardgrounds as having formed through early diagenesis,only a little distance below the sediment-water interface, during shallower water episodes. The return to more positive 6’ *O-values between -22.07 and -22.02 m could be interpreted as the result of diagenesisor of bioturbation, or due to a temporary reestablishment of marine conditions. As this return coincides with the peak in the negative excursion of the 6 13Cand a barren section in the nannofossil
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record, a more probable interpretation would be that the freshwater inflow had declined and meteoric evaporative (lagoonal) basin conditions developed. From - 22.01 m the freshwater flow again increased, to be gradually overtaken by a renewed marine transgression. Therefore, the sudden disappearance of all nannofossils can be related to the changeaway from marine conditions, which, neatly paralleled in the isotope and nannofossil (mainly “disaster” speciessuch as Thorucosphaeraand B. bigelowii) records, show a gradual resurgenceabove - 2 1.9 m. 4.2. Chemical data The carbonate-rich nature of the concretionary horizon shows a clear chemical fingerprint in the six-fold CaO increase. SiOz and A&O3 (and also MgO, not shown) are consistently lower in this interval (see Fig. 2). Petrographic observations indicate that this is almost certainly due to dilution of the clay and silt fraction of the sediment by the carbonate (as micritic calcite) content. The terrestrial origin of the carbonate is further suggestedby the markedly lower values of Br within the horizon. No “iridium anomaly” nor any enrichment of Co, Ni, Sb and As - all usually enhancedin K/T boundary clays - could be observed. Ir concentrations (Fig. 2 ) rise monotonously from < 1 ppb in the Maastrichtian to N 3.5 ppb in the lower Danian. The same trend is seen,if at much higher concentrations, for Cr. Other elements show slightly decreasedvalues in the hardground. An anomalous zone might have beenlost in the 10cm of unrecovered core material, but the inferred trends in oxygen and carbon isotopes and in chemical variations acrossthe hiatus would argue against this. 5. Discussion The negative excursion of amplitude > 1O?ho in 613C is the largest yet reported for a K/T
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boundary. More usual are rangesof m + 1oloofor some New Zealand sites (Wolbach et al., 1987; B.Th. Verhagen,unpublished data, 1988) and at most + 3%0for various Mediterranean and European sites (Perch-Nielsen et al., I982 ). At Caravaca, Biarritz and Lattengebirge, a strikingly parallel behaviour between6i80 and 6’ 3C is seenover the analysedsections.However, the S”O excursion rangesare greater (2-3%0) and the 613C ranges much smaller ( l-4%0) than observedat Richard’s Bay. Perch-Nielsen et al. ( 1982) found it “ ... interesting to note that there is frequently a positive correlation between the oxygen- and carbon-isotope changes...“,
but did not proffer any model which would accomodate such parallel behaviour of the two isotopes. More recently, Rampino and Volk ( 1988) proposed a mechanism, through dimethyl sulphide liberation and change in albedo, which links - albeit far from universally - decreasedmarine biological productivity and increasedocean temperature. However, in their review of carbon and oxygen isotope data at K/T transitions, theseisotopes are taken as independent indicators of biological activity and temperature of the ocean, respectively. Wolbach et al. (1987) suggestthat a terrestrial carbon source could have been responsible for a- l%o swing in their carbon isotope 6 values for a New Zealand K/T site, but do not consider the possible influence of associated meteoric water on their oxygen isotopic values. In a study of the carbon isotope shift at the Permian-Triassic boundary, Magaritz et al. ( 1988) present data which in one section shows a strong correlation between Si80 and 6i3C. However, in their discussion the authors discount the significance of this information and the 6 ’ 3C data are treated as through they behaved independently of S”O. A similar approach is taken by Holser et al. ( 1989). Gruszczynski et al. (1989) present strikingly similar excursionsof N 11%oin both S*‘0
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and 613Cin composite sections acrossthe upper Permian below the P/T transition. Having dismissed diagenetic processes,they do address the covariance of the curves by considering oxidation of marine organic matter and concomitant kinetic oxygen isotope fractionation. This in turn is postulated as having produced major global changesin the oxygen isotope composition of seawater. However, the data in the extant literature seem to discount such a possibility for the K/T boundary. If 6180-values are to be regarded as indicators of, for example, water temperature, this requires that y1oother mechanism is known to exist by which the oxygen isotopic value of the carbonate at the site could have been altered. Such a requirement is most reliably satisfied at deep ocean depositional sites, where these excursions are usually neither as pronounced nor parallel, e.g.the South Atlantic Deep SeaDrilling Project (DSDP) Site 524 (Hsii et al., 1982) or the north Pacific DSDP Site 577 (Zachos et al., 1989). Similarly, when variations in 6 13Cshow trends similar to those in 6”0, they cannot simply be treated in isolation but clearly have to be considered with referenceto the latter. Parallel behaviour of oxygen and carbon isotopes in carbonates suggestssome common driving mechanism. For the Richard’s Bay section there is independent mineralogical and chemical evidence which suggestsa marine to terrestrial transition. In cases of parallel isotopic behaviour at other K/T boundary sections, often representing shallow marine environments, the mechanism might be lessclearly discernible. This is probably the reason why, as far as we are aware, no previous analysis in terms of some unified model for concordant oxygen and carbon isotope changes at some K/T boundary sites has been attempted. 6. Concluding remarks For Richard’s Bay, we now have various indicators of the chain of events at this K/T
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boundary site. Most striking is the unprecedented carbon isotope excursion, which by its magnitude allows for practically no other interpretation than a change from marine to terrestrial carbonate sedimentation conditions and back again. The generally parallel oxygen isotope variation supports this, with the added evidence, in the trend reversal, of possible evaporative basin conditions. Therefore, taking our interpretation of a changeto a meteoric source of the water as probable, our 6’*0 data for the excursions at the transition cannot be interpreted in terms of water temperature; however, such interpretation might be possible for the isotopically more featureless (shallow marine) sections above and below the transition. The coincidence of the inferred marine regressionwith the K/T boundary needsto be discussed. According to Hallam ( 1987), increasedmantle plume activity has the “potential of causing uplift of extensive sectorsof continents and hence regression of epicontinental seas. Epeirogenic subsidence on the continents and marine transgression might be expected to follow episodes of substantial volcanic eruptions”,
which could have influenced the changes in fauna1and floral assemblagesat and following the K/T transition. Apart from the well-known El Kef section, the K/T transition was observed at two other African sites: the Majunga Basin in Malagasy and the Sokoto Basin in northern Nigeria (A.K. von Salis Perch-Nielsen, unpublished data, 1988). It was found to be barren in nannofossils in the former and non-marine in the latter. The - admittedly sparse- evidence therefore points towards a marine regression or uplift which could have involved a major part of the continent. There is little evidence for any gradual shallowing prior to the regression event inferred here. The entire section along the present coastline shows very uniform depositional conditions over extended periods. The micro-
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fossil evidence also shows no intermediate between well-preservedvery diverse fully marine speciesand the bottom of the hardground section which is barren. The absenceof a IO-cm section of core should be noted. Any assessment of the amount of sea-level change associated with the K/T regression would be totally speculative on the basis of available data. No direct evidence of fallout is seen in the Richard’s Bay section, either from volcanic activity on the southern continent or from the Deccan traps, India, which at that stage (65 Ma) would have been in relative proximity. The clay mineral assembly both below and above the boundary is very similar and dominated by smectite, with minor illite and kaolinite. This can be interpreted as an unchanged source of weathered material from the basalts of the nearby Lebombo range. Therefore, the clay matrix of the entire core would tend to mask a diffuse volcanic contribution around the transition. Similar conclusions suggesting local conditions were were drawn by Dia et al. ( 1989) from Pb isotopic data for the marine K/T boundary sites of Stevns Klint in Denmark, Caravacain Spain and Woodside Creek in New Zealand. The section of the Richard’s Bay sequence analysed in this contribution in all probability spansan extended period and could be better explained by events associatedwith volcanism which at the end of the Maastrichtian became particularly intense (Axelrod, 1981; Courtillot et al., 1986) - also in southern Africa (Scholtz, 1985) - than by an “impact” mechanism. An indirect indication of volcanism could be the relative persistenceof isotopically “lighter” carbonate values in the siltstones above the K/T boundary. It suggeststhat this, and possibly part of the large negative excursion in the carbon isotope profile of the hardground, might have been generatedin addition by higher atmospheric concentrations of isotopically “light” CO2 of volcanic origin. The somewhat heavier 6’*0-values (by N 0.69/00)of the carbonatein the siltstone above
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the boundary concretionary layer as compared to that below could tentatively be ascribed to a cooling of the ocean. However, the relatively shallow continental shelf/lagoonal conditions which seemto have persisted at this site would tend to restrict the global validity of such a conclusion. Acknowledgements The authors wish to expresstheir appreciation to R. Maud and A. Scholtz for making the core materials available and to G. Lambert for initial microfossil information. They also gratefully acknowledgefruitful discussionswith these three colleagues,and the helpful suggestions of an anonymous referee.0. Malinga and L. de Matos are thanked respectively for assistance which the isotopic measurementsand elemental analysis and the Department of Geology, University of the Witwatersrand, for XRF and XRD analyses.For financial support the authors are indebted to the University of Witwatersrand and the South African Foundation for ResearchDevelopment. References Axelrod, D.I., 1981. Role of volcanism in climate and evolution. Geol. Sot. Am., Spec.Pap. 185, 59 pp. Courtillot, B., Besse,J., Vandamme, D., Montigny, R., Jaeger, J-J. and Cappetta, H., 1986. Deccan flood basalts at the Cretaceous/Tertiary boundary? Earth Planet. Sci. Lett., 80: 361-374. Dia, A., Manhbs, G., Dupre, B. and All&e, C.J., 1989. The Cretaceous-Tertiary boundary problem: An assessmentfrom lead isotope analysis. Chem. Geol., 75: 291-304. Gruszczynski, M., Halas, S., Hoffman, A. and Malkowski, K., 1989. A brachipod calcite record of the oceanic carbon and oxygen isotope shift at the Permian/Triassic transition. Nature (London), 337: 6468. Hallam, A., 1987. End-Cretaceous mass extinction event: argument for terrestrial causation. Science,238: 12371242. Holser, W.T., Schonlaub, H-P., Attrep, M., Boeckelmann, K., Klein, P., Magaritz, M., Orth, C.J., Fenninger, A., Jenny, C., Kralik, M., Mauritsch, H., Pak, E., Schramm, J-M., Stattegger, K. and Schmiiller, R., 1989. A unique
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geochemical record at the Permian/Triassic boundary. Nature (London), 337: 39-44. Hsii, K.J., He, Q., McKenzie, J.A., Weissert, H., PerchNielsen, A.K., Oberhlnsli, H. et al., 1982. Mass mortality and its environmental consequences. Science, 216: 249-256. Kennedy, W.J. and Klinger, H.C., 1972. Hiatus concretions and hardground in the Cretaceous of Zululand (South Africa). Palaeontology, 15 (Part 4): 539-549. Magaritz, M., BL, R., Baud, A. and Holser, W.T., 1988. The carbon-isotope shift at the Permian/Triassic boundary in the southern Alps is gradual. Nature (London), 331: 337-339. Maud, R.R. and Orr, W.N., 1975. Aspectsof the post-Karoo geology in the Richard’s Bay area.Trans. Geol. Sot. S. Afr., 78: 101-109. Mazor, E., Verhagen, B.Th., Sellschop,J.P.F., Robins, N.S. and Hutton, L.C., 1974. Kalahari ground waters: their hydrogen, carbon and oxygen isotopes. In: Isotope Techniques in Groundwater Hydrology, 1. I.A.E.A. (Int. At. Energy Agency), Vienna, pp. 203-225. Netterberg, F., 1980. Geology of southern Africa calcretes, 1. Terminology, description, macrofeatures and
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classification. Trans. Geol. Sot. S. Afr., 83: 255-238. Orr, W.N. and Chapman, J.R., 1974. Danian (Palaeocene) marine rocks at Richard’s Bay, South Africa. S. Afr. J. Sci., 70( 8): 247-249. Perch-Nielsen, K., McKenzie, J. and He, Q., 1982. Biostratigraphy and isotope stratigraphy and the ‘catastrophic’ extinction of calcareousnannoplankton at the Cretaceous/Tertiary boundary. Geol. Sot. Am., Spec. Pap., 190: 353-371. Rampino, M.R. and Volk, T., 1988. Mass extinctions, atmospheric sulphur and climatic warming at the K/T boundary. Nature (London), 332: 63-65. Scholtz, A., 1985. The palynology of the upper lacustrine sediments of the Arnot pipe (Banke), Namaqualand. Ann. S. Afr. Mus., CapeTown, 95(Part 1): l-37. Wolbach, W.S., Gilmour, I. and Anders, E., 1987. Environmental changesacrossthe K-T boundary at Woodside Creek, New Zealand. Lunar Planet. Sci.Conf., 19: 1286-1287. Zachos, J.C., Arthur, M.A. and Dean, W.E., 1989. Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary. Nature (London), 337: 61-64.