Strontium isotope stratigraphy for the Late Cretaceous

Strontium isotope stratigraphy for the Late Cretaceous: a new curve, based on the English Chalk J . M . M c A R T H U R 1, M . F . T H I R L W A L L 2, A . S . G A L E 3, W . J . K E N N E D Y 4, J . A , B U R N E T T ~, D. M A T T E Y 2 & A . R . L O R D 1

Department of Geological Sciences, University College, London, Gower Street, London WC1 E 6B T, UK 2 Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, Surrey TW20 OEX, UK 3 Department of Geology, Royal School of Mines, Imperial College, Prince Consort Road, South Kensington, London SW7 2BP and Palaeontology Department, Natural History Museum, Exhibition Road, South Kensington, London SW7 5BD, UK 4 Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK Marine 87Sr/86Srdecreases from 0.70775 in the Cenomanian to 0.70730 in the middle Turonian before increasing in a near-linear manner to >0.70775 in the early Maastrichtian. This variation has been defined using samples from the English Chalk that are closely integrated with the macrofossil and microfossil biostratigraphy of northwestern Europe. With this new isotope curve a stratigraphic resolution is attainable in correlation that is typically _+0.8 Ma for the Santonian and Campanian stages. Isotopic and biostratigraphic correlations between Dorset and Norfolk, in the UK, agree within the limit of analytical error in S:Sr/S6Sr. Abstract:

Strontium isotope stratigraphy is used increasingly for correlation and dating of marine carbonates, evaporites and phosphates (Wickman 1948; Elderfield 1986; Veizer 1989; McArthur et al. 1990; Hodell et al. 1991; DePaolo & Finger 1991). The method works well for those periods for which isotopic calibration curves exist that are both accurate and calibrated against welldocumented biostratigraphy and magnetostratigraphy (eg. Miller et al. 1988). Neither condition currently applies to the Cretaceous. Extant data are poorly constrained biostratigraphically and either scatter too much to permit definition of a good curve (Burke et al. 1982; Koepnick et al. 1985) or are sparse and derive from recrystalized samples (Hess et al. 1986). As a first step towards providing a standard curve for the Late Cretaceous we have determined the 87Sr/a6Sr of nannofossil chalk and macrofossils through a section of the English Chalk, at Trunch in Norfolk, UK (Fig. 1), where the British Geological Survey (then the Institute of Geological Sciences) cored 469 m of Cenoraanian-Maastrichtian chalk. In order to provide a degree of objective assessment of the effect of diagenesis on S7Sr/86Sr we have compared 87Sr/S6Sr in macrofossils and their adhering nannofossil chalk. Furthermore, we have tested the quality of the isotope curve by comparing

isotopic and biostratigraphic correlations of the chalk strata of Norfolk to chalk strata at Studland Bay, Dorset, 350 km southwest of Trunch (Fig. 1). Our isotope curve for the Coniacian-early Maastrichtian is a good record of marine S7Sr/ 86Sr and provides a quantitative isotopic template with which to correlate Late Cretaceous strata of northwestern Europe with strata worldwide, and against which to match and integrate diverse stratigraphic schemes via common isotopic signatures. Previous publications of St-isotopic curves have universally plotted 87Sr/86Sr against a linear numerical age scale. This practise gives a specious impression of accuracy to such curves. It does so in two ways. Firstly, it obscures the considerable problems inherent in assigning numerical ages to biostratigraphic schemes, a process that requires, amongst other things, interpolation, extrapolation, and assumptions about sedimentation rate, all of which introduce unquantifiable error. Secondly, primary data (the physical position of a sample in a section) is made subordinate to derived secondary data (numerical age) without making the connection between them very clear (with, perhaps, the exception of Miller et al. 1988). To avoid these problems we plot our isotopic data against depth

From HAILWOOD,E. A. & KIDD, R. B. (eds), High Resolution Stratigraphy Geological Society Special Publication, No. 70, pp. 195-209.

195

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STRONTIUM ISOTOPE STRATIGRAPHY FOR THE LATE CRETACEOUS

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5000 so no corrections for radiogenic Sr have been made. In order to assess the maximum disturbance of carbonate-aTSr/S6Sr that dissolution of non-carbonate material could cause, should any be attacked by our dissolution procedure, sample 38 was also prepared by dissolution in 2.5M and 6M HCI (Table 1). Our acetic-acid preparation procedure yields the same 87Sr/86Sr for sample 38 as does the dissolution of carbonate in a weak-acid, ion-exchange resin at pH 5 (Table 1 ; Kralik 1984), a procedure that is unlikely to affect non-carbonate minerals. Stable isotopic data were obtained with a V G Prism three-collector gas-sourced mass spectrometer running an on-line, automated, H3PO4 acid-bath operating at 90 ° C. During the analysis NBS-18, run as sample, gave 8~3C = - 5 . 0 6 + 0.04%0 and ~ 8 0 = - 3 . 1 2 + 0.08%0; NBS-19 gave 8J3C = + 1.97 + 0.08%0 and

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STRONTIUM ISOTOPE STRATIGRAPHY FOR THE LATE CRETACEOUS

203

data using errors on 87Sr/86Sr of one s t a n d a r d deviation of the p e r i o d m e a n .

~ 8 0 = -2.64 ± 0.09%0. Analysis for Sr and Ca concentrations were done by flame AAS after dissolving samples in 1M HCI. Precision for both is