GeoResearch Forum Vol. 6 (2000) pp. /37-144

@ 2000 Trans Tech Publications, Switzerland

. Strontium Isotope Stratigraphy of the Aalenian/Bajocian Auxiliary Stratotype Point at Bearreraig, Isle of Skye, NW Scotland

J .M . McArthur1 , N. Morton 1 and M.F . Thirlwal l2

1 Research School of Geological and Geophysical Sciences, Birkbeck College and University College London, Gower Street, London WC1 E 6BT, UK

2 Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 OEX, UK

Keywords: Strontium, Isotope, Stratigraphy, Bearreraig , Skye, Aalenian, Bajocian

Abstract: The 87Sr/86Sr value of the Aalenian/Bajocian boundary is 0.707 288 ± 0.000 0 1 4 (2 s.e.), based on analysis of belemnites from the Auxiliary Stratotype Point (ASP) at Bearreraig, Isle of Skye. The previous value for this boundary, based on a statistical analysis of data for belemnites from the Inferior Oolite in Dorset, UK, is 0.707 287±0.000 020 (95% C.l .). During the Aalenian, the 87Sr/86Sr of marine strontium changed ·by 0.000 0 1 7; correlation and dating of Aalenian strata using strontium isotope stratigraphy is of little value, given the present practical limit of measurement of about ±0.000 004 on multiple repeat analysis and the present uncertainty of ±0.000 020 on the Aalenian Sr isotope calibration curve. Correlation within the Aalenian will be possible when the uncertainty on the Sr isotope calibration curve for the Aalenian approaches the uncertainty of ±0.000 003 currently attained for the best-defined parts of the Neogene isotope calibration curve.

Introduction Strontium isotope stratigraphy (SIS) is developing rapidly as a valuable tool for correlating and

dating marine sediments (for recent reviews see Veizer, 1 989; McArthur, 1 994). The correlative precision117and the temporal resolution, of SIS depends upon, amongst other things, the rate at which marine Sr/86Sr was changing when the, strata of interest were deposited. Correlation of Maastrichtian rocks using Sr isotopes has been shown to be possible to zonal level, i.e. as good as that achievable from ammonites and belemnites (Crame et al., in press), with associated dates being resolved to ± 0.2 myr or one belemnite biozone. For much of the Jurassic, the rate at which the marine 87Sr/86Sr was changing was greater than it was in the Late Cretaceous and correlation with 87Sr/86Sr can better that available with ammonites (McArthur et al., in press).

Successful correlation using SIS requires that 87Sr/86Sr be defined for well known sequences and especially for stratotypes from which correlations ultimately derive. The use of Sr isotopes to assign numerical ages to rocks requires, in addition, that an accurate age calibration be assigned to such sequences. These combined approaches should lead eventually to a standard global Sr isotope curve showing how 87Sr/86Sr of marine Sr has varied through time. As a contribution to the database underpinning any such global standard curve, we present an 87Sr/86Sr profile through the Aalenian/Bajocian boundary as it is exposed in outcrops of the Bearreraig Sandstone Formation at Bearreraig, Isle of Skye, NW Scotland, which is the Auxiliary Stratotype Point for this boundary.

Aalenian - Bajocian Boundary The Aalenian - Bajocian boundary (the base of the Bajocian Stage) has traditionally been

defined at the base of the Discites Zone. More recently, under the auspices of the Bajocian Working Group of the ISJS, the ammonite biostratigraphy has been refined and the base of the Discites Zone and of the Bajocian Stage is now recognised and correlated by the first occurrence of an ammonite assemblage including Hyperlioceras mundum and related species (Pavia and Enay, 1 997).

To establish the chronostratigraphical definition of the base of the Bajocian Stage, possible boundary stratotypes were investigated by the Bajocian Working Group. Two sections were proposed and accepted by the ISJS and the ISC. Murtinheira at Cabo Mondego in Portugal was

1 38 Sh Jurassic Symposium, Vancouver

selected as the Global boundary stratotype (GSSP) and Bearreraig, Isle of Skye, Scotland as the Auxiliary Stratotype Point. This paper gives new stratigraphic data on the Auxiliary Stratotype.

The Bearreraig Section The Aalenian and Lower Bajocian succession at Bearreraig comprises over 200m of sediments

in the Bearreraig Sandstone Formation (Morton, 1 990; Morton and Hudson, I 995). Three major coarsening-up siltstone-sandstone cycles occur, within which there are smaller-scale cycles (Fig. I ). Marine fossils, especially ammonites, belemnites and bivaives occur throughout, although the great thickness of sediment means that the distributions of the fossils are discontinuous. All the standard ammonite zones, and most of the subzones, of the Aalenian and Lower Bajocian of the NW European Province can be identified, as can the more detailed ammonite horizons in the uppermost Aalenian to lowermost Bajocian part of the sequence.

The uppermost subzone of the Aalenian Stage (Limitatum Subzone, Concavum Zone) and the lowermost subzone of the Bajocian Stage (Walkeri Subzone, Discites Zone) occur in a thick succession (ea. 25m) of silty shales and siltstones of the Udaim Shale Member. The succession summarised in Figure I is compiled from three outcrop sections within 400m of each other. There is a small fault, and a stratigraphic gap calculated to be less than I m of strata, between the middle section and the upper section (Fig. I ).

Samples We analysed samples of belemnites occurring between the base of the Murchisonae Zone

(Middle Aalenian) to the Ovalis Zone (Lower Bajocian). Many fossils in the Bearreraig section are very well preserved, especially those found in early-diagenetic carbonate nodules. Whilst aragonitic shells have inverted to calcite, and several phases of calcite cementation have been identified (Marshall, I98 I ), belemnites are well preserved. Except in the immediate vicinity of minor intrusions, such good preservation is not unexpected because fission-track data (Morton, I 990) suggest that the rocks have not been heated to temperatures above I 00°C, whilst organic geochemical data (Thrasher, I 992) suggest that they have not been heated above 50°C. .

The most detailed sampling was concentrated on strata immediately above and below the Aalenian - Bajocian boundary. The Bearreraig section has been accepted as the Auxiliary Stratotype Point for the base of the Bajocian Stage; therefore, the data presented here are located stratigraphically with respect to their height above or below this boundary. They enable a strati graphically precise evaluation of the 87Sr/86Sr value at the Aalenian - Bajocian boundary which confirms the accuracy of the 87Sr/86Sr value of 0.707 287±0.000 020 (95% C.I .) derived for this boundary by Howarth and McArthur ( I 997) from the data of Jones et al. ( I 994) and Engkilde (pers. Comm., I 997) on samples from the Inferior Oolite of Dorset, UK.

Analytical Methods and Results

Sample Preparation We analysed 1 4 belemnites. Prior to analysis, portions most susceptible to alteration (alveolus,

apex, exteriors, apical line) were removed using diamond cutting tools and discarded. The remaining portions were cleaned by brief immersion in I .2 molar hydrochloric acid, washed with ultra-pure water and then dried. Finally, the remaining pieces were individually fragmented into sub-mm-sized pieces using an agate pestle and mortar. The fragments were again cleaned by brief immersion in I .2 molar hydrochloric acid, washed with ultra-pure water, and finally dried in a clean environment. Fragments for analysis were selected under the binocular microscope.

Chemical Data and Isotopic Analysis For 87Sr/86Sr analysis, picked fragments were dissolved in 6 M nitric acid and Sr was separated

by standard ion-exchange chemistry. Values of 87Sr/86Sr were measured with a VG354 five­collector mass spectrometer using a multid�namic routine on three collectors (SrSLL) that include corrections for isobaric interference from Rb (Thirlwall, I 99 I ). Data have been normalised to a value of O. I I 94 for 87Sr/86Sr. During data collection the measured value for NIST 987 was within 0.000 025 of the value 0.7I 0248. Data reported in Table 1 have been adjusted to a value of 0.7 I 02 48 for NIST 987 which, in our laboratory, corresponds to a value of 0.709I 75 for modem seawater Sr. Based upon replicated analysis of standards, the precision of our measurements (2 s.e.) was better than ±0.000 O I 5 for single determinations and ±0.000 O I I for duplicates. Total blanks were

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<2 mg of Sr; sample contained >5 mg of Sr. For chemical analysis, fragments picked under the microscope were analysed by standard ICP-AES methods for their concentrations of Ca, Sr, Mg, Fe, and Mn, after dissolution in 1 0% acetic acid. Data are presented in Table 1 .

Age Level +1- "/o pp m t<;lement

Sample Zone Subzone M a Boundary 87Sri'6Sr Ca Mg Na Sr Fe Mn

8 1 6 B Ova/is - I 75.6 5 3 .2 0.707292 38.9 3330 3990 I 290 2 I O 28

a 1 75.6 5 3 .2 0.707280 8 1 4 j Discites walkeri I 76.3 I l .7 0.707286 86/5 0 Discites walkeri I 76.3 I O.O 0.707282 38.9 2 I 40 2680 I O I 5 770 1 9

c Discites wa/keri I 76.3 I O.O 0.707289 80/20 i Discites wa/keri I 76.4 7.5 0.707274 39.7 2750 2970 970 1 3 30 30

a Discites walkeri I 76.4 7.5 0.707282 80/2 1 n Discites walkeri I 76.4 7.0 0.707296 39.3 2530 2560 985 840 8 I

80/ 1 8 Discites walkeri I 76.5 O. I 0.707284

9015 Concavum limitatum I 76.5 -0.3 0.707283 39.4 2070 2850 I I 20 I l 5 3 Concavum /imitatum I 76.5 -0.3 0.707287

75/8 A Concavum limitatum I 76.6 -5.3 0.707294 39.6 I 980 2860 I O I O 57 7 a Concavum limitatum I 76.6 -5.3 0.707288

75/5 I Concavum limitatum I 76.7 -6.0 0.707295 39.8 2360 3930 I I I O 4 I < I e Concavum limitatum I 76.7 -6.0 0.707293

75/3 n Concavum limitatum I 76.7 -9. I 0.707298 40. I 3360 3790 1 360 40 3 i Concavum limitatum 1 76.7 -9. I 0.707298

80/22 a Concavum limitatum I 76.9 . - 1 3 . 5 0.7072 8 I No sample n Concavum concavum No sample Bradfordensis ·

65M Murchisonae murchisonae I 78.3 -27.7 0.707280 No sample Murchisonae obtusiformis

74/25 Murchisonae haugi I 79 . I -29.9 0.707297 74/24 Murchisonae haugi I 79 . I -30.4 0.707290

Table 1 . Isotopic and elemental data for belemnites from Bearreraig, Skye.

Results and Discussion

Sample Preservation All samples have concentrations of Ca, Mg, Na and Sr that are within the ranges reported for

other belemnites (Jones et al . , 1 994; McArthur et al . , in press) and the 87Sr/86Sr ratios for the samples are concordant and close to those found for this time interval by others (Jones et al., 1 994; Engkilde, pers. comm., 1 997). On these criteria, all samples would be regarded as well preserved.

Diagenetic alteration of skeletal carbonates usually increases the concentrations of Fe and Mn. Uncertainty exists as to the threshold concentration above which iron and manganese may be presumed to show that skeletal carbonate is altered (Veizer, 1 983). In recent pa!aeo-climatic studies, Anderson et al. ( 1 994) and Ditchfield ( 1 997) assumed that the thresholds are 1 00 ppm Mn and 300 ppm Fe; Jones et al. ( 1 994) set the limit for belemnites at 1 50 ppm Fe, whilst McArthur ( 1 994), dealing with SIS criteria for brachiopods, bivalves and ammonoids, set the level at 1 00 ppm for both elements. Our Bajocian samples contain concentrations of Fe that suggest alteration has occurred (samples 80/20, 1 330 ppm; sample 80/2 1 , 840 ppm). Our samples are well preserved on other criteria (see above; Table 1 ) so we speculate that either the high Fe concentrations are not giving a true view of alteration in these two belemnites and m� re�resent trace disseminated pyrite or iron oxide, or that alteration has been too slight to alter the Sr/ 6Sr values. We accept the ratios for these belemnites as being representative of Aalenian marine strontium.

Isotopic Ratios In Fig. 2, Sr isotopic ratios of samples are plotted against their stratigraphic level within the

Bearreraig section. The data show a trend to lower values at higher stratigraphic levels. A linear regression of 87Sr/86Sr on stratigraphic level has been fitted to the data and is also shown on Figure

1 40 fih Jurassic Symposium, Vancouver

2; this regression predicts a boundary value for 87Sr/86Sr of 0.707 288±0.000 0 1 4 (2 s.e.). The ratio at this boundary has previously been predicted by Howarth and McArthur ( 1 997) to be 0.707 287±0.000 020 (2 s.e.), based on a LOWESS statistical fit to the 87Sr/86Sr data of Jones et al. ( 1 994) and M. Engkilde (unpublished belemnite analysis; pers. comm, 1 997) for belemnites from the Inferior Oolite of Dorset. The agreement between these values is remarkable; our linear regression predicts a boundary ratio that is analytically indistinguishable from the ratio predicted by Howarth and McArthur ( 1 997), whilst reducing from ±0.000 020 to ±0.0000 1 4 the uncertainty associated

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Figure 1 . The Aalenian/Bajocian boundary succession at Bearreraig, Isle of Skye, UK.

GeoResearch Forum Vol. 6 1 4 1

----1 0.70733

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Figure 3. Comparison of our 87Sr/86Sr data (ovals) with the data of Jones et al. ( 1 994; triangles).

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1 42 !fh Jurassic Symposium, Vancouver

with the boundary ratio. A comparison of our data with that of Jones et al. ( 1 994) is given in Figure 3; in order to make the comparison possible, a numerical scale has been assigned to our samples on the assumption that subzones are of equal duration and the numerical ages of stage boundaries are those given by Gradstein et al. ( 1 994); 1 80. 1 Myr for the base of the Aalenian, 1 76.5 Myr for the base of the Bajocian, and 1 69.2 M�r for the base of the Bathonian stage.

Through the Aalenian, 87Sr/8 Sr changed by 0.000 0 1 7 (Jones et al., 1 994i M. Engkilde, pers. corn., 1 997; Howarth and McArthur, 1 997), mostly as a result of a decline in 7Sr/86Sr through the upper two thirds of the Stage. Coupled with an analytical uncertainty for 87Sr/86Sr of as little as ±0.000 004, a figure achievable by replicate analysis (Crame et al., in press), it seems that strontium isotope stratigraphy might be useful in the Aalenian. Unfortunately, the present strontium isotope calibration curve for the Aalenian carries a statistical uncertainty of ±0.000 020 owing to the small number of data for the interval; until this uncertainty decreases, even replicate analysis will not enable Sr isotope stratigraphy to resolve stratigraphic problems involving Aalenian strata. Clearly, more data are required for this interval. Correlation within the Aalenian will be possible when the uncertainty on its Sr isotope calibration curve approaches the uncertainty of ±0.000 003 currently attained for the best-defined parts of the Neogene isotope calibration curve.

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stable isotopic records of fossils from the Peterborough Member, Oxford Clay Formation (Jurassic), UK: palaeoenvironmental implications. Journal of the Geological Society, London, 1 5 1 : 1 25- 1 38 .

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CRAME J. A . , J. M. MCARTHUR, D. PIRRIE AND J. B . RIDING. ( In press). Strontium isotope correlation of the basal Maastrichtian stage in Antarctica to the European and US standard biostratigraphic schemes. Journal ofthe Geological Society, London.

GRADSTEIN, F. M., AGTERBERG, F. P., OGG, J. G., HARDENBOL, J . , V AN VEEN, . P. , AND HUANG, Z. 1 994. A Mesozoic Time Scale: Journal of Geophysical Research, 99:2405 1 -24074.

HOWARTH, M. K., AND J. M. MCARTHUR. 1 997. Statistics for strontium isotope stratigraphy: a robust LOWESS fit to the marine strontium isotope curve for the period 0 to 206 Ma, with look­up table for the derivation of numerical age. Journal of Geology, I 05:441 -456.

JONES, C. E., H. C. JENKYNS, A. L. COE, AND S. P. HESSELBO. 1 994. Strontium isotope variations in Jurassic and Cretaceous seawater: Geochimica et Cosmochimica Acta, 58 :306 1 -3074.

MARSHALL, J. D. 1 98 1 . Zoned calcites in Jurassic ammonite chambers: trace elements, isotopes and neomorphic origin. Sedimentology, 28:867-887.

McARTHUR, J. M. D. T. DONOVAN, M. F. THIRLWALL, B. FOUKE AND M. BROWNLESS. (in press). Strontium isotope stratigraphy in the early Toarcian - late Pliensbachian (Jurassic): timescale revision and its implications. Earth and Planetary Science Letters.

__ . 1 994. Recent trends in strontium isotope stratigraphy. Terra Nova, 6:33 1 -358. MORTON, N. 1 990. Bearreraig (Isle of Skye, NW Scotland) as boundary stratotype for the base of

the Bajocian Stage, p. 23-48. In Cresta and G. Pavia, (eds.) Atti del meetingsulla stratigrafia del Baiociano. Memorie Descrittive Carta Geologica d'ltalia, 40. __ AND J. D. HUDSON. 1 995. Field guide to the Jurassic of the Isles of Raasay and Skye, Inner Hebrides, NW Scotland, p. 209-280. In P. D. Taylor, (ed.), Field Geology of the British Jurassic. The Geological Society, London.

PAVIA, G. AND R. ENA Y. 1 997. Definition of the Aalenian-Bajocian Stage boundary. Episodes 20: 1 6-22.

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VEIZER, J. 1 983. Trace elements and isotopes in sedimentary carbonates, p. 265-299. In R.J. Reeder (ed.), Carbonates: mineralogy and chemistry. Reviews in Mineralogy, 1 1 .

__ . 1 989. Strontiwn isotopes in seawater through time. Annual Reviews of Earth and Planetary Science, 1 7 : 1 4 1 - 1 67.