Variation of seawater 87Sr/86Sr throughout Phanerozoic time W. H. Burke, R. E. Denison,* E. A. Hetherington, R. B. Koepnick, H. F. Nelson,* J. B. Otto Vtobil Research and Development Corporation, Field Research Laboratory, Dallas, Texas 75221 ABSTRACT Precise measurements of 786 marine carbonate, evaporite, and phosphate samples of known age provide a curve of seawater 87Sr/86Sr versus geologic time through the Phanerozoic. Many episodes of increasing and decreasing values of 87Sr/86Sr of seawater have occurred through the Phanerozoic. The Late Cambrian-Early Ordovician seawater ratios are approximately equal to the modern ratio of 0.70907. The lowest ratios, ~ 0.7068, occurred during the Jurassic and Late Permian. The configuration of the curve appears to be strongly influenced by the history of both plate interactions and seafloor spreading throughout the Phanerozoic. The curve provides a basis for dating many marine carbonate, evaporite, and phosphate samples. Furthermore, diagenetic modifications of original marine 87Sr/86Sr values are often interpretable. Analysis of 87Sr/86Sr data, therefore, may provide useful information on regional diagenetic patterns and processes. All of the Cenozoic samples and some of the Cretaceous samples are from Deep Sea Drilling Project (DSDP) cores. With the exception of the DSDP samples, the curve was constructed only from samples containing at least 200 ppm Sr and not more than 10% dilute acid insoluble material. All measurements are made by comparison with standard 87 86 SrC03 (NBS SRM 987) for which a Sr/ Sr of 0.71014 is assumed. Precision is estimated to be ± 0.00005 at the 95% confidence level. Measured ratios of 42 modern marine samples average 0.70907, with a standard deviation of 0.00004. INTRODUCTION of 87Rb to 87Sr would be reflected in an (1975), Clauer (1976), Faure and others We present here our estimate of the increase with time of 87Sr/86Sr within sea- (1978), and Kovach (1980). curve of seawater 87Sr/86Sr versus geologic water and within strontium-bearing marine time (Holocene through Late Cambrian), precipitates. Peterman and others (1970) SAMPLES which is based on measurements of 786 determined that 87Sr,86Sr of Phanerozoic Surface and subsurface samples of samples of marine origin. This data set marine fossils both increased and decreased marine origin were obtained from North enables us to define the curve more pre- with time. During the early 1970s, addi- America, Europe, Africa, and Asia. Addi- cisely and more completely than was possi- tional reports on the temporal variation of tional samples from the Atlantic, Indian, ble heretofore and to recognize structure seawater 87Sr/86Sr (Dasch and Biscaye, and Pacific Oceans and the Caribbean Sea not previously apparent. The curve reveals 1971; Veizer and Compston, 1974) gener- were obtained from Deep Sea Drilling Pro- systematic temporal variation of seawater ally supported the earlier findings of ject (DSDP) cores. All Cenozoic and some 87Sr/86Sr. Although few ages can be Peterman and others (1970). Veizer and Cretaceous samples are foram-nannofossil assigned on the basis of unique seawater Compston (1974) included a summary of chalk from DSDP cores. Non-DSDP sam- strontium isotopic values, the curve pro- earlier work and presented reasons for ples used to construct the Mesozoic and vides a basis for resolving specific problems believing that seawater 87Sr/ 86Sr is uniform Paleozoic parts of the curve are predomi- in temporal correlation of marine strata at any given time, that marine precipitates nantly limestone, but also include dolo- and constitutes an important source of will have the seawater ratio when formed, stone, carbonate megafossils, evaporites, information on large-scale geologic forces, and that diagenesis will generally either and conodont samples. that control curve shape. increase the ratio or leave it unaffected. To obtain relevant data for construction Wickman (1948) predicted that decay They pointed out, however, that under cer- of the 87Sr/ 86Sr seawater curve, nonmarine tain circumstances diagenesis can decrease samples and extensively altered marine the ratio (Veizer and Compston, 1974, p. samples were excluded from the data set. 1464). Since 1974, additional work on the For the Cenozoic part of the curve, we •Present addresses: (Denison) Suite 616, One 87 86 temporal variation of Sr/ Sr of marine used only DSDP samples because they are Energy Square, 4925 Greenville Avenue, Dallas, carbonates and phosphates has been pub- Texas 75206; (Nelson) 2516 West Five Mile better dated and are more likely to have Parkway, Dallas, Texas 75233. lished by Brass (1976), Tremba and others retained their original marine ratios than 516 GEOLOGY, v. 10. p. 516-519, OCTOBER 1982 MILLIONS OF YEARS Figure 1. Plot of 87Sr/86Sr age for 744 of 786 marine samples. 87Sr/86Sr values for the 42 modern marine samples (Table 1) are not shown. Mod- ern values, however, were accounted for in drawing band and line. For any given time, correct seawater ratio probably lies within band. Line represents our best estimate of seawater ratio versus time. Pre-Cenozoic ages are based on van Eysinga (1975). Cenozoic ages are based on time scale provided by L. B. Gibson (1980, personal commun.). Pliocene-Pleistocene boundary is at 1.62 m.y. B.P., and Tertiary stage boundar- ies are at 5.0, 23.5, 37.0, and 53.5 m.y. B.P. other samples available to us. For the most MEASUREMENTS RESULTS AND DISCUSSION part, our Mesozoic and Paleozoic samples The samples were dissolved with either Our data are presented graphically in are of lower quality because they have been hydrochloric or nitric acid, and Sr was Figure 1 (age, isotopic ratio, and other subjected to more complex postdeposi- separated from the bulk of other elements data for the individual samples will be tional alteration than our Cenozoic sam- either by means of ion exchange columns presented in a more complete future ples. We have found empirically that or by precipitation from approximately paper). Seawater Sr is assumed to have tighter clustering of 87Sr/86Sr values among 90% nitric acid. Isotope ratios were meas- been sufficiently well mixed at all times so coeval Mesozoic and Paleozoic samples is ured by comparison with standard SrCOi that for any particular time, seawater achieved when samples with low strontium (NBS SRM 987) for which a ratio of 87Sr/86Sr would have been essentially contents or high insoluble residues are 0.71014 has been assumed. Measurements constant throughout the oceans of the eliminated. Thus, our Mesozoic and were made using a second-order double world. General isotopic agreement among Paleozoic data are limited to samples that focusing Nier-Johnson-type mass spec- coeval samples from diverse geographic contain at least 200 ppm Sr and not more trometer with a 60°, 13-in. radius of curva- locations lends support to this assumption than 10% dilute acid insoluble residue. The ture magnetic sector and a 91°, 15.8-in. (Peterman and others, 1970; Veizer and probable explanation for the improved radius of curvature electric sector. Masses Compston, 1974; this paper). In addition, clustering is that the restriction decreases 85, 86, 87, and 88 were collected simul- measurements of 42 modern marine sam- the fraction of samples that have not taneously in four separate faraday cups. ples (Table 1) reaffirm the homogeneity retained the marine 87Sr/86Sr value charac- 87Sr/86Sr values have been corrected of 87Sr/86Sr in modern seawater. 87 teristic of their time of deposition. While for the presence of Rb and have been The gray band in Figure 1 includes most 86 88 this restriction reduces the amount of scat- normalized to Sr/ Sr = 0.1194. Precision of the data points (93%). For any given ter in the Mesozoic and Paleozoic data, it is estimated to be ± 0.00005 at the 95% time, the correct seawater ratio probably does not compensate for a basic difference confidence level. lies within this band. Points outside the in quality between DSDP samples and On the basis of a ratio of 0.71014 for band (7%) probably represent either non- other samples. This contrast is shown in standard SrC03 (NBS SRM 987), meas- marine samples that did not obtain the Figure I by a tighter clustering of coeval ured ratios of 42 modern marine samples 87Sr/86Sr of contemporaneous seawater data points over the Cenozoic part of the average 0.70907, with a standard deviation or diagenetically altered marine samples curve as compared with clustering of co- from the mean of 0.00004 (Table 1), and 36 whose strontium was contaminated by for- eval data points over the pre-Cenozoic part measurements of Eimer and Amend SrCOi eign strontium with a different ratio. of the curve. Analysis of the remaining lot 492327 average 0.70797, with a stand- Because most samples can be confidently sample variation is presented below. ard deviation from the mean of 0.00003. interpreted to be of marine origin, on the GEOLOGY, OCTOBER 1982 517 basis of other geologic criteria, diagenetic modification is the most likely cause of the scatter. Points occur above or below the TABLE I. 87Sr/86Sr VALUES FOR MODERN MARINE SAMPLES band, depending on the strontium isotopic composition of the terrain from which the Sample type Location 87Sr Sr/66SR diagenetic strontium was derived. Most of (ppm) Pelecypods these points occur above the band and probably reflect the predominance of sam- Donax variabiles texasiana Mustang Island, Texas 1,564 0. 70912 ples from cratonic areas. Old sialic rocks Donax variabiles Gulf Shores, Alabama 1,424 0. 70905 from cratons are important sources of Tel 1 ina radiata JouIters Cay, Bahamas 2,324 0. 70900 radiogenic 87Sr, and weathering of these Tel 1ina tayloriana Mustang Island, Texas 1,741 0. 70905 Myt i1 us edu1i s San Luis Obispo County, Cal ifornia 1,092 0.