High-resolution strontium isotope stratigraphy across the Cambrian-Ordovician transition

Abstract

We have analyzed 214 fossil apatite samples from nine stratigraphic sections worldwide that cover the lower Cambrian to lower Ordovician for their strontiumisotope ratios. Of these samples, 180 from six sections that cover the Cambrian-Ordoviciantransition (<5 Ma) reveal how the extent of isotopic alteration can differ greatly according to lithology and sample type. From three limestone sections, 126 euconodont samples yield the most consistent 87Sr/86Sr ratios and are used in this article to constrain seawater 87Sr/86Sr across the Cambrian-Ordovician boundary. By contrast, protoconodonts, paraconodonts, and inarticulate brachiopods are far less likely to retain a primary 87Sr/86Sr signature and appear to be of only limited value for chemostratigraphy. Seawater 87Sr/86Sr fell from maximum values for the Phanerozoic of more than 0.7092 during the early Late Cambrian to 0.7090 by the earliest Ordovician. High-resolution sampling has permitted the recognition of sinusoidal variations with a wavelength on the order of <1 Ma that are superimposed on the overall fall in 87Sr/86Sr. These variations may be caused by high-order cycles in seawater 87Sr/86Sr or diagenetic alteration.