Strontium isotope ratios from the Swabo-Franconian Basin (Germany) and a new compilation of marine 87Sr/86Sr signatures for the Upper Sinemurian to Toarcian: global uniformity and driving forces for marine 87Sr/86Sr

Sr-87/Sr-86 ratios have frequently been used for correlating marine sedimentary units and its fossils, especially where high fidelity reference curves exist. The latter are usually constructed from macrofossil calcite samples, which have been screened for post-depositional alteration. Here, belemnite rostra from Lower Jurassic (Upper Sinemurian-Upper Toarcian) siliciclastic and carbonate sediments of the Swabo-Franconian Basin, deposited at mid-latitudes in the Laurasian Seaway, were targeted to confirm the trends and absolute values of the marine Sr-87/Sr-86 reference curve throughout this interval. Sr-87/Sr-86 ratios are derived from 21 belemnite rostra from a wider set of fossil specimens for which preservation was controlled using Scanning Electron Microscopy (SEM) and element/Ca ratios. Taken together with published data, a compilation of >500 Sr-87/Sr-86 datapoints is established that helps verifying the global significance of the reported trends and allows for the best-constrained reconstruction of marine Sr-87/Sr-86 across the late Sinemurian-Toarcian interval yet. The obtained results are in excellent agreement with earlier measurements from basins in Portugal, Spain, UK, Germany, and Bulgaria and Canada. Sr-87/Sr-86 ratios follow the well-known decreasing trend, from 0.70747 in the latest Sinemurian to lowest ratios of 0.70708 around the Pliensbachian-Toarcian boundary. Also the subsequent increase throughout the early Toarcian reaching values of >0.7072 in the early Late Toarcian is found to match previously established datasets. The new results further corroborate a link between marine Sr-87/Sr-86 ratios and rifting related to the Central Atlantic Magmatic Province throughout the Early Jurassic and increased radiogenic influx, possibly linked to Karoo-Ferrar volcanism and associated climate change in the Early Toarcian which would have likely caused a weathering pulse. The very good agreement in trends and absolute values suggests that macrofossils tapped into an isotopically uniform Sr pool across the Laurasian Seaway even during phases of severe basinal restriction.

Automated summary

This study confirms the trends and absolute values of the marine Sr-87/Sr-86 reference curve in the Lower Jurassic sediments and provides evidence for the link between marine Sr-87/Sr-86 ratios and rifting, volcanic activity, and climate change.