Strontium isotopic variations in Jurassic and Cretaceous seawater

Abstract

A high-resolution seawater strontium isotope curve has been generated through the analysis of well-dated and well-preserved belemnites and oysters from the Middle and Upper Jurassic and the Lower Cretaceous of Great Britain. Analysis of Fe and Mn concentrations in these fossils has yielded criteria for eliminating samples that are diagenetically altered. The strontium isotope curve remains relatively flat through the Aalenian and early Bajocian, rapidly descends through the late Bajocian and Bathonian, and reaches a minimum in the Callovian and Oxfordian. It then begins a rapid increase in the Kimmeridgian and Portlandian that continues through much of the early Cretaceous. The curve levels off in the Barremian, suddenly dips downwards in the Aptian, and recovers gradually through the Albian. The strontium isotopic variations are sufficiently large and the data are presented with sufficient stratigraphic detail to allow precise correlation to the classic ammonite zones and lithologic sections of Great Britain using the techniques of strontium isotope stratigraphy. Model results indicate that much of the variation in seawater 87Sr 86Sr between 120 and 40 Ma can be explained by changing the intensity of mid-ocean ridge hydrothermal fluxes proportionally to estimated mid-ocean ridge crustal generation rates. It is also possible that the variations during the rest of the Mesozoic and the Permian are primarily reflections of changing hydrothermal inputs. The model results have several important implications. First, they provide an example in which the variations in the strontium isotope curve are not necessarily driven by changes in fluvial inputs. Second, they suggest that from at least the Aptian through the Eocene variations in continental weathering were minimal. This heightens the importance of the rapid rise in seawater 87Sr 86Sr beginning ~ 40 Ma as a significant transition to an extended period of increasing fluvial 87Sr fluxes continuing to the present. Finally, the results suggest that several documented short-term excursions towards lower 87Sr 86Sr in the latest Triassic, Pliensbachian-Toarcian, Callovian-Oxfordian, Aptian-Albian, and Cenomanian-Turonian are interpretable as pulses of seafloor hydrothermal activity. If so, the strontium isotope record offers a means of constraining the timing, duration, and magnitude of known or proposed hydrothermal events in the geological record.