STRONTIUM ISOTOPIC VARIATIONS IN JURASSIC AND CRETACEOUS SEAWATER

A high-resolution seawater strontium isotope curve has been generated through the analysis of well-dated and well-preserved belemnites and o ysters from the Middle and Upper Jurassic and the Lower Cretaceous of Great Britain. Analysis of Fe and Mn concentrations in these fossils h as yielded criteria for eliminating samples that are diagenetically al tered. The strontium isotope curve remains relatively flat through the Aalenian and early Bajocian, rapidly descends through the late Bajoci an 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 recov ers gradually through the Albian. The strontium isotopic variations ar e sufficiently large and the data are presented with sufficient strati graphic detail to allow precise correlation to the classic ammonite zo nes and lithologic sections of Great Britain using the techniques of s trontium isotope stratigraphy. Model results indicate that much of the variation in seawater Sr-87/Sr-86 between 120 and 40 Ma can be explai ned by changing the intensity of mid-ocean ridge hydrothermal fluxes p roportionally to estimated mid-ocean ridge crustal generation rates. I t is also possible that the variations during the rest of the Mesozoic and the Permian are primarily reflections of changing hydrothermal in puts. The model results have several important implications. First, th ey 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 variat ions in continental weathering were minimal. This heightens the import ance of the rapid rise in seawater 87 Sr/ 86 Sr beginning approximatel y 40 Ma as a significant transition to an extended period of increasin g fluvial Sr-87 fluxes continuing to the present. Finally, the results suggest that several documented short-term excursions towards lower S r-87/Sr-86 in the latest Triassic, Pliensbachian-Toarcian, Callovian-O xfordian, 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 magni tude of known or proposed hydrothermal events in the geological record .