Chemistry of Jurassic seas and its bearing on the existing organic life

Abstract

Study of Jurassic seawater chemistry comprised has revealed two successive stages of water mass organisation. The first stage, the unstable development of stratified zones, lasting until the mid Oxfordian, was characterised by large fluctuations of sea water temperatures and was interrupted by two episodes of opposite nature. The early Toarcian episode was related to expansion, whereas the Bajocian-Bathonian (B-B) episode was connected to temporary mixing of stratified seawater zones. The next stage, the breakdown of the stratified zones was characterised by stable,relatively warm seawater temperatures; it was also interrupted by boundary events, and depended, increasingly withtime, on oceanic circulation. The presence, development and retreat of stratified zones was manifested by massive CaCO₃ precipitation and CaCO₃ precipitation crises. Thus, interpretation of delta ¹³C in terms of burial and reburial of C_org in the sea, which requires existence and mixing of stratified seawater zones resulted in secular changes of CaCO₃ saturation states. This led to the picture of a Jurassic "aragonitic sea", contrary to the generally accepted model. Those CaCO₃ precipitation crises, emphasised by omission surfaces and specific mineral parageneses,suggested that Phanerozoic carbonate seas could have been suitable sites for "reverse weathering.

Although geochemical perturbations did not coincide with the major mass extinctions postulated for the Jurassic, they expressed changes, of primary productivity, and in the structure of organic assemblages. These changes as well as detailed palaeoenvironmental conditions, may be monitored by the elemental tracer i.e. Ba concentrations.