| Popis: |
A compilation of new and published stratigraphic, paleontological and geochemical data is used to detect the reciprocal influences of carbon cycling and global environmental changes in the Jurassic. A major perturbation of the surface carbon cycling accompanied by pronounced climate and sea level fluctuations (waxing and waning of continental ice?) affected Earth history around the Middle/Late Jurassic transition (MLJT). We establish the respective timing of changes of carbonate and organic matter sedimentation, and global fluctuations of sea surface temperatures (paleobiogeography and O-isotope paleothermometry) and sea level (sequence stratigraphy), so that causative mechanisms and feedback effects can be considered. It is apparent that the global sea level rise and warming initiated in the Late Bathonian led to a constriction of carbonate platforms to low latitudes and enhanced marine organic deposition. Sea level and temperature optima were achieved several million-years later during the Middle Callovian. A detailed record of sea surface temperatures in the Northern Hemisphere based on migration of marine fauna and isotopic thermometry indicates that a drastic climatic decline set in during the early Late Callovian, just post-dating the increased capture of organic matter by marine sediments. This decline in temperature is interpreted in terms of an inverse greenhouse effect, triggered by drawdown of CO 2 consequent upon excess carbon burial. The magnitude of refrigeration and its coincidence in time with an abrupt global-scale fall of sea level are both suggestive of continental ice formation at this time. Carbonate sedimentation was jeopardized at the MLJT as a result of both global cooling and presumed P C O 2 lowering, and resumed abruptly during the Middle Oxfordian by spreading again over mid-latitude zones. Salient conclusions are that (1) the pattern of excess carbon burial, coincident with elevated temperature but followed by climatic deterioration supports the general hypothesis that a major control on Mesozoic climate was the abundance of atmospheric CO 2 ; (2) significant masses of continental ice may have formed during this part of the Jurassic and correlatively, high CO 2 levels were certainly not sustained throughout this period; (3) the global carbonate sedimentation budget correlated with the surface temperature and sea level, but the latitudinal spreading of type-tropical carbonates was not simply related to the thermal status of seawaters; (4) on a global scale, the C o r g and C i n o r g burial rates were coupled, apparently through the correlation existing between the CO 2 level and surface temperature. |
