Late Jurassic climate and its impact on carbon cycling

The climate of the Late Jurassic has been characterized by high atmospheric CO2 levels and by a monsoonal rainfall pattern. In this study we traced the evolution of the global carbon cycle through the Late Jurassic with the help of carbonate carbon isotope stratigraphy. The Oxfordian-Tithonian δ13C curve is marked by one major positive carbon isotope excursion with an amplitude Δδ13C > 1.0‰ (Middle to Late Oxfordian) and a second, minor positive excursion with an amplitude Δδ13C < 1.0‰ (Late Kimmeridgian). The Early Kimmeridgian and Early Tithonian δ13C-values fluctuate around δ13C = +2‰±0.3‰ and contrast with the less positive δ13C-values of Early Oxfordian and Late Tithonian age (δ13C = +1‰±0.5‰). A comparison of the Late Jurassic carbonate carbon isotope curve with the occurrence of organic-rich sediments suggests that not only fluctuations in organic carbon burial but also in carbonate carbon burial had an impact on the C-isotope record. The Oxfordian C-isotope excursion appears to correspond to a time of overall increased organic carbon burial triggered by increased nutrient transfer from continents to oceans during a time of rising global sea level. However, episodes of enhanced organic carbon burial during the Kimmeridgian and Early Tithonian are not reflected by prominent spikes in the C-isotope record. Favourable conditions for carbonate platform growth at a time of high global sealevel may have resulted in the stabilisation of the C org/C carb burial ratio and hence maintained the δ13C record at steady but relatively positive values. The Middle and Late Tithonian C-isotope values drop below δ13C = +1.5‰. A similar shift to less positive C-isotope values was recognized in other C-isotope records from the Tethys and Atlantic Oceans and reflects a decrease in the C org/C carb burial ratio possibly related to a reorganisation of the global climate system. ly widespread marine black shale-mature quartzose sandstone assemblages suggest that the relative efficiency of the Late Jurassic carbon pumps was controlled by weathering, erosion and runoff causing widespread marine eutrophication. Eutrophication favoured the organic carbon pump but it diminished the carbonate-platform growth potential. The monsoonal rainfall distribution pattern may explain why Late Jurassic carbonate platforms experienced less severe growth crises than Early Cretaceous carbonate platforms.