PHOSPHOGENESIS, CARBON-ISOTOPE STRATIGRAPHY, AND CARBONATE-PLATFORM EVOLUTION ALONG THE LOWER CRETACEOUS NORTHERN TETHYAN MARGIN

We distinguish three different stages in the evolution of the Tithonian (Late Jurassic) to Aptian (Early Cretaceous) northern Tethyan carbonate platform: (1) carbonate production in the coral-oolite mode (late Tithonian to early Valanginian, early Barremian to early Aptian), (2) carbonate production in the crinoid-bryozoan mode (early Valanginian, Hauterivian, late Aptian), and (3) platform retrogradation and destruction, condensation, and phosphogenesis (that is, platform drowning; early Valanginian to early Hauterivian, middle Hauterivian, late Hauterivian to early Barremian, early to early late Aptian, and latest Aptian to earliest Albian). Transitions from the coral-oolite mode to the crinoid-bryozoan mode and consequently to platform drowning may have been driven by increases in nutrient levels on the shelf. Phases of relative sea-level rise in times of carbonate production in the coral-oolite mode are named constructive, because of the observed platform regeneration following phases of widespread emersion during late sea-level highstands. In contrast, phases of relative sea-level rise in times of platform drowning are termed destructive. The deltaC-13 stratigriphies obtained from Valanginian-Hauterivian and Aptian-Albian hemipelagic successions beyond the platform correlate well with the Early Cretaceous pelagic deltaC-13 record. Positive excursions in the pelagic deltaC-13 record correspond in time to episodes of platform drowning. This suggests the existence of a coupling mechanism between changes in the global carbon cycle and platform drowning. In our view, the Early Cretaceous crises in carbonate-platform growth were the consequence of reinforced greenhouse conditions, which may have been triggered by episodes of extensive, flood-basalt volcanism. Strong greenhouse conditions may have induced the following chain of feedback mechanisms, enabling the biosphere to return to normal conditions: climate warming --> sea-level rise, accelerated water cycle, intensified weathering --> nutrient mobilization --> platform destruction, increased productivity --> increased phosphogenesis and carbon burial --> weakened greenhouse conditions.