Phosphogenesis, carbon-isotope stratigraphy, and carbonate-platform evolution along the Lower Cretaceous northern Tethyan margin

Abstract

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, Haute-rivian, 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 δ 13 C stratigraphies obtained from Valanginian-Hauterivian and Aptian-Albian hemipelagic successions beyond the platform correlate well with the Early Cretaceous pelagic δ 13 C record. Positive excursions in the pelagic δ 13 C 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.