Earlier onset of the Early Cretaceous Equatorial humidity belt

Abstract

The long-standing view on the Cretaceous climate zones suggests the inception of an Equatorial Humid Belt (EHB) in the Albian. This inception contrasts with recent sedimentological and palynological evidence of sustained low-latitude, wetter paleoenvironment in the South American and African landmasses during the Aptian. The trigger for this early-stage EHB stands on two hypotheses: [1] the first major marine incursions to the equatorial Gondwana; [2] the southward displacement of a proto-Intertropical Convergence Zone (ITCZ). We provide new palynological, ichnological, and sedimentological data from the Codó Formation obtained from borehole 2-ANP-5-MA, drilled in the São Luís Basin, Brazilian equatorial margin. This integrated analysis allowed recovering well-preserved palynomorph assemblages of the upper Aptian Codó Formation. Based on three palynomorph-based climate indicators (sporomorph genera richness, the relative abundance of reworked palynomorphs, and climate-indicative palynofloras), and independent climate-indicative sedimentological indicators, our results show an interval of increasingly humid climate, which precedes the first major marine incursion. It also confirms that sporomorph genera richness scales directly with precipitation levels. Based on that, we reconstructed coeval late Aptian humidity patterns from thirteen South American and African sedimentary basins. Our findings support the establishment of the EHB in the late Aptian and challenge the view of a dominantly arid Equatorial Gondwana before the Albian. We argue that decreasing Proto-North Atlantic/South Atlantic sea-surface temperature (SST) gradient may have triggered a southward EHB displacement, leading to wetter, low-latitude biomes with broad and complex ecosystems. For the first time, a plausible mechanism is raised to explain the linkage between changing interhemispheric SST gradient and the dynamics of a proto-ITCZ in the Early Cretaceous.