No abstract

Greenhouse warming is a predicted consequence of the Chicxulub impact, but supporting data are sparse. This shortcoming compromises understanding of the impact's effects, and it has persisted due to an absence of sections that both contain suitable material for traditional carbonate- or organic-based paleothermometry and are complete and expanded enough to resolve changes on short time scales. We address the problem by analyzing the oxygen isotopic composition of fish debris, phosphatic microfossils that are relatively resistant to diagenetic alteration, from the Global Stratotype Section and Point for the Cretaceous/Paleogene boundary at El Kef, Tunisia. We report an ~1 per mil decrease in oxygen isotopic values (~5°C warming) beginning at the boundary and spanning ~300 centimeters of section (~100,000 years). The pattern found matches expectations for impact-initiated greenhouse warming.

During the Aptian, some carbonate platforms of the sub-tropical realm (for example, on the northern Tethys margin or in the Gulf of Mexico) were affected repeatedly by severe perturbations in the carbonate production factory and drowning, preferentially during global warming events such as the Early Aptian Oceanic Anoxic Event 1a and a prominent mid-Late Aptian warming interval. These platform growth crises have been explained mainly by strongly increased coastal runoff (for example, siliciclastics and nutrients) in combination with pronounced eustatic sea-level rises. In the last few years, increasing evidence suggests that carbonate platforms of lower latitudes were generally less or even not affected by environmental perturbations during these events. This raises the question as to the responsible factors that promoted platform growth or decline in different latitudinal areas. In this study, Upper Aptian (Middle Gargasian to Uppermost Clansayesian) inner-tropical carbonate ramp deposits of the Serdj Formation at Djebel Serdj, north-central Tunisia are studied in detail with regard to microfacies, lithology, biostratigraphy and chemostratigraphy. These data allow reconstruction of the palaeoenvironmental evolution of the Tunisian carbonate platform margin and investigation of its response to the prominent mid-Late Aptian warming interval. The unusually expanded, 600 m thick Serdj Formation consists of limestones, marlstones and siltstones, suggesting deposition within mid-ramp to inner-ramp palaeoenvironments. Deposits of the mid-Late Aptian are represented by quartz-rich platform carbonates and siltstones, probably resulting from increased coastal runoff on the Tunisian shelf as a response to global warming and accelerated water cycling. The siliciclastic input was accompanied by elevated nutrient levels as indicated by a partial decline in the abundance of oligotrophic biota and mass occurrences of orbitolines and green algae. Carbonate platform drowning during the mid-Late Aptian, as reported from the sub-tropical realm, has not been identified. A comparison with other tropical river-influenced platforms suggests that none of them drowned during the mid-Late Aptian. One important reason might be widespread arid to semi-arid climatic conditions within lower latitudes during that time, promoting platform growth due to comparably low nutrient runoff.

No abstract