Response of western South American epeiric-neritic ecosystem to middle Cretaceous Oceanic Anoxic Events

Little is known about the impact of the mid-Cretaceous Oceanic Anoxic Events (OAEs) on the neritic carbonate systems in South America. In order to fill this knowledge gap, the present paper reports on the record of environmental changes in the Albian–Turonian neritic carbonates from the western South American domain in Peru. Owing to the very expanded and well-exposed sections in the Oyon region of central Peru, the OAE 1d and 2 intervals were sampled at high temporal resolution for both bulk micrite and bulk organic matter carbon isotopes, allowing us to compare the fingerprint of these two events between the northern and central Peruvian regions. This suggests the installation of two marked depositional modes: 1) the Albian–Turonian formation of a regional facies belt constituted by oyster-rich mixed siliciclastic-carbonate deposition along the western South America platform; 2) a restricted oligotrophic environment, characterized by the mass occurrence of Perouvianella peruviana and associated miliolids in central Peru during the late Cenomanian–Turonian. These observations advocate for the following scenario: Global warming during the late Albian–early Turonian resulted in humid climate on the western platform. This in turn caused enhanced chemical weathering rates on the Brazilian Shield, resulting in high runoff of nutrients onto the western platform. Nutrient runoff promoted the diversification of benthic oyster communities. Due to the uplift of the Marañon Massif and the installation of the Huarmey Trough, central Peru was isolated from the Pacific and from eastern deltaic influx of the Brazilian continental basement, allowing the local development of oligotrophic conditions during OAE 2. Furthermore, an increased influx of argillaceous sediment and reduced carbonate production is recorded in northern Peru at the onset of OAE 2, marked by a prominent negative shift in δ¹³C. This negative carbon-isotope excursion has also been identified in other sections in the Pacific domain and can be linked to an increase in isotopically light pCO₂ induced by the formation of the Caribbean large igneous province.