Ongoing Cenomanian — Turonian heterozoan carbonate production in the neritic settings of Peru

Abstract

The present paper reports on the sedimentological and geochemical record of Albian–Turonian neritic carbonates from the eastern subequatorial Pacific domain in Peru. The focus is on one of the most extreme carbon cycle perturbations of the Phanerozoic, the Oceanic Anoxic Event 2 (late Cenomanian–early Turonian). Thanks to the very expanded and well-exposed sections in Peru, the OAE2 interval was sampled at high temporal resolution for both bulk micrite and bulk organic matter carbon isotopes. Despite the scarcity of significant amounts of organic matter or evidence for oxygen deficiency, the d13C curve matches well with global published high-resolution data for coeval successions such as those reported from the English Chalk and the Portland # 1 core. Biostratigraphic data and the detailed sequence stratigraphic interpretation of these sections are combined with the carbon-isotope chemostratigraphy documented here. Applying the characteristic peak and trough chemostratigraphic terminology for OAE2 (A–C), the following main environmental and carbon isotope stratigraphic features are observed from the late Albian to the early middle Turonian in Peru: (i) An Albian to early late Cenomanian heterozoan ramp recording the pre-OAE2 d13C excursions, specifically the Mid-Cenomanian Event. (ii) A late Cenomanian trough of d13C values (B) showing a progressive deepening leading to the short-lived establishment of middle ramp type sedimentation. (iii) A late Cenomanian to early Turonian d13C plateau (C) characterised by benthonic inner ramp sedimentation during a sea-level highstand phase. (iv) A recovery of d13C values at the end of OAE2 associated to a trophic change, increased influx of argillaceous facies and reduced carbonate production. (v) A early to middle Turonian fluctuating d13C curve, linked to a maximum flooding phase in the Mammites nodosoides Zone and carbonate production during the Collignoniceras woollgari Zone. The data shown here are particularly relevant as they come from very expanded neritic sections in the sub-equatorial eastern Pacific. Many of the features recognized share important similarities with Tethyan and Atlantic sections whilst the ramp system as such did not suffer from a carbonate crisis during OAE2 as recorded, for instance, in Mexico and Tibet.