A high-resolution timescale of the Cenomanian–Turonian Oceanic Anoxic Event 2: constructing a 3HeET timescale across the Bonarelli Level, Italy.

The Mesozoic Era was punctuated by periods of rapid environmental change, known asOceanic Anoxic Events (OAEs), during which a rapid rise in temperature was accompaniedby an accelerated hydrological cycle, enhanced continental weathering, increasednutrient input to the oceans, intensified upwelling, increased organic productivity andexpansion of marine anoxia/euxinia, all documented through a range of geochemicalproxies. However, a major issue in the study of OAEs is the absence of good temporal constraintson these proxy records, as timescales can be hard to construct over sections whereenvironmental perturbations have resulted in rapidly changing lithologies and, potentially,sedimentation rates. The use of extraterrestrial 3He concentration as a proxy for sedimentation rate in marinesediments allows the construction of high-resolution timescales. This methodology isparticularly suited for 'difficult’ sections, such as those recording OAEs, because aninstantaneous sedimentation rate is determined for each sample measured; timescales cantherefore be constructed over sections with rapid fluctuations in lithology where the cyclicalsignals required for cyclostratigraphic timescales may be difficult to identify. The resolutionof such a timescale is also therefore theoretically limited only by sampling density (and themixing effects of any bioturbation). We use this technique to construct a high-resolution timescale over the Bonarelli Level, a∼1 m-thick stratigraphic unit of millimetre-laminated organic-rich black shale andradiolarian sands which is the expression of the Cenomanian–Turonian OAE 2 in centralItaly, and represents a historically important and well-studied section. In order to measurehelium in the extremely organic carbon rich shales of the Bonarelli Level (> 30% TOClocally in some horizons), we build on previously established techniques and expand therange of lithologies to which this method has been applied to include organic-rich blackshales. Using our 3HeET timescale, we are able to provide an estimate for the timespanned by the Bonarelli Level, explore the possibility of hiatuses and condensedsedimentation at its top and base and examine the depositional rates of differentlithologies. Using our new data, we reconstruct organic-carbon mass accumulationrates in detail through OAE 2 and discuss the significance of these new data forunderstanding palaeoceanographic conditions during this major palaeoceanographic event. [ABSTRACT FROM AUTHOR]