| Summary: | <p>The concentration of extraterrestrial 3He (3HeET) measured in pelagic sediments may be used as a proxy for sedimentation rate, allowing the construction of high-resolution timescales over “short” geological intervals (100s kyr), with temporal resolution in theory limited only by sampling resolution. The construction of a 3HeET timescale through the Bonarelli Level, a ~1 m thick interval of interbedded black and grey shales and radiolarian sands which is the sedimentary expression of Oceanic Anoxic Event 2 (~94 Ma) in Umbria–Marche, Italy, provides not only a novel method for estimating the duration of this record (478±160 kyr), but also allows rates and durations to be attached to the environmental shifts recorded in changing styles of sedimentation and/or changing geochemical records. Slow-downs in sedimentation rates are associated with the intervals of biotic turnover represented by the lithological shift from limestone to black shale at the base of the Bonarelli Level, and from black shale to limestone at its top. A transient increase in the accumulation rate of carbonates above the Bonarelli Level likely reflects enhanced biogenic calcite preservation due to build-up of excess alkalinity during OAE 2.</p>
<p>Precession cycles within the rhythmically bedded Scaglia Bianca Formation, a sequence of pelagic limestones well characterised by cyclostratigraphy that underlie the Bonarelli Level, were also investigated using 3HeET timescales. Exploration of the timings and possible drivers of lithological change within these Scaglia Bianca precession cycles provides insight into how local palaeoclimatic and Earth System processes reacted to ‘base level’ orbital forcing in the Cenomanian prior to the perturbation associated with OAE 2. Precession cycles deposited under eccentricity maxima (expressed as limestone-black shale-black chert sequences) and minima (expressed as limestone laminated limestone-grey chert) were selected for comparison. Calculation of the proportion of time per cycle represented by silica- and organic-rich facies indicates that, when the amplitude of precessional insolation forcing was greatest (at eccentricity maxima), the palaeoenvironmental system spent longer in the more nutrient-rich environment under which silica- and organic-rich sediments were deposited, reflecting increased time spent above a ‘threshold’ insolation level.</p>
<p>This work expands the use of extraterrestrial timescales to stratigraphic sections with heterogeneous lithologies, including organic-rich black shales and cherts. 3HeET is shown not to be liberated during the diagenetic process of chert formation. Various methods of calculating 3HeET background flux in sections with heterogeneous lithology were also evaluated, and a best-practice method proposed.</p>
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