Basalt‐seawater interaction, the Plenus Cold Event, enhanced weathering and geochemical change: deconstructing Oceanic Anoxic Event 2 (Cenomanian–Turonian, Late Cretaceous)

Abstract

Oceanic Anoxic Event 2 (Cenomanian-Turonian: ca 94 Ma) represents a major palaeoceanographic phenomenon that took place during an interval of extreme global warmth when large amounts of organic matter entered the marine burial record, probably triggered by increased availability of nutrients for planktonic biota. Three sections (Eastbourne, Sussex, UK; Raia del Pedale, Campania, Italy; and Tarfaya, Morocco) recording this event illustrate the influence on marine geochemistry of mafic volcanic rock-seawater interaction, anoxia to euxinia, and re-oxygenation and cooling during the socalled 'Plenus Cold Event'. The Eastbourne section represents the organiclean epicontinental pelagic deposits of the English Chalk; the Raia del Pedale section represents a shallow-water platform carbonate on the Tethyan continental margin, also largely devoid of organic matter; and the Tarfaya core represents an Atlantic margin site where cyclically bedded organic-rich sediments were well developed. Correlation between all three sections is readily achieved by biostratigraphy and carbon-isotope stratigraphy (d 13 C carb and d 13 C org ) over the Oceanic Anoxic Event 2 interval, represented by a characteristic broad positive carbon-isotope excursion. The stratigraphic range of the Plenus Cold Event, defined by the presence, in two discrete levels, of boreal fauna and an excursion to heavier oxygen-isotope values in the English Chalk, can be identified in Raia del Pedale and Tarfaya by using the carbon-isotope curve as a correlative tool. Similarly, a section in southern France allows its co-existing osmium-isotope excursion to relatively unradiogenic values to be placed in the context of the Oceanic Anoxic Event in all three analysed sections. A fall to lower osmium-isotope values clearly predated the onset of Oceanic Anoxic Event 2, as defined by the initial rise in carbon-isotope values, allowing the putative magmatic/mafic event as a trigger for the Oceanic Anoxic Event. An initial drop in sulphur-isotope ratios (d 34 S CAS ) at Eastbourne correlates with the osmium-isotope curve, suggesting that isotopically light sulphur could have been derived from a mafic igneous source. Re-oxygenation of sediments of all three investigated sections during the Plenus Cold Event is variably illustrated by change in cerium:calcium, iodine:calcium, molybedenum:calcium and uranium:calcium ratios, according to the redox behaviour of the elements in question and whether controls on seawater chemistry were local or global in nature. Changes in molybdenum-isotope ratios from Tarfaya and portions of the sulphur-isotope curve from Eastbourne and Raia del Pedale also indicate the probable presence of more oxygen-rich bottom waters during the Plenus Cold Event. Oxidation by such waters of previously deposited organic-rich shales, as well as loss of anoxic/euxinic sinks, is credited with temporarily enriching global seawater in a range of other redox-sensitive trace metals (for example, V, Cr, Co, Ni, Cu, Zn and Cd) during ongoing basalt-seawater int...

The Albian to Turonian carbon isotope record from the Shilaif Basin (United Arab Emirates) and its regional and intercontinental correlation

“…It coincides with global deposition of organic‐rich laminated sediments (e.g. Gale et al ., ; Jenkyns et al ., , ; Tsikos et al ., ; Scopelliti et al ., ; Voigt et al ., ; Gebhardt et al ., ; Jarvis et al ., ; Bomou et al ., ; Dickson et al ., ; Owens et al ., ). Other observations associated with OAE 2 include an increase of siliceous plankton in the Umbria‐Marche Basin during the event (Mort et al ., ) or pre‐dating it in the Natih Basin (Van Buchem et al ., ; Wohlwend et al ., ), and blooms of pelagic bivalve filaments from the earliest Turonian across Northern Africa and Oman to Tibet (Negra et al ., ).…”

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“…These short-lived (<1 My) episodes of enhanced deposition of organic carbon are associated with carbon isotope excursions and high rates of species turnover (Jenkyns, 1980(Jenkyns, , 2010Arthur et al, 1988;Leckie et al, 2002;Jenkyns et al, 2012Jenkyns et al, , 2017. The sediment records cored at Sites U1513, U1514, and U1516 exceeded our expectations for this objective.…”

Mesozoic climates and oceans – a tribute to Hugh Jenkyns and Helmut Weissert

“…Jones et al ., ; Jenkyns et al ., , , ; Lu et al ., ; Blättler et al ., ; Pogge von Strandmann et al ., ; Zheng et al ., ). Throughout his work, Hugh Jenkyns has been able to draw on a wide variety of datasets and make links that provide deep insights into the workings of the Earth system during the Mesozoic, exemplified in this volume by his contribution on the variety of geochemical and sedimentological signatures associated with the Plenus cold event during OAE 2 (Jenkyns et al ., ).…”

“…In this aspect of OAE research, concentrations and isotopes of redox‐sensitive elements, such as Cr, Fe, I, Mn, Mo, N, S, Tl, U and V, have proven particularly valuable in reconstructing changing redox conditions both locally and globally (e.g. Jenkyns et al ., , , ; Kuypers et al ., ; Pearce et al ., ; Jenkyns, ; Lu et al ., ; Montoya‐Pino et al ., ; Gill et al ., ; Nielsen et al ., ; Owens et al ., , ; Westermann et al ., ; Zhou et al ., ; Dickson et al ., , ; Gomes et al ., ; Holmden et al ., ). Through the integration of the different geochemical systems discussed here, it has been possible to develop a detailed understanding of the temporal (and, arguably, mechanistic) links between changes in the physical environment, seawater chemistry and biogeochemical cycles during OAEs (e.g.…”

“…Although this simple model (albeit with added nuances) has been applied to many events, the fit to each event is variable. For example, OAE 2 in the Late Cenomanian conforms to the conceptual model well (Jenkyns et al ., ), except for the absence of a definitive negative δ 13 C excursion; in contrast, the Late Valanginian ‘Weissert’ Event, a prominent positive carbon‐isotope excursion (Fig. ), is not associated with a discrete period of time characterized by widespread black shale deposition, leading some to speculate that organic carbon was deposited on land instead (e.g.…”