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New constraints upon isotope variation during the early Cretaceous (Barremian–Cenomanian) from the Pacific Ocean

Department of Geological Sciences, The University of Plymouth, Drake Circus, Plymouth, PL4 8AA, UK

^* E-mail: g.price{at}plymouth.ac.uk

Carbon and oxygen isotope data from a succession of Cretaceous (Barremian–Cenomanian) age recovered from the Pacific Ocean (DSDP site 463) are presented. The carbon isotope curve reveals a large isotope excursion within the early Aptian where ¹³C values reach ~4.8 in the L. cabri–G. ferreolensis foraminifera zone. A decrease in ¹³C values is observed at the base G. algerianus zone, before a return to more positive values at the top of the G. algerianus–T. bejaouaensis zone. The pronounced early Aptian positive event is preceded by a large negative isotope excursion, confined to the G. blowi zone. Synchronous with this excursion are increased total organic carbon values and increases in Mn and Fe concentrations. Integrated biostratigraphic and magnetostratigraphic data, together with the carbon isotope profile, suggest that the organic-rich units of site 463 are correlatable with Oceanic Anoxic Event 1a. The input of isotopically light volcanic CO₂ in concert with the intensification and upwelling of intermediate water enriched in ¹²C could account for the observed trends. A potential trigger may have been the destabilization of the water column and the prodigious CO₂ emissions associated with hydrothermal activity and the emplacement of the Ontong Java Plateau. Coupled with faunal evidence, the subsequent positive carbon isotope excursion is interpreted to be resulting from high, but decreasing, productivity and possibly increasing ocean stratification resulting in strong carbon isotopic gradients and ¹³C-enriched surface waters. The decrease in ¹³C within the G. algerianus zone is coincident with more positive ¹⁸O values. If these are interpreted in terms of temperature this interval may be characterized by a period of cooling and possibly a waning of C_org cycling. A return to lower ¹³C values during the middle Albian is considered to be related to the increased influence of upwelling, as opposed to a waning of C_org cycling. Upwelling introduced isotopically light carbon to the surface, arresting the stratified oceanic conditions.

Key Words: Cretaceous • Aptian • oxygen • carbon • isotopes

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