We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian-Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak ä 13 C values for all components are followed by intervals of high, near-constant ä 13 C in the form of an isotopic plateau. Recognition of an unambiguous return to background ä 13 C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian-Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the ä 13 C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

A group of calcareous nannoplankton named nannoconids experienced a crisis in the early Aptian and recovered only later in the late Aptian after a period of virtual absence. Although no extinctions occurred, the widespread nature of the "nannoconid crisis" suggests a global causal factor. This crisis is recorded within the Chiastozygus litterarius nannofossil and Globigerinelloides blowi planktonic foraminiferal zones, postdates magnetic chronozone M0 by approximately 300 kyr, and precedes the oceanic anoxic subevent la and associated •5•3C anomaly by some 40-100 kyr. Selective dissolution and anoxia cannot explain the crisis, because nannoconids are dissolution-resistant forms and their crisis clearly precedes the deposition of anoxic sediments. At least 1 m.y. prior to the "nannoconid crisis," the onset of a nannoplankton speciation event may be the response of nannofloras to a major rise in relative sea level. The "nannoconid crisis" seems to be synchronous with the early Aptian volcanic eruptions in the Pacific Ocean. Hence calcareous nannoplankton were severely affected by the "superplume" volcanic episode. The coccolithophorid bloom/nannoconid crisis was possibly induced by the excessive CO 2 levels in the atmosphere and/or caused by changes in nutrient content of oceanic surface waters. Fertility was enhanced by rapid turnover of nutrients due to the abnormal volcanic activity and accelerated transfer of nutrients from the continents into the oceans under warm and humid conditions of the mid-Cretaceous greenhouse climate. The "nannoconid crisis" may represent a competition between phytoplankton groups for nutrients or, more likely, competition between different calcareous nannoplankton. The biologic affinity and mode of life of Nannoconus are unknown, because there is no modem analog of this genus. However, comparison of Lower Cretaceous nannofossil assemblages with modem nannoplankton cummunities suggests that nannoconids, like extant Florisphaera profunda, possibly inhabited the lower photic zone. Concentrations of nutrients in the upper euphotic zone may have triggered blooms of coccolithophorids and nannoconid depletion. This model implies that the "nannoconid crisis" is the result of an abrupt, major change in the structure of surface waters caused directly or indirectly by the "superplume." The adjustments of the biosphere to the new paleoceanographic and climatic conditions required some 40-100 kyr before changing into abnormally high primary productivity and deposition of organic carbon-rich sediments with dinoflagellates outcompeting nannoplankton. tion of the Darwin Rise [Menard, 1964; Schlanger et al., 1981]. Recently, Larson [1991 a, b] quantified the dramatic Paper number 94PA00258. 0883-8305/94/94PA-00258510.00 increase in ocean crust production and abnormal intraplate volcanism and related the mid-Cretaceous volcanic episode to the eruption of a mantle "superplume." Tarduno et al. [1991] and Mahoney et al. [1993a, b] estimated that the Ontong Java Plateau, which formed close to the center he...

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Ocean acidification induced by atmospheric CO2 may be a major threat to marine ecosystems, particularly to calcareous nannoplankton. We show that, during the Aptian (approximately 120 million years ago) Oceanic Anoxic Event 1a, which resulted from a massive addition of volcanic CO2, the morphological features of calcareous nannofossils traced the biological response to acidified surface waters. We observe the demise of heavily calcified nannoconids and reduced calcite paleofluxes at the beginning of a pre-anoxia calcification crisis. Ephemeral coccolith dwarfism and malformation represent species-specific adjustments to survive lower pH, whereas later, abundance peaks indicate intermittent alkalinity recovery. Deepwater acidification occurred with a delay of 25,000 to 30,000 years. After the dissolution climax, nannoplankton and carbonate recovery developed over approximately 160,000 years under persisting global dysoxia-anoxia.

Ma, 87Sr/86Sr declined rapidly and reached a minimum at about 116-113 Ma. We speculate that the intensity of these latter responses suggests a corresponding peak in volcanic/tectonic activity at about 121-119 Ma.