Among calcareous nannofossils, important primary producers in Jurassic and Cretaceousoceans, nannoconids were carbonate rock-forming organisms. During the late Barremian andearly Aptian (-126 to 122 million years ago), nannoconids went through a crisis culminatingduring the Oceanic Anoxic Event 1a. Here we present nannofossil and geochemical data froma section of early Barremian–early Aptian age from the North Sea, recording the earliestchalks ever known in the Boreal Realm. These middle-late Barremian chalks were generatedby blooming of endemic nannoconids under relative warm and arid conditions. A subsequentdecrease of nannoconids in the latest Barremian coincides with increased nutrient and clayinput. This nannoconid decline, also detected at low latitudes, was associated with theOntong Java Plateau emplacement. We conclude that nannoconids were rock forming also athigh latitudes, under clear and oligotrophic waters. Their decline was related to increasedcontinental runoff under reinforced greenhouse conditions.
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