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Jörg Mutterlose:
Early Cretaceous climates: from an icehouse to a greenhouse world
(Talk)

For a better understanding of the palaeoceanography and palaeoclimate of the Early Cretaceous, calcareous nannofossil data have been obtained from high-latitudinal sites, offshore mid Norway and from the Barents Sea. No consistent nannofossil data are yet available for Early Cretaceous high latitudes, which should more clearly reflect possible palaeoclimatic changes as recorded in fluctuations of diversity and abundance. These data have been complemented by material from NW Europe, Poland, France, Italy, Romania and various low-latitude ODP sites in order to gain a complete picture of the palaeobiogeography of calcareous nannofossils for the earliest Cretaceous.

The nannofossil assemblages recorded from the high latitudes are generally of low diversity and characterised by abundant Watznaueria barnesiae and Crucibiscutum salebrosum, whereas Biscutum constans is less common. C. salebrosum, rare or absent at low latitudes, is extremely common in the Norwegian samples. It shows a bipolar distribution especially during Valanginian-Hauterivian times. In the early Valanginian, a distinctive latitudinal gradient in the abundance of C. salebrosum, impoverished in low latitudes and abundant in high latitudes, possibly reflects latitudinal differences in temperature. Restricted palaeoceanographic settings, caused by a sea-level lowstand, may have amplified these thermal gradients. Palaeoclimatically, this implies the existence of climatic belts throughout parts of the Early Cretaceous (Early Valanginian, Hauterivian), resulting from considerable temperature gradients from N to S.

The Early Cretaceous biogeographic pattern is very similar to the one found in modern oceans. The modern Atlantic Ocean consists, from N to S, of a subarctic, a northern temperate, a northern subtropical, a tropical, a southern subtropical, a southern temperate, and a subantarctic belt. The palaeobiogeographic patterns described suggest considerable thermal gradients for the Early Cretaceous. The data gained from calcareous nannofossils support the idea of an icehouse world or a cold mode for the Early Valanginian and Early-mid Hauterivian, and clearly disagree with a warm, equable climate for the earliest Cretaceous (Valanginian-Hauterivian). Our findings are supported by strontium and delta¹⁸O data, suggesting periods of low temperatures or high ice-volume, or both, for the latest Berriasian and Early Valanginian.

The Aptian-Albian is a period of major turnover of marine biota which occurred simultaneously both in the Tethys and in the Boreal Realm. Nannofossils and other groups of organisms (e.g. planktonic foraminifera, cephalopods) show distinctive cosmopolitan affinities, where endemic taxa and bipolar belts are less widespread. This may be best explained by a global shift towards open oceanic, pelagic environments in a greenhouse world caused by tectonic processes. This, in turn, resulted in an adaptive radiation of planktonic organisms.

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 [Division of Micropalaeontology] [Department of Geosciences] [Bremen University]

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