Biotic changes in Cretaceous oceanic anoxic events of the Tethys

Universita di Milano, Dipartimento di Scienze della Terra, Milan, Italy

The two major episodes of C _org -rich black shales, namely the Selli (early Aptian) and Bonarelli (latest Cenomanian) Levels, corresponding to the anoxic Events OAE1a and OAE2, were studied for their calcareous and siliceous planktonic content in order to clarify the biotic changes associated to these extreme paleoceanographic environments. Each episode has its own peculiarities. In particular, the Selli Level is characterized by the temporary disappearance of nannoconids among the calcareous nannofossils and a decrease in diversity among the radiolarians and planktonic foraminifers. On the other hand, the Bonarelli Level coincides with a major turnover within the radiolarians and calcareous nannofossils and the extinction of the most specialized planktonic foraminifers, the rotaliporids, whereas the opportunist Heterohelix proliferates. The two black shales, however, share some common features, (1) in the interval immediately preceding both OAEs calcareous nannofossil assemblages reflect higher fertility of surface waters, (2) the selected decrease in diversity within the radiolarians, whose assemblages in the early to middle portion of each of them comprise only Nassellaria, interpreted as possible deep-dwelling taxa by analogy with the modern forms; (3) although planktonic foraminifers are the most susceptible to dissolution resulting in very scanty records, they also decrease in diversity associated with a relative increase in abundance of Leupoldina (Selli) and Schackoina (Bonarelli). While the decrease in radiolarian diversity occurs within the black shale levels, the changes, either as turnover or fluctuation in abundance of calcareous nannoflora and, to a minor extent, of planktonic foraminifers, precede the acme of C _org accumulation. Depauperate radiolarian assemblages, characterized by the supposed deep-dwelling forms, occur generally in association with maximum organic carbon values (up to 23% t.w.) and delta ¹³ C positive shift, but also occur in other organic-rich black shales prior to the Selli as far back as the late Hauterivian. The changes of calcareous and siliceous plankton observed within the OAEs all point to an increase in fertility of surface waters, possibly related to increased upwelling. The intensity of the perturbation, however, was much stronger during the Bonarelli deposition, resulting in a major turnover in siliceous and calcareous plankton and leading to anoxia at the sea-floor, whereas the perturbation was overall less intense during the Selli Event and paleoenvironmental conditions at the seafloor were only dysoxic in the absence of turnover in all the plankton groups.

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