The impact of calcareous nannofossils on the pelagic carbonate accumulation across the Jurassic-Cretaceous boundary

摘要

Calcareous nannofossils were important producers of pelagic carbonates in Mesozoic oceans. In order to better understand the origin of Mesozoic pelagic Carbonates wed Studied upper Jurassic to lower Cretaceous sediments in the Central Atlantic Ocean (DSDP Sites 105, 367, 534A) with respect to their content of calcareous nannofossils. THe interval under investigation is characterized by a significant increase in the deposition of carbonate-rich sediments. a trend that goes along with the rapid radiation of calcareous nannofossils. The assemblage composition and the size variation of common taxa were analyzed, and subsequently the contribution of this phytoplankton group to the pelagic carbonate accumulation was calculated. Results reveal two long-term changes of the nannofossil assemblage composition. (1) The early Tithonian coccolith assemblages are of low diversity Watznaueria spp., Cyclagelosphaera spp., Zeugrhabdotus spp.), whilst the mid- to late Tithonian assemblage is dominated by nannoliths (Counsphaera mexicana, Polycostella beckmannii, Nanoconus spp.) and large-sized Watznaueria. (2) The early Berriasian is characterized by a shift from the later Tithonian nannolity-rich assemblage to a highly diverse coccolith assemblage. Morphometric studies of the placolith genus Watznaueria show for all three DSDP sites large-sized forms of this genus in the mid-and later Tithoniam, followed by a decrease of up to 2 μm for the mean size in the earliest Berriasian. At DSDP Site 105 the studied nannolith taxa show an increase in size during the mid-Tithonian to Berriasian interval. The records of both nannofossil carbonate estimates and the measured bulk-rock carbonate reveal two periods of increase in the nannofossil carbonate record of DSDP Site 105. A first significant increase of the carbonate accumulation is observed in the mid-and late Tithonian, probably caused by mass occurrences of strongly calcified taxa (C. mexicana, p. beckmannii, Nannoconus spp., Watxnaueria cf. manivitae). This interval is here named 'Nannofossil Calcification Event' (NCE). The second carbonate maximum in the late Berriasian is related to a rise in absolute abundances of nannofossils. This peak is amplified by an overall increase of the sedimentation rate. The calculated accumulation rates of nannofossils, nannofossil carbonate and bulk-rock carbonate for he late Berriasian are on the same scale as values from recent ocean surface sediments. A comparison of nannofossil carbonate values with the bulk-rock carbonate content shows that on average only 27% of the total carbonate can be explained by our nannofossil carbonate estimates. This discrepancy is most likely caused by the high amount of unidentifiable micrite and fragments of calcareous nannofossils. Other factors contributing to the error are possible inaccuracies in the determination of absolute abundances and nannofossil volume calculations. The NCE occurs during a long-term sealevel fall, dry climate and presumed low pCO_2 levels. A decline in abundance of strongly calcified nannofossils coincides with the opening of the Pacific-Atlantic seaway via Central America, which may have had a substantial impact o the palaeoceanographic situation in the Central atlantic Ocean. These changes are here considered to have at least partly caused the shifts in abundance, size and assemblage composition of calcareous nannofossils observed across the Jurassic-Cretaceous boundary interval.