Discorhabdus as a key coccolith genus for paleoenvironmental reconstructions (Middle Jurassic, Lusitanian Basin): Biometry and taxonomic status

Biometric analyses of Discorhabdus coccoliths were performed on a set of 29 samples taken from the marl/limestone couplets of the reference section of Cabo Mondego (Lusitanian Basin, west Portugal), Late Aalenian to Early Bajocian in age. Mixture analysis performed on the whole dataset revealed a bimodal length size distribution for the Discorhabus pool composed of D. striatus and D. ignotus. The size limit at 5 μm proposed here can be reliably used to distinguish both populations, but optical characteristics, namely birefringence, form an additional criterion essential to discriminate the two species. An increase in Discorhabdus size occurred from the Late Aalenian to the Early Bajocian, due to a raise in size of the whole Discorhabdus pool and in the abundance of D. striatus. We propose that an increase in Discorhabdus size upward in the section is associated with a positive trend in paleoproductivity as supported by the increase of nannofossil accumulation rates as well as the δ13C values of bulk carbonates sampled in the Cabo Mondego section. However, the Discorhabdus size pattern may also represent a response to sea-surface temperature, salinity or an evolutionary trend. A relative supra-regional warming of sea-surface water masses is suggested by δ18O values of biogenic carbonates and apatites that were sampled in several Mediterranean Tethyan settings. Rise of sea-surface temperature may have triggered an increase in the abundances of D. striatus as well as of the size of the Discorhabdus pool. A hypothetical influence of changing sea-surface salinity on Discorhabdus size or species-specific Discorhabdus abundances is not supported by our micropaleontological data or by available geochemical data. Our results also reveal that the increase in Discorhabdus size matches the history of calcareous nannofossil turnover (radiation, diversity and increasing abundances) during the Middle Jurassic.