ALBERT

Description: Pleistocene and late Pliocene benthic foraminifera from the Manihiki Plateau (southwestern tropical Pacific) were studied at piston-core 34KL. A new benthic foraminiferal biozonation is proposed. It comprises the Nodogenerina sagriensis Partial Range Zone from core base to 566.5 cm and the Fissurina seminiformis Partial Range Zone from this level to core top. The boundary is defined by the last occurence of Nodogenerina sagriensis which is time equivalent to the ‘Stilostomella extinction’ in the Eastern Atlantic. High abundances of Cibicidoides wuellerstorfi indicate a strong influence of near-bottom currents. The absence of high-productivity sensitive species reveals a low flux of organic matter to the sea floor from which a considerable amount is adduced by lateral advection.

Description: The transition from benthos to plankton requires multiple adaptations, yet so far it remains unclear how these are acquired in the course of the transition. To investigate this process, we analyzed the genetic diversity and distribution patterns of a group of foraminifera of the genus Bolivina with a tychopelagic mode of life (same species occurring both in benthos and plankton). We assembled a global sequence dataset for this group from single-cell DNA extractions and occurrences in metabarcodes from pelagic environmental samples. The pelagic sequences all cluster within a single monophyletic clade within Bolivina. This clade harbors three distinct genetic lineages, which are associated with incipient morphological differentiation. All lineages occur in plankton and benthos, but only one lineage shows no limit to offshore dispersal and has been shown to grow in the plankton. These observations indicate that the emergence of buoyancy regulation within the clade preceded the evolution of pelagic feeding and that the evolution of both traits was not channeled into a full transition into the plankton. We infer that in foraminifera, colonization of the planktonic niche may occur by sequential cooptation of independently acquired traits, with holoplanktonic species being recruited from tychopelagic ancestors.

Description: Changes in Atlantic Meridional Overturning Circulation (AMOC) strength exert a major influence on global atmospheric circulation patterns. However, the pacing and mechanisms of low-latitude responses to high-latitude forcing are insufficiently constrained so far. To elucidate the interaction of atmospheric and oceanic forcing in tropical South America dur-ing periods of major AMOC reductions (Heinrich Stadial 1 and the Younger Dryas) we gen-erated a high-resolution foraminiferal multi-proxy record from off the Orinoco River based on Ba/Ca and Mg/Ca ratios, as well as stable isotope measurements. The data clearly indi-cate a three-phased structure of HS1 based on the reconfiguration of ocean currents in the tropical Atlantic Ocean. The initial phase (HS1a) is characterized by a diminished North Brazil Current, a southward displacement of the ITCZ, and moist conditions dominating northeastern Brazil. During subsequent HS1b, the NBC was even more diminished or yet reversed and the ITCZ shifted to its southernmost position. Hence, dryer conditions pre-vailed in northern South America, while eastern Brazil experienced maximally wet condi-tions. During the final stage, HS1c, conditions are similar to HS1a. The YD represents a smaller amplitude version of HS1 with a southward-shifted ITCZ. Our findings imply that the low-latitude continental climate response to high-latitude forcing is mediated by recon-figurations of surface ocean currents in low latitudes. Our new records demonstrate the ex-treme sensitivity of the terrestrial realm in tropical South America to abrupt perturbations in oceanic circulation during periods of unstable climate conditions.