AbstractsEarth & Environmental Science

Abyssal ocean currents develop unique physical and chemical properties, based on their geographic location of formation, circulation pathways, and the biogeochemical cycling of elements and their isotopes between different water masses. These distinct physiochemical properties enable water fingerprinting, the ability to identify and trace water masses as they circulate the globe, in their relentless attempt to redistribute the Earths heat, salt and biogeochemical agents. Over geological time, the chemical fingerprint of water masses has evolved in response to changing climatic regimes and tectonic events. Hydrogenous FMNs incorporate a record of these chemical fingerprints from the abyssal water masses in which they grow, as they accrete each successive growth layer from the elements and compounds available within ambient deep waters. Due to the exceptionally slow growth rate of these abyssal archives, FMNs provide insights on the chemical history of the deep ocean over millions of years. Such changes in FMN geochemistry have been previously linked to the development and demise of polar ice sheets and the opening and closing of ocean gateways. Here an attempt is made to recover the paleoenvironments recorded in the accretion of a large hydrogenous FMN recovered from the New Zealand Oceanic Gateway, where the conjoined flow of the Antarctic Circumpolar Current and Pacific Deep Western Boundary Current enter the Southwest Pacific from the Southern Ocean. This region of the deep ocean is of great interest, as it is the least explored ocean basin in terms of its elemental and radiogenic isotope composition and paleoceanographic evolution. The chemical and physical characteristics of these currents respond to environmental changes in their source area, Antarctica, as well as to global climatic and oceanographic events due to the effective mixing of all of the world’s major currents within the ACC. From a revision and assessment of beryllium cosmochronometry, analysis of macro- and micro- growth structures, authigenic and detrital nodule components and growth rates, analysis of major, minor and trace element chemistry via ICP-MS and Pb isotopic analysis via MC-ICP-MS, in addition to the application of multiple paleosource, paleocirculation proxies and novel application of paleoproductivity and redox, five major accretion periods and corresponding paleoenvironments can be ascertained for the late Neogene evolution of the PDWBC: Phase 1: The late mid-Miocene PDWBC – The first period of nodule growth is a faster accretionary period, distinguished by its calcareous shell fragment at the core, surrounded by dark red-brown Fe-Mn precipitates, and white-grey aluminosilicates and characterised by mottled microstructures due to high detrital incorporation. The physical and chemical archives of U1365B-M indicate that the PDWBC, during this phase of globally depressed atmospheric and oceanic temperatures, was characterised by the corrosive, vigorous, well-ventilated currents characterised by a shallow CCD. Paleocirculation…