Abyssal Archives: Unravelling the Late Neogene evolution of the Pacific Deep Western Boundary Current from the New Zealand sector of the Southern Ocean
Authors:   , Plant Amy Alexandra 
Published: November 2021

<p>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 proxies suggest the PDWBC transmits a strong NADW signal throughout this interval or, in the case of trends exhibited by Pb isotopic compositions, an increase in ice-sheet activity. Phase 2: The late Miocene PDWBC – The paleoceanographic conditions of the PDWBC established during the late mid-Miocene are strengthened over this second phase of accretion, distinguished by the lighter zone textures, in association with highest detrital incorporation percentages. The excursion toward radiogenic Pb values has been associated with further restriction of the Indonesian seaway to bottom and deep water circulation between the Pacific and Indian Oceans in addition to a phase of shoaling of the Panama Isthmus, leading to the increased divergence of warm tropic waters into the North Atlantic and strengthening of the ‘NADW’ fingerprint being exported to the Southern Ocean and incorporated into the PDWBC through effective mixing within the greatly sped-up ACC at this time. Additionally, the shift in Pb isotopic compositions in Pb-Pb space, indicate the PDWBC receives an increased aeolian flux in association with the colder, drier climate and shallow glacial ocean. Phase 3: The terminal late Miocene PDWBC – This phase of FMN growth is a slowing accretionary period, displaying reductions in detrital components and as such, microstructures grade from mottled to cuspate, as Mn and mangophile elements increase in concentration and the chemical evolution of PDWBC occurs in three main stages: 1) From [10 to 8.4 Ma] the PDWBC is in a transitional state, bottom water temperatures are reported to be higher and ocean oxygen (redox proxies) is significantly reduced as surface water productivity increases (productivity proxies). The large excursion to more radiogenic Pb is systematically and gradually reduced to those characteristic of the PDWBC before the excursion ccurred, potentially in response to: (a) closure of the Indonesian gateway and the resulting re-organisation and strengthening of Pacific circulation, strongly indicated by paleocirculation proxies which record an increasing ‘equatorial Pacific’ like signal from 10 Ma onward; (b) reduced spin up of the ACC during this period of relatively warmer conditions and thus a reduced NADW signal, indicated by the lack of regional hiatuses and declining detrital incorporation; (c) reduction of Antarctic ice-sheet activity after reaching a critical threshold at 10 Ma; (2) From [8 to 7 Ma] the CCD deepens, indicated by an increase in the Mn/Fe value as Fe delivery to the deep ocean is reduced due to decreased carbonate dissolution, and; (3) [7 – 6 Ma] when both detrital proportions and authigenic element concentrations increase potentially in response to a fresh influx of young AABW into the PDWBC as sheets are proposed to have increased once more, maintaining stable Ocean oxygen levels. Phase 4: The Miocene-Pliocene PDWBC – This fourth phase of growth is a slower accretionary period, marking the transition from previously higher accretion rates to those that are greatly reduced and the previously mottled and cuspate microstructures of the previous zones become continuously laminated and structured from this point forward. This phase of growth signifies a change in PDWBC chemistry, associated with the onset of modern thermohaline circulation, recorded in U1365B-M by transition from Atlantic to Pacific like Zr-Hf values, declining ocean oxygen as bottom waters become progressively frigid, and a biological boom in surface water productivity in response to: expansion of the west Antarctic ice sheet cooler, drier climates and enhanced AABW production. Phase 5: The Pliocene-Pleistocene PDWBC – The final phase of growth is the slowest accretionary period over which with microstructures become progressively more laminated on a finer micro-millimetre scale, indicative of strong PDWBC currents facilitating the accretion of a more compact and pure ferromanganese zone, displaying high concentrations of Mn (and associated elements, including group 3- Cu & Zn) and reductions in Fe (and associated elements). During the warm Pliocene [4.7 Ma to 3 Ma], a slight return of cuspate microstructures, reduction of detrital grains, and increase in the redox proxies over this interval, indicate a slower, warmer an increasingly ventilated Pliocene PDWBC. Significant reversals in long-term chemical trends occur at c. 4 Ma, most noticeable in the group 3 and HREE/LREE profiles, potentially recording the chemical response of the PDWBC to final closure of the Central American Seaway, and(or) the coeval shift and weakening of Indonesian Throughflow from a more southerly position to its more northern position of present, accompanied by a weakening of this flow. The transition from a warm Pliocene to a cold Pleistocene PDWBC is marked by cyclical spurts of increased Fe-Mn scavenging of tracer elements to greater levels of enrichment, which generally rise to their highest concentrations at 3 Ma (especially groups 4-6) and 1 Ma (especially groups 1-3), the latter of which is associated with the final shift in Mn/Fe values to double that of the terminal Miocene, indicating a further deepening of the CCD and thus decrease in PDWBC corrosivity. Productivity proxies are not in agreement over this period except for the decline in values to c. 1.5 Ma. Paleoredox proxies display a continued decrease in ocean oxygen as the oceans continued to cool and Paleocirculation proxies show a decrease from 3 Ma onwards to less radiogenic Pb values consistent with a reduced export of NADW during the Pleistocene as the AMOC is reduced to its shallow glacial mode of circulation.  In addition to reconstructing the paleoceanography in the Southwest Pacific, this thesis aims to improve our current knowledge about general sources and input mechanisms of elements to this region of the ocean and to broaden the range of possible applications of using the physiochemical archives of ferromanganese nodules in ocean and climate research in addition to providing a new technique.</p>