Browsing by Author "Neil, H"

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    Exploring the source-to-sink residence time of terrestrial pollen deposited offshore Westland, New Zealand
    (Elsevier, 2016-07-01) Ryan, MT; Newnham, RM; Dunbar, GB; Vandergoes, MJ; Rees, ABH; Neil, H; Callard, SL; Alloway, BV; Bostock, HC; Hua, Q; Anderson, BM
    The occurrence of terrestrial palynomorphs in Quaternary marine sedimentary sequences allows for direct land–sea correlations and provides a means for transferring Marine Isotope Stage chronologies to terrestrial records that extend beyond the range of radiocarbon dating. Both of these important applications require an implicit assumption that the lag between pollen release and final deposition on the seafloor – here referred to as source-to-sink residence time – is negligible in relation to the chronological resolution of the sedimentary sequence. Most studies implicitly assume zero lag, and where studies do take palynomorph residence time into account, its magnitude is rarely quantified. In Westland, New Zealand, fluvial transport is the main source of delivery of terrestrial pollen offshore to the adjacent East Tasman Sea. We radiocarbon-dated organic matter carried and deposited by contemporary Westland rivers that drain catchments with varying degrees of disturbance. The ages obtained ranged widely from essentially modern (i.e., − 57 ± 22 cal yr BP) to 3583 ± 188 cal yr BP, suggesting that precisely constraining the residence time in this region is unlikely to be achieved. We also compared the timing of four palynomorph events characterising Westland's late Pleistocene, along with the well-dated Kawakawa/Oruanui Tephra (KOT), between marine core MD06-2991 and four terrestrial records from Westland. Critically, all palynomorph events and the KOT are chronologically indistinguishable with respect to the independently dated marine and terrestrial records, supporting the general principle of transferring the marine chronology onto the terrestrial records in this setting. In other regions, particularly those lacking the high soil production and erosion rates that characterise Westland, we suggest that similar tests of marine residence time should be conducted before assumptions of zero or negligible lag are invoked.© 2016, Elsevier B.V.
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    Export production in the New-Zealand region since the last glacial maximum
    (Elsevier B. V., 2017-07-01) Durand, A; Chase, Z; Noble, TL; Bostock, HC; Jaccard, SL; Kitchener, P; Townsend, AT; Jansen, N; Kinsley, L; Jacobsen, GE; Johnson, S; Neil, H
    Increased export production (EP) in the Subantarctic Zone (SAZ) of the Southern Ocean due to iron fertilisation has been proposed as a key mechanism for explaining carbon drawdown during the last glacial maximum (LGM). This work reconstructs marine EP since the LGM at four sites around New Zealand. For the first time in this region, 230-Thorium-normalised fluxes of biogenic opal, carbonate, excess barium, and organic carbon are presented. In Subtropical Waters and the SAZ, these flux variations show that EP has not changed markedly since the LGM. The only exception is a site currently north of the subtropical front. Here we suggest the subtropical front shifted over the core site between 18 and 12 ka, driving increased EP. To understand why EP remained mostly low and constant elsewhere, lithogenic fluxes at the four sites were measured to investigate changes in dust deposition. At all sites, lithogenic fluxes were greater during the LGM compared to the Holocene. The positive temporal correlation between the Antarctic dust record and lithogenic flux at a site in the Tasman Sea shows that regionally, increased dust deposition contributed to the high glacial lithogenic fluxes. Additionally, it is inferred that lithogenic material from erosion and glacier melting deposited on the Campbell Plateau during the deglaciation (18–12 ka). From these observations, it is proposed that even though increased glacial dust deposition may have relieved iron limitation within the SAZ around New Zealand, the availability of silicic acid limited diatom growth and thus any resultant increase in carbon export during the LGM. Therefore, silicic acid concentrations have remained low since the LGM. This result suggests that both silicic acid and iron co-limit EP in the SAZ around New Zealand, consistent with modern process studies. © 2017 Elsevier B.V.
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    Mid-late quaternary vegetation and climate change reconstructed from palynology of marine cores off southwestern New Zealand
    (18th INQUA Congress, 2011-07-21) Ryan, MT; Newnham, RM; Dunbar, GB; Vandergoes, MJ; Neil, H; Bostock, HC
    This paper presents preliminary findings from a new PhD investigation that aims to produce a long continuous mid-late Quaternary record of terrestrial pollen, vegetation, and climate. Records are generated from marine cores collected ca. 100 km west of the central South Island of New Zealand (ca. 42 oS, 170 oE). These cores were collected as part of the 2006 MATACORE voyage of R/V Marion DuFresne. Core sites were located on channel levees that have aggraded with sea-level rise, and are fed by river discharge and littoral drift where they cross the shelf and upper slope. The main core to be used in this study extends from the present back to Marine Isotope Stage (MIS) 11, on the basis of preliminary ?18O stratigraphy, at an estimated sedimentation rate of 10 cm ka-1, which is comparatively high for the NZ region. Previous investigation of similar marine sequences has revealed their unexpected capacity for generating robust pollen records that mirror those produced at adjacent sites on land. This, allied with analysis of surface samples, has indicated the likely provenance and vegetation source area of the pollen. The marine sequences provide additional benefits to their terrestrial counterparts, notably the capacity to generate long, continuous pollen records with rigorous chronologies. These chronologies can be intimately tied to other marine proxies, investigated in the same sequences, and the climate reconstructions generated from them. These terrestrially influenced, high sedimentation, West Coast cores will enable stronger insights than were previously possible into marine and terrestrial climate in the southern mid-latitudes across the last 4-5 glacial-interglacial cycles. Of particular interest are the differences in timing of response to climate transitions, between the marine and terrestrial realms and between the southern mid-latitudes and northern hemisphere and Antarctic glaciations. Copyright (c) 2011 INQUA 18