Browsing by Author "Adams, PM"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    The Last Glacial transition and Holocene ecological change in Arthur’s Pass National Park, New Zealand
    (Australian Nuclear Science and Technology Organisation, 2021-11-17) Adams, PM; Fink, D; Shulmeister, J; Woodward, C; Fujioka, T; Simon, KJ
    This study improves our understanding of paleo-climate and -environmental change in the high-country South Island New Zealand focusing on formerly glaciated valleys near Arthur’s Pass. The area is of importance being positioned under a steep precipitation gradient on the lee side of the Southern Alps. Be-10 exposure ages (n=37) of ice limits, mapped as terminal moraines deposited during the last deglaciation through to late Holocene, span the past 17 000 years. The glacial record from well-preserved moraine sequences within the valley floor and glacial benches (850 to 1350m) demonstrates an initial post-LGM fast retreat that confirms previous studies (e.g. Shulmeister et al., 2010). Coupled with available in-valley exposure ages from Misery, Dobson and McGrath Moraines (Fink et al, 2017), a comprehensive regional deglacial pattern emerges. The Be-10 ages at Misery Moraines, subject of an early debate supporting a Younger Dryas glacial advance in the Southern Hemisphere (Ivy-Ochs et al. (1999), are re-dated to the Antarctic Cold Reversal (~14-15 ka). The record in headwater valleys of the Bealey and Otira rivers is largely obscured by slope-processes making conclusive observations relating to glaciation difficult. Consequently, early to middle Holocene moraines ages were not recovered in these valleys in contrast to nearby Dart Valley (Dowling et al., 2021). There is strong evidence at Otira headwall for a Little Ice Age glacial moraine limit. Several non-glacial boulders yield ages of 6.7ka, 4.2ka, 2.2ka and 1.7ka in the over steepened upper reaches of Bealey and Otira valleys that may reflect landslides associated with separate seismic events. In addition to our glacial chronology, we report on a well constrained record from Lances Tarn (Pb-210, and 37 radiocarbon dates over 4 m core length), a moraine-impounded bog at the saddle of Arthur’s Pass. This record spans ~13 000 years and displays post-glacial environmental change. The pollen record indicates typical succession from herb field through to closed forest, similar to other regional records (e.g. McGlone et al., 2004). The onset of Holocene like conditions is observed early c12ka, unlike central and lower South Island (~10ka). The area is influenced by persistent westerly flows through the entire record but with a reduction in flow in Early Holocene between 9.5 and 8.5ka, causing drier conditions in agreement with other records in Tasmania (Mariani and Fletcher, 2017 also in Patagonia (Moreno et al., 2020). There is little evidence of burning until c2.2ka when grass pollen rises sharply and charcoal counts increase, indicating a change in evapotranspiration (drying). Increased fires are evident from the onset of human arrival 750 years ago. The largest change in fire history and sediment transport occurs after European settlement ~ 1850’s, when intensive deforestation is evident in the pollen record. © The Authors
  • No Thumbnail Available
    Item
    A tale of two bogs - new 10Be production rates from UK and NZ calibrated by basal 14C ages
    (Copernicus GmbH, 2021-04-19) Fink, D; Hughes, PD; Fülöp, RH; Wilcken, KM; Adams, PM; Craig, W; Shulmeister, J; Fujioka, T; Ryan, P
    Cosmogenic production rates (PRs) are the essential conversion factor between AMS cosmogenic concentrations and absolute exposure ages. The accuracy of cosmogenic glacial chronologies and reliability in their comparison to other paleoclimate systems is largely contingent on the precision and accuracy of the adopted production rate. This is particularly critical in determining past glacial geochronologies at the scale of millennial temporal resolution. Most PR calibrations are carried out at deglaciation sites where radiocarbon provides the independent chronometric control usually based on 14C ages in basal sediments or varves from lake or bog cores which is assumed to represent the minimum age for glacial retreat. Under these conditions and hence provide PRs as maximum values. Given that today most AMS facilities can deliver 10-Be, 26-Al and 36-Cl data with total analytical errors less than 2% ( for 10 ka exposure), the precision of a PR remains largely dependent on the error in the independent chronology and accuracy of AMS standards. The history over the past 20 years of the ever-decreasing value of SLHL 10-Be cosmogenic spallation PRs from initial estimates of about 7 atoms/g/a to the current ‘accepted‘ (global average) values of ~4 atoms/g/a, is an interesting story in itself and demonstrates the complexity in such determinations. Over the past few years new web-based calculators are now available to calculate uniformly new production rates from either new data or combinations of any set of published data (CRONUS-Earth, CRONUS-UW, CosmoCalc, ICE-D, CREp). This delivers a means by which new production rates can be seamlessly integrated and compared using identical constants, methods and statistics that were used to generate (currently accepted) global average or regional production rates. For the British Isles, there are a number of 10-Be reference sites that give PRs (Lm scheme) between 3.89±3% atoms/g/a (Putnam, QG, v50, 2019) to 4.20±1% atoms/g/a (Small, JQS, v30, 2015) which convert to 3.95 and 4.28, respectively, using datasets in the ICE-D calculator). This difference in 10-Be spallation PRs has recently raised some debate and challenges for the timing of the local-LGM and demise of the British Ice Sheet. This work provides a new British Isles site specific 10Be PR from the Arenig Mountains in North Wales where radiocarbon dating of basal sediments from a bog core associated with a series of nearby cirque moraines provides independent age control. Similarly in the South Island of New Zealand, the current accepted 10Be PR is 3.76±2% (Putnam, QG 2009; converts to 3.94±1% using ICE-D) and is the only available PR that is used for these southern hemispheric glacial sites. This work provides a new Australasian site specific 10Be PR from Arthurs Pass retreat moraines where radiocarbon dating of basal sediments from three cores extracted from a bog impounded by the moraine provides independent age control.