No abstract

Calculation of surface exposure ages, using in situ cosmogenic nuclides, requires accurate knowledge of local production rates. Here, we report the first attempt to calibrate cosmogenic 3 He production in the south-west Pacific region. We present a new radiocarbon chronology that precisely dates the emplacement of the Murimotu Formation, a large debris avalanche deposit in central North Island, New Zealand (ca. 830 m asl; 39˚S), which occurred 10.4-10.6 cal ka BP. Measurements of cosmogenic 3 He in pyroxene separated from large andesitic blocks exposed during this event yield a sea-level high-latitude production rate of 120 AE 12 atoms g À1 a À1 ('Lm' scaling). This is consistent with a recent global compilation, comprised predominantly of calibration sites located in the Northern Hemisphere. Thus, we conclude that the globally compiled cosmogenic 3 He production rate is valid in the south-west Pacific. Using independent, proximal calibrations of cosmogenic isotopes

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 seafloorhere referred to as source-to-sink residence timeis 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 in this region. We also compared the timing of four palynomorph events characterising Westland's late Pleistocene, along with the well-dated A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTKawakawa/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.

Geological climate archives from the Holocene Epoch provide baseline information concerning natural climate variability. Temperate mountain glacier extent is sensitive to summer air temperature, thus geological records of past glacier length changes are a useful proxy for this climatic variable. Here we present a new cosmogenic 10 Be chronology of glacier length changes at Dart Glacier in the Southern Alps, New Zealand. Prominent moraines deposited 321 ± 44 yr ago (n ¼ 11) and 7.8 ± 0.3 ka (n ¼ 5) show glaciers during the Little Ice Age were less extensive than during the early Holocene. This pattern of net Holocene glacier retreat is consistent with emerging data from other catchments in New Zealand and across the southern mid-latitudes. Using the physical framework of a transient global climate model simulation, we suggest that cool summers in the early Holocene were promoted by the local summer insolation minimum, together with low atmospheric greenhouse gas concentrations, causing an early Holocene austral glacial maximum. An insolation-driven reduction in seasonality at southern midlatitudes may reconcile differences between early Holocene temperature reconstructions where climate proxies have different seasonal sensitivities. We suggest that rising greenhouse gas concentrations after 7 ka caused regional-scale glacier retreat and appear to be the dominant driver of multimillennial summer temperature trends in the southern mid-latitudes during the present interglacial.

No abstract