Robert Chisari

Tephrostratigraphy and geochronology of a ca. 120 ka terrestrial record at Lake Poukawa, North Island, New Zealand

Abstract A 198-m-long core was obtained from Lake Poukawa, Hawkes Bay, New Zealand for paleoclimatic analysis. A chronology extending back to ca. 120 ka has been developed using a combination of tephrostratigraphy, radiocarbon, optical, and U–Th disequilbrium dating. The core contains a new record of tephra beds, including temporal intervals poorly recorded elsewhere, and revises the dispersal for some known events. Thirty macroscopic tephra beds were identified, comprising 20 rhyolites with compositions consistent with previously studied tephra from Taupo and Okataina calderas, and 10 andesites–dacites compositionally similar to Tongariro and Egmont centre eruptions. Electron microprobe data provides evidence for a total of 24 rhyolite eruptions amongst the 20 macroscopic beds. Four widespread rhyolitic marker beds: Whakatane (4.6 ka), Kawakawa (22.6 ka), Tahuna (ca. 43 ka), and Rotoehu (ca. 50 ka) provide temporal constraints for the upper 40 m of the core. The occurrence of Opepe (9 ka) and Okaia (23 ka) tephra beds in this core extends their known dispersal to southern North Island. A previously unrecognised and chemically distinct rhyolite tephra (ca. 35 ka) was also found in the sequence. Twelve rhyolitic tephra occur in the interval 50–120 ka, a period in which the timing and nature of volcanic events is poorly understood at proximal sites of the Taupo Volcanic Zone. The Lake Poukawa core provides evidence for widely dispersed tephra-producing eruptions from Egmont volcano and Tongariro centre back to about 120 ka. The tephra are glassy, unlike many proximal deposits, and can be geochemically fingerprinted, thus providing an opportunity to develop a framework for eruptions not assessable in proximal localities. The pre-50 ka, high-K Egmont tephra are compositionally similar to younger (post-30 ka) proximal pyroclastics, but differ from contemporaneous low- and medium-K rocks that characterise the proximal ring-plain of the volcano. An average Holocene peat accumulate rate of 1.5 m/ka and an average post-50 ka sedimentation rate of 0.78 m/ka are implied from the ages of interbedded tephra. However, the depositional history of the core is complex because tephra at a depth of 40 m, and optical and U–Th disequilbrium ages at ca. 103 m are the same age within analytical uncertainties. This implies either rapid alluvial sedimentation or unrecognised problems in the dating methods. U–Th disequilbrium ages, together with paleoecological information, suggest that a peat interval at 143–146 m depth formed during the last interglacial maximum (oxygen isotope substage 5e).

Separation and measurement of thorium, plutonium, americium, uranium and strontium in environmental matrices

A technique for the isolation of thorium (Th), plutonium (Pu), americium (Am), uranium (U) and strontium (Sr) isotopes from various environmental matrices has been adapted from a previously published method specific to water samples (Maxwell, 2006). Separation and isolation of the various elemental fractions from a single sub-sample is possible, thereby eliminating the need for multiple analyses. The technique involves sample dissolution, concentration via calcium phosphate co-precipitation, rapid column extraction using TEVA™, TRU™ and Sr-Spec™ resin cartridges, alpha spectrometry for Th, Pu, U and Am and Cerenkov counting for Sr. Various standard reference materials were analysed and chemical yields are in the range of 70-80% for Th, Am, U and Sr and 50-60% for Pu. Sample sizes of up to 10 L for water, 5 g for dry soil and sediment and 10 g for dry vegetation and seaweed can be processed using this technique.

Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia

Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H2O stable isotopes, δ13CDIC, 3H, 14C and 87Sr/86Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing 3H and 14C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ∼50 m below ground surface in most areas and ∼90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO3-type waters, although with higher pH and SC, and no 3H and low 14C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times. Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO3− concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase.