Robert G. Ditchburn

Atmospheric deposition of 7Be and 10Be in New Zealand rain (1996-98)

Abstract Beryllium isotope concentrations were determined in monthly rainfall collections at three sites across New Zealand (36 to 45° S), from October 1996 to November 1998. At the northern sites of Leigh (near Auckland) and Gracefield (near Wellington), 7 Be and 10 Be concentrations are relatively constant at 1.2 to 1.4 × 10 7 atoms kg −1 rain and 2.1 to 2.6 × 10 7 atoms kg −1 rain, respectively. These concentrations correspond to annual flux rates of ∼15 and ∼27 × 10 9 atoms m −2 y −1 , respectively. At the southern site of Dunedin, concentrations are similar to those at the northern sites, but flux rates are significantly lower at ∼ 9 and ∼19 × 10 9 atoms m −2 y −1 , respectively, because of lower average rainfall east of the Southern Alps mountain range. These results are broadly similar to those reported by Brown et al. (1989) and Knies et al. (1994) for rain from midlatitude sites in the USA sampled from 1986 to 1994. Mean 7 Be/ 10 Be ratios for New Zealand (0.47 to 0.61) are, however, significantly lower than for the USA (0.69 to 0.78), due in part to the addition of 10 Be from re-suspended dust. Subtraction of the dust component increases the New Zealand 7 Be/ 10 Be ratios to 0.70 (Leigh), 0.65 (Gracefield) and 0.50 (Dunedin). The adjusted results provide evidence for transfer of older stratospheric air to the troposphere in late-spring-summer, an effect which is strongest in the north. The overall reduction of 7 Be/ 10 Be from north to south implies an increase in residence time from ∼ 80 to ∼100 d for Be isotopes in the atmosphere above New Zealand.

Validation of cosmogenic nuclide production rate scaling factors through direct measurement

Abstract 7Be produced in water targets by nuclear interactions of cosmic rays has been measured to determine cosmogenic nuclide production rates as a function of altitude (sea level to 2 km) and geomagnetic latitude (20–79°S). Relative intensities of low energy cosmic ray neutrons have at the same time been measured using neutron monitors based on IGY/NM-64 designed to efficiently thermalise ca. 2–30 MeV neutrons. The research is on-going and we present here preliminary data from the past two years. Water target and neutron flux results are in general agreement, and are consistent with the altitude-dependent scaling factors of Lal [Earth Planet. Sci. Lett. 104 (1991) 4241]. Significant differences between the sea level, latitude-dependent neutron flux data and Lals predictions are possibly related to the response function of the detector.

Rapid soil accumulation in a frozen landscape

Polygonal patterned ground is a dominant geomorphic feature in the McMurdo Dry Valleys, Antarctica. In central Beacon Valley, polygons are considered to be ancient expressions of stagnant surface processes and are thought to be stable for millions of years. However, the actual rate at which these polygons form and alter is not known. We describe a detailed cross section of a polygon, make several depth profiles of meteoric 10Be along this cross section, and use optically stimulated luminescence to date a few key samples at the center of this polygon. While confirming conclusions of previous studies that the polygon shoulders are stable on a 100 k.y. time scale and experience little vertical sediment mixing, our results also give clear evidence of eolian transport to the polygon center, leading to a sediment accumulation rate of 3 cm k.y.−1 over the past 15 k.y. Moreover, the data suggest that the accumulated material is locally derived, and, hence, surface erosion of the polygon shoulders must exist that cannot be recorded by meteoric 10Be. We conclude that polygon modification and soil accumulation under the apparent frozen conditions of Beacon Valley is an active and ongoing process.

210Pb chronology from Wellington Harbour, New Zealand

Abstract A sediment accumulation record based on the levels of “unsupported” 210Pb was obtained from a core taken from the centre of Wellington Harbour, New Zealand. Sediment accumulation rates (SARs) varied from 3 to 5 mm yr‐1. Before 1920, SARs were sufficiently high to dilute unsupported levels of 210Pb to background, but had fallen below the mean by 1925 maintaining constant SARs until 1953. These data provide a higher resolution chronology of early 20th century SARs than was previously available. Comparison with 137Cs data confirms that there was a significant increase in SARs between 1953 and 1967, but that these have remained fairly constant since 1974. Data from the two radionuclides are consistent after 1974, and 210Pb data are consistent with other chronological measurements before 1953. Therefore, it is suggested that the two radionuclides are immobile in the marine sediments of Wellington Harbour and that differences between the two SARs are real.

10Be investigations of sediments, soils and loess at GNS

Abstract Concentrations of 10Be have been measured in sediments and loess from the Wanganui region, New Zealand, and in soils from the Roberts Massif, Antarctica. The Wanganui sediments show a clear enhancement of 10Be over the period of the Brunhes-Matuyama geomagnetic field reversal, although the cause of the enhanced signal is not attributed directly to the field reversal event. Data from the Antarctic soils are consistent with an age of the landscape of the order of 12 million years.

Molten Sulfur Lakes of Intraoceanic Arc Volcanoes

Intraoceanic arcs of the world are dominated by submarine volcanoes, many of which host active hydrothermal systems. A considerable number of the morphological features common to subaerial volcanoes are also present on the submarine edifices, including summit craters. Surprisingly, some of the craters, such as at Daikoku and Nikko volcanoes of the Mariana Arc, and Macauley Cone of the Kermadec Arc, are host to lakes of molten sulfur, both ancient and modern. These lakes, up to ~200 m in diameter, act as condensers of gases that derive from the underlying magmas. Volcanic vents beneath these lakes provide a steady outflow of hot gases that continuously generate molten sulfur. At Daikoku, an extraordinary lake of liquid sulfur is in constant convective and gas escape-driven motion. Smaller pools of molten sulfur occur on Nikko, and there is evidence of older, larger lakes on both this volcano and Macauley, based on the accumulation of large quantities of sulfur in the subsurface. The elemental S at these sites is produced largely by the reaction 2H2S + SO2 = 3S + 2H2O and the disproportionation of magmatic SO2. Anomalous concentrations of Au and Cu in the lakes are most likely transported by vapor.