Albert Zondervan

New Marine ΔR Values for the South Pacific Subtropical Gyre Region

This paper presents 31 new ΔR results of known-age, pre-AD 1950 shells from the South Pacific subtropical gyre region, spanning from the Tuamotu Archipelago in the east to New Caledonia in the west. This doubles the number of available ΔR values for the Oceania region. These values indicate that the regional offset (ΔR) from the modeled radiocarbon marine age has remained relatively constant over the last 100 yr prior to 1950. Variation from the norm can be attributed to various influences including localized upwelling around islands, the presence of a hardwater effect, direct ingestion of old carbon by the live shellfish, or enhanced exchange with atmospheric CO2 as a consequence of photosynthetic activity or increased aeration.

Late Holocene uplift of beach ridges at Turakirae Head, south Wellington coast, New Zealand

Abstract Holocene terraces at Turakirae Head on the south coast of the North Island, New Zealand, record four recent earthquakes from simultaneous rupture of the Wairarapa Fault and flexure of the Rimutaka Anticline. The lowest tread and riser is the modern marine platform and storm beach that began forming when the area was raised during the Mw 8.2 Wairarapa earthquake of AD 1855 January. The remaining chronology is established by radiocarbon dating, in situ 10Be surface‐exposure dating, and slip‐predictable uplift estimation. Prior to AD 1855, uplifts occurred at 110–430 BC (max. 9.1 m), 2164–3468 BC (6.8 m), and 4660–4970 BC (7.3 m). Earlier uplift of unknown magnitude occurred at c. 7000 BC but went unrecorded because of rapidly rising sea level. Sea level was still rising when the two oldest surviving beach ridges were raised. Uplift at Turakirae Head in AD 1855 varied from 1.5 m at the Wainuiomata River to 6.4 m at the crest of the Rimutaka Anticline. Older beaches also are tilted, with the amount of tilt increasing with age. Coastal uplift at the anticline crest has averaged 3.32 ± 0.17 mm/yr over the past 9000 yr, and has changed little over the past 0.5 m.y. Uplift fits a slip‐predictable model of earthquake occurrence, and is log‐normally distributed with a mean of 7.3 ± 0.7 m. The most frequently occurring uplift is 7.1 ± 0.9 m. Uplift in AD 1855 was not significantly smaller than mean or mode, suggesting that the Turakirae Head sequence records four great earthquakes of at least similar magnitude to that of AD 1855. The mean earthquake recurrence interval is 2194 ± 117 yr; the modal interval is 2122 ± 193 yr. At the crest of the anticline, the coastal platform was cut entirely during the postglacial rise of sea level until shortly before 4660–4970 BC. Away from the crest, however, it may have been partially cut during low sea level of the penultimate glaciation. The open‐ocean radiocarbon reservoir correction (δR) for 10 14C dates of coastal marine shells that died in AD 1855 at Turakirae Head is 3 ± 14cal. yrBP(andnot‐31 ± 13 cal. yr BP, the currently accepted δR for central New Zealand coastal waters).

Orbital forcing of mid-latitude Southern Hemisphere glaciation since 100 ka inferred from cosmogenic nuclide ages of moraine boulders from the Cascade Plateau, southwest New Zealand

Cosmogenic nuclide (Be-10) exposure dating of moraine boulders in the Cascade Valley, southwest New Zealand, reveals three phases of glaciation with similar maximum magnitude since 100 ka. In this area, 8–10 lateral moraines were deposited during the Last Glacial Maximum (LGM) at 22–19 ka, and >15 lateral moraines and three end moraines were deposited during recession after the LGM. Also, three exposure ages of 29–33 ka from pre-LGM deposits may indicate increased weathering and erosion at the onset of the LGM in New Zealand, as has been suggested by other studies. An exposure age of 57.8 ± 2.7 ka from one of the highest moraines, combined with previous studies of cave speleothems, glacial features offset by the Alpine fault, the Vostok dust record, and sediment cores, supports the inference that a significant glacial phase culminated at 66–58 ka. A cluster of five exposure ages from older moraines reveals a glacial phase with at least three advance-retreat cycles at 79.0 ± 3.9 ka. Correlation between the ages of glacial periods and the timing of Southern Hemisphere summer insolation minima suggests that orbital forcing has played a first-order role in regulating glacial extent in New Zealand.

The marine reservoir effect in the Southern Ocean: An evaluation of extant and new ?R values and their application to archaeological chronologies

Abstract The last phase of human colonisation of the south‐west Pacific occurred around the 12th Century AD amongst the islands of the subtropical and temperate zones of the Southern Ocean (i.e., Norfolk Island, the Kermadec Islands, New Zealand and the Chatham Islands) (Fig. 1). Archaeological evidence indicates that initial colonisation of the region was rapid, possibly taking less than 100 years. Unfortunately, the chronology of these events has been largely reliant on charcoal radiocarbon determinations that have not been successful in delineating these changes. This paper investigates the use of marine shell for radiocarbon dating as an alternative to charcoal, with specific focus on variation in the marine 14C reservoir (specifically the ?R) of Raoul Island (Kermadec Islands), Norfolk Island and Chatham Island (Fig. 2). Results from known‐age, pre‐AD 1950 shellfish indicate that ?R values south of the South Pacific Convergence Zone are low compared to those recorded for islands within the South Pacific generally (Fig. 1) (see Petchey et al. in press). An average ?R value of ‐19 ± 13 14C yr is recorded for Raoul Island and ‐49 ± 10 14C yr for Norfolk Island. These values are attributed to heightened absorption of atmospheric CO2 in this region. Extant published ?R information from New Zealand (Fig. 2A) also suggests a low average ?R value for these southern waters, but the range of values indicates that considerable variability is possible because of the complex interplay of currents around the New Zealand coastline. In contrast, results from Chatham Island are more variable and much higher (average ?R = 134 ± 83 14C yr). Stable oxygen and carbon isotope data support the hypothesis that these ?R values are caused by upwelling and mixing of 14C‐depleted water along the Chatham Rise. Comparison of marine and charcoal 14C determinations from archaeological sites on Raoul Island support, within the limits of the available data, the ?R values obtained, but further analysis is required to establish the stability of this value over time.

New frontiers in glacier ice dating: Measurement of natural 32Si by AMS

Cosmogenic 32Si with a half-life of ca. 140 years is an excellent candidate to provide time information in the range 100–1000 years. Accelerator mass spectrometry (AMS) could greatly enhance its applicability as a natural clock due to the small sample size required for measurement. In this paper we describe the requirements for AMS measurement of natural samples, and we demonstrate the first 32Si AMS measurement for rainwater and glacial ice and snow. The results indicate that with AMS measurement of ca. 1 kg of water, a period of seven half-lives (ca.1000 years) can be covered. The 32Si result on Fox Glacier ice (New Zealand, 43°S) indicates an ice residence time on the bottom of the glacier of more than 800 years.

First results from the Groningen AMS facility

A new generation accelerator mass spectrometer has been installed at the Centre for Isotope Research in Groningen, The Netherlands, It is a dedicated C-14 machine, with a capacity of measuring 3000 samples per year with high precision (<0.5%). The system has been in full operation since the summer of 1994. We present here a short summary of the results and performance obtained thus far.

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.

Development plans for the AMS facility at the Institute of Geological and Nuclear Sciences, New Zealand

Abstract An overview is given of the past, present and near-future status of and research with the AMS facility in New Zealand. Key elements of technological improvement are the new MC-SNICS source, Pelletron conversion of the EN tandem accelerator and modification of the terminal to include a gas stripper/recirculator. A proposal to add a gas-filled magnet for isobar suppression is under investigation. The research program, focussing on radiocarbon and 10 Be for archaeology and earth sciences studies, will be expanded to include 26 Al, 32 Si and 36 Cl.

High-Precision Atmospheric 14 CO 2 Measurement at the Rafter Radiocarbon Laboratory

This article describes a new capability for high-precision 14 C measurement of CO2 from air at the Rafter Radiocarbon Laboratory, GNS Science, New Zealand. We evaluate the short-term within-wheel repeatability and long-term between-wheel repeatability from measurements of multiple aliquots of control materials sourced from whole air. Samples are typically measured to 650,000 14C counts, providing a nominal accelerator mass spectrometry (AMS) statistical uncer- tainty of 1.3‰. No additional uncertainty is required to explain the within-wheel variability. An additional uncertainty factor is needed to explain the long-term repeatability spanning multiple measurement wheels, bringing the overall repeatability to 1.8‰, comparable to other laboratories measuring air materials to high precision. This additional uncertainty factor appears to be due to variability in the measured 14 C content of OxI primary standard targets, likely from the combustion process. We observe an offset of 1.4‰ in our samples relative to those measured by the University of Colorado INSTAAR, comparable to interlaboratory offsets observed in recent intercomparison exercises. This article describes the high-precision atmospheric 14 CO 2 measurement capability at the Rafter Radiocarbon Laboratory at GNS Science, New Zealand. We discuss the two different methods we commonly use for CO 2 collection and subsequent extraction. We describe our recently upgraded graphitization system and detail our measurement protocols for samples requiring high precision. We use replicate measurements of CO 2 from a number of different control materials, all derived from whole air, to examine mean values and offsets between different standardization methods within our laboratory, as well as interlaboratory offsets based on an ongoing intercomparison with the University of Colorado INSTAAR. We then examine short-term within-wheel and long-term repeatability of our measurements, and the sources of uncertainty that contribute to these. METHODS CO 2 from air is collected either by in situ absorption of CO 2 into sodium hydroxide (NaOH) solution or by collection of whole air into flasks or pressurized cylinders. In addition, control materials, pri - mary standards, and process blanks are routinely analyzed. In this section, we describe the collection methods and protocols for extracting and purifying the CO 2 in the laboratory.