Alan G. Hogg

ShCal04 Southern Hemisphere Calibration, 0–11.0 Cal Kyr BP

Recent measurements on dendrochronologically-dated wood from the Southern Hemisphere have shown that there are differences between the structural form of the radiocarbon calibration curves from each hemisphere. Thus, it is desirable, when possible, to use calibration data obtained from secure dendrochronologically-dated wood from the corresponding hemisphere. In this paper, we outline the recent work and point the reader to the internationally recommended data set that should be used for future calibration of Southern Hemisphere (super 14) C dates.

A wiggle-match date for Polynesian settlement of New Zealand

Dating initial colonisation and environmental impacts by Polynesians in New Zealand is controversial. A key horizon is provided by the Kaharoa Tephra, deposited from an eruption of Mt Tarawera, because just underneath this layer are the first signs of forest clearance which imply human settlement. The authors used a log of celery pine from within Kaharoa deposits to derive a new precise date for the eruption via “wiggle-matching” – matching the radiocarbon dates of a sequence of samples from the log with the Southern Hemisphere calibration curve. The date obtained was 1314 ± 12 AD (2σ error), and the first environmental impacts and human occupation are argued to have occurred in the previous 50 years, i.e. in the late 13th – early 14th centuries AD. This date is contemporary with earliest settlement dates determined from archaeological sites in the New Zealand archipelago

Calibration of the radiocarbon time scale for the southern hemisphere: AD 1850-950.

We have conducted a series of radiocarbon measurements on decadal samples of dendrochronologically dated wood from both hemispheres, spanning 1000 years (McCormac et al. 1998; Hogg et al. this issue). Using the data presented in Hogg et al., we show that during the period AD 950-1850 the 14C offset between the hemispheres is not constant, but varies periodically (~130 yr periodicity) with amplitudes varying between 1 and 10‰ (i.e. 8-80 yr), with a consequent effect on the 14C calibration of material from the Southern Hemisphere. A large increase in the offset occurs between AD 1245 and 1355. In this paper, we present a Southern Hemisphere high-precision calibration data set (SHCal02) that comprises measurements from New Zealand, Chile, and South Africa. This data, and a new value of 41 ± 14 yr for correction of the IntCal98 data for the period outside the range given here, is proposed for use in calibrating Southern Hemisphere 14C dates.

High-precision radiocarbon measurements of contemporaneous tree-ring dated wood from the British Isles and New Zealand: AD 1850–950

The University of Waikato, Hamilton, New Zealand and The Queens University of Belfast, Northern Ireland radiocarbon dating laboratories have undertaken a series of high-precision measurements on decadal samples of dendrochronologically dated oak (Quercus petraea) from Great Britain and cedar (Libocedrus bidwillii) and silver pine (Lagarostrobos colensoi) from New Zealand. The results show an average hemispheric offset over the 900 yr of measurement of 40+ or -13 yr. This value is not constant but varies with a periodicity of about 130 yr. The Northern Hemisphere measurements confirm the validity of the Pearson et al. (1986) calibration dataset.

Radiocarbon age of the Kaharoa Tephra, a key marker for late-Holocene stratigraphy and archaeology in New Zealand:

The Kaharoa eruption, the most recent rhyolitic volcanic event in New Zealand, resulted in the deposition of the compositionally distinctive Kaharoa Tephra over at least 30 000 km2 of northern and eastern North Island. The tephra forms an isochronous marker bed for various late-Holocene stratigraphic and palaeoen vironmental studies but is particularly important for evaluating the chronology of New Zealands notably short prehistory because it was erupted within the last millennium, close to the time of first Polynesian colonization. We derive a definitive radiocarbon age, previously ambiguous, for the Kaharoa Tephra of 665615 BP using cluster analysis of 22 radiocarbon ages relevant to the Kaharoa eruptive episode. The error-weighted mean age obtained on unscreened ages, minus outliers, is supported by statistically identical ages obtained from three sets of screened ages selected to minimize the effects of inbuilt age or contamination. Based on the intercepts method and 1 sigma range of Northern Hemisphere calibration curves, and after subtraction of 27 years for the interhemispheric offset, the radiocarbon age 665615 BP corresponds to calibrated dates ranging fromc. 650–560 cal. BP (i.e. cal. ad 1300–1390). The approximate midpoint of this range provides a colloquial calen dar date for the Kaharoa Tephra ofc. 600 cal. BP (c. cal. ad 1350).

Revised calendar date for the Taupo eruption derived by 14C wiggle-matching using a New Zealand kauri 14C calibration data set

Taupo volcano in central North Island, New Zealand, is the most frequently active and productive rhyolite volcano on Earth. Its latest explosive activity about 1800 years ago generated the spectacular Taupo eruption, the most violent eruption known in the world in the last 5000 years. We present here a new accurate and precise eruption date of ad 232 ± 5 (1718 ± 5 cal. BP) for the Taupo event. This date was derived by wiggle-matching 25 high-precision 14C dates from decadal samples of Phyllocladus trichomanoides from the Pureora buried forest near Lake Taupo against the high-precision, first-millennium ad subfossil Agathis australis (kauri) calibration data set constructed by the Waikato Radiocarbon Laboratory. It shows that postulated dates for the eruption estimated previously from Greenland ice-core records (ad 181 ± 2) and putative historical records of unusual atmospheric phenomena in ancient Rome and China (c. ad 186) are both untenable. However, although their conclusion of a zero north–south 14C offset is erroneous, and their data exhibit a laboratory bias of about 38 years (too young), Sparks et al. (Sparks RJ, Melhuish WH, McKee JWA, Ogden J, Palmer JG and Molloy BPJ (1995) 14C calibration in the Southern Hemisphere and the date of the last Taupo eruption: Evidence from tree-ring sequences. Radiocarbon 37: 155–163) correctly utilized the Northern Hemisphere calibration curve of Stuiver and Becker (Stuiver M and Becker B (1993) High-precision decadal calibration of the radiocarbon timescale, AD 1950–6000 BC. Radiocarbon 35: 35–65) to obtain an accurate wiggle-match date for the eruption identical to ours but less precise (ad 232 ± 15). Our results demonstrate that high-agreement levels, indicated by either agreement indices or χ2 data, obtained from a 14C wiggle-match do not necessarily mean that age models are accurate. We also show that laboratory bias, if suspected, can be mitigated by applying the reservoir offset function with an appropriate error value (e.g. 0 ± 40 years). Ages for eruptives such as Taupo tephra that are based upon individual 14C dates should be considered as approximate only, and confined ideally to short-lived material (e.g. seeds, leaves, small branches or the outer rings of larger trees).

Evaluation of wood pretreatments on oak and cedar.

In a separate study, we conducted a series of high-precision radiocarbon measurements using wood from Britain and New Zealand to investigate interhemispheric offsets and possible temporal variations. To minimize variability associated with different species, the pretreatment of the oak (Quercus patraea) and cedar (Librocedrus bidwilli) was to a-cellulose for both. This study investigates the thoroughness of a range of pretreatment processes by the stable isotope analysis of the products.

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.

14C dating of modern marine and estuarine shellfish

We measured the 14C content of 36 living marine molluscs from Tairua Harbour and the rocky coast on the Coromandel Peninsula of New Zealand. We identified species suitable for radiocarbon dating and show that the open marine intertidal zone is enriched in 14C compared to the open marine subtidal zone or estuary. We also found a uniform 14C distri- bution in the Tairua Harbour, by analyzing samples of the estuarine bivalveAustrovenus stutchburyi collected up to 5 km from the harbor entrance.

Variations of radiocarbon in tree rings: Southern Hemisphere offset preliminary results

The Queens University of Belfast, Northern Ireland and University of Waikato, Hamilton. New Zealand radiocarbon laboratories have undertaken a series of high-precision measurements on decadal samples of dendrochronologically dated oak (Quercus patrea) and cedar (Libocedrus bidwillii) from Great Britain and New Zealand, respectively. The results show a real atmospheric offset of 3.4 ± 0.6% (27.2 ± 4.7 14C yr) between the two locations for the interval AD 1725 to AD 1885, with the Southern Hemisphere being depleted in 14C. This result is less than the value currently used to correct Southern Hemisphere calibrations, possibly indicating a gradient in Δ14C within the Southern Hemisphere.