Henry Polach

Discussion; reporting of C-14 data.

Standards for reporting C-14 age determinations are discussed. All dates should be related either directly or indirectly to the NBS oxalic acid standard. Corrections for isotopic fractionation are also desirable. For some materials, particularly marine shell, corrections for reservoir effect are necessary, but these should always be reported separately from the conventional radiocarbon age. The statistical uncertainty (plus or minus one standard deviation) expresses counting errors, inaccuracies in voltage, pressure, temperature, dilution, and should include errors in C-13 ratios. Errors can be significant when isotope ratios are estimated rather than measured directly. The error in the conventional C-14 half life is not included. The article includes tables indicating what data should be reported.

Radiocarbon Dating of Arid-Zone Calcareous Paleosols

Field study and radiocarbon dating of authigenic carbonate in arid-zone paleosols and calcretes have been carried out in Australia to determine the reliability of this method of soil dating. Three calcareous paleosols of late Quaternary age are developed within alluvial and eolian deposits on the Lake Torrens plain in arid South Australia. Dates for organic carbon obtained from the sediments show that soil-forming episodes took place shortly after 30,000 yrs B.P., between about 16,000 and 12,000 yrs B.P., and between 6,000 and 1,500 yrs B.P. Respective mean radiocarbon ages determined for pedogenic carbonate are (with approximate scatter) 27,450 (± 7,000) yrs B.P., 13,900 (± 2,000) yrs B.P., and 7,750 yrs (± 1,000) B.P. Although the great majority of the individual carbonate dates and the three mean dates are sound as relative ages, they are less reliable as absolute ages. A review of the literature reveals that most carbonate dates for arid-zone paleosols and calcretes which have been dated by coexisting organic carbon are from about 500 to 7,000 radiocarbon years too old. This is attributed to an initial low C 14 /C 12 ratio in soil carbonate, reflecting low C 14 content of source carbonate. Postformation contamination with C 14 commonly appears to be minimal in the arid zone, where dates for micritic carbonate from well-drained sites should approximate to or be up to several thousand years older than true ages of soil formation. Age correction based on δC 13 values for soil carbonate is invalid.

Radiocarbon evidence for the timing and rate of Island development, beach-rock formation and phosphatization at Lady Elliot Island, Queensland, Australia

Abstract Lady Elliot Island (Great Barrier Reef) is a coral cay constructed from near-concentric shingle ridges composed of coral, Tridacna shells and bioclastic sand bound by guano-derived phosphatic cement. Radiocarbon dating of individual Tridacna samples is used to investigate the evolution of the island and place constraints on the variations in storminess of the last 3200 years. A shallow platform reef existed at Lady Elliot since at least 6500 yrs B.P. Shortly before 3200 yrs B.P. island growth by shingle progradation commenced and proceeded to the present at a rather uniform lateral rate (60 m ka −1 to leeward and 90 m ka −1 windward). Phosphatization may have occurred continuously or episodically at any time throughout the last 3200 years. The youngest cemented ridge top is less than 770 years, whereas at the base of uncemented ridges, phosphatization is a modern (∼1950 A.D.) process. Similarly, low-level beach rock has continued to form within the last 25 years. A significant period of instability occurred less than 500 years ago during which marine erosion truncated the southwestern extremity of Lady Elliot Island, formed an eastern “spit” and stranded high-level beach rock. However, the radiocarbon ages of Tridacna samples from shingle ridges from Lady Elliot and Curacoa Islands are uniformly distributed (Kolmogorov-Smirnov test) throughout the last 4000 years. We interpret this to indicate no significant variation in ridge-forming processes or fluctuation in average storminess during this interval.

Some effects of partial recrystallisation on 14C dating Late Pleistocene corals and molluscs

The issue of sea level during the last interstadial revolves largely around the problem of achieving reliable 14 C dates for shell carbonate from Late Pleistocene shallow marine and littoral deposits. A set of 27 samples were collected from Late Pleistocene reefs in New Guinea, and measurements made of 13 C, 14 C, plus the degree of recrystallisation (determined by X-ray diffraction). The original fibrous aragonite structure of the samples (corals and clams) is seen in thin section to recrystallise in two quite different modes. The carbon isotope results strongly suggest that one mode, the sparry calcite recrystallisation, represents an open geochemical system which allows contamination by more recent 14 C, while the subtle coarsening mode of recrystallisation represents a closed system, often yielding reliable results. The reliability of the latter can be validated if a similarly recrystallised sample, known to be outside the range of 14 C dating, shows a background 14 C count.

Improved synthesis techniques for methane and benzene radiocarbon dating

Abstract A chemical preparation system based on a dual purpose reaction vessel is shown in detail. Described are: (1) The effect of water removal and ruthenium catalyst temperature on CH 4 synthesis rates using CO 2 + H 2 ; 101. of CH 4 were quantitatively produced in 1 hr at a temperature of 400–550°C; (2) Li + CO 2 reaction mechanism at elevated temperature (800–900°C); 101. CO 2 were reacted quantitatively with only 10 per cent excess Li in 4 min; (3) C 6 H 6 synthesis on vanadium catalyst using water cooled columns; near quantitative (96 per cent) C 6 H 6 yields at reaction rates of 100–150 ml C 2 H 2 /min were obtained.

Sea level change in the Holocene on the northern Great Barrier Reef

Detailed studies, utilizing a range of both well controlled sea level criteria and dates, are required if Holocene time-sea level curves are to be established with any degree of confidence. This paper is restricted to an interpretation of Expedition results from the northern Great Barrier Reef, excluding those from the drill core. Extensive colonies of emergent fossil corals in growth position indicate that present sea level was first reached about 6000 a b. p. Elevations of cay surfaces, cemented rubble platforms, microatolls, coral shingle ridges, reef flats and mangrove swamps, referenced to present sea level show an array of heights. However, levels of particular features are accordant on many reefs: it is believed that these can be related to particular sea levels. Radiometric dating provides the time framework. Ages of samples from similar deposits on different reefs are surprisingly consistent. Oscillations in sea level since 6000 a b.p ., relative to present sea level, are identified with varying degrees of confidence. This history of relative sea level does not separate eustatic from noneustatic components.

ANU Radiocarbon Date List X

Carbon-14 ages obtained in the Radiocarbon Dating Laboratory of the Australian National University for geologic and archaeologic samples from localities in Australia and elsewhere are reported.

The stratigraphy of coastal carbonate banks and Holocene sea levels of northern Spencer Gulf, South Australia

Abstract Stratigraphic and chronologic studies of the coastal sediments of northern Spencer Gulf in the vicinity of Redcliff are described. Two Pleistocene marine units are recognised beneath an extensive Holocene peritidal sequence. The Holocene sequence is dominated by sediments of the Posidonia australis seagrass facies which form a distinctive carbonate bank fringing the coastline. Peritidal sedimentation commenced prior to 6600 radiocarbon yrs B.P. and a + 2.5 m higher relative sea level existed from about 6000 until around 1700 yrs B.P. The subsequent relative fall to present level probably resulted from local tectonic uplift. After the regression, mangrove and samphire colonization, and beach ridge progradation, occurred over the former seagrass bank.

Holocene sea-level change and coral-reef growth at Huon Peninsula, Papua New Guinea

At the tectonically rising terraced coast of Huon Peninsula, Papua New Guinea, a Holocene reef is emerging by as much as 12 m. A clean seacliff section 8 m high provides detailed evidence of the relationship between reef development and early Holocene sea-level changes, relative to the rising coast. The section shows a shallow-water reef-crest facies that grew steadily upward while relative sea level rose approximately 7 m. Reef growth terminated with emplacement of a fluvial gravel cap, which was followed by emergence. Twelve C 14 dates and six Th 230 /U 234 dates put the base of the cliff section at 8200 C 14 yr B.P. and the crest at 6600 C 14 yr B.P. Four Th 230 /U 234 dates from near the base average 9,400 yr old, but C 14 ages from the same samples average 7,600 yr old; the cause of the discrepancy is not known. Uplift of the section is known from previous studies of the Pleistocene reefs in the area to be approximately 1.9 m/1,000 yr. If uplift has been at a uniform rate, then the position of sea level at 6,000 C 14 yr ago (around northeast Papua New Guinea) was about −4 m; 8,000 yr ago it was about −14 m. Mean upward growth rate of the reef was 4.7 m/1,000 yr, and the maximum rate was about 8 m/1,000 yr.

Radiocarbon dating of flood events, Katherine Gorge, Northern Territory, Australia

Radiocarbon analyses of organic materials provide accurate dating of flood events in the Katherine Gorge, Northern Territory, Australia. Selected flood events in the period 1955–1980 can be very precisely dated, essentially to the calendar year, through the use of curves showing the time variation of anomalously high 14 C activity in the troposphere that was generated by nuclear testing. Conventional radiocarbon analysis of alluvial charcoal and organic litter is used to establish dates for the largest Katherine River floods of the past several centuries. The improved geochronologic techniques demonstrated here will help improve paleoflood hydrologic interpretations.