Quan Hua

Review of tropospheric bomb 14C data for carbon cycle modeling and age calibration purposes

Comprehensive published radiocarbon data from selected atmospheric records, tree rings, and recent organic matter were analyzed and grouped into 4 different zones (three for the Northern Hemisphere and one for the whole Southern Hemisphere). These C-14 data for the summer season of each hemisphere were employed to construct zonal, hemispheric, and global data sets for use in regional and global carbon model calculations including calibrating and comparing carbon cycle models. In addition, extended monthly atmospheric C-14 data sets for 4 different zones were compiled for age calibration purposes. This is the first time these data sets were constructed to facilitate the dating of recent organic material using the bomb C-14 curves. The distribution of bomb C-14 reflects the major zones of atmospheric circulation.

Optical and radiocarbon dating at Jinmium rock shelter in northern Australia

The Jinmium rock shelter is located in the Kimberley region of northern Australia. Claims for ancient rock art and an early human presence at this site were based on thermoluminescence ages of 50–75 thousand years (kyr) for quartz sands associated with buried circular engravings (pecked cupules) and on thermoluminescence ages of 116–176 kyr for the underlying artefact-bearing deposits. Here we report substantially younger optical ages for quartz sand, and ages based on measurements of radioactive carbon in charcoal fragments, from the occupation deposit. Using conventional (multiple-grain) optical dating methods, we estimate that the base of the deposit is 22 kyr. However, dating of individual grains shows that some have been buried more recently. The single-grain optical ages indicate that the Jinmium deposit is younger than 10 kyr. This result is in agreement with the late-Holocene ages obtained for the upper two-thirds of the deposit from radiocarbon measurements. We suggest that some grains have older optical ages because they receivedinsufficient exposure to sunlight before burial. The presence of such grains in a sample will cause age overestimates using multiple-grain methods, whether using thermoluminescence or optical dating.

Progress In Radiocarbon Target Preparation At The Antares Ams Centre

We present routine methods of target preparation for radiocarbon analysis at the ANTARES Accelerator Mass Spectrometry (AMS) Centre, as well as recent developments which have decreased our procedural blank level and improved our ability to process small samples containing less than 200 μg of carbon. Routine methods of 14 C sample preparation include sample pretreatment, CO 2 extraction (combustion, hydrolysis and water stripping) and conversion to graphite (graphitization). A new method of cleaning glassware and reagents used in sample processing, by baking them under a stream of oxygen, is described. The results show significant improvements in our procedural blanks. In addition, a new graphitization system dedicated to small samples, using H 2 /Fe reduction of CO 2 , has been commissioned. The technical details of this system, the graphite yield and the level of fractionation of the targets are discussed.

Holocene marine 14C reservoir age variability: Evidence from 230Th-dated corals in the South China Sea

The South China Sea (SCS) is well connected with the western Pacific and influenced by the East Asian monsoon. We have examined temporal variations in radiocarbon marine reservoir ages (R) and regional marine reservoir corrections (Delta R) of the SCS during the Holocene using paired measurements of AMS C-14 and TIMS Th-230 on 20 pristine corals. The results show large fluctuations in both R and Delta R values over the past 7500 years (yrs) with two distinct plateaus during 7.5-5.6 and 3.5-2.5 thousand calendar years before present (cal ka BP). The respective weighted mean Delta R values of these plateaus are 151 +/- 85 and 89 +/- 59 yrs, which are significantly higher than its modern value of -23 +/- 52 yrs. This suggests that using a constant modern Delta R value to calibrate C-14 dates of the SCS marine samples will introduce additional errors to the calibrated ages. Our results provide the first database for the Holocene R and Delta R values of the SCS for improved radiocarbon calibration of marine samples. We interpret the two Delta R plateaus as being related to two intervals with weakened El Nino - Southern Oscillation (ENSO) and intensified East Asian summer monsoon (EASM). This is because the C-14 content of the SCS surface water is controlled by both the C-14 concentration of the Pacific North Equatorial Current (NEC) which is in turn influenced by ENSO-induced upwelling along the Pacific equator and vertical upwelling within the SCS as a result of moisture transportation to midlatitude region to supply the EASM rainfall.

Human occupation of northern Australia by 65,000 years ago

The time of arrival of people in Australia is an unresolved question. It is relevant to debates about when modern humans first dispersed out of Africa and when their descendants incorporated genetic material from Neanderthals, Denisovans and possibly other hominins. Humans have also been implicated in the extinction of Australia’s megafauna. Here we report the results of new excavations conducted at Madjedbebe, a rock shelter in northern Australia. Artefacts in primary depositional context are concentrated in three dense bands, with the stratigraphic integrity of the deposit demonstrated by artefact refits and by optical dating and other analyses of the sediments. Human occupation began around 65,000 years ago, with a distinctive stone tool assemblage including grinding stones, ground ochres, reflective additives and ground-edge hatchet heads. This evidence sets a new minimum age for the arrival of humans in Australia, the dispersal of modern humans out of Africa, and the subsequent interactions of modern humans with Neanderthals and Denisovans.

Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef

Received 21 May 2010; revised 20 June 2010; accepted 6 July 2010; published 3 August 2010. [1] Coral reefs track sea level and are particularly sensitive to changes in climate. Reefs are threatened by global warming, with many experiencing increased coral bleaching. Warmer sea surface temperatures might enable reef expansion into mid latitudes. Here we report multibeam sonar and coring that reveal an extensive relict coral reef around Lord Howe Island, which is fringed by the southernmost reef in the Pacific Ocean. The relict reef, in water depths of 25–50 m, flourished in early Holocene and covered an area more than 20 times larger than the modern reef. Radiocarbon and uranium‐series dating indicates that corals grew between 9000 and 7000 years ago. The reef was subsequently drowned, and backstepped to its modern limited extent. This relict reef, with localised re‐establishment of corals in the past three millennia, could become a substrate for reef expansion in response to warmer temperatures, anticipated later this century and beyond, if corals are able to recolonise its surface. Citation: Woodroffe, C. D., B. P. Brooke, M. Linklater, D. M. Kennedy, B. G. Jones, C. Buchanan, R. Mleczko, Q. Hua, and J. Zhao (2010), Response of coral reefs to climate change: Expansion and demise of the southernmost Pacific coral reef, Geophys. Res. Lett., 37, L15602, doi:10.1029/2010GL044067.

14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources.

Radiocarbon measurements show that wetlands were responsible for the rapid increase of atmospheric methane concentration during the last deglaciation. Methane from Wetlands At the end of the cold climate interval called the Younger Dryas, approximately 11,600 years ago, global temperatures began their final ascent to the warmth of the Holocene, and the concentration of methane in the atmosphere increased rapidly and substantially. There has been much speculation about the cause of that increase, with most recent evidence pointing to wetlands as the source. The most direct proof of that explanation requires the measurement of the radiocarbon content of that methane. Petrenko et al. (p. 506; see the Perspective by Nisbet and Chappellaz) analyzed 1000 kilogramsized samples of Greenland ice, which have sufficient methane to allow measurement of its 14C content. They show that wetland sources indeed must have been responsible for the majority of the rise in atmospheric methane levels at the end of the Younger Dryas. The cause of a large increase of atmospheric methane concentration during the Younger Dryas–Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas–Preboreal CH4 rise.

AMS at ANTARES – The first 10 years

Abstract The status and capabilities of the ANTARES AMS facility after 10 years are reviewed. The common AMS radioisotopes, 10 Be, 14 C, 26 Al, 36 Cl and 129 I, are routinely analysed. A capability for the detection of 236 U and other actinide isotopes has been developed. The measurement program includes support to Quaternary science projects at Australian universities and to ANSTO projects in global climate change and nuclear safeguards.

Taphonomic bias and time-averaging in tropical molluscan death assemblages: differential shell half-lives in Great Barrier Reef sediment

Abstract Radiocarbon-calibrated amino acid racemization ages of 428 individually dated shells representing four molluscan taxa are used to quantify time-averaging and shell half-lives with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contains a distinct, essentially modern assemblage. Shells recovered at depths from 25 to 125 cm are age-homogeneous and significantly older than the surface layer. Taxon age distributions within sedimentary layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a sub-century scale. The age distributions and shell half-lives of four taxa (Ethalia, Natica, Tellina, and Turbo) are found to be largely distinct. Shell half-lives do not coincide with any single morphological characteristic thought to infer greater durability, but they are strongly related to a combined durability score based on shell density, thickness, and shape. These results illustrate the importance of bioturbation in tropical sedimentary environments, indicate that age estimates in this depositional setting are sensitive to taxon choice, and quantify a taxon-dependent bias in shell longevity and death assemblage formation.

Sediment mixing and stratigraphic disorder revealed by the age-structure of Tellina shells in Great Barrier Reef sediment

Radiocarbon-calibrated amino acid racemization ages of 250 individually dated Tellina shells from two sediment cores are used to quantify molluscan time averaging with increasing burial depth in the shallow-water carbonate lagoon of Rib Reef, central Great Barrier Reef, Australia. The top 20 cm of sediment contain a distinct, essentially modern assemblage with a median age of only 5 yr. Sediment between 20 and 125 cm are age-homogeneous and significantly older than the surface sediment (median age 189 yr). Shell age distributions within layers indicate that the top 125 cm of lagoonal sediment is thoroughly mixed on a subcentennial scale. Shell size is an important correlate of shell half-life and an important determinant of the inferred age of sedimentary layers. These results illustrate the importance of bioturbation in these environments, indicate that age estimates in this depositional setting are sensitive to specimen choice, and document a size-dependent bias in death assemblage formation.