Bayesian evaluation of the southern hemisphere radiocarbon offset during the holocene

While an interhemispheric offset in atmospheric radiocarbon levels from AD 1950–950 is now well established, its existence earlier in the Holocene is less clear, with some studies reporting globally uniform 14C levels while others finding Southern Hemisphere samples older by a few decades. In this paper, we present a method for wiggle-matching Southern Hemisphere data sets against Northern Hemisphere curves, using the Bayesian calibration program OxCal 4.1 with the Reservoir Offset function accommodating a potential interhemispheric offset. The accuracy and robustness of this approach is confirmed by wiggle-matching known-calendar age sequences of the Southern Hemisphere calibration curve SHCal04 against the Northern Hemisphere curve IntCal04. We also show that 5 of 9 Holocene Southern Hemisphere data sets are capable of yielding reliable offset information. Those data sets that are accurate and precise show that interhemispheric offset levels in the Early Holocene are similar to modern levels, confirming SHCal04 as the curve of choice for calibrating Southern Hemisphere samples

Is there any Evidence for Regional Atmospheric 14C Offsets in the Southern Hemisphere?

Center for Accelerator Mass Spectrometry (CAMS) Tasmanian huon pine (Lagarostrobos franklinii) decadal measurements for the interval AD 745–855 suggest a mean interhemispheric radiocarbon offset (20 ± 5 yr), which is considerably lower than the previously reported mean interhemispheric offset for the last 2 millennia (44 ± 17 yr). However, comparable University of Waikato (Wk) New Zealand kauri (Agathis australis) measurements show significantly higher values (56 ± 6 yr), suggesting the possibility of a temporary geographic (intrahemispheric) offset between Tasmania, Australia, and Northland, New Zealand, during at least 1 common time interval. Here, we report 9 new Wk Tasmanian huon pine measurements from the decades showing the largest huon/kauri difference. We show statistically indistinguishable Wk huon and Wk kauri 14C ages, thus dispelling the suggestion of a 14C geographic offset between Tasmania and Northland

Robust radiocarbon dating of wood samples by high-sensitivity liquid scintillation spectroscopy in the 50–70 kyr age range

Although high-sensitivity liquid scintillation (LS) spectroscopy is theoretically capable of producing finite radiocarbon ages in the 50,000- to 70,000-yr range, there is little evidence in the literature that meaningful dates in this time period have been obtained. The pressing need to undertake calibration beyond 26 kyr has resulted in the regular publication of ¹⁴C results in excess of 50 kyr, yet very little effort has been made to demonstrate their accuracy or precision. There is a paucity of systematic studies of the techniques required to produce reliable dates close to background and the methods needed to assess contamination from either in situ sources or laboratory handling and processing. We have studied the requirements for producing accurate and reliable dates beyond 50 kyr. Laboratory procedures include optimization of LS spectrometers to obtain low and stable non-¹⁴C background count rates, use of low-background counting vials, large benzene volumes, long counting times, and preconditioning of vacuum lines. We also discuss the need for multiple analyses of a suitable material containing no original ¹⁴C (background blank) and the application of an appropriate statistical model to compensate for variability in background contamination beyond counting statistics. Accurate and reproducible finite ages >60 kyr are indeed possible by high-sensitivity LS spectroscopy, but require corroborating background blank data to be defensible

Evidence for suppressed mid-Holocene northeastern Australian monsoon variability from coral luminescence

Summer monsoon rainfall in northeastern (NE) Australia exhibits substantial interannual variability resulting in highly variable river flows. The occurrence and magnitude of these seasonal river flows are reliably recorded in modern inshore corals as luminescent lines. Here we present reconstructed annual river flows for two ~120 year mid-Holocene windows based on luminescence measurements from five cores obtained from three separate coral colonies. We were able to cross-date the luminescence signatures in four cores from two of the colonies, providing confidence in the derived reconstruction. Present-day NE Australian rainfall and river flow are sensitive to El Niño–Southern Oscillation (ENSO) variability, with La Niña (El Niño) events typically associated with wetter (drier) monsoon seasons. Thus, our replicated and annually resolved coral records provide valuable insights into the northern Australian summer monsoon and ENSO variability at a key period (6 ka) when greenhouse gas levels and ice sheet cover were comparable to the preindustrial period but orbital forcing was different. Average modern and mid-Holocene growth characteristics were very similar, suggesting that sea surface temperatures off NE Australia at 6 kyr were also close to present values. The reconstructed river flow record suggests, however, that the mid-Holocene Australian summer monsoon was weaker, less variable from year to year (possibly indicative of reduced ENSO variability), and characterized by more within-season flood pulses than present. In contrast to today, the delivery of moisture appears to have been dominated by eastward propagating convective coupled waves associated with the Madden-Julian Oscillation

Towards a radiocarbon calibration for oxygen isotope stage 3 using New Zealand kauri (Agathis australis)

It is well known that radiocarbon years do not directly equate to calendar time. As a result, considerable effort has been devoted to generating a decadally resolved calibration curve for the Holocene and latter part of the last termination. A calibration curve that can be unambiguously attributed to changes in atmospheric ¹⁴C content has not, however, been generated beyond 26 kyr cal BP, despite the urgent need to rigorously test climatic, environmental, and archaeological models. Here, we discuss the potential of New Zealand kauri (Agathis australis) to define the structure of the ¹⁴C calibration curve using annually resolved tree rings and thereby provide an absolute measure of atmospheric ¹⁴C. We report bidecadally sampled ¹⁴C measurements obtained from a floating 1050-yr chronology, demonstrating repeatable ¹⁴C measurements near the present limits of the dating method. The results indicate that considerable scope exists for a high-resolution ¹⁴C calibration curve back through OIS-3 using subfossil wood from this source

Sea-level change and demography during the last glacial termination and early Holocene across the Australian continent

Future changes in sea-level are projected to have significant environmental and social impacts, but we have limited understanding of comparable rates of change in the past. Using comprehensive palaeoenvironmental and archaeological datasets, we report the first quantitative model of the timing, spatial extent and pace of sea-level change in the Sahul region between 35-8 ka, and explore its effects on hunter-gatherer populations. Results show that the continental landmass (excluding New Guinea) increased to 9.80 million km2 during the Last Glacial Maximum (LGM), before a reduction of 2.12 million km2 (or ~21.6%) to the early Holocene (8 ka). Almost 90% of this inundation occurs during and immediately following Meltwater Pulse (MWP) 1a between 14.6 and 8 ka. The location of coastlines changed on average by 139 km between the LGM and early Holocene, with some areas >300 km, and at a rate of up to 23.7 m per year (~0.6 km land lost every 25-year generation). Spatially, inundation was highly variable, with greatest impacts across the northern half of Australia, while large parts of the east, south and west coastal margins were relatively unaffected. Hunter-gatherer populations remained low throughout (<30,000), but following MWP1a, increasing archaeological use of the landscape, comparable to a four-fold increase in populations, and indicative of large-scale migration away from inundated regions (notably the Bass Strait) are evident. Increasing population density resulting from MWP1a (from 1/655 km2 to 1/71 km2) may be implicated in the development of large and complex societies later in the Holocene. Our data support the hypothesis that late Pleistocene coastal populations were low, with use of coastal resources embedded in broad-ranging foraging strategies, and which would have been severely disrupted in some regions and at some time periods by sea-level change outpacing tolerances of mangals and other near-shore ecological communities

Near-threshold electron injection in the laser-plasma wakefield accelerator leading to femtosecond bunches

We gratefully acknowledge the support of the UK EPSRC (grant no. EP/J018171/1), the EU FP7 programmes: the Extreme Light Infrastructure (ELI) project, the Laserlab-Europe (no. 284464), and the EUCARD-2 project (no. 312453).The laser-plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator.Publisher PDFPeer reviewe

A comprehensive approach to simulation of cartridge filtration using CFD

Household water treatment (HWT) systems are widely used for the provision of potable water in many countries with their low-cost key to attaining universal and equitable access to safe and affordable drinking water, Sustainable Development Goal 6.1. Removal of suspended particles (turbidity) from water sources via cartridge filters is often the first step of a HWT system, with the primary treatment increasing the efficiency of a subsequent disinfection step. Whilst the performance of cartridge filters (removal efficiency and pressure drop) can be determined experimentally in long experiments with high volumes of water, numerical simulation adds fundamental insight to the influence of fluid dynamics on particle deposition and vice versa. In this study, a novel computational fluid dynamic (CFD) model was developed to simulate the fundamental mechanisms underpinning the removal of particles within the widely used 10 in. cartridge filter, informed by and complemented with laboratory validation. The Eulerian approach was used to simulate fluid flow with the Lagrangian approach adopted for particle tracking. Rosin-Rammler distribution was implemented with respect to the particle size distribution of the diatomaceous earth particles used in the experiments. Given particles were non-spherical (disk shape), Wadell's sphericity was included to account for the effect of particle shape on drag force. A porous domain was implemented to simulate the filter element through addition of a source term to the momentum equations, with the likelihood of particle deposition, detachment and rebound also considered. Laboratory based validation studies confirmed the novel CFD model to accurately model removal of turbidity and predict the pressure drop across the filter with Root Mean Square Percentage Error (RMSPE) of less than 3%. The simulated location of particle deposition on the filter elements closely matched images taken at several stages during filtration experiments with the model aiding understanding of pattern of particle removal along and within the porous filter structure. This novel and comprehensive modelling methodology can be utilized to simulate the filtration process at the macro-scale, permitting evaluation of new filter designs and materials for advanced filtration systems; ultimately improving HWT system performance and reducing costs to users.</p

SHCal13 Southern Hemisphere calibration, 0–50,000 years cal BP

The Southern Hemisphere SHCal04 radiocarbon calibration curve has been updated with the addition of new data sets extending measurements to 2145 cal BP and including the ANSTO Younger Dryas Huon pine data set. Outside the range of measured data, the curve is based upon the Northern Hemisphere data sets as presented in IntCal13, with an interhemispheric offset averaging 43 ± 23 yr modeled by an autoregressive process to represent the short-term correlations in the offset