Influence of mollusk species on marine DELTA R determinations

Radiocarbon ages were measured on replicate samples of burnt grain and 5 mollusk species collected from a single sealed layer at an archaeological site (Hornish Point) on the west coast of South Uist, Scotland. The aim was to examine the impact of using different mollusk species on ΔR determinations that are calculated using the paired terrestrial/marine sample approach. The mollusk species examined inhabit a range of environments and utilize a variety of food sources within the intertidal zone. Several authors have suggested that these factors may be responsible for observed variations in the 14C activity of mollusk shells that were contemporaneous in a single location. This study found no significant variation in the <sup>14</sup>C ages of the mollusk species, and consequently, no significant variation in calculated values of ΔR. The implication is that in an area where there are no carboniferous rocks or significant local inputs of freshwater to the surface ocean, any of a range of marine mollusk species can be used in combination with short-lived terrestrial material from the same secure archaeological context to accurately determine a ΔR value for a particular geographic location and period in time

Deglacial history of the Pensacola Mountains, Antarctica from glacial geomorphology and cosmogenic nuclide surface exposure dating

The retreat history of the Antarctic Ice Sheet is important for understanding rapid deglaciation, as well as to constrain numerical ice sheet models and ice loading models required for glacial isostatic adjustment modelling. There is particular debate about the extent of grounded ice in the Weddell Sea embayment at the Last Glacial Maximum, and its subsequent deglacial history. Here we provide a new dataset of geomorphological observations and cosmogenic nuclide surface exposure ages of erratic samples that constrain the deglacial history of the Pensacola Mountains, adjacent to the present day Foundation Ice Stream and Academy Glacier in the southern Weddell Sea embayment. We show there is evidence of at least two glaciations, the first of which was relatively old and warm-based, and a more recent cold-based glaciation. During the most recent glaciation ice thickened by at least 450 m in the Williams Hills and at least 380 m on Mt Bragg. Progressive thinning from these sites was well underway by 10 ka BP and ice reached present levels by 2.5 ka BP, and is broadly similar to the relatively modest thinning histories in the southern Ellsworth Mountains. The thinning history is consistent with, but does not mandate, a Late Holocene retreat of the grounding line to a smaller-than-present configuration, as has been recently hypothesized based on ice sheet and glacial isostatic modelling. The data also show that clasts with complex exposure histories are pervasive and that clast recycling is highly site-dependent. These new data provide constraints on a reconstruction of the retreat history of the formerly-expanded Foundation Ice Stream, derived using a numerical flowband model

¹⁴C AMS at SUERC: improving QA data from the 5 MV tandem AMS and 250 kV SSAMS

In 2003, a National Electrostatics Corporation (NEC) 5MV tandem accelerator mass spectrometer was installed at SUERC, providing the radiocarbon laboratory with 14C measurements to 4–5‰ repeatability. In 2007, a 250kV single-stage accelerator mass spectrometer (SSAMS) was added to provide additional 14C capability and is now the preferred system for 14C analysis. Changes to the technology and to our operations are evident in our copious quality assurance data: typically, we now use the 134-position MC-SNICS source, which is filled to capacity. Measurement of standards shows that spectrometer running without the complication of on-line δ13C evaluation is a good operational compromise. Currently, 3‰ 14C/13C measurements are routinely achieved for samples up to nearly 3 half-lives old by consistent sample preparation and an automated data acquisition algorithm with sample random access for measurement repeats. Background and known-age standard data are presented for the period 2003–2008 for the 5MV system and 2007–2008 for the SSAMS, to demonstrate the improvements in data quality

Imaging SIMS with an accelerator (and biomedical applications)

The combination of a scanning secondary ion source and an accelerator mass spectrometer is a powerful tool for performing imaging secondary ion mass spectrometry, offering various fundamental and operational advantages over conventional high performance magnetic sector instruments. Not only are lower concentrations detectable by AMS, but even at conventionally measurable levels the analysis of desirable surface areas (even greater than can be achieved conventionally with dynamic emittance matching) can be simpler, more sensitive and/or faster by AMS. This is principally a consequence of the large and constant spectrometer acceptance. These benefits are being explored with developments of the Oxford 14C-AMS system. This method is ideally suited to quantitative imaging of radiocarbon labels widely employed in biomédical research to tag specific molecules, is capable of the most sensitive detection and submicron resolution and potentially offers considerable advantages over traditional radiographic imaging

Imaging AMS

The benefits of simultaneous high effective mass resolution and large spectrometer acceptance that accelerator mass spectrometry has afforded the bulk analysis of material samples by secondary ion mass spectrometry may also be applied to imaging SIMS. We are exploring imaging AMS with the addition to the Oxford 14C-AMS system of a scanning secondary ion source. It employs a sub-micron probe and separate apparatus for caesiating sample surfaces to further increase the useful ion yield. The source has been accommodated on the system by directly injecting sputtered ions into the accelerator without mass analysis. They are detected with a range of devices including new Si detectors. Qualitative mass spectra may be easily generated by varying only the post-accelerator analysis magnet. Selected ion signals may be used for imaging. In developing the instrument for bioscience research we are establishing its capability for measuring the lighter elements prevalent in biological tissue. Importantly, the machine can map the distributions of radiocarbon labeled compounds with an efficiency of about 1‰. A background due to misidentification of non-14C ions as a result of the reduced ion mass filtering is too small to hinder high magnification microscopy

The design of a radiocarbon muprobe for tracer mapping in biological specimens

Biological scientists use radioisotopes to label and subsequently monitor specific molecules within living systems. We aim to improve on conventional methods of imaging 14C tracers, in both sensitivity and resolution, by combining a new scanning-beam ion source generating C− secondary ions with our AMS system. Our measurements, including that of the generation of sufficient C− ions to the quantity of tissue sputtered, demonstrate that the radiocarbon muprobe is feasible, and we expect its sensitivity to be 1 14C label atom in 2000. We have employed the negative ion production enhancing effect of surface caesiation, investigating the potential of a Cs vapour spray for this purpose. As suitable Cs+ ion sources are unavailable, the muprobe will feature a Cs spray to compensate for the lack of primary beam surface caesiation, in conjunction with a 0.2 μm spot Ga+ beam

Single-stage accelerator mass spectrometer radiocarbon-interference identification and positive-ionisation characterisation

A single-stageaccelerator mass spectrometer (SSAMS) is a good alternative to conventional spectrometers based on tandem electrostatic acceleration for radiocarbon measurement and permits experimentation with both negative and positive carbon ions. However, such <sup>14</sup>C AMS of either polarity ions is limited by an interference. In the case of anion acceleration we have newly determined this to be summed <sup>13</sup>C and <sup>16</sup>O by improvising an additional Wien filter on our SSAMS deck. Also, <sup>14</sup>C AMS might be improved by removing its dependency on negative-ionisation in a sputter ion source. This requires negative-ionisation of sample atoms elsewhere to suppress the <sup>14</sup>N interference, which we accomplish by transmitting initially positive ions through a thin membrane. The ionisation dependence on ion-energy is found to be consistent with previous experimentation with vapours and thicker foils

Positive ion AMS with single stage accelerator and RF-plasma ion source at SUERC

The new single-stageaccelerator mass spectrometer at SUERC can also accept positiveions. Potential benefits of positiveion measurement with suitable ionsources are more convenient sample form, smaller sample size, a variety of available charge states and maybe new applications. We have begun with oxygen isotope analyses to demonstrate destruction of interfering molecules

Microbeam PIXE analysis using wavelength dispersive spectrometry

Wavelength dispersive X-ray spectrometry (WDS) offers significant advantages over energy dispersive X-ray spectrometry for PIXE analysis in some situations. Compared with energy dispersive Si(Li) detectors, the better energy resolution of WDS results in better separation of closely spaced adjacent X-ray peaks and enhanced signal to background ratios. These can result in greater accuracy in quantitative analysis, and increased sensitivity for trace element analysis. In addition, it is possible to analyze elements from beryllium upwards (Z ≥ 4). We have installed a commercial WDS detector on a nuclear microprobe system located at Lawrence Livermore National Laboratory. Advantages and limitations of wavelength dispersive spectrometry and considerations for calibration and operation are discussed. Representative results are presented for micro-PIXE analysis