Approaches to Water Content Correction and Calibration for µXRF Core Scanning: Comparing X-ray Scattering with Simple Regression of Elemental Concentrations

Abstract

Geochemical evaluation of sediment records traditionally exploits dry mass concentration data; the new generation of scanning XRF devices, however, are generally presented with wet sediment cores. Therefore, conversion of wet core measured XRF data to dry mass concentrations will aid the palaeoenvironmental interpretation, provided the method used is reliable and avoids loss of data quality. Here, using data from a GEOTEK/Olympus DELTA scanning µXRF device (approximately 5 mm resolution), we compare two methods: (1) correction by simple regression, calibrated using dry sediment elemental concentration data measured for a 'training set' of subsamples, and (2) a novel technique that corrects for water content estimated using X-ray scattering data obtained during scanning. We show that where sediment water contents are highly variable the regression method fails while water content correction methods can be highly effective. Where water sediment water contents are relatively constant, the elemental regression is as effective and introduces less noise.

Quantifying system disturbance and recovery from historical mining-derived metal contamination at Brotherswater, northwest England

“…3; Table 3). Notwithstanding slight overestimation in the conversion of lXRF scan data to equivalent dry mass concentrations at maximum Pb values (Boyle et al 2015b), the Pb peak exceeds 10,000 lg g -1 in delta-proximal cores and exceeds 4000 lg g -1 more widely across the basin. Peak Pb occurs at 82 cm depth in BW11-3, contrasting with its appearance at depths of 17-20 cm beyond 350 m from the inflow.…”

“…The long cores BW11-1, BW11-4, BW11-5 and BW12-9 were lXRF-scanned (Olympus Delta ED-XRF) on a wet sediment basis at 0.5-cm intervals using a Geotek MSCL-XZ core scanner and wet sediment samples from core BW11-3 were measured manually on a Thermo-Niton ED-XRF. Wet sediment element concentrations were converted to dry-weight equivalent (Boyle et al 2015b) using a training set of dried samples (BW12-9A) measured on a Bruker S2 Ranger ED-XRF analyser equipped with a Pd X-ray tube and Peltier-cooled silicon drift detector. Dry mass concentrations were corrected for organic matter content (Boyle 2000).…”

“…All core and sediment trap samples that were measured on the S2 Ranger ED-XRF had previously been freeze-dried and their moisture contents and dry bulk densities (assuming average grain density = 2.65 g cm -3 ) calculated. Each XRF analyser undergoes a daily standardization procedure using certified reference materials (Boyle et al 2015b). …”

“…Profiles have been cutoff at 100 cm depth to highlight the major feature, which almost certainly corresponds to the episode of intense ore extraction during the 1860-1870s at Hartsop Hall Mine. Where XRF measurements were performed on a wet-sediment basis, concentrations have been converted to dry weight equivalent following the procedures of Boyle et al (2015b). As a result, the expected pattern of more diluted Pb concentrations with distance from the inflow is not fully coherent due to over-estimation of higher values in the regression model core into 5-cm-thick sections and estimated the accumulation rate for each segment using a Markov Chain Monte Carlo (MCMC) approach, constrained by prior information on accumulation rate (a gamma distribution with mean 5-year cm -1 and shape 2) and its variability (memory, a beta distribution with mean 0.5 and shape 20).…”

Future Developments and Innovations in High-Resolution Core Scanning

“…Many researchers continue to use either count or count-rate data as supplied by the instrument's software or they may adopt a semi-empirical approach by analyzing a small number of sub-samples using WD-XRF, ED-XRF or ICP-OES to calibrate their data (e.g. Boyle et al 2015;Croudace et al 2015). A more advanced approach based on the use of multi-variate statistics is described by Weltje et al (2015) and represents a significant advance that could become increasingly used.…”