A synthetic standard for the analysis of carbon isotopes of carbon in silicates, and the observation of a significant water-associated matrix effect

Christopher H House

doi:10.1186/s12932-015-0029-x

Abstract

BackgroundDue to the biogeochemical fractionation of isotopes, organic material can be heterogeneous at the microscale. Because this heterogentiy preserves in the rock record, the microscale measurement of carbon isotopes is an important frontier of geobiology. Such analyses via secondary ion mass spectrometry (SIMS) have been, however, held back by the lack of an appropriate homogeneous synthetic standard that can be shared between laboratories. Such a standard would need to yield a carbon signal intensity within the same instrument dynamic range as that found for typical rocks, exhibit minimal matrix effects under typical SIMS conditions, and be widely available. In this work, five possible standards were tested with repeated δ13C ion microprobe measurements against the PPRG #215-1 Precambrian chert that has been used as a working standard for these types of analyses by several laboratories.ResultsResults showed that silica powder, pressed, and bonded with Ceramacast 905 produced a useful synthetic standard. The material produced has a secondary ion carbon yield of only about 15× that of the PPRG #215-1 organic-rich chert. Finally, the material, once dried sufficiently, did not demonstrate an observable matrix effect when the carbon isotopes were measured. Another similar material (silica nanopowder, pressed, and bonded with Aremco-Bond 526N) appears to have retained its hydration after a substantial effect to dry it. The isotopes measurements of this more hydrated material showed a significant matrix effect that was diminished by intense pre-sputtering. The results indicate water can affect SIMS carbon isotopic measurements, and an intense beam reduces the effect. A hydrated standard might be useful to monitor the effect.ConclusionsA suitable artificial standard for SIMS isotopic measurements of organic material in rocks has been found, and it will allow an acute growth in both the quantity and quality of studies of ancient carbon at the microscale. Also, this work has revealed a novel water-associated matrix effect for carbon isotopes. This newly revealed matrix effect is important because it might have misled previous research. The effect could lead to increased observed heterogeneity of partially hydrated samples and/or produced systematic differences between natural targets and the standards used.

Highlights

Due to the biogeochemical fractionation of isotopes, organic material can be heterogeneous at the microscale
House Geochem Trans (2015) 16:14 bulk carbon isotope measurements are, limited because they represent an average of carbon in rocks, which is a mix of carbon from indigenous organisms, reworked organic material, and potentially post-depositional contaminants
Because PEEK GF30 is a commercially available high performance glass-filled plastic, its overall elemental composition is similar to natural samples and it is readily available

Summary

Introduction

Due to the biogeochemical fractionation of isotopes, organic material can be heterogeneous at the microscale Because this heterogentiy preserves in the rock record, the microscale measurement of carbon isotopes is an important frontier of geobiology. Such analyses via secondary ion mass spectrometry (SIMS) have been, held back by the lack of an appropriate homogeneous synthetic standard that can be shared between laboratories. The work published to date demonstrates that there is a great deal of such heterogeneity that is masked in a bulk analysis (e.g., [1,2,3,4,5,6,7]) This heterogeneity could, if measured, be a signature of particular microbial processes occurring when the sediments were deposited. Different carbon fixation pathways impart demonstrated different carbon isotopic fractionations [8,9,10], and different sources of carbon could result in sedimentary organic material with distinct carbon isotopic compositions (e.g., [11,12,13])

Methods

Results

Conclusion