PEDOGENIC SIDERITE AND THE REPRODUCIBILITY OF PALEOSOL ORGANIC MATTER D13C VALUES

Measurements of the carbon isotopic composition (δ¹³C) of bulk organic matter are routinely used to identify the carbon isotopic excursion associated with the Paleocene-Eocene Thermal Maximum. However, obtaining low (less than 0.5 ‰) intra-sample reproducibility in Paleocene-Eocene paleosol samples from North and South America (Piceance Basin, Bighorn Basin, US and Bogota Basin, Colombia) has been a challenge. Previous studies have suggested that the presence of pedogenic siderite (FeCO₃) can contaminate carbon isotopic measurements of organic matter, particularly at low total organic carbon content, resulting in low intra-sample reproducibility. This hypothesis is reasonable as siderite is difficult to remove with standard HCl pretreatment techniques. To determine if poor reproducibility of δ¹³C values of the paleosol samples was caused by the contamination by pedogenic siderite, we analyzed the samples using X-ray diffraction (XRD) and low temperature rock magnetic measurements. We calibrated these detection methods using synthetic standards containing increasing proportions of siderite (0.1%, 1%, 5%, 10%, 25%) and decreasing proportions of kaolinite. Results from the synthetic siderite standards show that XRD has a detection limit of 5% by mass, while the low temperature rock magnetic methods have a sensitivity of 1 part per thousand by mass. Measurements of the paleosols show no evidence from either XRD or magnetic data for the presence of siderite in the samples. Therefore, in this instance we suggest that the low reproducibility of the δ¹³C data is more likely related to the low organic carbon content (mean=0.05%) of all the samples. While the acidification pretreatment method may remove intra-sample variability, the actual mineral phases being removed may not relate to isotopic contamination, suggesting the possibility of other effects on the sample. We propose that characterization of the mineral phase of paleosol samples, such as the magnetic components as in this study, is an important adjunct to interpretation of conventional carbon isotope analysis of bulk organic matter.