Standardizing Organic Carbon Measurements for Modern and Geologic Timescales

Abstract:

Accurate reconstruction of carbon isotope records (as well as accurate characterization of the modern carbon cycle, e.g., in soils) relies on reliably separating organic carbon (C_org) from carbonate-derived carbon (C_carb). These fractions are characterized by very different isotope composition, so small carbonate contamination can strongly bias C_org results, and vice versa. Several criteria must be met for accurate %C and d¹³C analysis. In the case of analyzing C_org, these include: (1) C_carb must be removed through a process called “decarbonation.” (2) C_carb can be removed by acid dissolution, but if the acid is too strong then the C_org itself may solubilize, causing inaccurate results. (3) The preparation process for decarbonation can also unintentionally add carbon to samples and create a methodological blank that also will bias results.

This study tested decarbonation methods with the above criteria in mind. The focus was on (i) heated treatment with weak liquid acid, e.g., 1M HCl (“liquid phase decarbonation”) and (ii) heated treatment with vapor from concentrated acid (“vapor phase decarbonation”). Our results confirm that heated treatment is critical to producing reliable records; recalcitrant carbonate phases are not removed during room temperature decarbonation and can bias carbon isotope values. Vapor phase decarbonation may prevent loss in solution that is known to occur using liquid phase methods. However, our results show that blanks must be very carefully monitored and can be a concern during vapor phase treatment. Moreover, we still observe some loss of organics during vapor phase treatment, as evidenced by changes in C_org and d¹³C with length of reaction time. The length of vapor phase treatment must be carefully considered depending on the type of sample being tested. Overall, our work emphasizes the importance of carefully considering sample-specific decarbonation methodology in order to produce reliable values for %C_org and d¹³C.