Carbonate removal from coastal sediments for the determination of organic carbon and its isotopic signatures, δ¹³C and Δ¹⁴C: comparison of fumigation and direct acidification by hydrochloric acid

Abstract

Acidification of a solid sample to separate inorganic carbon (IC) from organic carbon (OC) is a widely encountered procedure in limnology and oceanography. Traditionally, OC was isolated to determine the weight‐% of OC (%OC), but it is becoming increasingly common to determine the isotopic signatures of the OC (δ 13 C OC and Δ 14 C OC ). This raises a need for a closer scrutiny of the currently used acidification methods. First, because IC and OC typically have distinct carbon‐isotopic signatures, δ 13 C OC and Δ 14 C OC values can be compromised if IC is not completely removed. Second, it is possible to isotopically fractionate a sample if a small portion of OC is lost during acidification, because OC itself is both chemically and isotopically heterogeneous. This study evaluated two acidification methods by HCl—in the vaporous (HCl vap ) and aqueous (HCl aq ) phases—to determine %OC, δ 13 C OC , and Δ 14 C OC in coastal sediments. Each method was assessed according to the criteria that it (1) has low blank levels, (2) is able to remove dolomite, (3) yields accurate %OC, and (4) yields accurate δ 13 C OC and Δ 14 C OC values. HCl vap fulfilled all criteria, given that the samples were not overexposed to acid. Overexposure led to underestimation of δ 13 C OC and Δ 14 C OC values. HCl aq gave similar results but was less reliable in that it consistently underestimated %OC and yielded inaccurate δ 13 C OC value for one test sample. It is recommended that an optimal acid exposure is carefully determined for each sample type to obtain most accurate δ 13 C OC and Δ 14 C OC values.