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

Tomoko Komada, Mark R. Anderson,Corin L. Dorfmeier

Limnol. Oceanogr. Methods 6:254-262 (2008) | DOI: 10.4319/lom.2008.6.254

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 (δ13COC and Δ14COC). 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, δ13COC and Δ14COC 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 (HClvap) and aqueous (HClaq) phases—to determine %OC, δ13COC, and Δ14COC 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 δ13COC and Δ14COC values. HClvap fulfilled all criteria, given that the samples were not overexposed to acid. Overexposure led to underestimation of δ13COC and Δ14COC values. HClaq gave similar results but was less reliable in that it consistently underestimated %OC and yielded inaccurate δ13COC value for one test sample. It is recommended that an optimal acid exposure is carefully determined for each sample type to obtain most accurate δ13COC and Δ14COC values.