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Carbonate removal from coastal sediments for the determination of organic carbon and its isotopic signatures, δ 13 C and Δ 14 C: comparison of fumigation and direct acidification by hydrochloric acid
Author(s) -
Komada Tomoko,
Anderson Mark R.,
Dorfmeier Corin L.
Publication year - 2008
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.4319/lom.2008.6.254
Subject(s) - chemistry , total inorganic carbon , hydrochloric acid , total organic carbon , environmental chemistry , dissolved organic carbon , carbonate , dolomite , isotopes of carbon , carbon dioxide , mineralogy , inorganic chemistry , organic chemistry
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.