Hydrogen Peroxide Treatment of Natural Lake Sediment Prior to Carbon and Oxygen Stable Isotope Analysis of Calcium Carbonate

Abstract

The carbon and oxygen stable isotope ratios (δ13C and δ18O) of authigenic and biogenic lacustrine carbonates are commonly used as past climate proxies, and these carbonates are often preserved within organic‐rich bulk sediment. We measured the δ13C and δ18O of carbon dioxide evolved from fine‐grained crystalline calcite and biogenic aragonite, mixed with natural organic‐rich lake sediment. We found that if the ratio of total inorganic carbon (TIC) to total organic carbon (TOC) in lacustrine bulk sediment is low, then organic compounds evolve detectable CO2 during phosphoric acid digestion, leading to an “organic bias” in the measured δ13C and δ18O. We tested the effect of oxidative pretreatment of the bulk sediment with acidic or alkaline hydrogen peroxide (H2O2), at a range of temperatures. Pretreatment with acidic H2O2 not only had a negligible effect on the TIC/TOC but also resulted in dissolution and reprecipitation of carbonate, and a consequent “treatment bias” that was particularly strong for δ18O. Oxidation with alkaline H2O2 removed a greater proportion of organic material, with no evidence for carbonate dissolution at temperatures of ≤50 °C. The δ13C and δ18O values obtained from sediment treated with alkaline H2O2 at 50 °C were both accurate and precise, even for sediment with very low initial TIC/TOC. Our results show that it is possible to obtain accurate δ13C and δ18O values from carbonates preserved within organic‐rich lacustrine sediment, suitable for use in paleoclimate reconstructions.

Effects of organic removal techniques prior to carbonate stable isotope analysis of lacustrine marls: A case study from palaeo‐lake Fucino (central Italy)

“…However, this could be an artefact due to the concomitant removal of larger amounts of OM in samples 2, 4, 5 and 7. Previous studies have shown that H 2 O 2 tends to dissolve carbonates, especially in its acidic form, while NaOCl has little or no interaction with carbonates . However, our data indicate that carbonates also undergo dissolution during the N pre‐treatment, although the extent of dissolution and the number of samples where this occurs is smaller than during the H pre‐treatment.…”

“…Studies of the effects induced by sample pre‐treatment on lacustrine carbonates have so far focused on artificial sediments obtained by mixing pure crystalline calcite (e.g. shells, synthetic calcite, ground marble or limestone) with variable amounts of the organic compounds which are typically found in lacustrine sediments, or with decarbonated lacustrine sediments . By analysing artificial calcite–OM mixtures with variable total inorganic carbon/total organic carbon (TIC/TOC) ratios, Oehlerich and co‐authors observed that δ 13 C and δ 18 O values are biased only up to TIC/TOC = 0.3, and they advised only pre‐treating samples lying below this threshold .…”

“…2,6 For these reasons, various physical and chemical pre-treatment methods have been used to remove OM from geologic materials in palaeoenvironmental and pedological studies. 4,7 Except for the method recently proposed by Falster and co-authors, 8 there is no universally accepted method for completely removing OM and, at the same time, not changing the isotope composition of the sample to be analysed. 7,9 Studies of the effects induced by sample pre-treatment on lacustrine carbonates have so far focused on artificial sediments obtained by mixing pure crystalline calcite (e.g.…”

“…shells, synthetic calcite, ground marble or limestone) with variable amounts of the organic compounds which are typically found in lacustrine sediments, 4,5 or with decarbonated lacustrine sediments. 8 By analysing artificial calcite-OM mixtures with variable total inorganic carbon/total organic carbon (TIC/TOC) ratios, Oehlerich and coauthors observed that δ 13 C and δ 18 O values are biased only up to TIC/TOC = 0.3, and they advised only pre-treating samples lying below this threshold. 5 However, chemical pre-treatment with sodium hypochlorite (NaOCl) or hydrogen peroxide (H 2 O 2 ) is still routinely applied in palaeolimnological studies as its effect on geologic materials is poorly understood.…”

“…For these reasons, various physical and chemical pre‐treatment methods have been used to remove OM from geologic materials in palaeoenvironmental and pedological studies . Except for the method recently proposed by Falster and co‐authors, there is no universally accepted method for completely removing OM and, at the same time, not changing the isotope composition of the sample to be analysed …”