Hydrogen Peroxide Treatment of Natural Lake Sediment Prior to Carbon and Oxygen Stable Isotope Analysis of Calcium Carbonate. (19th September 2018)

• Record Type: Journal Article
• Authors: Falster, Georgina; Delean, Steven; Tyler, Jonathan
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 19:Number 9(2018)
• Page Start: 3583
• Page End: 3595
• Publication Date: 2018-09-19
• Subjects: stable isotopes -- calcium carbonate -- lake -- paleoclimate -- scanning electron microscopy -- pretreatment; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2018GC007575
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The carbon and oxygen stable isotope ratios (δ 13 C and δ 18 O) 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 δ 13 C and δ 18 O 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 δ 13 C and δ 18 O. We tested the effect of oxidative pretreatment of the bulk sediment with acidic or alkaline hydrogen peroxide (H2 O2 ), at a range of temperatures. Pretreatment with acidic H2 O2 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 δ 18 O. Oxidation with alkaline H2 O2 removed a greater proportion of organic material, with no evidence for carbonate dissolution at temperatures of ≤50 °C. The δ 13 C and δ 18 O values obtained from sediment treated with alkaline H2 O2 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 δ 13 C and δ 18 O values from carbonates preserved within organic‐rich lacustrine sediment, suitable for use in paleoclimate reconstructions. Key Points: Stable isotope values from carbonate preserved within natural lake sediment may be biased by release of CO2 from organic compounds Pretreatment of sediment with acidic hydrogen peroxide causes a treatment bias due partly to carbonate dissolution and reprecipitation Pretreatment of sediment with alkaline hydrogen peroxide at 50 °C results in accurate stable isotope values