109-5 Triple Oxygen and Carbon Isotope Effects of Chemical Pretreatments on Biominerals

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives (Posters)

Presenting Author:

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Abstract:

Oxygen and carbon isotope values of biominerals (e.g., tooth enamel bioapatite and eggshell calcite) are commonly used as proxies for paleoecological and paleoclimatic conditions. However, the presence of organics or secondary carbonate within the mineral structure can impact the measured isotopic values. Commonly, researchers chemically pretreat biominerals to eliminate or reduce the potential contaminants. Chemical treatment tends to result in mass loss, a potential problem in mass-limited samples. Potential mineralogical alteration is also a constant concern. In recent decades, the effects of chemical pretreatment procedures on mineralogy and measured oxygen and carbon isotope values have been published extensively. However, it is unknown how, or if, chemical pretreatments impact biomineral triple oxygen isotope values (Δ'¹⁷O, paired δ¹⁷O-δ¹⁸O measurements where Δ'¹⁷O=ln(δ¹⁷O+1)-0.528xln(δ¹⁸O+1)). Recently, biomineral Δ'¹⁷O values has emerged as an important proxy for paleo- aridity, atmospheric pCO₂ reconstructions, global primary production changes, and paleohydrology. 

Here, we apply six chemical treatments (including a control treatment) to a suite of modern and fossil tooth enamel and eggshell. The treatments used different chemical combinations and concentrations of oxidants and acetic acid commonly used in the literature to eliminate organics and secondary carbonate, respectively. The δ¹³C, δ¹⁸O, and Δ'¹⁷O values were measured using a Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) instrument. Additionally, infrared spectra were collected before and after pretreatments to identify changes in mineralogy and/or organics. Changes in sample mass were determined following both the organic and secondary carbonate removal. Relatively little mass was lost following organic treatment, while more mass was lost following acetic acid treatment, though the precise amount varied with both treatment conditions and sample type. In general, each of the chemical treatments yielded similar δ¹³C, δ¹⁸O, and Δ'¹⁷O values except for fossil tooth enamel, probably due to the presence of secondary carbonate. The impact was minimal for the Δ'¹⁷O values and most clearly seen in the δ¹⁸O values between the pretreated and the control fossil horse tooth enamel. We conclude that it is important to (at minimum) remove secondary carbonate before δ¹³C, δ¹⁸O, and Δ'¹⁷O analysis of fossil samples. 

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10778

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