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Kinetics and fractionation of carbon and oxygen isotopes during the solid-phase transformation of biogenic aragonite to calcite: The effect of organic matter
Palaeogeography, Palaeoclimatology, Palaeoecology ( IF 3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.palaeo.2020.109876
Chenglong Li , Hua Shen , Xuefen Sheng , Haizhen Wei , Jun Chen

Abstract The phase transformation of biogenic shells from aragonite to calcite can modify the primary carbon and oxygen isotopic values (δ13C and δ18O), and thus affect their significance as paleoenvironmental proxies. While the biasing of δ13C and δ18O by dissolution and reprecipitation phase transformation (DR) has been extensively studied, thermal solid-phase transformation (SPT) in archaeological shell middens is rarely considered. In this study, the effects of intra-crystalline organic matter on the kinetics and carbon and oxygen isotopic fractionation during thermal SPT are evaluated by comparing two sets of experiments on powdered aragonite snail shells heated in air and pure nitrogen atmospheres. The results show that during heating temperature below 400 °C, the intra-crystalline organic matter could inhibit the phase transformation due to the shielding effect of organic matter, while during heating temperature above 400 °C, the decomposition of the intra-crystalline organic matter increases the kinetic rate of the phase transformation. The activation energy for the phase transformation was determined to be +150.5 kJ·mol−1, lower than that of +247 kJ·mol−1 obtained for abiogenic polycrystalline aragonite and close to that of +158 to +163 kJ·mol−1 for abiogenic single crystal aragonite. This is attributed to the larger lattice parameter c in biogenic aragonite and the increasing porosity of calcium carbonate with the decomposition of organic matter. In a nitrogen atmosphere, the deviation of δ13C and δ18O between the heated sample (the mixture of aragonite and calcite) and the original shell aragonite (Δ13CM-A and Δ18OM-A) is negligible during heating temperature below 400 °C; however, Δ13CM-A (from −0.43‰ to 0.00‰) and Δ18OM-A (from −1.54‰ to −0.02‰) are significant during heating temperature above 400 °C. The temperature-dependence of Δ13CM-A in biogenic carbonates is much more significant than that in abiogenic carbonates, which clearly reflects the occurrence of isotopic exchange between the intra-crystalline organic matter and the carbonates. In air, the heating-induced calcium carbonate is depleted in δ13C by up to −0.48‰ and in δ18O by up to −3.46‰, compared with the original samples, which is mainly caused by isotopic exchange with the external carbon dioxide in air and the intra-crystalline organic matter, with the latter being the dominant source. Our results demonstrate that the decomposition of the intra-crystalline organic matter increases the kinetic rate and induces carbon and oxygen isotope fractionation during SPT from aragonite to calcite. This study provides direct evidence for evaluating the preservation of carbon and oxygen isotopes of biogenic calcium carbonate shells which have undergone the SPT process in archaeological shell middens.

中文翻译:

生物源文石向方解石固相转化过程中碳氧同位素的动力学和分馏:有机质的影响

摘要 生物壳从文石到方解石的相变可以改变主要的碳和氧同位素值(δ13C 和 δ18O),从而影响它们作为古环境代理的意义。虽然已经广泛研究了溶解和再沉淀相变 (DR) 对 δ13C 和 δ18O 的偏置,但很少考虑考古贝壳中的热固相转变 (SPT)。在这项研究中,通过比较两组在空气和纯氮气氛中加热的粉状文石蜗牛壳的实验,评估了晶内有机物对热 SPT 过程中动力学和碳和氧同位素分馏的影响。结果表明,在加热温度低于 400 ℃时,由于有机物的屏蔽作用,晶内有机物可以抑制相变,而在加热温度超过400℃时,晶内有机物的分解增加了相变的动力学速率。相变的活化能确定为+150.5 kJ·mol-1,低于非生物多晶文石的+247 kJ·mol-1,接近+158至+163 kJ·mol-1为非生物单晶文石。这归因于生物源文石中较大的晶格参数 c 和碳酸钙的孔隙率随着有机物的分解而增加。在氮气气氛中,加热温度低于400℃时,加热样品(文石和方解石的混合物)与原始壳文石(Δ13CM-A和Δ18OM-A)的δ13C和δ18O偏差可以忽略不计;然而,Δ13CM-A(从-0.43‰到0.00‰)和Δ18OM-A(从-1.54‰到-0.02‰)在加热温度高于400°C时是显着的。Δ13CM-A在生物碳酸盐岩中的温度依赖性比非生物碳酸盐显着,这清楚地反映了晶内有机质与碳酸盐之间同位素交换的发生。在空气中,与原始样品相比,加热诱导的碳酸钙在 δ13C 中消耗高达 -0.48‰,在 δ18O 中消耗高达 -3.46‰,这主要是由于与空气中外部二氧化碳的同位素交换和晶内有机质,后者是主要来源。我们的结果表明,在从文石到方解石的 SPT 过程中,晶内有机物的分解增加了动力学速率并诱导了碳和氧同位素分馏。该研究为评估在考古贝壳中经过 SPT 过程的生物碳酸钙贝壳的碳和氧同位素的保存提供了直接证据。
更新日期:2020-10-01
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