Abstract Long-term paleovegetational records from continental settings help in comprehending regional and global forcing on the abundance of C3-C4 plants in the past. The carbon isotopic composition of soil carbonates (δ13CSC), soil organic matter (δ13CSOM), organic matter occluded in soil carbonate nodules (δ13CNOM) and biomarkers in paleosol organic matter (long-chain fatty acid; δ13CFAME) are often used to estimate changes in the past-vegetational composition. However, it has been observed that depending on the type of proxy, the estimated abundance of C3-C4 plants varies, which can lead to uncertainty in paleovegetational records. Hence, the present study aims to comprehend the factors affecting the δ13CSC, δ13CSOM, δ13CNOM and δ13CFAME values within a paleosol. In this context, available δ13CSC, δ13CSOM, δ13CFAME and newly measured δ13CNOM values from the late Quaternary sequences of the Ganga Plain, India has been used. The abundance of C4 plants calculated from the δ13CSC and δ13CFAME values is ~2% to 89% higher compared to the δ13CSOM and δ13CNOM values-based estimates. Even with a common source of organic matter, the δ13CSOM values indicate a higher abundance of C4 plants (~2% to 50%) compared to the estimates from δ13CNOM values. We suggest that the disparity is due to the variation in the response of proxies to perturbations in the paleovegetational regime, growing season condition, and isotopic fractionation during decomposition and incorporation of organic matter into the soil. For example, the organic matter is incorporated into the soil throughout the year and represents average annual biomass, whereas SC precipitates under warmer and often drier condition when the ratio of C4 to C3 plant respiration is higher. Additionally, preferential degradation of 13C enriched labile compounds and C4 plants derived organic matter may lower the δ13CNOM values resulting in an under estimation of C4 plants in a mixed C3-C4 environment. The higher abundance of C4 plants estimated from the δ13CFAME values is due to the isotopic fractionation (13C enrichment of ~2‰ to 7‰) during incorporation of plant-derived long-chain fatty acids into the soil. The disparity in the abundance of C4 plants estimated from δ13CSOM and δ13CNOM values is due to the difference in the preservation of SOM and NOM. Contrary to the NOM (which is in a closed system), SOM in open system within the soil matrix is subjected to 13C enrichment due to the long-term humification of organic matter. Various factors such as grain size and pedogenesis that are inherent to the depositional environment also control the δ13C values of paleosol components. Considering the uncertainties associated with the δ13C values of paleosol components, reporting the absolute abundance of C4 plants would be uncertain. Therefore, we recommend presenting the relative change in abundance of C3-C4 plants during paleovegetational reconstruction.

中文翻译：

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使用古土壤组分的碳同位素组成估算的C4植物丰度差异

摘要 来自大陆环境的长期古植被记录有助于理解区域和全球强迫过去对 C3-C4 植物丰度的影响。土壤碳酸盐（δ13CSC）、土壤有机质（δ13CSOM）、土壤碳酸盐结核中的有机质（δ13CNOM）和古土壤有机质（长链脂肪酸；δ13CFAME）中的生物标志物的碳同位素组成通常用于估计过去的植被组成。然而，据观察，根据代理的类型，C3-C4 植物的估计丰度会有所不同，这可能导致古植被记录的不确定性。因此，本研究旨在了解影响古土壤中δ13CSC、δ13CSOM、δ13CNOM和δ13CFAME值的因素。在此背景下，可用 δ13CSC、δ13CSOM、已经使用了来自印度恒河平原晚第四纪层序的 δ13CFAME 和新测量的 δ13CNOM 值。与基于 δ13CSOM 和 δ13CNOM 值的估计值相比，根据 δ13CSC 和 δ13CFAME 值计算出的 C4 植物丰度高约 2% 至 89%。即使具有共同的有机质来源，δ13CSOM 值也表明与 δ13CNOM 值的估计值相比，C4 植物的丰度更高（~2% 至 50%）。我们认为这种差异是由于代用物对古植被制度、生长季节条件和有机质分解和掺入土壤中同位素分馏过程中的扰动的响应的变化。例如，有机质全年都被纳入土壤，代表年均生物量，而当 C4 与 C3 植物呼吸的比率较高时，SC 在较温暖且通常较干燥的条件下沉淀。此外，富含 13C 的不稳定化合物和 C4 植物衍生的有机物的优先降解可能会降低 δ13CNOM 值，从而导致对混合 C3-C4 环境中的 C4 植物的估计不足。根据 δ13CFAME 值估计的 C4 植物丰度较高是由于在将植物来源的长链脂肪酸掺入土壤期间的同位素分馏（13C 富集约 2‰ 至 7‰）。δ13​​CSOM 和 δ13CNOM 值估计的 C4 植物丰度的差异是由于 SOM 和 NOM 的保存情况不同。与 NOM（处于封闭系统中）相反，由于有机质的长期腐殖化，土壤基质内开放系统中的 SOM 受到 13C 富集。沉积环境固有的各种因素，如粒度和成土作用，也控制着古土壤成分的 δ13C 值。考虑到与古土壤成分的 δ13C 值相关的不确定性，报告 C4 植物的绝对丰度将是不确定的。因此，我们建议在古植被重建过程中呈现 C3-C4 植物丰度的相对变化。报告 C4 植物的绝对丰度将是不确定的。因此，我们建议在古植被重建过程中呈现 C3-C4 植物丰度的相对变化。报告 C4 植物的绝对丰度将是不确定的。因此，我们建议在古植被重建过程中呈现 C3-C4 植物丰度的相对变化。