Stable carbon isotope of black carbon in lake sediments as an indicator of terrestrial environmental changes: An evaluation on paleorecord from Daihai Lake, Inner Mongolia, China

Abstract

We measured the carbon isotope ratio of black carbon (BC) from the Daihai Lake sediment core (DH99a) in north-central China with an objective to examine the effectiveness and sensitivity of the δ13C values of BC (δ13CBC) as a potential indicator of terrestrial environmental changes. We first performed a statistical study on the available data regarding carbon isotope fractionation (CIF) during the conversion of C3 and C4 vegetation to BC and observed that the mean CIF for BC produced from C3 plants is −0.3‰, whereas that for BC from C4 plants is −1.7‰. This result provides a solid reference for reconstructing vegetation and environmental changes using the δ13CBC values. The δ13CBC record in the DH99a sediment core spanning the last ca 10,000years displayed large variations from −23.7‰ to −29.2‰, which suggests that C3 plants dominantly occupied the Daihai Lake region during the Holocene. The most negative δ13CBC peaks coincided with high values of tree percentages and grain sizes, which occurred under relatively wetter climatic conditions during the middle Holocene (ca 6500–3200cal.yrBP) and an interval between 1700 and 1350cal.yrBP. In contrast, the least negative δ13CBC values corresponded to low values of tree percentages and grain sizes during relatively drier phases of the early and late Holocene. The generally negative correlation of the δ13CBC values with the tree percentages and grain sizes was thought to reflect a negative correlation of the δ13CBC values with the monsoon precipitation. This correlation is consistent with the response of carbon isotope in modern C3 plants to precipitation in north China. Therefore, we developed a computational model to reconstruct the changes in annual precipitation over the Daihai Lake region using the δ13CBC values. The results indicated that the annual precipitation was highly variable, ranging from 170mm lower to 310mm higher than that at present during the middle Holocene, whereas the annual precipitation was generally ~70mm lower than that at present during the early and late Holocene. The general features of the inferred precipitation changes using the δ13CBC values are generally consistent with those reconstructed using pollen data of the same sediment core. Meanwhile, the δ13CBC values tend to register some extreme variations of monsoon precipitation, which were not reflected in the pollen assemblages. We conclude that the δ13CBC values in the Daihai Lake sediments may serve as a sensitive and reliable proxy for monitoring monsoon precipitation.