Paired stable carbon isotope in carbonate and Cladophora: A novel and quantitative proxy for palaeolake level variations on the northeastern Qinghai-Tibet Plateau

Abstract

Stable organic carbon isotopes (δ¹³C_org) in lacustrine sediments have been widely used for palaeoenvironmental reconstructions, with bulk δ¹³C_org values inherently integrating signals from diverse organic sources. Cladophora, a common alga genus in Qinghai-Tibet Plateau (QTP) lakes, potentially serves as a robust archive for source-specific δ¹³C_org. In this study, we reevaluated the potential factors influencing Cladophora-derived δ¹³C (δ¹³C_Clad) by analyzing samples collected from varying water depths in three northeastern QTP lakes (Qinghai Lake, Tuosu Lake, and Gahai Lake). Our results show that δ¹³C_Clad is not only strongly correlated with the photosynthetic activities modulated by water depth, but with the δ¹³C values of dissolved inorganic carbon (DIC) in different lakes. Therefore, to minimize interference from lake-level signals influenced by catchment-scale carbon cycle perturbations in δ¹³C_Clad-based reconstructions, we further established paired stable organic carbon isotopes (Δ¹³C_Clad), derived by subtracting carbonate carbon isotopes (δ¹³C_DIC) from δ¹³C_Clad, as a novel and quantitative proxy for palaeolake level variations on the northeastern QTP. Then, we reconstructed lake-level fluctuations using this equation in Lake Tuosu since 1600 CE (known as the Little Ice Age). The record reveals four distinct phases: (1) an initial post-1600 CE rise culminating in 1700 CE; (2) sustained high levels until 1885 CE; (3) a decline to minimal levels during 1885–1965 CE; and (4) a post-1965 CE recovery likely driven by anthropogenic and climatic drivers. These fluctuations exhibit coherence with tree-ring based precipitation reconstructions across the eastern QTP but display temporally asymmetric responses to solar irradiance—showing strong synchronization during the 1645–1715 CE Maunder Minimum yet decoupling thereafter. This “phase-mismatch” pattern underscores the transitional climate regime of the northeastern QTP, where interactions between westerly circulation and monsoon dynamics modulate hydrological responses to external forcing.