IMIS - Marine Research Groups

The large difference in the fractionation of stable carbon isotopes between C₃ and C₄ plants is widely used in vegetation reconstructions, where the predominance of C₃ plants suggests wetter and that of C₄ plants drier conditions. The stable carbon isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the stable carbon isotopic value of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the bulk carbon isotopic values of C₃ andC₄ plants (δ¹³C) and of organic carbon ( δ¹³C_org) in soils, river suspended particulate matter (SPM) and riverbed sediments to gain insight into thecontrol of precipitation on C₃ and C₄ plantδ¹³C values and to assess changes in δ ¹³C_org values along the plant–soil–river continuum.This information allows us to elucidate the implications of different δ¹³C end-members on C₃ / C₄ vegetation reconstructions. Our analysis was performed in the Godavari River basin, located in the core monsoon zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The basin has distinct wet and dry seasons and is characterised by natural gradients in soil type (from clay-rich to sandy), precipitation (∼ 500 to 1500 mm yr⁻¹) and vegetation type (from mixed C₃ / C₄ to primarily C₃) from the upper to the lower basin. The δ¹³C values of Godavari C₃ plants were strongly controlled by mean annual precipitation (MAP), showing an isotopic enrichment of ∼ 2.2 ‰ from ∼ 1500 to 500 mm yr⁻¹. Tracing δ¹³C_org values from plant to soils and rivers revealed that soils and riverbedsediments reflected the transition from mixed C₃ and C ₄ vegetation in the dry upper basin to more C₃ vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered theplant-derived δ¹³C signal. Phytoplankton dominated the δ¹³C_org signal carried by SPM in the dry season and year-round in the upper basin. Application of a linear mixing model showed that the %C₄ plants in the different subbasins was ∼ 7 %–15 % higher using plant end-members based on measurement of the Godavari vegetation and tailored to local moisture availability than using those derived from data compilations of global vegetation. Including a correction for the ¹³C enrichment in Godavari C₃plants due to drought resulted in maximally 6 % lower estimated C ₄ plant cover. Our results from the Godavari basin underline theimportance of making informed choices about the plant δ ¹³C end-members for vegetation reconstructions, considering characteristics of the regional vegetation and environmental factors such as MAP in monsoonal regions.