Linking sedimentary total organic carbon to 210Pbex chronology from Changshou Lake in the Three Gorges Reservoir Region, China.

The influences of total organic carbon (TOC) and total nitrogen (TN) on Lead-210 (210Pb) dating have recently been of increasing concern in lacustrine research. Sediment core from Changshou Lake in the Longxi catchment was investigated for influence of TOC on 210Pb dating. Lead-210 excess (210Pbex), Cesium-137 (137Cs) activities, TOC, TN, and particle size were measured. We proposed a dating index based on 137Cs chronology and particle size distribution of the lake sediment profile and rainfall erosivities calculated from Longxi catchment metrological records. Increasing trends in TOC and TN were specifically caused by commercial cage fish farming after 1989. The statistically significant correlation between 210Pbex activity, TOC (0.61, p = 0.04) and TN (0.51, p = 0.04), respectively explained post-1989 210Pb scavenging. The 210Pbex activity was closely related with coupled peaks of TOC and TN from mass depth 5-10 g cm-2. Higher TOC/TN ratio (8.33) indicated submerged macrophytes and native aquatic algal growth as main source of carbon from enhanced primary productivity because of massive fertilizer use and coherent climate warming. The study supported key hypothesis on vital role of fertilizer usage and algal derived TOC in controlling sedimentary 210Pbex activity at Changshou Lake sediment. 137Cs profile and erosive events as time markers provided reliable and consistent sedimentation rate of (1.6 cm y-1). 210Pbex activity decayed exponentially after peak at mass depth 5.68 g cm-2. Therefore, violation of 210Pb dating primary assumptions made it inappropriate for sediment dating at Changshou Lake. TOC content must be considered while using 210Pb as dating tool for lake sediment profiles.