Lake nutrient variability inferred from elemental (C, N, S) and isotopic (δ 13C, δ 15N) analyses of aquatic plant macrofossils

Abstract

This paper aims to highlight the potential of using elemental and stable isotope analyses of aquatic macrophytes in palaeolimnological studies. Potamogeton pectinatus material was collected from modern plants ( n = 68) and from late glacial and Holocene-aged sediments from Koucha Lake (northeastern Tibetan Plateau; 34.0°N; 97.2°E; 4540 m a.s.l.). It was analyzed for δ 13C Potamogeton (modern: −23 to 0‰, fossil: −19 to −4‰) and δ 15N Potamogeton (modern: −11.0 to +13.8‰, fossil: −9.5 to +6.7‰) in addition to elemental carbon and nitrogen (modern C/N Potamogeton : 7 to 29; fossil: 13 to 68) and sulfur (fossil: 188–899 μmol/g dry weight). Fossil data were interpreted in terms of palaeo-nutrient availability and palaeo-productivity based on the modern relationships between various proxies and certain environmental data. Productivity of Potamogeton pectinatus mats at Koucha Lake as indicated by palaeo-ɛ Potamogeton -TIC (i.e. the enrichment of δ 13C Potamogeton relative to the δ 13C TIC) was reduced during periods of high conductivity, especially between 10.3 and 7.4 cal kyr BP. Potamogeton pectinatus material from these periods was also characterized by high S Potamogeton indicating high sulfide concentrations and anoxic conditions within the sediments. However, C/N Potamogeton ratios and δ 15N Potamogeton from the lower core section were found to have been altered by decompositional processes. A pronounced shift in the aquatic productivity of Lake Koucha occurred at ∼7.4 cal kyr BP when the hydrological conditions shifted towards an open lake system and water depth increased. At this time a strong increase in productivity led to a strong decrease in the water HCO 3 – concentration as inferred from the application of a ɛ Potamogeton -TIC-lnHCO 3 − transfer function. A comparison of reconstructed productivity changes from Koucha Lake with further environmental proxies suggests that primary productivity changes are probably a function of internal lake dynamics and were only indirectly triggered by climate change.