Seasonal Variations in δ18O of Inflowing River Water, Lake Water, Sediment Trap Material and Ostracod Shells of Lake Nam Co and its Catchment (Tibet, China) — a Proxy Calibration Study
Abstract:δ18O is a very common proxy used in palaeoclimate studies all over the world, but for most parts of the world and especially the Tibetan Plateau modern studies that trace the pathway of this isotope from sink to source are lacking. In this study, financed by the BMBF (German Ministry of Science and Education, Project CADY), we present data that fill this gap. The study was performed in the terminal lake Nam Co, the biggest lake on the central Tibetan Plateau and its catchment. Water of the main tributaries to the lake, lake water at three different stations and from different water depths, bulk carbonate collected from different water depths with sediment traps and shells of living ostracods were collected for one year at lake Nam Co in approx. monthly intervals and analyzed for their δ18O values.
The inflowing river waters mainly reflect the precipitation and show regional as well as seasonal variations depending on their location and on Monsoon or Westerly season but are in general on the global meteoric waterline. The lake water shows only minor vertical and spatial variations and is clearly off the meteoric waterline due to strong evaporation effects in this terminal lake. δ18O values of the bulk carbonate of sediment collected in sediment traps show only minor vertical and spatial variations during the year and the δ18O offset compared to the lake water is with around 1 ‰ quite small. Most probably this is attributed to chemical fractionation during formation of monohydrocalcite in the water column. In contrast δ18O values of ostracod shells are significant heavier than the lake water indicating isotopic fractionation (vital effect) during shell formation. In addition a seasonal variability is visible.
The data of this proxy calibration study give now for the first time the opportunity to validate δ18O proxy data measured on core material from Lake Nam Co for their palaeoclimatic significance.