Modern stable isotopic (δ18O, δ2H, δ13C) variation in terrestrial, fluvial, estuarine and marine waters from north‐central Sarawak, Malaysian Borneo

Abstract

AbstractStable isotope data on humid tropical hydrology are scarce and, at present, no such data exist for Borneo. Delta18O, δ2H and δ13C were analysed on 22 water samples from different parts of the Sungai (river) Niah basin (rain, cave drip, rainforest pool, tributary stream, river, estuary, sea) in north‐central Sarawak, Malaysian Borneo. This was done to improve understanding of the modern stable isotope systematics of the Sungai Niah basin, essential for the palaeoenvironmental interpretation of the Late Quaternary stable isotope proxies preserved in the Great Cave of Niah. The Niah hydrology data are put into a regional context using the meteoric water line for Southeast Asia, as derived from International Atomic Energy Agency/World Meteorological Organization isotopes in precipitation network data. Although the Niah hydrological data‐set is relatively small, spatial isotopic variability was found for the different subenvironments of the Sungai Niah basin. A progressive enrichment occurs towards the South China Sea (δ18O −4·6‰; δ2H −29·3‰; δ13C −4·8‰) from the tributary stream (δ18O −8·4‰; δ2H −54·7‰; δ13C −14·5‰) to up‐river (δ18O c. −8‰; δ2H c. −51‰; δ13C c. −12‰) and down‐river values (δ18O c. −7·5‰; δ2H c. −45‰; δ13C c. −11‰). This is thought to reflect differential evaporation and mixing of different components of the water cycle and a combination of depleted biogenic δ13C (plant respiration and decay) with enriched δ13C values (due to photosynthesis, atmospheric exchange, mixing with limestone and marine waters) downstream. Cave drip waters are relatively enriched in δ13C as compared to the surface waters. This may indicate rapid degassing of the cave drips as they enter the cave atmosphere. Copyright © 2005 John Wiley & Sons, Ltd.