Water Isotope Variability Across Individual Precipitation Events in Borneo

Abstract:

The composition of hydrogen and oxygen isotopes in rainwater (measured as δ¹⁸O and δD) provides vital information about current hydrological dynamics, and forms the basis for many paleoclimate reconstructions of hydroclimate variability. However, many factors – both local and remote – govern water isotope fractionation, complicating the interpretation of water isotope records. While Raleigh distillation serves as a key first-order driver of the well-noted “amount effect”, post-condensation evaporative processes are an important influence on intra-event isotope variations (e.g. Moerman, et al. 2013). To further resolve the processes driving this variability, rainwater isotopes from Gunung Mulu National Park in northern Borneo (4°N, 115 °E) were analyzed at one-minute intervals across nine rain events in 2012. To assess the influence of large-scale, remote fractionation processes versus those that act locally, our intra-event time series was compared to daily-resolved isotope records over the same time interval. We quantify a large range of water isotopic compositions over the sampling period (-13.1‰ to 0.2‰ in δ¹⁸O and -88.3‰ to -1.2‰ in δD). There is appreciable evidence for evaporative enrichment at our site, with δ¹⁸O vs. δD slopes significantly less than eight – the slope of the Global Meteoric Water Line. Large differences in the shape of the intra-event profile, ranging from monotonically increasing to “V-shaped” (e.g. Celle-Jeanton, et al. 2004) to monotonically decreasing isotopic values indicate that different precipitation regimes have a profound impact on water isotope evolution through a precipitation event. As such, we use a suite of meteorological data including in-situ observations, satellite imagery, model reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF), and NOAA HYSPLIT water vapor back-trajectories to provide an interpretive framework for the observed intra-event isotopic variability. Our study demonstrates the utility of investigating the relationships between small-scale, local rainwater isotopic compositions and large-scale regional weather and climate patterns for resolving key dynamical processes driving tropical convection, in both the present and the past.