Seasonal and spatial variability of modeled leaf water δD values in the Himalaya

Abstract:

The hydrogen isotope composition (δD) of leaf wax n-alkanes in sedimentary archives is commonly used as a qualitative paleohydrology proxy due to its relationship to δD values of water taken up by plants. In addition to source water δD values the climatic and plant physiologically-dependent leaf water D-enrichment is a key process controlling the leaf wax isotopic composition. As leaf water δD values are strongly variable on diurnal to seasonal timescales, it is not possible to capture the full temporal and spatial leaf water variability in the Himalaya by sampling. Modeled δD values in turn can be used to understand the temporal and spatial distribution of leaf water isotopes and enable us to better understand the influence of seasonal variation in source water δD and D/H fractionation processes on the plant leaf water. Moreover seasonally resolved modeled leaf water δD values can be used to understand the seasonal origin of leaf wax n-alkane δD values.

The Himalaya provides an excellent natural laboratory to study spatial and temporal leaf water isotope patterns, because it is characterized by large gradients in altitude, precipitation, vegetation type, and temperature. The Indian Summer Monsoon transports moisture from the Bay of Bengal along the Southern Himalayan Front causing strong orographic rainfall with additional winter precipitation derived through the Westerlies.

To capture the effect of this spatially and temporally varying hydrology along and across the Himalayan orogeny on leaf water isotope enrichment, we utilized calibrated satellite data (TRMM, MODIS) to drive a steady-state two-pool Craig-Gordon leaf water model (spatial resolution: 5x5 km² and 8-day temporal resolution). Our model yields more D-depleted leaf water throughout the year towards the Tibetan Plateau reflecting the orographic effect. Compared to the wet summer season winter leaf waters are generally more D-enriched with a less pronounced isotopic lapse rate across the Himalayan range. These different seasonal responses stress the importance of understanding the seasonal origin of the leaf wax δD signal for paleoclimatic studies as well as paleoalitimetry.