Reconstructing precipitation in the tropical South Pacific from dinosterol H-2/H-1 ratios in lake sediment

The South Pacific Convergence Zone (SPCZ) is the Southern Hemisphere's largest precipitation feature supplying freshwater to 11 million people. Despite its significance, little is known about the location and intensity of SPCZ precipitation prior to instrumental records, hindering attempts to predict precipitation changes in a warming world. Here we use sedimentary molecular fossils to establish a tool for extending the historical record of precipitation. Freshwater lake sediments and water samples were collected from 30 lakes that span a 4.6 mm d(-1)- range in precipitation rates from the Global Precipitation Climatology Project (GPCP). delta H-2(lakewater) values from 29 lakes ranged from -29 to +23 parts per thousand and were inversely correlated (r = -0.51, p < 0.001) with precipitation rates, likely due to the combination of the amount of precipitation plus evaporation. .delta H-2(lakewater) values of the dinoflagellate sterol dinosterol in surficial sediments from 21 lakes ranged from -316 parts per thousand in the Solomon Islands to -245 parts per thousand in French Polynesia. These delta H-2(dimosterol) values were significantly correlated (r = 0.71, p < 0.001) with delta H-2(lakewater) and inversely correlated (r = -0.77, p < 0.001) with mean annual precipitation rates with a sensitivity of -12.1 +/- 2.6 parts per thousand (mm d(-1))(-1). Fractionation between dinosterol and lake water (epsilon(dinosterol/lakewater)) decreased at the driest lake sites (r = - 0.70, p < 0.001). The empirical relationship bet ween delta H-2(dmosterol) and GPCP rainfall, although indirect, provides a means of quantitatively reconstructing past precipitation in the SPCZ region with an uncertainty of less than 3.1 mm which compares favorably to the 1.5 mm d(-1) uncertainty for the satellite-gauge based GPCP precipitation data.