Stable isotopic compositions of silica phytoliths and plant water in grasses: Implications for the study of paleoclimate

Information regarding climatic conditions during plant growth is preserved by the oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The O-isotope composition of phytoliths is dependent on soil-water δ 18O values, relative humidity, evapotranspiration and temperature during plant growth. The compositions of plant water from several grass species at Pinery Provincial Park were examined to determine the variability in δ 18O and δD values from which biogenic silica may precipitate. Here, stem water was unfractionated from soil-water δ18O values. Hence, the δ18O values of stem silica can provide a proxy of the soil water available for root uptake during the growing season. Phytolith and plant-water δ18O values for C3 ( Ammophila breviligulata) and C4 (Calamovilfa longifolia) grasses from southwestern Ontario were also used to compare the isotopic fractionation between biogenic silica and water in various plant tissues. Temperatures calculated from δ18O_silica and δ18O _{plant water} values in non-transpiring tissues matched measured growing temperatures. The isotopic heterogeneity of water within individual leaves and the diurnal variations in δ18O_{leaf water} values that result from transpiration processes complicate interpretations. Water within the sheath, and lower and upper leaf tissues experiences continual evaporation, hence becoming progressively enriched in 18O and D as it moves towards the tip of the leaf. However, the water from which leaf silica precipitates has not acquired the extreme 18O-enrichment predicted using steady-state models, or measured for mid-day leaf water. There may be a secluded water fraction within the leaf, which experiences smaller diurnal variations in isotopic composition than leaf water at sites of evaporation.; C. longifolia was collected across North America to investigate the effects of climate and soil-water δ18O values on the oxygen-isotope composition of phytoliths. Phytoliths at all sites have a similar pattern of δ18O values within an individual plant, but the isotopic separation between leaf and stem silica increases as relative humidity decreases. Once the phytoliths are transferred to the soil, the temperature and soil-water δ18O signals carried by the phytoliths from stems, sheaths and rhizomes may be masked by the δ18O values of phytoliths from leaves and inflorescence, which are variably further enriched, depending on relative humidity. The difference between growing temperature and that calculated using measured δ 18O values for stem silica and local meteoric water became larger as relative humidity decreased, likely because of evaporative 18O-enrichment of soil water.