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

Information about climatic conditions during plant growth is preserved by t he oxygen-isotope composition of biogenic silica (phytoliths) deposited in grasses. The oxygen-isotope composition of phytolith silica is dependent on soil-water delta(18)O values, relative humidity and evapotranspiration, an d temperature during plant growth. Phytolith and plant-water delta(18)O val ues for C3 (A. breviligulata) and C4 (C. longifolia) grasses from natural a nd greenhouse sites in southwestern Ontario were used to compare the isotop ic fractionation between biogenic silica and water in various parts of thes e living plants. For non or weakly transpiring tissues (rhizomes, stems, sh eaths) in both grass species, the Delta(18)O(silica-plant water) remained c onstant at similar to 34 parts per thousand, and the delta(18)O and delta D values of plant water collected from pre-dawn and mid-day samplings showed little variation. These plant waters were only slightly enriched in O-18 a nd D relative to water provided to the grasses. Isotopic temperatures calcu lated from the silica and plant-water isotopic data matched measured growin g temperatures for the region. By comparison, the upper leaf water was extr emely enriched in oxygen-18 and deuterium at maximum rates of transpiration relative to water from non-transpiring tissues, as were the calculated, st eady-state values for leaf-water delta(18)O and delta D. Silica produced in the transpiring tissues (leaf, inflorescence) has higher delta(18)O values than silica from non-transpiring tissues, but the enrichment is modest com pared to upper leaf water under mid-day conditions. Leaf phytoliths have fo rmed from plant water typical of average conditions in the lower leaf, wher e the extreme O-18-enrichment is not encountered. C. longifolia was also co llected from Alberta and Nebraska, where growing conditions an different fr om southwestern Ontario. Phytoliths at all three sites have a similar patte rn of delta(18)O values within the plants, but the isotopic separation betw een leaf and stem silica increases from 4 to 8 parts per thousand as averag e relative humidity decreases. The difference between actual growing temper ature and that calculated using measured delta(18)O values for stem silica and local meteoric water became progressively larger as relative humidity d ecreased, likely because of evaporative O-18-enrichment of soil water. Such effects are most pronounced in arid environments and pertinent in grasslan ds where much of the active rooting zone can be situated at the shallower d epths most affected by the O-18-enrichment of soil water. Copyright (C) 200 0 Elsevier Science Ltd.