Oxygen isotopes in cellulose from modern and quaternary intertropical peatbogs: Implications for palaeohydrology

Cellulose delta(18)O-values were measured in modern peatbog plants, in recent peats and in mountain peatbog cores from Burundi in a study of the hydrologic control of the O-18/O-16 ratio in intertropical peat cellulose. Modern plants and waters were collected under various altitudes (800-2500 m a.s.l.) with mean annual relative humidity ranging from 0.72 to 0.82. The water delta(18)O-values (delta(in)) vary between -4 and -2.5 parts per thousand. The cellulose-water enrichment is of +25.3 +/- 0.5 parts per thousand for aquatic plants, of +24.6 parts per thousand for a Sphagnum sample, and much higher (+26.1 to +36.2 parts per thousand) for emergent plant species. The scatter in the cellulose delta(18)O-values of emergent plants collected in a peatbog is less than or equal to 5 parts per thousand. Cellulose delta(18)O-values of emergent plants tend to decrease with increasing elevation (-0.6 to -0.07 parts per thousand per 100 m) and relative humidity (-0.5 to -0.1 parts per thousand per %). Mean emergent plant delta(18)O-values are approximated within +/-0.6 parts per thousand by available cellulose delta(18)O models relating cellulose delta(18)O to the relative humidity (h) and the environmental water delta(18)O, except in a large swampy system. Feats were subsampled in core Ka-2 (Kashiru, Burundi, 3 degrees 28'S, 29 degrees 34'E, 890 cm long, 2240 m a.s.l. altitude) and, for control, in the topmost 50 cm of core Ku-2 (Kuruyangwe, 3 degrees 35'S, 29 degrees 41'E, 2020 m a.s.l. altitude). Similar delta(18)O-values (+26 to +28.5 parts per thousand) were found in the topmost 50 cm of both cores. They compare well with the mean delta(18)O for modern emergent plants at Kashiru. AMS-C-14 dating showed that core Ka-2 spans the last 30 ka. Celluloses are mainly derived from emergent plants and yield delta(18)O-values ranging from +19 to +29 parts per thousand with the highest values between 2.1 and 0 ka and the lowest values (+19 to +23.5 parts per thousand) between 6 and 4.5 ka. For the period between 6 and 4.5 ka, the models yield a decrease in delta(in) of -4 to -2.5 parts per thousand relative to present, for an h increase of 6% to 11%. Before 11.7 ka, the models yield a delta(in)-value close to the present value, for an h increase of 6%.