Late Quaternary Biomass Changes from 13C Measurements in a Highland Peatbog from Equatorial Africa (Burundi)

Abstract

Stable carbon isotope ratios of total organic matter were measured in two cores collected from the Kashiru peatbog in Burundi, Equatorial Africa. The record, which spans at least the last 40,000 yr, documents the C₃-C₄ biomass balance in the organic sediment. Among the major modern peat formers, most plants are C₃ species and are characterized by δ¹³C values of -25.5 ± 2.3% (vs PDB). The C₄ plants, which are characterized by higher δ¹³C values (-11.3 ± 0.7%) belong to the Gramineae (Miscanthidium sp.) and Cyperaceae families (Cyperus latifolius, C. papyrus, Pycreus nigricans). In the fossil record, δ¹³C values of total organic matter vary between -28 and -15% in response to the relative fluxes of C₃ and C₄ plants. Before 30,000 yr B.P., low δ¹³C values (-23.5 ± 1.1%) match high arboreal pollen contents. From 30,000 to 15,000 yr B.P., higher δ¹³C values (-17.6 ± 1.1%) correspond to a significant increase in percentages of grass pollen. During this episode, a short and sharp shift toward lighter carbon isotopic compositions at 21,000 yr B.P. is synchronous with higher input of arboreal pollen. From 15,000 to 12,000 yr B.P., the ¹³C content decreases (δ¹³C = -22.9 ± 1.4%). This shift, which cannot be explained by an increase in the arboreal vegetation, could be explained by the spreading of C₃ Gramineae or C₃ Cyperaceae. The interval from 12,000 to 7000 yr B.P. is poorly documented in these cores due to much lower organic matter accumulation. Low δ¹³C values (δ¹³C = -25.2 ± 1.3%) are observed from 7000 to 5000 yr B.P., when the pollen data show development of C₃ mountain forest. The Late Holocene is characterized by a mixed C₃-C₄ organic matter accumulation (δ¹³C = -20.9 ± 1.6%). This study depicts a change in the dominant photosynthetic pathway among the herbaceous components, notably at the glacial-interglacial transition, when C₃ plants were favored by increased water supply and/or higher atmospheric CO₂ concentration.