Sources and accumulation rates of organic carbon in an equatorial peat bog (Burundi, East Africa) during the Holocene: carbon isotope constraints

Abstract

The relationship between organic carbon accumulation rates and 13C/ 12C ratios of total organic carbon (TOC) was investigated in an highland peat bog core (Ru-3) from Equatorial Africa. This core yielded a sequence spanning the last 14 kyr and was analysed with a 100–300 yr resolution for TOC-δ 13C values. The Holocene section shows contrasted TOC accumulation regimes and TOC δ 13C varying between −28.5 and −19.5‰ with a few very short `isotopic excursions' (dated at ca. 9.3, 7.5, 4.2 ka B.P.). The organic carbon accumulation rates range from 2 to 20 mg C cm −2 yr −1. They increase when TOC becomes more depleted in 13C, notably between 12 and 9.8 ka B.P., 8.5 and 7.8 ka B.P. and after 1.6 ka B.P. Periods of restricted carbon storage correspond to heavier TOC accumulation at 9.3, and between 7.5 and 1.6 ka B.P. At the study site, the δ-variations can be related to variable C4-plant inputs, and possibly, to changes in the fractionation between CO 2 and the organic carbon in C3 vascular plants. The Ru-3 record indicates restricted carbon storage during the periods of increased contribution from C4 plants and/or of decreased fractionation between CO 2 and organic carbon in C3 plants. Changes in TOC-δ 13C values in core Ru-3 seem to match fluctuations of East Equatorial African lakes. High lake stands correspond to low δ 13C intervals and vice versa. This points to indirect climatic forcing of δ 13C changes in intertropical peats.