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

The relationship between organic carbon accumulation rates and C-13/C-12 ra tios 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-delta (13)C values. The Holocene section shows contrasted TOC accumulation regime s and TOC delta(13)C varying between -28.5 and -19.5 parts per thousand wit h a few very shea '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 C-13, 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 re stricted 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 delta-variations can be related to variable C4-plant inputs, and possibly, to changes in the fracti onation between CO2 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 CO2 a nd organic carbon in C3 plants. Changes in TOC-delta(13)C values in core Ru -3 seem to match fluctuations of East Equatorial African lakes. High lake s tands correspond to low delta(13)C intervals and vice versa. This points to indirect climatic forcing of delta(13)C changes in intertropical pears. (C ) 1999 Elsevier Science B.V. All rights reserved.