Résumé
Most terrestrial plants producing large amounts of organic matter in the East African Rift follow the Calvin (C3) photosynthetic pathway. Their end products have
δ
13C values of ca.
−27 ± 2‰ (vs. PDB). On the contrary, most Cyperaceae (notably
Cyperus papyrus and
C. latifolius) are characterized by higher
13C contents
°
13
C = −10.5 ± 1‰
) in relation to their Hatch and Slack (C4) photosynthetic cycle. In consequence,
δ
13C values in total organic matter (TOM) from peat bog or lake cores essentially responded to the proportion of detritus from C4-Cyperaceae. Immediate evidence of the development or disappearance of Cyperaceae around lake margins or in peat bogs can be found in pollen assemblages. Lag times between pollen signals and correlative °
13C shifts in TOM from cores are therefore indicative of the residence time of organic matter prior to its sedimentation. Delayed sedimentation of TOM will result in
14C anomalies which depend on several parameters, most of them being site specific as shown by examples from a peat bog in Burundi and from southern Lake Tanganyika. An independent assessment of the chronology by high resolution paleomagnetic correlations indicates a ca. 1.5 ka apparent
14C age of TOM in Lake Tanganyika at the Pleistocene-Holocene transition.