Pigment Analysis of Short Cores from the Central Ethiopian Rift Valley Lakes

Abstract

Pigment analysis (Chlorophyll Derivatives — CD, and Total Carotenoids — TC) from surface and core sediments of three lakes: Langano, Abijata and Shalla in the Central Ethiopian Rift Valley are presented. The results show that pigment concentration is very low in modern sediments with CD generally higher than TC. This is in accordance with the present low productivity of the lakes. Within laminated portions of the cores, collected from Lakes Langano and Shalla, the concentration of pigments is as much as 5 to 40 times higher than in the modern sediments. This is interpreted as resulting from persistent lake stratification due to higher lake levels. This allowed the conservation of both laminae and pigments in a reducing environment. The fact that CD and TC values increased contemporaneously could also imply a high terrestrial organic matter input into the lakes as well as a high nutrient supply, which might have led to enhanced lake productivity. This was more so when pigment concentration was the highest in cores from Shalla and Langano at around 2000 yr BP. The above interpretation is supported by data from lake level variations showing that there was a rising trend at around 2000 yr BP with inference of humid conditions coming from pollen and other proxies in the study area and in the region. Pigment concentration declined after about 1000 YR BP when the CD/TC ratio became high. This could result from a fall in lake productivity shown by a low TC value and a poor conservation condition shown by a more homogeneous sediment.