Sediment reflectance spectroscopy as a paleo-hydrological proxy in East Africa

Abstract

This study presents the merit of visible-spectrum scanning reflectance spectroscopy (VIS-RS) as a rapid technique for determining the concentration of chlorophyll a (Chl a) and its derivatives in lake sediments. In a 25,000-yr sediment record from Lake Challa, in East Africa, we found that VIS-RS values correlate with Chl a concentrations measured by High Performance/Pressure Liquid Chromatography. Observed variation in sedimentary values of RABD660;670 appears linked to proxies of climatic moisture balance as well as to a seismically-derived lake-level reconstruction over this period, indicating that VIS-RS can be used as a proxy for long-term paleo-hydrological change. Changes in the reflectance of Lake Challa sediments are likely related to differences in the preservation/degradation of algal pigments in response to changes in water-column depth and the stability of stratification. The connecting mechanism may be that during lake lowstands, more frequent injection of oxygen to (near-) bottom waters enhance the breakdown of labile organic compounds before permanent burial. We suggest that fast and inexpensive VIS-RS scanning, although less specific than HPLC in quantifying individual pigments, provides accurate data on down-core variations in the concentration of Chl a and its derivatives in lake sediments, hence allows to reconstruct long-term changes in the hydrology of climate-sensitive lakes. The main prerequisite for its successful application is that temporal variation in lake hydrology over the period of interest has not appreciably affected sedimentation dynamics at the core site, since major changes in sediment texture and organic content are likely to create confounding effects in the VIS-RS signature.