Holocene evolution of the crater lake at Malha, Northwest Sudan

Abstract

A reconstruction of the hydrological and environmental evolution of the crater lake at Malha (Northern Darfur, Sudan) resulted from the mineralogical and biological study of a 9.21 m section of lake sediments, representing an uninterrupted sequence of lacustrine deposition since 8 290 14C years BP.Important changes in water supply and conditions of sedimentation are reflected in the nature of the sediments and the morphology and stratigraphical distribution of various salt minerals. Additional information on lake circulation patterns and salinity conditions are obtained from associated benthic paleocommunities, represented by ostracods and dipterid larvae. Combining both lines of evidence, the studied sequence can be divided in six distinct sections, which correspond to six successive periods in the lake's Holocene history. The first three periods, generally characterized by high lake levels, represent three generations of a meromictic lake, two of which have ended with a complete desiccation of the lake basin. Meromixis was stable during Period I, due to wind shelter and pronounced density stratification. In the course of Periods II and III stratification was repeatedly interrupted. During Period II, the disruptions were accompanied by important water budget fluctuations; a superimposed gradual decrease in net water supply eventually resulted in holomictic conditions terminating this period. Evidence of turbulence periodically affecting profundal waters is recorded in the sediments of Period III, suggesting that disruptions of stratification were now initiated by very strong winds. Between Period I and Period III, the littoral mixolimnion gradually evolved from near fresh to mesosaline. In Periods IV to VI, lake level was intermediate to low. The lake was holomictic for most of the time and meso-to hypersaline; during Period V, it repeatedly shrunk to a shallow brine pool.The Holocene evolution of Malha Crater Lake illustrates the progressive increase in aridity over most of North Africa following a well-established, early-to mid-Holocene major humid episode. The uninterrupted sedimentary sequence lends itself for detailed reconstruction of Holocene climatic evolution in arid Northeast Africa, a region where records of continuous lacustrine deposition are extremely scarce. As the chronology of critical events in the lake's history remains as yet unsupported by radiocarbon dates, correlation with other Holocene sequences in the eastern Sahara is highly tentative at this point.

Midge-and diatom-based palaeosalinity reconstructions for Mahoney Lake, Okanagan Valley, British Columbia, Canada

“…Diatoms have frequently been used as palaeosalinity indicators, and several quantitative models now exist for reconstructing past salinities from their remains (e.g., Cumming and Smol, 1993;Fritz et al 1991Fritz et al , 1993Gasse et al, 1995;Wilson et al, 1994Wilson et al, , 1996. In contrast, aquatic midges (principally chironomids) have rarely been used as palaeosalinity indicators (Paterson and Walker, 1974;Mees et al, 1991;Verschuren, 1994), but a model developed by Walker et al (1995) for British Columbia has recently made it possible to quantitatively reconstruct salinities from assemblages of their larval remains.…”

Palaeoenvironmental implications of evaporative gaylussite crystals from Lonar Lake, central India

“…Investigations on Lonar sediment cores revealed abundant gaylussite crystals (Na 2 CO 3 · CaCO 3 .5H 2 O) in some sections -the first such discovery from the Indian subcontinent. The primary gaylussite mineral is mostly precipitated by the gradual evaporative concentration of the brine (Hardie and Eugster, 1970;Eugster and Hardie, 1978;Mees et al, 1991) at the sediment-water interface (Bischoff et al, 1991) in contrast to calcite and aragonite that form in surface waters (McConnaughey et al, 1994;Leng and Marshall, 2004;Reddy and Hoch, 2011). Gaylussite usually succeeds the precipitation of calcite and is the first sodium carbonate mineral to be precipitated when saline, alkaline waters are concentrated by evaporation (Rankama and Sahama, 1964;Renaut et al, 1986).…”

“…Due to its rare occurrence in nature (e.g. Eugster and Hardie, 1978;Renaut et al, 1986;Bischoff et al, 1991;Mees et al, 1991;Partridge et al, 1993), only one study (Mees et al, 1998) has as yet investigated the potential of isotope data from gaylussite for palaeoclimate reconstruction.…”

Midges: Chironomidae and Related Diptera

“…Verschuren and co-workers have pioneered similar research in east Africa (Mees et al, 1991;Verschuren, 1994Verschuren, , 1997Verschuren et al, 1999Verschuren et al, , 2000aVerschuren et al, , 2000bDe Deyne, 1999). Surface samples collected from diverse habitats have clearly demonstrated the importance of salinity, water depth and specific benthic habitats to the midge fauna of Lake Naivasha, Kenya (De Deyne, 1999).…”