Differential diatom dissolution in Late Quaternary sediments from Lake Manyara, Tanzania: an experimental approach

Abstract

Diatom dissolution in saline lakes represents an important obstacle to the quantitative reconstruction of water chemistry and climate from lake sediment archives. This problem is here approached experimentally by artificially dissolving diatom-bearing core sediment from Lake Manyara, Tanzania. Manyara holds one of the longest continuous palaeolimnological records from tropical Africa although its interpretation is based on a fragmentary diatom record due to frustule dissolution. These experiments have revealed clear changes in assemblage composition as dissolution operated differentially with respect to diatom taxa. Differential dissolution has considerable impact on the water chemistry estimates derived from transfer functions. Taphonomy, rather than environmental change, may have been responsible for minor fluctuations in the diatom assemblages from Manyara, although major palaeohydrological changes during the Late Pleistocene and Early Holocene can be identified. Particularly well represented by MANE-87 is a period of intermediate lake level between 27 500 and 23 000 ~4C yr BP which has regional palaeohydrological significance.

Diatomaceous sediments and environmental change in the Pleistocene Olorgesailie Formation, southern Kenya Rift Valley

“…However, their accumulation within sediments is also the result of a variety of processes that control deposition and preservation (Owen, 2002). Differential dissolution can, for example, cause significant changes in assemblage compositions (Schrader, 1971;Barker, 1992;Legesse et al, 2002). This may take place within the water column during sinking (especially in very deep lakes), or after burial where alkaline pore waters occur.…”

“…These studies have largely been based on data from cores recovered from late Pleistocene to Holocene sediments in modern lakes (Richardson and Richardson, 1972;Barker, 1992;Johnson, 1996;Stager et al, 1997;Telford et al, 1999;Chalié and Gasse, 2002;Johnson et al, 2002;Legesse et al, 2002;Russell et al, 2003;Cohen et al, 2007), and to a lesser extent within wetland areas Driese et al, 2004;. Miocene to Pleistocene stratigraphic records have also been developed from lacustrine rocks exposed in outcrops as a result of faulting and/or erosion, although the latter studies tend to reconstruct environmental conditions at discrete intervals rather than attempt to develop detailed stratigraphies through temporally long sequences (Gasse, 1974;Owen, 1981;Owen and Renaut, 1986;Renaut et al, 2000;Trauth et al, 2003).…”

Experimental diatom dissolution and the quantification of microfossil preservation in sediments

“…As assemblages will be biased towards robust taxa as they become progressively more dissolved, this will have a disproportionate effect on composition-based inferences in unpredictable ways (Barker, 1992). Even with significant diatom dissolution (F values > 0.5), the salinity model performs well at this site ( Figure 5), but in more dissolved samples, inferred values lose accuracy, even at lower salinity in the early 20th century that are well modeled in upper parts of the core.…”

“…The proportional rate of population decline is greater than BiSi dissolved in the earlier stages (cf. Barker, 1992), which may reflect losses of both numerically important, but weakly silicified, taxa and partly dissolved valves. Dissolution indices show little change in initially well-preserved assemblages until between 20 and 50% of valves are lost, in agreement with other experimental observations (Johnson 1974;Adelseck 1978).…”

“…For example, McIntyre and McIntyre (1971) observed a sequence of degradation for several heterococcoliths from deep-sea core tops and experiments, and ranked the taxa according to physical resistance to destruction. More recently, Battarbee (1988), Barker (1992), Flower (1993), and Barker et al (1994) have demonstrated steps in the dissolution process for several diatom taxa and used these stages to construct dissolution indices (Barker et al 1994;Flower and Likhoshway, 1993), based on changes in diatom morphology caused by dissolution. This paper evaluates two approaches for assessing diatom preservation in athalassic lakes using a series of laboratory dissolution experiments on fresh diatoms from subsaline to saline lakes in the Northern Great Plains of North America (Fritz et al, 1993).…”