Mg/Ca ratios in freshwater microbial carbonates: Thermodynamic, kinetic and vital effects

The ratio of magnesium to calcium (Mg/Ca) in carbonate minerals in an abiotic setting is conventionally assumed to be predominantly controlled by (Mg/Ca)solution and a temperature dependant partition coefficient. This temperature dependence suggests that both marine (e.g. foraminiferal calcite and corals) and freshwater (e.g. speleothems and surface freshwater deposits, “tufas”) carbonate deposits may be important archives of palaeotemperature data. However, there is considerable uncertainty in all these settings. In surface freshwater deposits this uncertainty is focussed on the influence of microbial biofilms. Biogenic or “vital” effects may arise from microbial metabolic activity and/or the presence of extracellular polymeric substances (EPS). This study addresses this key question for the first time, via a series of unique through-flow microcosm and agitated flask experiments where freshwater calcite was precipitated under controlled conditions. These experiments reveal there is no strong relationship between (Mg/Ca)calcite and temperature, so the assumption of thermodynamic fractionation is not viable. However, there is a pronounced influence on (Mg/Ca)calcite from precipitation rate, so that rapidly forming precipitates develop with very low magnesium content indicating kinetic control on fractionation. Calcite precipitation rate in these experiments (where the solution is only moderately supersaturated) is controlled by biofilm growth rate, but occurs even when light is excluded indicating that photosynthetic influences are not critical. Our results thus suggest the apparent kinetic fractionation arises from the electrochemical activity of EPS molecules, and are therefore likely to occur wherever these molecules occur, including stromatolites, soil and lake carbonates and (via colloidal EPS) speleothems

Lubricating Bacteria Model for Branching growth of Bacterial Colonies

Various bacterial strains (e.g. strains belonging to the genera Bacillus, Paenibacillus, Serratia and Salmonella) exhibit colonial branching patterns during growth on poor semi-solid substrates. These patterns reflect the bacterial cooperative self-organization. Central part of the cooperation is the collective formation of lubricant on top of the agar which enables the bacteria to swim. Hence it provides the colony means to advance towards the food. One method of modeling the colonial development is via coupled reaction-diffusion equations which describe the time evolution of the bacterial density and the concentrations of the relevant chemical fields. This idea has been pursued by a number of groups. Here we present an additional model which specifically includes an evolution equation for the lubricant excreted by the bacteria. We show that when the diffusion of the fluid is governed by nonlinear diffusion coefficient branching patterns evolves. We study the effect of the rates of emission and decomposition of the lubricant fluid on the observed patterns. The results are compared with experimental observations. We also include fields of chemotactic agents and food chemotaxis and conclude that these features are needed in order to explain the observations.Comment: 1 latex file, 16 jpeg files, submitted to Phys. Rev.

Multispectral remote sensing of nearshore suspended sediments: a pilot study

The till that comprises the coastal cliffs at Holderness is being eroded and transported southwards by waves and currents within the nearshore zone. As a first step towards estimating the magnitude of this movement, airborne multispectral scanner data were collected. The spectral radiance ( Lλ ) recorded by this sensor was positively correlated with the suspended sediment concentration ( SSC) at the 95 percent confidence level. However, when the relationships between SSC and Lλ were inverted for the estimation of SSC, errors of the order of 343-494 percent were observed. It is recommended that for future studies there is a need to reduce the errors consequent upon the areal, vertical and temporal variability in SSC, the problems of inversion and the presence of white-caps