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

Growing spherulitic calcite grains in saline, hyperalkaline lakes: experimental evaluation of the effects of Mg-clays and organic acids

The origin of spherical-radial calcite bodies – spherulites – in sublacustrine, hyperalkaline and saline systems is unclear, and therefore their palaeoenvironmental significance as allochems is disputed. Here, we experimentally investigate two hypotheses concerning the origin of spherulites. The first is that spherulites precipitate from solutions super-saturated with respect to magnesium-silicate clays, such as stevensite. The second is that spherulite precipitation happens in the presence of dissolved, organic acid molecules. In both cases, experiments were performed under sterile conditions using large batches of a synthetic and cell-free solution replicating waters found in hyperalkaline, saline lakes (such as Mono Lake, California). Our experimental results show that a highly alkaline and highly saline solution supersaturated with respect to calcite (control solution) will precipitate euhedral to subhedral rhombic and trigonal bladed calcite crystals. The same solution supersaturated with respect to stevensite precipitates sheet-like stevensite crystals rather than a gel, and calcite precipitation is reduced by ~ 50% compared to the control solution, producing a mixture of patchy prismatic subhedral to euhedral, and minor needle-like, calcite crystals. Enhanced magnesium concentration in solution is the likely the cause of decreased volumes of calcite precipitation, as this raised equilibrium ion activity ratio in the solution. On the other hand, when alginic acid was present then the result was widespread development of micron-size calcium carbonate spherulite bodies. With further growth time, but falling supersaturation, these spherules fused into botryoidal-topped crusts made of micron-size fibro-radial calcite crystals. We conclude that the simplest tested mechanism to deposit significant spherical-radial calcite bodies is to begin with a strongly supersaturated solution that contains specific but environmentally-common organic acids. Furthermore, we found that this morphology is not a universal consequence of having organic acids dissolved in the solution, but rather spherulite development requires specific binding behaviour. Finally, we found that the location of calcite precipitation was altered from the air:water interface to the surface of the glassware when organic acids were present, implying that attached calcite precipitates reflect precipitation via metal–organic intermediaries, rather than direct forcing via gas exchange

Pollen and mollusc records for environmental change in central Spain during the mid- and late Holocene

Holocene85605-612HOLO

Petrology of a barrage tufa system (Pleistocene to recent) in the Ruidera Lakes Natural Park (Central Spain)

Publicación derivada del V Congreso Geológico de España (Alicante, 10-14 julio 2000). M.A.G del C, S.O. and J.A.G.M. were supported by Spanish CICYT research project grant CL 195-1853. H.M.R was assisted in the sedimentological study by NERC grant GR9/421 «Sedimentology of Fluvial Tufas».The Ruidera Lakes Natural Park, Central Spain, contains a well-exposed Pleistocene to Recent freshwater carbonate succession in a fluvio-lacustrine setting. Carbonate precipitation continues today and is manifested in active phytoherm barrage constructions, marginal lacustrine stromatolitic terraces, lacustrine lime muds, and sand-grade detrital tufa. Phytoherm barrage deposits consist of coarse sparite fringe in vigorous flow areas associated with phytoherm framestones, and alternating sparite and micrite fringes, which are most common in areas with lower velocity flow. Holocene lake margin deposits consist of laterally growing mammilate stromatolites. These are developed as pinnacle stromatolites at the foot of overhangs associated with the lake margins. Pre-Holocene marginal lacustrine terraces have no associated stromatolite rims but contain abundant oncoidal deposits close to their base. In some ephemeral channels and pools located on the active phytoherm barrages stromatolitic pavements are developed which are comprised of elongate domes composed of fibrous calcite crystals encrusting cyanobacteriaI filaments. The erosion of phytoherms and continous rain of fragments from pinnacles, pavement and marginal stromatolites in the active system provides abundant detrital tufa. These allochthonous detrital tufas are volumetrically the most important carbonate deposits in Ruidera Lakes and dominate the lower reaches of the Upper Guadiana River valley. The lake depocentres accumulate carbonate muds, which are locally rich in Chara.Comisión Interministerial de Ciencia y Tecnología (España)Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu