Precipitation Of Ca Research Articles

Lagged atmospheric circulation response in the Black Sea region to Greenland Interstadial 10

Northern Hemispheric high-latitude climate variations during the last glacial are expected to propagate globally in a complex way. Investigating the evolution of these variations requires a precise synchronization of the considered environmental archives. Aligning the globally common production rate variations of the cosmogenic radionuclide 10Be in different archives provides a tool for such synchronizations. Here, we present a 10Be record at <40-y resolution along with subdecadal proxy records from one Black Sea sediment core around Greenland Interstadial 10 (GI-10) ∼41 ka BP and the Laschamp geomagnetic excursion. We synchronized our 10Be record to that from Greenland ice cores based on its globally common production rate variations. The synchronized environmental proxy records reveal a bipartite climate response in the Black Sea region at the onset of GI-10. First, in phase with Greenland warming, reduced sedimentary coastal ice rafted detritus contents indicate less severe winters. Second, and with a lag of 190 (± 44) y, an increase in the detrital K/Ti ratio and authigenic Ca precipitation point to enhanced regional precipitation and warmer lake surface temperatures. We explain the lagged climatic response by a shift in the dominant mode of atmospheric circulation, likely connected with a time-transgressive adjustment of the regional thermal ocean interior to interstadial conditions.

Mitigation of soil salinization and alkalization by bacterium-induced inhibition of evaporation and salt crystallization

Soil salinization and alkalization is one of the most devastating environmental problems, threatening the sustainable development of agriculture. Bio-amelioration using microorganisms such as bacteria is a promising method for the remediation of calcareous sodic and saline-sodic soil due to its high efficiency, low cost and environmental-friendly characteristics. In the present study, a salt resistant bacterium, Bacillus subtilis BSN-1, was isolated from arid region in Xinjiang, China, and its effects on salt crystallization during evaporation crystallization of saline-alkali soil solution were examined. It was found that the fermentation products of B. subtilis BSN-1, such as glutamic acid, significantly lowered the pH of saline soil solution because of the ionization of carboxyl. The complexation between Ca2+ and fermentation products inhibited the precipitation of Ca–P compounds as well, since the binding sites supplied for Ca2+ is one or two orders of magnitude than that for HPO42−. Moreover, the increased content of active phosphate is attributed to the chelation and adsorption exerted through carboxyl and amide bonds. These findings demonstrated that Bacillus subtilis BSN-1 suppressed the crystallization of phosphate and therefor increased the content of active phosphate, which may provide a promising solution for amendment and remediation of saline-alkali soil.

Removal of Ca2+, Mg2+, Ba2+ and Sr2+ from shale gas flowback water from the Sichuan Basin in China by chemical softening under the guidance of OLI Stream Analyzer: precipitation behaviors and optimization study

The disposal of flowback water is recognized as a key issue for the sustainable shale gas development and discharge after reasonable treatment is considered as a feasible pathway. One of the challenges during treatment is the severe mineral scaling potential in reverse osmosis desalination, especially with high amounts of Ca2+, Mg2+, Ba2+ and Sr2+ in flowback water. In this study, precipitation behaviors of Ca2+, Mg2+, Ba2+ and Sr2+ during traditional chemical softening was evaluated so as to achieve optimal chemical dosage. Both jar tests and OLI Stream Analyzer simulation revealed that the main precipitates were CaCO3, SrCO3 and BaSO4 during Na2CO3 addition, and Ba2+ could not be removed efficiently by Na2CO3 unless a high dosage was applied since Ba2+ would react after the precipitation of Ca2+ and Sr2+. Reverse Osmosis System Analysis simulation indicated that Ba2+ was a concern because Ba2+ would form tenacious BaSO4 scale on the reverse osmosis membranes. Finally, the Na2SO4-NaOH-Na2CO3 process was proposed for chemical softening as it has a high removal efficiency and low chemical cost. Overall, this study presents an effective chemical softening method and OLI Stream Analyzer could serve as a reliable tool for the calculation, which would finally improve the design and operation of shale gas flowback water treatment.

Introduction of an interface layer on hydroxyapatite whisker/poly(L-lactide) composite and its contribution for improved bioactivity and mechanical properties

A hydroxyapatite whisker (w-HA) was synthesized via dissolution-precipitation by forming calcium-ethylene diamine tetra acetic acid (Ca-EDTA) complexing. The hydroxyapatite whisker was formed with precipitation of Ca2+ along the c-axis due to the space inhibition of Ca-EDTA complex during refluxing. The op-w-HA (oligomeric poly(lactic acid) modified w-HA), p-w-HA (poly(L-lactide) modified w-HA) and pc-w-HA (poly(L-lactide) and cyclodextrin modified w-HA) were obtained via the surface modification of w-HA. The particle size, surface charge and biocompatibility of theses modified w-HA particles were successfully adjusted. Among these materials, pc-w-HA exhibited nearly no toxicity, better adhesion to mesenchymal stem cells (MSCs) (5 times better than w-HA) and greater osteoinductivity among the obtained materials (40% of mineralized extracellular matrix higher than w-HA) due to better surface properties. Different kinds of powders (w-HA, p-w-HA and pc-w-HA) were blended with PLLA (poly(L-Lactide)) to form a composite material, respectively. The pc-w-HA/PLLA composite showed better mechanical properties (tensile strength of the pc-w-HA/PLLA composite was 22.3% higher than that of w-HA/PLLA), which could be attributed to mainly two factors including the structure preservation of w-HA bundles and pseudorotaxane linkage between PLA-cyclodextrin and PLLA. The MSCs adhesion of the pc-w-HA/PLLA composite was much better due to balanced hydrophilicity/hydrophobicity and surface roughness. This surface modification method could provide a new and effective strategy for the preparation of bioresorbable composite material with great bioactivity and mechanical property, which has great potential in the medical device industry.

Ceramic Materials in the Tricalcium Phosphate–Trimagnesium Phosphate System

We have synthesized powders and produced ceramic materials in the tricalcium phosphate–trimagnesium phosphate system with a degree of Mg2+ substitution for Ca2+ up to 40 wt %. It has been shown that increasing the percentage of magnesium in the materials reduces their sintering temperature from 1300 to 1100°C. We have determined the phase composition of the materials and examined their microstructure. The results demonstrate that the addition of Mg2+ ions reduces the precipitation of Ca2+ ions from the ceramics to model fluids and leads to a considerable increase in their three-point bend strength: to 95–120 MPa.

Pengaruh Penambahan NaOH dan Ca(OH)2 Terhadap Penurunan Kadar Logam Berat (Fe) di Kolong Tambang 23 Desa Kimhin Kecamatan Sungailiat

Tin mining activity causes a lot of opening pit known as kolong in Bangka Island. Ecologically, kolong has function as retention reservoir that contain harmful acid water. Tin mining ex-pit potentially causes environment effects especially the quality and quantity of water contained heavy metal. One of kolong in Tambang 23 Kimhin Village of Sungailiat subdistrict contain a quite high of heavy metal of Fe in amount 2.18 mg/L that tested from the result of testing of Health Official Laboratorium of Bangka Regency. Therefore, this research conducted to decrease metal grade in the water kolong. This research focused based on metal grade of Fe value. Analysis conducted by counting the decrease of percentage of Fe metal grade. The method used in lowering metal grade was precipitation method using NaOH and Ca(OH)2 solution. Metal grade analysis used spectrophotometer. The aim of this research was to determine the influence of solution concentration, precipitation time and solution influence of NaOH and Ca(OH)2 in lowering Fe grade. The result of research showed the influence of adding precipitation of NaOH and Ca(OH)2 with the precipitation time. The longer time of precitipation, the bigger percentage of lowering Fe metal. The biggest percentage of Fe metal grade was on concentration of NaOH 6% and Ca(OH)2 3% with the lowering percentage of 88.99% and 96.33% respectively. The optimum time to precipitate Fe metal was in 60 minute. Based on research result showed that precitipation of NaOH better than Ca(OH)2.

Using hydrogeochemical indicators to interpret groundwater flow and geochemical evolution of a freshwater lens on Majuro Atoll, Republic of the Marshall Islands

Groundwater flow and the geochemical evolution of a freshwater lens in an aquifer on a Pacific atoll were investigated by hydrogeochemical surveys. Sulfur hexafluoride measurements showed that deeper groundwater and groundwater at the periphery of the lens are older, consistent with a downward and outward groundwater flow scheme. This is the typical flow scheme on Pacific atolls where a Holocene–Pleistocene unconformity restricts the shape of the freshwater lens. Enrichment of Mg/Ca in the groundwater is another indicator of a longer residence time, because contact between the groundwater and the carbonate sediments composing the aquifer leads to the release of Mg from high-Mg calcite and the precipitation of Ca as low-Mg calcite. Groundwater quality was also affected by anthropogenic nitrogen loading and aboveground organic matter, which were altered by denitrification and sulfate reduction in the aquifer, especially in the older groundwater. The chemical composition of the groundwater in the center of the island, where saline water is up-coning, implies that freshwater recharge dilutes the older saline water, which as time passes will eventually be replaced by newly recharged freshwater.

C15–Al&lt;sub&gt;2&lt;/sub&gt;Ca Precipitation in a Mg–Al–Ca Alloy

The coarsening kinetics of a fine C15–Al2Ca Laves phase with a plate-like morphology precipitated within the primary α-Mg grains were investigated for Mg–5Al–1.5Ca alloy aged at 523 K. The Al2Ca precipitate coarsened as its coherence was retained in the aging time below 300 h, and a quantitative relationship was obtained between precipitate length (l) and aging time (t) as l ∝ t0.23. However, the α/C15 coherent interface changed into a semi-coherent interface by introduction of misfit dislocations on the planar surface of the precipitates in the aging time above 300 h, which resulted in the promotion of Al2Ca coarsening. The Al2Ca phase was assumed to precipitate through a nucleation and growth mechanism rather than spinodal decomposition, and its coarsening was explained using the terrace-ledge-kink mechanism. The result shows that the aspect ratio of the Al2Ca precipitates was predominantly determined by the aging temperature, and it decreased at higher aging temperatures.

Treatment of tannery unhairing wastewater using carbon dioxide and zinc cations for greenhouse gas capture, pollution removal and water recycling

The aim of this study is to treat the tannery unhairing wastewaters (WW) issued from an important leather tanning plant, which is a large environmental threat and a severe industrial issue. High levels of pH, chemical oxygen demand (COD) mainly as alkaline proteins, conductivity, suspended solids and sulfide are the main characteristics of these highly polluted wastewaters. This study is based on the coupling of CO2 neutralization with Zn2+ precipitation. The use of CO2 leads to the neutralization of the WW alkalinities, and the precipitation of Ca2+ and COD to CaCO3 and solid proteins respectively, while Zn2+ leads to the precipitation of sulfide as ZnS avoiding any H2S gas evolution. The originality of this paper comes from the simultaneous capture of greenhouse gas, the removal of WW pollution, and the recycling of treated water. The pollutants removal efficiency is evident and can be seen through the decrease of pH from 13 to 6.5, a decrease in the conductivity from 8000 to 2000 μS, and in COD from 20000 to 80000 to 3000 mg O2 L−1, as well as obtaining pristine final water for recycling. In addition to the simplicity and cost-effectiveness of these results, the kinetics of the CO2 and Zn2+ reactions is instantaneous, contrary to the conventional biological slow process and the catalytic sulfide oxidation reaction. CO2 and Zn2+ have never been studied as a treatment method for tannery WW. This will open promising perspectives for industrial applications.

A novel UASB-MFC dual sensors system for wastewater treatment: On-line sensor recovery and electrode cleaning in the long-term operation.

In this research, the UASB-MFC dual sensors system was established and treatment the brewery wastewater. The COD removal rate attain about 90% and the NH4+-N concentration less than 15mg/L, MFCs has a voltage range of 0.34-0.42V. Meanwhile, as the biosensor for coupling system, MFCs can be used to make simultaneous monitor COD and TVFA. The potential distribution can in-situ accelerate the reattachment of micro-organisms, which shorten the recovery time to 55% of the original. The long-term performance of MFCs were tested by electrochemical methods and found that the degradation of biosensors was mainly caused by the precipitation of Ca2+ and Mg2+ on the cathode surface and affected by concentration. More importantly, cleaning the electrode by an self-enhanced method without external assistance ECS (Electrodes Connection Switching) can improve the MFCs performance to 83.2 %-84.6%. Dual sensors system in UASB gives a novel possibility for UASB-MFC sensor self-sustaining in a long-term.

Analysis of uncontrolled phosphorus precipitation in anaerobic digesters under thermophilic and mesophilic conditions

ABSTRACT This study compares the operation of mesophilic and thermophilic anaerobic digestion of sewage sludge and their effects in uncontrolled phosphorus precipitation. The research has been carried out using a pilot plant consisting of two digesters of 1.6 m3 working volume, treating the mixed sludge of Alzira WWTP (Valencia, Spain). The digesters were operated in parallel, at different conditions: mesophilic (38 ± 2.0°C) and thermophilic (55 ± 2.5°C) temperatures and organic loading rates (OLR) ranging from 1.1 to 1.7 kg volatile solids (VS) m−3 d−1 and different hydraulic retention times (HRT) 20, 15 and 12 days. Uncontrolled precipitation was evaluated through P, Mg and Ca mass balances in both digesters. The results revealed that up to 82% of the available P and 81% of the available Mg precipitated in the mesophilic digester at HRT = 20 days which suggests the possible formation of struvite in both digesters. At lower HRT (HRT = 12 days) Mg and Ca precipitation was negligible and P fixation has been attributed to the possible formation of iron phosphates or adsorption processes on solid surfaces.

Effect of Ca selective chelator BAPTA as depressant on flotation separation of magnesite from dolomite

With the depletion of high-grade magnesite mineral resources, improving the grade of magnesite obtained from gangue minerals such as dolomite has been attracting considerable attention from the industry. However, it is difficult to achieve the effective separation of magnesite from dolomite because of their similar chemical and crystal properties. In this study, 1, 2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetraacetic acid (BAPTA) was used as a depressant for dolomite to improve the separation of magnesite from dolomite. Microflotation tests of the single minerals and artificially mixed minerals using oleic acid as a collector were performed to evaluate the selective depression effect of BAPTA. The results indicated that the BAPTA depressant had a selective effect on dolomite, thereby suggesting that a magnesite grade of 87.5%, recovery of 95.8%, and CaO content of 3.8% can be achieved with a BAPTA dosage of 62.5 mg/L. The depression mechanism of BAPTA was investigated using zeta-potential measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results indicated that BAPTA was selectively adsorbed on the surface of dolomite. The selective depression effect can be attributed to the selective chelating formation of BAPTA on Ca rather than Mg, which not only adsorbed BAPTA decreasing the adsorption sites of oleic acid on the dolomite surface, but also prevented the precipitation of Ca on the surface of the magnesite.

The Lateglacial-Early Holocene dynamics of the sedimentation environment based on the multi-proxy abiotic study of Lieporiai palaeolake, Northern Lithuania

New data were obtained from the sedimentary sequence study of Lieporiai palaeolake, Northern Lithuania, employing a multi-proxy abiotic approach, for the description of the Lateglacial-Early Holocene palaeoenvironmental dynamics in the basin. The study reveals significant differences in sediments deposited in the Lateglacial, at the end of Lateglacial/Early Holocene and in the Late Holocene time periods. Six main environmental stages are described. After glacial retreat, the formed landscape was re-organized by very fast currents that might have appeared later than previously thought. Rapid water flow stabilized and lacustrine sedimentation began together with the appearance of scarce pioneer vegetation shortly before 14 600 cal yr BP, (GI-1e). The development of the lake with mostly undisturbed sedimentation continued up to the final stages of the Lateglacial Interstadial. The warm period caused maximum precipitation of Ca and Sr carbonates, which strongly affected sediment saturation with other components. Increase in humidity in the later Lateglacial Stadial (GS-1) period is indicated by the abrupt appearance of the coarser-sized mineral matter accompanied by weathering elements, i.e. Ti, Al, Si, Mg, and the early immigration of Picea. About 11 500 cal yr BP, the mineral matter input started decreasing, and stabilization of the climate regime began about 10 200 cal yr BP. (A)biotic proxies highlight slow sedimentation mechanisms that recover shallow- and trophic-lake stages. The Lieporiai palaeolake record clearly shows the sedimentation hiatus between the Early and the Middle Holocene. Further stages are characterized by paludification processes. Finally, peat accumulation was interrupted by humans.

Formation and characterization of collagen and hydroxyapatite composite

The formation of composite of collagen and hydroxyapatite (HA) was investigated by ex-situ precipitation method. Synthesis of HA was carried out by precipitation of Ca(OH)2 1 M and (NH4)2.HPO4 0.6 M with the molar ratio of Ca/P 1.67, the mixture was exposed to microwave irradiation (450 W) for 45 min. Subsequently, the result was sintered to produce the powders of HA. Verification of HA powders was performed using X-Ray Diffraction (XRD). Chicken collagen was prepared in our laboratory (Physics Department, Universitas Indonesia). Composite was synthesized by ex-situ precipitation method with a mass fraction of collagen and HA was 2:1. The mixture was stirring continuously for 2 h and aged for 20 h at room temperature. The precipitate was dried by freeze-drying method. The composite result was examined by Scanning Electron Microscopy (SEM). XRD pattern of sintered powder shown as HA crystal. The unit cell of HA crystals was evaluated by High Score Plus Program (a=b=9.425 and c=6.884) with the accuracy 98.175% for a and b lattice and 98.406% for c lattice. The collagen had been verified by SDS PAGE as type 1 collagen and the micrograph illustrated the presence of micropores. Based on SEM examination, the composite revealed that HA had been deposited onto collagen and filled part of void space of collagen. The preliminary study will be beneficial for leading the formation of composites of collagen-HA as biomaterials.

Pulp mineralogy and chemistry, leaching and rheological behaviour relationships of refractory gold ore dispersions

In this paper, the influential role of ore mineralogy, pulp interfacial chemistry and alkali type (cautic soda and quicklime) in temporal rheological behaviour of two refractory flotation concentrates and their bio-oxidised mineral dispersions have been investigated. Caustic soda modified pulps showed different particle zeta potentials from the quicklime modified pulps over pH range 2–12. At high pH (>10), the two flotation concentrates displayed positive zeta potentials, regardless of the pH modifier. On the other hand, quicklime pH modified bio-oxidised pulp particles were electropositive at high pH whilst the caustic soda modified pulps showed deprotonated particles. In addition, the predominant chemical species leached from the flotation concentrates and their bio-oxidised products at pH 10.5 were Ca and S, followed by incongruent leaching of Cu, K, As, Ni, Zn, Fe, Al and Au. This encouraged lower zeta potential and reduced the particle electrostatic repulsions within the ore dispersion. Significant back precipitation of the Ca ions during leaching was mainly observed when caustic soda was deployed in comparison with the quicklime modified pulp. Mineralogically-driven, rapid back precipitation of Ca, As, Cu and S was, however, noticeable in only the jarosite-bearing bio-oxidised product when quicklime was employed. Generally, it was evident that the alkaline cyanide leaching contact time, mineralogy and alkali-type markedly influenced the pulp chemistry and particle interactions. Non-Newtonian, plastic, thixotropic rheological behaviour was displayed by the pulps at pH 10.5 over a 480 min leaching time. The flotation concentrate dispersions showed shear thickening, time-invariant behaviour, however, the bio-oxidised pulps exhibited pH modifier, leaching time, shear and ore mineralogy dependent rheological behaviour. The results suggest emergence of van der Waals and non-DLVO attractive forces such as adsorbed Ca(II) ion-particle bridging, surface nucleation and cementation for the stronger rheology of quicklime modified pulps, in contrast with caustic soda modified pulps.

Application of Acidizing and Plugging Removal Technology in Chang 2 Reservoir of CH Oilfield

In view of the serious scale and blockage in Chang 2 reservoir of a certain oil production plant, in recent years, based on the scientific and technological project “Acidification technology test of Chang 2 reservoir in XX-YY District” completed in 2013, a series of blockage removal workers suited to different blockage characteristics in this area have been formed by studying and analyzing a large number of indoor data, optimizing the acidification formula. The dosage of additives is increased, thus protecting oil pipes, rods and pumps, and preventing new pollution caused by precipitation of Ca2+, Mg2+. In view of the specific situation of reservoir blockage, 222 wells have been carried out with various blockage removal measures, such as compound acidification, blockage removal and so on. The cumulative oil increase is 7894.8 tons, with an average of 38.7 tons per well, which has made a positive contribution to the stable production of oil production plants

Chemical composition of salt lakes in East Transbaikalia (Russia)

We have developed a new concept on the formation of diverse geochemical types of salt lakes, in particular for those in the Transbaikal region. This concept explains the nature of their different chemical composition under the close landscape-climatic and geological-geochemical conditions. The extensive hydrogeochemical field data and thermodynamic calculations demonstrate that in addition to evaporation, interactions with rocks play a substantial role in the formation of the composition of these salt lakes. Such processes are most widespread in soda lakes, which is confirmed by the highest pH values (9.0–10.7). These lakes provide evidence of the interaction of the lake waters with aluminosilicates; their hydrolysis provides a pH increase and consequently a high concentration of carbonate ions. These high pH and HCO3− and CO32− contents in turn lead to the maximum precipitation of Ca, Mg and Fe carbonates. Soda lakes also contain high chloride ion concentrations, since a small portion of those ions evaporate under the lake conditions. If evaporation continues and the amount of carbonate ions in the lake does not increase, or increases slowly due to binding with carbonate minerals, the chlorine content becomes higher, the pH lowers to less than 9.0, and the lakes become chloride. Another fundamental process in these lake environments is sulphate reduction, which results in the production of hydrogen sulphide, fundamentally changing the geochemical environment from oxidizing to reducing. The presence of an oxidizing environment and sulphides in rocks gives additional sources for sulphates, which ensures the formation of sulphate-type lakes.

In vitro bioactivity of laser surface-treated Ti6Al4V alloy.

The effects of lasing parameters on the precipitation of hydroxyapatite (HA) on the commercial Ti6Al4V alloy in simulated body fluid (SBF) were investigated. Ti6Al4V plates were polished and ultrasonically cleaned in acetone and ethyl alcohol, respectively. The specimen surfaces were treated with Er:YAG laser using super short pulse (SSP, 50μs) and very short pulse (VSP, 100μs) modes. Surface roughness was measured before and after laser treatment. The specimens were immersed in simulated body fluid (SBF) for 1, 3, and 7days and, then the amount of Ca and P precipitation on specimens was determined using SEM/EDS analysis. An average roughness varying between 0.19 and 0.81μm in surface roughness was detected in all laser-treated specimens depending on the lasing parameters. The highest surface roughness and Ca precipitation were found in VSP group (20Hz and 5W). Laser treatment of specimen surfaces has dramatically increased the HA precipitation due to the increasing surface roughness. It is also concluded that the immersion time was effective on the HA precipitation as well.

Mechanisms of potassium release from calcareous soils to different salt, organic acid and inorganic acid solutions

Mechanisms of potassium (K) release and fixation in calcareous soils may differ from non-calcareous soils. In the current investigation, four soils with different properties were extracted three and 10 times (45 and 150min, defined as short- and long-term respectively) with 0.025molL−1 CaCl2, HCl and oxalic and citric acid solutions and the contents of calcium (Ca), magnesium (Mg), aluminium (Al) and K in the extracts were determined. Immediately after K extraction, 500mg kg–1 of K (as KCl) was added to the remaining soils and shaken for 24h and K-fixation capacity was determined. No significant difference was observed between CaCl2 and HCl solutions in K extraction from soils in the short- and long-term experiments; the concentrations of Ca, Mg and Al in the extractants showed that HCl was not able to dissolve soil minerals and the soils released K via exchange reaction of Ca (originated from CaCO3 dissolution by HCl) with interlayer K. The organic acids had similar behaviour to each other in the short-term experiment; however, partial dissolution of minerals by citric acid was observed. Oxalic acid extracted less K than other solutions in the short-term experiment due to precipitation of soluble Ca as oxalate salt and thereby less exchange of soluble Ca with non-exchangeable K. Oxalic and citric acids dissolved K-bearing minerals and released K in the long-term experiment, but oxalic acid was more effective. The K-fixation capacity of soils was lower when treated with organic acids than with CaCl2 and HCl, possibly due to the dissolution of K-fixing minerals like smectite and illite by oxalate and citrate. Long-term treatment of soils with different solutions decreased K-fixation capacity of soils due to dissolution of K-fixing minerals or flocculation of minerals by Ca and slower K diffusion to the interlayer. With respect to the highly calcareous nature of the studied soils, the soluble cations in calcareous soils (predominantly Ca and Mg) may have been exchanged with non-exchangeable K of clay minerals and buffered soluble K as well as organic acids produced by plant roots and microorganisms.

Carbonates dissolution and precipitation in hemipelagic sediments overlaid by supersaturated bottom-waters – Gulf of Aqaba, Red Sea

Abstract Whether CaCO3 dissolves within the top centimeters of marine sediments overlaid by deep, supersaturated bottom waters remains an area of debate in geochemistry. This uncertainty stems from the fact that different methods used to assess CaCO3 dissolution rates often provide what appear to be profoundly different results. Here we combine microelectrode and porewater chemistry profiles, core incubation experiments, mineral characterizations and observations of the state of preservation of coccolithophorid exoskeletons for a holistic view of carbonate reactions within the top 30 centimeters of hemipelagic sediments from the Gulf of Aqaba, Red Sea. Calculations based on pH and O2 microelectrode data suggest that rapid metabolic dissolution of carbonate minerals occurs in these sediments within the top two millimeters. Porewater chemistry supports these calculations. The porewater-based observations are further supported by sedimentological characteristics such as aragonite content, and dissolution pitting and fragmentation of coccoliths in sediment layers deposited over the last 200 y. Dissolution appears to be occurring today within surface sediments despite the bulk porewater solution being supersaturated with respect to aragonite and Mg-calcite. In spite of intense dissolution within the sediments, there is no evidence for significant alkalinity and/or calcium fluxes (transport) into bottom waters. It appears that the supersaturated bottom water promotes the removal of all excess alkalinity and calcium produced within the sediment, by CaCO3 precipitation at or above the sediment/bottom water interface. The precipitation mechanism may be by either benthic organisms (biogenic precipitation) or inorganically (direct precipitation on settling CaCO3 grains). We suggest that authigenic precipitation of (Ca,Mn)CO3 as it becomes supersaturated below 3 cm in the sediments can reconcile the evidence for carbonate dissolution in what appears to be supersaturated conditions. This means that MnCO3 replaces CaCO3 within the nanofossils below ∼3 cm, and that part of the manganese rich CaCO3 is bioturbated upwards into undersaturated conditions, facilitating dissolution of these fossils. Diminished calcite and aragonite concentrations in sediments deposited in recent decades are proposed to be a result of increased manganese cycling rates and greater rates of coupled dissolution within the interfacial sediments, possibly combined with diminished calcareous plankton productivity, in response to increased surface water primary productivity.