The carbonate system in the central South China Sea

The Clements of the carbonate system (HCO3- , CO32-, CO2, ·CO2 and Pco2) in the central South China Sea have been calculated by determining the pH values and total alkalinity of the seawater samples collected at 42 stations, combining simultaneously with the data of the temperatures, salinity and depths. The distributions of their characteristics have been briefly described and discussed. The saturation degrees (Ω) of calcite and aragonite in the studied area have also been evaluated. The ΩcaIc- and Ωarag. in the surface waters are about 5.8 and 3.9, respectively. The saturation depth is about 2200m for calcite and 1200m for aragonite. The lysocline in the studied area probably lies between 3000 m and 4000 m where the saturation degree of calcite is approximately 0.78.     

Shell preservation of Limacina inflata (Pteropoda) in surface sediments from the Central and South Atlantic Ocean: a new proxy to determine the aragonite saturation state of water masses

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive “metabolic” aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and δ¹³C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO₂ release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.     

The steady-state calcium carbonate ion activity product of recent shallow water carbonate sediments in seawater: a comparison of field observations and laboratory experiments

Closed system equilibration experiments between natural seawater and shallow water calcium carbonate-rich sediments from the Bahamas yielded steady-state calcium carbonate ion activity products (CCIAP). Results obtained from initially supersaturated and undersaturated solutions were in good agreement. Experiments conducted with the addition of a biocide and/or the destruction of sediment organic matter gave results similar to those obtained in systems where these treatments were not used. Excellent agreement was also found between CCIAP values for 8 day and more than 50 day equilibration times. Our results, therefore, meet the major criteria for at least metastable equilibrium between the solution and carbonate sediment.Fine-grained samples produced a CCIAP close to the value predicted for aragonite, which is the major carbonate phase in all samples. Coarse-grained sediments produced larger CCIAP values of up to 2.8 times that predicted for aragonite equilibrium. The CCIAP for the coarse-grained sediments is probably produced by high-Mg calcite which is a significant component of these sediments. Oolite samples were among the coarse-grained sediment samples studied. They also produced results much greater than expected for aragonite equilibrium. This brings into question their use as material for measuring aragonite solubility as has been done in the past.The CCIAP measured in the laboratory experiments are in good agreement with field observations of pore-water CCIAP values from the fine-grained sediments. Coarse-grained sediments showed greater variability, with higher CCIAP values generally occurring in the pore waters than in the laboratory experiments. Since the overlying waters were always at a higher CCIAP than the pore waters, the major factor causing this difference is believed to be the short residence time of pore waters in the coarse-grained sediments, which is the result of the high-energy hydrodynamic environments in which they reside.     

Cementation in cold-water bryozoan sand, Tasmania, Australia

Cold-water (<3–11°C) carbonate is the predominant sediment on the Tasmanian shelf. Calcitic skeletal grains (bryozoa, foraminifera, echinoderms, etc.) predominate over aragonitic (gastropods, etc.) ones. Non-skeletal grains are mostly micritic intraclasts with some pellets.

Fibrous spherulitic and rhombohedral calcite submarine cements range up to 90% in the bryozoan sand. X-ray analyses show that the bryozoan sand is characterized by a spectrum of calcites (low to high magnesian) and some aragonite.

A uniform spread of Mg concentrations from 0.06 to 2.48 wt.% indicates <3–10°C ambient water temperatures. The Mn (10–360 ppm) and Fe (176–2499 ppm) concentrations increase with increasing Mg values due to the formation of impure CaCO₃ phases. The Sr content in bryozoan sand (bryozoa = 3200 ppm Sr) decreases with increasing rhombohedral calcite cement, as low Mg-calcite precipitating from 3° C sea water would have 1350 ppm Sr. The bryozoan sand grains with fibrous spherulitic calcite cements have high Sr concentrations (4470–7000 ppm), in the same range as in aragonitic (detected only by X-ray analyses) bryozoan sand grains. The spherulitic calcite cements are either pseudomorphs after original aragonite cements or these calcite cements and aragonite were inverted from fibrous spherulitic vaterite, a predominant CaCO₃ polymorph at temperatures <10°C.     

Dissolution of carbonate minerals in a subtropical shallow marine environment

Carbonate saturation state with respect to calcite and three different biogenic magnesian calcites has been determined by a modified saturometry technique. Measurements were made in the water column of Devil's Hole, a 25 m-deep basin in the Harrington Sound (Bermuda), which exhibits oxygen depletion in subthermocline waters during summer. Supersaturation in the entire water column was attained with respect to marble calcite. The biogenic carbonates analyzed became undersaturated below the thermocline within a narrow depth range, soon after P_CO2 exceeded 600 ppm. It is concluded that high magnesian calcites can be attacked during a significant portion of the year in these waters. Possible implications for the global CO₂ cycle are discussed.     

The influence of sea level changes on sediment carbonate mineralogy,Tongue of the Ocean,Bahamas

Large fluctuations occur in the aragonite content of fine silt and clay-sized material sampled by piston cores in Tongue of the Ocean, Bahamas. Electron microscopy reveals that sedimentary sequences with high aragonite content are characterized by abundant aragonite needles. Sedimentary sequences containing small amounts of aragonite but large amounts of low-Mg calcite are dominated by coccoliths. The variation in both mineralogy and grain morphology is interpreted to be related to sea level fluctuations and the consequent presence or absence of sediment contribution (needles) from the adjacent Bahama Banks. These fluctuations are useful chronological indicators in these cores and should be applicable in other areas adjacent to carbonate banks. In particular, the last rise in sea level is clearly marked in all the cores.     

南沙珊瑚礁对大气CO2含量上升的响应

本文利用南沙海域的碳酸盐化学以及相关调查资料，分析了表层CaCO3饱和度的分布特征，通过碳酸盐的热力学计算模式，定量评估了未来大气CO2增加对表层CaCO3饱和度的潜在影响，利用CaCO3饱和度与珊瑚钙化速率的经验关系式，进一步预测了珊瑚礁对大气CO2上升可能产生的生物地球化学响应．初步结果显示，工业革命前至2100年，南沙海域的CaCO3饱和度将下降43％左右，从而将引起珊瑚礁的平均钙化速率减少33％．如果未来大气CO2浓度继续保持目前的上升趋势，南沙海域珊瑚礁可能会停止生长，甚至某些造礁生物面，临灭绝的危险．     

Oxygen-carbon dioxide-nutrients relationships in the Southeastern Region of the Bering Sea

The vertical distribution of density, salinity, temperature, dissolved oxygen, apparent oxygen utilization, nutrients, preformed phosphate, pH, alkalinity, alkalinity: chlorinity ratio, in situ partial pressure of carbon dioxide, and percent saturation of calcite and aragonite, for the Southeastern Bering Sea, is studied and explained in terms of biological and physical processes. Some hydrological interactions between the Bering Sea and the North Pacific Ocean are explained. The horizontal distribution of dissolved oxygen at 2000 and 2500 m depths, throughout the Bering Sea, indicates that deep water is flowing from the Pacific, through the Kamchatka Strait, and then northward and eastward in the Bering Sea. Based on the dissolved oxygen distribution we estimate roughly that it takes 20 years for the deep waters to move from the Kamchatka Strait to the Southeastern part of the eastern basin. The surface concentration of nutrients is higher in the Bering Sea than in the North Pacific Ocean, probably because of upwelling and intense vertical mixing in the Bering Sea. A multivariable regression analysis of dissolved oxygen as a function of phosphate concentration and potential temperature was applied for the region where the potential temperature-salinity diagram is straight, and the confidence interval of the PO₄ coefficient, at the 95% probability level, was found consistent with theRedfield biochemical oxidation model. The calcium carbonate saturation calculations show that the Bering Sea is supersaturated with aragonite in the upper 100 m, and with calcite in the upper 200 m. Below these depths seawater is undersaturated with respect to these two minerals.     

Interstitial water chemistry of Jamaican reef sediment: sulfate reduction and submarine cementation

Chemical and isotopic analyses of pore waters from Jamaican reef sediment suggest the importance of microbial sulfate reduction as a major control upon the origin, distribution, and composition of submarine cements in this fringing reef setting. Fore-reef sediment pore waters exhibit active sulfate reduction and enrichment in ¹⁸O which is consistent with associated active magnesian calcite cementation, alkalinity consumption, and cement enrichment in ¹⁸O. Conversely, lack of widespread submarine cementation in the back-reef corresponds to the diminished resupply of sulfate coupled with input of CO₂-charged meteoric water from a nearby unconfined aquifer into the more stagnant pore waters which lower pore-water magnesian calcite saturation states and preclude active submarine cementation.     

Arctic ocean shelf–basin interaction: An active continental shelf CO2 pump and its impact on the degree of calcium carbonate solubility

The Arctic Ocean has wide shelf areas with extensive biological activity including a high primary productivity and an active microbial loop within the surface sediment. This in combination with brine production during sea ice formation result in the decay products exiting from the shelf into the deep basin typically at a depth of about 150 m and over a wide salinity range centered around S ～33. We present data from the Beringia cruise in 2005 along a section in the Canada Basin from the continental margin north of Alaska towards the north and from the International Siberian Shelf Study in 2008 (ISSS-08) to illustrate the impact of these processes. The water rich in decay products, nutrients and dissolved inorganic carbon (DIC), exits the shelf not only from the Chukchi Sea, as has been shown earlier, but also from the East Siberian Sea. The excess of DIC found in the Canada Basin in a depth range of about 50–250 m amounts to 90±40 g C m^?2. If this excess is integrated over the whole Canadian Basin the excess equals 320±140×10¹² g C. The high DIC concentration layer also has low pH and consequently a low degree of calcium carbonate saturation, with minimum aragonite values of 60% saturation and calcite values just below saturation. The mean age of the waters in the top 300 m was calculated using the transit time distribution method. By applying a future exponential increase of atmospheric CO₂ the invasion of anthropogenic carbon into these waters will result in an under-saturated surface water with respect to aragonite by the year 2050, even without any freshening caused by melting sea ice or increased river discharge.     

极低饱和度和近饱和状态下海水中方解石和文石溶解动力学研究

利用free-drift开放反应系统,在恒压力(101.325 kPa)和恒温度(25.0±0.2)℃环境条件下,研究人工海水中二氧化碳分压(pCO2)的变化对方解石和文石的溶解速率及反应级数的影响.研究结果表明:如果溶解实验中pCO2未达到完全平衡,计算得出的反应级数偏小,且反应液pCO2不平衡是造成不同反应阶段反应...     

Formation of chemogenic calcite in super-anoxic seawater — Framvaren, southern Norway

Framvaren, a super-anoxic fjord in southern Norway, contains 7–8 mmoll⁻¹ of sulphide and a total carbonate concentration of 18.5 mmol kg⁻¹ in the bottom water. The chemistry of calcium has been studied, considering sources, biogenic and chemical processes and sedimentary sinks. Calcium associated with the bacteria biomass at the redox interface (18m depth) appears to be the primary source of dissolved calcium in the deep, anoxic water. Excess calcium and high total carbonate cause supersaturation of calcite, which is precipitated chemogenically. Calcite (and presumably some aragonite) is identified both in sediment trap material and the bottom sediments below the depth of supersaturation.     

Mineral assemblages occurring around hydrocarbon vents in the northern Gulf of Mexico

Magnesian calcites are the most abundant authigenic minerals associated with hydrocarbon vents at 25 sites, in water depths ranging from 100 to 600 m in the Green Canyon area and about 2200 m in the Alaminos Canyon area on the Continental Slope of the northern Gulf of Mexico. The most frequently encountered magnesian calcites have 10–15 mol% MgCO₃ and the apparent structural disorder revealed by XRD peak widths increases with Mg substitution. There are no systematic variations in Mg content with respect to water depth or geographic location. The calcite saturation state of the precipitating fluid is primarily determined by the nature of the fluids escaping from the vents, not the ambient seawater.     

Hardground formation in the Bannock Basin, Eastern Mediterranean

Twenty kilogrammes of crusts and slabs of indurated carbonate sediment, usually referred to as hardgrounds, were dredged along the eastern steep wall of the Bannock Basin during the 1984 cruise of R.V. Bannock.

The crusts range in thickness from one to a few centimetres and the fragments of these crusts are irregular in shape. Their surface is always uneven and their colour ranges from white to brownish dark grey. Some slabs are impregnated along one side by ferromanganese sesquioxides, and borings occur in several samples. Serpulid tubes have been observed in one instance. The borings and serpulids suggest formation of the hardgrounds at or close to the sediment/water interface and exposure at the seafloor.

The degree of lithification is generally different on the inferred upper and lower sides of the slabs. An upward increase of lithification across the slabs is reflected by mineralogy, ultrastructure and stable isotope composition of the carbonate. X-ray diffraction analyses indicate high-magnesian calcite as the predominant carbonate with minor amounts of low-magnesian calcite and dolomite. Occasionally, large gypsum crystals are attached to the hardgrounds and sometimes smaller ones are dispersed through the carbonate matrix.

An increase in diagenesis is reflected by the passage from friable, nodular nannofossil chalk to nannofossil limestone and hard xenotopic calcite micrite. Overgrowth of coccoliths and internal cementation of the tests of planktonic foraminifera by high-Mg calcite increase from chalk to limestone. In the hard, fully cemented micrites, coccoliths can no longer be recognised in the xenotopic fabric. Pteropods occur as dissolution moulds with aragonite preserved as only tiny relics.

Carbon and oxygen isotope analyses were performed on different samples. The progressive lithification to chalk and limestone is marked by a shift in the δ¹⁸O values from +1.2‰ to +5.4‰ (PDB). This change indicates that precipitation of high-Mg calcite and possibly also recrystallisation of the original biogenic carbonate took place within cold and hypersaline brines which were enriched in ¹⁸O. The oxygen isotope data suggest that lithification and gypsum precipitation occurred under identical conditions. The carbon isotope data show progressive diagenetic change from values near +1‰ to values of +3‰. This change may reflect a contribution of methanogenetic CO₂ to the hypersaline brine.     

红海及亚丁湾间之海水交换

分析表观耗氧量、滴定碱度及总二氧化碳量等资料来研判红海及亚丁湾间之海水交换。结果显示,红海深层水的方解石及霰石饱和度均比亚丁湾和阿拉伯海深层水的饱和度高。红海全水柱之方解石和霰石都处於过饱和状态,亚丁湾和阿拉伯海中各深度之方解石亦呈过饱和状态,但霰石的饱和探度则大约在500m左右。分析深层水之生物体无机碳与有机碳的分解比值,可以发现此地区深层水中,大约有25％的总二氧化碳增加量是由无机碳酸钙溶解而来。     

印尼贯穿流源区马鲁古海与哈马黑拉海水团来源的季节和年际变化

So far, large uncertainties of the Indonesian throughflow(ITF) reside in the eastern Indonesian seas, such as the Maluku Sea and the Halmahera Sea. In this study, the water sources of the Maluku Sea and the Halmahera Sea are diagnosed at seasonal and interannual timescales and at different vertical layers, using the state-of-the-art simulations of the Ocean General Circulation Model(OGCM) for Earth Simulator(OFES). Asian monsoon leaves clear seasonal footprints on the eastern Indonesian seas. Consequently, the subsurface waters(around 24.5σ_θ and at ～150 m) in both the Maluku Sea and the Halmahera Sea stem from the South Pacific(SP) during winter monsoon, but during summer monsoon the Maluku Sea is from the North Pacific(NP), and the Halmahera Sea is a mixture of waters originating from the NP and the SP. The monsoon impact decreases with depth, so that in the Maluku Sea, the intermediate water(around 26.8σ_θ and at ～480 m) is always from the northern Banda Sea and the Halmahera Sea water is mainly from the SP in winter and the Banda Sea in summer. The deep waters(around27.2σ_θ and at ～1 040 m) in both seas are from the SP, with weak seasonal variability. At the interannual timescale,the subsurface water in the Maluku Sea originates from the NP/SP during El Ni?o/La Ni?a, while the subsurface water in the Halmahera Sea always originates from the SP. Similar to the seasonal variability, the intermediate water in Maluku Sea mainly comes from the Banda Sea and the Halmahera Sea always originates from the SP. The deep waters in both seas are from the SP. Our findings are helpful for drawing a comprehensive picture of the water properties in the Indonesian seas and will contribute to a better understanding of the ocean-atmosphere interaction over the maritime continent.     

Carbonate cement fabrics displayed: A traverse across the margin of the Bahamas Platform near Lee Stocking Island in the Exuma Cays

Consolidated to friable carbonate rocks found in the Lee Stocking Island area in the Exuma Cays include: (1) reef rock, (2) channel stromatolites, (3) shallow-water hardgrounds, (4) beachrock rimming the islands and (5) Pleistocene bedrock.

The most common cement fabrics observed are: aragonitic fibers, which include acicular fan-druse and square-tipped coarse fibers cementing beachrock and stromatolites; and an isopachous needle-fiber rim cementing hardgrounds and stromatolites.

Less common are high-Mg calcite bladed textures of the reef rock and stromatolites. Two types of blades are present: the more common stubby variety of either high-Mg or low-Mg calcite, and an elongated variety of high-Mg calcite which was found in only three beachrock samples.

Aragonitic micrite envelopes usually surround grains in beachrock, hardgrounds and stromatolites, but only in association with fibrous cement. An aragonitic crust cements the surfaces of lime mud beds of the tidal channel, while a high-Mg calcite cryptocrystalline cement occurs in all the rock types. Calcified algal filaments of high-Mg calcite, from the abundant green and blue-green algae in the area, are a primary cement in stromatolites and a secondary cement in hardgrounds and beachrock. A low-Mg calcite equant spar cements the Pleistocene samples and is associated with meteoric diagenesis and cementation of the Pleistocene surface.

Cement precipitation coincides with the path of the cool oceanic water from Exuma Sound as it warms and loses CO₂ and moves up onto the bank near Lee Stocking with the incoming tide. Cryptocrystalline cement is the first and commonest cement forming to the seaward while platformward, fibrous cements become predominant. As suggested by their crystal size and location on the shelf margin, we think that the reef rock cryptocrystalline material are the fastest forming of the cements, where the incoming oceanic water is more saturated with respect to calcium carbonate and undergoes the most significant warming. The rate of the warming and degassing process is thought to increase in the tidal channel though the cementation rate is thought to fall slightly in response to a reduced availability of calcium carbonate. On the platform interior further warming and degassing are believed to cause cement precipitation and the development of hardgrounds, but these may form at a slower rate than that of the margin, though this rate is still quite high. Cementation gradients occur from the tidal channel to the intertidal zones of: (1) west Norman's Pond Cay, where cement fabric suggests a reduced calcium carbonate availability, and (2) west Lee Stocking Island, where a change in mineralogy suggests a change in water chemistry.

Thus, a sequence of cement fabrics and mineralogies can be traced. Micritic textures occur in a more seaward position; fine, fibrous aragonite fibers in a more lagoonal and levee position; and coarser aragonite fibers and Mg-calcite cements in the intertidal and supratidal position. This sequence is thought to track the evolution of the water mass.     

Behavior of bromide ions during the formation of calcium carbonate

Coprecipitation of bromide ions with calcium carbonate, formed from a calcium bicarbonate solution containing bromide ions, and the influence of bromide ions in a parent solution on the crystal form of the calcium carbonate were studied experimentally. The following results were obtained: the coprecipitation is affected by the crystal form of the calcium carbonate and the concentration of sodium chloride dissolved in a parent solution. The bromide ions are more easily coprecipitated with aragonite than with calcite. The amount of bromide ions coprecipitated with aragonite decreases significantly with increasing concentration of sodium chloride in a parent solution. The presence of bromide ions in a parent solution has a slight influence on the crystal form of calcium carbonate to favor calcite formation and to inhibit aragonite formation.The role of marine carbonate sediments in the removal of bromide ions from sea water is discussed briefly.     

A literature review of the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on OTEC heat exchangers

An investigation has been made of available data on the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on Ocean Thermal Energy Conversion (OTEC) heat exchangers. Pertinent oceanographic data is lacking at or near potential OTEC sites for the calculation of the degree of saturation of seawater with respect to calcium carbonate. Consequently, only “extrapolated” saturation values can be used. These indicate that near surface seawater is probably supersaturated, with respect to the calcium carbonate phases calcite and aragonite, at all potential OTEC sites. The deep seawater that would be brought to the surface at the potential Atlantic Ocean sites is also likely to be supersaturated with respect to calcium carbonate. The deep seawater at the potential Pacific Ocean sites may be slightly undersaturated.The fact that OTEC heat exchangers will be operating in seawater, which is supersaturated with respect to calcium carbonate, means that if nucleation of calcite or aragonite occurs on the heat exchanger surfaces, significant growth rates of calcium carbonate scale may be expected. The potential for calcium carbonate nucleation is highest at cathodic metal surface locations, which are produced as the result of aluminum corrosion in seawater. Consequently, corrosion and scale formation may be closely related. What the possible effects of biofouling may be on this process are not known.     

The saturation of calcite and aragonite in the Arctic Ocean

We report on the chemical saturation of CaCO₃ in the waters of the Arctic Ocean calculated from total alkalinity (A_T) and total dissolved inorganic carbon (C_T). Data based on four different expeditions are presented: International Arctic Ocean Expedition (IAOE-91), Arctic Ocean Section 94 (AOS94), Polarstern Arctic '96 expedition (ACSYS 96), and Joint Ocean Ice Study 97 (JOIS 97). The results show a lysocline at around 3500 m for aragonite and that most of the Arctic Ocean sea floor lies above the lysocline for calcite. The only anomaly is the low degree of saturation at the shelf break depth in the Canadian Basin seen in the sections of the AOS94 and JOIS 97 cruises, correlated with nutrient maxima and very low O₂ concentration, suggesting decomposition of organic matter. The insignificant variability in degree of saturation between the deep waters of the different basins in the Arctic Ocean indicates a very low sedimentation/remineralisation of organic soft matter.