Source Of Alkalinity Research Articles

Alkalinity sources in the Dutch Wadden Sea

Abstract. Total alkalinity (TA) is an important chemical property that plays a decisive role in the oceanic buffering capacity with respect to CO2. TA is mainly generated by weathering on land as well as by various anaerobic metabolic processes in the water and sediments. The Wadden Sea, located in the southern North Sea, is hypothesized to be a source of TA for the North Sea, but quantifications are scarce. This study shows observations of TA, dissolved inorganic carbon (DIC), and nutrients in the Dutch Wadden Sea in May 2019. Surface samples were taken along several transects in order to investigate spatial distribution patterns and compare them with data from the late 1980s. A tidal cycle was sampled to further shed light on TA generation and potential TA sources. We identified the Dutch Wadden Sea as a source of TA and estimated an export of 6.6 Mmol TA per tide to the North Sea. TA was generated in the sediments, with deep pore water flow during low tide enriching the surface water. A combination of anaerobic processes and CaCO3 dissolution were potential TA sources in the sediments. We deduce that seasonality and the associated nitrate availability specifically influence TA generation by denitrification, which is low in spring and summer.

Influence of ocean alkalinity enhancement with olivine or steel slag on a coastal plankton community in Tasmania

Abstract. Ocean alkalinity enhancement (OAE) aims to increase atmospheric CO2 sequestration in the oceans through the acceleration of chemical rock weathering. This could be achieved by grinding rocks containing alkaline minerals and adding the rock powder to the surface ocean where it dissolves and chemically locks CO2 in seawater as bicarbonate. However, CO2 sequestration during dissolution coincides with the release of potentially bioactive chemicals and may induce side effects. Here, we used 53 L microcosms to test how coastal plankton communities from Tasmania respond to OAE with olivine (mainly Mg2SiO4) or steel slag (mainly CaO and Ca(OH)2) as alkalinity sources. Three microcosms were left unperturbed and served as a control, three were enriched with olivine powder (1.9 g L−1), and three were enriched with steel slag powder (0.038 g L−1). Olivine and steel slag powders were of similar grain size. Olivine was added in a higher amount than the steel slag with the aim of compensating for the lower efficiency of olivine to deliver alkalinity over the 3-week experiment. Phytoplankton and zooplankton community responses as well as some biogeochemical parameters were monitored. Olivine and steel slag additions increased total alkalinity by 29 and 361 µmol kg−1, respectively, corresponding to a respective theoretical increase of 0.9 % and 14.8 % of the seawater storage capacity for atmospheric CO2. Olivine and steel slag released silicate nutrients into the seawater, but steel slag released considerably more and also significant amounts of phosphate. After 21 d, no significant difference was found in dissolved iron concentrations (>100 nmol L−1) in the treatments and the control. The slag addition increased dissolved manganese concentrations (771 nmol L−1), while olivine increased dissolved nickel concentrations (37 nmol L−1). There was no significant difference in total chlorophyll-a concentrations between the treatments and the control, likely due to nitrogen limitation of the phytoplankton community. However, flow cytometry results indicated an increase in the cellular abundance of several smaller (∼<20 µm) phytoplankton groups in the olivine treatment. The abundance of larger phytoplankton (∼>20 µm) decreased much more in the control than in the treatments after day 10. Furthermore, the maximum quantum yields of photosystem II (Fv/Fm) were higher in slag and olivine treatments, suggesting that mineral additions increased photosynthetic performance. The zooplankton community composition was also affected, with the most notable changes being observed in the dinoflagellate Noctiluca scintillans and the appendicularian Oikopleura sp. in the olivine treatment. Overall, the steel slag used here was more efficient for CO2 removal with OAE than the olivine over the 3-week timescale of the experiment. Furthermore, the steel slag appeared to induce less change in the plankton community than the olivine when comparing the CO2 removal potential of both minerals with the level of environmental impact that they caused.

A new method of alkalinity remediation for Cd-contaminated groundwater by PAAS-modified MgCO3/Mg(OH)2 colloid

Mg(OH)2 dissolves slowly and can provide a long-term source of alkalinity, thus a promising alternative reagent for the in situ remediation of heavy metal polluted groundwater. Unfortunately, it exhibits a relatively poor stabilization effect on heavy metal Cd due to the higher solubility of the resulting stabilized product, Cd(OH)2. To overcome this limitation, we investigated the use of MgCO3/Mg(OH)2 colloid modified by sodium polyacrylate (PAAS) to remove Cd from groundwater. Through ultrasonic dispersion, the molecular chains of PAAS are broken, causing a transformation from flocculation to surface modification, resulting in the production of a stable colloid. The colloidal particles of MgCO3/Mg(OH)2 have a smaller size and a negatively charged surface, which significantly enhances their migration ability in aquifers. The combination of MgCO3 and Mg(OH)2 provides a complementary effect, where MgCO3 effectively precipitates Cd in the aquifer while Mg(OH)2 maintains the required pH level for stabilization. The optimal compounding ratio of MgCO3 to Mg(OH)2 for achieving the best stabilization effect on Cd is found to be 1:1. Column experiments demonstrate that the injection of MgCO3/Mg(OH)2 colloid substantially enhances Cd stability, reducing the exchangeable fraction of Cd in aquifer media from 88.61% to a range of 22.50–34.38%. Based on these results, the MgCO3/Mg(OH)2 colloid shows great potential as a reactive medium for remediating Cd-contaminated groundwater.

Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation

Abstract The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea's passive continental margins, and by assessing Neoproterozoic reconstructions of tectonic plates. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.

The additionality problem of ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is an emerging approach for atmospheric carbon dioxide removal (CDR). The net climatic benefit of OAE depends on how much it can increase CO2 sequestration relative to a baseline state without OAE. This so-called “additionality” can be calculated as follows: Additionality=COAE-ΔCbaseline. So far, feasibility studies on OAE have mainly focussed on enhancing alkalinity in the oceans to stimulate CO2 sequestration (COAE); however, the primary focus has not been on how such anthropogenic alkalinity would modify the natural alkalinity cycle and associated baseline CO2 sequestration (ΔCbaseline). Here, I present incubation experiments in which materials considered for OAE (sodium hydroxide, steel slag, and olivine) are exposed to beach sand to investigate the influence of anthropogenic alkalinity on natural alkalinity sources and sinks. The experiments show that anthropogenic alkalinity can strongly reduce the generation of natural alkalinity, thereby reducing additionality. This is because the anthropogenic alkalinity increases the calcium carbonate saturation state, which reduces the dissolution of calcium carbonate from sand, a natural alkalinity source. I argue that this “additionality problem” of OAE is potentially widespread and applies to many marine systems where OAE implementation is considered – far beyond the beach scenario investigated in this study. However, the problem can potentially be mitigated by dilute dosing of anthropogenic alkalinity into the ocean environment and the avoidance of OAE in natural alkalinity cycling hotspots, such as in marine sediments. Understanding a potential slowdown of the natural alkalinity cycle through the introduction of an anthropogenic alkalinity cycle will be crucial for the assessment of OAE.

High-resolution X-ray fluorescence-based provenance mapping of Eocene fluvial distributary fans that fed ancient Gosiute Lake, Wyoming, USA

The Green River Formation of Wyoming, USA, is host to the world’s largest known lacustrine sodium carbonate deposits, which accumulated in a closed basin during the early Eocene greenhouse. Alkaline brines are hypothesized to have been delivered to ancient Gosiute Lake by the Aspen paleoriver that flowed from the Colorado Mineral Belt. To precisely trace fluvial provenance in the resulting deposits, we conducted X-ray fluorescence analyses and petrographic studies across a suite of well-dated sandstone marker beds of the Wilkins Peak Member of the Green River Formation. Principal component analysis reveals strong correlation among elemental abundances, grain composition, and sedimentary lithofacies. To isolate a detrital signal, elements least affected by authigenic minerals, weathering, and other processes were included in a principal component analysis, the results of which are consistent with petrographic sandstone modes and detrital zircon chronofacies of the basin. Sandstone marker beds formed during eccentricity-paced lacustrine lowstands and record the migration of fluvial distributary channel networks from multiple catchments around a migrating depocenter, including two major paleorivers. The depositional topography of these convergent fluvial fans would have inversely defined bathymetric lows during subsequent phases of lacustrine inundation, locations where trona could accumulate below a thermocline. Provenance mapping verifies fluvial connectivity to the Aspen paleoriver and to sources of alkalinity in the Colorado Mineral Belt across Wilkins Peak Member deposition, and shows that the greatest volumes of sediment were delivered from the Aspen paleoriver during deposition of marker beds A, B, D, and I, each of which were deposited coincident with prominent “hyperthermal” isotopic excursions documented in oceanic cores.

Assessing the technical aspects of ocean-alkalinity-enhancement approaches

Abstract. Ocean alkalinity enhancement (OAE) is an emerging strategy that aims to mitigate climate change by increasing the alkalinity of seawater. This approach involves increasing the alkalinity of the ocean to enhance its capacity to absorb and store carbon dioxide (CO2) from the atmosphere. This chapter presents an overview of the technical aspects associated with the full range of OAE methods being pursued and discusses implications for undertaking research on these approaches. Various methods have been developed to implement OAE, including the direct injection of alkaline liquid into the surface ocean; dispersal of alkaline particles from ships, platforms, or pipes; the addition of minerals to coastal environments; and the electrochemical removal of acid from seawater. Each method has its advantages and challenges, such as scalability, cost effectiveness, and potential environmental impacts. The choice of technique may depend on factors such as regional oceanographic conditions, alkalinity source availability, and engineering feasibility. This chapter considers electrochemical methods, the accelerated weathering of limestone, ocean liming, the creation of hydrated carbonates, and the addition of minerals to coastal environments. In each case, the technical aspects of the technologies are considered, and implications for best-practice research are drawn. The environmental and social impacts of OAE will likely depend on the specific technology and the local context in which it is deployed. Therefore, it is essential that the technical feasibility of OAE is undertaken in parallel with, and informed by, wider impact assessments. While OAE shows promise as a potential climate change mitigation strategy, it is essential to acknowledge its limitations and uncertainties. Further research and development are needed to understand the long-term effects, optimize techniques, and address potential unintended consequences. OAE should be viewed as complementary to extensive emission reductions, and its feasibility may be improved if it is operated using energy and supply chains with minimal CO2 emissions.

Ammonia gas treatment in low cost biological reactor

Ammonia gas contributes to a number of environmental and human health concerns. The use of chalk, a cheap alkalinity source may reduce costs for biological systems. This research studies a closed liquid flow reactor to treat ammonia gas using chalk as biomass media and alkalinity source with high value calcium nitrate fertilizer production.The proposed reactor showed complete ammonia gas removal at high rate (500 mg N/L/day) and with low cost; where chalk dissolution and ammonia gas absorption contributed to alkalinity in the water for nitrification. High calcium ion concentration (up to 10,000 mg Ca2+ as CaCO3/L) showed only minor effects on ammonia absorption and nitrification rate.

Carbon and oxygen isotope microanalysis of calcite in the Permian Kupferschiefer system, Saale subbasin, eastern Germany

The Kupferschiefer district in Central Europe contains some of the world's highest-grade sediment-hosted stratiform Cu (SSC) deposits. In the Saale subbasin (eastern Germany), high-grade sulfides formed via replacement of calcite cement in the continental sandstones of the uppermost Rotliegend (S1), the overlying organic-rich marine mudstones of the Kupferschiefer (T1) and the Zechstein Limestone (Ca1) units. The spatial distribution of the calcite cement, therefore, had a fundamental role to play in the Cu mineralizing system. In this study, we investigate the origin of the calcite cement (and crosscutting calcite veins) using detailed petrography (cathodoluminescence, CL; scanning electron microscopy, SEM), major element chemistry (electron probe microanalyzer, EPMA), and secondary ion mass spectrometry (SIMS) microanalyses of δ13C and δ18O values in drill core samples (n = 47) from the Saale subbasin. The calcite cement in the S1, T1 and Ca1 has a similar CL response and major element chemistry, suggestive of a common origin. Overlapping δ13C and δ18O values in calcite cement in samples from the S1 and T1 in the Sangerhausen and Wallendorf drill cores also suggest that the calcite cement was derived from fluids of similar composition. The low δ13C values of calcite cement in samples from the S1 (−13‰ to 4.3‰, VPDB) and T1 (−10‰ to 0.7‰) indicates carbonate alkalinity was sourced mainly from seawater-derived fluids and the oxidation of organic matter. The wide range of δ18O values in the calcite cement in the S1 (∼18‰ to 31‰, VSMOW) and T1 (∼ 22‰ to 31‰) samples suggest they are derived from pore fluids with a chemical composition influenced by early diagenetic alteration of detrital clasts, mainly dissolution of volcanic rock fragments, with minor contributions from the influx of meteoric waters and evaporated seawater. The negative δ13C values (down to −15‰) in calcite veins from the T1 and Ca1 indicate sources of carbonate alkalinity derived from organic matter degradation. Our data demonstrate that no isotopic hydrothermal alteration haloes can be inferred from the δ13C and δ18O values in calcite cement associated with the high-grade sulfide mineralization. The lack of systematic isotopic variability in the calcite cement likely indicates the mineralizing fluid flux or temperature was not sufficient to overprint the background sources of isotopic variability, which may help to explain the modest size of SSC deposits in this part of the Kupferschiefer district.

Insight into the effect of rice-straw ash on enhancing the anaerobic digestion performance of high salinity organic wastewater

Anaerobic digestion is an economic method for treating high salinity organic wastewater (HSOW), but performance enhancement is needed because of the inhibitory effect of high salinity. In this study, rice-straw ash (RSA) was applied to alleviate the inhibitory effect during HSOW anaerobic digestion. The results showed that, when the NaCl content increased from 0% to 3.0%, the methane production decreased by 87.35%, and the TOC removal rate decreased to 34.12%. As a K+ and alkalinity source, RSA addition enhanced the anaerobic digestion performance, and the optimal dosage was 0.88 g/L. Under this dosage, the methane production increased by 221.60%, and TOC removal rate reached 66.42% at 3.0% salinity. The addition of RSA increased the proportion of living cells in the high salinity environment, and enhanced the activity of key enzymes and electron transfer efficiency in the anaerobic digestion process. The addition of RSA with a dosage of 0.88 g/L promoted the accumulation of acetoclastic methanogen Methanothrix. The abundance of substrate transporters, ion transporters and electron transfer related functional genes were enriched, which might be key for promoting HSOW anaerobic digestion performance. The results also showed that RSA addition played an important role in maintaining the stability of the anaerobic digestion system, and it could be a potential strategy for enhancing the anaerobic digestion performance under high salinity conditions.

Modulation of struvite composition in full-scale nutrient recovery system using source-separated urine.

Using urine collected from a public restroom at a highway rest stop in Northern California, a full-scale nutrient recovery system involving a two-part system consisting of urine distillation followed by the precipitation of struvite crystals was characterized. The study examined the effects of different operational parameters of the coupled ammonium distillation and struvite process (CADSP) on the composition of struvite crystals and the overall nutrient recovery rates. System inputs that were investigated included the feed:steam (F:S), alkalinity source, and urinary pH. Overall, the findings demonstrate that the composition of the struvite produced can be varied by adjusting the preceding distillation unit process. Low F:S distillation operations result in high ammonium distillation rates which coincided with increased formation of magnesium potassium phosphate (MPP) while higher F:S operations led to conditions favorable to magnesium ammonium phosphate (MAP) precipitation. Therefore, low energy distillation operations (high F:S) are more conducive to MAP formation and high phosphorus recovery rates while high energy operations (Iow F:S) are more conducive to MPP formation. Sodium-based alkalinity sources should also be avoided in instances of low ammonium concentrations in the struvite precipitation tank to minimize the co-precipitation of magnesium sodium phosphate (MSP) with MPP.

A biogeochemical model of mineral-based ocean alkalinity enhancement: impacts on the biological pump and ocean carbon uptake

Minimizing anthropogenic climate disruption in the coming century will likely require carbon dioxide removal (CDR) from Earth’s atmosphere in addition to deep and rapid cuts to greenhouse gas emissions. Ocean alkalinity enhancement—the modification of surface ocean chemistry to drive marine uptake of atmospheric CO2—is seen as a potentially significant component of ocean-based CDR portfolios. However, there has been limited mechanistic exploration of the large-scale CDR potential of mineral-based ocean alkalinity enhancement, potential bottlenecks in alkalinity release, and the biophysical impacts of alkaline mineral feedstocks on marine ecology and the marine biological carbon pump. Here we a series of biogeochemical models to evaluate the gross CDR potential and environmental impacts of ocean alkalinity enhancement using solid mineral feedstocks. We find that natural alkalinity sources—basalt and olivine—lead to very low CDR efficiency while strongly perturbing marine food quality and fecal pellet production by marine zooplankton. Artificial alkalinity sources—the synthetic metal oxides MgO and CaO—are potentially capable of significant CDR with reduced environmental impact, but their deployment at scale faces major challenges associated with substrate limitation and process CO2 emissions during feedstock production. Taken together, our results highlight distinct challenges for ocean alkalinity enhancement as a CDR strategy and indicate that mineral-based ocean alkalinity enhancement should be pursued with caution.

OrgAlkCalc: Estimation of organic alkalinity quantities and acid-base properties with proof of concept in Dublin Bay

The presence and influence of organic species is generally omitted in total alkalinity (TA) analysis. This has direct implications to calculated carbonate system parameters and to key descriptors of ocean acidification, especially in coastal waters where organic alkalinity (OrgAlk) can contribute significantly to TA. As titratable charge groups of OrgAlk can act as unknown seawater acid-base systems, the inclusion of the total concentration and apparent dissociation constants of OrgAlk in carbonate calculations involving TA is required to minimise uncertainty in computed speciation. Here we present OrgAlkCalc, an open-source Python based programme that can be used in conjunction with simply modified Global Ocean Acidification Observing Network (GOA-ON) TA titration apparatus to measure TA and OrgAlk, as well as return estimations of associated acid-base properties. The modified titration apparatus and OrgAlkCalc were tested using samples collected from the transitional waters of Dublin Bay, Ireland over a 8 month period (n = 100). TA values ranged from 2257 to 4692 μmol·kg−1 and indicated that freshwater inputs pose a significant source of carbonate alkalinity to Dublin Bay. OrgAlk values ranged from 46 to 234 μmol·kg−1 and were generally observed to be higher in more saline waters, with elevated levels in the Autumn/Winter period. The dissociation constants of two distinct OrgAlk charge groups were identified, with pK values in agreement with previously reported values for humic substances. The majority of OrgAlk charge group concentrations were associated with carboxyl-like charge groups.

Groundwater discharge and streams drive spatial alkalinity and pCO2 dynamics in two contrasting tropical lagoons

Coral reef lagoons are areas of complex carbon cycling, however, regional (e.g. land use) and global (e.g. climate) factors, including land runoff and ocean acidification, are adversely affecting carbonate-building coral reef systems. Coupled with this, surplus nutrients entering coastal waters can prompt excess algae growth, which can stimulate further carbon dioxide (CO2) production in the water column, thus enhancing coral/sediment dissolution. However, new inputs of alkalinity into coastal systems can buffer against acidification. By combining the natural groundwater tracer radon (222Rn) with carbonate chemistry in two contrasting Cook Island lagoons (the fringing Muri Lagoon on Rarotonga and the comparatively larger Aitutaki near-atoll lagoon), we were able to identify multiple drivers of coral reef acidification and regulation. Despite the lagoons having similar rates of submarine groundwater discharge (3.1 to 3.3 cm d−1; although the rate in Aitutaki is a maximum rate based on an assumed 100 m seepage face), groundwater inputs of CO2 and alkalinity were primarily driven by different sources (discrete offshore seeps in Muri Lagoon and dredged channels in Aitutaki Lagoon). Streams delivered low alkalinity water to both lagoons, but high pCO2 waters to the Aitutaki Lagoon in contrast to Muri Lagoon. Aragonite saturation states (ΩAr) ranged between 2.2 and 5.2, with areas of low ΩAr corresponding to areas of high radon and excess algal growth in Muri Lagoon, and areas that receive low alkalinity surface water in Aitutaki. Time-series sampling indicated that tidal heights and the ability of seawater to overtop the fringing reef influenced groundwater dynamics, lagoon hydrodynamics and carbonate chemistry. Groundwater discharge and stream flows were a significant freshwater source of new geologic CO2 and alkalinity to each lagoon, while recirculated seawater is likely a significant source of biologic CO2 driven by microbial respiration in sediments. The study found that while groundwater inputs of alkalinity may reduce acidification, they do not fully counteract ongoing acidification and CO2 inputs. This study also highlighted the need for future studies to undertake detailed spatial measurements to accurately characterise tropical island carbon dynamics due to the heterogeneous nature of these environments.

Lateral Export and Sources of Subsurface Dissolved Carbon and Alkalinity in Mangroves: Revising the Blue Carbon Budget

Mangroves are carbon-rich ecosystems that store large quantities of carbon, mostly in soils. Early carbon (C) budgets indicated that >50% of mangrove C fixation was unaccounted for. This ‘missing C’ has now been discovered to be a large release (423 Tg C a−1) of porewater dissolved DIC (dissolved inorganic carbon), dissolved organic carbon (DOC), and total alkalinity (17 TMOL a−1) via lateral export derived from bacterial decomposition of soil organic matter. This large export originates from DIC produced over at least a 1.0–1.5 m soil profile (280–420 Tg C a−1) via decomposition of autochthonous and allochthonous inputs and/or likely mineralization in deep (≥1 m) ancient soils. DOC and DIC export from mangroves equate to 41% and ≈100% of export from the world’s tropical rivers, respectively. A newly revised blue carbon budget for the world’s mangroves indicates a mean ecosystem gross primary production (GPPE) to ecosystem respiration (RE) ratio of 1.35 and a net ecosystem production (NEP) of 794 g C m−2 a−1 (= global NEP of 117 Tg C a−1), reflecting net autotrophy. CORG burial is 5% and 9% of GPPE and NEPE, respectively. Mean RE/GPPE is 0.74 and carbon use efficiency averages 0.57, higher than for tropical humid forests (0.35).

Non-Redfieldian carbon model for the Baltic Sea (ERGOM version 1.2) – implementation and budget estimates

Abstract. Marine biogeochemical models based on Redfield stoichiometry suffer from underestimating carbon fixation by primary production. The most pronounced indication of this is the overestimation of the dissolved inorganic carbon (DIC) concentration and, consequently, the partial pressure of carbon dioxide in surface waters. The reduced production of organic carbon will impact most biogeochemical processes. We propose a marine biogeochemical model allowing for a non-Redfieldian carbon fixation. The updated model is able to reproduce observed partial pressure of carbon dioxide and other variables of the ecosystem, like nutrients and oxygen, reasonably well. The additional carbon uptake is realized in the model by an extracellular release (ER) of dissolved organic matter (DOM) from phytoplankton. Dissolved organic matter is subject to flocculation and the sinking particles remove carbon from surface waters. This approach is mechanistically different from existing non-Redfieldian models which allow for flexible elemental ratios for the living cells of the phytoplankton itself. The performance of the model is demonstrated as an example for the Baltic Sea. We have chosen this approach because of a reduced computational effort which is beneficial for large-scale and long-term model simulations. Budget estimates for carbon illustrate that the Baltic Sea acts as a carbon sink. For alkalinity, the Baltic Sea is a source due to internal alkalinity generation by denitrification. Owing to the underestimated model alkalinity, an unknown alkalinity source or underestimated land-based fluxes still exist.

Evaluation of alkalinity sources to Cryogenian cap carbonates, and implications for cap carbonate formation models

Current cap carbonate formation models invoke a number of different alkalinity sources for the Neoproterozoic deglacial ocean (e.g., weathering alkalinity, hydrothermal alkalinity, microbial sulfate reduction (MSR) alkalinity, and methane oxidation alkalinity). Assessment of the competing models depends in part on an evaluation of their proposed alkalinity source(s). In the Nanhua rift basin of South China, recently discovered Sturtian cap carbonates on horsts have the potential to address this issue owing to sedimentological and geochemical differences between them and correlative deepwater deposits from adjacent grabens. Cap carbonates at three sites in northeastern Guizhou Province, China were selected for petrographic, XRD, XRF, ICP-MS, in-situ LA-ICP-MS, and Sr and Nd isotope analyses to evaluate the conditions and processes under which they formed: (1) JJS, a high-energy shallow-water (platform) setting that accumulated white laminated dolostone and pisolitic grainstone, (2) ZK01, an intermediate-depth (upper slope) setting that accumulated dark-gray laminated dolostone, and (3) ZK4207, a deep-water (basinal) setting that accumulated organic-rich Mn‑carbonates of the basal Datangpo Formation. The salinity proxy B/Ga records brackish conditions at JJS (4.7 ± 1.2) versus fully marine conditions at ZK01 (7.5 ± 0.8) and ZK4207 (9.1 ± 2.0). Hydrothermal influences, as proxied by high Eu/Eu*, trace-metal (Co, Ni, Cu, V) enrichments, and radiogenic SrNd isotopic compositions, were strong in the latter sections but absent at JJS. These observations document the existence in the post-Sturtian Nanhua Basin of a reduced-salinity surface layer presumably containing a large glacial meltwater component and lacking hydrothermal inputs, and a fully marine deep watermass subject to strong hydrothermal inputs. Carbonate δ13C variation between JJS (−2 to +2‰), ZK01 (−4 to 0‰), and ZK4207 (−7 to −6‰) records a pronounced vertical carbon-isotope gradient in the Nanhua Basin, consistent with elevated marine productivity in the context of strong salinity/density stratification. Massive inputs of alkalinity from meltwater sources are supported by a ∼ 3.5× greater flux to surface-layer dolomitic cap carbonates than to basinal Mn‑carbonates during the Sturtian deglaciation, although strong correlations between intermediate/deep-water carbonates and hydrothermal proxies imply that basin-floor vents were an additional source of alkalinity. Our study provides insight into the mechanisms of formation of cap carbonates generally, with implications for both Sturtian and Marinoan cap carbonate deposits on other cratons.

Erosion of carbonate-bearing sedimentary rocks may close the alkalinity budget of the Baltic Sea and support atmospheric CO2 uptake in coastal seas

High alkalinity values observed in coastal seas promote the uptake of CO2 from the atmosphere. However, the alkalinity budget of coastal areas and marginal seas is poorly understood, even though some of the recently observed alkalinity enhancement can be ascribed to riverine fluxes and anaerobic processes in shelf sediments. Here, we investigate the alkalinity budget of the Baltic Sea to identify previously unrecognized alkalinity sources. We quantify the generation of alkalinity and dissolved calcium (Ca) in this marginal sea applying simple mass balance calculations. Using this approach, we identify alkalinity and Ca sources of approximately 324 Gmol yr-1 and 122 Gmol yr-1, respectively, that cannot be ascribed to the riverine input. The magnitude of the Ca source suggests that a major fraction of the excess alkalinity (244 Gmol yr-1) is induced by the dissolution of calcium carbonate (CaCO3). A review of available field data shows that carbonate-bearing rocks at the coast and the seabed of the Baltic Sea are rapidly eroded and may provide sufficient CaCO3 to close the Ca budget. Hence, dissolution of eroded CaCO3 is the most likely source for the Ca enrichment observed in Baltic Sea water. This hypothesis is supported by mass accumulation rates of sediments derived from radioisotope data that are evaluated to derive a basin-wide rate of mud to muddy sand accumulation at the bottom of the Baltic Sea. The resulting value (139 Tg yr-1) exceeds current estimates of riverine particle fluxes into the Baltic Sea by more than one order of magnitude and confirms that rates of till erosion are sufficiently high to account for the Ca and most of the alkalinity excess in Baltic Sea water. Finally, we show that deliberate addition of CaCO3 to sediments deposited in the Baltic Sea could neutralize significant amounts of CO2 and help to achieve net-zero greenhouse gas emissions in the Baltic region.

Biogas production from anaerobic co-digestion of sewage sludge and food waste in continuously stirred tank reactor

The purpose of current investigation was to explore the scope of integrating an anaerobic digester in a decentralized municipal wastewater treatment plant for in-situ treatment of household waste. The present work intends to identify the potential of sewage sludge and food waste for biogas generation. The study was conducted in lab scale continuous stirred tank reactor having a working volume of 6.5 L at mesophilic temperature. The paper mill sludge was used as inoculum and was stabilised using food waste substrate for initial 50 days in batch mode. Co-digestion of food waste (FW) and bio-flocculated sewage sludge (BFS) was performed using continuous feeding for maintaining OLR of 2.5 gVSL−1D−1at HRT of 4 days for 120 days. 2% FW and 98% BFS (by volume) was feeded in the reactor throughout the experimental duration. The highest VFA accumulation of 1902 mg L−1 were observed and maximum bio-methane yield was found to be 127.05 mLCH4 g−1 VSadded. The pH with current feeding ratio was found stable during the reactor operation without adding external alkalinity source.

Benthic foraminifera and pore water carbonate chemistry on a tidal flat and salt marsh at Ria Formosa, Algarve, Portugal

Benthic foraminifera showed a vertical zonation in tidally influenced salt marshes, which has been used for sea level reconstructions. Growing evidence suggested that freshwater influx, salinity, or the pH of interstitial waters has also an impact on the foraminiferal distribution. A tidal flat and salt marsh transect was investigated in the north-western Ria Formosa coastal lagoon, Algarve, Portugal, to constrain the relationship of benthic foraminifera, halophytes, and pore water properties. The dominance of saltworts from the subfamily Salicornioideae and landward increasing soil salinities depicted evaporation as governing environmental factor. The carbonate chemistry from lagoonal and pore waters identified anoxic tidal flat sediments of as main source of total alkalinity. The alkalinity was lower in the salt marsh, where the pCO2 was extremely high. Salt marsh pore waters showed a high variability of carbonate system parameters, which mirrored small-scale spatial heterogeneities in the soil. The distribution of textulariid salt marsh foraminifera was confined to the vegetated zones, where their abundance increased with elevation. Calcareous species were frequent on the tidal flat and in the highest salt marsh. Many of them were specialised to high salinities or to extreme and variable environmental conditions. Two levels of faunal change in the salt marsh coincide with vegetation zonal boundaries, mean tide or mean high water levels. The two other faunal changes were related to changes in calcite saturation state or organic carbon concentrations. The proportion of textulariids showed a negative correlation with submergence time or elevation, and a significant correlation with pore water pCO2. The faunal distribution, pore water calcite saturation, and Ammonia dissolution patterns indicated that calcareous species specialised to tolerate carbonate-corrosive conditions prevailed even at lowest saturation levels.