Water Dissolved Inorganic Carbon Research Articles

High precision determination of carbon isotope composition and concentration of dissolved inorganic carbon in seawaters

Determination of the isotopic composition and concentration of dissolved inorganic carbon (DIC) in sea water requires not only high measurement accuracy, but also the development of unified approaches to data standardization and material collection. In this work, we studied the effect of sampling methods (“in a container” and “in a vial with acid”, with and without the toxin) on the results of determining the values of δ13C(DIC) and the concentration of DIC in sea water. The analytical protocol was described in detail, based on many years of experience in the selection, measurement and standardization of data obtained for a large number of water samples of the Arctic seas. According to this protocol, the values of δ13C(DIC) and [DIC] can be determined with an error of less than 0.05 ‰ (1σ) and 4.5 rel.% respectively. It has been shown that sampling “in a vial with acid” with their storage for 4 months is accompanied by significant contamination by atmospheric carbon dioxide with an underestimation of δ13C(DIC) values by an average of 0.3–0.8 ‰ and an overestimation of [DIC] values by an average of two times. The absence of the toxin that stops biological activity does not lead to significant shifts in the DIC concentration, but it strongly affects the δ13C(DIC) values, which become underestimated by an average of 1 ‰. Using sampling “in a container”, with toxin addition, and following other recommendations, the samples retain the isotope and concentration parameters of DIC for at least year.

Response of dissolved inorganic carbon dynamics to simulated tidal hydrological processes in coastal wetlands

To clarify the impacts of tidal hydrological process shifts caused by sea level rise on the blue carbon cycle, a typical coastal wetland in Jiaozhou Bay was selected for this study. The soils of Suaeda salsa (SS) and Phragmites australis (PA) wetlands were collected to simulate the effects of three types of tidal hydrological processes (Neap tide group, NT; Middle tide group, MT; Spring tide group, ST) on the soil-water dissolved inorganic carbon (DIC) dynamic. The results showed that the concentration of water dissolved inorganic carbon (WDIC) increased rapidly (115% higher) at early stage (days 0–4) under the influence of the tidal hydrological processes. Significant differences were found in WDIC concentration during different tidal hydrological processes (P < 0.05), which were expressed as MT (52.7 ± 13.3 mg L−1) > ST (52.5 ± 12.9 mg L−1) > NT (48.4 ± 10.1 mg L−1). After experiencing the tidal hydrological processes, the soil DIC content showed a net accumulation (55.1 ± 1.29 mg L−1vs. 46.7 ± 1.76 mg L−1, P < 0.001), whereas the soil inorganic carbon (SIC) decreased (2.73 ± 1.64 mg L−1vs. 4.61 ± 1.71 mg L−1), which may be attributed to the dissolution of SIC caused by the uptake of CO2 to form DIC. The accumulation of soil DIC was directly related to the SIC (λ = 1.03, P < 0.01), and indirectly related to soil nutrients (SOC substrate, λ = −0.003) and microbes (microbial biomass, λ = −0.10), and was mainly dominated by abiotic processes (abiotic: 58.1 ± 1.8% to 82.7 ± 2.46% vs. biotic: 17.4 ± 2.46% to 41.9 ± 1.76%). The increase of tidal frequency generally inhibited the accumulation of soil DIC content and promoted the output of WDIC. However, the response of soil DIC in different wetland types to tidal frequency was divergent, which was mainly regulated by the trade-off between soil nutrients and SIC content. Taken together, tidal hydrological processes and their frequency changes reshaped DIC dynamics, promoted the dissolution of SIC and the potential uptake of CO2. These findings enhance the comprehension of the inorganic carbon cycle within coastal wetlands, particularly amidst the backdrop of climate change and the rising sea levels.

East Asian summer monsoon and early anthropogenic impacts on the lacustrine carbon cycle in the Otindag Sandy Land since the middle Holocene

Lake systems play a pivotal role in the global carbon cycle, yet the biogeochemical processes in alkaline and saline lakes have received little attention. In this study, we present a comprehensive analysis of δ13Ccarb and δ18Ocarb records from Lake Chagan in the Otindag Sandy Land, located in North China, spanning approximately 7000 years. Our aim is to explore how the lacustrine inorganic carbon cycle responded to both climate change and human activity. The findings reveal a notable highstand in Lake Chagan around 6.9–6.0 cal kyr BP, followed by a gradual regression leading to an unstable state over the past 1.6 cal kyr BP. The hydroclimate variations inferred from Lake Chagan align with other regional records, suggesting that long-term East Asian summer monsoon (EASM) precipitation trends were influenced by decreasing insolation and variations in solar irradiance on a centennial timescale. The sources of dissolved inorganic carbon (DIC) in lake water generally mirrored regional hydroclimate changes since the middle Holocene. Our evidence indicates that during periods of strong EASM activity, more allochthonous organic matter was converted to lake water DIC, coinciding with high vegetation cover and intense soil erosion. While hydroclimatic changes significantly impacted the lacustrine carbon cycle, the isotopic evidence also suggests substantial anthropogenic influences post-1.6 cal kyr BP, particularly in terms of the input of allochthonous organic matter into the lakes of the Otindag Sandy Land. These findings underscore the potential for anthropogenic-driven terrestrial carbon cycle feedback to global climate change over millennia.

Simple bulk pigment analysis suggests microphytobenthos contributions to food webs may be underestimated due to isotopic contamination by remineralized wetland carbon

Stable isotope studies have revealed the importance of microphytobenthos (MPB) in coastal food webs. Microalgae typically have δ13C values between depleted C3 wetland/terrestrial macrophytes, and enriched C4 macrophytes and seagrasses. However, the challenges of obtaining clean samples of microalgae from sediments means they are often represented by limited sampling in many food web studies; consequently, we have a limited understanding of spatial and temporal variation in their δ13C values. We tested a simple technique to measure the δ13C of bulk pigments extracted from surficial sediments to represent MPB and applied it to quantify fine scale spatial variation in MPB δ13C around salt marshes. The bulk extraction method is logistically simple, and drives substantial but relatively consistent fractionation in δ13C of −3.5 ± 0.13 ‰ (mean ± 1 S.E., range = 2.3–4.4 ‰, n = 18 paired comparisons) compared to whole cell values. The consistency in fractionation suggests that spatial and temporal δ13C patterns measured in field samples should reflect real variation in source values, and that measured values could be corrected and incorporated into isotope mixing models. In 88 MPB samples among four marsh sites over two summers, MPB δ13C in marsh creeks was lower by an average of 4.4 ± 0.72 ‰ and up to 8.4 ‰ compared to sites along the outer marsh-open water fringe 10's of m away. Few food web studies incorporate this magnitude of variation in their MPB source estimates into mixing models. Over three weekly samplings at one marsh creek site, low tide dissolved inorganic carbon (DIC) δ13C was similarly lower by 4.8 ± 0.36 ‰ and up to 6.2 ‰ compared to high tide and adjacent open water DIC values. The significant small-scale variability in MPB δ13C appears to be driven by remineralized marsh carbon which depletes the DIC δ13C in the marsh creeks, a phenomenon that has long been recognized but is rarely considered in food web studies. Mixing models that assume a narrower range in MPB source values will erroneously attribute isotopically variable MPB contributions to end-member production sources thereby clouding our understanding of energy flows through coastal seascapes.

Impacts of Basal Melting of the Totten Ice Shelf and Biological Productivity on Marine Biogeochemical Components in Sabrina Coast, East Antarctica

AbstractTo clarify the impacts of basal melting of the Antarctic ice sheet and biological productivity on biogeochemical processes in Antarctic coastal waters, concentrations of dissolved inorganic carbon (DIC), total alkalinity (TA), inorganic nutrients, chlorophyll a, and stable oxygen isotopic ratios (δ18O) were measured from the offshore slope to the ice front of the Totten Ice Shelf (TIS) during the spring/summer of 2018, 2019, and 2020. Modified Circumpolar Deep Water (mCDW) intruded onto the continental shelf off the TIS and flowed along bathymetric troughs into the TIS cavity, where it formed a buoyant mixture with glacial meltwater from the ice shelf base. Physical oceanographic processes mostly determined the distributions of DIC, TA, and nutrient concentrations. However, photosynthesis and dilution by meltwater from sea ice and the ice shelf base decreased DIC, TA, and nutrient concentrations in surface water near the ice front. These causes also reduced the partial pressure of CO2 (pCO2) in surface water by more than 100 μatm with respect to mCDW in austral summer of 2018 and 2020, and the surface water became a strong CO2 sink for the atmosphere. Phytoplankton photosynthesis changed DIC and TA in a molar ratio of 106:16. Thus, pCO2 decreased mostly as a result of photosynthesis while dilution by glacial and sea ice meltwater had a small effect. The nutrient consumption ratio suggested that photosynthesis was stimulated by iron in the water column, supplied to the surface layer via buoyancy‐driven upwelling and basal ice shelf meltwater in addition to sea ice meltwater.

A high-throughput atomic emission analyzer for simultaneous field detection of dissolved inorganic and organic carbon in seawater and lake water

Dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) are two important indicators of global carbon cycle. However, there are no portable analyzers available to simultaneously accomplish high-throughput field detection of them in the same sample. Herein, a simple analyzer comprising a dual-mode reactor to accomplish both chemical vapor generation and headspace sampling, and a miniature point discharge optical emission spectrometer (μPD-OES) was developed for simultaneous and high-throughput detection of DIC and DOC in seawater and lake water. Phosphoric acid and persulfate were successively injected into sample solutions to convert DIC and DOC to CO2 under the conditions of magnetic stirring and UV irradiation, respectively. Subsequently, the generated CO2 was swept into the μPD-OES for quantitation of DIC and DOC via monitoring carbon atomic emission at 193.0 nm. Under optimal conditions, limits of detection for DIC and DOC (as C) were both 0.01 mg L−1 with relative standard deviations (n = 20) better than 5% and sample throughput of 80 samples per hour. Compared to conventional analyzers, the proposed instrument provides the advantages of high throughput, compactness, low energy consumption and eliminates expensive instruments. The accuracy of the system was validated by simultaneous determination of DIC and DOC in various water samples in laboratory and field environments.

HgCl2 addition to pore water samples from cold seeps can affect the geochemistry of dissolved inorganic carbon ([DIC], δ13CDIC)

The conventional use of mercuric chloride (HgCl2) as an antimicrobial agent in water samples for dissolved inorganic carbon (DIC) analysis poses health and environmental risks related to its handling and disposal. Even though there is an increasing interest in quantifying pore water DIC contribution to the ocean carbon cycle and deep-sea acidification, the paucity of comparative studies on marine pore waters prevents the modification of sampling protocols and HgCl2 still remains widely used. Here, we compared DIC concentrations and δ13CDIC composition in pore water samples from methane seepage areas in the Barents Sea and offshore N. Svalbard. Samples were extracted using 0.15 μm rhizon filters and split into two aliquots with 2–3 replicates each. Only one aliquot was treated with 10 μL saturated HgCl2(aq) and all samples were stored in the dark at 4 °C, prior to measurements ∼30 days later. The samples yielded extremely wide ranges of DIC concentrations, from 1.8 mM to 20.1 mM, and δ13CDIC values, from −36.0‰ to −1.6‰ (VPDB), due to variable contributions of methane-derived DIC to the pore water system. Overall, we obtained lower [DIC] (77% samples; N = 26) and heavier δ13C values (79% samples; N = 42) in Hg-treated samples. Isotopic and concentration differences larger than the uncertainties on the mean of replicated measurements (±0.2–0.5‰; ± 0.5 mM) and analytical precision (0.15‰; 0.71 mM) represent the 38% and 19% of the samples, with offsets of up to 7.4‰ and 1.9 mM, respectively. The largest offsets are in agreement with our CO2-degassing model, suggesting an interaction between mercuric chloride and dissolved hydrogen sulfide released by sulfate-driven methane oxidation. We therefore caution against further use of HgCl2 for DIC studies of marine pore waters from cold seeps.

Distance-based paper analytical device for the determination of dissolved inorganic carbon concentration in freshwater

Dissolved inorganic carbon (DIC) levels in freshwaters play a key role in the equilibrium of the carbon cycle between the atmosphere, water and living beings. Standard classical methods for DIC determination generally involve bulky and expensive equipment used in centralized laboratories, resulting in time-consuming processes that do not allow for adequate monitoring in the field. In order to address this challenge, we have developed a distance-based paper analytical device (PAD) for on-site determination of DIC in water. The portable, cost-effective and easy-to-use device was based on the miniaturization and integration of a classical acid-base colorimetric titration on a paper channel, enabling an accurate determination of DIC in less than 20 min. The length of the blue colored line in the detection channel after being filled with the sample was related to the DIC concentration in the sample. The reagent solution used to modify the titration channel was optimized so that DIC concentrations in the range 50–1000 mg L−1 could be measured. The long-term stability of the paper-based device was also evaluated, demonstrating a working stability for more than 70 days after their fabrication, an important characteristic for in-the-field analysis. Finally, the PAD was validated with different water samples, i.e. tap water, commercial bottled drinking water and water samples from a mine, with excellent agreement between the results obtained from the PAD and the standard method. This demonstrates the high potential of the proposed paper analytical device to quantify DIC in situ by minimally-trained personnel without the need for peripheral equipment, which represents an important advance compared to the current limited analysis systems.

Inorganic Carbon Transport and Dynamics in the Florida Straits

AbstractOcean heat and carbon are transported through the Florida Straits, contributing to the Atlantic Meridional Overturning Circulation, and playing an important role in climate. Insufficient observations of carbonate chemistry within the Florida Straits have limited our understanding of ocean acidification within this region. To examine carbonate chemistry and carbon transport dynamics within this region, we developed an algorithm to estimate dissolved inorganic carbon (DIC) using more routinely measured input parameters (temperature, salinity, and dissolved oxygen [DO]) and the corresponding sampling date, depth, and longitude. The developed DIC algorithm output demonstrates good agreement with limited existing in situ observations. By applying this algorithm, we developed a seasonally resolved time series of DIC spanning from 2002 to 2018 for the Florida Straits at 27°N. This time series suggests that short‐term variations in surface water DO and DIC were strongly influenced by the Florida Current transport. The long‐term increase in DIC was mainly caused by anthropogenic carbon accumulation and DO decrease. The highest increasing rate in DIC was found in North Atlantic Central Water where DO decrease was fastest while the decreasing rate in pH was highest in Antarctic Intermediate Water (AAIW) because of the lower buffer capacity of this water mass. The long‐term pH decrease, especially in AAIW, can impact the health of deep corals in the Florida Straits. Quantifying carbon transport between the coast of Florida and the Bahamas is important to understanding the carbonate chemistry dynamics and the long‐term acidification of this important region.

The evolution of dissolved inorganic carbon (DIC) and δ13CDIC in acid mine drainage polluted karst rivers: Preliminary findings from laboratory experiments

• The water chemistry evolution in AMD-karst river mixture water are main influenced by DO and Fe 2+ . • Mean loss of 41.0 ± 11.8 % occurred in the initial DIC in AMD mixed with karst river water. • δ 13 C DIC was gradually enriched with increasing rate of DIC loss. • The main effect mechanism of DIC in AMD-karst river mixture water are HCO 3 − dehydration and exchange of CO 2(aq) . Acid mine drainage (AMD) has caused serious pollution in the surface water environments of karst areas, but the effect of AMD on the dissolved inorganic carbon (DIC) dynamics in the receiving karst rivers remains poorly understood. The AMD and karst river water were sampled from the Yangliujie River watershed in Southwest China, and mixed different proportions of AMD and karst river water in a laboratory experiment to simulate seasonal hydrological conditions in the river, in an attempt to explore the impact of AMD on the loss rate and evolution of DIC in karst river water. The results showed that AMD can cause a rapid pH drop in karst river water, resulting in approximately 41.0 ± 11.8 % DIC loss in the early stage of mixing as CO 2(g) release to the atmosphere. With the biological or chemical oxidation of Fe 2+ , and followed by Fe 3+ biogenic formation of schwertmannite or hydrolysis, the pH decreased and the DIC concentration further decreased in the mixed samples, the degree of decrease was related to the oxidation rate of Fe 2+ . With decreasing DIC concentration, δ 13 C DIC enrichment gradually increased. When the mixing ratio of AMD was large, the total H + in AMD and H + generated by the oxidation of Fe 2+ exceeded the HCO 3 − content in karst river water, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was small, and the DIC transformation was mainly dominated by the loss of CO 2(g) . When a small proportion of AMD was mixed in, the HCO 3 − concentration in the mixed water sample was in excess of the H + produced by Fe 2+ and the total H + in AMD, the isotopic enrichment coefficient caused by the initial mixing and the end of the reaction was larger, and the DIC conversion was mainly affected by HCO 3 − dehydration. These preliminary findings support evidence showing DIC conversion and loss in AMD-affected karst rivers increase the transfer of carbon dioxide to the atmosphere, which diminishes net carbon sink fluxes in karst river basins, so reasonable AMD treatment methods should be applied to minimize these negative impacts.

Origin of Deep Methane from Active Faults along the Itoigawa–Shizuoka Tectonic Line between the Eurasian and North American Plates: 13C/12C and 14C/12C Methane Profiles from a Pull-Apart Basin at Lake Suwa

Stable and radiocarbon isotope analyses provide valuable insights into carbon dynamics in hydrosphere ecosystems. A lake containing seeps of deep methane (CH4) may provide an ideal environment for elucidating such dynamics, including the involvement of deep carbon in the surface ecosystem and its food web. In this study, we performed carbon isotope analysis of seep gas, lake water, and a cyanobacterial bloom sample collected at Lake Suwa, a pull-apart basin on the Itoigawa–Shizuoka tectonic line, Japan. CH4 collected from two seep sites 100 m from the lakeshore was 14C-depleted (Δ14C = −972.1 to −989.8‰), indicating that the origins of seep CH4 and benthic bubble CH4 (Δ14C = +21.7‰) collected near the lakeshore differed. The results suggested that benthic bubble CH4 was produced by methanogenic archaea in benthic sediments and that seep CH4 was produced by methanogenic archaea using relic carbon. The Δ14C values of dissolved inorganic carbon (DIC) in surface lake water collected at seep sites (Δ14C = −103.1 to −630.6‰) and near the lakeshore (Δ14C = −100.9‰) suggested that the influence of deep carbon occurred not only around the seep sites but also at the lakeshore. The Δ14C values of the cyanobacterial bloom sample also showed values similar to those of DIC (Δ14C = −110.3‰), suggesting that 10–63% of DIC in surface water and 10% in primary producers originated from relic carbon, including CH4 and CO2, and was incorporated into the surface ecosystem of Lake Suwa.

Seasonal and Spatial Variations of δ13CDIC Values in the Mun River, Northeast Thailand

As an important part of the global carbon cycle, dissolved inorganic carbon (DIC) concentration and its stable carbon isotopic composition (δ13CDIC) have been used to constrain the sources of DIC in rivers. In this study, we systematically investigated the water chemistry, DIC contents, and δ13CDIC values in a tropical agricultural river in northeast Thailand. The water temperature ranged from 20.3 to 31.3 °C, and water pH values ranged from 6.4 to 8.4, with seasonal variations. Based on the major ion compositions, the hydro-chemical type of the Mun River water was a unique Na–Ca–Cl–HCO3 type, controlled by evaporite and silicate weathering. Seasonal variation of DIC concentrations and its carbon isotopic composition was obvious; DIC and δ13CDIC were significantly lower in the wet season (135 to 3146 μmol/L and −31.0‰ to −7.0‰) compared to the dry season (185 to 5897 μmol/L and −19.6‰ to −2.7‰). A high level of 12C-enriched DIC/CO2 from soil respiration and organic matter oxidation may cause the low pH values, δ13CDIC values, and high partial pressure of CO2 (pCO2) in the middle and lower reaches during the wet/rainy season compared to the dry season. This may be responsible for the seasonal and spatial variations of DIC concentrations and δ13CDIC values in the Mun River. According to the relationship between pCO2 and δ13CDIC values, CO2 outgassing may be more significant in the dry season, due to the greater influx of groundwater with higher pCO2 levels; and the rapid CO2 diffusion into the atmosphere will continuously increase the δ13CDIC values and decrease pCO2 levels. These results show that riverine biologic effects and CO2 outgassing play important roles in the DIC and δ13CDIC evolution of this typical agriculturally-dominated watershed.

Sulfur and Oxygen Isotope Records of Sulfate-Driven Anaerobic Oxidation of Methane in Diffusion-Dominated Marine Sediments

Organoclastic sulfate reduction (OSR) and sulfate-driven anaerobic oxidation of methane (SD-AOM) are the two major microbial pathways for sulfate consumption in marine sulfur cycle. The relative changes of sulfur and oxygen isotope ratios in pore water sulfate are affected by the mode of microbial sulfate reduction and have been applied as an indicator for assessing methane excess environments. However, so far, this isotope proxy fails to distinguish sulfate reduction processes fueled by the oxidation of organic matter or by diffusing methane. To better understand the mechanism of sulfur and oxygen isotope partitioning during OSR and SD-AOM, coupled sulfur and oxygen isotopic compositions of pore water sulfate (δ34SSO4 and δ18OSO4) were investigated from four methane diffusing sites (CL56, CL57, CL59, and CL60) of the South China Sea, supplemented by carbon isotopic composition of dissolved inorganic carbon (DIC) and sulfur isotopic composition of pyrite in bulk sediments. Pore water sulfate and DIC concentrations, as well as calculated net sulfate reduction rates suggest that the sulfate reduction at site CL57 was mainly dominated by OSR, whereas sites CL56, CL59, and CL60 were likely impacted by both OSR and SD-AOM. Furthermore, the trend of cross-plotting δ18OSO4 versus δ34SSO4 values from site CL57 was distinguishable from sites CL56, CL59, and CL60, although all study sites show similar patterns to those derived from methane limited environments. This further indicates the trajectory of sulfur and oxygen isotope partitioning was affected by the mode of sulfate reduction (i.e., OSR vs. SD-AOM). At site CL57, the low net sulfate reduction rate would lead to enhanced oxidation of intermediate sulfur species during OSR, thus leading to a higher slope in the δ18OSO4 vs. δ34SSO4 cross-plot (1.26). In contrast, the higher net sulfate reduction rates at sites CL56, CL59, and CL60 due to the impact from SD-AOM would lead to lower slopes in the δ18OSO4 vs. δ34SSO4 cross-plots (0.78 ± 0.11). This study provides new insights into the sulfur and oxygen isotope systematics during microbial sulfate reduction processes in methane diffusing environments.

Lake water dissolved inorganic carbon dynamics revealed from monthly measurements of radiocarbon in the Fuji Five Lakes, Japan

Lakes are sensitive recorders of anthropogenic activities, as human society often develops in their vicinity. Lake sediments thus have been widely used to reconstruct the history of environmental changes in the past, anthropogenic, or otherwise, and radiocarbon dating provides chronological control of the samples. However, specific values of radiocarbon in different carbon reservoirs due to the different pathways of radiocarbon from the upper atmosphere to the lake, called the radiocarbon reservoir age, is always difficult to evaluate because of dynamic processes in and around lakes. There are few systematic studies on radiocarbon reservoir ages for lakes owing to the complex radiocarbon transfer processes for lakes. Here, we investigate lake waters of the Fuji Five Lakes with monthly monitoring of the radiocarbon reservoir effects. Radiocarbon from dissolved inorganic carbon (DIC) for groundwater and river water is also measured, with resulting concentrations (Δ14C) at their lowest at Lake Kawaguchi in August 2018 (–122.4 ± 3.2‰), and at their highest at Lake Motosu in January 2019 (–22.4 ± 2.5‰), despite a distance of 25 km. However, winter values in both lakes show similar trends of rising Δ14C (about 20‰). Our lake water DIC Δ14C results are compared to previously published records obtained from sediments in Lake Motosu and Lake Kawaguchi. These suggest that total organic carbon and compound-specific radiocarbon found in sediments are heavily influenced by summer blooms of aquatic organisms that fix DIC in water. Thus, future studies to conduct similar analyses at the various lakes would be able to provide further insights into the carbon cycle around inland water, namely understanding the nature of radiocarbon reservoir ages.

Total Dissolved Inorganic Carbon Sensor Based on Amperometric CO2 Microsensor and Local Acidification.

We present a dipping probe total dissolved inorganic carbon (DIC) microsensor based on a localized acidic microenvironment in front of an amperometric CO2 microsensor. The acidic milieu facilitates conversion of bicarbonate and carbonate to CO2, which in turn is reduced at a silver cathode. Interfering oxygen is removed by an acidic CrCl2 oxygen trap. Theoretical simulations of microsensor functioning were performed to find a suitable compromise between response time and near-complete conversion of bicarbonate to CO2. The sensor exhibited a linear response over a wide range of 0-8 mM DIC, with a calculated LOD of 5 μM and a 90% response time of 150 s. The sensor was successfully tested in measuring DIC in bottled mineral water and seawater. This DIC microsensor holds the potential to become an important tool in environmental sensing and beyond for measurements of DIC at high spatial and temporal resolution.

A SIMPLE CO2 EXTRACTION METHOD FOR RADIOCARBON ANALYSES OF DISSOLVED INORGANIC CARBON IN WATER SAMPLES WITHOUT A CARRIER GAS

ABSTRACTWe developed a simple and cost-effective method for extracting carbon from dissolved inorganic carbon (DIC) in water samples without a carrier gas. This method only slightly modifies the existing vacuum line for CO2 purification in radiocarbon research laboratories by connecting several reservoirs and traps. The procedure consists of repeated cycles of CO2 extraction from water into the headspace of the reaction container, expansion of the extracted gas into the vacuum line, and cryogenic trapping of CO2. High CO2 yield (∼98%) was obtained from a variety of water samples with a wide range of DIC concentrations (0.4–100 mmol·L−1, in the case of 1.2 mgC). The δ13C fractionation depended on the CO2 yield, while the 14C concentration was constant within the error range, regardless of the CO2 yield. The average δ13C discrepancy between the results of this method and direct analyses made using the GC-IRMS was 0.02 ± 0.06‰. The standard deviations (1σ) in fraction of modern carbon (F14C) ranged from 0.0002 to 0.0004 for waters below 0.01 of F14C, and below 0.8% of F14C values for waters above 0.1. We conclude that this method is useful for effectively extracting CO2 from DIC in water and yields accurate 14C data.

Pore water exchange‐driven inorganic carbon export from intertidal salt marshes

AbstractRespiration in intertidal salt marshes generates dissolved inorganic carbon (DIC) that is exported to the coastal ocean by tidal exchange with the marsh platform. Understanding the link between physical drivers of water exchange and chemical flux is a key to constraining coastal wetland contributions to regional carbon budgets. The spatial and temporal (seasonal, annual) variability of marsh pore water exchange and DIC export was assessed from a microtidal salt marsh (Sage Lot Pond, Massachusetts). Spatial variability was constrained from 224Ra : 228Th disequilibria across two hydrologic units within the marsh sediments. Disequilibrium between the more soluble 224Ra and its sediment‐bound parent 228Th reveals significant pore water exchange in the upper 5 cm of the marsh surface (0–36 L m−2 d−1) that is most intense in low marsh elevation zones, driven by tidal overtopping. Surficial sediment DIC transport ranges from 0.0 to 0.7 g C m−2 d−1. The sub‐surface sediment horizon intersected by mean low tide was disproportionately impacted by tidal pumping (20–80 L m−2 d−1) and supplied a seasonal DIC flux of 1.7–5.4 g C m−2 d−1. Export exceeded 10 g C m−2 d−1 for another marsh unit, demonstrating that fluxes can vary substantially across salt marshes under similar conditions within the same estuary. Seasonal and annual variability in marsh pore water exchange, constrained from tidal time‐series of radium isotopes, was driven in part by variability in mean sea level. Rising sea levels will further inundate high marsh elevation zones, which may lead to greater DIC export.

Sources of Corrosive Bottom Water to Bellingham Bay, Washington State

The Salish Sea, spanning Washington State and British Columbia, receives relatively low pH water from upwelling in the northeast Pacific Ocean and the acidity of bottom water increases within the sea, particularly in winter. In order to better understand the processes that lead to bottom water acidification in this region, I quantified the sources of dissolved inorganic carbon (DIC) and alkalinity to bottom water in Bellingham Bay, a small embayment that is nevertheless representative of the broader Salish Sea. Corrosive bottom water with a pH of 7.5 and an aragonite saturation state of 0.51 was observed in deep water the northern portion of the bay. Water column respiration contributed 6.8 mmol DIC m−3 d−1 to bottom water while sediments added both DIC (13.6 mmol m−2 d−1) and alkalinity (9 mmol m−2 d−1). A two-layer box model assessed the relative importance of sources of corrosive water to the bay and calculated how bottom water pH and aragonite saturation state vary during spring. The 30-day model run calculated increases in bottom water DIC and alkalinity but decreases in bottom water pH and aragonite saturation state along with oscillations of these parameters with a period of approximately 6 days. A sensitivity analysis of the model demonstrated that water column respiration was the primary driver of corrosive bottom-water production. Sediment respiration contributed less to the production of corrosive water but was a source of alkalinity. The accumulation of corrosive bottom water in Bellingham Bay was also a function of mean circulation; the slowing of bottom water flow led to longer bottom-water residence time and an increase in bottom water corrosivity.

Deepening roots can enhance carbonate weathering by amplifying CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;-rich recharge

Abstract. Carbonate weathering is essential in regulating atmospheric CO2 and carbon cycle at the century timescale. Plant roots accelerate weathering by elevating soil CO2 via respiration. It however remains poorly understood how and how much rooting characteristics (e.g., depth and density distribution) modify flow paths and weathering. We address this knowledge gap using field data from and reactive transport numerical experiments at the Konza Prairie Biological Station (Konza), Kansas (USA), a site where woody encroachment into grasslands is surmised to deepen roots. Results indicate that deepening roots can enhance weathering in two ways. First, deepening roots can control thermodynamic limits of carbonate dissolution by regulating how much CO2 transports vertical downward to the deeper carbonate-rich zone. The base-case data and model from Konza reveal that concentrations of Ca and dissolved inorganic carbon (DIC) are regulated by soil pCO2 driven by the seasonal soil respiration. This relationship can be encapsulated in equations derived in this work describing the dependence of Ca and DIC on temperature and soil CO2. The relationship can explain spring water Ca and DIC concentrations from multiple carbonate-dominated catchments. Second, numerical experiments show that roots control weathering rates by regulating recharge (or vertical water fluxes) into the deeper carbonate zone and export reaction products at dissolution equilibrium. The numerical experiments explored the potential effects of partitioning 40 % of infiltrated water to depth in woodlands compared to 5 % in grasslands. Soil CO2 data suggest relatively similar soil CO2 distribution over depth, which in woodlands and grasslands leads only to 1 % to ∼ 12 % difference in weathering rates if flow partitioning was kept the same between the two land covers. In contrast, deepening roots can enhance weathering by ∼ 17 % to 200 % as infiltration rates increased from 3.7 × 10−2 to 3.7 m/a. Weathering rates in these cases however are more than an order of magnitude higher than a case without roots at all, underscoring the essential role of roots in general. Numerical experiments also indicate that weathering fronts in woodlands propagated &gt; 2 times deeper compared to grasslands after 300 years at an infiltration rate of 0.37 m/a. These differences in weathering fronts are ultimately caused by the differences in the contact times of CO2-charged water with carbonate in the deep subsurface. Within the limitation of modeling exercises, these data and numerical experiments prompt the hypothesis that (1) deepening roots in woodlands can enhance carbonate weathering by promoting recharge and CO2–carbonate contact in the deep subsurface and (2) the hydrological impacts of rooting characteristics can be more influential than those of soil CO2 distribution in modulating weathering rates. We call for colocated characterizations of roots, subsurface structure, and soil CO2 levels, as well as their linkage to water and water chemistry. These measurements will be essential to illuminate feedback mechanisms of land cover changes, chemical weathering, global carbon cycle, and climate.

The carbonate system of the northern South China Sea: Seasonality and exchange with the western North Pacific

Using a most comprehensive, high quality, high-resolution dataset for any marginal sea up to depths >2000 db, we examined the seasonality of the carbonate system in the northern South China Sea (nSCS) and exchange with the West Philippine Sea (WPS) during 2009–2011. The carbonate system dynamics demonstrated evident spatial and seasonal variations. Winter exhibited the highest average surface dissolved inorganic carbon (DIC) concentrations (1936 ± 15 μmol kg−1), and summer had the lowest (1882 ± 12 μmol kg−1), primarily associated with more abundant freshwater inputs in summer. At 100 db depth, decreased DIC and total alkalinity (TA) values were observed within the Luzon Strait vicinity due to the influence of WPS waters. Higher DIC and TA concentrations were found within the central nSCS basin. The average Kuroshio contribution to the DIC inventory in the upper 150 db was seasonally significant, ranging between 11 and 32%, with the highest contributions during spring and winter. Below 2000 db, nSCS basin-averaged DIC was significantly higher than WPS-averaged DIC (∼23 µmol kg−1 difference) due to more organic matter decomposition in the nSCS basin. Within the basin, average deep water DIC values were highest in autumn, and averaged concentrations at >18.5°N were lower than at ≤18.5°N. Our datasets and analysis imply that (i) the significant seasonal and spatial patterns of carbonate chemistry in the nSCS are controlled by a combination of large-scale and smaller mesoscale physical processes; (ii) extrinsically from Asian monsoons via seasonal freshwater discharge and dynamic exchanges with open ocean waters; and (iii) intrinsically, through seasonal vertical mixing as well as mesoscale processes and their subsequent new productions. The seasonal and spatial variability in carbonate parameters established here serves as an essential baseline to monitor future changes to the nSCS and to compare with other marginal sea systems.