Dissolved Inorganic Carbon Measurements Research Articles

Development of an autonomous on-site dissolved inorganic carbon analyzer using conductometric detection

BackgroundThe increase in anthropogenic CO2 concentrations in the Earth's atmosphere since the industrial revolution has resulted in an increased uptake of CO2 by the oceans, leading to ocean acidification. Dissolved Inorganic Carbon (DIC) is one of the key variables to characterize the seawater carbonate system. High quality DIC observations at a high spatial-temporal resolution is required to improve our understanding of the marine carbonate system. To meet the requirements, autonomous DIC analyzers are needed which offer a high sampling frequency, are cost-effective and have a low reagent and power consumption. ResultsWe present the development and validation of a novel analyzer for autonomous measurements of DIC in seawater using conductometric detection. The analyzer employs a gas diffusion sequential injection approach in a “Tube In A Tube” configuration that facilitates diffusion of gaseous CO2 from an acidified sample through a gas permeable membrane into a stream of an alkaline solution. The change in conductivity in the alkaline medium is proportional to the DIC concentration of the sample and is measured using a detection cell constructed of 4 hollow brass electrodes. Physical and chemical optimizations of the analyzer yielded a sampling frequency of 4 samples h−1 using sub mL reagent volumes for each measurement. Temperature and salinity effects on DIC measurements were mathematically corrected to increase accuracy. Analytical precision of ±4.9 μmol kg−1 and ±9.7 μmol kg−1 were achieved from measurements of a DIC reference material in the laboratory and during a field deployment in the southwest Baltic Sea, respectively. SignificanceThis study describes a simple, cost-effective, autonomous, on-site benchtop DIC analyzer capable of measuring DIC in seawater at a high temporal resolution as a step towards an underwater DIC sensor. The analyzer is able to measure a wide range of DIC concentrations in both fresh and marine waters. The achieved accuracy and precision offer an excellent opportunity to employ the analyzer for ocean acidification studies and CO2 leakage detection in the context of Carbon Capture and Storage operations.

High Resolution Estimation of Ocean Dissolved Inorganic Carbon, Total Alkalinity and pH Based on Deep Learning

This study combines measurements of dissolved inorganic carbon (DIC), total alkalinity (TA), pH, earth observation (EO), and ocean model products with deep learning to provide a good step forward in detecting changes in the ocean carbonate system parameters at a high spatial and temporal resolution in the North Atlantic region (Long. −61.00° to −50.04° W; Lat. 24.99° to 34.96° N). The in situ reference dataset that was used for this study provided discrete underway measurements of DIC, TA, and pH collected by M/V Equinox in the North Atlantic Ocean. A unique list of co-temporal and co-located global daily environmental drivers derived from independent sources (using satellite remote sensing, model reanalyses, empirical algorithms, and depth soundings) were collected for this study at the highest possible spatial resolution (0.04° × 0.04°). The resulting ANN-estimated DIC, TA, and pH obtained by deep learning shows a high correspondence when verified against observations. This study demonstrates how a select number of geophysical information derived from EO and model reanalysis data can be used to estimate and understand the spatiotemporal variability of the oceanic carbonate system at a high spatiotemporal resolution. Further methodological improvements are being suggested.

Comparison of discrete and underway CO2 measurements: Inferences on the temperature dependence of the fugacity of CO2 in seawater

The fugacity or partial pressure of CO2 in surface water (fCO2w) is a key parameter to determine air-sea CO2 fluxes and the evolution of ocean acidification. Despite its importance some key physical chemical characteristics are not fully resolved, notably its dependence on temperature. The fCO2w is mostly measured by autonomous underway systems near in situ sea surface temperature (SST). Subsurface measurements are commonly carried out on individual (discrete) samples at a fixed temperature, normally 20 °C. Here, the underway system observations are compared with co-located discrete observations to determine the consistency of these types of measurements. The co-located discrete fCO2w at 20 °C and underway fCO2w measurements at SST are used to infer the temperature dependence of CO2. In addition, calculated fCO2w from total alkalinity (TA) and total dissolved inorganic carbon (DIC) are compared with the underway and discrete fCO2w measurements. For 21 cruises spanning the major ocean basins from 1992 to 2020 a temperature dependence of 4.13 ± 0.01% °C−1 is determined in close agreement with a widely used previous empirical estimate of 4.23 ± 0.02% °C−1 for North Atlantic surface water. The temperature dependency of calculated fCO2w from TA and DIC using recommended constants is 4.10% °C−1 for 17 cruises where there are co-located measurements of fCO2w, TA and DIC.

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 Membrane Microfluidic Sensor for Conductivity Measurement of Ocean Dissolved Inorganic Carbon

Atmospheric CO2 has increased by > 40% since pre-industrial times leading to significant global warming. The ocean absorption of CO2 is saturating, possibly leading to much more serious consequences than predicted. The latest IPCC special report SR15 warns that a 1.5 oC average temperature increase by 2030 is likely and associated CO2 ocean acidification is projected to amplify the adverse effects of warming. The ocean CO2 system is characterized by measuring at least 2 out of 4 canonical “master variables”: total dissolved inorganic carbon (DIC), total alkalinity (AT), partial CO2 (pCO2), and acidity (pH). Understanding this system is of fundamental importance yet CO2 continues to be chronically under-sampled in both space (including depth profiles) and time. Unfortunately, pCO2 and pH are very sensitive to temperature/pressure, hence autonomous measurements of DIC or AT are much preferred but have yet to be realized. Observations are almost exclusively limited to the surface and use large sensors, e.g. spectrophotometry, NDIR and mass spectrometry, attached to expensive research vessels. These large systems are unlikely candidates for miniaturisation. Very large uncertainties therefore still remain in the CO2 budget.The Argo float network, a global array of >3,000 untethered battery-operated floats with satellite data transfer, enables continuous monitoring of ocean salinity and temperature depth profiles. This float infrastructure awaits new autonomous chemical sensor technology but will impose severe constraints on device footprint, volume, power consumption and long-term environmental stability under high pressure cycling (up to 200 atm). Instantaneous measurement of DIC during float ascent or descent is not possible as the CO2 has to be extracted from the seawater first. Hence at each depth, an autonomous device would have to collect and store a seawater sample for subsequent analysis while the float is at park depth (~1500m). This analysis involves several chemical reactions and partial or complete equilibration across a membrane with high precision conductivity measurements. Microfluidic ocean-relevant chemical sensors have received limited attention. Research efforts have concentrated on colorimetric or luminescence techniques at shallow depths.We investigate a potential miniaturised version of a standard bench technique for DIC measurement based on flow injection conductimetric measurements.[1] To achieve a fully functioning DIC profiling system requires microfluidic lab on chip structures with sample volumes ≤ 1uL (to minimise 3 years of reagent payload), precision channel patterning, multi-layer thermoplastic bonding, and liquid-liquid membrane separation of CO2, with incompatible gas-permeable ion blocking materials, suitable for >3-year ocean deployment at high pressures. Recently we demonstrated long-term PDMS membrane bonding within a thermoplastic manifold [2] as well high pressure resilient thermoplastic bonding for multi-layer and multi-channel devices. [3] We have established the principle of precision micro-metering to inject HCl acid into the seawater sample. [4] The detection principle is based on CO2, transferred across the membrane, dissolves in high strength NaOH leading to a reduction in conductivity. [5] Conductivity electrodes are limited in size by sample volume and must operate continuously in a highly corrosive environment (NaOH, pH12). Previously we attempted a capacitively-coupled approach using thin-film protective insulating layer on copper tracks. However, this resulted in severe signal attenuation. Here we report direct metal contact conductivity cells with active volumes between 0.5 mL and 2.0 mL for DIC detection in the seawater range 1900 – 2400 mmol Kg-1. For these volumes, the use of macroscopic wires or plates as electrodes is not feasible and instead, we developed a thin film metallisation process onto PMMA using a sputtered thin film of gold (< 200 nm) on top of a 10 nm adhesion promoting inter-layer of sputtered Ti.After transfer across the membrane, the eluted CO2 in a NaOH carrier, was drawn through a < 2 mL conductivity cell where the change in impedance versus time was measured. Minimum precision values obtained from relative standard deviation were ~ 0.2 % from peak height and 0.5% from area under curve. This compares favourably with precision values of ~ 0.25 % obtained using a large C4D electrophoresis headstage of similar active measurement volume. The required sample and reagent volumes amounted to ~ 500 mL in total due to the incorporation of a planar membrane into a small volume exchange cell. This compares very favourably with reported attempts at conductivity based DIC analysis where total volumes between 5,000 mL and 10,000 mL were required as, in order to achieve the required precision, bulk wire electrodes and 20cm long membrane tubes were used. We show therefore the first microfluidic-based DIC sensor with accuracy, precision and reagent payload that offers a route to global autonomous ocean CO2 depth profiling. P. Hall and R. Aller, Limnol. Oceanogr., 1992, 37, 1113 https://doi.org/10.4319/lo.1992.37.5.1113 M. Tweedie et al, Lab Chip, 2019, 1287 https://doi.org/10.1039/C9LC00123A D. Sun et al., Microfluid. Nanofluid., 2015, 913 https://doi.org/10.1007/s10404-015-1620-2 M. Tweedie et al. https://arxiv.org/abs/1909.01845 M. Tweedie et al., https://doi.org/10.26434/chemrxiv.8852087.v1 Figure 1

Seasonal Carbon Dynamics in the Near‐Global Ocean

The seasonal cycle represents one of the largest signals of dissolved inorganic carbon (DIC) in the ocean, yet these seasonal variations are not well established at a global scale. Here, we present the Mapped Observation‐Based Oceanic DIC (MOBO‐DIC) product, a monthly DIC climatology developed based on the DIC measurements from GLODAPv2.2019 and a two‐step neural network method to interpolate and map the measurements. MOBO‐DIC extends from the surface down to 2,000 m and from 65°N to 65°S. We find the largest seasonal amplitudes of surface DIC in the northern high‐latitude Pacific (∼30 to &gt;50 μmol kg−1). Surface DIC maxima occur in hemispheric spring and minima in fall, driven by the input of DIC into the upper ocean by mixing during winter, and net community production (NCP) driven drawdown of DIC over summer. The seasonal pattern seen at the surface extends to a nodal depth of &lt;50 m in the tropics and several hundred meters in the subtropics. Below the nodal depth, the seasonal cycle of DIC has the opposite phase, primarily owing to the seasonal accumulation of DIC stemming from the remineralization of sinking organic matter. The well‐captured seasonal drawdown of DIC in the mid‐latitudes (23° to 65°) allows us to estimate the spring‐to‐fall NCP in this region. We find a spatially relatively uniform spring‐to‐fall NCP of 1.9 ± 1.3 mol C m−2 yr−1, which sums to 3.9 ± 2.7 Pg C yr−1 over this region. This corresponds to a global spring‐to‐fall NCP of 8.2 ± 5.6 Pg C yr−1.

Modeling inorganic carbon dynamics in the Seine River continuum in France

Abstract. Inland waters are an active component of the carbon cycle where transformations and transports are associated with carbon dioxide (CO2) outgassing. This study estimated CO2 emissions from the human-impacted Seine River (France) and provided a detailed budget of aquatic carbon transfers for organic and inorganic forms, including the in-stream metabolism along the whole Seine River network. The existing process-based biogeochemical pyNuts-Riverstrahler model was supplemented with a newly developed inorganic carbon module and simulations were performed for the recent time period 2010–2013. New input constraints for the modeling of riverine inorganic carbon were documented by field measurements and complemented by analysis of existing databases. The resulting dissolved inorganic carbon (DIC) concentrations in the Seine aquifers ranged from 25 to 92 mg C L−1, while in wastewater treatment plant (WWTP) effluents our DIC measurements averaged 70 mg C L−1. Along the main stem of the Seine River, simulations of DIC, total alkalinity, pH and CO2 concentrations were of the same order of magnitude as the observations, but seasonal variability was not always well reproduced. Our simulations demonstrated the CO2 supersaturation with respect to atmospheric concentrations over the entire Seine River network. The most significant outgassing was in lower-order streams while peaks were simulated downstream of the major WWTP effluent. For the period studied (2010–2013), the annual average of simulated CO2 emissions from the Seine drainage network were estimated at 364±99 Gg C yr−1. Results from metabolism analysis in the Seine hydrographic network highlighted the importance of benthic activities in headwaters while planktonic activities occurred mainly downstream in larger rivers. The net ecosystem productivity remained negative throughout the 4 simulated years and over the entire drainage network, highlighting the heterotrophy of the basin. Highlights CO2 emission from the Seine River was estimated at 364±99 Gg C yr−1 with the Riverstrahler model. CO2 riverine concentrations are modulated by groundwater discharge and instream metabolism. CO2 emissions account for 31 % of inorganic carbon exports, the rest being exported as DIC.

A Membrane Microfluidic Sensor for Conductivity Measurement of Ocean Dissolved Inorganic Carbon

Atmospheric CO2 has increased by > 40% since pre-industrial times leading to significant global warming. The ocean absorption of CO2 is saturating, possibly leading to much more serious consequences than predicted. The latest IPCC special report SR15 warns that a 1.5 oC average temperature increase by 2030 is likely and associated CO2 ocean acidification is projected to amplify the adverse effects of warming. The ocean CO2 system is characterized by measuring at least 2 out of 4 canonical “master variables”: total dissolved inorganic carbon (DIC), total alkalinity (AT), partial CO2 (pCO2), and acidity (pH). Understanding this system is of fundamental importance yet CO2 continues to be chronically under-sampled in both space (including depth profiles) and time. Unfortunately, pCO2 and pH are very sensitive to temperature/pressure, hence autonomous measurements of DIC or AT are much preferred but have yet to be realized. Observations are almost exclusively limited to the surface and use large sensors, e.g. spectrophotometry, NDIR and mass spectrometry, attached to expensive research vessels. These large systems are unlikely candidates for miniaturisation. Very large uncertainties therefore still remain in the CO2 budget.The Argo float network, a global array of >3,000 untethered battery-operated floats with satellite data transfer, enables continuous monitoring of ocean salinity and temperature depth profiles. This float infrastructure awaits new autonomous chemical sensor technology but will impose severe constraints on device footprint, volume, power consumption and long-term environmental stability under high pressure cycling (up to 200 atm). Instantaneous measurement of DIC during float ascent or descent is not possible as the CO2 has to be extracted from the seawater first. Hence at each depth, an autonomous device would have to collect and store a seawater sample for subsequent analysis while the float is at park depth (~1500m). This analysis involves several chemical reactions and partial or complete equilibration across a membrane with high precision conductivity measurements. Microfluidic ocean-relevant chemical sensors have received limited attention. Research efforts have concentrated on colorimetric or luminescence techniques at shallow depths.We investigate a potential miniaturised version of a standard bench technique for DIC measurement based on flow injection conductimetric measurements.[1] To achieve a fully functioning DIC profiling system requires microfluidic lab on chip structures with sample volumes ≤ 1uL (to minimise 3 years of reagent payload), precision channel patterning, multi-layer thermoplastic bonding, and liquid-liquid membrane separation of CO2, with incompatible gas-permeable ion blocking materials, suitable for >3-year ocean deployment at high pressures. Recently we demonstrated long-term PDMS membrane bonding within a thermoplastic manifold [2] as well high pressure resilient thermoplastic bonding for multi-layer and multi-channel devices. [3] We have established the principle of precision micro-metering to inject HCl acid into the seawater sample. [4] The detection principle is based on CO2, transferred across the membrane, dissolves in high strength NaOH leading to a reduction in conductivity. [5] Conductivity electrodes are limited in size by sample volume and must operate continuously in a highly corrosive environment (NaOH, pH12). Previously we attempted a capacitively-coupled approach using thin-film protective insulating layer on copper tracks. However, this resulted in severe signal attenuation. Here we report direct metal contact conductivity cells with active volumes between 0.5 mL and 2.0 mL for DIC detection in the seawater range 1900 – 2400 mmol Kg-1. For these volumes, the use of macroscopic wires or plates as electrodes is not feasible and instead, we developed a thin film metallisation process onto PMMA using a sputtered thin film of gold (< 200 nm) on top of a 10 nm adhesion promoting inter-layer of sputtered Ti.After transfer across the membrane, the eluted CO2 in a NaOH carrier, was drawn through a < 2 mL conductivity cell where the change in impedance versus time was measured. Minimum precision values obtained from relative standard deviation were ~ 0.2 % from peak height and 0.5% from area under curve. This compares favourably with precision values of ~ 0.25 % obtained using a large C4D electrophoresis headstage of similar active measurement volume. The required sample and reagent volumes amounted to ~ 500 mL in total due to the incorporation of a planar membrane into a small volume exchange cell. This compares very favourably with reported attempts at conductivity based DIC analysis where total volumes between 5,000 mL and 10,000 mL were required as, in order to achieve the required precision, bulk wire electrodes and 20cm long membrane tubes were used. We show therefore the first microfluidic-based DIC sensor with accuracy, precision and reagent payload that offers a route to global autonomous ocean CO2 depth profiling. 1. P. Hall and R. Aller, Limnol. Oceanogr., 1992, 37, 1113 https://doi.org/10.4319/lo.1992.37.5.11132. M. Tweedie et al, Lab Chip, 2019, 1287 https://doi.org/10.1039/C9LC00123A3. D. Sun et al., Microfluid. Nanofluid., 2015, 913 https://doi.org/10.1007/s10404-015-1620-24. M. Tweedie et al. https://arxiv.org/abs/1909.018455. M. Tweedie et al., https://doi.org/10.26434/chemrxiv.8852087.v1 Figure 1

Assessment of pH dependent errors in spectrophotometric pH measurements of seawater

A recent analysis of full water column hydrographic data revealed a pH-dependent discrepancy between spectrophotometrically measured pH using purified meta-cresol purple and pH calculated from dissolved inorganic carbon (DIC) and total alkalinity (TA). The discrepancy (pHspec – pHTA,DIC) is approximately −0.018 and 0.014 at pH 7.4 and 8.2, respectively. This discrepancy has a wide range of implications for marine inorganic carbon measurements, such as establishing robust calibration protocols for pH sensors operating on profiling floats. Here, we conducted a series of lab based experiments to assess the magnitude of pH-dependent errors for spectrophotometric pH measurements in seawater by directly comparing its performance to pH measured by an Ion Sensitive Field Effect Transistor (ISFET) pH sensor known to have Nernstian behavior. Natural seawater was titrated with high CO2 seawater while simultaneously measuring pH using spectrophotometry and an ISFET sensor over a large range in pH (7–8.5) and temperature (5–30 °C). The two pH measurements were consistent to better than ±0.003 (range) at all temperatures except at 5 and 10 °C and very low and high pH, where discrepancies were as large as ±0.005. These results demonstrate that pH-dependent errors in spectrophotometric pH measurements can be rejected as the cause of the pH-dependent discrepancy between pHspec and pHTA,DIC. The cause of this discrepancy is thus likely due to our incomplete understanding of the marine inorganic carbon model that could include errors in thermodynamic constants, concentrations of major ions in seawater, systematic biases in measurements of TA or DIC, or contributions of organic compounds that are not accounted for in the definition of total alkalinity. This should be a research priority for the inorganic carbon community.

Dynamics of the Carbonate System Across the Peruvian Oxygen Minimum Zone

The oxygen minimum zone (OMZ) of Peru is recognized as a source of CO2 to the atmosphere due to upwelling that brings water with high concentrations of dissolved inorganic carbon (DIC) to the surface. However, the influence of OMZ dynamics on the carbonate system remains poorly understood given a lack of direct observations. This study examines the influence of a coastal Eastern South Pacific OMZ on carbonate system dynamics based on a multidisciplinary cruise that took place in 2014. During the cruise, onboard DIC and pH measurements were used to estimate pCO2 and to calculate the calcium carbonate saturation state (Ω aragonite and calcite). South of Chimbote (9oS), water stratification decreased and both the oxycline and carbocline moved from 150 m depth to 20-50 m below the surface. The aragonite saturation depth was observed to be close to 50 m. However, values less than 1.2 were detected close to 20 m along with low pH (minimum of 7.5), high pCO2 (maximum 1250 μatm), and high DIC concentrations (maximum 2300 μmol kg-1). These chemical characteristics are shown to be associated with Equatorial Subsurface Water (ESSW). Large spatial variability in surface values was also found. Part of this variability can be attributed to the influence of mesoscale eddies, which can modify the distribution of biogeochemical variables, such as the aragonite saturation horizon, in response to shallower (cyclonic eddies) or deeper (anticyclonic eddies) thermoclines. The analysis of a 21-year (1993-2014) data set of mean sea surface level anomalies (SSHa) derived from altimetry data indicated that a large variance associated with interannual timescales was present near the coast. However, 2014 was characterized by weak Kelvin activity, and physical forcing was more associated with eddy activity. Mesoscale activity modulates the position of the upper boundary of ESSW, which is associated with high DIC and influences the carbocline and aragonite saturation depths. Weighing the relative importance of each individual signal results in a better understanding of the biogeochemical processes present in the area.

Deciphering the dynamics of inorganic carbon export from intertidal salt marshes using high-frequency measurements

The lateral export of carbon from coastal marshes via tidal exchange is a key component of the marsh carbon budget and coastal carbon cycles. However, the magnitude of this export has been difficult to accurately quantify due to complex tidal dynamics and seasonal cycling of carbon. In this study, we use in situ, high-frequency measurements of dissolved inorganic carbon (DIC) and water fluxes to estimate lateral DIC fluxes from a U.S. northeastern salt marsh. DIC was measured by a CHANnelized Optical Sensor (CHANOS) that provided an in situ concentration measurement at 15-min intervals, during periods in summer (July – August) and late fall (December). Seasonal changes in the marsh had strong effects on DIC concentrations, while tidally-driven water fluxes were the fundamental vehicle of marsh carbon export. Episodic events, such as groundwater discharge and mean sea water level changes, can impact DIC flux through altered DIC concentrations and water flow. Variability between individual tides within each season was comparable to mean variability between the two seasons. Estimated mean DIC fluxes based on a multiple linear regression (MLR) model of DIC concentrations and high-frequency water fluxes agreed reasonably well with those derived from CHANOS DIC measurements for both study periods, indicating that high-frequency, modeled DIC concentrations, coupled with continuous water flux measurements and a hydrodynamic model, provide a robust estimate of DIC flux. Additionally, an analysis of sampling strategies revealed that DIC fluxes calculated using conventional sampling frequencies (hourly to two-hourly) of a single tidal cycle are unlikely to capture a representative mean DIC flux compared to longer-term measurements across multiple tidal cycles with sampling frequency on the order of tens of minutes. This results from a disproportionately large amount of the net DIC flux occurring over a small number of tidal cycles, while most tides have a near-zero DIC export. Thus, high-frequency measurements (on the order of tens of minutes or better) over the time period of interest are necessary to accurately quantify tidal exports of carbon species from salt marshes.

Carbonate system parameters of an algal-dominated reef along West Maui

Abstract. Constraining coral reef metabolism and carbon chemistry dynamics are fundamental for understanding and predicting reef vulnerability to rising coastal CO2 concentrations and decreasing seawater pH. However, few studies exist along reefs occupying densely inhabited shorelines with known input from land-based sources of pollution. The shallow coral reefs off Kahekili, West Maui, are exposed to nutrient-enriched, low-pH submarine groundwater discharge (SGD) and are particularly vulnerable to the compounding stressors from land-based sources of pollution and lower seawater pH. To constrain the carbonate chemistry system, nutrients and carbonate chemistry were measured along the Kahekili reef flat every 4 h over a 6-day sampling period in March 2016. Abiotic process – primarily SGD fluxes – controlled the carbonate chemistry adjacent to the primary SGD vent site, with nutrient-laden freshwater decreasing pH levels and favoring undersaturated aragonite saturation (Ωarag) conditions. In contrast, diurnal variability in the carbonate chemistry at other sites along the reef flat was driven by reef community metabolism. Superimposed on the diurnal signal was a transition during the second sampling period to a surplus of total alkalinity (TA) and dissolved inorganic carbon (DIC) compared to ocean endmember TA and DIC measurements. A shift from positive net community production and positive net community calcification to negative net community production and negative net community calcification was identified. This transition occurred during a period of increased SGD-driven nutrient loading, lower wave height, and reduced current speeds. This detailed study of carbon chemistry dynamics highlights the need to incorporate local effects of nearshore oceanographic processes into predictions of coral reef vulnerability and resilience.

Gas Diffusion Cell Geometry for a Microfluidic Dissolved Inorganic Carbon Analyzer

Variable gas diffusion cell geometries were tested for the extraction of dissolved inorganic carbon (DIC) from a microfluidic (tens to hundreds of microliters) seawater sample. With a focus on optimization of diffusion cell geometry, we compare five unique diffusion cell designs. Using 3-D printing technology to streamline the prototyping and testing process, we were able to conceive, design, fabricate, and thoroughly evaluate each design over the course of about one month. In total, 1043 DIC measurements were carried out in 109 experiments for the five working manifolds. We find that a small diameter, cylindrical diffusion cell design offers several advantages over its planar counterparts and a larger diameter cylindrical cell, most notably the ability to increase the ratio of the exchange membrane’s contact surface area to solution volume (the “aspect ratio”) without sacrificing channel integrity. Multiple designs approached short-term repeatability of $\mu \text{L}$ of sample.

Long-lived coralline alga records multidecadal variability in Labrador Sea carbon isotopes

While the recent decline in the δ13C composition of oceanic dissolved inorganic carbon (DIC) can be attributed to increasing anthropogenic CO2 emissions (13C Suess effect), the causes of natural variability in the δ13C of oceanic DIC (δ13CDIC) are far less understood. Unfortunately, instrumental oceanic DIC measurements are not available prior to the 1970s, prohibiting the observation and study of long-term variability in oceanic carbon isotope dynamics. Thus, in order to identify the main driving forces of changes in oceanic δ13CDIC, multicentury carbon isotope time series that extend from the present into the preindustrial period are required. Such time series may be extracted from the carbonate skeletons of long-lived marine organisms, which have been shown to be robust recorders of fluctuations and trends in oceanic δ13CDIC. In this study, we use an annually-banded coralline alga live-collected from the Labrador shelf to generate a 266-year time series of δ13CDIC changes in the Labrador Sea. Our results indicate that from the 1960s onwards, the rate of δ13CDIC decline in the Labrador Sea slightly exceeds the rate of δ13C decline in the atmosphere, providing support for the enhanced CO2 uptake ability of the Labrador Sea. In addition, the detrended algal δ13C time series displays multidecadal variability with typical Atlantic Multidecadal Oscillation (AMO) frequencies. We show that prior to the late 1980s, algal δ13C compositions significantly correlate with regional sea ice cover (SIC) variability, post-1850 instrumental, reconstructed AMO indices, and solar variability. We speculate that these low-frequency oscillations in δ13C reflect changes in marine primary productivity modulated by a mechanism involving solar changes, the AMO and SIC variability. Our algal carbon isotope time series suggests that while the anthropogenic Suess effect has influenced Labrador Sea δ13CDIC since the 1960s, its influence may have been obscured by the effects of natural climatic variability up until the late 1980s.

Submarine groundwater discharge drives biogeochemistry in two Hawaiian reefs

AbstractGroundwater inputs are typically overlooked as drivers of environmental change in coastal reef studies. To assess the impact of groundwater discharge on reef biogeochemistry, we examined two fringing reef environments, located in Maunalua Bay on the south shore of O‘ahu, Hawai‘i, that receive large inputs of submarine groundwater discharge. We supplemented 25‐ and 30‐d time series measurements of salinity, water temperature, pH, dissolved oxygen, and 222Rn with high‐resolution 24‐h nutrient, dissolved inorganic carbon (DIC), total alkalinity (TA), and δ13C–DIC measurements to evaluate both groundwater‐induced and biologically‐driven changes in coastal carbonate chemistry across salinity gradients. Submarine groundwater discharge at these two locations was characterized by low pHT (7.36–7.62), and variable DIC (1734–3046 μM) and TA (1716–2958 μM) content relative to ambient seawater. Groundwater‐driven variability in coastal carbonate system parameters was generally on the same order of magnitude as biologically‐driven variability in carbonate system parameters at our study locations. Further, our data revealed a shift in reef metabolism from net dissolution to net calcification across this groundwater‐driven physicochemical gradient. At sites with high levels of groundwater exposure, net community production and calcification rates were reduced. Our findings shed light on the importance of considering groundwater inputs when examining coastal carbonate chemistry.

Evasion of CO2 and dissolved carbon in river waters of three small catchments in an area occupied by small family farms in the eastern Amazon

CO2 effluxes from streams and rivers has been hypothesized to be a critical pathway of carbon flow from the biosphere back to the atmosphere. This study was conducted in three Amazonian small catchments to evaluate carbon evasion and dynamics, where land-use change has occurred on small family-farms. Monthly field campaigns were conducted from June 2006 to May 2007 in the Cumaru (CM), Pachibá (PB) and São João (SJ) streams. Electrical conductivity, pH, temperature, and dissolved oxygen measurements were done in situ, while water samples were collected to determine dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations, as well as carbon dioxide partial pressures (pCO2) and CO2 evasion fluxes. Instantaneous discharge measured by a current meter was used to calculate DOC fluxes. The sites’ DOC, DIC, pCO2, and CO2 flux measurements ranged as follows, respectively: 0.27 - 12.13 mg L-1; 3.5 - 38.9 mg L-1; 2,265 - 26,974 ppm; and 3.39 - 75.35 μmol m-2 s-1. DOC annual flux estimates for CM, SJ and PB were, respectively, 281, 245, and 169 kg C ha-1. CO2 evasion fluxes ranged from 3.39 to 75.35 μmol m-2 s-1, with an average of 22.70 ± 1.67 μmol m-2 s-1. These CO2 evasion fluxes per unit area were similar to those measured for major Amazonian rivers, thus confirming our hypothesis that small streams can evade substantial quantities of CO2. As secondary vegetation is abundant as a result of family farming management in the region, we conclude that this vegetation can be a major driver of an abundant carbon cycle.

Robust sensor for extended autonomous measurements of surface ocean dissolved inorganic carbon.

Ocean carbon monitoring efforts have increased dramatically in the past few decades in response to the need for better marine carbon cycle characterization. Autonomous pH and carbon dioxide (CO2) sensors capable of yearlong deployments are now commercially available; however, due to their strong covariance, this is the least desirable pair of carbonate system parameters to measure for high-quality, in situ, carbon-cycle studies. To expand the number of tools available for autonomous carbonate system observations, we have developed a robust surface ocean dissolved inorganic carbon (DIC) sensor capable of extended (>year) field deployments with a laboratory determined uncertainty of ±5 μmol kg(-1). Results from the first two field tests of this prototype sensor indicate that measurements of DIC are ∼90% more accurate than estimates of DIC calculated from contemporaneous and collocated measurements of pH and CO2. The improved accuracy from directly measuring DIC gives rise to new opportunities for quantitative, autonomous carbon-cycle studies.

Drivers of carbon isotopic fractionation in a coral reef lagoon: Predominance of demand over supply

The carbon isotopic signature (δ13C) of coral skeletons is influenced by isotopic fractionation (εorg) during photosynthetic dissolved inorganic carbon (DIC) fixation, but only a few direct εorg measurements are available in coral communities. In particular, observations of εorg at the ecosystem scale are lacking. Here we present high frequency (hourly) measurements of DIC and its δ13C in the water column and benthic chambers in a highly productive coral reef lagoon (Heron Island, Great Barrier Reef, Australia) and apply simple molar balance calculations to infer community εorg. Variation in εorg was between 3.7‰ and 25.2‰ in the open lagoon, with lower values during the mid-afternoon and higher values in early morning and evening. The εorg range was broader (0.3–30.1‰) in enclosed benthic chambers with a similar diel pattern. There was a strong correlation between carbon uptake rates and εorg in closed incubations, suggesting that C demand largely controlled εorg. Benthic chamber incubations revealed increased εorg as water circulation increased, implying that C supply to photosynthesizing algae on the sediment also influenced εorg. Hysteresis in carbon uptake through the day complicated the expected straightforward influence of irradiance on C demand, and consequently on εorg. These results highlight the need for more in depth understanding on carbon uptake rates to fully understand δ13C variation in coral paleo-records.

Net community production in the North Atlantic Ocean derived from Volunteer Observing Ship data

AbstractThe magnitude of marine plankton net community production (NCP) is indicative of both the biologically driven exchange of carbon dioxide between the atmosphere and the surface ocean and the export of organic carbon from the surface ocean to the ocean interior. In this study the seasonal variability in the NCP of five biogeochemical regions in the North Atlantic was determined from measurements of surface water dissolved oxygen and dissolved inorganic carbon (DIC) sampled from a Volunteer Observing Ship (VOS). The magnitude of NCP derived from dissolved oxygen measurements (NCP) was consistent with previous geochemical estimates of NCP in the North Atlantic, with an average annual NCP of 9.5 ± 6.5 mmol O2 m−2 d−1. Annual NCP did not vary significantly over 35° of latitude and was not significantly different from NCP derived from DIC measurements (NCPDIC). The relatively simple method described here is applicable to any VOS route on which surface water dissolved oxygen concentrations can be accurately measured, thus providing estimates of NCP at higher spatial and temporal resolution than currently achieved.

Determining in situ pH values of pressurised fluids using stable carbon isotope techniques

Accurate determination of pH values is fundamentally important for understanding and predicting geochemical processes in surface and subsurface environments. However, conducting in situ pH measurements of fluids from geological formations exposed to high CO2 pressures and elevated temperatures is technically and logistically challenging. Here we introduce a proxy-method for determining in situ pH values based on stable carbon equilibrium isotope fractionation (103lnα13C) between dissolved inorganic carbon (DIC) and headspace CO2, which is valid for a DIC system composed of a mixture of H2CO3⁎ (CO2(aq)+H2CO3) and HCO3−. The method is based on determination of DIC speciation and δ13C measurements of DIC and headspace CO2 on de-pressurised fluid samples in the laboratory. Since 103lnα13CDIC–CO2 values vary predictably dependent upon temperature and pH, DIC speciation and carbon isotope ratio measurements on two carbon species (CO2 and DIC) enable the determination of in-situ pH values if temperatures are known. This approach was verified in a series of laboratory experiments with water samples equilibrated with gaseous CO2 at 55bars for various temperatures and time periods. Concentration and carbon isotope measurements upon opening of the reaction vessels yielded 103lnα13CDIC–CO2(g) values between −1.3±0.1 and +1.4±0.2‰ corresponding to reconstructed in situ pH values ranging from 5.5 to 7.4 at temperatures between 20 and 120°C. Measurements with a pH-metre immediately after opening the reaction vessels were on average 1.4pH units higher than those determined by the proxy method due to CO2 outgassing that increased the measured pH. This CO2 degassing effect does not impact the here presented carbon isotope-based pH reconstruction method, which constitutes a viable alternative to direct pH measurements.