Posts Tagged 'methods'

Spectrophotometric calibration procedures to enable calibration-free measurements of seawater calcium carbonate saturation states


• A new method for measuring seawater carbonate saturation states is described.
• No calibrations or volumetric or gravimetric measurements are required.
• Solution pH and titrant additions are measured spectrophotometrically.
• Saturation state precision is ±0.02; accuracy is ±1.0 %.
• The measurement protocol is suitable for in situ measurements.


A simple protocol was developed to measure seawater calcium carbonate saturation states (Ωspec) spectrophotometrically. Saturation states are typically derived from the separate measurement of two other carbon system parameters, with each requiring unique instrumentation and often complex measurement protocols. Using the new protocol, the only required equipment is a thermostatted laboratory spectrophotometer. For each seawater sample, spectrophotometric measurements of pH (visible absorbance) are made in paired optical cells, one with and one without added nitric acid. Ultraviolet absorbance is measured to determine the amount of added acid based on the direct proportionality between nitrate concentration and UV absorbance. Coupled measurements of pH and the alkalinity change that accompanies the nitric acid addition allow calculation of a seawater sample’s original carbonate ion concentration and saturation state. These paired absorbance measurements yield Ωspec (and other carbonate system parameters), with each sample requiring about 12 min processing time. Initially, an instrument-specific nitrate molar absorptivity coefficient must be determined (due to small but significant discrepancies in instrumental wavelength calibrations), but thereafter no further calibration is needed. In this work, the 1σ precision of replicate measurements of aragonite saturation state was found to be 0.020, and the average difference between Ωspec and Ω calculated conventionally from measured total alkalinity and pH (Ωcalc) was −0.11% ± 0.96% (a level of accuracy comparable to that obtained from spectrophotometric measurements of carbonate ion concentration). Over the entire range of experimental conditions, 0.97 < Ω < 3.17 (n = 125), all measurements attained the Global Ocean Acidification Observing Network’s “weather level” goal for accuracy and 90% attained the more stringent “climate level” goal. When Ωspec was calculated from averages of duplicate samples (n = 56), the precision improved to 0.014 and the average difference between Ωspec and Ωcalc improved to −0.11% ± 0.73%. Additionally, 97% of the duplicate-based Ωspec measurements attained the “climate level” accuracy goal. These results indicate that the simple measurement protocol developed in this work should be widely applicable for monitoring fundamental seawater changes associated with ocean acidification.

Continue reading ‘Spectrophotometric calibration procedures to enable calibration-free measurements of seawater calcium carbonate saturation states’

Gastropod shell dissolution as a tool for biomonitoring marine acidification, with reference to coastal geochemical discharge

Marine water pH is becoming progressively reduced in response to atmospheric CO2 elevation. Considering that marine environments support a vast global biodiversity and provide a variety of ecosystem functions and services, monitoring of the coastal and intertidal water pH assumes obvious significance. Because current monitoring approaches using meters and loggers are typically limited in application in heterogeneous environments and are financially prohibitive, we sought to evaluate an approach to acidification biomonitoring using living gastropod shells. We investigated snail populations exposed naturally to corrosive water in Brunei (Borneo, South East Asia). We show that surface erosion features of shells are generally more sensitive to acidic water exposure than other attributes (shell mass) in a study of rocky-shore snail populations (Nerita chamaeleon) exposed to greater or lesser coastal geochemical acidification (acid sulphate soil seepage, ASS), by virtue of their spatial separation. We develop a novel digital approach to measuring the surface area of shell erosion. Surficial shell erosion of a muddy-sediment estuarine snail, Umbonium vestiarium, is shown to capture variation in acidic water exposure for the timeframe of a decade. Shell dissolution in Neripteron violaceum from an extremely acidic estuarine habitat, directly influenced by ASS inflows, was high variable among individuals. In conclusion, gastropod shell dissolution potentially provides a powerful and cost-effective tool for rapidly assessing marine pH change across a range of spatial and temporal frameworks and coastal intertidal environments. We discuss caveats when interpreting gastropod shell dissolution patterns.

Continue reading ‘Gastropod shell dissolution as a tool for biomonitoring marine acidification, with reference to coastal geochemical discharge’

δ11B as monitor of calcification site pH in divergent marine calcifying organisms

The boron isotope composition (δ11B) of marine biogenic carbonates has been predominantly studied as a proxy for monitoring past changes in seawater pH and carbonate chemistry. However, a number of assumptions regarding chemical kinetics and thermodynamic isotope exchange reactions are required to derive seawater pH from δ11B biogenic carbonates. It is also probable that δ11B of biogenic carbonate reflects seawater pH at the organism’s site of calcification, which may or may not reflect seawater pH. Here, we report the development of methodology for measuring the δ11B of biogenic carbonate samples at the multi-collector inductively coupled mass spectrometry facility at Ifremer (Plouzané, France) and the evaluation of δ11BCaCO3 in a diverse range of marine calcifying organisms reared for 60 days in isothermal seawater (25 °C) equilibrated with an atmospheric pCO2 of ca. 409 µatm. Average δ11BCaCO3 composition for all species evaluated in this study range from 16.27 to 35.09 ‰, including, in decreasing order, coralline red alga Neogoniolithion sp. (35.89 ± 3.71 ‰), temperate coral Oculina arbuscula (24.12 ± 0.19 ‰), serpulid worm Hydroides crucigera (19.26 ± 0.16 ‰), tropical urchin Eucidaris tribuloides (18.71 ± 0.26 ‰), temperate urchin Arbacia punctulata (16.28 ± 0.86 ‰), and temperate oyster Crassostrea virginica (16.03 ‰). These results are discussed in the context of each species’ proposed mechanism of biocalcification and other factors that could influence skeletal and shell δ11B, including calcifying site pH, the proposed direct incorporation of isotopically enriched boric acid (instead of borate) into biogenic calcium carbonate, and differences in shell/skeleton polymorph mineralogy. We conclude that the large inter-species variability in δ11BCaCO3 (ca. 20 ‰) and significant discrepancies between measured δ11BCaCO3 and δ11BCaCO3 expected from established relationships between abiogenic δ11BCaCO3 and seawater pH arise primarily from fundamental differences in calcifying site pH amongst the different species. These results highlight the potential utility of δ11B as a proxy of calcifying site pH for a wide range of calcifying taxa and underscore the importance of using species-specific seawater-pH–δ11BCaCO3 calibrations when reconstructing seawater pH from δ11B of biogenic carbonates.

Continue reading ‘δ11B as monitor of calcification site pH in divergent marine calcifying organisms’

Workshop on Durafet-based pH sensors highlights importance of collecting reference samples

Ocean pH sensor technology has advanced quickly over the last few years. This is predominately driven by the integration of Honeywell™ Durafet® pH electrodes in oceanographic sensor packages. Durafets sensors contain Ion Sensitive Field Effect Transistor (ISFET) technology, which produces highly precise pH measurements in seawater. Precision is better than 0.005 units pHT and stability of the calibration can be maintained for several months (Martz et al. 2010; Takeshita et al. 2014). These characteristics have made Durafet-based sensors popular among oceanographers and marine biologists alike.

Since 2010, multiple Durafet-based pH sensors have become commercially available. Sensors are currently used in estuarine, coastal, and deep-sea ecosystems, underway measurements, profiling floats, and laboratory-based ocean acidification experiments. Durafet-based sensors require specialized expertise for optimal operation and high-quality data acquisition (McLaughlin et al. 2017). The crux of data quality lies in the intricacies associated with calibration of these sensors. While commercial sensors have a factory calibration, this may not be accurate enough for certain marine applications (Figure 1). Most often, users need to perform a calibration using multiple, independent, seawater samples which are then analyzed for pH in the laboratory and used in data processing (Bresnahan et al. 2014). Using such reference samples for calibration, and following proper maintenance, a single SeaFET™ pH sensor can produce high-quality data for multiple years in a coastal environment (Kapsenberg et al. 2017a). Currently, data quality varies among user groups and is associated with experience level (McLaughlin et al. 2017). For this reason, hands-on training provides a beneficial supplement to recently published best practices (Bresnahan et al. 2014; Kapsenberg et al. 2017b; Rivest et al. 2016).

Continue reading ‘Workshop on Durafet-based pH sensors highlights importance of collecting reference samples’

Carbonate chemistry of an in-situ free-ocean CO2 enrichment experiment (antFOCE) in comparison to short term variation in Antarctic coastal waters

Free-ocean CO2 enrichment (FOCE) experiments have been deployed in marine ecosystems to manipulate carbonate system conditions to those predicted in future oceans. We investigated whether the pH/carbonate chemistry of extremely cold polar waters can be manipulated in an ecologically relevant way, to represent conditions under future atmospheric CO2 levels, in an in-situ FOCE experiment in Antarctica. We examined spatial and temporal variation in local ambient carbonate chemistry at hourly intervals at two sites between December and February and compared these with experimental conditions. We successfully maintained a mean pH offset in acidified benthic chambers of −0.38 (±0.07) from ambient for approximately 8 weeks. Local diel and seasonal fluctuations in ambient pH were duplicated in the FOCE system. Large temporal variability in acidified chambers resulted from system stoppages. The mean pH, Ωarag and fCO2 values in the acidified chambers were 7.688 ± 0.079, 0.62 ± 0.13 and 912 ± 150 µatm, respectively. Variation in ambient pH appeared to be mainly driven by salinity and biological production and ranged from 8.019 to 8.192 with significant spatio-temporal variation. This experiment demonstrates the utility of FOCE systems to create conditions expected in future oceans that represent ecologically relevant variation, even under polar conditions.

Continue reading ‘Carbonate chemistry of an in-situ free-ocean CO2 enrichment experiment (antFOCE) in comparison to short term variation in Antarctic coastal waters’

Towards an autonomous microfluidic sensor for dissolved carbon dioxide determination


•A microfluidic sensor for dissolved carbon dioxide determination in aqueous environments has been developed.
•The design of a microfluidic chip coupled with a white LED and a colour detector successfully formed the detection system.
•This is the first microfluidic device for the determination of dissolved CO2 that measures colour.
•The stability of the selected reagent has been studied for three months so far and is still stable.
•The result is a portable system for the determination of dissolved carbon dioxide from 7.2 ppm to 425.6 ppm.

In this work, we present a new system for the determination of dissolved carbon dioxide (from 7.2 ppm to 425.6 ppm) in aqueous environments. Microfluidic technology has been incorporated in sensor design to reduce the volume of samples and reagents. Moreover, a detection system has been integrated in the chip, consisting of a white light-emitting diode as a light source and a high-resolution digital colour sensor as the detector, which are able to detect changes in colour produced by the reaction of the sensing chemistry and carbon dioxide in water.
The optimised parameters found for the system are: flow rate 0.6 mL·min−1, integration time 30 s and the time for pumping of solutions was 3 min, obtaining a LOD of 7.2 ppm. The CO2 response, reproducibility, precision, and stability of the sensing chemistry have been studied and compared with those obtained using bench-top instrumentation (i.e. a spectrophotometer), obtaining good agreement.

Continue reading ‘Towards an autonomous microfluidic sensor for dissolved carbon dioxide determination’

Boron isotope-based seasonal paleo-pH reconstruction for the Southeast Atlantic – a multispecies approach using habitat preference of planktonic foraminifera


• Coretop foraminifera δ11B accounting for intra-annual peak flux variations.
• Coretop data in agreement with established boron isotope calibrations.
• Multispecies downcore record demonstrates seasonal changes in SST and pCO2.
• Seasonal pCO2 changes probably due to SST and hence CO2 disequilibrium


The boron isotopic composition of planktonic foraminiferal shell calcite (δ11BCc) provides valuable information on the pH of ambient water at the time of calcification. Hence, δ11BCc of fossil surface-dwelling planktonic foraminifera can be used to reconstruct ancient aqueous pCO2 if information on a second carbonate system parameter, temperature and salinity is available. However, pH and pCO2 of surface waters may vary seasonally, largely due to changes in temperature, DIC, and alkalinity. As also the shell fluxes of planktonic foraminifera show species-specific seasonal patterns that are linked to intra-annual changes in temperature, it is obvious that δ11BCc of a certain species reflects the pH and thus pCO2 biased towards a specific time period within a year. This is important to consider for the interpretation of fossil δ11BCc records that may mirror seasonal pH signals. Here we present new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) δ11BCc coretop data for the planktonic foraminifera species Globigerina bulloides, Globigerinoides ruber, Trilobatus sacculifer and Orbulina universa   and compare them with δ11Bborate derived from seasonally resolved carbonate system parameters. We show that the inferred season-adjusted δ11BCc/δ11Bborate relationships are similar to existing calibrations and can be combined with published δ11BCc field and culture data to augment paleo-pH calibrations. To test the applicability of these calibrations, we used a core drilled on the Walvis Ridge in the Southeast Atlantic spanning the last 330,000 years to reconstruct changes in surface-water pCO2. The reconstruction based on G. bulloides, which reflects the austral spring season, was shown to yield values that closely resemble the Vostok ice-core data indicating that surface-water pCO2 was close to equilibrium with the atmosphere during the cooler spring season. In contrast, pCO2 estimated from δ11BCc of O. universa, T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up to ∼50 ppmv higher values than the Vostok ice-core data. This is probably due to the higher austral summer and fall temperatures, as shown by Mg/Ca to be on average ∼4 °C higher than during the cooler spring season, accounting for an increase in pCO2 of ∼4% per 1 °C. Our results demonstrate that paleo-pH estimates based on δ11BCc contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species.

Continue reading ‘Boron isotope-based seasonal paleo-pH reconstruction for the Southeast Atlantic – a multispecies approach using habitat preference of planktonic foraminifera’

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Ocean acidification in the IPCC AR5 WG II

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