Determination of the CO2 system at sub-zero temperatures in seawater and seawater-derived brines

Lead Research Organisation: Bangor University
Department Name: College of Natural Sciences


Our understanding of the biogeochemical cycling of carbon in the oceans has been revolutionised through our ability to analyse several of the parameters that describe the carbonate system via gas exchange and the aqueous acid-base thermodynamic equilibria. Thus, the individual, or more commonly, combined measurement of dissolved inorganic carbon (DIC), hydrogen ion concentration (pH), total alkalinity (TA) and the partial pressure of carbon dioxide (pCO2) has provided us with the ability to determine the influence that primary production, respiration, and calcium carbonate precipitation and dissolution have on the chemistry of the oceans.

Although the geographical and temporal data coverage of the CO2 system has increased since the inception of techniques to measure all its directly observable parameters, large gaps still exist in the oceanic data base. Particular black spots are the polar oceans and especially under sea ice cover. This is an important consideration, especially as the polar oceans are experiencing environmental change as a result of ocean acidification, which is particularly rapid in the land-locked Arctic Ocean. In addition, the presence of sea ice adds complexity to the polar environment as it consists of a dynamic environment of numerous inter-connected or isolated micro-habitats that expand and contract during the seasonal cycle of formation and decay of sea ice. The study of the complex, sea ice environment is important as it in now recognized as an active interface in the interaction between the ocean and the atmosphere, through which carbon species, transform and migrate. The biogeochemical information about the polar oceans is limited in part due to its relative inaccessibility, especially when there is ice cover, the complexity of the environment and the difficulty in working in harsh conditions, but also due to a lack of appropriate methods to work at these temperatures and knowledge of the change in the value of equilibrium constants used in determining parameters of the CO2 system under these conditions. Thus, our knowledge of the CO2 system at near-zero polar waters and the sub-zero temperatures in the brine enriched micro-habitats of sea ice is currently rudimentary compared with that in oceanic waters where the temperature is above-zero.As not all of the parameters that can describe the CO2 system fully (TA, DIC, pH, pCO2) can be reliably measured in some of the polar environments, this has meant that the value of the unmeasured or unmeasurable parameters must be calculated, a process that requires extrapolation of physical-chemical equations that really should only be used with above-zero temperatures and salinity less than 50. This type of extrapolation of can lead to large differences in the calculated pCO2 and pH. Thus, the aim of our research is to provide the necessary analytical tools and experimental data so that the CO2 system in polar environments can be investigated with the same degree of sophistication as that currently afforded in temperate and tropical temperature and salinity conditions. To be able to achieve this, we have chosen existing methods of measuring pH and pCO2 in ocean waters, which we can reliable modify to measure the same parameters in brine enriched solutions at sub-zero temperatures. Using our high quality measurements, we will determine the coefficients that are essential for the determination of CO2 system and subsequently test the validity of this approach by measuring any 2 (out of 4) directly observable physical-chemical parameters of the CO2 system to predict the remaining two. In the marine community, the use of these constants, tools, and analytical methodology will aid investigation of ongoing and future changes in the CO2 chemistry, carbon-based fluxes, and saturation with respect to calcium carbonate minerals in high latitude oceans, setting important constraints on model predictions of past, present, and future climate excursions.

Planned Impact

Understanding the role of the polar marine environment in the sequestration of carbon dioxide (CO2) is key in constraining past and present climate sensitivity to different forcing factors. This requires complete investigation of the processes that modulate carbon distribution and fluxes in ice-covered polar oceans. Methodological and theoretical constraints have hampered a full examination of the CO2 system in near-freezing seawater and the seawater derived brines found in sea ice.. As a result, data sets often contain only two of the four directly measurable parameters of the CO2 system and scientists have relied on predictive equations to determine the concentration of the other two parameters. But, as yet, we cannot accurately test the veracity of these predictions. The current proposal will deliver practical ways to measure the two least measured parameters of the CO2 system in near-zero and sub-zero temperature conditions. It will also allow the reliable determination of any pair of unmeasured parameters from any pair of measured parameters for the first time in these conditions. Our experimental outcomes will offer a more realistic predictive capacity of the flow of carbon as CO2 gas and dissolved CO2 species in ice-covered high latitude marine environments.
Of our methodological developments, a pH sensing system with excellent performance over a wide temperature range, which has the capacity for miniaturization, will be of benefit to a wide range of users. We will engage directly (through our workshop) and indirectly (through dissemination of our results) with end users to ensure that the impact of our research is fully developed. The marketing potential of the pH sensing system will also result in partnership with companies keen to exploit this technology. Postgraduate students and early career scientists working with the CO2 system will benefit from our proposed workshop, where they will be exposed to the state-of-the-art-technology and topical information by scientific leaders in the field of carbon chemistry. This experience will give participants an appreciation, and will enhance their understanding, of the CO2 system, especially in cold marine environments. The ability of the oceans to sequester CO2 and its relationship to climate change is at the forefront of current scientific and public debate. To unlock a wider engagement of the public with these issues, the PIs will maintain contact with the press office at their university and will publish their research in an accessible format on the web, and in newspaper articles and society newsletters.
Description Our findings will help biogeochemists investigate the inorganic carbon system in cold environments, where temperatures are typically near and below zero. Such environments can be collectively characterized as the cryosphere and, on Earth, are found in the high latitude oceans and lakes of the polar regions. Our key findings will be especially useful to this end in sea ice research. Sea ice results from seawater freezing and covers tens of million square kilometres of high latitude oceans every year. It contributes to dense seawater formation associated with the thermohaline circulation and to the CO2 cycle in the polar regions in substantial quantities via biological and non-biological reactions that occur in the internal sea ice brines. The internal brines are the residual liquid forming during seawater freezing as a result of rejection of the dissolved sea salts from the he pure water crystal matrix of sea ice. Data for key parameters of the dissolved CO2 system in cryospheric conditions are provided for the first time in this investigation.

Our key findings pertain to three key parameters of the aquatic CO2 system, (i) the proton concentration, known as its negative common logarithm (pH = -log[H]) and regulator of geochemical reactions (weathering, metal complexation, atmospheric CO2 dissolution, biological activity), and (ii) the first and second dissociation constants of dissolved CO2 that control its speciation and reactions in natural waters. We determined all three at near-zero and below-zero temperatures and concentrated salt solutions such as the brines that result from seawater freezing or evaporation. In this respect, our findings expand the existing oceanographic data base into the freezing temperatures. Our key finding are: (i) the value of calibration pH buffers traceable and characterized to international standards, (ii) the value of the optical and thermodynamic properties of the pH indicator dye used in the spectrophotometric determination of pH in natural waters, and (iii) the value of the first and second dissociation constant of dissolved CO2.
Exploitation Route The findings of our study will be shortly available in peer reviewed journals as algebraic functions for the computation of the value of (i) the pH of traceable calibration buffers, and (ii) the dissociation constants of dissolved CO2, both characterized to international standards, extending knowledge and application to near-zero and below-zero temperatures to the freezing point of a maximum practical salinity of 100. Our protocols and findings can (i) form the basis for the marine chemistry community to expand the data base for traceable pH calibration buffers and the dissociation constants of dissolved CO2 into the coldest region of the marine temperature spectrum to the (fully frozen, no liquid phase) seawater eutectic point, and (ii) expand the deficient database of the dissociation constants of the remainder weak acids and bases of the marine CO2 system (boric, phosphoric, and silicic acids) to the below-zero temperatures of cryospheric environments. They can also be useful to the physical chemistry community in their venture to revive the field of electrolyte chemistry in their aim at generating open access thermodynamic models of seawater chemistry.
Sectors Environment