Graduation Year

2022

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Robert H. Byrne, Ph.D.

Committee Member

Kristen Buck, Ph.D.

Committee Member

Richard Feely, Ph.D.

Committee Member

Gary Mitchum, Ph.D.

Committee Member

Rik Wanninkhof, Ph.D.

Committee Member

Kimberly Yates, Ph.D.

Keywords

bicarbonate dissociation constant, conductometry, internal consistency, ionic strength, ocean carbon system, pH

Abstract

Accurate characterizations of chemical equilibria and thermodynamic processes in natural waters are key components for assessing the current state of our global climate and predicting future changes, especially for observations of small rates of change over long time scales. One of the most important chemical systems in natural waters is the system of inorganic carbon species comprised of carbon dioxide (CO2), bicarbonate, and carbonate. The bicarbonate dissociation constant, K2, describes the relationship between pH and the relative concentrations of bicarbonate and carbonate ions at thermodynamic equilibrium. The carbonate/bicarbonate equilibrium quotient is required in nearly all CO2 system calculations relevant to freshwater and seawater (i.e., relationships between pH, total dissolved inorganic carbon (CT), total alkalinity (AT), and CO2 fugacity (fCO2)). There have been many previous characterizations of K2 in seawater, but most of them are not in statistical agreement with one another. This incongruity has resulted in ongoing speculation as to which K2 parameterization to use at different salinity and temperature conditions.

One focus of my dissertation was development of techniques to improve the accuracy and precision of K2 measurements in seawater. Chapter Two describes the development of a novel spectrophotometric method, based on the potentiometric method used by Mehrbach et al. (1973), to experimentally determine K2 for a range of temperatures (15 ≤ t ≤ 35 °C) and salinities (20 ≤ S ≤ 40). The K2 values determined in this work, each based on hundreds of spectrophotometric pH measurements, were used to produce a new K2 parameterization. The residuals (observations minus predictions) of my K2 parameterization in terms of salinity and temperature are much smaller than the residuals obtained in previous works, indicating that my characterization is based on more precise measurements than other available K2 characterizations.

In Chapter Three, this novel spectrophotometric method was adapted to determine K2 under conditions more applicable to polar surface waters and the deep ocean (i.e., 3 ≤ t ≤ 15 °C). Typical spectrophotometric measurements of pH at temperatures below ~15 °C are challenging because condensation on optical surfaces can impede measurement accuracy. To address this challenge, the spectrophotometric method was modified to specifically work at low temperatures. This work involved the use of an environmentally controlled cold room (reducing ambient humidity) and directing moisture-free nitrogen gas on optical surfaces to preclude condensation. The combination of observations described in Chapters Two and Three resulted in a new K2 parameterization relevant to the open ocean at atmospheric pressure with an associated random uncertainty component that is greatly reduced compared to previous works. This parameterization can be used by oceanographers who make CO2 system calculations for biogeochemical process studies, carbon budgets, and assessments of both current and future climate.

Another emphasis of this dissertation was assessment of the accuracy of CO2 system models via examinations of internal consistency (i.e., comparison of measured and calculated CO2 system parameters). In Chapter Two, several large oceanographic data sets were used to assess the level of internal consistency of CO2 system calculations obtained with alternative sets of CO2 system dissociation constant parameterizations. These analyses demonstrated that improved CO2 system internal consistency was obtained using the set of constants which includes the K2 parameterization produced with the spectrophotometric procedures described in this work. Importantly, though, these internal consistency assessments (and essentially all assessments found in the literature) are limited to 20 or 25 °C as these are the traditional measurement temperatures of fCO2 and pH, respectively. In Chapter Three, results from NOAA’s 2021 West Coast Ocean Acidification cruise are presented, where paired pH measurements were made at both the traditional measurement temperature of 25 °C and at an alternative, lower, temperature of 12 °C. These paired measurements, combined with measurements of AT and CT, allowed assessment of internal consistency for conditions more representative of the subsurface open ocean. These comparisons of calculations and direct measurements indicated that pH should be directly measured rather than calculated from the AT and CT pair. The assessments also demonstrated that pH measurements at either 25 °C or 12 °C can be used to obtain reliable calculations of CO2 system parameters at in situ conditions (i.e., pH, fCO2, and calcium carbonate saturation state (Ω)).

Under conditions appropriate to the open ocean, equilibrium constants such as K2 are generally weakly dependent on salinity or, equivalently, ionic strength (I). In contrast, equilibrium constants vary strongly with ionic strength in dilute solutions such as rivers and lakes. As fresh waters typically have an ionic strength between 0 ≤ I ≤ 0.01 mol kg−1, the ability to detect small differences in ionic strength can be critical to understanding freshwater chemistry and associated implications for freshwater organisms. A third focus of this dissertation was development of an improved method to measure the ionic strength of natural freshwaters. The novel method described in Chapter Four is based on a combination of conductometric and spectrophotometric measurements. The spectrophotometric method is based on innovative procedures involving observations of the dissociation characteristics of phosphate buffers. The hybrid (conductometric/spectrophotometric) procedure described in Chapter Four is shown to be both more precise and more accurate than the conductometric procedures that have previously been used to determine I.

This dissertation research exemplifies how analytical techniques, in particular spectrophotometric methods, can be used in a wide variety of studies to better understand the chemical processes and equilibria of natural waters from rivers to the sea.

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