Calcium carbonates are among the most abundant and reactive minerals on Earth, and their dissolution/preservation in the ocean helps to regulate changes in atmospheric pCO2. The chemistry of the oceans has varied significantly over the past several billion years, and it is changing at an unprecedented rate today in response to anthropogenic burning of fossil fuels. The excess CO2 from human activities is acidifying the oceans and decreasing the saturation state (Ω = ([Ca2+][CO32-])/Ksp’) of marine carbonates, increasing their propensity to dissolve. Despite its importance, the rate of carbonate dissolution in seawater is still described by a purely empirical expression, and the physical and chemical mechanisms setting the overall kinetics remain unknown. This stands in contrast to calcite dissolution in freshwater, where fully coupled surface-solution models have been identified. The lack of mechanistic understanding in seawater limits our ability to predict how carbonate dissolution kinetics, and therefore the buffering capacity of the ocean, are affected by changes in chemistry. This thesis advances our knowledge of the physical and chemical mechanisms responsible for carbonate dissolution by making new measurements in seawater both in the lab and in-situ.
I first probe the activation energy of the reaction in seawater by dissolving 13C-labeled CaCO3 across the full range of Ω at 5, 12, 21, and 37°C. I find that a surface-based framework is required to explain the strong non-linearity of the data near equilibrium. In this framework, dissolution proceeds by the retreat of pre-existing steps for 0.9<Ω<1, defect-assisted etch pit formation for 0.75<Ω<0.9, and homogenous etch pit formation for 0<Ω<0.75. I provide the first seawater estimates of kinetic coefficients (β), nucleation site densities (ns), and step edge free energies (α) for each mechanism, as well as the activation energy for detachment from steps (ϵstep) and the kinetic energy barrier to etch pit initiation (ϵinit).
Next, I use a custom designed in-situ reactor to measure calcite dissolution rates across a transect of the North Pacific. I find that the same surface mechanisms and “critical” Ωs identified in lab also govern the dissolution of calcite in the open ocean. In-situ dissolution rates are ~4x slower than in the lab, but I use a combination of chemical spike experiments and measurements in archived seawater to show that this discrepancy can be explained by the presence of dissolved organic carbon in-situ. I propose an empirical rate equation that describes all previous in-situ measurements of inorganic calcite dissolution rates.
Changes in the relation between dissolution rate and Ω can be explained by the activation of different surface processes, but the surface theory cannot account for much of the near-equilibrium dissolution behavior and temperature dependence. I therefore continue on in this thesis to combine the latest speciation models with dissolution measurements in artificial seawater of varying sulfate concentrations. I find that low sulfate solutions suppress dissolution rates by two orders of magnitude near equilibrium, while dissolution rates in the same solutions are enhanced far-from-equilibrium. Using these results, I fit a mechanistic model of dissolution that couples surface and solution processes. The model satisfies the principle of microscopic reversibility, provides an excellent estimate of calcite solubility product in seawater, and explains near equilibrium (Ω > 0.75) dissolution rates in 0, 14, and 28 mM [SO42-] seawater at 21°C. The model cannot explain dissolution rates for Ω < 0.75 when etch pits begin opening homogenously across the surface, so I suggest areas of improvement for future models.
Previous work has demonstrated that calcite dissolution rates are enhanced in the presence of the enzyme carbonic anhydrase (CA). In the final chapter of this thesis, I evaluate the mechanism of CA rate enhancement by comparing the catalytic effects of freely dissolved CA, CA immobilized within hydrogels, and CA chemically bound onto porous silica beads. At the same time, I design and test a fluidized bed reactor and demonstrate its efficacy as a carbon capture device by attaching it directly to the Caltech cogeneration power plant smokestack. I find that dissolution rates within the reactor are only enhanced when CA is freely dissolved, strongly suggesting that the catalytic mechanism is direct proton transfer from the enzyme to the calcite surface.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/54TA-JK92, author = {Chen, Sang}, title = {Understanding Geochemical Tracers in Deep-Sea Corals from a Biomineralization Perspective}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/54TA-JK92}, url = {https://resolver.caltech.edu/CaltechTHESIS:06072019-145745731}, abstract = {Deep-sea corals have been developed as a useful archive of the chemistry and circulation of intermediate and deep waters in past oceans over the last three decades. However, applications of traditional paleoceanographic tracers in deep-sea corals remain a challenge due to our incomplete understanding of the biomineralization mechanisms underlying the incorporation of these tracers and their variabilities in the coral skeletons (a.k.a. the “vital effects”). In this thesis, an effort was made to understand the vital effects associated with the stable isotope as well as minor and trace element compositions of the aragonitic skeletons of the deep-sea coral species Desmophyllum dianthus, through a combination of empirical observations and a numerical model of coral calcification. Observations of the chemical and isotopic compositions of the coral skeletons were performed on four different spatial scales in a suite of modern D. dianthus specimens: bulk samples, micromilled samples, SIMS and nanoSIMS. These observations reveal tracer correlations in deep-sea corals that are coherent over different spatial scales and point toward a universal mechanism of the incorporation of these tracers through the biomineralization process. A few tracers emerge as promising proxies for the temperature (Li/Mg, Sr/Ca) and carbonate chemistry (U/Ca, B/Ca, Ba/Ca) of the oceans. The numerical model for coral calcification explains the strong δ18O and δ13C vital effects in individual deep-sea corals with an updated physicochemical basis, and carbonic anhydrase is found to play a key role in setting the slopes of the strong δ18O-δ13C correlations in different biogenic carbonates. The model also constrains the key physical parameters in the biomineralization process and is extended to explain the observed minor and trace element variabilities and correlations in deep-sea corals. The model can qualitatively explain the observed correlation patterns between Mg/Ca, Li/Ca, B/Ca and Sr/Ca in the coral skeletons, but quantitative data-model comparison is limited by both deficiencies in high-quality data and a lack of a well-constrained inorganic reference frame for aragonite. Future improvements in the geochemical tracers in biogenic carbonates will benefit from more extended empirical calibrations as well as a more complete mechanistic understanding of the key physicochemical and biological processes underlying the incorporation of tracers.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/6SFR-EX25, author = {Present, Theodore Michael}, title = {Controls on the Sulfur Isotopic Composition of Carbonate-Associated Sulfate}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/6SFR-EX25}, url = {https://resolver.caltech.edu/CaltechTHESIS:04042018-153105432}, abstract = {Sulfate in the modern ocean has a homogenous concentration and sulfur isotopic composition. It is well-mixed because rivers and mantle degassing deliver small amounts relative to its mass in the ocean. A similar small amount of sulfate is removed as biologic, sedimentary, and hydrothermal processes oxidize and reduce sulfur, carbon, and iron. These sulfur fluxes may have changed along with the carbon and oxygen cycles during ancient evolutionary, extinction, climatic, and tectonic transitions. The changing budget of marine sulfate is therefore key to understanding biogeochemical processes that control Earth’s surface environment. The sulfur isotopic composition of marine sulfate reflects the proportion of sulfur partitioned into reduced minerals, especially pyrite, in marine sediments and weathering rocks.
In this thesis, I examine how the sulfur isotopic compositions of ancient oceans is recorded in the sedimentary rock record and examine local and global effects on the sulfur isotopic composition of Paleozoic and the Mesoproterozoic sedimentary rocks. Carbonate minerals form in many depositional environments throughout Earth history and their chemical compositions relate to that of the fluid in which they formed. Much of my thesis focuses on the sulfur isotopic composition of minor amounts of sulfate incorporated into calcite, dolomite, and aragonite called carbonate-associated sulfate. Unpacking the local biogeochemical processes and global budgets affecting the sulfur isotopic composition of ancient carbonates enriches and clarifies the paleoenvironmental information preserved in the sedimentary record.
Chapter 1 is a compilation and critical comparison of proxy records of the sulfur isotopic composition of Phanerozoic seawater sulfate. I compared data from marine evaporites, barite, and carbonate-associated sulfate and showed where each record is prone to biases and which processes create variance. Only carbonate-associated sulfate data fills critical periods of biogeochemical change, but it is the most susceptible to sources of variance other than passively recording the composition of ancient oceans. However, this additional variance reflects changes in the biogeochemical processes during early diagenesis in penecontemporaneous sediments, which are the locus of the pyrite burial and sulfide reoxidation fluxes pulling on the global sulfur budget.
Chapter 2 utilizes a recently-developed analytical technique to compare the carbonate-associated sulfate of diagenetic carbonates and primary marine biogenic carbonates from latest Ordovician and earliest Silurian strata on Anticosti Island, Quebec. These samples span the duration of the Hirnantian Stage glaciation of Gondwana, which coincided with and possibly caused the Late Ordovician Mass Extinction. Much of the variance observed in bulk carbonate-associated sulfate is imparted during early diagenesis and burial diagenesis, and the best-preserved calcite from ancient brachiopods faithfully reflects seawater’s sulfur isotopic composition. Seawater sulfate’s isotopic composition did not change during the glaciation and extinction, supporting prior constraints on the mass of the marine sulfate reservoir and the magnitude of sulfur flux changes.
In Chapter 3, we extended the record of seawater sulfate’s sulfur isotopic composition from well-preserved brachiopod calcite from the Cincinnati Arch, Indiana-Ohio-Kentucky and Gotland, Sweden. We demonstrated that marine sulfate likely remained globally well-mixed with a constant isotopic composition for at least 30 Myr, from the earliest Late Ordovician through the late Silurian. The ocean’s sulfur isotope composition likely changed little during multiple biotic crises, periods of basin restriction, oceanographic circulation changes, and sea level and climate changes. However, the first replicate carbonate-associated sulfate measurements of individual brachiopods indicate that even the best-preserved calcite is prone to diagenetic alteration that may obscure small changes in the ocean sulfate budget.
Exquisitely-preserved biogenic calcite is rare in the rock record and absent in Precambrian strata, but bulk limestones and dolomites may record changes in the composition of ancient oceans. Chapter 4 compares the sulfur isotopic composition of carbonate-associated sulfate from limestones and dolostone deposited in peritidal to basinal environments on the Capitan Reef carbonate platform in the Guadalupe Mountains, west Texas. Rocks formed in different environments at the same time have carbonate-associated sulfate with different sulfur isotope compositions. Carbonate-associated sulfate is incorporated into bulk limestone and dolostone during early marine diagenesis, and its sulfur isotopic composition reflects the diagenetic and depositional environment. Carbonates recrystallizing in low-energy environments may incorporate marine pore fluids whose sulfur isotopic compositions evolved by the action of microbial sulfate reducing organisms. The sulfur isotopic composition of rocks deposited in high-energy environments, however, reflects that of seawater sulfate because the diagenetic fluid is open to the ocean and has the same sulfur isotopic composition of seawater. Later meteoric and burial diagenetic processes to which other geochemical tracers, such as carbon and oxygen isotopes, are sensitive do not greatly affect carbonate-associated sulfate. Thus, a record of the evolution of the sulfur, carbon, and oxygen isotopic composition of ancient oceans cannot come from the same sedimentary archives.
Chapter 5 considers the range of hydrothermal and sedimentary reactions that fractionate sulfur isotopes to understand the origin of unusual millimeter-scale pyrite tubes associated with a Mesoproterozoic massive sulfide deposit in the Newland Formation, Belt Supergroup, Meagher County, Montana. The petrography and sedimentology of the tubes indicates that they formed on the seafloor or in the uppermost unlithified sediments from the effluence of metalliferous fluids into euxinic seawater. The texture-specific sulfur isotopic compositions of diagenetic barite, carbonate-associated sulfate, and diagenetic and hydrothermal pyrite indicates that there was an active microbial sulfate reducing community in the sediments and possibly colonizing the vents. A dynamic set of oxidation and reduction interactions between hydrothermal fluids and seawater were controlled by this community, leading to the novel morphology and texture of vent structures.
This work indicates that combining sedimentological and petrographic observations with sulfur isotope data can constrain a wide range of biogeochemical processes. It guides future sulfur geochemical examination of parts of the rock record, especially the Precambrian, with few traditional archives of ancient seawater sulfate’s chemistry. Information on both local and global controls on the sulfur isotopic composition of carbonate-associated sulfate, barite, and pyrite helps to resolve paleoenvironmental change.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, } @phdthesis{10.7907/Z9RF5S72, author = {Hines, Sophia Katharine Vizza}, title = {Glacial Ocean Dynamics: Insight from Deep-Sea Coral Reconstructions and A Time-Dependent Dynamical Box Model}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z9RF5S72}, url = {https://resolver.caltech.edu/CaltechTHESIS:10242017-152242759}, abstract = {Glacial-interglacial cycles, occurring at a period of approximately 100,000 years, have dominated Earth’s climate over the past 800,000 years. These cycles involve major changes in land ice, global sea level, ocean circulation, and the carbon cycle. While it is generally agreed that the ultimate driver of global climate is changes in insolation, glacial cycles do not look like insolation forcing. Notably, there is a highly non-linear warming response at 100,000 years to a relatively small forcing, implicating a more complicated system of biogeochemical and physical drivers. The ocean plays a pivotal role in glacial-interglacial climate through direct equator-to-pole transport of heat and its role in the carbon cycle. The deep ocean contains 60 times more carbon than the atmosphere, and therefore even small changes in ocean circulation can have a large impact on atmospheric CO2, a crucial amplifier in the climate system. In order to better understand the role that ocean circulation plays in glacial-interglacial climate we focus on the last glacial-interglacial transition. In this thesis, we present reconstructions of changes in intermediate water circulation and explore a new time-dependent dynamical box model. We reconstruct circulation using radiocarbon and clumped isotope measurements on U/Th dated deep-sea corals from the New England and Corner Rise Seamounts in the western basin of the North Atlantic and from south of Tasmania in the Indo-Pacific sector of the Southern Ocean. Our new time-dependent model contains key aspects of ocean physics, including Southern Ocean Residual Mean theory, and allows us to explore dynamical mechanisms which drive abrupt climate transitions during the last glacial period.
In Chapter 2 we present a compilation of reconnaissance dated deep-sea corals from the Caltech collection. Reconnaissance dating facilitates sample selection for our high-precision radiocarbon and temperature time series and patterns in the depth distribution of deep-sea corals over time contain additional relevant climate information. In Chapter 3, we present a high-resolution radiocarbon record from south of Tasmania which highlights variability in Southern Ocean Intermediate Water radiocarbon during the deglaciation, particularly during the Antarctic Cold Reversal. We use our radiocarbon data, in combination with other deglacial climate records, to infer changes in overturning circulation configuration across this time interval. In Chapter 4 we present our time-dependent dynamical box model. Our model displays hysteresis in basin stratification and Southern Ocean isopycnal outcrop position as a function of North Atlantic Deep Water formation rate. In a dynamical system, hysteresis implies that there are multiple stable states, and switches between these states can lead to abrupt transitions, such as those observed during the middle of the last glacial period. In Chapter 5 we present paired radiocarbon and temperature time series from the North Atlantic and Southern Ocean spanning the late part of the last glacial. We explore the mechanisms driving trends in radiocarbon and temperature by looking at cross-plots of the data, and we make inferences about changes in circulation configuration using insight gained from our dynamical box model.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/Z93X84P3, author = {Subhas, Adam Vinay}, title = {Chemical Controls on the Dissolution Kinetics of Calcite in Seawater}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z93X84P3}, url = {https://resolver.caltech.edu/CaltechTHESIS:06092017-091849904}, abstract = {Calcium carbonate minerals are abundant on the earth’s surface. Delivery of alkalinity to the oceans is balanced by the production and burial of calcium carbonate in marine sediments, which results in a large reservoir of sedimentary calcium carbonate both in the ocean and in terrestrial rocks. Alkalinity also provides oceanic buffering capacity, which today results in about 60 times more dissolved carbon dioxide in the world oceans than is present as carbon dioxide gas in the atmosphere. Because calcium carbonate formation removes alkalinity from the oceans, calcium carbonate precipitation leads to the outgassing of carbon dioxide from the ocean into the atmosphere. Likewise, the dissolution of calcium carbonate adds alkalinity to the oceans, leading to an increased buffering capacity and a drawdown of atmospheric carbon dioxide concentration.
Calcium carbonate precipitation in the form of calcite and aragonite is almost exclusively mediated by biological organisms such as corals, coccoliths, and foraminifera, which use these minerals as components in their shells. calcium carbonate is overproduced by organisms in the ocean relative to the flux of alkalinity delivered to the oceans by rivers. Thus, a significant portion of calcium carbonate must be dissolved back into seawater for the ocean alkalinity cycle to come into steady state. Because of the link between alkalinity and carbon dioxide, the ocean alkalinity cycle has a direct effect on atmospheric carbon dioxide concentration especially on timescales less than 100,000 years.
How fast calcium carbonate dissolves back into seawater is thus a crucial rate in determining the response of the oceanic system to perturbations in either alkalinity or carbon dioxide input to the ocean-atmosphere system. We are testing the kinetics of this system with the large amount of carbon dioxide emitted from fossil fuel burning, about one third of which has dissolved into the surface ocean. This process is known as ocean acidification, as carbon dioxide is an acid, soaking up buffering capacity and dropping ocean pH. This carbon dioxide will eventually be neutralized through the dissolution of carbonate rich deep-sea sediments, but the process will take a long time. This thesis makes new measurements calcite dissolution in seawater, in an attempt to build an understanding of the chemical processes responsible for dissolution kinetics.
I first introduce the new method, in which carbon-13 labeled calcium carbonate is dissolved in undersaturated seawater. Mass loss is directly traced by measuring the appearance of carbon-13 in seawater over time. The dissolution rate of calcite is a highly nonlinear function of calcite saturation state.
Next, I show that this tracer can tell us about the balance of precipitation and dissolution at the mineral surface. I use this balance to constrain mass fluxes due to precipitation and dissolution as a function of saturation state. I also show that the enzyme Carbonic Anhydrase (CA), which rapidly equilibrates carbon dioxide and carbonic acid, greatly enhances the rate of calcite dissolution especially near equilibrium. A model of dissolution is presented in which CA is most effective in the region where dissolution proceeds via etch pit nucleation at surface defects.
The dissolution behavior of biogenic carbonates is also investigated using the carbon-13 method. I cultured coccoliths, foraminifera, and soft corals in carbon-13-labeled seawater so that their skeletons incorporated the carbon-13 tracer. These skeletons were then used in dissolution experiments. I show that both magnesium and organic matter contained within the calcite lattice have large effects on the dissolution behavior of biogenic carbonates. Magnesium content generally increases dissolution rate, and it is hypothesized that highly soluble magnesium-rich phases are preferentially removed from dissolving carbonates. Organic content generally decreases dissolution rate. It is hypothesized that organic matrices within the calcite lattice promote re-precipitation reactions, due to the balance of dissolution and precipitation rates in our data, and their promotion of precipitation during biomineralization.
I then analyze in 2- and 3-dimensions dissolved foraminiferal tests to locate where and how mass is being lost. It is shown that dissolution proceeds along specific layers, that are consistent with the size and location of Mg-rich carbonate spherules that are initially deposited during chamber formation. Surface topography generation of foraminiferal tests shows that sub-micron features are formed rapidly and then quickly eroded into larger pits and channels. These larger channels then propagate and cover the test surface at higher amounts of mass loss.
Finally, the involvement of CA in carbonate dissolution necessitates the measurement of CA activity in the environment, especially in carbonate-rich ecosystems such as reefs, carbonate-rich sediments, and carbonate-rich marine particles. To this end, I survey a number of available techniques for measuring CA activity. In the end, it is shown that the most effective method is based on measuring the depletion of oxygen-18 from carbon-13- and oxygen-18-labeled DIC, as measured by membrane inlet mass spectrometry (MIMS). This method is promising and shows about 0.1 nM CA present in unfiltered surface seawater collected from San Pedro Basin.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/QHD5-FH77, author = {Miller, Madeline Diane}, title = {The Deep Ocean Density Structure at the Last Glacial Maximum: What Was It and Why?}, school = {California Institute of Technology}, year = {2014}, doi = {10.7907/QHD5-FH77}, url = {https://resolver.caltech.edu/CaltechTHESIS:10312013-000733635}, abstract = {The search for reliable proxies of past deep ocean temperature and salinity has proved difficult, thereby limiting our ability to understand the coupling of ocean circulation and climate over glacial-interglacial timescales. Previous inferences of deep ocean temperature and salinity from sediment pore fluid oxygen isotopes and chlorinity indicate that the deep ocean density structure at the Last Glacial Maximum (LGM, approximately 20,000 years BP) was set by salinity, and that the density contrast between northern and southern sourced deep waters was markedly greater than in the modern ocean. High density stratification could help explain the marked contrast in carbon isotope distribution recorded in the LGM ocean relative to that we observe today, but what made the ocean’s density structure so different at the LGM? How did it evolve from one state to another? Further, given the sparsity of the LGM temperature and salinity data set, what else can we learn by increasing the spatial density of proxy records?
We investigate the cause and feasibility of a highly and salinity stratified deep ocean at the LGM and we work to increase the amount of information we can glean about the past ocean from pore fluid profiles of oxygen isotopes and chloride. Using a coupled ocean–sea ice–ice shelf cavity model we test whether the deep ocean density structure at the LGM can be explained by ice–ocean interactions over the Antarctic continental shelves, and show that a large contribution of the LGM salinity stratification can be explained through lower ocean temperature. In order to extract the maximum information from pore fluid profiles of oxygen isotopes and chloride we evaluate several inverse methods for ill-posed problems and their ability to recover bottom water histories from sediment pore fluid profiles. We demonstrate that Bayesian Markov Chain Monte Carlo parameter estimation techniques enable us to robustly recover the full solution space of bottom water histories, not only at the LGM, but through the most recent deglaciation and the Holocene up to the present. Finally, we evaluate a non-destructive pore fluid sampling technique, Rhizon samplers, in comparison to traditional squeezing methods and show that despite their promise, Rhizons are unlikely to be a good sampling tool for pore fluid measurements of oxygen isotopes and chloride.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/0CAB-KF69, author = {Beck, Anna Rose}, title = {Iron in the Ocean: Laboratory Experiments of Iron Geochemistry in the Presence of Marine Particles}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/0CAB-KF69}, url = {https://resolver.caltech.edu/CaltechTHESIS:09282011-141454915}, abstract = {Iron (Fe) is an important micronutrient for primary productivity in the ocean. The Fe cycle in the ocean is relatively unconstrained, especially when it comes to quantifying sources and sinks related to exchange with particulate matter. This thesis attempts to constrain some of the kinetic and equilibrium particle interactions with Fe bound to the siderophore desferrioxamine B (DFB). Out of five inorganic particle types investigated, ferrihydrite, goethite, opal, foraminifera, and montmorillonite, ferrihydrite has the largest, extended impact on dissolved FeDFB. From experimental and modeling results, ferrihydrite has two primary exchange pathways, absorption, with a rate of 4 ± 2 x 10-4 /(mg/L) per day, and dissolution, with a rate of 0.015 ± 0.01 per day. Uptake appears irreversible and follows a colloidal pumping model. Isotopic fractionation is also the greatest in the presence of ferrihydrite with signals up to +1‰ or higher with excess ligand. Dry montmorillonite has the biggest initial impact on FeDFB, resulting in a nearly instantaneous equilibrium and little isotopic fractionation. Goethite, opal, and foraminifera all have a minimal impact on FeDFB and show slight enriched isotopic fractionation, +0.15‰, in the presence of large particle concentrations. DFB seems to induce heavy Fe desorption or dissolution, while particle uptake seems to favor transfer of lighter Fe. These isotopic and kinetic parameters are important constraints on the ability of particles to control dissolved Fe, since they fall through the water column faster than equilibrium will be obtained.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/Z9CV4FR0, author = {Thiagarajan, Nivedita}, title = {Using Clumped Isotopes and Radiocarbon to Characterize Rapid Climate Change During the Last Glacial Cycle}, school = {California Institute of Technology}, year = {2012}, doi = {10.7907/Z9CV4FR0}, url = {https://resolver.caltech.edu/CaltechTHESIS:05312012-104144486}, abstract = {We generated records of carbonate clumped isotopes and radiocarbon in deep-sea corals to investigate the role of the deep ocean during rapid climate change events. First we calibrated the carbonate clumped isotope thermometer in modern deep-sea corals. We examined 11 specimens of three species of deep-sea corals and one species of a surface coral spanning a total range in growth temperature of 2–25°C. We find that skeletal carbonate from deep-sea corals shows the same relationship of Δ47 to temperature as does inorganic calcite. We explore several reasons why the clumped isotope compositions of deep-sea coral skeletons exhibit no evidence of a vital effect despite having large conventional isotopic vital effects.
We also used a new dating technique, called the reconnaissance dating method to investigate the ecological response of deep-sea coral communities in the North Atlantic and Southern Ocean to both glaciation and rapid climate change. We find that the deep-sea coral populations of D. dianthus in both the North Atlantic and the Southern Ocean expand at times of rapid climate change. The most important factors for controlling deep-sea coral distributions are likely climatically driven changes in productivity, [O2] and [CO32-].
We take 14 deep-sea corals that we had dated to the Younger Dryas (YD) and Heinrich 1 (H1), two rapid climate change events during the last deglaciation and make U-series dates and measure clumped isotopes in them. We find that temperatures during the YD and H1 are cooler than modern and that H1 exhibits warming with depth. We place our record in the context of atmospheric and marine benthic Δ14C, δ13C, and δ18O records during the deglaciation to understand the role of the deep North Atlantic during the deglaciation.
We also investigated the role of climate change in the distribution of terrestrial megafauna. To help with this, we also developed a method for compound-specific radiocarbon dating of hydroxyproline extracted from bones in the La Brea Tar Pits. We find that the radiocarbon chronologies of megafauna from several locations around the world, including the La Brea Tar Pits, exhibit an increase in abundance of megafauna during Heinrich events.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/N1MW-8Q84, author = {Gagnon, Alexander C.}, title = {Geochemical Mechanisms of Biomineralization from Analysis of Deep-Sea and Laboratory Cultured Corals}, school = {California Institute of Technology}, year = {2010}, doi = {10.7907/N1MW-8Q84}, url = {https://resolver.caltech.edu/CaltechTHESIS:05102010-102555148}, abstract = {The ocean is a major component of global heat transport and represents a large exchangeable reservoir of CO₂. The importance of these effects on climate can be quantified with records of ocean temperature, chemistry and dynamics spanning past climate change. One approach to reconstruct past ocean conditions relies on the chemical composition of CaCO₃ skeletons from coral. Despite the utility of these geochemical proxies, several lines of evidence suggest that biomineralization, the process corals use to build their skeletons, also influences composition, complicating the interpretation of past records. Coral grown under constant environmental conditions, either collected from the deep-sea or cultured in the laboratory, are used to quantify and spatially map the effects of biomineralization on skeletal composition.
In modern deep-sea coral, Mg/Ca increases with decreasing Sr/Ca in most the skeleton, consistent with closed-system (Rayleigh) precipitation. Results also show composition strongly follows skeletal architecture. Centers of calcification (COCs) are small regions of disorganized crystals thought to be the initial stage of skeletal extension. Unlike the rest of the skeleton, Mg/Ca ratios vary more than two fold within the COCs while Sr/Ca is near constant. Our data provide new constraints on a number of possible mechanisms for this effect.
In a complementary set of experiments the nanoSIMS, a new instrument capable of accurate sub-micron compositional analysis, is applied to adult cultured surface coral (1) mapping the pattern of metal ion incorporation in new growth and showing that the calcifying fluid is likely in direct exchange with seawater; and (2) testing the sensitivity of Me/Ca ratios to aragonite saturation Ω. Despite a large range of Ω and calcification rates, the average Sr/Ca of nanoSIMS spot measurements in cultured coral are within 1.2% (2 sigma std. dev. of the 5 means). These data suggest that temperature is a more significant control on Sr/Ca than aragonite saturation between Ω = 2.5–5. Within the framework of a closed-system (Rayleigh) model for biomineralization the results constrain explanations for the sensitivity of coral calcification rates to ocean acidification, improving our understanding of how anthropogenic CO₂ will impact coral reefs.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/Q7JK-MV77, author = {Mendez, Jeffrey Michael}, title = {Iron and Manganese in the Ocean: Investigation of Atmospheric Input by Dust and Coastal Ocean Time Series}, school = {California Institute of Technology}, year = {2008}, doi = {10.7907/Q7JK-MV77}, url = {https://resolver.caltech.edu/CaltechETD:etd-05022008-220144}, abstract = {Trace metals such as iron (Fe) and manganese (Mn) are essential micronutrients in the biogeochemistry of the ocean (Turner and Hunter, 2001), and dry deposition is a substantial source of both Fe and Mn to the surface ocean (Duce and Tindale, 1991; Guieu et al., 1994). Kinetic and thermodynamic values for the release of metals from dust are needed for computer models which incorporate dust as part of their ocean system. Here we investigate the thermodynamic and kinetics parameters involved in the dissolution of metals from dust in seawater. We added dust from the Sahara and the Western United States to seawater in a variety of ways to investigate the dissolution patterns of Fe and Mn. Results show different apparent thermodynamic constants for manganese (Mn) and iron (Fe). The final Mn concentrations are proportional to the added dust concentration and light intensity, and independent of initial dissolution rate. Fe concentrations in fresh seawater reach a maximum concentration of less than 2 nM. However, depletion of organic ligands lead to the precipitation of Fe oxide from solution, and the addition of siderophores enhanced both the total Fe capacity of the seawater and the rate of Fe dissolution from dust. The first order rate constant for the dissolution of dust differed by dust source and was dependent on oxalate concentration and intensity of natural UV light. We conclude that final Mn concentrations are limited by available Mn on the dust surface, while Fe concentrations are limited by the ligand concentrations in the seawater, which ultimately are determined by the biological community. Because the coastal ocean plays a significant role in global biogeochemical cycles, (Smith and Hollibaugh, 1993; Tsunogai and Noriki, 1991), we conducted a coastal ocean time series to investigate the basic modes and cycles which characterize the ocean. We found that Mn is highly dependent on seasonal rain events, with surface water concentrations observed as high as 30 nM after rain events. Fe within the coastal ocean is highly variable and can be used as a tool to track water mass movements and mixing patterns.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, } @phdthesis{10.7907/ZXHZ-EH83, author = {Eltgroth, Selene Farrell}, title = {Unraveling Deep-Ocean Connections to Climate with Deep-Sea Coral Records of Radiocarbon and Cd/Ca}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/ZXHZ-EH83}, url = {https://resolver.caltech.edu/CaltechETD:etd-05262006-110220}, abstract = {We generated records of radiocarbon and trace metals in deep-sea corals to investigate the role of the deep ocean during episodes of rapid environmental change. Our record of radiocarbon ages measured in a modern deep-sea coral from the northeastern Atlantic shows the transfer of bomb radiocarbon from the atmosphere to the deep ocean. We detect bomb radiocarbon at the coral growth site starting in 1975–1979. Our record documents a Delta14C increase from –80 ± 1‰ (average 1930–1979) to a plateau at –39 ± 2‰ (average 1994–2001). From a suite of fossil deep-sea corals, variability in North Atlantic intermediate water Delta14C during the Younger Dryas (13.0–11.5 ka) supports a link between abrupt climate change and intermediate ocean circulation. We observe rapid shifts in deep-sea Delta14C that require the repositioning of large Delta14C gradients within the North Atlantic. The shifts are consistent with changes in the rate of North Atlantic Deep Water formation. We also observe a decadal scale event at 12.0 ka that is marked by the transient return of radiocarbon to the eastern and western basins of the North Atlantic.
To develop a nutrient proxy for use in deep-sea corals, we measured Cd/Ca in 14 modern corals. Several of these corals had anomalously high Cd/Ca that we explain with a systematic bias in Cd/Ca obscuring the signal of seawater Cd/Ca. When these high Cd/Ca corals are removed from the calibration, the best-fit coral-water partition coefficient is 1.3 ± 0.1. Examining Cd/Ca in fossil deep-sea corals, we find that our coral from the Younger Dryas (12.0 ka) resembles the high Cd/Ca corals of the modern calibration and probably does not reflect seawater Cd/Ca. The Cd/Ca record from a 15.4 ka coral resembles our low Cd/Ca calibration samples and probably reflects average seawater Cd/Ca.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Adkins, Jess F.}, }