Earth’s climate is intricately linked to the carbon cycle through the radiative effect of atmospheric carbon dioxide. The ocean plays a central role in this climate-carbon system; as oceans store ∼50 times more carbon than the atmosphere, even small changes in ocean chemistry could greatly affect global climate. Understanding how the oceanic carbon reservoir has evolved across changing climates is thus critical for both constraining mechanisms of climate change and predicting impacts from anthropogenic carbon addition. This dissertation contributes to knowledge of the ocean carbon reservoir’s evolution across the last 1.5 million years of Earth’s history, with a particular focus on two key intervals of climatic change: 1) Present day, when a large, human-sourced perturbation to the carbon cycle is underway, the effects of which are not yet fully realized; and 2) The mid-Pleistocene transition (MPT; ∼900,000 years ago), when natural cycles of global warming and cooling increased in intensity and duration. Without direct observations for both these time intervals, I focus on documenting changes to ocean carbon chemistry using proxies for seawater composition. The primary tools for this purpose are boron concentrations (B/Ca ratios) and the boron isotopic composition (δ11B) of carbonate skeletons produced by marine organisms. These tools are rooted in the aqueous chemistry of boron, in which the speciation and isotopic composition of boron compounds change with seawater pH. To test present-day changes in the oceanic carbon reservoir, I measured δ11B on the calcitic skeletons of deep-sea corals (genus Keratoisis). Results show that while coral δ11B does correlate with deep ocean pH, δ11B variations within coral skeletons are too large to be explained by changes in deep ocean pH over the corals’ lifespan. These variations most likely reflect the biology of the coral organism, suggesting that δ11B measurements in Keraotisis cannot be utilized to track ocean pH until coral growth mechanisms are better understood. To complement these δ11B data, I measured the radiocarbon (14C) content of Keratoisis skeletons. Results show that coral skeletal 14C tightly correlates to the 14C content of the deep ocean, and that bamboo corals live for 50 to 300 years with radial growth rates of 10 to 80 μm per year. This supports the use of 14C for generating bamboo coral ages and growth rates, and for tracking perturbations to the 14C content of the deep ocean. Through my deep-sea coral study, I learned the importance of accurate and precise δ11B measurements for sound interpretations of ocean carbon chemistry. These interpretations necessitate highly specialized analysis protocols. While two protocols are commonly applied for δ11B measurements, existing comparisons found relatively large offsets between both protocols. To trace the cause and implications of this offset, I established a new δ11B measurement protocol and performed an internal comparison between the new and existing measurement protocols. Results confirm that carbonate δ11B values are significantly offset between techniques. Although the nature of this offset remains enigmatic, I show that both techniques show the same δ11B-to-pH sensitivity, and consistent pH estimates are obtained when a protocol-specific constant offset is applied. This suggests that both δ11B analysis protocols can be applied for reconstructing pH with equal confidence. To test for changes in the ocean carbon reservoir across the MPT, I investigated the B/Ca and Cd/Ca composition of the benthic foraminifer Cibicidoides wuellerstorfi to track deep ocean carbonate saturation state (∆[CO32−]) and nutrient inventories. At 4.3 km water depth in the South Atlantic Ocean, B/Ca abruptly decreased by 20% and Cd/Ca increased by 40% between 950 and 900 ka, equivalent to a 60 μmol/kg increase in abyssal ocean carbon storage. Coincident shifts in deep ocean circulation and atmospheric pCO2 around 900 ka suggest that a new regime of deep ocean carbon sequestration developed during the MPT. I argue that this regime was intricately linked with the increased magnitude and duration of glacial cycles following the MPT.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Pb and Pb isotopes in the ocean have varied on decadal to centennial time scales due to anthropogenic Pb inputs. Thus, tracing the temporal variation of Pb and Pb isotopes in the ocean provides information on the major sources of Pb and the transport of Pb from sources to the ocean surface and into the ocean interior. In this thesis study, first, a method was developed for the analysis of dissolved Pb and other trace elements in seawater using single batch nitrilotriacetate resin extraction and isotope dilution ICP-MS, which was applied in analyzing seawater Pb concentrations in the rest of the study. A -550 year history of the Pb and Pb isotopes in the deep North Atlantic Ocean is reconstructed using a deep-sea coral, showing the infiltration of anthropogenic Pb to deep sea. Comparing the results to the surface North Atlantic Ocean Pb record using a Transit Time Distribution model, the mean transit time of Pb is estimated to be -64 years. This is longer than the transit time estimate assuming simple advection from a source, showing the importance of advective-diffusive mixing in the transport of Pb to the ocean interior. The later part of the thesis investigates Pb in the Indian Ocean, where no useful Pb data have been previously reported. First, using annually-banded surface growing corals, I reconstruct variations of Pb and isotopes in the surface waters of the central and eastern Indian Oceans during the past half-century. Results of the study show the increase of Pb concentrations from the mid-1970s, and major sources of the Pb are discussed, including leaded gasoline and coal burning, based on their emission histories and Pb isotope signatures. Second, Pb concentration and isotope profiles are presented from the northern and western Indian Oceans. Higher Pb concentrations and lower Pb isotope ratios (206Pb/ 207Pb, 208Pb/207Pb) are found in the upper water column (<I 500m) as a result of anthropogenic Pb inputs, and their distributions are largely controlled by the water circulation in the Indian Ocean. / by Jong-Mi Lee. / Ph.D.

The marine chemistry of dissolved iron (Fe) was examined in two studies conducted off the coast of British Columbia in the northeast Pacific. Dissolved (<0.4 µm) Fe was measured along inshore-offshore transects across the continental shelf in Queen Charlotte Sound. A benthic layer of high Fe, enriched due to exchange with shelf sediments, was found to extend across the shelf-slope break into offshore waters. Advection of these shelf waters may be a significant source of Fe to the nearby Fe-poor region of the northeast Pacific. In this Fe-limited gyre, little is known about the chemical speciation and bioavailability of Fe. The fractionation of dissolved Fe into inorganic and organically bound species along Line P was established by competitive ligand equilibration coupled with cathodic stripping voltammetry (CLE-CSV), and the results confirm the presence of biogenic Fe-binding ligands at concentrations in excess of dissolved Fe with conditional stability constants between 8.3 x 1010 and 5.6 x 1012 for the Fe-ligand complexes. The temporal and spatial variations observed are consistent with either a biological source of the ligands or an atmospheric source where greatest deposition occurs during the summer when stratification of the upper water column is most pronounced.

The ocean is the only cumulative sink of atmospheric CO2. It has absorbed approximately 40% of the CO2 from fossil fuel burning and cement production, lowering atmospheric CO2 and limiting climate change. Here we will examine the regional and global mechanisms controlling the evolution of ocean uptake of this additional carbon from human activities (anthropogenic carbon, Cant) using ocean models and observations. Cant is rapidly injected into the deep ocean, sequestering it from the atmosphere for centuries. It is currently uncertain whether any of this sequestered Cant was absorbed from the atmosphere in the subpolar North Atlantic. Here we present evidence that the upper limb of the ocean’s overturning circulation supplies the subpolar North Atlantic with capacity to absorb Cant from the atmosphere. Using a coupled ocean model, we find that surface freshening of the subpolar North Atlantic reduces the volume available for Cant storage. We also investigate whether global ocean Cant uptake is reduced due to changing ocean circulation, this time across multiple emission scenarios, including scenarios with aggressive emission mitigation. Though it is clear that emission mitigation will reduce the magnitude of the ocean carbon sink, the mechanisms governing the decline in uptake have not been studied in detail. We find that the ocean sink becomes less efficient due to kinematic effects wherein Cant escapes from the surface ocean as atmospheric CO2 plateaus and then declines. In emission scenarios ranging from high to low emissions, projected changes in global Cant uptake due to ocean circulation are small. This is in contrast with the subpolar North Atlantic, where future circulation change plays a important role in the declining Cant uptake.

Thesis (Ph. D.)--Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. / Includes bibliographical references (p. 215-225). / The five noble gases (helium, neon, argon, krypton, and xenon) are biologically and chemically inert, making them ideal oceanographic tracers. Additionally, the noble gases have a wide range of solubilities and molecular diffusivities, and thus respond differently to physical forcing. Tritium, an isotope of hydrogen, is useful in tandem with its daughter helium-3 as a tracer for water mass ages. In this thesis, a fourteen month time-series of the five noble gases, helium-3 and tritium was measured at the Bermuda Atlantic Time-series Study (BATS) site. The time-series of five noble gases was used to develop a parameterization of air-sea gas exchange for oligotrophic waters and wind speeds between 0 and 13 m s-1 that explicitly includes bubble processes and that constrains diffusive gas exchange to ± 6% and complete and partial air injection processes to ± 15%. Additionally, the parameterization is based on weeks to seasonal time scales, matching the time scales of many relevant biogeochemical cycles. The time-series of helium isotopes, tritium, argon, and oxygen was used to constrain upper ocean biological production. Specifically, the helium flux gauge technique was used to estimate new production, apparent oxygen utilization rates were used to quantify export production, and euphotic zone seasonal cycles of oxygen and argon were used to determine net community production. The concurrent use of these three methods allows examination of the relationship between the types of production and begins to address a number of apparent inconsistencies in the elemental budgets of carbon, oxygen, and nitrogen. / by Rachel H.R. Stanley. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Halogenated 1'-methyl-1,2'-bipyrroles (MBPs) are a distinctive class of marine organic compounds. They are naturally produced, they have a unique carbon structure, they are highly halogenated, and they bioaccumulate in upper trophic levels. MBPs share many characteristics with persistent organic pollutants (POPs), and may prove to be useful natural analogues for these anthropogenic compounds. Further, their unique structure suggests that their biosynthetic organism(s) may have new genes to add to current knowledge of biosynthetic chemistry. The objectives of this dissertation were to further clarify the environmental distribution of MBPs, to examine whether MBPs biomagnify, and to investigate possible origins of these compounds through their stable nitrogen isotopic signatures. Results from these investigations have shown that over 40 highly brominated MBP congeners are present in marine mammals, fish, and squid from the Northwestern Atlantic Ocean. The most abundant MBPs do appear to biomagnify through the food web to reach the concentrations observed in marine mammals. This additional evidence affords greater confidence in the use of MBPs as natural analogues for POPs. However, differences in the environmental chemistry of MBPs and anthropogenic compounds are also evident, and may be due to these compounds' different origins, or to the capacity of degradative enzymes to act upon them. / (cont.) Finally, compound-specific nitrogen isotope analyses on MBPs isolated from dolphin blubber show that these compounds are dramatically enriched in 15N relative to other biosynthetic organic compounds. This enrichment is likely a signal imparted during biosynthesis, and may assist in elucidating the organism(s) and mechanism(s) responsible for the biosynthesis of MBPs. / by Kristin C. Pangallo. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2007. / Includes bibliographical references. / Present-day expeditionary oceanography is beginning to shift from a focus on short-term ship and submersible deployments to an ocean observatory mode where long-term temporally-focused studies are feasible. As a result, a critical need for in situ chemical sensors is evolving. New sensors take a significant amount of time to develop; thus, the evaluation of techniques in the laboratory for use in the ocean environment is becoming increasingly important. Laser-induced breakdown spectroscopy (LIBS) possesses many of the characteristics required for such in situ chemical sensing, and is a promising technique for field measurements in extreme environments. Although many LIBS researchers have focused their work on liquid jets or surfaces, little attention has been paid to bulk liquid analysis, and especially to the effect of oceanic pressures on LIBS signals. In this work, laboratory experiments validate the LIBS technique in a simulated deep ocean environment to pressures up to 2.76 x 10⁷ Pa. A key focus of this work is the validation that select elements important for understanding hydrothermal vent fluid chemistry (Na, Ca, Mn, Mg, K, and Li) are detectable using LIBS. A data processing scheme that accurately deals with the extreme nature of laser-induced plasma formation was developed that allows for statistically accurate comparisons of spectra. The use of both single and double pulse LIBS for high pressure bulk aqueous solutions is explored and the system parameters needed for the detection of the key analytes are optimized. Using both single and double pulse LIBS, the limits of detection were found to be higher than expected as a result of the spectrometer used in this experimentation. However, the results of this validation show that LIBS possesses the characteristics to be a viable chemical sensing method for in situ analyte detection in high pressure environments like the deep ocean. / by Anna Pauline Miranda Michel. / Ph.D.

Thesis (Ph.D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2000. / Vita. / Includes bibliographical references (leaves 266-286). / Phosphorus, an essential nutrient, is removed from the oceans only through burial with marine sediments. Organic phosphorus (Prog) constitutes an important fraction (ca. 25%) of total-P in marine sediments. However, given the inherent lability of primary Prog biochemicals, it is a puzzle that any Porg is preserved in marine sediments. The goal of this thesis was to address this apparent paradox by linking bulk and molecular-level Porg information. A newly-developed sequential extraction method, which isolates sedimentary Pol reservoirs based on solubility, was used in concert with Prog nuclear magnetic resonance spectroscopy (31P-NMR) to quantify Prog functional group concentrations. The coupled extraction/ 31P-NMR method was applied to three sediment cores from the Santa Barbara Basin, and the first-ever high-resolution depth profiles of molecular-level Porg distribution during diagenesis were generated. These depth profiles were used to consider regulation of Prog distribution by biomass abundance, chemical structure, and physical protection mechanisms. Biomass cannot account for more than a few percent of sedimentary Prog. No evidence for direct structural control on remineralization of Porg was found. Instead, sorptive protection appears to be an important mechanism for Prog preservation, and structure may act as a secondary control due to preferential sorption of specific Porg compound classes. / by Kirsten Lynn Laarkamp. / Ph.D.

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