Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: George W. Luther, III., College of Marine & Earth Studies. Includes bibliographical references.

Thesis: Ph. D., Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / This thesis examines the controls on organic, inorganic, and volatile species distributions in hydrothermal fluids venting at Von Damm and Piccard, two recently discovered vent fields at the ultra slow spreading Mid-Cayman Rise, Earth's deepest mid-ocean ridge. A wide variety of possible temperatures and substrates for fluid/rock reaction exist at ultraslow spreading ridges. The flux of chemicals delivered to the ocean by circulating vent fluids exerts a major control on mass transfer into and out of the oceanic crust and supports chemosynthetic ecosystems. In Chapter 2, abiotic organic synthesis is shown to occur via two distinct mechanisms in the serpentinizing Von Damm system. Longstanding questions concerning the spatial, temporal, and mechanistic nature of carbon transformations in deep-sea hot springs are addressed. In contrast with the current paradigm, CH4 is not actively forming during circulation of H2-rich vent fluids, but instead is derived from fluid inclusions in the host rocks. Chapters 3 and 4 present in-depth studies of the chemical and isotopic compositions of aqueous species in vent fluids at Von Damm and Piccard to elucidate the role of reaction temperature, pressure, substrate composition, and water/rock mass ratios during the chemical evolution of hydrothermal fluids at oceanic spreading centers. At Von Damm, sequential reaction of gabbroic and peridotite substrates at intermediate temperatures can explain generation of the observed fluids. Geochemical modeling shows that talc-quartz assemblage is expected to precipitate during fluid mixing with seawater at the seafloor. At Piccard, extremely high temperature subsurface water/rock reaction results in high temperature fluids that are richer in dissolved H2 than any previously observed fluids worldwide. At both locations, high-H2 conditions promote the abiotic reduction of [Epsilon]CO2 to formate species, which may fuel a subsurface biosphere. In Chapter 5, multiple sulfur isotopes were measured on metal sulfide deposits, So, and fluid H2S to constrain sulfur sources and the isotopic systematics of precipitation in a wide variety of seafloor hydrothermal vents. Areas studied include the eastern Manus Basin and Lau Basin back-arc spreading centers, the unsedimented basalt-hosted Southern East Pacific Rise, and the sediment-hosted Guaymas Basin mid-ocean ridge spreading centers. Limited isotope fractionation between fluid H2S and precipitating chalcopyrite implies that sulfur isotopes in a chimney lining may record past hydrothermal activity. / by Jill Marie McDermott. / Ph. D.

Thesis: Ph. D., Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016. / Cataloged from PDF version of thesis. / Includes bibliographical references. / In the spring and summer within the ablation zone of the Greenland Ice Sheet (GrIS), meltwater drains to the ice sheet bed through an evolving network of efficient channelized and inefficient distributed drainage systems. Distributed system drainage is a key component in stabilizing GrIS velocity on interannual time scales and controlling geochemical fluxes. During the spring and summer of 2011 and 2012, I conducted fieldwork at a large outlet glacier in southwest Greenland underlain by metamorphic silicate rocks. Data collected from a continuous 222Rn monitor in the proglacial river were used as a component of a mass balance model. I demonstrated that Jdis, the 222Rn fraction derived from the distributed system, was >90% of the 222Rn flux on average, and therefore, 222Rn can be used as a passive flow tracer of distributed system drainage. Supraglacial meltwater runoff estimated using two independent models was compared with ice velocity measurements across the glacier's catchment. Major spikes of Jdis, occurred after rapid supraglacial meltwater runoff inputs and during the expansion of the subglacial channelized system. While increases in meltwater runoff induced ice acceleration, they also resulted in the formation of efficient subglacial channels and increased drainage from the distributed system, mechanisms known to cause slower late summer to winter velocities. Sr, U, and Ra isotopes and major and trace element chemistry were used to investigate the impact of glacial hydrology on subglacial weathering. Analysis of partial and total digestions of the riverine suspended load (SSL) found that trace carbonates within the silicate watershed largely controlled the 'Sr/'Sr ratio in the dissolved load. Experiments and sampling transects downstream from the GrIS demonstrated that [delta]234U in the dissolved phase decreased with increasing interaction with the SSL. The (2 2 8Ra/2 26Ra) value of the dissolved load was significantly higher than that of the SSL and therefore, was not the result of the source rock material but of extensive mineral surface weathering and the faster ingrowth rate of 228Ra (t1 2=5.75 y) relative to 22 6Ra (t112=1600 y). In summary, extensive, repeated cycles of rapid supraglacial meltwater runoff to subglacial drainage networks leads to increased distributed system drainage and mineral weathering. / by Benjamin Shawn Linhoff. / Ph. D.

<p>The Arctic Ocean and Western Antarctic Peninsula (WAP) are the fastest warming regions on the planet and are undergoing rapid climate and ecosystem changes. Until we can fully resolve the coupling between biological and physical processes we cannot predict how warming will influence carbon cycling and ecosystem function and structure in these sensitive and climactically important regions. My dissertation centers on the use of high-resolution measurements of surface dissolved gases, primarily O2 and Ar, as tracers or physical and biological functioning that we measure underway using an optode and Equilibrator Inlet Mass Spectrometry (EIMS). Total O2 measurements are common throughout the historical and autonomous record but are influenced by biological (net metabolic balance) and physical (temperature, salinity, pressure changes, ice melt/freeze, mixing, bubbles and diffusive gas exchange) processes. We use Ar, an inert gas with similar solubility properties to O2, to devolve distinct records of biological (O2/Ar) and physical (Ar) oxygen. These high-resolution measurements that expose intersystem coupling and submesoscale variability were central to studies in the Arctic Ocean, WAP and open Southern Ocean that make up this dissertation. </p><p>Key findings of this work include the documentation of under ice and ice-edge blooms and basin scale net sea ice freeze/melt processes in the Arctic Ocean. In the WAP O2 and pCO2 are both biologically driven and net community production (NCP) variability is controlled by Fe and light availability tied to glacial and sea ice meltwater input. Further, we present a feasibility study that shows the ability to use modeled Ar to derive NCP from total O2 records. This approach has the potential to unlock critical carbon flux estimates from historical and autonomous O2 measurements in the global oceans.</p> / Dissertation

The use of planktonic foraminiferal Mg/Ca ratios to reconstruct past sea surface temperatures (SST) is prevalent in the literature. The perceived simplicity of the underlying chemistry and ease of measurements are alluring. Canonically, temperature is thought to be the primary control on the shell Mg/Ca values. Additionally, an appeal of this proxy is that it can be combined with shell ä18O values to reconstruct changes in the local ä18O of seawater, a proxy for salinity. However, we have identified a salinity effect on the Mg/Ca signal recorded in planktonic foraminifera influencing samples from open ocean locations. This effect causes excess Mg incorporation, higher than predicted by theory, in high salinity regions for the planktonic foraminiferal species, Globigerinoides ruber (white). The shell "excess Mg/Ca" resides within the primary calcite lattice of the shell itself and may be related to the observed cyclic banding of high and low Mg/Ca calcite with the foraminiferal shells. We derive new equations which describe the relationship between shell Mg/Ca, ocean sea surface temperature, salinity, and bottom water ÄCO32-. We also define new equations for ocean temperature and salinity using shell Mg/Ca, ä18O, and bottom water ÄCO32-, which take advantage of the dual sensitivity of shell Mg/Ca and ä18O to temperature and salinity. We apply these results downcore at several locations to assess the influence on paleo-reconstructions. These results are widely applicable to paleoceanographic studies and should allow more accurate reconstructions of both temperature and salinity. Below are brief outlines of the dissertation chapters: 1) A poor correlation between Mg/Ca derived and observed (WOA05) SST was found for 64 coretops in the (sub)tropical Atlantic. Shell-derived SST values from the subtropical gyres were overestimated and the residual "excess Mg/Ca" was well correlated with surface salinity. In this chapter, new calibration equations are developed for the Atlantic Ocean using paired Mg/Ca and ä18O measurements, along with the bottom water ÄCO32-, to predict temperature and salinity. These equations are validated using published coretop data and yield accurate estimates for SST and salinity. 2) The ITCZ is clearly identified in the oceans as the region where temperatures are the highest and salinities are the lowest. These oceanographic fingerprints can be used to track ITCZ variability over the ocean through time. Both canonical equations and the new equations from chapter 1 are used here to reconstruct SST and ä18Oseawater/Salinity gradients since the LGM in the equatorial Atlantic. The marine Atlantic ITCZ migrated in excess of 10° latitude away from its modern position, during both the LGM and early Holocene, supporting climate model results as well as coastal and terrestrial paleohydrological records that document the sensitivity of ITCZ position to both high- and low-latitude forcing. 3) The nature of the excess shell Mg/Ca and the mechanism for incorporation is poorly understood. We investigated excess Mg/Ca using SEM, flow through ICP-MS (FT-ICP-MS) and electron microprobe analyses. SEM and FT-ICP-MS results suggest the excess shell Mg resides within the primary structure of the calcite lattice. Electron microprobe maps of shell Mg/Ca confirm that the excess Mg/Ca lies within the shell itself, likely within the high Mg/Ca calcite bands. These findings suggest the incorporation of shell "excess Mg/Ca" first identified in chapter 1 is not related to post-depositional diagenetic alteration. These results will help elucidate the mechanism responsible for enhanced Mg uptake in high salinity settings. 4) Currently there exist no globally applicable calibration equations relating oceanographic parameters to foraminiferal shell Mg/Ca. In this chapter, we develop new, global calibration equations for G. ruber (white) following the methods of chapter 1. We find that the relationship between shell Mg/Ca and salinity is non-linear, with a threshold value near a salinity of 35, below which there is little influence of salinity on shell Mg/Ca. These equations were validated with published data and appear to be robust. By accounting for the additional influence, alkenone and foraminiferal Mg-Ca derived SST records may be reconciled in for some locations, particularly where there were likely to have been large variations of salinity in the past. These results represent a significant advance for the paleoceanographic community.

Planktonic organisms form the base of the marine food web and may be impacted by environmental change in many ways. The interactive effects of multiple, simultaneous climate-driven changes on these organisms are not well understood. This dissertation examined the impacts of ocean acidification in combination with other environmental stressors on marine plankton and determined spatial patterns of one of these potential interactive drivers. Chapter 2 investigated the synergistic effects of ocean acidification and hypoxia on the harmful dinoflagellate Amphidinium carterae. Findings indicated that empirical studies may be crucial to accurately predict organismal responses to multi-stressors. Results also suggested that photorespiration may serve a previously unrecognized role in dinoflagellate metabolism. Chapter 3 examined the combined effects of ocean acidification and lithogenic trace metals on the growth of another harmful dinoflagellate, Cochlodinium polykrikoides. Results indicated that high suspended sediment loads may deliver toxic concentrations of trace elements to marine phytoplankton in acidified coastal ecosystems. Chapter 4 examined the interactive effects of ocean acidification and bacteria on the severity and extent of dissolution in the shells of larval gastropods and the adult pteropod Limacina helicina. Research findings indicated that microbial communities on the shell surfaces of some planktonic molluscs may mediate certain types of shell dissolution in acidified, upwelled waters. Chapter 5 explored the use of thorium isotope fluxes as a proxy for dust and lithogenic iron in the Indian Ocean. Results suggested that the gradient of dust fluxes in the region could impose thresholds for biological productivity. Together, these interdisciplinary studies demonstrate coupled biological and chemical changes in marine ecosystems as a result of increased anthropogenic environmental change.

The ocean is both a repository and reactor for chemicals at the Earth’s surface. As chemicals enter the ocean they are taken up by organisms, transported by currents, reacted with particle surfaces, and eventually buried at the seafloor. This dynamic set of chemical processes and exchanges are encapsulated by the term biogeochemistry. Marine biogeochemistry can be broadly deconstructed into two parts: ocean interfaces and internal cycling. Ocean interfaces are where chemical constituents enter and leave the ocean, including the air-sea boundary, mid-ocean ridges, continental margins, and rivers. Internal cycling is how chemical constituents are reacted, transported, taken up by organisms, and redistributed within the ocean. For a complete understanding of marine biogeochemical cycles, the input, output, and internal cycling rates of major and trace elements must be quantified. However, this rate information is difficult to infer from the observational snapshots of chemical concentrations typically collected on oceanographic expeditions. The long-lived radioisotopes of thorium (Th) and protactinium (Pa) provide an opportunity to quantify these elusive biogeochemical rates. The radiogenic isotopes 230Th and 231Pa are produced at a uniform rate throughout the water column by uranium decay. A third isotope, 232Th, is primordial and brought to the ocean by the dissolution of lithogenic matter. While uranium is highly soluble, Th and Pa are highly insoluble, and are rapidly removed from solution by adsorption onto settling particle matter. Due to their insolubility and known input rates, 230Th and 231Pa have well-constrained 1-d mass budgets between radiogenic production and scavenging removal. This thesis explores new ways Th and Pa isotopes can be used to understand and quantify rates of biogeochemical processes in the South Pacific Ocean, and to assess how measurements of sedimentary Th and Pa isotopes can be used to study these processes in the geologic past. In chapter 1, I characterize the effects of submarine hydrothermal activity on the distributions of 230Th and 231Pa, finding strong removal due to adsorption by Fe and Mn oxide particles. In chapter two, I utilize the radioactive disequilibria of two additional radiogenic thorium isotopes with much shorter half-lives, 234Th and 228Th, to constrain the kinetics of Th scavenging by hydrothermal particles. Chapter three switches gears towards quantifying the internal cycling of particulate organic carbon in the subtropical South Pacific. Using a new method based on measurements of particulate 230Th, I generated high-resolution water column profiles of particulate organic carbon flux to constrain carbon regeneration lengthscales in both oligotrophic and oxygen minimum zone settings. In chapter 4, I demonstrate the importance of isopycnal mixing in transporting 230Th, 231Pa, and 232Th into the Pacific Southern Ocean, showing the first high-resolution dissolved Th and Pa data from the region. Chapter 5 provides estimates of dust input spanning the South Pacific using two methods based on paired 230Th-232Th, evaluates the flux of dust-borne iron, and discusses the impacts on measured and modeled nitrogen fixation rates in the South Pacific gyre. Finally, in chapter 6 I present enigmatic profiles of Th and Pa isotopes from the semi-enclosed Peru and Bauer Basins, with anomalous Th and Pa removal extending 1-2km above the seafloor. I hypothesize that these depletions are related to the extent of water mass contact the seafloor, allowing for scavenging removal of Th and Pa by resuspended sediments.

Data from the Surveyor 1968 Spring cruise were used to study the vertical distribution of salinity, temperature, dissolved oxygen, apparent oxygen utilization, pH, alkalinity, specific alkalinity and percent saturation of calcite in two sections, one along 162°W from 35°N to 45°N and the other along 180°W from 35°N to 50°N. Data from this cruise and additional data from the Surveyor 1968 Fall cruise, YALOC 66 cruise (summer) (Barstowetal, , 1968) and Boreas cruise (winter, 1966) (SIO reports, 1966) were used to study the distribution of salinity, temperature, apparent oxygen utilization, preformed phosphate and depth on the sigma-t surfaces of 26.8 and 27.3 in an area between 35°N and 52°N and 162°W and 155°E. In both sections the vertical distribution of the physico-chemical parameters is such that in general there is a tendency for the isograms to slope upward from south to north following the same trend of the sigma-t surfaces. This indicates that mixing and advection along the sigma-t surfaces play an important role on the distribution of these parameters. AOU data from YALOC 66 cruise (summer) compared to that from Surveyor 1968 Spring cruise suggest that aeration by mixing, eddy diffusivity and conductivity takes place to more than 500 meters depth at about 50°N. It also suggests that the changes of organic primary production at the euphotic zone during different seasons of the year may affect to a great extent the AOU distribution on the 26.8 sigma-t surface and to a very small extent on the 27.3 sigma-t surface. The direction of flow suggested by the AOU distribution on the 26.8 and 27.3 sigma-t surfaces was compared to that indicated by the acceleration potential contours on the S[subscript t] = 125 cl/ton and S[subscript t] = 80 cl/ton surfaces drawn by Reid (1965). The disagreements were explained in terms of mixing and possible gradients of primary production at the sea surface. On the 26.8 sigma-t surface a southward flow connecting the westward flow south of the Aleutian chain and the eastward flow farther south, between 175°E and 180°W is suggested by the AOU distribution but not by the acceleration potential contours. If the circulation pattern at this density surface is similar to that at the sea surface, this linkage is very likely to be real. / Graduation date: 1970

Graduation date: 1987

Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2007. / Title from Web page (viewed on Mar. 24, 2008). "September 2007." Includes bibliographical references.