Biogeochemistry and geochemical paleoceanography of the South Pacific Gyre

Pelagic clays cover nearly one half of the ocean floor, but are rarely used for paleoceanographic research because of their extremely slow sedimentation rates, post-depositional alteration(s), and the lack of biogenic material available to provide ages. My dissertation develops and applies approaches to study pelagic clays by targeting the largest marine sediment province in the world: the South Pacific Gyre (SPG). I present an unprecedented spatially and temporally extensive paleoceanographic history of the SPG and discuss authigenic processes in pelagic clays that are linked to changes in global seawater composition through the Cenozoic. My research was based on an extensive inorganic geochemical dataset I developed from samples gathered during Integrated Ocean Drilling Program Expedition 329. I applied multivariate statistical techniques (e.g., Q-mode factor analysis and constrained least squares multiple linear regression (CLS)) to the dataset in order to (a) identify the existence of six end-members in pelagic clay (namely, eolian dust, Fe/Mn-oxyhydroxides, apatite, excess Si, and two types of volcanic ash), (b) quantify their abundances, (c) determine their mass accumulation rates, and (d) infer major features in the paleoceanographic evolution of the SPG. Key parts of my research also developed improved MATLAB codes to facilitate and speed the search for best fitting end-member combinations in CLS modeling. Additionally, I expanded the natural gamma radiation instrumental capabilities on the D/V JOIDES Resolution to quantify concentrations of uranium, thorium, and potassium. I dated the pelagic clay at four of the IODP sites with a cobalt-based age model that I developed, and documented that the seawater behavior of cobalt determines the extent to which this method can be applied. Collectively, the results track the spatial extent of dust deposition in the SPG during the aridification of Australia, dispersed ash accumulation from episodes of Southern Hemisphere volcanism, and other features of Earth’s evolution during the Cenozoic. I further quantified two geochemically distinct types of authigenic ash alterations within the pelagic clay, indicating that altered ashes may be a significant and variable sink of magnesium in seawater over geologic timescales

Pliocene expansion of C-4 vegetation in the core monsoon zone on the Indian Peninsula

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Dunlea, A. G., Giosan, L., & Huang, Y. Pliocene expansion of C-4 vegetation in the core monsoon zone on the Indian Peninsula. Climate of the Past, 16(6), (2020): 2533-2546, https://doi.org/10.5194/cp-16-2533-2020.The expansion of C4 vegetation during the Neogene was one of the largest reorganizations of Earth's terrestrial biome. Once thought to be globally synchronous in the late Miocene, site-specific studies have revealed differences in the timing of the expansion and suggest that local conditions play a substantial role. Here, we examine the expansion of C4 vegetation on the Indian Peninsula since the late Miocene by constructing a ∼6-million-year paleorecord with marine sediment from the Bay of Bengal at Site U1445, drilled during International Ocean Discovery Program Expedition 353. Analyses of element concentrations indicate that the marine sediment originates from the Mahanadi River in the Core Monsoon Zone (CMZ) of the Indian Peninsula. Hydrogen isotopes of the fatty acids of leaf waxes reveal an overall decrease in the CMZ precipitation since the late Miocene. Carbon isotopes of the leaf wax fatty acids suggest C4 vegetation on the Indian Peninsula existed before the end of the Miocene but expanded to even higher abundances during the mid-Pliocene to mid-Pleistocene (∼3.5 to 1.5 million years ago). Similar to the CMZ on the Indian Peninsula, a Pliocene expansion or re-expansion has previously been observed in northwest Australia and in East Africa, suggesting that these tropical ecosystems surrounding the Indian Ocean remained highly sensitive to changes in hydroclimate after the initial spread of C4 plants in late Miocene.This research has been supported by the Ocean and Climate Change Institute Postdoctoral Scholarship at Woods Hole Oceanographic Institution to Ann Dunlea, and the U.S. National Science Foundation to Liviu Giosan (grant no. NSF OCE-0652315). USSSP post-cruise support was provided to Expedition 353 shipboard participants Liviu Giosan and Yongsong Huang

Cenozoic global cooling and increased seawater Mg/Ca via reduced reverse weathering

© The Author(s), 2017. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 8 (2017): 844, doi:10.1038/s41467-017-00853-5.Authigenic clay minerals formed on or in the seafloor occur in every type of marine sediment. They are recognized to be a major sink of many elements in the ocean but are difficult to study directly due to dilution by detrital clay minerals. The extremely low dust fluxes and marine sedimentation rates in the South Pacific Gyre (SPG) provide a unique opportunity to examine relatively undiluted authigenic clay. Here, using Mg isotopes and element concentrations combined with multivariate statistical modeling, we fingerprint and quantify the abundance of authigenic clay within SPG sediment. Key reactants include volcanic ash (source of reactive aluminium) and reactive biogenic silica on or shallowly buried within the seafloor. Our results, together with previous studies, suggest that global reorganizations of biogenic silica burial over the Cenozoic reduced marine authigenic clay formation, contributing to the rise in seawater Mg/Ca and decline in atmospheric CO2 over the past 50 million years.Funding for this research was provided by the U.S. National Science Foundation to R.W.M. (OCE1130531) and to J.A.H. (OCE1654571)

Radiolytic Hydrogen Production in the Subseafloor Basaltic Aquifer

Hydrogen (H2) is produced in geological settings by dissociation of water due to radiation from radioactive decay of naturally occurring uranium (238U, 235U), thorium (232Th) and potassium (40K). To quantify the potential significance of radiolytic H2 as an electron donor for microbes within the South Pacific subseafloor basaltic aquifer, we use radionuclide concentrations of 43 basalt samples from IODP Expedition 329 to calculate radiolytic H2 production rates in basement fractures. The samples are from three sites with very different basement ages and a wide range of alteration types. U, Th, and K concentrations vary by up to an order of magnitude from sample to sample at each site. Comparison of our samples to each other and to the results of previous studies of unaltered East Pacific Rise basalt suggests that significant variations in radionuclide concentrations are due to differences in initial (unaltered basalt) concentrations (which can vary between eruptive events) and post-emplacement alteration. However, there is no clear relationship between alteration type and calculated radiolytic yields. Local maxima in U, Th, and K produce hotspots of H2production, causing calculated radiolytic rates to differ by up to a factor of 80 from sample to sample. Fracture width also greatly influences H2 production, where microfractures are hotspots for radiolytic H2 production. For example, H2 production rates normalized to water volume are 190 times higher in 1 μm wide fractures than in fractures that are 10 cm wide. To assess the importance of water radiolysis for microbial communities in subseafloor basaltic aquifers, we compare electron transfer rates from radiolysis to rates from iron oxidation in subseafloor basalt. Radiolysis appears likely to be a more important electron donor source than iron oxidation in old (\u3e10 Ma) basement basalt. Radiolytic H2 production in the volume of water adjacent to a square cm of the most radioactive SPG basalt may support as many as 1500 cells

Dust, Volcanic Ash, and the Evolution of the South Pacific Gyre through the Cenozoic

We examine the 0–100 Ma paleoceanographic record retained in pelagic clay from the South Pacific Gyre (SPG) by analyzing 47 major, trace, and rare earth elements in bulk sediment in 206 samples from seven sites drilled during Integrated Ocean Drilling Program Expedition 329. We use multivariate statistical analyses (Q-mode factor analysis and multiple linear regression) of the geochemical data to construct a model of bulk pelagic clay composition and mass accumulation rates (MAR) of six end-members, (post-Archean average Australian shale, rhyolite, basalt, Fe-Mn-oxyhydroxides, apatite, and excess Si). Integrating the results with Co-based age models at Sites U1365, U1366, U1369, and U1370, we link changes in MAR of these components to global oceanographic, terrestrial, and climatic transformations through the Cenozoic. Our results track the spatial extent (thousands of kilometers) of dust deposition in the SPG during the aridification of Australia. Dispersed ash is a significant component of the pelagic clay, often comprising \u3e50% by mass, and records episodes of Southern Hemisphere volcanism. Because both are transported by wind, the correlation of dust and ash MAR depends on the site\u27s latitude and suggests meridional shifts in the position of atmospheric circulation cells. The hydrothermal MARs provide evidence for rapid deposition from the Osbourn Trough spreading ridge before it went extinct. Excess Si MARs show that the abrupt increase in siliceous productivity observed at Site U1371 also extended at least as far north as Sites U1369 and U1370, suggesting large-scale reorganizations of oceanic Si distributions ~10–8 Ma in the southern SPG

Assessment and Use of NGR Instrumentation on the JOIDES Resolution to Quantify U, Th, and K Concentrations in Marine Sediment

No abstract available. doi:10.2204/iodp.sd.15.05.2013</a

The Contribution of Water Radiolysis to Marine Sedimentary Life

Water radiolysis continuously produces H2 and oxidized chemicals in wet sediment and rock. Radiolytic H2 has been identified as the primary electron donor (food) for microorganisms in continental aquifers kilometers below Earth’s surface. Radiolytic products may also be significant for sustaining life in subseafloor sediment and subsurface environments of other planets. However, the extent to which most subsurface ecosystems rely on radiolytic products has been poorly constrained, due to incomplete understanding of radiolytic chemical yields in natural environments. Here we show that all common marine sediment types catalyse radiolytic H2 production, amplifying yields by up to 27X relative to pure water. In electron equivalents, the global rate of radiolytic H2 production in marine sediment appears to be 1-2% of the global organic flux to the seafloor. However, most organic matter is consumed at or near the seafloor, whereas radiolytic H2 is produced at all sediment depths. Comparison of radiolytic H2 consumption rates to organic oxidation rates suggests that water radiolysis is the principal source of biologically accessible energy for microbial communities in marine sediment older than a few million years. Where water permeates similarly catalytic material on other worlds, life may also be sustained by water radiolysis

Hydrogen Utilization Potential in Subsurface Sediments

Subsurface microbial communities undertake many terminal electron-accepting processes, often simultaneously. Using a tritium-based assay, we measured the potential hydrogen oxidation catalyzed by hydrogenase enzymes in several subsurface sedimentary environments (Lake Van, Barents Sea, Equatorial Pacific, and Gulf of Mexico) with different predominant electron-acceptors. Hydrogenases constitute a diverse family of enzymes expressed by microorganisms that utilize molecular hydrogen as a metabolic substrate, product, or intermediate. The assay reveals the potential for utilizing molecular hydrogen and allows qualitative detection of microbial activity irrespective of the predominant electron-accepting process. Because the method only requires samples frozen immediately after recovery, the assay can be used for identifying microbial activity in subsurface ecosystems without the need to preserve live material. We measured potential hydrogen oxidation rates in all samples from multiple depths at several sites that collectively span a wide range of environmental conditions and biogeochemical zones. Potential activity normalized to total cell abundance ranges over five orders of magnitude and varies, dependent upon the predominant terminal electron acceptor. Lowest per-cell potential rates characterize the zone of nitrate reduction and highest per-cell potential rates occur in the methanogenic zone. Possible reasons for this relationship to predominant electron acceptor include (i) increasing importance of fermentation in successively deeper biogeochemical zones and (ii) adaptation of H2ases to successively higher concentrations of H2 in successively deeper zones

Dust, volcanic ash, and the evolution of the South Pacific Gyre through the Cenozoic

We examine the 0–100 Ma paleoceanographic record retained in pelagic clay from the South Pacific Gyre (SPG) by analyzing 47 major, trace, and rare earth elements in bulk sediment in 206 samples from seven sites drilled during Integrated Ocean Drilling Program Expedition 329. We use multivariate statistical analyses (Q-mode factor analysis and multiple linear regression) of the geochemical data to construct a model of bulk pelagic clay composition and mass accumulation rates (MAR) of six end-members, (post-Archean average Australian shale, rhyolite, basalt, Fe-Mn-oxyhydroxides, apatite, and excess Si). Integrating the results with Co-based age models at Sites U1365, U1366, U1369, and U1370, we link changes in MAR of these components to global oceanographic, terrestrial, and climatic transformations through the Cenozoic. Our results track the spatial extent (thousands of kilometers) of dust deposition in the SPG during the aridification of Australia. Dispersed ash is a significant component of the pelagic clay, often comprising >50% by mass, and records episodes of Southern Hemisphere volcanism. Because both are transported by wind, the correlation of dust and ash MAR depends on the site's latitude and suggests meridional shifts in the position of atmospheric circulation cells. The hydrothermal MARs provide evidence for rapid deposition from the Osbourn Trough spreading ridge before it went extinct. Excess Si MARs show that the abrupt increase in siliceous productivity observed at Site U1371 also extended at least as far north as Sites U1369 and U1370, suggesting large-scale reorganizations of oceanic Si distributions ~10–8 Ma in the southern SPGThis is the publisher’s final pdf. The article is copyrighted by the American Geophysical UnionSociety and published by John Wiley & Sons, Inc. It can be found at: http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186