Insights Into the Deglacial Variability of Phytoplankton Community Structure in the Eastern Equatorial Pacific Ocean Using [\u3csup\u3e231\u3c/sup\u3ePa/\u3csup\u3e230\u3c/sup\u3eTh]xs and Opal-Carbonate Fluxes

Fully and accurately reconstructing changes in oceanic productivity and carbon export and their controls is critical to determining the efficiency of the biological pump and its role in the global carbon cycle through time, particularly in modern CO2 source regions like the eastern equatorial Pacific (EEP). Here we present new high-resolution records of sedimentary 230Th-normalized opal and nannofossil carbonate fluxes and [231Pa/230Th]xs ratios from site MV1014-02-17JC in the Panama Basin. We find that, across the last deglaciation, phytoplankton community structure is driven by changing patterns of nutrient (nitrate, iron, and silica) availability which, in turn, are caused by variability in the position of the Intertropical Convergence Zone (ITCZ) and associated changes in biogeochemical cycling and circulation in the Southern Ocean. Our multi-proxy work suggests greater scrutiny is required in the interpretation of common geochemical proxies of productivity and carbon export in the EEP

Sedimentary Radiogenic Isotopes as Tracers of Oceanographic and Atmospheric Change in the Eastern Equatorial Pacific Over the Last Glacial Period

The purpose of this research is to investigate atmospheric and oceanographic change that affected the eastern equatorial Pacific Ocean (EEP), as archived in marine sediments from a single site in the Panama Basin, over the past 83,000 years. I focus my work exclusively on site MV1014-02-17JC (17JC) for its high sedimentation rates (18 cm kyr-1 on average) that allow me to construct very high-resolution geochemical records of climatic change. The primary concern is to evaluate how the EEP was impacted, if at all, by abrupt climate changes over the last glacial period and onset of the Holocene that include: Heinrich Stadial events, the last glacial maximum, the Bølling-Allerød, the Younger Dryas, and the early Holocene/African Humid Period. To this end, I explore how export production and phytoplankton community structure at 17JC varied in response to rapid fluctuations in nutrient delivery to the EEP with shifts in the Intertropical Convergence Zone (ITCZ) and changes in Southern Ocean climate over the deglaciation (25,000 years ago to present) using biogenic particle fluxes and sedimentary Pa/Th ratios. I also evaluate controls on sedimentary Pa/Th variability over the entire 83,000-year record. Finally, I utilize detrital Nd and Pb isotope ratios to consider dust provenance changes to 17JC that occurred with latitudinal migrations of the ITCZ and the southern westerly wind belt over the past 30,000 years. Overall, this work is significant in contributing a robust multi-proxy record of coeval oceanographic and atmospheric change that occurred in response to abrupt and glacial-interglacial climate change over the past 83,000 years in the EEP

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst–support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.This work was supported by the Center for Alkaline-Based Energy Solutions, an Energy Frontier Research Center program supported by the U.S. Department of Energy, under Grant DE-SC0019445. This work acknowledges the long-term support of TEM facilities at the Cornell Center for Materials Research (CCMR) which are supported through the National Science Foundation Materials Research Science and Engineering Center (NSF MRSEC) program (DMR1719875), and Cornell high-energy synchrotron sources (CHESS), which is supported by the National Science Foundation under Award DMR-1332208