The Arctic Ocean is currently experiencing increases in temperature twice as fast as lower latitudes. Changes in air and surface water temperatures as well as fluctuations in rates and volume of deep-water formation in the North Atlantic will have substantial impacts on global climate. Therefore, understanding how the structure of the Arctic Ocean has changed in response to variations in past climate will help us to better predict what changes we can expect as the Earth warms over the next century. To do this, we rely on one of the fundamental tools used to reconstruct past ocean conditions- the geochemistry of foraminiferal calcite. Neogloboquadrina pachyderma is the species of planktonic foraminifera that dominates high latitude assemblages and is therefore the principal species utilized to study the paleoceanography of the polar regions. The calcification of N. pachyderma is unusual in that it grows its primary (lamellar) calcite in the surface waters and subsequently adds a thick CaCO3 crust at depths below the mixed layer. Therefore, geochemical analyses on whole shells produce results that are difficult to interpret as they are derived from mixed lamellar and crust calcite contributions that are unequal in mass. Analytical tools capable of high spatial resolution such as electron microprobe, laser ablation ICPMS (LA-ICPMS) and secondary ion mass spectrometry (SIMS) can measure the geochemistry of discrete regions of N. pachyderma shells, therefore allowing us to analyze ontogenetic and crust calcite separately. For the first portion of my dissertation, I analyze the in situ Mg/Ca and 18O in discrete calcite zones using LA-ICPMS, EPMA and SIMS within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. I compare shell geochemistry to the measured temperature, salinity and 18O sw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. The results include a new relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. Further, this data revealed that the 18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma 18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions. Next, I evaluate intrashell trace element (TE)/Ca data from N.pachyderma shells that were grown and subsequently produced a crust in culture at various temperatures. These shells reveal marked shifts in the Mg/Ca, Ba/Ca, Zn/Ca, Al/Ca, and Mn/Ca between the lamellar and crust calcites grown in culture with no associated changes in either the environmental conditions or trace element concentrations of the seawater. The TE/Ca data were compared to the hydrologic conditions (i.e. temperature, salinity, pH, [TE]), to better understand the relationships between the crust and lamellar geochemistry and the environment in which the shells calcified. My results demonstrate that the Mg/Ca in the LC agrees with the calibration relationship previously described in Livsey et al. (2020), while the study reveals that Mg/Ca in the crust calcite is not primarily controlled by temperature. Though these results cannot definitively resolve what is controlling the transition in geochemistry between the LC and crust, they do reveal that neither changes in the environmental conditions, [Te]sw, nor a change in the calcification rate are the dominant factors. Finally, I quantified patterns of freshwater outflow from the Arctic associated with the draining of glacial Lake Agassiz. This meltwater event at ~11.5 ka likely triggered a slowdown of global ocean currents and initiated the northern hemisphere cold interval known as the Younger Dryas. I combine LA-ICPMS trace element measurements with δ18Ocalcite analyses on N. pachyderma shells to reconstruct the δ18O of Lake Agassiz-derived Mackenzie River meltwater. Remarkably, these results indicate that the geochemistry of the meltwater entering the Arctic Ocean varies throughout the Younger Dryas event, revealing the inaccuracy of current model simulations. Further, I illustrate that the meltwater pulse was not continuous throughout the interval, with at least one cessation of drainage that is linked to a shift in the chemistry of the meltwater. Overall, the time-resolved isotopic evolution of meltwater in the Arctic Ocean through the Younger Dryas reveals the unexpected complexities of meltwater pulses, which have large implications for our understanding of volumes and rates of freshwater discharge into the North Atlantic in the past and provides insights on the source of the meltwater for the Younger Dryas event.

Anthropogenic climate change is altering global biogeochemical cycles and climate processes with critical implications for marine and terrestrial ecosystems and human communities. Investigation of climate records from the Holocene (11.75 ka to present) provides critical insights into contextualizing modern climate change and understanding biogeochemical and ecosystem responses to environmental change. Here, I investigated changes in marine oxygenation along the Southern California margin and explored Northeast Pacific marine-terrestrial interactions throughout the Holocene. Through investigation of marine sediment archives, specifically benthic foraminifera, I quantitatively refined oxygenation proxies, examined changes in the oxygen minimum zone (OMZ) in past 1.5 ka, and reconstructed oxygenation of waters below 1000 m throughout the Holocene. In the first chapter, I conducted paired spatial and temporal analysis of benthic foraminiferal assemblages across a modern oxygen gradient on the San Diego margin. Results show that modern oxygenation gradients structure foraminiferal species distributions, shell size, and diversity. Centennial-scale variability in the upper margin of the OMZ occurred in the past 1.5 ka, while the core of the OMZ and below the OMZ remained stable. A recent expansion of the OMZ and decline in oxygenation at 528 m in the last 400 years on the San Diego margin is synchronous with regional records of deoxygenation. In the second chapter, I analyzed oxygen and carbon stable isotope records from planktic and benthic foraminifera and benthic foraminiferal assemblages from three silled basins in the Southern California Borderlands through the Holocene to constrain timing and implications of oxygenation and ventilation change across depth. Reconstructed oxygenation at depth (from 1100-1800m) was consistently within a range of intermediate hypoxia (0.5-1.5 ml L-1 [O2]) through the Holocene and had reduced variability relative to shallower basins. To contextualize these marine changes within a linked marine-terrestrial system, in my third chapter I conducted a systematic review of Holocene climate and oceanography of Western North America and the Northeast Pacific. Findings reveal that the early Holocene is characterized by warming relative to pre-Holocene conditions, including warm sea surface conditions, a warm and dry Pacific Northwest, a warm and wet Southwest, and overall spatial and temporal stability. In the mid Holocene, these patterns reverse; this interval is characterized by cool sea surface temperatures, a cool and wet Pacific Northwest and warm and dry Southwest. The late Holocene is the most variable interval spatially and temporally, and a new spatial trend appears characterized by warmer coastal areas and cooler inland areas. This synthesis of climate and oceanographic processes and resultant impacts on ecosystems and human communities provides a window into understanding modern patterns of climate and environmental change in Western North America.

Anthropogenic emissions of carbon dioxide are warming, deoxygenating, and acidifying the global ocean. In coastal areas, these global trends are much more complicated. Climate change trends can be attenuated, exacerbated, or entirely overridden by local influences such as rivers, pollution, complex circulation patterns, or biological productivity. As such, climate stress in the coastal ocean is more accurately described as a mosaic of stress loci and refugia that shift in space and time. Capturing this complexity is difficult, but is critical for coastal resource managers tasked with sustaining local fisheries and ecosystems over small spatial and temporal scales. In this work, I investigate and identify regions of climate stress in two coastal ecosystems highly vulnerable to warming and acidification: the eastern Bering Sea and the California Current System. In the Bering Sea, I develop species distribution models for adult red king crab to evaluate the effectiveness of modeled oceanographic conditions to predict crab distributions. In the California Current System, I develop the largest synthesis of ocean acidification- and hypoxia-relevant coastal climate observations to date, then use my synthesis to evaluate spatial patterns of vulnerability to single and multiple stressors in four widely dispersed shellfish species. Throughout this work, I interrogate the utility of high-resolution maps to inform resource management decisions and find that, while spatially-explicit information about climate stresses is important, its utility for management is often undermined by uncertainty about species sensitivity to climate stress or distribution. In the California Current System, I also find that accounting for uncertainty shows where oceanographic observations versus better precision in species sensitivity metrics produces the biggest gains in information.

The deep sea is often thought of as removed from terrestrial and nearshore processes. Despite imaginaries of discontinuity, connecting seemingly separate systems informs us on how best to relate to far, or not easily accessible, regions. Such expansive views of interconnections aid in the holistic understanding of whether and how to manage areas both far and near. Toward this, I first illuminate the surface-deep connections through the biogeochemical history of deep sea coral organic skeletons off of North-Central California that reflect overlying surface water processes over the past century. I then investigate the chemical oceanographic changes of the overlying surface waters within the past decade, and attend to the accelerating geopolitical tensions of the deep sea due to demands on land. I document a shift in coral isotopic signatures over the 20th century and modified surface and subsurface waters over the past decade. I present evidence for changes in upwelling with implications for both deep water communities and surface ocean acidification during marine heatwaves. Lastly, I will show the utility of incorporating multiple perspectives to inform 1) the contextualization of deep sea mining, 2) ongoing deep sea mining discussions, and 3) the selection of the overarching goal i.e. centering climate justice rather than green futures.

The California mussel (Mytilus californianus) is an ecologically and culturally important bivalve mollusc species that provides a habitat for hundreds of other taxa throughout rocky intertidal environments along the Pacific Coast of North America. Mytilus californianus is a well-studied organism in the fields of coastal archaeology and marine ecology, but an improved understanding of its shell structure, life-history traits, and calcification patterns could provide valuable insights paleo-seasonality or paleoceanography of Holocene coastal environments. Here, I investigated shell characteristics of M. californianus across a variety of spatial and temporal scales in order to assess and optimize its utility as a biogenic archive. By applying morphometrics, optical microscopy, and geochemical data-synthesis methods to modern, historic, and Holocene M. californianus shells collected throughout the California Current System, I established relationships between growth banding and seasonality, documented the influence of micro- habitat on shell growth, characterized stable isotopic variability over seasonal, ontogenetic, and millennial scales, and identified regional differences in calcification patterns. I present evidence for rapidly shifting calcification patterns in northern California mussels from the early 2000s to 2020, including thinner inner calcite layers and lowered contrast between dark-light growth bands. Shifting shell characteristics were correlated with greater temperature extremes, heightened temperature variability, and more intense upwelling in the past few years relative to the early 2000s. Northern populations of M. californianus preferentially grow their shells during stable, moderate periods (mean monthly temperature near 13°C) with low variability (< 5 C° seasonal temperature range), as indicated by calcite-rich light bands. I then expanded the geographic and temporal scope to characterize geochemical variability of M. californianus shells throughout southern California over millennial scales. By synthesizing the ample published stable isotope records from archaeological mussel shells of the Channel Islands, California, I found that individual analysis of complete stable isotope profiles of many shells offers more valuable paleobiological and paleo-seasonal insights, including an δ18O-inferred annual seawater temperature range of ~ 5°C in the Channel Islands. Collectively, stable isotope data from mussels revealed both millennial-scale isotopic variability indicative of Holocene warm-cool oscillations and an east-west temperature gradient characteristic of the modern-day Channel Islands. However, M. californianus shells exhibited highly variable intra-individual δ18O values depending on micro-environment and ontogeny, complicating the use of aggregated stable oxygen and carbon isotope data from many individuals as an annually resolved climate proxy. Finally, I investigated changes in shell morphology, growth banding, and microstructure over the twentieth century in M. californianus shells from the southern portion of the California Current System (Santa Barbara through Baja California Sur). Shell thickness, morphology, the percentage of calcite-rich light bands, and the contrast between dark-light banding remained unchanged throughout all study sites in southern California and the Baja California peninsula. It is likely that southern populations of M. californianus are adapted to warmer conditions, while mussels from regions farther north in the California Current System may be more susceptible to warming waters. Mytilus californianus is a complex yet useful archive of environmental variability. Its shell records its environment over seasonal to millennial scales, depending on a variety of factors such as ontogenetic age, tidal position, and the sampling technique. I conclude that M. californianus can serve as a valuable record when multiple approaches are applied in tandem: analysis of the growth band pattern, ontogenetic stable isotope profiles, shell morphometrics, and microstructural imaging in one individual, and then across multiple individuals from many sites through time.