Long sediment records from offshore and terrestrial basins in California show a history of vegetation and climatic change since the last interglacial (130,000 years BP). Vegetation sensitive to temperature and hydroclimatic change tended to be basin-specific, though the expansion of shrubs and herbs universally signalled arid conditions, and landscpe conversion to steppe. Multi-proxy analyses were conducted on two cores from the Big Bear Valley in the San Bernardino Mountains to reconstruct a 125,000-year history for alpine southern California, at the transition between mediterranean alpine forest and Mojave desert. Age control was based upon radiocarbon and luminescence dating. Loss-on-ignition, magnetic susceptibility, grain size, x-ray fluorescence, pollen, biogenic silica, and charcoal analyses showed that the paleoclimate of the San Bernardino Mountains was highly subject to globally pervasive forcing mechanisms that register in northern hemispheric oceans. Primary productivity in Baldwin Lake during most of its history showed a strong correlation to historic fluctuations in local summer solar radiation values. Rapid organic perturbations in the Baldwin Lake core were coeval with Dansgaard-Oeschger (D-O) events from the North Atlantic, which were evident in records from the Santa Barbara Basin. The predominant vegetation signal at Baldwin Lake was one of temperate conifer forest expansion during moist conditions, and contraction during dry conditions. This expansion and contraction was paced with summer insolation fluctuations during Marine Isotope Stage 5 (110,000 – 71,000 years BP), before a regime change towards more rapid, shorter-lived hydroclimate extremes. Wildfire is an important agent of landscape change throughout the Valley’s history, with two hiatuses during cold and moist conditions from 25,000 – 14,000 years BP, and arid conditions 7,000 – 3,000 years BP. Taken together, this multi-proxy dataset suggests that paleoclimatic changes in alpine southern California have been highly sensitive to three climate drivers: 1) shifts in local summer insolation, 2) rapid warming in the North Atlantic, and 3) changes in the strength of the California Current. The record also showed a wide range of possible moisture states in southern California’s past that vary from present conditions, including multi-millennial states of long-term aridity and pre-glacial deep lake conditions.

The San Francisco Bay has the largest concentration of salt marshes in the state of California, representing a diversity of marsh habit. Protecting these environments is critical, as salt marshes provide refuge to endangered species, absorb carbon from the atmosphere, and preserve detailed evidence of past climatic, hydrologic, geomorphic, and ecologic conditions. However, much of the marshes have been impacted by pollutants, altered or lost entirely due to human activity over the past 150 years, making their prehistoric conditions, ecological trajectories and resilience to disturbance uncertain. In this study, I collected data from Petaluma Marsh, one of the oldest marshes in the Bay Area, to document the geochemical changes including heavy metal concentrations as a response to climatic and anthropogenic changes since the mid-Holocene. Loss-on ignition, pXRF, and magnetic susceptibility data were collected at high resolution from a 12-meter, 6109-year-old, sedimentary core recovered from this tidal marsh located along the Petaluma river in the northern Bay region. Average rate of sediment accretion is 3.6 mm yr-1 � 0.8. Our results show that the introduction of European land-use and activities over the last 150-200 years resulted in disturbance of marsh biota and increases in heavy metals with unprecedented levels of lead and zinc. However, based on statistical time-series analysis of long-core elemental concentrations, results show that modern geochemical conditions are not so far removed compared to prehistoric conditions, as often suggested by century-scale analyses. Modern heavy metal concentrations (e.g., Cr, Fe, Sr, Ba, Zr, Rb and Ni) match concentration levels from 4000-6000 YBP. However, Pb levels in the marsh post-European land use are dramatically higher now than ever before. Our results show the effects of anthropogenic changes on this ancient and ecologically important marsh in the north San Francisco Bay area that can help better inform restoration ecologists and policy makers, specifically in terms of heavy-metal pollutants.

Coastal salt marsh ecosystems in California are at risk from projected rates of sea-level rise (SLR) of up to an order of magnitude higher than rates seen over the past 6,000 years of stable sea levels (Griggs, Cayan, Tebaldi, Fricker, & �rvai, 2017). With rates of this magnitude, salt marsh area, already limited by land use changes in the 19th and 20th centuries, could be completely lost by 2100 (Thorne et al., 2018). To better understand how California salt marshes are adapting to modern acceleration of SLR, over 100 sediment cores were collected from 13 salt marsh sites, ranging from Humboldt Bay to Tijuana River Estuary. Sediment accretion rates over the past several hundred years were measured using radiocesium, radiolead, and radiocarbon dating on 32 cores. Valuation of the carbon storage, an ecosystem service known as blue carbon provided by salt marshes, presents an opportunity to help preserve and restore sites threatened by SLR through carbon credits (Bear, 2017; Callaway, Borgnis, Turner, & Milan, 2012; Mcleod et al., 2011), but there are many questions which much be addressed before this can become a reality for the state of California (Macreadie et al., 2019). A standardized protocol for estimation of carbon content from loss-on-ignition (LOI) was developed with an emphasis on quantifying error and uncertainty in carbon measurements for blue carbon purposes. Using a conversion between soil organic matter and soil organic carbon shown to be effective for California salt marshes, carbon content was estimated through LOI analysis of 61 sediment cores. The impact of climate change in these ecosystems was further explored in the first documented record of a fire in a Pacific coast salt marsh at Mugu Lagoon.

California salt marsh sediment accretion averages at 2.93 � 1.9 mm yr-1, which is lower than average rates from regions such as the US Gulf and East coasts. Rates of accretion and relative SLR (RSLR) show a non-linear relationship with highest accretion occurring at rates of RSLR from 2 – 6 mm yr-1. Linear relationships between SLR and accretion are comparatively weak, but are stronger in the low elevations of salt marsh habitat. Salt marshes in the state annually sequester about 0.08% of state-wide annual greenhouse gas emissions and store about 23% of one year’s emissions in their soils (as compared to 2016 emissions). Because of limited area, these habitats will not serve as an effective mitigation strategy at the state level, but loss of this habitat may release up to 27 � 0.3 Tg stored carbon, potentially valued at about $1.4 billion (using an estimate of $15/tonne CO2 �equivalent). Preservation of current habitat through facilitation of sediment accretion will have the largest positive impact on carbon storage and sequestration, as well as protect salt marsh habitat from being lost to SLR. Analysis of the persistent effects of a recent marsh fire at Mugu Lagoon demonstrates that drought-stress may slow California salt marsh response to disturbance by one or more growing seasons and highlights the uncertain impacts of climate change on system function. This dissertation provides important baseline data for salt marsh sediment accretion, salt marsh carbon stocks and sequestrations rates, recommends best practices for use of LOI as a measure of soil organic carbon, and examines ecosystem recovery under multiple stressors. This work can be used in vulnerability assessments, ecosystem models, and valuation of ecosystem services for California salt marshes.

Coastal wetlands are considered one of the most productive natural ecological infrastructures in the world. Although coastal ecosystems total only 6% of global surface area, they provide an estimated 38% of global ecosystem services. Despite their environmental and societal value, coastlines and coastal habitats are increasingly threatened by human activity. Human threats include proximal disruptions such as wetland removal, changes in sedimentation and chemical pollution. Additionally, climate change, and more specifically sea-level rise (SLR) poses one of the greatest global threats to coastal marshes. Estimates for future SLR rates range anywhere from 0.3 m to over 1.3 m by the end of the 21st century. While historical observations have shown that tidal wetlands can tolerate and dynamically adjust in elevation to some rate of SLR, there are limits. Human population growth, coastal development and the concept of coastal squeeze constrain landward vertical migration of marshes and bring in additional factors that challenge efforts to understand and manage future salt marsh trajectories. Indeed, humans are integrated into the very fabric of major processes governing wetland stability, which can have major impacts on ecogeomorphic feedback systems and overall marsh resiliency. Thus, local anthropogenic stressors should be coupled with climate change impacts in management and conservation efforts, as they often interact synergistically. However, to do so effectively requires communication and unified actions by stakeholders, managers, and scientists. In the following dissertation, I plan to tie these themes together by researching the multiple human and climatic stressors on California coastal marshes and creating knowledge that can be used in science-policy settings. Furthermore, I use a participant-observer approach to study the communication and planning for mitigating coastal threats in California. First, I obtain high-resolution geochemical data from three coastal marshes in the San Francisco Bay to look at responses to recent anthropogenic changes in sedimentation and pollutant loadings in the context of marsh conditions and histories since the mid-Holocene. Next, I look at attempts to mitigate the impacts of SLR through a large-scale sediment addition project in Seal Beach National Wildlife Refuge, CA. I use the analysis of sediment cores, to understand natural accretion and variability over time and how it compares to the artificial accretion and sedimentation from sediment addition. Lastly, I utilize information from a longer-term participant-observer project updated and augmented with my own participant-observer experience with the SWCASC funded coastal workshops to provide an analysis on knowledge co-production efforts in coastal management settings to understand what makes knowledge relevant in management and policy contexts.

In recent years, California has been suffering from severe hydroclimatic related challenges, such as droughts, fires, and deluges. Despite many ongoing efforts, uncertainties remain regarding the natural variability of hydroclimatic system and how anthropogenic impacts will affect future climate dynamics. To better understand the issues, it is essential to understand paleo-hydroclimatic variability over the long-term during the period that human beings had little impact on the climate. This dissertation aims to investigate long-term centennial to millennial hydroclimatic changes to understand natural variability to help differentiate this from anthropogenic impacts on the current climate and to plan for future climate change (e.g. formulating relevant policies, proposing contingency plans, etc.). Lake sediments preserve long hydroclimate records useful for resolving long-term variability.Among the feasible proxy indicators used to reconstruct paleo-hydroclimate, I utilized diatom microfossils (microscopic algae) because they have been shown to record lake level and salinity changes within lakes that reflect changes in precipitation and evaporation. I analyzed the fossil and modern diatom flora from Kelly Lake in Northern California and from Baldwin Lake in Southern California. This dissertation focuses on the late Quaternary overall while focusing specifically on the Holocene changes in Northern California and Pleistocene changes in Southern California. From Kelly Lake, I established diatom-inferred water depth transfer functions to quantitatively infer the past lake level changes during the Holocene. The results show a meaningful correlation between modern diatom assemblages and water depths, allowing the construction of statistical transfer functions with high R2 values (>0.95). This is the first such single-lake transfer function developed in California and one of only a handful worldwide. The transfer functions are then applied to the fossil diatom flora from a Holocene-length sediment core from Kelly Lake. The quantitatively reconstructed lake level changes in Kelly Lake indicate appreciable variations over the Holocene and suggest the influences from global drivers such as solar radiative and El Ni�o-Southern Oscillation forcings to the hydroclimatic variability in the Northern Coast Ranges. In Southern California, the qualitatively inferred salinity and water depth changes in Baldwin Lake showed abrupt change at about 70,000 cal yr BP when the orbitally controlled radiative forcing transitioned high seasonal variability to low seasonal variability. In both Northern and Southern California, the diatom records indicate a high degree of sensitivity by hydroclimate to external drivers related to external forcing factors such as ocean conditions and orbital variations.

Plant macrofossils can be powerful proxies for environmental change, but their preservation in the fossil record does not always allow for long-term reconstructions. The La Brea Tar Pits are unique not just for the sheer abundance of fossils they preserve from the Late Pleistocene and Holocene, but also for the particular taphonomic conditions that allow for continuous preservation of an ecologically diverse fossil assemblage across a 55 ka timespan (Chester Stock, 1992). This proves especially valuable in the Mediterranean region of coastal Southern California, where long-term terrestrial records are spatially scarce (Glover et al., 2020; Kirby et al., 2018). Hydrocarbon preservation of cellulose and lignin allows for the preservation of plant macrofossils (fossils visible with the naked-eye and identifiable using low-magnification microscopy) which have the potential to provide species specific data on plant presence through time (S. T. Jackson et al., 1997). Rich ecological data through the Late Pleistocene and Holocene provides much needed information on how plant communities were responding during a climatically active time of glacial growth and decline, and millennial-scale shifts in atmospheric and sea surface temperatures (SST), as well as important environmental context for development and the extinction of Ice Age megafauna and migration of human populations (Barnosky et al., 2017; Meltzer, 2020).This dissertation explores the viability of La Brea Tar Pit plant macrofossils as environmental and plant community proxies. Through active seep observation and exploration of taphonomic processes of plant deposition in modern hydrocarbon seeps, multi-method approaches to identification, and direct accelerator mass spectrometry (AMS) radiocarbon dating of preserved plant material, this project constructs a high taxonomic resolution, 55 ka long, record of vegetation change for the Los Angeles Basin. This dissertation finds plant material deposition in asphalt seeps to be highly localized and representative of flora fruiting in warmer months. Through the La Brea Tar Pits vegetation record we have found species response in the Los Angeles Basin to be sensitive to climatic changes driven by orbital forcings, and potentially variations in the Northeast Pacific and North Atlantic sea surface temperatures. Substantial changes in the flora and vegetation cover are recorded in the LBTP record and some of these shifts are contemporaneous with local extinction of mega-fauna both during the transition from the Pleistocene to the Holocene (Marine Isotope Stages MIS2 to MIS1 - ~12 ka) and in the earlier Pleistocene. The disappearance of large-bodied herbivores at the start of the Holocene, coupled with increased fires, possible increasing in frequency due to the arrival of humans, potentially also played a role in the vegetation changes observed, most notably the extirpation of juniper from the Los Angeles Basin. This local-scale record of environmental shifts from the Late Pleistocene into the Holocene at the LBTP’s provides a dynamic baseline of plant species presence for future environmental and ecological studies of paleo-coastal Southern California, and possibly assessing the sensitivity of current vegetation to ongoing and future climatic changes.

Sediment cores were collected from wetlands along a headwaters-to-estuary gradient within the San Joaquin watershed, California, to examine environmental changes, and carbon (C) storage and sequestration rates over the Holocene. Local hydrogeomorphic conditions were the primary drivers of C sequestration at seven wetland systems analyzed Paleoecological assays of C and multiple paleo-indices were conducted on two cores from the Sacramento-San Joaquin Delta, three cores from Yosemite National Park, and on two cores from Sequoia-Kings Canyon National Park to reconstruct C storage and ecosystem functioning of wetland systems in the watershed over the Holocene. Bayesian age-depth models were developed using cesium 137/lead 210 and radiocarbon dating. Paleo proxies such as loss-on-ignition, pollen, x-ray fluorescence, and charcoal showed that local hydrogeomorphic conditions were the primary drivers of variability in C sequestration at 7 wetland systems analyzed. There is some evidence of C sequestration responding to past warming by changing vegetation types, altering geochemical concentrations of crustal elements, and varying the frequency of wildfires. A 300 cm sediment core retrieved from Dana Meadow, Yosemite National Park, spans the past 9000 years and indicates carbon storage was ≥ 24 g m2 yr-1 during the Holocene Thermal Maximum (~8000 – 3000 years ago). By the mid-Holocene, terrestrial plants like Quercus and Tsuga mertensiana expanded upslope amid prolonged warming. Average rates of C sequestration and vertical accretion from the five Yosemite and Sequoia-Kings Canyon wet meadows indicate regional similarities to rates of C storage from subalpine fens in the mountains of western North America as far north as Alberta, Canada. The two cores from the Sacramento-San Joaquin Delta present no clear relationships between C content and climate change. In the Delta, decreases in C content and sedimentation rates occurred after Euro-American settlement in the 19th century, trends that continue during the 21st century. The findings of this dissertation on the whole suggest: 1) no regionally coherent climatic response to past climatic changes was shown in the paleo indices measured for the seven sediment cores from the Delta and Sierra; 2) there is much local variability in the histories of these wetlands and their C sequestration records; 3) local variations in hydrogeomorphology appears to be a primary driver of natural C sequestration and storage for each site; 4) such variability likely reflects differing responses and sensitivities to the impacts past climatic change as well as local hydrogeomorphology; and 5) although this study detected no widespread coherent response of the San Joaquin wetlands C dynamics to past Holocene climatic changes, documenting and helping to understand the potential variability of such environmental systems isarguably just as important in anticipating California’s climate change future and its potential variability of responses.

Coastal marshes are complex ecogeomorphic feedback systems that require further investigation on the Southern California coast to understand potential responses to sea level rise (SLR). Long-term accretion rates - deposition and erosion of mineral and organic matter - form a basis of understanding processes in the marsh related to SLR responses. From sediment cores, I reconstruct the net accretion rates of three marshes using radiocarbon dating methods and analyze loss on ignition (LOI) data to understand the physical properties of the sedimentary record in the three marshes. Average net accretion rates for Tijuana Estuary are 1.0 ± 0.94 mm yr¬-1, for Upper Newport Bay are 1.0 ± 0.4 mm yr-1, and for Morro Bay are 8.0 ± 8.3 mm yr-1. Over the past 2000 cal YBP, all net accretion rates kept pace or exceed rates of SLR (when compared to historic SLR of 0.6 to 2 mm yr-1); however, only Morro Bay exhibits historic net accretion rates high enough to compare to possible rates of SLR associated with projected sea level gains of 0.3 to 1.6 m on the Pacific coast through 2100. Core stratigraphies indicate marsh conditions change frequently and the current Spartina spp. and Salicornia spp. marsh vegetation communities are geologically recent features in their present locations, existing from 700 to 1000 cal YBP. The future under continued human modification of coastal systems, climate change, and accelerated SLR merit continued research into the dynamics of coastal salt marsh systems on the California coast.

In chapter 1, Fremont Ancestral Puebloan (AP) maize (Zea mays) cumulative growing degree days (cGDD) were reconstructed for the years 850-1449 CE by statistically downscaling temperature dailies from a state-of-the-art, fully coupled climate model (CESM LME). In addition, radiocarbon dates from geolocated Fremont artifacts (n=982) used were to map time series of spatially discrete summed probability distributions (SPD). Fremont occupation of the Colorado Plateau was shown to coincide with warming conditions during the early Medieval Climate Anomaly (MCA), ~850-1050 CE, and abandonment with shorter and increasingly variable growing seasons after ~1050 CE. The disappearance of Fremont-identified material culture from Utah was shown to coincide with increased hydrologic variability on the Colorado Plateau with the transition to the Little Ice Age (LIA, ~1350-1850 CE). In chapter 2, the downscaled climate model was used to drive a process-based crop model (EPIC) to reconstruct maize yields under 7 different crop management strategies for irrigated and non-irrigated cases. Individual sites were clustered within three separate subregions for analysis (i.e., the Great Salt Lake shoreline, the highland Uinta Basin, and an archetypal Fremont canyon occupation at Range Creek). All occupations analyzed were caught within the same agro-ecological vise of increasing exogenous climatic and endogenous (anthropogenic) soil nutrient degradation, with varying degrees of intensity. Among the subregions, Range Creek was the most constrained by exogenous changes in climate (cooling and increasing variability at the MCA-LIA transition), and the Great Salt Lake shoreline the least; but the Great Salt Lake sites were the most sensitive to endogenous soil degradation. The methods employed in chapters 1 and 2 are broadly applicable to reconstruct GDD and crop yields, especially in other low-technology subsistence dryland agricultural contexts. In chapter 3, subfossil chironomid (Insecta: Diptera: Chironomidae) assemblages from sediments were analyzed to infer mean July air temperatures for 3 subalpine lakes in Utah. Results showed warm and cool periods over the Markagunt Plateau (i.e., the location of Alpine Pond) in southwestern Utah and over the Wasatch Plateau (i.e., Blue Lake) in southern central Utah were consistent with those expected from other proxy records during the MCA and LIA. (The record for Denise Lake, in the Uinta Mountains of northeastern Utah, was discontinuous.) Further, warming over the past century was unprecedented over the past 1570 years.