Soil building processes were studied in reconstructed tree islands in the Loxahatchee Impoundment Landscape Assessment, Florida. Soil building was evaluated by measuring litter production, litter decomposition, soil accretion, and changes in soil elevation under different hydrologic conditions, and by determining physicochemical characteristics of newly accreted soils. Tree islands showed higher litter production and soil accretion but a larger loss of soil elevation caused by subsidence at higher elevations and shorter inundation periods. Newly accreted soils exhibited higher nutrient concentrations, and organic matter (OM) than older soils. Most of the soil phosphorus was stored in the organic fraction. A positive correlation was found between soil nutrients and OM. Reconstructed tree islands are increasing in soil OM and nutrients, generating a positive feedback that increases tree productivity, and soil building. These findings contribute to the understanding of tree islands’ dynamics and can be used by managers for restoration efforts in the Everglades.

abstract: Archaeologists have long contended that large-scale human migrations played an essential role in the cultural development of pre-Hispanic central Mexico. During the Epiclassic period (600-900 CE), migration is implicated in the appearance of new forms of material culture, sociopolitical disruptions, and the emergence of new regional polities. Sweeping social changes accompanied these developments, including demographic reorganization and increased levels of violence. Research across the social sciences finds that violence directed at individuals perceived as categorically distinct also typically increases during such periods of socio-political upheaval. This dissertation investigates identity-based violence in the Epiclassic Basin of Mexico to consider how diverse social identities contributed to the selection of victims of ritual violence. This research examines the skeletal remains from a sacrificial deposit at the Epiclassic shrine site of Non-Grid 4 in the Basin of Mexico, where a minimum of 180 human crania were interred as ritual offerings. The project reconstructs patterns of paleomobility and biological relatedness to determine whether individuals with distinct categorical social identities were more likely to become victims of human sacrifice. It answers the questions: (1) Were the sacrificed individuals predominantly locals who lived in the Basin of Mexico throughout their lives?; (2) Were the sacrificed individuals comprised of a single kin-group biologically continuous with pre-extant populations in the Basin of Mexico?; and (3) If victims were migrants biologically discontinuous with antecedent populations, from where in ancient Mesoamerica did they originate? Results indicate that a majority of sacrificial victims were immigrants originating north and south of the Basin of Mexico. Biogeochemical analyses of sacrificed individuals find that 80% are non-local migrants into the Basin, suggesting that they were likely targeted for violence based on their divergent residential histories. Multi-scalar biodistance analyses of Non-Grid 4 sacrificial victims demonstrate that they represent two biologically distinct groups. There was evidence, however, for both biological continuity among victims and pre-extant central Mexican populations, as well as for migration from northern and southern Mexico. This project therefore not only improves knowledge of migration during the central Mexican Epiclassic, but also contributes to broader anthropological understandings of the social context of violence. / Dissertation/Thesis / Doctoral Dissertation Anthropology 2020

abstract: Nitrous oxide (N2O) is an important greenhouse gas and an oxidant respired by a diverse range of anaerobic microbes, but its sources and sinks are poorly understood. The overarching goal of my dissertation is to explore abiotic N2O formation and microbial N2O consumption across reducing environments of the early and modern Earth. By combining experiments as well as diffusion and atmospheric modeling, I present evidence that N2O production can be catalyzed on iron mineral surfaces that may have been present in shallow waters of the Archean ocean. Using photochemical models, I showed that tropospheric N2O concentrations close to modern ones (ppb range) were possible before O2 accumulated. In peatlands of the Amazon basin (modern Earth), unexpected abiotic activity became apparent under anoxic conditions. However, care has to be taken to adequately disentangle abiotic from biotic reactions. I identified significant sterilant-induced changes in Fe2+ and dissolved organic matter pools (determined by fluorescence spectroscopy). Among all chemical and physical sterilants tested, γ - irradiation showed the least effect on reactant pools. Targeting geochemically diverse peatlands across Central and South America, I present evidence that coupled abiotic and biotic cycling of N2O could be a widespread phenomenon. Using isotopic tracers in the field, I showed that abiotic N2O fluxes rival biotic ones under in-situ conditions. Moreover, once N2O is produced, it is rapidly consumed by N2O-reducing microbes. Using amplicon sequencing and metagenomics, I demonstrated that this surprising N2O sink potential is associated with diverse bacteria, including the recently discovered clade II that is present in high proportions at Amazonian sites based on nosZ quantities. Finally, to evaluate the impact of nitrogen oxides on methane production in peatlands, I characterized soil nitrite (NO2–) and N2O abundances along soil profiles. I complemented field analyses with molecular work by deploying amplicon-based 16S rRNA and mcrA sequencing. The diversity and activity of soil methanogens was affected by the presence of NO2– and N2O, suggesting that methane emissions could be influenced by N2O cycling dynamics. Overall, my work proposes a key role for N2O in Earth systems across time and a central position in tropical microbial ecosystems. / Dissertation/Thesis / Doctoral Dissertation Microbiology 2020

Tropical coral reefs provide important ecosystem goods and services that are supported by one or more ecosystem functions (e.g., recruitment, primary production, calcification, and nutrient recycling). Scleractinian corals drive most of these functions, but a combination of global and local anthropogenic stressors has caused persistent shifts from coral- to algae-dominated benthic reef communities globally. Such phase shifts likely have major consequences for ecosystem functions; yet, related knowledge is scarce in general, but particularly at the community level, under ‘in situ’ conditions, and under the influence of changing environmental variables. Thus, we conducted a series of interconnected in situ experiments in coral- and algae-dominated reef communities in the central Red Sea, combining traditional community ecology approaches with novel metabolic and biogeochemical assessments from December 2016 to January 2018. Specifically, we (i) examined the influence of coral-algal phase shifts on recruitment and succession patterns, (ii) assessed the role of benthic pioneer communities in reef carbon and nitrogen dynamics, (iii) developed a novel approach to measure functions of structurally complex reef communities in situ, and (iv) quantified biogeochemical functions of mature coral- and algae-dominated reef communities. The findings suggest that coral-algal phase shifts fundamentally modify critical reef functions at different levels of biological organization, namely from pioneer to mature reef communities. For example, community shifts, through a lower habitat complexity and grazing pressure, decreased the number of coral recruits by >50 %, thereby inhibiting the replenishment of adult coral populations. At the same time, a 30 % higher productivity (annual mean) and increased organic carbon retention in algae-dominated communities supported a fast biomass accumulation and community growth, altering the habitat-specific community metabolism and reef biogeochemistry. Seasonal warming amplified these functional differences between coral- and algae-dominated communities, likely promoting a positive feedback loop of reef degradation under predicted ocean warming. Overall, this dissertation provides quantitative data on critical functions of classical and phase shifted novel reef communities, on tipping points for the collapse of community functions, and potential future winners and losers. The knowledge gained with this thesis helps, thereby, to understand how phase-shifted reef ecosystems function and which services will be generated in comparison to coral-dominated reefs under near-future stress scenarios.

Sea level regression during the Last Glacial Maximum exposed the Bering Land Bridge, and cut off the connection between the North Pacific and Arctic Ocean, ending the exchange of North Pacific Water through the Bering Strait. Exchange of North Pacific Water comprises a major portion of fresh water input to the Arctic Ocean, and is of vital importance to North Atlantic Deep Water formation, a vital component of Atlantic Meridional Overturning Circulation. Bering Strait throughflow thus plays an integral role in global climate stability. A suite of four cores was selected, three in the Bering Sea and one in the Chukchi Sea, to bracket the Bering Strait in order to elucidate changes in sediment delivery, productivity and regional oceanography as the Bering Land Bridge flooded and modern ocean circulation was established during the last deglaciation. The arrival of nutrient rich North Pacific Water in the Chukchi Sea is recorded around 8 ka by organic carbon isotope depletion and an increase in total organic carbon and organic nitrogen, reflecting an increasingly marine isotopic signal and increased productivity. In the Bering Sea, the early deglaciation is marked by depleted organic carbon isotopes that indicate increasing terrestrial input, and increased total organic carbon. Principal component analysis of sedimentologic, geochemical and isotopic data clearly captures discrete sediment populations that correspond to key climatic intervals, representing changes in sediment delivery, productivity and circulation during the last deglaciation. In the Bering Sea we observe that deglaciation began in earnest around 18–17 ka, but lack of confidence in our age control does not allow for a precise date. Our results suggest that modern circulation through the Bering Strait, and thus for the Bering and Chukchi Seas, was established ~8 ka. Prior to 8 ka there is an interval of sediment that appears record a possible reversal of flow through the Bering Strait corresponding to the 8.2 ka event.

Future impacts from climate change can be better understood by placing modern climate trends into perspective through extension of the short instrumental records of climate variability. This is especially true for extreme climatic events, such as extreme precipitation and wildfires, as the period of instrumental records provides only a few examples and these have likely have been influenced by anthropogenic warming. Multi-parameter records showing the past range of climate variability can be obtained from lakes. Lakes are particularly good recorders of climate variability because sediment from the surrounding environment accumulates in lakes, making them sensitive recorders of climate variability and providing high-resolution histories of local environmental conditions in the past. In some cases, such as at Basin Pond, sediment is persevered efficiently enough to produce distinguishable annual laminations (varves) in the sedimentary record. The varved record at Basin Pond was used to construct an accurate, highly-resolved age-to-depth model over the past 300 years. Using a multi-proxy analysis, including organic biomarker analysis of molecular compounds and sedimentological features preserved in the sediment record, a history of environmental and climatic change at Basin Pond was constructed. These analyses were compared with the record of known extreme events (from instrumental measurements and historical documents), including 129 years of high-resolution precipitation and temperature meteorological data, 19 tropical systems over the past 145 years, and two known wildfire events over the past 200 years. Long-term trends in precipitation, including the increase in precipitation seen throughout the last half of the 20th century and the drought of the 1940’s, were captured in the analysis of long-chain n-alkane distributions and through varve thickness measurements obtained through X-Ray Fluorescence analysis. Furthermore, Polycyclic Aromatic Hydrocarbons (PAHs), a class of organic compounds that can be used to trace combustion activity, were found in abundance in the Basin Pond sedimentary record. Peaks in the abundances of two PAHs (retene and chrysene) and the ratio retene/(retene + chrysene) were found to be highly correlated with the known wildfire events occurring in the historical period, giving promise as using these compounds and ratio as a robust proxy for regional wildfire events in the northeastern U.S.

Because Ga shares many physicochemical properties with Al and Fe, Ga may be able to incorporate into Al and Fe oxy-hydroxides. Understanding how Ga incorporates into these oxy-hydroxides may be crucial for finding Ga-rich bauxite deposits. In order to find the difference in Ga inclusion rates into oxy-hydroxides, as well as understand the mechanisms for this Ga inclusion, Al and Fe oxy-hydroxides were synthesized in the lab with Ga additions of 2 mol % Ga and 20 mol % Ga for a low-Ga and high-Ga treatment, respectively, along with a no added Ga control. X-Ray diffraction analyses confirmed the formation of bayerite (α-Al(OH)3) and goethite (FeOOH) after 100 days (goethite long synthesis [LS]). A second batch of goethite was synthesized in the lab and aged for 60 hours (goethite short synthesis [SS]). Results showed the highest Ga inclusion rates in goethite LS minerals at 0.89 mol % / mol % Ga, then 0.17 mol % / mol % Ga in goethite SS, and 0.50 mol % / mol % Ga in bayerite. Scanning electron microscopy and electron microprobe analyses determined co-precipitation of Ga was the dominant Ga incorporation mechanism in bayerite over isomorphic substitution, where needle-like mineral assemblages began to form in the high-Ga treatments. Isomorphic substitution vii was dominant in both goethite batches. Additionally, Ga mol % in the high-Ga goethite LS and goethite SS minerals revealed a temporal aspect to Ga inclusion in goethite. Goethite LS high-Ga treatment minerals had Ga mol % of 16.8 ± 0.23 % compared to 3.34 ± 0.03 % for high-Ga treatment goethite SS minerals. This study highlights an advance in knowledge of Ga incorporation mechanisms into Al and Fe oxy-hydroxides and provides a basis for future studies to expand on these efforts.

Critical water resources, such as groundwater, are undergoing a period of intense and global environmental change, driven by climate change, anthropogenic impacts and exploitation, and perturbations to interactions of fundamental processes that are affected by hydrological, mineralogical and biogeochemical factors. Arsenic contamination is a significant threat to these water resources and the populations who depend on them, yet there are few studies directly linking water quality with changes in hydrology and geochemistry in sediments on varying scales. My research explores environmental variability in hydrology and redox processes that regulate soluble arsenic concentrations at the pore scale (µm to mm), and develops methods of upscaling these mechanistic studies to understand heterogeneity in groundwater arsenic levels and their impacts on public health at larger scales (a couple of meters to hundreds of kilometers). Specifically, my research examines the interaction of redox processes in the Earth’s subsurface that drive the release of arsenic into groundwater. Naturally-occurring, or geogenic, arsenic contamination is the main source of arsenic release into groundwater that affects human health, with possible anthropogenic exacerbation of this natural contamination. Throughout this dissertation, I have developed a suite of data-driven approaches to understand and quantify the highly variable factors that underlie the mechanisms of geogenic arsenic release into groundwater and its migration in the environment. In Chapter 1, I investigate the effects of hydrologic perturbations on formerly uncontaminated aquifers that release arsenic due to increased groundwater pumping in the Red River Delta, Vietnam. To compare the effect of hydrologic processes to measured groundwater arsenic concentrations, I used Monte Carlo simulations in an end-member mixing model and quantified fraction of different recharge sources into an aquifer based on stable water isotopes. I find that changing flow patterns due to groundwater abstraction have increased the extent of arsenic release into groundwater and also changed the location of where arsenic contamination originates. In Chapter 2, I characterize iron mineralogy associated with arsenic release through sampling of sediment cores across a lateral redox gradient in Vietnam with extensive spectroscopy measurements. Through hierarchical cluster analysis on this data set of X-ray absorption spectroscopy (XAS) measurements of borehole cuttings paired with dissolved groundwater measurements, I reveal signatures of iron mineral reduction that could cause or exacerbate arsenic release. This was upscaled to other deltaic aquifers in South and Southeast Asia based on groundwater data to identify aquifers at risk of arsenic release. I showed that the extent of older and previously pristine aquifers that have been contaminated may have been misclassified and thus underrepresented in deltaic aquifers throughout South and Southeast Asia, disrupting the assumption that older and deeper aquifers are oxidized and thus guarded against arsenic release. In Chapter 3, I use process-based reactive transport modeling of a laboratory-scale experiment to mechanistically explain the infiltration of contaminated water into uncontaminated aquifers and find that arsenic contamination cannot be explained by the commonly invoked mechanism of iron reducing bacteria only, but instead relies on sulfate reduction and complexation of aqueous arsenic in solution. The role of sulfate reduction in mobilizing arsenic in groundwater is in stark contrast to and undermines the previous use of sulfate reduction as strategy for arsenic remediation. Finally, in Chapter 4, I quantitatively examine the processes that release arsenic across different arsenic-impacted aquifers, based on the relationships between redox status of iron and arsenic mineralogy and groundwater concentrations. Synthesis of X-ray absorption spectra of the deltaic aquifers of Southeast Asia and the glacial aquifer system in the Northern United States shows that arsenic release occurs in similar geochemical environments in both systems, and is highly generalizable via statistical and unsupervised machine learning approaches. This dissertation demonstrates that common assumptions behind geogenic arsenic release must be tested: from which aquifers are low in arsenic to the commonly assumed mechanism of arsenic release by iron reducing bacteria. These findings also reveal that the extent of anthropogenic impact on geogenic arsenic contamination is detectable: from changes in recharge sources to changes in mineralogy that affect arsenic concentrations and human health. The next step is to use these data driven and machine learning approaches to quantify the vulnerability of affected aquifers, to mitigate the risk of those currently reliant on contaminated groundwater, to reduce the risks of future contamination and, ultimately, to protect human health.

No description available.

Thesis advisor: Hilary Palevsky / The ocean plays a key role in global carbon cycling, taking up CO2 from the atmosphere. A fraction of this CO2 is converted into organic carbon through primary production in the surface ocean and sequestered in the deep ocean through a process known as the biological pump. The ability of the biological pump to sequester carbon away from the atmosphere is influenced by the interaction between the annual cycle of ocean mixed layer depth (MLD), primary production, and ecosystem processes that influence export efficiency. Gravitational sinking of particulate organic carbon (POC) is the largest component of the biological pump and the aspect that is best represented in Earth System Models (ESMs). I use ESM data from CESM2, an ESM participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6), to investigate how a high-emissions climate change scenario will impact POC flux globally and regionally over the 21st century. The model simulates a 4.4% decrease in global POC flux at the 100 m depth horizon, from 7.12 Pg C/yr in the short-term (2014-2034) to 6.81 Pg C/yr in the long-term (2079-2099), indicating that the biological pump will become less efficient overall at sequestering carbon. However, the extent of change varies across the globe, including the largest POC flux declines in the North Atlantic, where the maximum annual MLD is projected to shoal immensely. In the future, a multi-model comparison across ESMs will allow for further analysis on the variability of these changes to the biological pump. / Thesis (BS) — Boston College, 2021. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Earth and Environmental Science.