Uranium-series isotopes are an emerging tool to characterize weathering and soil forming processes in the Critical Zone, the dynamic region of earth's surface where bedrock, water, soil, and life chemically and physically interact to support ecosystems. Understanding controls on the U-series composition of soils across a range of lithologies and climate is critical to applying existing mass balance models of U-series isotopes to calculate soil formation rates in these systems. This study seeks to understand the behavior of U-series isotopes in soil profiles in the semi-arid complex volcanic terrain of the Valles Caldera, NM. (²³⁴U/²³⁸U) measured in soils ranged from 0.90 to 1.56 and (²³⁰Th/²³⁸U) values ranged from 0.48 to 1.39. Significant ²³⁰Th enrichment in upper soil profiles was interpreted as evidence of mixing with ²³⁰Th-enriched volcanic ash and significant ²³⁴U enrichment in one soil profile was interpreted as evidence of addition of U to soils from ²³⁴U-enriched soil solutions. A simple U isotope mass balance model was applied to estimate soil residence time based on U addition, which yielded a minimum residence time of ~10ka. Evidence of past episodic mixing of volcanic ash in these soils suggests modeling soil formation using a mass balance approach is problematic, and future applications of existing models in other heterogeneous volcanic soils should be applied cautiously. U-series isotopes have also shown promise as a tracer of residence time in shallow groundwater and streams. In this study, (²³⁴U/²³⁸U) in dissolved U is used to trace seasonal variation in source water contributions to streamflow in a small (3.29km²), headwater catchment in the Jemez River Basin Critical Zone Observatory within the Valles Caldera. Systematically lower (²³⁴U/²³⁸U) values in dissolved U were observed in spring and stream waters in conjunction with greater contributions of longer residence time waters during snowmelt ((²³⁴U/²³⁸U) ranged 1.7 to 2.8) vs. dry seasons ((²³⁴U/²³⁸U) ranged 1.9 to 3.1). The lower (²³⁴U/²³⁸U) values in longer residence time waters were attributed to progressive depletion of easily-weathered ²³⁴U with increasing duration of water rock interaction. Further studies with more quantitative age tracers, such as ³H, could help to establish (²³⁴U/²³⁸U) values as a powerful tracer of water sources and residence time in streamwaters at the catchment scale.

Radionuclides from natural U-series in sediments from two river catchments in the UK have been studied. The aim was to gain insight into the behaviour of 238U, 234U, 230Th and 226Ra in real natural systems enriched in uranium. A radiochemical method for radium separation followed by alpha spectrometric measurement has been developed. The method allowed use of 225Ra, in equilibrium with the parent 229Th, as a yield determinant, and has been applied in 226Ra concentrations measurements in the selected areas of study.U-series progeny, 238U, 234U, 230Th and 226Ra, in totally dissolved sediments from the valley of the River Noe and the fraction leached by aqua regia, have been measured. Total sediment contents ranged from 9 ± 2 to 184 ± 8 Bq.kg-1 for uranium, 9 ± 3 to 200 ± 13 Bq.kg-1 for thorium and 18 ± 1 to 179 ± 8 Bq.kg-1 for radium. The activity concentrations in the leached fractions, compared with the total, were 46% for uranium, 54% for thorium and 56% for radium, on average. The radionuclides showed extensive disequilibrium and this suggested a complex leaching/accumulation of uranium as well as an impact of organic matter and secondary minerals.Uranium and radium have been geochemically characterised in sediments from near the South Terras abandoned uranium mine, Cornwall. Background activity concentration levels of uranium in sediments ranged from 13 ± 3 to 290 ± 14 Bq.kg-1, with radium from 42 ± 4 to 424 ± 23 Bq.kg-1. Elevated concentrations of uranium and radium were measured in two samples, S3 with 1820 ± 36 Bq.kg-1 for uranium and 940 ± 53 Bq.kg-1 for radium; and S7 with 4350 ± 53 Bq.kg-1 for uranium and 1765 ± 48 Bq.kg-1 for radium. Sequential chemical extraction for the two samples revealed that both uranium and radium were associated with organic and carbonate fractions, with 25 % of the uranium in the resistant phase of S7. 234U/238U activity ratios of the sequential extraction fractions showed different trends in the sediments, and this was linked to the impact of organic matter and/or exchange between water and sediment. Uranium-bearing minerals in association with potassium, calcium, iron, manganese and arsenic have been identified in these sediments.

Trajectories of decline have been described in coral reefs throughout the Indo-Pacific region, with long-term losses of abundance, diversity and habitat structure. Since European settlement of the Queensland coastline in the mid-18th century, widespread changes in land use have occurred within Great Barrier Reef (GBR) catchments (e.g. extensive land clearing, agriculture and grazing). Despite direct and indirect evidence indicating decline of inshore reefs, it has proven difficult to ascertain links between land use changes, terrestrial discharge, water quality and the decline of coral reefs at regional scales, and the contributions of anthropogenic influences to the disturbance regimes of inshore reefs remains highly controversial. This thesis uses palaeoecological reconstruction of coral assemblages and high-precision U-Series dating to examine changes in community structure of inshore coral reefs in the Palm Islands region across decadal and centennial scales. Comparisons of modern and historical coral assemblages provide evidence of a collapse of Acropora coral communities at Pelorus Reef in the early 20th Century. Fossil assemblages in an adjacent site at Pelorus provide further evidence of an extrinsic shift from historical Acropora assemblages to more sediment-tolerant corals in modern assemblages, a change without precedence in 800 years of record. Sediment cores (2-5m length) were extracted from Pandora and Havannah reefs to determine long-term rates of reef growth throughout the late Holocene. Computer Axial Tomography (CAT) scans of cores revealed a framework dominated by coral fragments, and U-Series dating of corals revealed rapid and continuous reef growth at both reefs throughout the last 1000 yrs. Comparisons of reef accretion (m ka-1) from cores with published accretion rates from early-mid Holocene inshore GBR reefs show that reef slope environments are now accreting at rates equal to and exceeding those of the last 8000 years. This result contradicts assumptions that inshore reefs are undergoing or have undergone natural trajectories of geomorphic decline. A bayesian approach to determine stability of coral communities from the Pandora and Havannah cores indicates that those coral communities have existed in stable states for upwards of 400yrs, punctuated by periods of instability. Contrary to some paradigms of biodiversity, the high diversity communities were unstable on centennial scales, 4 yet the low diversity assemblages did not necessarily confer stability to these assemblages. Growth rates of inshore reefs were independent of diversity or community structure. From a management perspective, these results provide a longer-term (decades to centuries) understanding of coral community dynamics on inshore reefs of the GBR that provides a basis for detecting and understanding changes following European settlement, and a baseline for the management and potential restoration of coral communities at local and regional scales.

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[pt] O presente trabalho aborda as determinações de idades de espeleotemas e corais, usando o método de desequilíbrio da série urânio, mais especificamente, o método de deficiência de filhos daughter deficient DD, em particular o método 230Th/234U. Foram testadas diferentes metodologias empregando separação por extração cromatográfica e quantificação por espectrometria alfa e ICP-MS. Foram testados três procedimentos diferentes: separação em batelada empregando-se colunas com Tri-octil óxido de fosfina (TOPO) em silica- gel e a coluna TRU comercializada pela Eichrom(R), composta de octil-N,N,- isobutil carbamoil óxido de fosfina dissolvido em fosfato de tributila (TBP) e quantificação por espectrometria alfa; separação em batelada com os mesmos sistemas mas quantificação por ICP-MS e separação em linha (flow injection) com cartuchos de TRU e quantificação por ICP-MS. As metodologias desenvolvidas foram validadas empregando-se amostras de referência certificada, IAEA-327 (solo) tendo sido obtidas incertezas de 2% para o método empregando a separação em linha (flow injection) com cartuchos de TRU e quantificação por ICP-MS. As amostras de espeleotema analisadas foram coletadas no Carste de Lagoa Santa, MG, pelo grupo do Prof. Luis Piló (USP) e suas idades variaram de 15,2±2,2 kanos a >350 kanos. Estes valores estão na faixa dos valores encontrados na literatura para amostras de espeleotema coletadas pelo mesmo grupo na mesma região. A amostra de coral, coletada na Bacia de Campos (RJ) era da espécie Lophelia pertusa, foi subdividida segundo suas ramificações (primária, secundária e terciária) e o ramo principal foi datado em 9,4 ± 0,3 kanos. / [en] This paper discusses the age determination of speleothems and corals using Uranium-series- imbalance method, more specifically the daughter deficient method - DD, particularly the 230Th/234U method. Different methodologies were tested using chromatographic extraction separation and quantification by alpha spectrometry and ICP-MS. Three different procedures were tested: batch separation using columns with Tri-N-Octylphosphine Oxide (TOPO) in silica gel and TRU column commercialized by Eichrom(R) composed of octyl-N,N,-isobutyl carbamoylphosphine oxide dissolved in tri-n-butyl phosphate (TBP) and with quantification by alpha spectrometry; batch separation with the same systems, but with quantification by ICP-MS; and flow injection with TRU cartridges and qualification by ICP-MS. The methodologies developed were verified using certified reference samples, IAEA-327 (soil) with 2% imprecision rate for the method using flow injection with TRU cartridges and qualification by ICP-MS. The speleothem samples analyzed were collected at Carste de Lagoa Santa, in the state of Minas Gerais, by Prof. Luis Piló`s (USP) group, and their ages varied from 15,2±2,2 kyears to >350 kyears. These numbers are within the range found in the literature for speleothem samples collected by the same group in the same region. The coral sample, collected at Bacia de Campos (RJ), and from the Lophelia pertusa species, was subdivided according to its ramifications (primary, secondary and tertiary), and the main ramification dated 9,4 ± 0,3 kyears.

Understanding U-series isotopic disequilibria of partially melted crust is integral for determining the effect that crustal assimilation has on the U-series signature of magmas. In this work, U, Th and Ba (as a proxy for Ra) elemental abundances were gathered on the quenched glass in partially melted crustal xenoliths of granitic composition using microbeam techniques. The crustal xenoliths, which are found in basaltic lava, from Mirador Volcano in Chile, are old, and can be assumed to be at secular equilibrium, whereas melting occurred during eruption of Mirador in 1979. A comparison of the ratios Ba/Th and U/Th in the partial melts with those of the whole rock reveal how much fractionation has occurred during partial melting. Different ratios of U, Th and Ba compared to the whole rock substantiate fractionation via partial melting. Thus, assimilation of partial melts of crust can play a role in U-series isotopic disequilibria.

Variability in the tropical ocean-atmospheric system causes global scale climate anomalies, most evident in the El Niño-Southern Oscillation’s coupled climate feedbacks. Despite being an area of high interest, many questions still remain regarding the west Pacific warm pool’s response to external forcing, particularly its response to increases in anthropogenic greenhouse gases. Paleoclimate reconstructions coupled with model simulations provide insight into the tropical Pacific’s role in past climate variability necessary to the development of robust climate projections. Most paleoclimate records, however, still lack the resolution, length, and chronological control to resolve rapid variability against a background of orbital-scale variations. Here we present stalagmite oxygen isotope (δ18O) reconstructions from Gunung Mulu National Park (4oN, 115oE ), in northern Borneo, that provide reproducible centennial-scale records of western Pacific hydrologic variability that are precisely U/Th-dated and continuous throughout most of the late Pleistocene (0-160 thousand years ago, kybp). The record comprises an entire glacial-interglacial cycle, which allows us to investigate orbital-scale climate forcings and compare two well-dated glacial terminations in the western tropical Pacific. The ice- volume-corrected δ18O records suggest that glacial boundary condtions, which include significantly lower atmospheric carbon dioxide levels, did not drive significant changes in Mulu rainfall δ18O. Similarly, Borneo stalagmite δ18O is poorly correlated to either global sea level shifts or Sunda Shelf areal exposure is not evident. The Borneo record does vary in phase with local mid-fall equatorial insolation, suggesting that precessional forcing may impart a strong influence on hydroclimate variability in the warm pool. This is best illustrated across Glacial Termination II, when the oscillation of equatorial fall insolation is large and out of phase with ice sheet decay. We also use a subset of well-dated, high-resolution stalagmite δ18O records from Mulu to investigate millennial-scale climate variability during Marine Isotope Stages 3-5 (30-100kybp). We find that regional convection likely decreased during the six massive iceberg discharges defined in the North Atlantic sediment records (“Heinrich events”). The inferred drying (increased stalagmite δ18O) during Heinrich events is consistent with a southward shift of the Intertropical Convergence Zone – the dominant paradigm to explain global climate anomalies originating in the north Atlantic (ref). However, any hydrologic variability related to Dansgaad-Oeschgar (D/O) events, millennial-scale sawtooth temperature anomalies of the last glacial period first evident in the Greenland ice records, is notably absent in the stalagmite records. . The Mulu stalagmite record’s absence of D/O signal, however, is in marked contrast to the regional west Pacific marine records and suggests D/O events and Heinrich events may be characterized by fundamentally different climate mechanisms and feedbacks.

Cette thèse se propose de définir les temps caractéristiques des processus d’altération à l'échelle de profil d'altération (bassin versant de Rio Icacos à Porto Rico) et de transfert sédimentaire subséquent dans les plaines alluviales (bassins du Gange et du Brahmapoutre). Cette problématique est abordée par l’analyse des nucléides des séries de désintégration radioactives de l’uranium-238. L’étude du système d’altération sphéroïdale développée sur le site de Porto Rico a permis de développer une méthode numérique de type quasi-Newton afin de résoudre le système d'équations qui rend compte du comportement des nucléides et du taux de formation de la saprolite et des couronnes d’altération.Pour les bassins du Gange et du Brahmapoutre, les sédiments ont été collectés sur différents affluents et à son exutoire. La variation des déséquilibres 238U-234U-230Th dans les sédiments, différente pour les sédiments en suspension et les sédiments à grains grossiers d’amont en aval, semble être une caractéristique générale des rivières himalayennes coulant dans la plaine Indo-gangétique. Par ce travail de thèse, nous avons montré l’utilité d’inclure l’analyse du 226Ra à l’analyse des autres nucléides de la chaîne de décroissance radioactive de l’238U (l’238U -234U-230Th 230Th) pour déterminer un taux d’altération dans un système bien défini. Néanmoins, l’étude du système du Brahmapoutre montre que la méthode proposée demande à être affinée car le transfert sédimentaire est un processus plus complexe dû à la variabilité minéralogique, granulométrique et à la dynamique sédimentaire (locale) des échantillons récoltés. / This thesis proposes to define the time-scales of weathering processes on a spheroidal weathering profile, (Rio Icacos watershed, Puerto Rico ) and the transfer time of sediments within alluvial plains (Ganges and Brahmaputra basins). This issue is addressed through the analysis of the nuclides of the 238U decay chain. The study of a spheroidal weathering system developed on the site of Puerto Rico permit to develop a limited memory quasi-Newton algorithm in order to solve the system of equations that reflects the behavior of nuclides and the rate of formation of the saprolite and the rindlet zone. For the Ganges and Brahmaputra basins, sediments were collected from different tributaries and their outlets. The variation of 238U - 234U - 230Th disequilibria in the sediments, with different variation trends for suspended and coarse-grained sediments, is probably a general feature of all Himalayan rivers flowing across the Indo-Gangetic plain. For this work, we have shown the usefulness of including the analysis of 226Ra analysis of other nuclides in the chain of radioactive decay of 238U (238U - 234U - 230Th) to determine the weathering rate in a well-defined system. Nevertheless, the study of the Brahmaputra system shows that proposed method needs to be refined as the sediment transfer is a complex process due to the mineralogical variability, size and (local) dynamics of sediments samples collected.

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