Four sediment cores and ten sediment trap samples were collected from Yax Chen cave, part of the Ox Bel Ha cave system on the Yucatan Peninsula in Mexico, for microfossil and micro-XRF geochemical analysis. In cores, Cl ̄trends (measuring salinity changes in meteoric water mass) show a direct relationship with microfossil (specifically the foraminifer Elphidium spp.) and weathering proxies (K, Ti, and Fe) used for measuring wet/dry conditions. Micro-XRF measurements of Cl ̄are derived from pore water in the cores, allowing for a more accurate observation of change in the cave system. Conversely, weathering proxies are measured from transported sediments. All four cores show a general decrease in groundwater salinity up core. Two of the cores, which were collected above the present halocline (~11m), show salinity changes in the meteoric water mass. One core was collected at the halocline and measures its movement in response to precipitation and Holocene sea-level rise. These findings are in agreement with other studies from the Yucatan coast (e.g. cenotes Aktun Ha and Sac Actum) as well as other paleoclimate records in the region. Additionally, these findings correlate with archaeological evidence, revealing several droughts that occurred throughout late Pre-Classic to late Classical Maya. High resolution analysis of Cl ̄offers new insights on how coastal aquifers respond to climate change and may have implications for regional archaeology. The use of micro-XRF geochemical analysis is an effective tool which can be used to understand the spatial and temporal complexities of Yucatan coastal aquifers. / Thesis / Master of Science (MSc)

Thesis (Ph. D.)--University of Wisconsin--Madison, 1975. / Vita. Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

The highly porous karst limestone of the Yucatan Peninsula promotes infiltration of rainwater into the subsurface, where it becomes part of the Yucatan Aquifer. The combination of high subsurface porosity, high evapotranspiration, and seasonal droughts results in relative scarcity of lakes or drinkable water at the surface. The majority of past and present people living on the Yucatan Peninsula have depended on groundwater resources for domestic purposes. Whereas coastal karst aquifers such as the one in the Yucatan Peninsula are important water resources, they are highly vulnerable to climate-related changes such as sea level (SL) rise, increased hurricane intensity and extended droughts. With ongoing development along the eastern coast of the Yucatan Peninsula (Quintana Roo), predicted increases in storm intensity, and rising population and potential pollutant output, it has become imperative to study the seasonal and long-term effects of climate and human activity on the Yucatan Aquifer. Like many coastal karst aquifers, the Yucatan Aquifer is stratified according to density, with the Meteoric Water Mass (MeWM) flowing towards the coast on top, and the Marine Water Mass (MaWM) flowing inland on the bottom. The current basis of our knowledge about how the two water masses interact has been from short-term instrumental monitoring and numerical modelling, which is useful for understanding straightforward relationships between salinity, precipitation and temperature across the two water masses and have paved the way for more complex analyses to be completed using the simple principles to guide geochemical studies of sediment within the systems. Generally, sediment cores have been analysed discretely using various methods including grain size analysis, micropaleontology, WD-XRF, and others, however the recent rise of µXRF Core Scanning provides a quicker, more cost effective and higher-resolution method for studying climate-related patterns in sediment cores. This thesis outlines and provides robust evidence for three new methods of using µXRF-CS to determine past and present climatological changes and their relationship to sediment elemental counts. We provide the first outline of the seasonal and spatial controls of geochemical changes in sedimentation in a coastal cave system (Yax Chen), using four years of in situ sediment collection. We then provide the first calibrated record of past salinity based on Cl counts within sediment cores taken from shallow lakes. Although instrumental monitoring has provided evidence that the aquifer is impacted by modern wet and dry periods, the effect of past climate on the aquifer has not been investigated. We provide the first record of water-column mixing at three locations within coastal Quintana Roo. This demonstrates that there has been a long-term climate impacts to coastal Yucatan Peninsula groundwater, which may be scaled to other karst islands and provides evidence that increased hurricane frequency and/or magnitude could change the baseline salinity of the fresh MeWM. / Dissertation / Doctor of Philosophy (PhD) / The porous limestone (karst) geology of the Yucatan Peninsula results in percolation of rainwater down into the subsurface where it is held in the vast Yucatan Aquifer that past and present people have depended on for water resources. Such aquifers are highly vulnerable to human activity and also climate change via increased sea level rise, hurricane intensity and droughts. The Yucatan Aquifer is made up of two separate water masses that have different salinities, and can interact physically and chemically due to changes in climate and weather. Sediment cores can be used to look at subannual geochemical changes which reflect long term behaviour of the aquifer, but first the relationship between sediment change and climate must be established. This thesis uses an iTRAX X-Ray Fluorescence Core Scanner (XRF-CS) to investigate changes in surface water and groundwater over the past 6000 years, and provides the first record of rainfall-induced mixing in a coastal karst aquifer. We also provide evidence of rapid salinity change in shallow lakes associated with intense dry periods, and reveal relationships between cave sedimentation and surface vegetation coverage.

Geological and geophysical investigations of the Chicxulub meteorite impact structure have been ongoing since its scientific recognition in 1991 Hildebrand et al. 1991). The structure is of important significance because it is currently the only known impact crater that is linked to a global catastrophe, the Cretaceous/Tertiary boundary that occurred 65 Ma years ago. Major climatic and biological changes occurred at this interval that include the disappearance of 70% of all living species, in particular the dinosaurs. A global iridium anomaly along with the occurrence of shocked quartz grains characterize a thin clay layer this interval that led to the search for a large meteorite impact crater on continental crust. A large “volcanic” igneous province identified by oil exploration boreholes on the NW region of the Yucatán Peninsula was eventually recognized as a vast impactite deposit associated with a 180 km wide crater. Until 2002, only small grab and chip samples had been described from Chicxulub. This lack of sampling and, thus, poor understanding of the cratering conditions at Chicxulub led the International Continental Drilling Program (ICDP) to fund and drill the Yaxcopoil-1 borehole. The Yaxcopoil-1 (Yax-1) borehole was drilled 60 km south-southwest from the center of the Chicxulub meteoritic impact. It intersected 794.63 m of post-impact cover rocks, 100.31 m of impactites, and 616.03 m of Cretaceous target rocks, terminating at a final depth of 1510.97 m. The impactite interval, as well as several selected samples from the Cretaceous target rocks, is the focus of this scientific investigation. In conjunction with this work, the Yax-1 core was studied by numerous international research groups and is, thus, currently one of the best studied continuous diamond drill core from an impact crater. This petrographic and geochemical investigation provides further understanding on the primary and secondary conditions that influenced the formation of the Yax-1 impactites and selected target rocks. Five units have been recognized in the impactite interval. These subdivisions are based on macro- and microscopic observations and are complemented by geochemical characteristics. Unit 1 (795-822 m) comprises subrounded melt rock particles that are poorly sorted, yet show a progressive gradation with height, are self supported, show perlitic devitrification texture, and are generally fine-grained. Unit 2 (823-846 m) and Unit 3 (846-861 m) are relatively similar, as they both consist of a groundmass-supported breccia with melt rock particles that are angular, fluidal, and vesiculated in texture. The groundmass in both units is pervaded by numerous carbonate-veinlets and decreases in volume towards Unit 3 because of compaction. Unit 2 and Unit 1 are both altered to a predominantly green colour by the pervasive conversion of silicate phases from clay minerals. Unit 3 is of a variegated character and is suggested to be the less altered unit bove Unit 4. Unit 4 (861-885 m) comprises a massive yet brecciated microcrystalline impact melt rock. It is primarily of a silicate composition and contains only minor secondary carbonate crystals. All lithic fragments are of silicate compositions. Unit 5 (885-895 m) shows the greatest variation in the proportion of melt rock particles and lithic fragments. The melt rock particles contain numerous microlites that crystallized below the glass-transition temperature. These are suspended in a carbonate groundmass that is either of a primary impact melt origin or of a secondary nature. Units 1 and 5 both contain foraminifera fossils and greater proportions of carbonate clasts than any other units. All unit show shock metamorphic characteristics, i.e., planar deformation features, ballenquartz, and checkerboard feldspar. Geochemical results have been obtained by various analytical techniques in order to constrain cratering and alteration processes at various sampling scales. Main results reveal that samples from units 1 and 2 have been leached of their alkali elements, show negative Ce anomalies on a microscopic scale, and show less major element variation on a bulk sample scale than lower units. The groundmass in units 1 to 3 comprises a microcrystalline calcite and altered alkali element-, Ca- and Si- rich cement. In units 2, 3, and 5 melt rock particles are of a heterogeneous composition. In Unit 1, melt rock particles are highly altered, therefore volatile rich, and are of a more homogeneous composition than those of other units. On a bulk sampling scale, the silicate component for the whole impactite sequence shows remarkable homogeneity. Major and trace element compositions show that this component and Unit 4 are typical of the upper continental crust. The carbonate component is more calcite rich than dolomitic and most likely represents strong secondary alteration. No significant sulfur content was measured compared to published known target rock values. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax-1 impactites. Cretaceous rocks were also sampled in order to provide compositional constraints with the impactites and observe any shock related metamorphic features. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from pre- to post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, no evidence was found from samples located between 1347–1348 m depth for the presence of melt breccia. An emplacement mechanism for the impactite sequence is proposed with regards to cratering. Unit 5 is interpreted as an early ejecta deposit that was emplaced following the passage of the initial ejecta curtain during the excavation stage of cratering. Unit 4 is an allogenic siliceous melt rock body that originated primarily from the fusion of the silicate crystalline basement. The origin of Unit 4 is based on geochemical and petrographic arguments, i.e., no carbonate component to the melt could be detected and only igneous/metamorphic mineral/rock fragments were observed in it. It is suggested Unit 4 was emplaced as an outward flow of fused crystalline basement rocks from the collapsing central uplift or it may have also been deposited from the fallback of a large melt bomb. Brecciation occurred post-deposition as fragments fit together like pieces of a jigsaw puzzle. Units 2 and 3 represent unreworked fallback suevite deposits. Vesiculated melt rock particles are a testimony of the volatile rich nature of the collapsing impact plume. Volatiles are believed to have helped disperse the suevite and inhibited the melt rock particles from undergoing compositional homogenization. Unit 1 represents a reworked fallback deposit that formed from the resurge of seawater into the impact basin. Unit 2 is the altered equivalent of Unit 3 and along with Unit 1 underwent significant post-depositional phyllosilicate alteration from circulating fluids at the top of the suevite pile.

Background: Coastal ecosystems in Mexico remain understudied in spite of their ecological, economic and conservation value and are being impacted by human activities along the coast. Knowledge on spatial patterns of plant species distribution that helps preserve these fragile ecosystems is crucial. Aims: We evaluated differences in species richness, species diversity and species dominance patterns in 16 plant communities as well as the degree to which differences were driven by climatic conditions in sandy dunes in Yucatán. We evaluated the importance of invasive species in mediating patterns of species diversity and species dominance patterns. Results: We found wide variation in plant species richness, species diversity and species dominance patterns among communities that stems from broad climatic differences along dune systems. Invasive plants represent almost one-third of total species richness and seem to be drastically changing the species dominance patterns in these communities. Conclusions: Regional climatic differences along the Yucatán north coast seems to be a major driver of plant diversity and species composition. Our findings suggest that invasive plants have successfully colonised and spread along the coast over the past 30 years. Even though invasive species do not alter spatial patterns of species diversity, they are becoming more dominant with potential detrimental consequences for native plants.

Harmful Algal Blooms (HABs) in the Gulf of Mexico (GOM) are natural phenomena that can have negative impacts on marine ecosystems on which human health and the economy of some Gulf States depends. Many of the HABs in the GOM are dominated by the toxic dinoflagellate Karenia brevis. Non-toxic phytoplankton taxa such as Scrippsiella sp. also form intense blooms off the Mexican coast that result in massive fish mortality and economic losses, particularly as they may lead to anoxia. The main objectives of this dissertation were to (1) evaluate and improve the techniques developed for detection of Karenia spp. blooms on the West Florida Shelf (WFS) using satellite remote sensing methods, (2) test the use of these methods for waters in the GOM, and (3) use the output of these techniques to better understand the dynamics and evolution of Karenia spp. blooms in the WFS and off Mexico. The first chapter of this dissertation examines the performance of several Karenia HABs detection techniques using Moderate Resolution Imaging Spectroradiometer (MODIS) satellite images and historical ground truth observations collected on the WFS from August 2002 to December 2011. A total of 2323 in situ samples collected by the Florida Fish and Wildlife Research Institute to test for Karenia spp. matched pixels with valid ocean color satellite observations over this period. This dataset was used to systematically optimize variables and coefficients used in five published HAB detection methods. Each technique was tested using a set of metrics that included the F-Measure (FM). Before optimization, the average FM for all techniques was 0.47. After optimization, the average FM increased to 0.59, and false positives decreased ~50%. The addition of a Fluorescence Line Height (FLH) criterion improved the performance of every method. A new practical method was developed using a combination of FLH and Remote Sensing Reflectance at 555 nm (Rrs555-FLH). The new method resulted in an FM of 0.62 and 3% false negatives, similar to those from more complex techniques. The first chapter concludes with a series of recommendations on how to improve the detection techniques and how to take these results a step further into a Gulf wide observing systems for HABs. In chapter two, ocean color techniques were used to examine the extension, evolution and displacement of four Karenia spp. events that occurred in the WFS between 2004 and 2011. Blooms were identified in the imagery using the new Rrs-FLH method and validated using in situ phytoplankton cell counts. The spatial extension of each event was followed in time by delineating the blooms. In 2004 and 2005, the WFS was affected by a series of hurricanes that led to high river discharge and intense sediment resuspension events. Both processes had an impact on HAB occurrence. For example, I tracked a Karenia spp. bloom found in late December 2004 approximately 40-80 km offshore Saint Petersburg, which then expanded reaching an extension of >8000 km2 in February 2005. The bloom weakened in spring 2005 and intensified again in summer reaching >42,000 km2 after the passage of hurricane Katrina in August 2005. This bloom covered the WFS from Charlotte Harbor to the Florida Panhandle. Two other cases were studied in the WFS. The results of the Hybrid Coordinate Ocean Model from the U.S. Navy aid understanding the dispersal of the blooms. During fall 2011, three field campaigns to study HABs in Mexico were conducted to do an analysis of optical properties and explore the possibility of using ocean color techniques to distinguish between the main phytoplankton blooms in that region. Three main bloom scenarios were observed in the Campeche Bank region: massive diatom blooms, blooms dominated by Scrippsiella spp., and Karenia spp. blooms. The normalized specific phytoplankton absorption spectra were found to be different for Karenia spp. and Scrippsiella sp. blooms. A new technique that combines phytoplankton absorption derived from MODIS data and the new technique developed in Chapter One showed potential for a detection technique that can distinguish between Karenia and Scrippsiella blooms. Additional work is needed to improve the new technique developed for Mexican waters, but results show potential for detection techniques that can be used Gulf-wide. This will help better understand the dynamic and possible connectivity of phytoplankton blooms in the GOM.

Background: Tropical sand dunes are ideal systems for understanding drivers of community assembly as dunes are subject to both deterministic and stochastic processes. However, studies that evaluate the factors that mediate plant community assembly in these ecosystems are few. Aims: We evaluated phylogenetic community structure to elucidate the role of deterministic and stochastic processes in mediating the assembly of plant communities along the north of the Yucatan Peninsula, Mexico. Methods: We used plastid genetic markers to evaluate phylogenetic relationships in 16 sand-dune communities. To evaluate the role of climate in shaping plant community structure we carried out linear regressions between climatic variables and mean phylogenetic distance. We estimated the Net Relatedness Index and Nearest Taxon Index to identify ecological processes mediating community assembly. Results: Observed phylogenetic structure was not different from random, suggesting that stochastic processes are the major determinants of community assembly. Climate was slightly correlated with phylogenetic diversity suggesting that abiotic environment plays a minimal role in community assembly. Conclusions: Random assembly appears to be the primary factor structuring the studied sand dune plant communities. Environmental filters may represent a secondary factor contributing to the observed phylogenetic structure. Thus, both processes may act simultaneously to mediate the assembly of sand-dune plant communities.

No description available.

Bahia de la Ascension (BA) is a pristine, shallow, karst bay located in the Mexican Caribbean, a region experiencing rapid population growth stimulated by intense tourism development. The overall objective of this study was to address the natural hydrographic variability of this inherently vulnerable ecosystem and assess its influence on a key habitat, the seagrass. The chapters follow the three-branched nature of the study which tackled the connected ecosystem issues of coastal hydrology, physical dynamics of flow and circulation, and the ecological dynamics of the seagrass species Thalassia testudinum in BA. Freshwater input to BA is primarily by submerged groundwater discharges and surface runoff; both sources are derived from fissures in the aquifer but feature distinct water quality due to the interaction with adjacent wetlands. Hurricanes explain 36 percent of the interannual precipitation variability in the region. The water balance indicates a persistent net outflow from BA to the adjacent shelf, suggesting an intense exchange across inlets. Both diurnal and semidiurnal tidal frequencies are attenuated in the inner bay, where a meteorologically-induced subtidal water level increase may occur during four-day southeasterly winds. A clear SW-NE salinity gradient was established during dry and rainy seasons, with a strong tidally-driven marine influence throughout the central basin, and a perennial mesohaline ambient in the southwestern-most bay, where hydrodynamics are primarily controlled by wind stress. Thalassia testudinum is the dominant seagrass species in BA, occupying ~90 percent of the substrate, including the freshwater-influenced inner bay. High nutrient inputs, including phosphorus which might have limiting effects in karst environments, along with the wind-driven circulation controlling water residence times are associated with the successful development of T. testudinum (up to 1,461.23 g DW m-2) within the SW bay. Farthest into the central basin, Thalassia consistently exhibited an inverse correlation between abundance and density of shoots. This pattern was enhanced under exceptional precipitation and inputs of denuded organic matter resulting from hurricanes making landfall on this region. The relationship between nutrient distribution and the above/belowground ratio suggested that Thalassia growing in BA favors the development of the aerial component as nutrients availability increases. This study provides a basic understanding of the most important processes molding the patterns of variability exhibited by T. testudinum in Bahia de la Ascension. The salinity gradient and external nutrient supply, along with the hydrodynamic component, define the spatial scale at which the connectivity between the adjacent wetland, the bay, and the shelf may occur.