This project expands on the 4200-year history of Laguna Los Mangos by adding diatom analysis to existing records of pollen, charcoal, elemental composition, and stable light isotope analysis. Diatoms were counted in peroxide-treated samples, and data were analyzed using cluster analysis, PCA, CCA, and Pearson correlation. Diatom variability was correlated with nitrogen and organic material. Before 3430 cal yr BP, diatoms reflect landscape instability with higher lake levels and macrophyte cover. This period was followed by a period of agriculture-induced nutrient pollution, reduced pH, and increased precipitation until about 2450 cal yr BP. Peaks in A. granulata may indicate periods of increased mixing driven by precipitation. Diatom composition reflects reduced agricultural activity and lake desiccation during the TCD and increased precipitation and agricultural collapse during the LIA. Overall, this record reflects a history of slightly alkaline, eutrophic conditions, increasing salinity, and human disturbance from maize agriculture and deforestation. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2021. / FAU Electronic Theses and Dissertations Collection

Results from the palaeoenvironmental investigations into the Holocene vegetation history of three sites in eastern lowland Scotland are presented. Two of the sites, Cruvie and Pitbladdo, are located in north Fife; the third site, Methvern is situated in south Perthshire. Cruvie is located in a kettle-hole and provides data extending from the Late-glacial to ea. 3900 BP. Pitbladdo is a former bog and cores from this site provide data on the period from ca. 8000 to 3900 BP. Methvern is a well-maintained raised bog and provides data that spans the entire Holocene. Relative, concentration and pollen preservation data are supplemented by loss-on-ignition, pH and magnetic susceptibility analyses. Microscopic charcoal data are also recorded. Radiocarbon dates allow comparisons to be made between similar events at different sites, resulting in a detailed picture of temporal and spatial patterns of palaeoecological change within a small geographical area. Attention is focused upon the identification of human impact on the environment during the early to mid Holocene. The influences of succession and climate change in determining patterns of vegetation change are also considered. The data obtained indicate that human activity may have had a limited impact on the environment in this area during the Mesolithic, but no unequivocal evidence is recorded. Anthropogenic impacts are more clearly identified during the Neolithic period and from the late Neolithic/early Bronze Age, human activity is considerable and includes pastoral and mixed farming. The value of tephra as a dating tool in this area of eastern Scotland is considered. The absence of tephra at the three sites investigated has led to the formulation of a hypothesis linking patterns of orographic rainfall and tephra deposition within Scotland. The study highlights the difficulties of determining the causal factors of vegetation change and the limitations of palaeoecological data in the identification of anthropogenic activity during the early Holocene. The recognition of climate signals is discussed and the routine counting of microscopic charcoal at all sites is proposed. It is suggested that further research is required to clarify the boundaries of tephra deposition in Britain. Finally the diverse patterns of change recorded within the study area emphasise the need for a network of closely spaced and well dated palaeoenvironmental sites covering the regions of Scotland, leading to the recognition of local patterns of environmental change.

In the semi-arid southwestern US, the lack of continuous records of climate over the last glacial cycle has precluded a complete understanding of the rates and timing of past regional changes in climate. Speleothems can provide high-resolution, continuous record of moisture, temperature, and, potentially, vegetation variations and can be precisely dated by uranium-series disequilibrium. We have produced two U-series dated speleothem δ¹⁸O records from Cave of the Bells (COB). COB is located in Santa Cruz County, Arizona on the east side of the Santa Rita Mountains (31°45'N, 110°45'W; 1700 m). The glacial speleothem δ¹⁸O record (53 to 8.5 ka) confirms that deglaciation in the Southwest proceeded via a stepwise shift, mirroring the Bølling-Allerød warming and Younger Dryas cooling, beginning around 15 ka. There is no evidence of early warming before the decline of the large ice sheets. In Marine Isotope Stage 3 (MIS3; 53 to 30 ka), we observe millennial variations similar to Dansgaard-Oeschger (DO) events first seen in Greenland ice core δ¹⁸O records with wet/cold conditions indicated by our cave record during glacial stadials (cold periods) and dry/warm during glacial interstadials (warmer periods). High-resolution U-series dating allows for refinement of the timing of DO events in MIS3, and spectral analysis confirms the presence of a 1515-year climate cycle during this time. The δ¹⁸O data from a Holocene stalagmite (~6.9 to 3.5 ka) average ~3‰ higher than modern and exhibit substantial multidecadal to multicentury variation. We propose that in addition to drier/warmer conditions in the winter, a stronger summer monsoon and perhaps warmer summer temperatures supplied waters with higher δ¹⁸O values to the cave during the mid-Holocene. Spectral analysis of early part of the δ¹⁸O record reveals variability at periods of 233 years and at 142 and 52. After ~4.9 ka a prominent shift from centennial to multidecadal periods of variability (a 70 to 50-year cycle) is observed and there is a slight decrease in average δ¹⁸O values. This shift is coincident with a hypothesized increase in El Niño activity, which is correlated to wet winters in the modern southwest, in the tropical Pacific at ~5 ka.

The Antarctic Peninsula is one of the fastest warming regions on the planet. In the past 50 years, the temperature has increased by more than 2⁰C, leading to the retreat of large areas of the ice shelves fringing the Antarctic Peninsula. Recent environmental changes in the Antarctic Peninsula are well documented by meteorological and remote sensing data, but the behavior of the Holocene atmosphere-ocean-cryosphere system is not well understood. In this study foraminifera are used as a proxy for Holocene oceanographic conditions in the Gerlache Strait, western Antarctic Peninsula. The most abundant foraminifera identified in this study include the agglutinated taxa Miliammina arenacea and Paratrochammina lepida, which are associated with cold, saline water masses and periods of high sea-ice production. The most abundant calcareous species identified is the opportunistic Fursenkoina spp., which is associated with ice-proximal conditions and fresh water input due to glacial melting. Deglaciation of the Gerlache following the Last Glacial Maximum is indicated by the appearance of foraminifera and diatoms at ~7700 years BP. The Post-Deglaciation period is characterized by high frequency variation in foraminiferal assemblages between abundant agglutinated and calcareous taxa, indicating unstable glacial conditions. The beginning of the Mid-Holocene Climactic Optimum (MHCO) is indicated by a substantial decrease in sedimentation rates and a shift to more stable foraminiferal assemblages. A decline in diatom abundance and the absence of calcareous foraminifera indicates a glacial readvance at 6030 years BP. At 4470 years BP the calcareous taxa including Fursenkoina spp. become dominant, indicating glacial retreat and input of fresh water into the water column. After 3240 years BP agglutinated taxa are once again dominant and calcareous taxa absent. This marks the beginning of the Neoglacial period and the presence of colder, more saline shelf waters in the Gerlache Strait. Stratification of the water column is apparent during the Post-Deglaciation period and the latter part of the MCHO. A difference in δ18O values of >0.5 per mille between benthic and planktonic foraminifera indicates the presence of a less saline surface water layer which may be the result of freshwater input due to glacial melting and an estuarine circulation regime.

A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The fine-grained flood deposits used to reconstruct these flood histories selectively preserve evidence of only the largest events. Large floods were frequent on rivers throughout the region from 4.8-3.6 ka (¹⁴C yrs). In contrast, the period from 3.4-2.2 ka is marked by a significant decrease in the number of large floods on virtually all of the rivers. The frequency of large floods increased after about 2.2 ka, with particularly prominent peaks around 1 ka and after 500 yrs BP, separated by a sharp decrease between 600 and 800 yrs BP. The storms that generate large floods (≥ 10-year) in this region fall into three general categories: 1) winter North Pacific extratropical storms, 2) late-summer and fall storms that draw on moisture from recurved Pacific tropical cyclones, and 3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the very largest floods on the rivers with paleoflood records, and are the most probable causes of the late-Holocene paleofloods. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mb heights and monthly sea-level pressure indicate that the winter floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. Shifts in the locations of the low- and high-pressure anomalies over the North Pacific appear to control which subregions of the southwestern U.S. experience floods. Composite 700-mb anomalies during tropical cyclone floods show a similar pattern, with a variation to a blocking high-pressure anomaly in the central North Pacific and a low-pressure anomaly over the western U.S. There is a strong connection between the negative phase of the Southern Oscillation Index and the large floods associated with winter storms and tropical cyclones. Over the last 5000 years, the episodes of more frequent large floods coincide with cool, wet, neoglacial periods. Warm periods are times of dramatic decreases in the number of paleofloods. Although the floods record individual extreme storms, they cluster in times of generally moister conditions in the region. A clear positive relationship exists between floods and low-frequency variations in El Niño over the last 1000 years. Warm coastal sea-surface temperatures indicative of El Niño-like conditions are associated with more frequent large floods over at least the last 2000 years. The paleoflood records demonstrate centennial-scale variations in the conditions conducive to the occurrence of large floods in this region.

An archival-stratigraphic investigation of the middle Gila River provides insights into late-Holocene channel behavior and flood-plain formation. Historical records detail changes in channel patterns that correlate with changing frequency of large floods, but channel sensitivity is also affected by factors such as flood seasonality, changes in sediment load, human disturbances, and internal thresholds. Because the frequency of large floods is the dominant factor in channel changes, radiocarbon-dated flood deposits in late-Holocene alluvial terraces allow for a reconstruction of prehistoric channel behavior. A period of reduced large flood frequency and channel stability 4,000-1,000 years BP separates periods of increased large flood frequency and channel instability 5,000-4,000 and 1,000-0 years BP. Transformations between braided and single channel morphologies affect the conveyance of floods and change the spatial characteristics of flood hazards. These channel dynamics are also important in analyzing changes in Hohokam-Pima irrigation technology and settlement patterns.

Thesis submitted in fulfilment of the academic requirements for the degree of Master of Science. Department of Chemistry University of Witwatersrand, Johannesburg April 2017. / The Mkhuze River discharges into the most northern part of Lake St Lucia, via a contemporary bayhead delta. The delta formed in response to sea level rise during the last deglaciation and today exerts great influence on the functioning of Lake St Lucia, one of the largest estuarine systems in Africa and a globally important conservation area. A sediment core (11.5 m) was extracted from the distal end of the delta to examine the geomorphic evolution of the Mkhuze River Delta and links with variations in Holocene sea level and climate. Radiocarbon and optically-stimulated luminescence dating show that the core captured the entire Holocene infill and documents changes in sedimentation over the last ~13.8 kyr. Grain size and high resolution XRF analysis indicates that initiation of the modern delta occurred since ~7200 cal yr BP , when deglacial sea-level rise reached present-day level. Initial Holocene aged sediments are dominated by clay and silt material that was deposited when seawater intruded into Lake St Lucia via a palaeo-river connection to the ocean at Leven Point. The influx of silt and clay material was accompanied by the emergence of an onshore proto-barrier that created a sheltered lagoonal environment and promoted the accumulation of fine fluvial sediment. The presence of discrete, coarse-grained horizons enriched in zircon identifies a period of increased marine palaeostorm activity between 4700 and 2500 cal yr BP. This period is characterised by the presence of discrete shell fragment accumulations and is interpreted to reflect a strongly positive Indian Ocean dipole anomaly, which resulted in warmer sea surface temperatures and an increase in regional cyclone activity and frequency. The upper part of the core is characterized by generally fine silt and is marked by a decrease in sedimentation rate that corresponds to a phase of lateral delta progradation. The last ~1700 cal yr BP years of the record identify with subtle changes in grain size that can be attributed to a strengthening in El Niño–Southern Oscillation (ENSO) activity, which is known to be associated with prolonged drought and wind erosion in eastern South Africa. This study highlights the usefulness of coastal geochemical records in identifying environmental changes and related climate signals at a regional scale. / GR2018

Volcanic ash, or tephra, blankets the local and regional landscape following a volcanic eruption. If this ash layer is preserved and identified, it can act as a time synchronous marker bed (isochron) for correlation between marine, terrestrial, glacial and lacustrine deposits. This can be a powerful tool when attempting to determine the true age of a marine sample (e.g. clam or mollusk), affected by the marine reservoir effect (MRE). The MRE causes dated radiocarbon to appear the age that carbon was last in equilibrium with the atmosphere rather than the time that a dated material was deposited. The offset (in years) caused by the MRE is referred to as ΔR. Presented in this study is new data on the lowermost part of SWERUS-L2-2-PC1 (2PC), a marine sedimentary core retrieved from the Chukchi shelf north of Siberia. By using quantification of rhyolitic tephra to locate tephra-rich layers for further study, results show a thick layer that is interpreted to have originated from the caldera-forming eruption of Aniakchak (Aniakchak II). The geochemical identification of the tephra was done using electron probe micro-analysis. A grain size analysis was also conducted to learn more about the sedimentology of 2PC and the possible proxies that can be used when trying to determine where to place the isochron. The isochron was finally placed with the help of relevant literature and the results from this study. That position shifted the previous age model of 2PC at a position to yield a ΔR of 482 years for the Chukchi Sea during this time period.

Previous studies on reconstructing the paleoclimate on the Tibetan Plateauare commonly based on a single proxy, and have thus limited the usage of their results. Therefore this study was designed firstly to evaluate the applicability of organic carbon isotopes from modern plants in the Tibetan Plateau as a paleo-environmental proxy, and secondly to apply palynology, organic geochemistry and biomarkers to the reconstructions ofpaleo-environmental history of central Tibet. The new, comprehensive records reveal the history of regional precipitation, temperature and vegetation change in responding to Holocene climatic changesin the region. Organic carbon isotope (δ13C) and lipid n-alkanes were measured from plant samples collected from various altitudes and environments across the study area. The δ13C results indicate the importance of availability of water to C3/C4 herbs distribution in this alpine region. The molecular distributions of trees, shrubs, herbs and submergent macrophytes show distinctive features in a number of indices. These indices are considered to be useful for paleo-environmental reconstructions. A 5.8m-long peat core, which were dated back to 95,000 yr BP was obtained from a river valley at an altitude of 4300 m above sea level. The pollen data from the core reveals a sparse herbaceous vegetation in the catchment area during early Holocene, suggesting non-favourable conditions for vegetation growth. The environment became wetter during the early mid-Holocene around 8100-7000 cal. yr BP, and since then a sedge-dominated wetland habitat was developed. This vegetation changed little until about 1100 cal. yr BP; from which an increase of drought-tolerant herbs was indicated by the pollen data, implying a slightly drier condition of the latter part of late Holocene. The pollen record suggests that the optimum precipitation in central Tibet occurred later and lasted longer than that in the East Asian Monsoon region. Finally, bulk sample carbon geochemistry(TOC, δ13C) as well as the biomarkers analysis on n-alkanes and GDGTs as paleo-limnology and paleo-temperature indicators respectively were applied to the core. Before 1500 cal. yr BP, the wetland was dominated by both vascular herbs as suggested by the δ13Cdata and aquatic floating/submerged macrophytes as indicated by mid-chain n-alkanes. The relative proportion of the two sources of organic matter input varied slightly, and vascular herbs increased in two episodes(5400 -4700 and 2800 -2300 cal. yr BP) indicated by the increase of long-chain n-alkanes (>n-C25). Considering of the soil water content variation by the ratios of iGDGTs to bGDGTs, the change between the two plant communities could to be caused by different lake status. GDGTs paleo-temperature reconstruction indicates a warm period from 6700 to 6000 cal. yr BP, which is believed to be the warmest and highly humid in central Tibetan region. Then the temperature decreased towards the late Holocene, consistent with other paleo climate records across the Plateau region, i.e. largely controlled by change of solar insolation. Along the decreasing trend, a warm episode at about 1200 yr BP that inferred from MAAT was suggested corresponding with the North Hemisphere Medieval Warming. / published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

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