<p> Paleoclimate reconstructions are essential to contextualize recent warming that is affecting the Arctic region faster than anywhere else on Earth. To better understand this rapidly changing landscape, I used sedimentological evidence from Lake Peters and Lake Schrader in the northeastern Brooks Range to infer changing environmental conditions over the past 16,000 years (16 ka). Across five core sites, distinct changes in the visual stratigraphy and physical sediment properties including sediment bulk density, organic-matter content, and grain-size distributions record changing environmental conditions. The oldest sediments accumulated rapidly and contain little organic matter, interpreted to represent a landscape dominated by glacial and paraglacial processes associated with the rapid upvalley retreat of glaciers. No robust evidence was found for a climate fluctuation concurrent with the Younger Dryas. A peak in organic-matter abundance between 12&ndash;10 ka is attributed to a maximum in Northern Hemisphere summer insolation and accords with other regional paleoclimate reconstructions. Following this, conditions appear to have become drier as indicated by sediments with high density and low organic content until 5 ka. Alternatively, these sediments could represent a glacial advance, but this is unlikely as regional evidence for cooler conditions is absent. From 5&ndash;2 ka, organic matter consistently increases in several cores and is attributed to increased river discharge, which carried terrestrial organic matter into the lakes, or to increased summer temperatures, which led to higher productivity, or both. After 2 ka, sediments increase in density and decrease in organic content, which suggests the growth of glaciers within the catchment. Moraine mapping and lichenometry confirm previous studies and accord with changes in lake sediments. Rhizocarpon geographicum thallus diameters on boulders located on the outermost moraine crest suggest the maximum Holocene glacial extent is associated with increased moraine frequency from elsewhere in the Brooks Range dating to 2.5&ndash;1.9 ka. New measurements from a distinct moraine crest from an additional valley show consistency within the basin and are likely associated with a regional increase in moraine frequency from 1.1&ndash;1.0 ka. Increased sedimentation rates and organic productivity in both lakes may record glacier retreat during the last century. This study provides a framework for future research and fills an important gap in paleoenvironmental records for the northeastern Brooks Range.</p><p>

<p> Recent research has proposed that an increase in sea surface temperatures (SSTs) interpreted to be caused by anthropogenic climate change has lead to an increase in the frequency of intense hurricanes. However, this theory has been challenged on the basis that the instrumental record is too short (approximately 160 years) and unreliable to reveal any long-term trends in intense hurricane activity. This limitation can be addressed by the means of paleotempestology, a field that studies past hurricane activity by means of geological and biological proxy techniques. Hurricane-induced overwash deposits that become preserved in the sediments of back-barrier lagoons, lakes and marshes can provide scientists a unique opportunity to study past hurricane landfalls. It also provides an opportunity to study their associated climate drivers over much longer time-scales (centuries to millennia). </p><p> This study investigates overwash deposits (paleo-tempestites) at 10 sites along the Southwest Florida coastline, focusing on two. The Sanibel Island marsh and Keewaydin Island lagoon have a high potential for recording hurricane-induced paleo-tempestites. The Sanibel Island marsh record was constructed using loss-on-ignition, grain size analysis, percent calcium carbonate, and chronologically dated using 210Pb analysis. Proxy and dating results of three sediment cores revealed two prominent paleo-tempestites&mdash;likely representing Hurricane Donna (1960) and the Great Miami Hurricane of 1926. These layers were deposited as both fine-grained sand and shell hash, and contribute to our understanding of storm overwash in the modern record. Three sediment cores were also extracted from a back-barrier lagoon (Island Bay), behind Keewaydin Island in Collier County, Florida. Core samples were analyzed for grain size, percent calcium carbonate, fossil shells species and dated using the ²¹⁰Pb and ¹⁴C dating methods. These methods revealed a 1 thousand year old record of hurricane overwash. Two prominent paleo-tempestites, deposited as both fine-grained sand and shell hash, were also observed at this site and may possibly replicate the most recent storm events documented on Sanibel Island. This suggests that only the most intense hurricanes are being recorded in the geologic record. </p><p> "Active" (1000-500 yrs. BP) and "inactive" (500-0 yrs. BP) periods of hurricane overwash were identified in the Island Bay record. These correlate well with the reconstructed SSTs from the Main Development Region in the North Atlantic Ocean during the Medieval Warm Period and Little Ice Age. Results from this initial Southwest Florida study point to SSTs of the North Atlantic Main Development Region as a potential climatic driver of hurricane landfalls in Southwest Florida over the past 1 thousand years. This is in opposition to SSTs in the Gulf of Mexico and El Ni&ntilde;o Southern Oscillation as hypothesized by other studies in the Gulf of Mexico and Caribbean regions. These results are the first proxy records of past hurricane strikes in Southwest Florida. </p>

<p> Periodic temperature measurements in the DOI/GTN-P Deep Borehole Array on the western Arctic Slope of Alaska have shown a strong near-surface permafrost warming over the last 40 years, particularly since &sim; 1990. Due to the manner in which these deep wells were drilled, the portion of the observed permafrost warming caused by climate change has remained unclear. Other factors that have strongly influenced temperatures near the wellbores include the heat deposited into permafrost during drilling and local-landscape changes associated with drilling operations (creation of reserve pits and drill pads). Multidimensional heat-transfer models capable of assessing the magnitude of the drilling and local-landscape disturbances near the wellbores have not been available. For the western Arctic Slope, such models must be capable of simulating heat-transfer processes in layered fine-grained mudrocks whose thermal properties are highly nonlinear due to the occurrence of unfrozen water at temperatures well below 0&deg;C. An assessment of the drilling and landscape-change effects also requires knowledge of the specific thermophysical properties occurring at the well sites. Little information has been available about these properties on the western Arctic Slope.</p><p> To establish the portion of the observed permafrost warming related to drilling and landscape-change effects, multidimensional (2-D cylindrical, 3-D cartesian) numerical heat-transfer models were created that simulate heat flow in layered heterogenous materials surrounding a wellbore, phase changes, and the unfrozen water properties of a wide range of fine-grained sediments. Using these models in conjunction with the borehole temperature measurements, the mean thermophysical properties of permafrost rock units on the western Arctic Slope were determined using an optimization process. Incorporation of local meteorological information into the optimization allows a more refined estimate of the thermal properties to be determined at a well site. Applying this methodology to the East Simpson #1 well on the Beaufort Sea coast (70&deg;55.046'N, 154&deg;37.286'W), the freezing point of permafrost is found to be &ndash;1.05&deg;C at this site and thermal diffusivities range 0.22&ndash;0.40 &times; 10^&ndash;6 m² s^&ndash;1. Accounting for the drilling and landscape-change effects, tundra adjacent to East Simpson is found to have warmed 5.1 K since the mid-1880s. Of this, 3.1 K (60%) of the warming has occurred since 1970.</p>

<p> A sediment core from Clear Lake, CA that was drilled in 2012 is the centerpiece of a current interdisciplinary paleoclimate investigation in northern California. Two similar research projects were conducted in Clear Lake in 1973 and 1980, but inconsistent core recovery and a possible hiatus in sedimentation introduced uncertainty about the length of time represented by recovered sediments. Two cores recovered in 2012 were drilled in effort to create a continuous splice and attain a complete paleoenvironmental record for the Clear Lake area. Until now, the core has been radiocarbon dated to about 55 meters below lake floor (mblf) and one ash has been identified at 63 mblf, but the lower 90 meters of sediment remained undated. </p><p> Paleomagnetic research provides a chronostratigraphic framework for the collaborative project and reveals variations in sediment flux and environmental conditions over time. In Clear Lake sediments, magnetic minerals are ubiquitous and demonstrate reliable magnetic behavior for magnetostratigraphic interpretation, such as relative paleointensity (RPI) correlation. RPI correlation provides continuous chronostratigraphy for the core to 108 mblf, which corresponds to more than 200 ka. </p><p> Magnetic mineral concentration and grain domain size of Clear Lake sediments reflect zones of varying environmental conditions within the watershed that correlate with previous pollen research. These zones can be correlated to select Marine Isotope Stages (MIS), providing a direct link between paleoclimatic conditions in the marine and terrestrial environment in California. The boundary of MIS 1 and MIS 2, known to occur at 14 ka, is clearly shown as an increase in magnetic mineral concentrations responding to wetter conditions in MIS 2. These results are consistent with previous palynological interpretations from Clear Lake [Adam and Robinson, 1988]. Magnetic mineral concentrations reflect five zones of distinct environmental conditions and also suggests that sedimentation rate increases dramatically in the lower 30-40 meters of the core. </p><p> <b>The environmental magnetic response of Clear Lake sediments, in conjunction with a magnetostratigraphic interpretation, provides a unique opportunity to investigate the timing of environmental change in the lake region.</b> California&rsquo;s current drought may be a new norm of extreme weather in response to higher average temperatures associated with climate change, and paleoenvironmental research like this provides insight on whole-system responses to rapid environmental change.</p>

<p>ABSTRACT Carbon isotope measurements (&#948;13C) across tree rings retain long-term annual and seasonal climate trends that can be used to extend historical records in regions where instrumental observations are limited or unavailable. The &#948;13C value measured from successive tree-rings reflects changes in stomatal conductance, which varies as a stress response to changes in environmental moisture. In relatively dry environments, a decrease in moisture leads to a reduction in stomatal conductance and causes an observable increase in the measured tree-ring &#948;13C composition. Thus, changes in fractionation can be an indicator of varying water-stress associated with changing temperatures. Here, I investigate the use of annually resolved &#948;13C data to identify twentieth century Arctic temperature trends. I present a new annually resolved &#948;13C dataset spanning 50 years (1912-1961) from three Larix cajanderi tree cores collected in far northeastern Siberia. The dataset yields a strong correlation (r = 0.55) with an increase in temperatures associated with the Early Twentieth Century Warming (ETCW) event (1925 to 1946). In order to investigate whether this Arctic-wide temperature anomaly can be identified from other Arctic tree-ring sites, I compiled &#948;13C data from thirteen previously published high-latitude (>62 &#730;N) tree-ring chronologies. The combined dataset, which spanned nearly the entire twentieth century (1900-1998), identified a strong negative relationship (r = -0.53, p < 0.01) between net carbon isotope fractionation and temperature. This Arctic-wide tree-ring dataset showed strong correlation across the ETCW in particular (r = -0.86), as well as across the interval of rapid late twentieth century anthropogenic warming (r = -0.50). Identification of both the natural ETCW and the current anthropogenic Late Twentieth Century Warming (LTCW) event (1966 to 1998) demonstrates the potential for tree-ring &#948;13C records to extend our knowledge of Arctic temperature change beyond the limited historical record.

<p> A &sim;2500 year record of hydrologic change from southern Iran is inferred from the mineralogy and stable isotopic composition of bulk and biogenic carbonates archived in Lake Hirom (27&ordm; 57&rsquo;N, 53&ordm;52&rsquo;E). This change is related to regional variations in moisture and to the larger Indian Summer Monsoon circulation (ISM). During the early Holocene, increased summer insolation from ~10,000 to 8,000 yr BP contributed to the intensification of the ISM. This intensification may have increased summer precipitation north of the modern ISM limit. Evidence of wetter lake conditions in Lake Hirom occur from 8,800 to 7,800 yr BP. Drier conditions occur and persist from 7,800 to 6,300 yr BP, until lacustrine marl changes to peat, indicating maximum aridity. The timing of the drying trend in the mid-Holocene of Lake Hirom correlates with cave and lake records in the Arabian Peninsula, and lake records in northern Iran, indicating a regional drying event.</p>

<p> Hydrogen isotope ratios (&delta;D) in various types of leaf waxes, including n-alkanoic acids, extracted from lacustrine sediments are becoming increasingly popular for understanding past climate changes. Leaf-wax &delta;D values track precipitation &delta;D, and provided that controls on precipitation &delta;D are known, changes in leaf wax &delta;D can be used to reconstruct these changes in the past. Seventy-six sediment samples from Andrew Lake, Adak Island, Alaska, extending to 4800 years ago (4.8 ka) were analyzed for &delta;D of <i> n</i>-alkanoic acids. &delta;D values of isolated C₂₈ <i> n</i>-alkanoic acids show a strong inverse correlation with October-May storminess (days with >19 mm of precipitation) over the meteorological record (r² = 0.58, p &lt; 0.02), and a similarly strong correlation with total precipitation amount. This implies that isotopes in precipitation on Adak Island are strongly influenced by the amount effect. Shifts in precipitation amount and storminess are associated with shifts in the North Pacific hydroclimate, which is driven by the Aleutian Low during fall and winter. Low &delta;D values indicate high precipitation amount/storminess on Adak Island, which correlates well to a weak or westward Aleutian Low as inferred from other sites, while high &delta;D values indicate lower precipitation amount and generally correlate with a stronger or more eastward Aleutian Low. Results from Adak Island were combined with evidence from previously published paleoclimate studies from southern Alaska and the Yukon to reveal a more complete spatial picture of hydroclimatological changes for the last 5000 years. Variability in &delta;D since 3.5 ka is related to changes in North Pacific atmospheric circulation patterns including the Aleutian Low. High precipitation amount/storminess during the Medieval Climate Anomaly (&sim;850 to 1050 AD) on Adak Island is consistent with evidence from other studies that suggest a weakened Aleutian Low. Evidence of decrease precipitation during the early Little Ice Age (&sim;1200 to 1500 AD) gave way to wetter conditions during the latter half (&sim;1500 to 1900 AD). A wet late LIA is consistent with the results from coastal studies in southern Alaska, suggesting another period of a weakened Aleutian Low. This study is part of a multi-proxy investigation involving two lakes on Adak Island including analyses of pollen, biogenic silica, chironomids, isotopes in diatoms, and other proxies to help increase our knowledge about past hydroclimate in the North Pacific.</p>

<p> The Paleocene-Eocene Thermal Maximum (PETM) was followed by a lesser hyperthermal event, called ETM2, at &sim;53.7 Ma (Zachos et al., 2010). The carbon isotope excursion and global temperature increases for ETM2 were approximately half those of the PETM (Stap et al., 2010). The paleohydrologic response to this event in the continental interior of western North America is less well understood than the response to PETM warming. Although ETM2 is better known from marine than continental strata, the hyperthermal has been identified from outcrops of the alluvial Willwood Formation from the Deer Creek and Gilmore Hill sections of the McCullough Peaks area in the Bighorn Basin, Wyoming (Abels et al., 2012). The presence of ETM2 in Willwood Formation strata provides a rare opportunity to examine local continental climactic and sedimentary response to this hyperthermal. </p><p> Core drilled at Gilmore Hill was described and analyzed geochemically. The core consists of paleosols formed on mudrocks that are interbedded with siltstones and sandstones. Carbon isotope analysis of carbonate nodules from paleosols in the core shows that the top of the core, below a prominent yellow sandstone, most likely records the very beginning of the carbon isotope excursion that marks ETM2 (Maibauer and Bowen, unpublished data).The rest of the CIE was likely either not recorded due to sandstone deposition or removed by erosion prior to the deposition of the sandstone. </p><p> Analysis of bulk oxides in the paleosols using the methods of Sheldon et al. (2002) and Nordt and Driese (2010b) provides quantitative estimates of precipitation through the core section. The estimates reveal drying over the &sim;15m leading up to ETM2. Red and brown paleosols, attributed to generally dry conditions, dominate the entire section below the onset of ETM2 and confirm drier conditions. In contrast, thick purple paleosols are associated with ETM2 at the Deer Creek site and suggest wetter conditions during most of the ETM2 interval. The prominent yellow sandstone at the top of the Gilmore Hill core was probably deposited during those wetter climate conditions. </p><p> The core displays distinct changes in stratigraphic architecture: the bottom &sim;100m is mudrock-dominated and the top &sim;100m is sandstone dominated. Several PETM studies have suggested that sediment coarsening in continental basins in the US and Spain developed in response to precipitation changes associated with global warming. Analysis of the Gilmore Hill core's stratigraphic architecture in conjunction with carbon isotope and precipitation data shows that the prominent sandstone in the position of ETM2 was not caused by climate change. The sandstone is the uppermost part of the sandstone-rich interval whose base underlies ETM2 by more than 50m. This study shows that the shift from mudrock- to sandstone-dominated stratigraphy at Gilmore Hill, and possibly throughout the McCullough Peaks area, was not caused by climactic change associated with ETM2. While studies of PETM sections have suggested that the hyperthermal caused sediment coarsening in several different basins including the Bighorn Basin (e.g., Schmitz and Pujalte, 2007; Smith et al., 2008b; Foreman et al., 2012), this study suggests that the lesser magnitude ETM2 did not cross the necessary threshold to provoke a sedimentological response in the Bighorn Basin.</p>

<p> Millennial-scale variability in the Indian monsoon was temporally linked to changes in global ocean circulation during the last glacial period, as evidenced by planktic-benthic foraminiferal stable isotope and trace element results from an intermediate depth sediment core from the northwestern Bay of Bengal (Core NGHP-01-19B; 18&deg;58'N, 85&deg;39'E; 1,422 m). Paired planktic foraminiferal Mg/Ca and &delta;¹⁸O_c of <i> G. ruber</i> constrain sea surface temperatures and isolate millennial-scale variations in the &delta;¹⁸O of surface waters (&delta;¹⁸O_sw) which resulted from changes in river runoff in the northwestern Bay. Concurrently with low &delta;¹⁸O_sw events, benthic foraminiferal &delta;¹³C of <i>Cibicidoides </i> spp. decreased, suggesting an increased influence of an aged water mass at this intermediate depth site during the low salinity events. Benthic foraminiferal Cd/Ca of <i>H. elegans</i> supports the identification of this water mass as aged Glacial Antarctic Intermediate Water (GAAIW). Lagged correlation analysis (r= 0.41) indicates that changes in subsurface properties led changes in surface properties by an average of 380 years. The implication is that Southern Hemisphere climate exerted a controlling influence on the Indian monsoon during the last glacial period.</p>

<p> In southwestern United States, the North American monsoon (NAM) is the main driver of severe weather in the Southwest. How the monsoon has behaved in the past and how it will change in the future is a question of importance for natural resource management and infrastructural planning. In this dissertation, I present the results of three studies that have investigated NAM variability and change from the perspective of paleoclimate records, future climate change projections, and simulation of the low-frequency variability with the longest retrospective atmospheric reanalysis. </p><p> In the first study, a monsoon-sensitive network of tree-ring chronologies is evaluated within its ability to reproduce NAM variability during the past four centuries. The tree-ring chronologies can reasonable characterizes the dominant modes of NAM climate variability and reveal low-frequency climate variability at decadal and longer timescales that is beyond the ability of the instrumental record to temporally well resolve. This low-frequency climate variability seems to coincide with the occurrence of multiyear persistent droughts. </p><p> In the second study, we consider the modes of climate variability to assess the degree of physical uncertainty in climate change projections models used in the North American Regional Climate Change Assessment Program (NARCCAP). NARCCAP models are evaluated mainly on their ability to represent warm season driven by quasi-stationary Rossby wave trains and El Ni&ntilde;o Southern Oscillation &ndash; Pacific Decadal Variability (ENSO-PDV). Only one out of eight NARCCAP models has a reasonable representation of the seasonal cycle of monsoon precipitation and ENSO-driven variability in both the 20^th and 21^st centuries. No decadal variability was observed in any of the NARCCAP models. </p><p> In the third study, the low-frequency drought signal found with tree-ring chronologies is further explored within the framework of a regional climate modeling. The Twentieth-Century Reanalysis is dynamically downscaled (DD-20CR) and its statistic analysis suggests that low-frequency drought signal in the Southwest is driven by atmospheric circulation changes on global to continental scales that affect precipitation in Central American as well. Low-frequency climate variability is therefore likely responsible for the multiyear persistent droughts in the last four centuries, as independently evaluated from the tree-ring monsoon-sensitive network.</p>