The stable isotopes 2H and 18O, combined with noble gases and radioisotopes (e.g., 3H, 14C, 36Cl), are used to infer groundwater age and climate during recharge. Flow regimes within low-velocity flowpaths and long residence times could allow an aquitard-aquifer diffusive flux to alter isotope abundance. Consequently, the diffusion of isotopes (e.g., 14C, 2H and 18O) between aquifers and confining layers needs to be considered in such conditions. In this study, COMSOL Multiphysics was used to determine if diffusion of 18O (and 2H by proxy) from a bounding aquitard could explain observed downgradient enrichment of 2H and 18O within a regional aquifer. Using the geologic and hydraulic properties of the lower Wilcox aquifer of the Mississippi Embayment aquifer system in Missouri and Arkansas, the advection-dispersion equation was solved along a 1-D groundwater flow domain, coupled with a cross-contact aquitard-aquifer diffusive flux. Although the observed signal within the lower Wilcox was not matched, a sensitivity analysis indicated the importance of the isotope composition gradient between the aquifer and aquitard. Furthermore, groundwater velocity was suggested as a controlling influence on aquitard-aquifer exchange that could alter aquifer isotope composition.

Authigenic clays are potential proxies for paleoclimate change when formed under saline conditions. Between 1.9 -1.7 mya, Olduvai was a closed-hydrologic basin into which Al- and Fe- rich detrital clays were transported from surrounding soils. Authigenic Mg-rich smectite formed within the basin due to chemical alteration in alkaline and saline conditions. 40 samples were chosen from a stratigraphic section in the center of the paleolake and analyzed to characterize the geochemical facies of the material using XRD, EMPA, and FTIR. ~70% of samples were Mg-rich with clay d060 XRD peaks between 1.506-1.523Å. EMPA indicated (Al2O3 + Fe2O3)/MgO ratios ranged from 0.2-4.8, 21 had ratios >1.0, indicating higher (Al2O3 + Fe2O3) content. These clay data correlated with bulk XRF data, indicating diagenetic controls on bulk composition. Therefore, bulk geochemistry can potentially indicate paleoclimate change when influenced by Mg-rich phyllosilicate formation.

Tree-ring chronologies are widely used to reconstruct high-to low-frequency variations in growing season temperatures over centuries to millennia. The relevance of these timeseries in large-scale climate reconstructions is often determined by the strength of their correlation against instrumental temperature data. However, this single criterion ignores several important quantitative and qualitative characteristics of tree-ring chronologies. Those characteristics are (i) data homogeneity, (ii) sample replication, (iii) growth coherence, (iv) chronology development, and (v) climate signal including the correlation with instrumental data. Based on these 5 characteristics, a reconstruction-scoring scheme is proposed and applied to 39 published, millennial-length temperature reconstructions from Asia, Europe, North America, and the Southern Hemisphere. Results reveal no reconstruction scores highest in every category and each has their own strengths and weaknesses. Reconstructions that perform better overall include N-Scan and Finland from Europe, E-Canada from North America, Yamal and Dzhelo from Asia. Reconstructions performing less well include W-Himalaya and Karakorum from Asia, Tatra and S-Finland from Europe, and Great Basin from North America. By providing a comprehensive set of criteria to evaluate tree-ring chronologies we hope to improve the development of large-scale temperature reconstructions spanning the past millennium. All reconstructions and their corresponding scores are provided at www.blogs.uni-mainz.de/fb09climatology. (C) 2016 Elsevier Ltd. All rights reserved.

Thesis advisor: Amy E. Frappier / A ~7,000 year stable isotope record from a Central American stalagmite is presented as a record of rainfall and consequently Intertropical Convergence Zone (ITCZ) tropical rain belt strength over the late Pleistocene to mid-Holocene. The "amount effect" explains the well-documented inverse relationship between rainfall amount and stable oxygen isotope values observed in tropical monsoon regions and consequently in stalagmite calcite from those regions. ITCZ rainfall influences much of the Central American tropical region and here a ~7,000 year stable isotope record from stalagmite ATM1 harvested from Actún Tunichil Muknal Cave in Belize is presented as a record of ITCZ influenced rainfall during the late Pleistocene to mid-Holocene (5,561 ± 2,488 BP - 12,605 ± 284 BP). Three major oxygen isotope excursions occur within the record. These excursions correspond to the global cold Younger Dryas and 8.2 ka events and a relatively undocumented wet period around 6,300 bp. The Younger Dryas manifests as a relatively moist period in central Belize while the 8.2 ka event manifests as a relatively dry period. The reason for the opposite responses to cooling elsewhere in the globe is posited to be due to orbital forcings. The 6,300 bp relatively wet period appears to be synoptic in scale and two possible triggers for the isotope excursion are presented: eustatic sea level rise causing lagoonal constriction, warming of water off the coast of Belize, and thus increased evaporation and precipitation over the study region; and hurricane clusters, evidenced in the region in the succeeding 1,000 years, in which the location of the Azores High funnels hurricanes to make landfall near the central Belize region. ATM1 provides evidence for tropical leads and/or lags to global climate events and bolsters the idea that high and low latitude climate relationships are complexly interlinked. / Thesis (MS) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Geology and Geophysics.

Thesis advisor: Jeremy Shakun / The mechanisms of abrupt climate change during the last glacial period are not yet fully understood. The objective of this research is to use oxygen isotope and magnesium/calcium ratios from foraminifera in a marine sediment core <200 km southeast of New York City (Ocean Drilling Program 174 Site 1073A) to test the hypothesis that changes in freshwater run-off patterns during intermediate extensions of the Laurentide Ice Sheet caused abrupt climate change by disrupting the Atlantic thermohaline circulation. The combination of foraminiferal δ18O and Mg/Ca yields salinity as an isolated variable, which is used as a proxy for Hudson River discharge through ~42,000-28,000 years ago. This thesis reviews the literature on abrupt climate change and compares the Hudson River paleodischarge record to established records of abrupt climate events observed in Greenland ice cores. It concludes that a higher resolution of data points is required to evaluate the impact of Hudson River discharge on abrupt climate change. / Thesis (BS) — Boston College, 2015. / Submitted to: Boston College. College of Arts and Sciences. / Discipline: Departmental Honors. / Discipline: Earth and Environmental Sciences.

Des del 2002, s'han desenvolupat una sèrie de proxies climàtiques basades en biomarcadors derivats d'arquees i bacteris. Aquests biomarcadors són els glicerol dialquil glicerol tetraèters o GDGTs. D'entre aquestes proxies, l'índex TEX86 s'aplica en sediments per tal de reconstruir les temperatures superficials del mar i dels llacs; els índexs MBT i CBT són utilitzats conjuntament per a reconstruir la temperatura superficial de l'aire i l'índex BIT permet avaluar la contribució relativa de material provinent del sòl i material pelàgic en els sediments. A causa del recent desenvolupament d'aquestes proxies, hi ha encara un gran nombre d'aspectes metodològics i d'aplicació que han estat poc estudiats. Per això, l'objectiu d'aquesta tesi ha estat la comparació i optimització de diversos procediments analítics implicats en la determinació dels GDGTs, així com l'estudi d'aquests biomarcadors en nous ambients, amb especial atenció en els llacs.En la part analítica d'aquesta tesi, s'ha comparat el mètode estàndard de clean-up de les mostres basat en el fraccionament de lípids en alúmina amb la hidròlisi alcalina; a més, s'han comparat diversos adsorbents i graus d'activació per al fraccionament dels lípids. Els resultats demostren que el TEX86 és robust respecte a aquestes tècniques, mentres que l'índex BIT depèn del grau d'activació de l'adsorbent. En la cromatografia, s'han dut a terme una reducció del flux i un augment de la velocitat dels anàlisis sense pèrdua de resolució cromatogràfica. Finalment, s'han utilitzat dos espectròmetres de masses de disseny diferent per a mesurar els índexs TEX86 i BIT i se n'ha confirmat la comparabilitat. Per altra banda, s'ha observat una sensibilitat extrema del BIT respecte a les condicions d'optimització dels espectròmetres.S'ha estudiat les concentracions dels GDGTs en els sediments superficials d'un conjunt de 38 llacs de la Península Ibèrica per tal d'identificar els factors que en determinen la distribució relativa. L'anàlisi de components principals indica que la major part de la variança pot explicar-se amb l'origen dels GDGTs (del sòl o lacustre), el grau de ciclització interna i el grau de metilació dels GDGTs. Els resultats demostren que les proxies MBT/CBT i el TEX86 tenen una utilitat limitada en la predicció de les temperatures d'aquests llacs, possiblement perquè la distribució de GDGTs és determinada per una mescla complexa de factors. Utilitzant l'anàlisi de clusters s'ha identificat un subconjunt de llacs on els índexs TEX86 i MBT/CBT mantenen una relació linial forta amb les temperatures instrumentals de l'aire.L'estudi dut a terme al llac Baikal (Sibèria) combina els resultats de trampes de sediment, sediments superficials i dos testimonis sedimentaris que corresponen a l'últim cicle glacial-interglacial. Els valors per a l'índex BIT al sediment suggereixen una contribució minsa de material provinent del sòl i les temperatures superficials del llac derivades de l'índex TEX86 es troben dins el rang anual de les mesures de temperatures superficials. En els testimonis sedimentaris, les variacions en profunditat del TEX86 i l'MBT/CBT revelen un senyal climàtic que es concreta en el registre de la transició de condicions glacials a interglacials i l'acord parcial amb les reconstruccions climàtiques existents per a la regió del llac Baikal derivades d'altres proxies.En darrer lloc, l'estudi exploratori dut a terme al cràter del Bosumtwi (Àfrica) revela la presència de GDGTs en les roques impactades i es discuteixen tres hipòtesis en referència a l'origen dels GDGTs trobats.El treball presentat amplia el rang de tècniques analítiques que poden utilitzar-se de forma fiable per a la mesura dels índexs basats en GDGTs i profunditza en l'aplicació i validació dels GDGTs com a proxies climàtiques en sediments lacustres, particularment de la Península Ibèrica i Sibèria. / Since 2002, several climate proxies based on biomarkers of Archaea and Bacteria, the GDGTs, have been developed and applied in sedimentary archives worldwide. Among the proposed proxies, the TEX86 index is applied in sediments to reconstruct sea surface temperatures (SST) and lake surface temperatures (LST); The MBT and CBT indices are together applied as a proxy for past surface air temperature and the BIT index is measured in sediments to assess qualitatively relative inputs of pelagic-originated material versus soil material. Given the novelty of these proxies, there is plenty of scope to improve upon a range of issues regarding their measurement and application. Consequently, the aim of this thesis has been to tackle analytical methodological issues to gain in analytical reliability and sample throughput, and expand the range of environments where GDGT proxies have been appraised, particularly in lake environments.In the analytical section of the thesis, two sample clean-up methods, i.e. alumina fractionation and alkaline hydrolysis are compared. Additional tests compare alumina and silica in their activated and deactivated states as adsorbents onto which GDGTs are fractionated. Results show that TEX86 values are robust using these methodologies, while BIT values are dependant on adsorbent activation state. Regarding the chromatographic analysis, an increase in the analysis velocity and a reduction of flow are effectively implemented without loss of chromatographic peak resolution. Additionally, two mass spectrometer designs, i.e. an ion-trap system and a quadrupole system, are used for the measurement of a wide range of values of TEX86 and BIT indices, and results confirm the comparability of both systems. However, BIT values are found to be extremely sensitive to the MS tuning conditions. In order to find out the main factors influencing the GDGTs distribution in lacustrine environments, a suite of surface sediments from 38 Iberian lakes are surveyed. Principal component analysis of the relative concentrations of GDGTs indicates that terrestrial versus lacustrine origin, combined with degree of cyclization and degree of methylation of the lipids are the principal factors accounting for the GDGT distribution in the sediments. The measurement of the MBT/CBT proxy represents a pioneer application of these indices in lake sediments. TEX86 and MBT/CBT are shown to have a limited applicability in predicting the lake temperature, likely due to a complex combination of factors influencing the GDGT abundances. Using cluster analysis a subset of lakes is selected with a rather linear relationship of TEX86 and CBT/MBT with instrumental air temperatures. In the study conducted in lake Baikal in Siberia, sediment traps, surface sediments and two cores spanning the last glacial-interglacial cycle are analysed in the north and south basins of the lake. BIT values both for the particulate material collected and the surface sediments suggest low influence of soil-derived material. TEX86 derived LST from the water column particulate material and sediments are in the range of measured annual LST. The down-core variations of the TEX86 and MBT/CBT proxies reveal a climatic signal, inferred both from the reconstructed transition from glacial to interglacial conditions and the partial agreement with reconstructed changes in Lake Baikal region derived from other proxies applied in the sediments. The work presented extends the range of analytical techniques that can be reliably used for the measurement of the GDGT indices and provides deeper knowledge on the application and validation of GDGTs as climate proxies in lacustrine sediments, in particular from the Iberian and Siberian regions.

Instrumental and observational records of climate in West Africa suggest that this region may be susceptible to abrupt, decades-long drought events, with potentially catastrophic impacts for the people living in this region. However, because of the dearth of long, continuous and high quality climate records from sub-Saharan Africa, little is known about the long-term frequency and persistence of drought events in this region. It is also unclear whether observed 20th century droughts are natural or due to human impacts. In the present study, we use several complementary approaches to develop a high-resolution record of paleoclimatic changes in West Africa from the geological record preserved at Lake Bosumtwi, Ghana.Our results suggest that West Africa has undergone significant hydrologic variations over the last ca. 10,000 years. The dominant influence on hydrologic changes over this interval was changes in northern hemisphere summer insolation and the associated feedback processes acting in the oceans and on land. This led to a more northerly position of the Intertropical Convergence Zone (ITCZ) and increased precipitation during the early to mid-Holocene. In the late Holocene, a second increase in precipitation occurred along the Guinea coast as a result of the southward migration of the ITCZ from its northern position. This maximum was followed by an abrupt decrease in precipitation at ca. 2.5-3 kyr.The West African monsoon also varies on timescales from millennia to decades. Millennial and century-scale variations appear to be partly paced by changes in solar irradiance, either directly or indirectly. On decadal timescales, variability appears to be dominated by changes in Atlantic sea surface temperatures. The dominant mode is a ca. 40 year oscillation, which in strongly coherent and in phase with the Atlantic Multidecadal Oscillation (AMO). It is unclear from this study, however, if drought conditions over the last century are related to this multidecadal oscillation, or if they are forced by anthropogenic changes.

<p> Long-term records of cyclic droughts are valuable for understanding future changes in hydrologic patterns and constraining climate models. A 3,300-year long record of &delta;¹⁸O and &delta;¹³C values from endogenic carbonate from Foy Lake, Montana is used to infer such droughts during the early Holocene. From 10.8 to 9.6 kyr BP, &delta;¹⁸O and &delta;¹³C values are low, indicating a period dominated by a cooler, less evaporative climate. Both records exhibit strong cyclicity in the ~200 yr range, which is inferred to be a solar cycle. A dramatic shift towards a warmer, drier climate occurs after 9.6 kyr BP in a step pattern, dramatic in the &delta;¹³C record. Warming occurs after the transition. Stochastic cyclicity dominates with weaker, but statistically significant, periodicities ranging from 40-70 yrs. These are believed to be an expression of the Atlantic Multidecadal Oscillation. This larger synoptic climate signal may be important to the overall Pacific Northwest climate. </p>

<p>Lake sediments and glacier extents were used to reconstruct late Holocene climate changes from Kurupa Lake (N 68.35&deg;, W 154.61&deg;; 920 m asl) in the north-central Brooks Range, Arctic Alaska. The lake is fed by several tributaries, including meltwater from eight rapidly disappearing cirque glaciers. An age-depth model for the 622-cm-long sediment core was developed using five radiocarbon (¹⁴C) ages and a 239+240Pu profile. Sedimentary chlorin (chlorophyll a and derivatives) content was analyzed at high resolution (0.2 cm = ~2 yr intervals) with visible reflectance spectroscopy for the past 5.7 kyr. Sedimentary bulk density was analyzed with gamma-ray attenuation (1.0 cm = ~10 yr intervals). </p><p> High-resolution measurements of chlorin content from the last 122 yr (top 16 cm of surface sediment) were correlated with a suite of instrumental climate records from Barrow and Bettles, National Center for Environmental Prediction reanalysis data (grid = N&deg;68 to N&deg;70 &times; W&deg;156 to W&deg;154) and with published climate indices for the Arctic (e.g., Arctic Oscillation, North Atlantic Oscillation). Chlorin content correlates most strongly to average June + September temperature (r = 0.68) and a linear inference model was developed using this regression to quantitatively reconstruct summer temperatures for the past 5.7 kyr. The temperature reconstruction (50-year smoothed) shows minimum summer temperature (5.6 &plusmn; 0.9&deg;C) at 650 AD and maximum summer temperature (10.7 &plusmn; 0.9&deg;C) 2.9 ka. </p><p> Fluctuations in bulk density over the past 5.7 kyr were interpreted to reflect changes in up-valley glacier extent. The accumulation-area ratio (AAR) method was used to estimate former equilibrium-line altitudes (ELAs) for the maximum Neoglacial and modern extents of eight cirque glaciers in the study area. Independently dated moraines enable the history of glacier fluctuations to be linked to the lake sediment sequence (r2 = 0.74), and this integrated approach provides a 5.7 kyr quantitative record of centennial-scale ELA variability for Kurupa River valley. The reconstruction supports the previously established onset of Neoglaciation around 4.2 ka, as well as maximum and minimum glacier extents around 700 AD and the present, respectively. The calculated ELA lowering at 700 AD and the LIA, relative to 1982, was 63 and 55 m, respectively. </p>

No description available.