The effects of dissolution on the silicon and oxygen isotope compositions of silica phytoliths

The δ30Si and δ18O values of silica phytoliths have applications for reconstructing paleoenvironmental conditions. This study examines the effect of partial dissolution and burning of phytoliths on their isotopic compositions, dissolution behaviour, and physical characteristics (specific surface area, mean particle size, and visual appearance) and discusses problems with the use of phytolith δ18O and δ30Si values that have been modified in soils in paleoclimate reconstruction. Dissolution experiments were conducted in batch reactors under a range of pH (4-10) and temperature (4-44˚C) conditions. The δ18O and δ30Si values of fresh phytoliths behave similarly as dissolution progresses, with values increasing until the solution is approximately 30-40% saturated with silicic acid. During this phase of the experiment the isotopic composition of the remaining silica is dominated by dissolution preferentially removing the light isotope (16O and 28Si) to the solution. After ~30-40% saturation back reactions begin to affect the isotopic composition of remaining silica, despite net movement in the forward direction (i.e. dissolution). The δ18O values of precipitated silica are determined by the δ18O value of water in the solution and the temperature of the experiment. The δ30Si values of precipitated silica are determined by the δ30Si value of silicic acid. Phytoliths subjected to burning at 700˚C have δ18O values that are 2.6 ‰ lower than unburned phytoliths while their δ30Si values remain unchanged. This suggests that heating results in the incorporation of 18O-depleted hydroxyl groups into the silica structure. Dissolution of burned phytoliths progressed more slowly than dissolution of fresh phytoliths in conditions that are less favourable for dissolution (i.e. low pH and T) and more quickly in conditions that are favourable (i.e. high pH and T). The δ18O values of partially dissolved burned phytoliths follow the same general trend as those of unburned phytoliths but with less overall change in δ18O values. Burning may increase silanol sites that are more susceptible to dissolution. We recommend caution in using the δ18O and δ30Si values of soil phytoliths in paleoclimate reconstructions. Care must be taken to identify alteration by dissolution or burning, which may not always be visually evident

Triple Oxygen Isotope Composition of Carbonates

This dissertation presents a method of analyzing the triple oxygen isotope compositions of carbonates, presents an empirical calibration of the carbonate-water equilibrium fractionation line, presents a triple oxygen isotope equipped fluid-rock mixing model for carbonates to see-through diagenesis, and applies all these findings to ancient carbonate samples. Using modern carbonates and associate water, the following equations are calculated to describe equilibrium triple oxygen isotope fractionation of carbonates: 1000lnalpha18Occ-wt=2.84x106/T2-2.96 1), Thetacc-wt=-1.39/T+0.5305 2). Using these fractionation equations provides an extremely useful tool to determine whether a carbonate sample is altered or preserves its original isotopic composition. In samples that are altered, a fluid-rock mixing model is used to see-through the diagenesis. Applying these tools to ancient carbonate rocks shows that many samples thought to be pristine are altered and are confusing paleoenvironmental interpretations. This work shows that seawater temperature and isotopic composition is unchanged over the Phanerozoic, an important consideration when reconstruction paleoenvironments

Geochemistry of Vein Calcites Hosted in the Troodos Pillow Lavas and Their Implications for the Timing and Physicochemical Environment of Fracturing, Fluid Circulation, and Vein Mineral Growth

Calcite veins hosted in pillow lavas of the Late Cretaceous Troodos suprasubduction zone ophiolite provide insights into the timing and physicochemical environment of postmagmatic fracturing and fluid circulation through oceanic crust. This study presents rare earth element and yttrium (REE+Y) concentrations, δ13C, δ18O, 87Sr/86Sr, and clumped isotopic (Δ47) compositions of vein calcites in order to investigate their fluid sources, formation temperatures, and precipitation ages. These geochemical data are combined with microtextural analyses. Intersections of 87Sr/86Sr ratios of vein calcites with the Sr isotope seawater curve suggest two distinct calcite veining phases. Major calcite veining within an interval of ~10 Myr after crust formation is characterized by microtextures that point to extensional fracturing related to crack and sealing, host rock brecciation, and advective fluid flow. These vein calcites show REE+Y characteristics, 87Sr/86Sr ratios, and clumped isotopic compositions indicative of precipitation from seawater at <50 °C. Extended fluid residence times intensified fluid‐rock interactions and lowered Y/Ho ratios of some blocky vein calcites, whereas crack and sealing resulted in pristine seawater signatures. Low 87Sr/86Sr ratios of localized high‐temperature blocky vein calcites point to the involvement of hydrothermal fluids. These calcites show Mn‐controlled oscillatory growth zonations that probably developed in a closed system out of equilibrium. Later calcite veining (<75 Ma) may have coincided with rotation and/or uplift of the Troodos ophiolite. Microtextures of these vein calcites indicate fluid diffusion and fracture‐independent crystallization pressure‐driven veining. Their variably modified seawater signatures resulted from diffusion‐related fluid interaction with hydrothermal sediments

A geochemical study of the Maquoketa formation in Pike County, Missouri

Statistical interpretation of geochemical data was used to determine the environment of deposition of the Maquoketa Formation in Pike County, Missouri. A cyclic sequence was noted which can be best explained by cyclic climatic changes. During the study it was found that elemental relationships which would be expected in a freshly deposited sediment can be found in core samples. In samples from outcrops and quarry faces these relationships have been destroyed. As these elemental relationships reflect mineralogical relationships, serious doubts have arisen as to the validity of petrographic studies of samples taken from outcrops. This study indicates weathering may be responsible for many features which previously have been described as diagenetic--Abstract, page ii

Variation In Gold Content Of Minerals Of The Marysville Quartz Diorite Stock, Montana

Neutron activation analysis for gold has been carried out on 135 mineral samples from throughout the Marysville stock with the following results: (1) the gold content of 44 biotite ranges from 0·002 to 0·924 ppm with an average of 0·076 ppm; (2) 37 hornblende samples gave a range of 0·003-0·823 ppm and an average of 0·100 ppm; (3) for 44 magnetite samples the range was 0·003-0·329 ppm and the average 0·037 ppm; (4) 10 quartz-feldspar samples varied from 0·006 to 0·176 ppm with an average of 0·065 ppm. High gold values mainly occur near the edge of the stock and the lowest values in the center of the stock. The highest values are found near the old gold mines located along the periphery of the stock. Because it could not form bonds in crystallizing minerals, the gold was concentrated in the residual fluids of the Marysville magma and was finally deposited in quartz veins at the edge of the stock. The gold in the minerals is probably entrapped as uncharged gold atoms. The amount of gold in a given mineral was determined by the structure of the mineral and the concentration of gold in the magma at the time of crystallization of that mineral. © 1967

Novel Paleoclimate and Paleoenvironmental Applications of Stable and Radiogenic Isotope and Elemental Geochemistry from the Holocene through the Cretaceous

Anthropogenic influence on modern climate and the environment is unambiguous and bears profound implications for agriculture, water availability, and natural resource management. These influences are global in extent and require critical examinations of past climatic and environmental perturbations in the geological record to predict the magnitude of anticipated changes (e.g. surface temperature increase) that will impact humanity in the future. This dissertation explores the use of existing and novel geochemical proxies for environmental information ranging from atmospheric circulation patterns of the Pleistocene/Holocene to coastal marine temperatures and volcanism of the Late Cretaceous. The initial focus of this research has centered on the application of carbonate clumped isotope (Δ47) paleothermometry to marine mollusk fossils in order to reconstruct marine temperatures preceding, during, and after the Cretaceous-Paleogene (K-Pg) extinction. The K-Pg boundary interval serves as an important analogue to modern and future climate projections, with atmospheric CO2 concentrations estimated at ~400 to 1100 ppm (compared to an annual average of 402.8 ppm in 2016) and a world lacking significant continental ice sheets. The K-Pg boundary is also known for pervasive terrestrial and marine extinctions including the demise of dinosaurian groups. These methods revealed Δ47-derived coastal marine temperatures from the ancient Mississippi Embayment and Atlantic Coast ranging from 7 to 25 °C, which compare closely to modern values in the range of 10 to 29 °C. The similarity between modern and reconstructed paleotemperatures suggests that the surface temperatures required at the subtropics for an unglaciated world may be closer to those observed in the modern than originally believed and implies a shallow equator-to-pole thermal gradient. Cold paleotemperatures along the Atlantic Coast near New Jersey (3 to 14 °C) may provide evidence for the presence of the Gulf Stream current, and subsequent North Atlantic return flow as early as the Campanian/Maastrichtian. The temperature ranges reconstructed for these sites also compare well to marine temperatures determined using other proxy methods, with the exception of the TEX86 organic geochemical temperature proxy, which may be seasonally biased towards summer temperatures. Coupled with the temperatures reconstructed from Δ47 values, we have developed a unique proxy for environmental mercury concentrations, [Hg], in the past using the same mollusk fossils from before, during, and after the K-Pg boundary interval. This work represents the first deep-time application of [Hg] records in biogenic carbonates. We have measured both [Hg] and Δ47 values from globally-distributed specimens across a latitudinal range from 70 °N to 67 °S. We found covariation between elevated marine temperatures (~7 to 10 °C excursions) and peak [Hg] between 17 to 42 ng g-1 (relative to a background of ~0 to 5 ng g-1) immediately prior to the K-Pg extinction and coinciding with the onset of the main eruptive phase of the Deccan Traps Large Igneous Province (LIP). Evidence of a global signal of volcanogenic emissions (e.g. CO2 and gaseous elemental Hg) and changing climate prior to the K-Pg boundary suggest causality and serve to clarify the role of the Deccan Traps in terms of extinction patterns. This work also reveals the potential of [Hg] in biominerals at other critical intervals of the geological record in addressing questions relating to LIP volcanism and environmental mercury cycling.PHDEarth and Environmental SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144022/1/meyerkw_1.pd

Stable Isotope Paleolimnology of Barry Lake, Ontario, Canada Since AD - 1268

The paleolimnology of Barry Lake, SE Ontario, Canada is described using mineralogy, magnetic susceptibility, carbon:nitrogen ratio, mass accumulation rates, grain-size, δ18O and δ13C of authigenic calcite and mollusc aragonite, δ13C and δ15N of organic matter, and archival records. These sediments span the Medieval Warm Period (MWP), Little Ice Age (LIA), and human settlement. Interval I proxies (AD 1268-1350, MWP) indicate warmer and drier conditions and elevated lacustrine production. Interval II (AD 1350-1615) was cooler and wetter, with lower lacustrine production and low-oxygen conditions causing loss of shelly fauna. Interval III (AD 1615-1850, LIA) was colder, with lower lacustrine production beginning at AD 1720, coincident with European activity beginning at AD 1830. Interval IV (AD 1850-2011) is marked by rising temperature and lacustrine production, declining human impact, and since AD ~1950, new nitrogen input (fertilizer?). These data provide a baseline for future change in climatic and anthropogenic factors affecting Barry Lake

Assessing the Paleoceanographic Potential of the coral Montipora venosa at Fanning Atoll, Central Equatorial Pacific

As interest in global climate change increases, so does the need for better and more extensive climate proxies. The central equatorial Pacific has been established as the region with the largest ENSO-related sea surface temperature (SST) and precipitation (PPT) anomalies, which are known to impact global interannual climate variability. To date, the coral genus Porites has been most commonly utilized for coral-based paleo-reconstructions of ENSO and lower frequency phenomena. However, due to questions of biological artifacts in coral, to more fully understand coral-based reconstruction, different coral genera need to be analyzed. In this study, oxygen (δ18O) and carbon (δ13C) isotopic time series as well as Sr/Ca time series, were generated from a colony of the massive hermatypic coral Montipora venosa, (a genus not previously studied) from an ENSO sensitive site. This coral core was recovered from Fanning Island (3º 52’N, 159º 20’W) in the central equatorial Pacific, located in the heart of the important Niño 3.4 region for monitoring ENSO. Oxygen isotopic data from Montipora venosa core (FI4) is compared to previously unpublished δ18O data from a Porites sp. core (FI5) taken at the same location. Even though core FI4 M. venosa is the shorter core (662mm), it spans 111yr (1997-1887), while the longer FI5 Porites core (1260mm) spans only 75 yr (1997-1922). Core FI5 Porites contains a deviation of undetermined cause towards increased δ18O values between 1950 and 1951, which is not recorded in core FI4 Montipora venosa. This deviation is not found among other central equatorial Pacific Porites records and thus supports the need for a multi-core replication strategy. There has been a suggestion by Cane et al. (1997) that the pattern of 20th century SST change across the Pacific may be due to an increase in the west to east temperature gradient. However, comparison of the M. venosacore FI4 δ18O time series with δ18O time series from Maiana (1ºN, 173ºE) and Urvina Bay (0º 24.5’S, 91º 14’W) indicates there has been no change in SST gradient across the Pacific during the studied time period (1997-1887), assuming that δ18O is primarily affected by temperature at all these sites.Coral data from several locations throughout the Pacific Ocean show a shift in 1976 towards lighter δ18O values, which is often interpreted as a trend towards warmer SSTs. However, it is not clear how much of this trend is real and how much might be attributable to biological effects. The results of this study indicate that the magnitude of the shift varies not only by location, but also by coral genus. Again, this substantiates the need for a replication strategy