Rachel Murray

Advisor: Ray Rogers
Senior Honors Thesis: 2009

A Geochemical Framework For Fossilization In The Upper Cretaceous Two Medicine And Judith River Formations of Montana

Rachel MurrayThe rare earth element concentration of vertebrate fossil material is a good indicator of early taphonomic and diagenetic conditions. Previous studies of trace elements in fossils have considered the bone provenance and paleoenvironment in the absence of in-depth sediment or bone mineral characterization. The objective of this study is to contribute the sedimentological and biomineralogical context for taphonomic work on fossil material from the Upper Cretaceous Judith River and Two Medicine Formations of Montana. The final goal is to identify the source and cause of fossil rare earth element enrichment, and to clarify the relationship between early fossil diagenesis and the paleoenvironment.

Four previous Macalester senior theses characterized the early diagenetic conditions of fossil-bearing deposits from the Two Medicine and Judith River Formations through laser ablation-inductively coupled plasma-mass spectrometry analysis of rare earth elements of vertebrate fossils. Building upon the work of my colleagues, I use solution-based inductively coupled mass spectrometry (ICP-MS) to determine the rare earth element (REE) concentrations of the associated host rock material. This data provides perspective on the geochemistry of the natural environment, and a more comprehensive framework for my colleagues’ paleoenvironmental hypotheses.

Fourier Transform Infrared Spectroscopy offers a detailed look at the crystallinity, molecular bond arrangement, and ion replacement of the bone mineral lattice. This technique was employed to determine the extent of diagenesis from the original bioapatite to francolite. FT-IR spectroscopy was used to identify the mineral species transitions within fossil bone material during recrystallization, with the goal of tracking the underlying path of REE enrichment. In this study, infrared spectroscopy failed to provide a good measure of fossil diagenesis, due to contamination by authigenic calcite and method-based discrepancies. However, infrared analysis demonstrated that the fossils were composed of francolite and had likely obtained their high REE concentrations through recrystallization during diagenesis.

Rare Earth Elements signatures from the rocks of the JRF and TMF generally support the paleoenvironmental conditions interpreted from fossil REE data. Rock material from both Formations show evidence for oxidizing paleoenvironmental conditions. The relationship between rock REE content and fossil REE uptake seems to be dependent on the rock grain size, ratio of fossil to rock material, and pore water-sediment interactions. The complex interaction of several factors produces a range of different types of correlation between rock and fossil REE signatures, from direct correlation to mirror-like reverse correlation.

Rare earth element concentrations of fine-grained floodplain mudstones are higher than the REE concentrations of coarse-grained channel-hosted and marine sandstones. This indicates that rare earth elements are concentrated in the clay and silt size fractions of the host material. The mineralogical composition of the clay suggests that the rare earth elements may be volcanic in provenance.

Hannah Wydeven

Advisor: Kelly MacGregor
Senior Honors Thesis: 2009

Paleoenvironmental change over the past 4000 years in Glacier National Park, Montana: carbon and nitrogen in lake cores from Lake Josephine and Swiftcurrent Lake

Hannah WydevenLake cores are important indicators of environmental and climate change, and can be used to understand detailed histories of local environments. Lake core data from Glacier National Park, Montana, is especially useful, as there are very little high-resolution data records of the Holocene.

A core collected downstream of Grinnell Glacier from Lake Josephine in 2005 was analyzed in order to describe local change through the late Holocene. C/N analysis was performed in order to understand terrestrial versus aquatic sources of lake input using an Elemental Analyzer. %TOC data was retrieved through carbon coulometry in order to compare its relevance as a proxy for climate change through solar forcing. An age model was also developed using XRD data, which revealed a maximum core age of 3,020 years BP.

Results show that there is a distinct relationship between the variance of solar forcing and that of %TOC. C/N ratios reveal an abundance of terrestrial inputs and rooted aquatic material in Lake Josephine, and are anti-correlated with the presence of dolomite, which acts as a signal of glacial fluctuations. A major carbon event was identified as an in-wash event of terrestrial, possibly hill-slope material. Josephine data proves useful as a proxy for climate and environmental change.

Alana Bartolai

Advisor: Kelly MacGregor
Senior Honors Thesis: 2009

Reconstructing The Paleoenvironment Of The Great Salt Pond On Block Island, Rhode Island: A Stable Isotope Analysis Of Foraminifera Elphidium Excavatum Over The Last 1750 Years

Alana BartolaiTwo sediment cores were collected from the Great Salt Pond on Block Island, Rhode Island, and reflect climatic and geomorphic changes over the past ˜1750 years. Radiocarbon dating and mercury analysis provides a basal age of 1740 years BP on GSP3 and mercury analysis dates GSP2 back to ˜130 years BP. Sand layers in GSP3 likely represents storm deposits indicating instantaneous deposits, however, these sand layers remain in our age model since they could not be correlated to GSP2. These two cores allow paleoclimatic data from stable isotopes to be compared to global climate changes as well as to the local signal of environmental impacts on the Great Salt Pond.

Shifts in oxygen isotope ratios in core GSP3 are contemporaneous with the Little Ice Age and Medieval Warm Period that occur 150-550 years BP and 1100-700 years BP respectively. d18O values during these time periods reach their largest values, of 1.35‰, and their lowest values of -.11‰ indicating a cooler overall climate. d18O values in core GSP2 reflect precipitation events on a local time scale, which reflect salinity changes in the pond. Salinity oscillations were found to correlate with historical and archeological evidence for the opening of the oceanic breach from the Great Salt Pond to the Atlantic Ocean.

d13C values in both cores show a decrease towards the present indicating a decrease in the ponds productivity. Declined percent oxygen saturation levels derived from anthropogenic eutrophication, with declined percent oxygen saturation levels derived d13C values show a similar decrease towards the present, indicating possible anthropogenic eutrophication. The declines in percent oxygen saturation occur in the past 100 years and the more recent 40 years, reaching values as low as 40% saturation. This time period correlates with tourism booms on Block Island. The paleoenvironment of the Great Salt Pond has been impacted by local climatic events such as precipitation as well as the effect of human settlements on the island.

Kenneth Nelson

Advisor: Ray Rogers
Senior Honors Thesis: 2009

Clay Mineralogy Across A Sequence Boundary In The Upper Cretaceous (Campanian) Judith River Formation, North-Central Montana

Kenneth NelsonClay minerals from a complete reference section of the nonmarine Upper Cretaceous (Campanian) Judith River Formation (JRF; Western Interior Basin, north-central Montana) were semi-quantitatively characterized by the way of X-ray diffraction (XRD) to determine whether or not their trends indicate the presence of a third-order sequence boundary (D1) marked by a macroscopic shift from sandstone- to mudstone-dominated facies and a sudden shift in well-log properties. The technique of clay mineral semi-quantification used here involved oriented specimens of clay-sized extracts and the normalized relative intensity ratio (RIR) method. Analyses of four known mixtures composed of Source Clays from the Clay Minerals Society suggest that the uncertainty of the semi-quantitative results presented is +/-6.3 Wt.%.

The four clay mineral groups that comprise the JRF clay mineral suite are kaolin, smectite, illitic material, and chlorite. Kaolin is interpreted to be primarily of an early diagenetic origin and is most abundant in sandstone lithofacies whereas the other three clay mineral groups are largely detrital in nature and are most abundant in mudstone lithofacies.

The results of a non-parametric statistical test including all samples point out that there is a significant decrease in the average relative abundance of kaolin upward through D1 and an increase in that of smectite at the 95% confidence level. An identical test concerning just sandstone bodies indicates that the average relative abundance of kaolin is greater in sandstones below D1 than those above. Since abundant kaolin and smectite are restricted to sandstones and mudstones, respectively, the results of the former test simply reflect the fact that the JRF is dominated by sandstones below D1 and mudstones above. The reasoning for the decrease in the average relative abundance of kaolin within sandstones alone is explained here as a consequence of higher rates of deposition above D1 as higher rates of deposition remove sediment from the zone of meteoric water flushing and thus early diagenetic processes more rapidly. Regardless of the exact explanations, the semi-quantitative clay mineral trends of the JRF indicate the presence of discontinuity D1 and a change in accommodation space.

The results of this study fall in line with the only predictive model concerning clay mineralogy in a sequence stratigraphic framework which states that the abundance of early diagenetic clay minerals (i.e., kaolin) should decrease across sequence stratigraphic surfaces. However, this study alone cannot validate that prediction and future work is needed to determine the utility of semi-quantitative clay mineral trends for identifying sequence boundaries within the JRF and other formations.