Rock magnetic techniques are one of the most sensitive means for detecting subtle changes in grain size, sedimentation rate, or pedogenic development. We expect that the environmental changes (wildfires, vegetation loss, erosion/deposition rates) associated with an impact or airburst event of this magnitude would be preserved in the magnetic record. Hysteresis loops and frequency dependence of susceptibility measurements are used to evaluate the concentration, grain size, and composition of magnetic material, while low-temperature magnetic properties, SEM images and EDS spectra speak to magnetic mineralogy.

We observe gradual decreases in the amount of magnetic material and shifts towards smaller, more magnetically hard (high-coercivity) minerals with depth at both sites. These trends are consistent with a history of uninterrupted soil development rather than a catastrophic impact event. Magnetic extracts were also prepared from bulk soil collected at one site, following the methods of previous investigators in order to explore the possibility of sampling bias during magnetic extraction. Extracted magnetic grains are larger and more magnetically hard than in-situ magnetic material, and are predominantly detrital ilmenites with varying degrees of alteration to high-titanium products. Many grains exhibit hematite exolution lamellae, which gives rise to lamellar magnetic coupling. Pedogenesis dominates the magnetic properties at both sites, producing profiles with near-surface enrichment of fine-grained magnetically hard material that is progressively oxidized with depth.

For the comparative study, sediment from the fluvial site (UC-941) and lacustrine site [UC-8303) was collected and sieved in a custom oscillatory device that gently dunked sediment in water with minimal fossil breakage. The results of the comparative study show that significantly more fluvial microfossils are polished compared with lacustrine microfossils. Of the bones from the fluvial site, 78% were polished, versus 3% of bones from the lacustrine site. Additionally, only 14% of bones from UC-941 retained identifying features, as opposed to 47% of bones from UC-8303.

In the experimental study, bones ranging from 2-7 mm in size were selected from three sites. Recrystallized bone was taken from UC-8303, a fossil locality in the Judith River Formation; dry bone stripped of collagen was selected from Homestead Cave, Utah (a 13,000 year old site); and fresh bone was taken from a deer metatarsal recovered form the Upper Missouri River Breaks National Monument, Montana. The bones were run in a vibratory rock tumbler for 72 hours, and periodically removed to be photographed. SEM images were taken at the beginning and end of the experiment. The results show that collagen-free bone became polished most quickly, in an average of 3 hours, followed by fossil bone, which required an average of 6 hours. The fresh bone polished most slowly, after an average of 14 hours of tumbling.

The comparative results show polish is a feature found mainly in fluvial microfossil bonebeds, suggesting that polish is imparted by interaction with channel sediments. Contrary to previous studies that speculated that polish was acquired through predation and digestion, it appears that abrasion is also a major source of polish. The fast rate of polishing in the experiment demonstrates that a relatively short period of time is required to polish small vertebrate microfossils. Microfossils could exist in an active channel environment for as little as hours to days and become polished. This is consistent with previous studies that have suggested that fluvial microfossils are sourced by lacustrine sites and undergo minimal transport before final burial. Surface alteration appears to occur on relatively short timescales, and while bone could exist at the base of a channel for longer periods, this is not required for abrasion and polish to occur.

This study examines the vegetation history of Many Glacier Valley using pollen extracted from a sediment core taken from Swiftcurrent Lake (SWF) in Glacier National Park, MT. Pollen is used to create a vegetation reconstruction focusing on the past 1200 years, which includes both the MWP and LIA. Changes in vegetation through time reflect vegetation’s response to climate, specifically temperature and precipitation, and thus can be used to broadly reconstruct past climate. Evidence from pollen indicates that the region surrounding SWF during the MWP was likely drier and warmer than present climate, and more moist and cool during the LIA. This is consistent with other results from the Western United States.

Magnetic susceptibility and stable carbon isotopes were studied in a sequence of late Ordovician carbonate and clastic strata from the Shine Jinst Area of southern Mongolia where the Daravgai and Gashuunovoo formations are exposed. The rocks at this locality are typical of carbonate shelf deposits and primarily exhibit interbedded carbonate and shale. Six sections capped by faults were measured in this structurally complex area.

Magnetic susceptibility is used as a proxy for sea level and tectonic effects that drive terrestrial weathering and detrital input to the marine system. Preliminary hyteresis measurements identify the primary iron-carrying mineral as hematite. The basal section of the Daravgai Fm. exhibits anomalously high values that average 4.01x10-8 m3/kg. The two overlying sections exhibit values averaging 1.29x10-8 m3/kg and include no major excursions. Missing section in the Daravgai Fm. make climate and sea level interpretations problematic. Meter-scale magnetic susceptibility oscillations were identified in the Gashuunovoo Formation that likely represent transgressive/regressive cycles, which are reflected lithologically as siliciclastic pulses. Significant positive excursions were identified which alternate with low value, relatively stable periods averaging 2.08x10-8 m3/kg. These data can be used in future studies for lateral correlation of other sections near Shine Jinst.

Stable carbon isotope ratios are used to interpret climate trends and attempt regional and global correlation of the section. The Daravgai Fm. exhibits anomalously low carbon isotope values in the basal section averaging -2.79 ‰ followed by relatively stable values averaging 1.61 ‰ in the overlying two sections. The lower part of the Gashuunovoo Fm. exhibits an increasing trend and a major positive excursion averaging 2.32 ‰. This excursion likely corresponds to one of the five major excursions identified worldwide in the late Ordovician, and is tentatively correlated with a lower Ashgill excursion. Carbon isotopic data for the upper sections of the Gashuunovoo Fm. would enhance the dataset and strengthen the possibilities for correlation of the sequence.

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 Dl 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 Dl 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 the higher rates of deposition naboveD1ashigherrates 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.