The Department of Earth and Environmental Sciences will hear three mini-talks from current graduate students.

Adriana Brown: The evolution of seawater temperature and oxygen isotopes in the Cretaceous Western Interior Seaway using carbonate clumped isotopes

The Western Interior Seaway (WIS) was a vast epicontinental sea that inundated North America in a series of transgressive-regressive marine cycles from the Early to Late Cretaceous. The WIS has been studied as a classic shallow Cretaceous seaway with a complex stratigraphic record of the interplay between tectonics, climate, sea level, basin evolution, and faunal evolution. However, fundamental questions remain unresolved regarding the WIS, including how seawater temperatures and oxygen isotope compositions (δ18Ow) varied latitudinally and through time. A robust record of reconstructed temperature and water isotopes could help explain paleoceanographic and environmental conditions in the WIS, but this has traditionally been challenging due to limited sample coverage and poor constraints on δ18Ow. Here, we use carbonate clumped isotopes (𝚫47) measurements of 119 well-preserved oysters to reconstruct temperature and δ18Ow from the Albian to Maastrichtian between 25 to 57 °N. This dataset approximately triples the amount of 𝚫47 WIS temperature and δ18Ow values published to date. From this, we find that in each time period the seaway remained consistently warm through all U.S. latitudes and may have been influenced by freshwater runoff from emerging highlands. Canadian temperatures indicate a cooler and potentially less saline water source entering the seaway from the North. We relate these changes in temperature and isotopic composition to global and regional sea level fluctuations, tectonic regimes, and paleocirculation dynamics.

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Rory Sweedler: Topographic diversity gradients revealed among latest Cretaceous multituberculate mammals

Today, mountainous regions are hotspots of mammalian biodiversity, forming patterns known as topographic diversity gradients; however, interactions between mountain uplift and mammalian biodiversity in deep time are poorly understood. I compare the taxonomic diversity of a group of mammals, multituberculates, from two sites from the latest Cretaceous of Wyoming during the uplift of the North American Cordillera. Higher species richness of multituberculates at the mountain-proximal site compared to the mountain-distal site, nearly 400 km to the east, is interpreted as evidence of a topographic diversity gradient. Similarities in faunal composition between mountain-proximal sites across the latest Cretaceous Western Interior of North America suggest tectonic processes as important drivers of mammalian biodiversity and community composition in the past and present.

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Mack Taylor: The Failed Supervolcano Spirit Mountain (Avi Kwa Ame) and The Critical Role of Basalt and Pre-existing Aplite Dikes in Producing Eruptible High-SiO 2 Rhyolite

High-SiO2 (≥76 wt%) rhyolite (HSR) is the most differentiated magma on Earth. Generally restricted to thin (<20 cm) aplite dikes within arc granitoids, voluminous accumulations of HSR in plutonic rocks are rare at subduction zones but do occur in regions of continental extension. Among the few examples known is the Miocene bimodal (granite-diorite) Spirit Mountain pluton of the Colorado River Extensional Corridor, NV. Evidence from the literature shows the ~6 km thick (x ~25 km) pluton assembled incrementally via sheets of low-SiO2 rhyolite over a 2 m.y. interval. It features a ~1.5 km thick leucogranite cap (76-78 wt% SiO2) that zones down through ≥4.5 km of coarse granite into a quartz monzonite, which contains abundant mafic enclaves. Within the coarse granite, discrete silicic segregations and aplite dikes are found with a HSR composition, like the leucogranite cap. However, there are distinct compositional differences, including a relative depletion of middle rare earth elements, reflecting the presence of titanite during melt segregation. This horseshoe REE pattern is not seen in the leucogranite cap, which instead features a seagull pattern, indicating that titanite melted out when it formed. Previous studies have emphasized the role of compaction in a crystal-rich mush to drive segregation of interstitial melts, producing the leucogranite cap and quartz monzonite (cumulate) base. In this study, we test a modification of this model and examine whether the HSR leucogranite cap formed by episodic partial melting of the granitic base, driven by the influx of hot, H2O-rich fluids from deeper, degassed basaltic intrusions. We further explore the role of pre-existing aplite dikes during partial melting of the granitoid host. These eutectic HSR aplite dikes would have melted completely when the host granitoid was only partially molten (with titanite melted out). The initial ascent of the aplite dikes would have drawn voluminous partial melt out of the host granite. We show that resulting molten high-SiO2 rhyolite dikes ≥0.5 m width (exceeding critical widths) could have ascended through several km of sub-solidus granite at ≥ 600°C. Presumably, if an influx of basalt had intruded beneath the leucogranite cap, releasing hot fluid, it could have induced its rapid melting and thus eruption of HSR with a seagull REE pattern.