A Proposed Formation of Unique Crystallographic Ikaite as a Key Proxy for Identifying Frigid Ancient Climates in Creede, Colorado During the Late Oligocene

Romly, Amir

Undergraduate Honors Thesis

A Proposed Formation of Unique Crystallographic Ikaite as a Key Proxy for Identifying Frigid Ancient Climates in Creede, Colorado During the Late Oligocene Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/tb09j736b

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Abstract

The metastable pseudomorph of calcium carbonate known as ikaite (CaCO3.6H2O) is a reliable tool for recognizing frigid ancient climates. Its recognition is aided by the unique guttulatic structure. This structure appears as ellipsoidal overgrowths surrounding pseudohexagonal to ellipsoidal cores. These cores are surrounded by more Mg-rich sparry or micritic cement. This study employs Transmitted Light (TL), Cathodoluminescence (CL) with Plane Polarized Light (PPL) and Cross Polarized Light (XPL), alongside Isotope δ¹³C ‰VPDB and δ¹⁸O ‰ VSMOW analyses. These are used to investigate the crystallography of ikaite. The aim is to discern the frigid ancient climate of Creede, Colorado during the Oligocene period. A key focus is on identifying the guttulatic crystal structure of ikaite. Traditionally, it has been described as possessing a sole pseudo-hexagonal core with overgrowth in various cold locations. However, our examination reveals core variations in Creede. These include tear-drop, oval, triangular, circular almond, and irregular rounded corner shapes. This indicates the presence of guttulatic structures as the second most abundant form. Furthermore, our analysis of the water chemistry of ancient Lake Creede suggests complexities in interpretation. We observe a dark core of ikaite, identified as vaterite. We hypothesize its formation in a highly oxygenated environment. Subsequent overgrowth with lighter orange coloration suggest a transition to a low-oxygen environment. Macroscopic observations of ikaite pseudomorphs, specifically glendonite formation, offer insights of environmental conditions. The transformation of guttulatic crystals into stable vaterite hexagonal cores with overgrowth, accompanied by sparry calcite filling, suggests warming and loss of water. Calcite with cleavage angles exceeding 78° and 102° indicates temperatures surpassing 8°C of the stability range of ikaite. While a dislocation of guttulatic crystal is hypothesized to be deformed during uplift event such as the Neogene uplift and Rio Grande incision. Analysis of δ¹³C and δ¹⁸O values indicating a cold-water environment during the Oligocene. Low δ¹³C values imply organic carbon incorporation, possibly influenced by decaying organic matter in mud deposits. Higher δ¹⁸O values suggest colder climate conditions, indicative of a cold and dry evaporative ancient Creede Lake approximately 26.9 million years ago. This comprehensive approach enhances our understanding of Creede's paleoclimate and geological evolution.

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