Earth and Planetary Sciences ETDs

Phil Ragonese

Publication Date

7-1-2014

Abstract

Decades of study on Phanerozoic sedimentary deposits indicates that changes in Earth's orbital parameters are one of the main drivers of climate change on the 104-105 yr time scales, however understanding the relationships between climate change on coeval marine and continental deposits is difficult due to post-depositional erosion and tectonic processes. Nd isotopes measured in marine carbonates are a proxy for regional-scale continental weathering flux (CWF) and can help resolve these difficulties because its short residence time (less than ocean mixing times). This study focuses on cyclic Middle Pennsylvanian (Desmoinesian) marine successions from the Pedregosa (Arizona) and Bird Spring (southern Nevada) basin to address the interbasin and intrabasin response of Nd. Two types of orbital-scale subtidal cycles (1-7 m thick) are observed. Transgressive-regressive (TR) cycles (44%) are characterized by basal transgressive shallow subtidal deposits, deep subtidal deposits and capped by shallow subtidal deposits. Regressive (R) cycles (56%) are composed of basal deep subtidal deposits capped by shallow subtidal deposits; approximately 55% of T-R and R cycle caps show evidence of subaerial exposure. Due to the interpreted large magnitude sea-level changes and the position of study areas along the inner/middle platform, the cycles are dominated by highstand (or interglacial) deposition. Seventeen cycles were sampled (<0.5 m sample resolution) for Nd-isotope analysis of whole rock limestones. єNd values for all three locations range between -12.3 to -5.95 and two contrasting orbital-scale єNd trends are recognized. Trend 1 (30%) is characterized by low єNd (high CWF) during sea-level highstands (interglacial stages) and trend 2 (45%) is defined by high єNd (low CWF) during sea-level highstands (interglacial stages). The absence of fluvial deposits within either studied basin combined with previous reports of thick Pennsylvanian loess deposits throughout the U.S. Southwest suggests an eolian source for the observed CWF. If an eolian interpretation is correct and we utilize recent climate modeling results for the late Paleozoic from Horton et al. (2012), which incorporates glacial-interglacial pCO2 changes which influences the position of the intertropical convergence zone (ITCZ) and low-latitude precipitation changes, then cycles that record high CWF (low єNd) during interglacials (trend 1) represents drier/more windy climates in upwind source areas, whereas low CWF (high єNd) during interglacials (trend 2) implies wetter/less windy climates in upwind source areas. This study illustrates the use of Nd isotopes in marine carbonates as novel proxy for detecting orbital-scale CWF changes in ancient marine systems.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Fawcett, Peter

Second Committee Member

Asmerom, Yemane

Language

English

Keywords

Nd Isotopes, Geochemistry, Carbonates, Stratigraphy

Document Type

Thesis

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