Award Date

12-15-2025

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geoscience

First Committee Member

Matthew Lachniet

Second Committee Member

Michael Nicholl

Third Committee Member

Ganqing Jiang

Fourth Committee Member

Amos Winter

Fifth Committee Member

Haroon Stephen

Number of Pages

235

Abstract

Tropical monsoon rainfall is a key component of Earth’s climate system and supports the livelihood and economy of billions of people. Yet, projections of future monsoons remain highly uncertain, particularly in regions with distinct physiography and complex land-ocean-atmosphere interactions like Central America, where long and continuous high-resolution paleoclimate data to test climate model simulations are still limited. This series of studies aims to reconstruct monsoon dynamics and underlying forcing controls in Central America on multiple timescales by the analysis and integration of stable isotope data in modern and paleo-rainfall records from Guatemala.

In Chapter 2, I present a daily-resolution, two-year-long stable isotope (δ¹⁸O) record of rainfall collected in Cobán, Mayan highlands of Guatemala. The main objective of this study is to identify the controls of rainfall δ 18O to improve the interpretation of speleothems paleo-rainfall records. The isotope data reveal a strong seasonal cycle, with daily δ¹⁸Oₚ showing a weak relationship with local rainfall amount, suggesting that local precipitation intensity is not the main control on isotope variability. In contrast, rainfall δ¹⁸O shows strong correlations with regionalscale precipitation and convective intensity, indicating that upstream rainfall and large-scale circulation exert greater control. These findings support the interpretation of speleothem δ¹⁸O as a proxy for regional monsoon convection and upstream rainfall amount.

In Chapter 3, I present a 140,000-year-long δ¹⁸O record using multiple replicated stalagmites from three cave sites near Cobán, representing the longest high-resolution record from Central America. The data show that Central American monsoon variability was not primarily controlled by orbital precession, which dominates other subtropical monsoon regions. Instead, monsoon intensity in Central America was closely linked to Atlantic Ocean dynamics, and specifically, to changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) and tropical Atlantic-Caribbean sea surface temperatures. Weakened AMOC phases during North Atlantic cold events (Heinrich Events) correspond to reduced regional convection and drier conditions in Central America, while stronger AMOC phases promoted enhanced monsoon rainfall. These findings are supported by isotope-enabled climate model simulations, which validate the dominant role of ocean circulation in modulating Central American rainfall on orbital-to-millennial timescales.

In Chapter 4, I integrate δ¹⁸O with carbon and uranium isotope data from four stalagmites from Grutas del Rey Marcos to reconstruct hydroclimate and environmental response to regional monsoon changes during the last deglaciation. The multi-proxy data reveal that variations in groundwater infiltration and soil productivity were closely tied to shifts in regional monsoon intensity. Drier conditions during Heinrich Stadial 1 and the Younger Dryas are reflected in reduced infiltration and soil moisture, while wetter conditions during the Bølling–Allerød and early Holocene indicate stronger convection, greater water recharge, and enhanced vegetation productivity. Further, carbon isotope values suggest a relatively stable vegetation in the Maya highlands through the deglacial period, in contrast with nearby records from lowland regions, implying a potential role of the Guatemalan highlands as ecological refugia of forested species during dry and cold intervals.

From the results of this dissertation, the Central American monsoon emerges as a thermally-controlled monsoon system in which changes in the Atlantic Ocean thermal states are primary drivers, and orbital summer insolation influence is limited. These findings provide critical insights into how future changes in Atlantic Ocean circulation—and especially AMOC weakening—could potentially impact freshwater availability across Central America, and offer powerful tools for testing regional climate models under different future climate conditions.

Keywords

Atlantic Ocean; Caves; Climate Change; Isotopes; Mesoamerica; Tropics

Disciplines

Climate | Geochemistry | Geology

File Format

PDF

File Size

11700 KB

Degree Grantor

University of Nevada, Las Vegas

Language

English

Rights

IN COPYRIGHT. For more information about this rights statement, please visit http://rightsstatements.org/vocab/InC/1.0/

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