Date of Award

5-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean Engineering and Marine Sciences

First Advisor

Mark B. Bush

Second Advisor

Robert van Woesik

Third Advisor

Richard B. Aronson

Fourth Advisor

Brooke E. Wheeler

Abstract

The Chocó biogeographic region of northwestern Ecuador is known for its high biodiversity and endemism, yet modern forest destruction has caused it to be listed as a conservation hotspot. The Chocó has received far less scientific study than Amazonia, but it is located in a region that is more directly sensitive to El Niño-Southern Oscillation (ENSO) and lay on Pre-Columbian trade routes. This combination makes it an ideal location to investigate long-term relationships between people and ENSO.

In the Chocó the positive phase of El Niño-Southern Oscillation (ENSO) makes these rainforests even wetter through extreme precipitation and flooding. Despite its importance, high-resolution, long-term, continental records of El Niño and its climatic effects remain rare and are often based on locations that do not lie in the epicenter of ENSO effects. Coastal locations in the Choco have provided a basic timeline for human occupation, but the human history of the hyper-humid Chocó rainforest along the western Andean flank is practically unknown. Consequently, interactions between past climatic changes and human presence and their influence on forest composition are even less well understood. In this study we address the following questions: (1) How and when did El Niño and human influence alter forest composition within the last 5100 years, and 2) Was the ENSO expression coherent between different geographic localities along a Chocó-Andes-Amazonia transect ?

Here, I present the first high-resolution paleoecological record from the Chocó region, obtained from Lake Cube, Ecuador. Sub-annual data used for XRF fluorescence (XRF) geochemical analysis, and high-resolution layer analysis, combined with decadal pollen analysis and fire reconstructions, reveal clear signatures of ENSO variability preserved in the sediments. The observed events closely track established paleo-ENSO reconstructions though because of their consistently high temporal resolution offer new insights. The sediment analysis also revealed human presence and landscape modification around the lake from c. 4200 yr BP onward, with clear phases of occupation and abandonment. Notably, after c.1800 yr BP, we observe an increase in the frequency and variability of El Niño-related signals, coincident with pronounced changes in forest structure and a reduction in the commonly used human-related proxies. We propose that intensified El Niño-driven precipitation and persistently wetter conditions altered human land use. During this period, humans ceased maize cultivation and landscape burning but likely continued managing forest resources, including timber and non-timber products, which resulted in altered forest composition. During colonial times, the forests were used again, and the cumulative legacy effects of past human land use are still clear today with modern forest assemblages noticeably different from the onset of the record.

In a comparative study, I provide three new paleo-ENSO records that when combined with a classic record of paleo-ENSO activity creates a transect of four sites from Amazonia across the Andes to the coastal plain of Ecuador. I find evidence that regional coherence is possible between proxy records of paleo-ENSO variability across the Andes, but that these may be sensitive to chronological uncertainties and depositional processes. Analysis of inferred ENSO frequency using wavelet analysis highlights the strong sensitivity of these results to even minor variations in depositional rates and to artefacts introduced through interpolation. I suggest that erosive events along the Pacific coastal plain reflect wet anomalies associated with El Niño conditions, whereas in western Amazonia the ENSO signal was probably driven by La Niña conditions.

My results highlight the sensitivity of human-related activities to hydroclimatic extremes and emphasize the role of ENSO variability in shaping human settlement dynamics during the late Holocene. Additionally, the results indicate that ENSO variability during the late Holocene is highly non-stationary and spatially heterogeneous.

Download

Available for download on Tuesday, May 09, 2028

Share

COinS