Dissertations, Theses, and Capstone Projects

Date of Degree

2-2024

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor

Stephen F. Pekar

Committee Members

Ceclia M. McHugh

Jacob Mey

Subject Categories

Environmental Sciences

Keywords

100 kyr Cycles, Caribbean Carbonate Crash, Central American Seaway, Great American Biotic Interchange, Negative Carbon Isotope Excursion, North Atlantic Deep Water

Abstract

The state of the middle late Miocene global ocean circulations which regulated climate and affected deep ocean chemical environments were impacted by geographic changes. During the Miocene, cirum-equatorial circulations slowly decreased, which included through the seaway between the Americas called the Central American Seaway (CAS). Its shoaling and ultimate closing led to the onset of modern day oceanic circulation as characterized by the global thermohaline circulation. However, this seaway has a long hotly debated history concerning the timing of its restriction and shallowing owing in part to the shifting of tectonic plates.

The goal of this research focuses on investigating the shoaling history of the CAS between 12.1 and 9.3 Ma, a critical time of this seaway as new studies suggests that major changes occurred in the CAS that permanently altered global oceanic circulation closer to the modern state that exists today. In addition, carbonate crashes observed in various ocean basins, including the Caribbean, a newly discovered negative carbon isotope excursion, carbon isotope maxima events, and δ18O Mi events that occurred during this time are also investigated and are shown to have common causal mechanisms. This was accomplished by developing high v resolution (~4 ky) integrated data sets (eg., δ18O, δ13C, CaCO3 wt %, benthic planktic foraminiferal ratios, and percent sand size fraction) at ODP Site 1000 located in the western Caribbean in 916 m water depth. From these new data sets a new picture of the middle late Miocene transition emerges.

The first important conclusion was to identify a new shoaling event for the CAS at 11.6 – 11.4 Ma. As the CAS shoaled and constricted, less Pacific sourced waters entered the Atlantic as reductions in Atlantic sourced waters entered the Pacific. The result was the occurrences of increased Northern Component Waters (i.e., Proto-North Atlantic Deep Water). After this event, the first significant basin to basin fractionation occurred as observed in the δ13C records between Site 1000 and Pacific ODP Site 1146 and Indian Ocean IODP Site U1443. This was interpreted as a slow down of deep ocean ventilation that was coeval with the timing of the shoaling of the CAS.

A second shoaling event of the CAS is documented at 10.8 Ma and is concomitant with a global negative carbon isotope excursion (NCIE) first identified at the Pacific ODP Site 1146. This event has only been firmly identified at Site 1000 and is evidenced in Indian Ocean Site IODP U1443. This allowed positive correlation to the second shoaling event. Importantly, the NCIE occurred coeval with the timing of the documented shoaling of the CAS to ~50 – 200 m based on neodymium isotopic and modeling data. The importance of these findings infers the timing of when the reorganization of global ocean circulations finally approached the modern patterns of today. This study also identified a NCIE, dubbed the NCIE 2, occurring at ~9.8 Ma.

This study also investigated and reexamined the Caribbean carbonate crash. Analyses of the original data recognized the timings of carbonate dissolutions in intermediate and deep water were asynchronous. This is ascribed to the shoaling of CAS affecting deep water masses first vi followed by intermediate waters. The high resolution data from this study indicates that the carbonate crashes occurred owing to changes in Caribbean water masses at 100 ky shorteccentricity orbital timescales as well due to the long term shoaling of the CAS. Finally, correlating the data from Site 1000 to sites in other ocean basins shows that the shoaling had a direct impact on global ocean circulation changes experienced during this time interval and may have possibly had a role to play in causing the global carbonate crash during this interval.

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