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Author ORCID Identifier

https://orcid.org/ 0000-0002-5257-3463

Date Available

4-30-2026

Year of Publication

2026

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (PhD)

College

Arts and Sciences

Department/School/Program

Earth and Environmental Sciences (Geology)

Faculty

Andrea Erhardt

Abstract

Marine sediments serve as an important geochemical record used to study how the Earth’s climate and oceans have changed throughout geologic time. As such, it is critical to understand how these sediments can be altered following deposition on the seafloor. This dissertation targeted two key geochemical archives, carbonate and pyrite, to link changing burial conditions with the resulting marine sediment chemistry. In particular, primary carbonate minerals record several indicators of past ocean chemistry, while pyrite preserves information related to the redox state of the sediment during deposition and burial. Additionally, secondary processes associated with changes in pore fluid chemistry and redox state can alter sediment chemistry, beginning at the sediment-water interface and extending deep into the buried sediment. This dissertation was divided into three papers addressing these questions at two ocean drilling sites, IODP Site U1153 on the Southern Campbell Plateau and ODP Site 1172 on the East Tasman Plateau. The first study examined how a 26-million-year unconformity at IODP Site U1553 impacted the chemistry of carbonate sediments in the region. The unconformity at this site resulted in the exposure of older sediments to more recent seawater which could lead to alteration of the carbonate minerals. This study used a combination of bulk sediment chemistry, pore water chemistry, and isotopic indicators to investigate the effects of this unconformity on the shallow burial conditions in the region. Pore water analyses showed expanded manganese, iron, and sulfate reduction windows which are attributed to the low total organic content limiting the influence of redox processes on the sediments. The sulfur isotope analyses in the pore water sulfate and carbonate associated sulfate (CAS) showed no correlation. This indicates there is likely little to no interaction between the sulfate in the CAS and pore fluids. This suggests the influence of more recent pore waters intruding into the older rocks is limited and the CAS record is resistant to alteration as burial conditions change through time. The second study built on the first by examining the influence the expanded redox zones had on the deep-burial conditions at IODP Site U1553. The presence of dissolved sulfate at depths >400 mbsf and abundant methane below 520 mbsf suggests the presence of a deeply buried (>500 mbsf) sulfate-methane transition zone (SMTZ). This study used sulfur isotopes in chromium reducible sulfur and bulk sediment geochemistry to identify the modern SMTZ as well as three intervals where the SMTZ was in the past. The apparent upward migration of the STMZ suggests that the changing conditions at the sediment-water interface may play a role in controlling the depth of the SMTZ over time if it impacts the sulfate supply. Deeply buried SMTZs are poorly understood, however, they may play an important role in the carbon and sulfur cycles in marine sediments. The third study examined carbonate sediments and pore waters on the East Tasman Plateau to evaluate how carbonate recrystallization rates vary around the Tasman Gateway. Carbonate minerals are highly susceptible to rapid alteration shortly after burial as the minerals are converted into more stable forms such as low-Mg calcite. This study examined these processes at ODP Site 1172 using the analysis of δ44Ca, 87Sr/86Sr, as well as bulk carbonate sediment and pore water chemistry. Based on the dissolved Sr profile at this site and comparisons with published results from ODP Sites 1170 and 1171, the pore water δ44Ca at Site 1172 should have equilibrated with those in the carbonate sediment around 20-25 mbsf. However, they do not equilibrate until 120 mbsf. This suggests that short-term recrystallization at this site is much slower than anticipated. The Sr composition indicates the longer-term recrystallization rate should align with other sites. This highlights the need for further study of how carbonate minerals are affected by alteration following deposition.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2026.202

Archival?

Archival

Funding Information

This study was supported by the United States Science Support Program in 2020, including salary, travel, and research support. 

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