Date of Award

Spring 2024

Project Type

Thesis

Program or Major

Earth Sciences

Degree Name

Master of Science

First Advisor

Joel E Johnson

Second Advisor

Robert Letscher

Third Advisor

William Clyde

Abstract

In deep marine subduction zone settings, the movement of sediment pore fluids, as well as sediment transport and deposition, are likely to occur during earthquake ruptures and leave evidence in the sedimentary record. This seismic activity could influence the depth of the sulfate methane transition zone (SMTZ), a moving diagenetic front that produces sulfide and carbonate minerals through time. One method for the reconstruction of paleo-SMTZ positions in marine sediments is the measurement of downcore bulk sulfur isotopic compositions. Sulfide produced by organoclastic sulfate reduction (OSR) is less enriched in δ34S while sulfide produced by anaerobic oxidation of methane (AOM) at the SMTZ tends to be enriched in δ34S. Determining these zones of enrichment and reconciling them with other depositional and diagenetic characteristics of the sediments could provide a reliable record of paleo-SMTZs through time. In this research, paleo-SMTZ positions are reconstructed in an ~11,000 year marine sediment record found in a series of 40 m sediment cores collected along the Japan Trench. These cores were taken during IODP Expedition 386 Japan Trench Paleoseismology, which is focused on developing a long timescale paleoseismic record within the Japan Trench subduction zone. To determine the total sulfur (TS) content and sulfur isotopic signatures throughout the Japan Trench sediment record, an elemental analyzer and mass spectrometer were utilized to measure a high resolution (1 sample/meter) downcore record of bulk sediment TS at 15 sites and δ34S at 6 sites. These results were examined with previously determined IODP Exp. 386 bulk sediment TS, bulk sediment total organic carbon (TOC), porewater sulfate, porewater methane, porewater barium, core sedimentology, and an age model derived from biostratigraphy, ash chronology, and magnetostratigraphy. Enrichments in δ34S along the Japan Trench at 6 of 15 sites indicate that multiple paleo-SMTZ are preserved in these records and there is an apparent decrease in paleo-SMTZs from south to north during the last ~6,500 years, with a variation in spacing between the past locations of these diagenetic fronts. The causes of these variations are likely due to differing organic carbon and methane availability at each site along the trench. These availabilities are, in turn, influenced by changes in climate or depositional environment, which can include impacts from earthquakes. The number of interpreted paleo-SMTZ positions and interpreted earthquake related event deposits are the same in 4 of the 6 sites where sulfur isotopes were measured, suggesting that the delivery of sulfate via sedimentation plays a key role in the development of SMTZs through time in the Japan Trench. Reconstructing paleo-SMTZ locations within the Japan Trench and in other subduction zone settings allows us to determine the evolution of these diagenetic fronts through time and their potential linkages to earthquake driven event deposition. Ultimately, this effort may help us more fully understand the sedimentary response during earthquakes and further test and refine paleoseismic records based on other proxies.

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