A MULTI-PROXY ANALYSIS OF NORTHERN PACIFIC STEMSEAS SEDIMENT CORE LITHOLOGY AND PERMEABILITY

In May 2016 the STEMSEAS expedition traveled from San Diego to Honolulu on the R/V Oceanus and recovered two sediment cores. While the purpose of the expedition was for students to experience science at sea, an added benefit was that core samples were available for student research post-expedition. As part of a JMU paleoclimate undergraduate course-based research project, we conducted a multi-proxy analysis of the two cores to: (1) determine the lithological distinctions of sediments from the continental margin and the deeper basin, and (2) determine if the shipboard measured x-ray fluorescence (XRF) Cl signal is a proxy for sediment permeability. Previous research has shown that Cl is a proxy for sediment seawater content (Tjallingii et al 2007). We tested a hypothesis posed shipboard that porewater (seawater content) and the Cl signal increased because of increased permeability.

Sites 1 and 2 have different geologic settings, provenance, and transport mechanisms. Site 1 is 1337 mbsl in the Tanner Basin in the CA Borderlands and Site 2 is 1,070 km from the CA margin at 4458 mbsl. Composition of Sites 1 and 2 were compared using smear slides, binocular microscope examination, and XRD analysis of clays; a comparative grain size analysis was also undertaken. Site 1 is a nanno-rich silty clay that is high in foraminifera and fecal pellets, whereas Site 2 is a red clay with rare biogenic material. The smectite-rich composition may result from river runoff from the Palos Verdes Headlands. In contrast, the fine-grained and illite-dominated composition of sediments at Site 2 are most likely derived from eolian dust originating in Asia.

To address research question 2, we performed grain size analysis of 40 samples using a Laser Diffraction Particle Size Analyzer and compared the results with the shipboard XRF data. We found larger and more uniform grain sizes correlate to higher Cl. Because sediments with larger particles allow more water to penetrate them, we infer that the XRF Cl signal may be used as a proxy for sediment permeability. Higher permeability may be controlled by an increase in the abundance of fecal pellets and/or by submarine sediment transport via turbidity flows to the Tanner Basin. This correlation between the Cl signal and permeability has significant implications for future studies of marine sediments.