Identifying Slope Failure Deposits from a Potentially Mixed Magnetic Susceptibility Signal in Gas Hydrate Bearing Regions

The marine gas hydrate stability zone (GHSZ) occurs in the slope environment along many active and passive continental margins. In this environment, slope failures are common and can occur near the shelf slope break, within submarine canyons, or on the flanks of bathymetric highs, resulting in a spectrum of slope failure deposits from landslides to turbidites. On the Cascadia margin, the GHSZ occurs within the bathymetric thrust ridges and slope basins of the accretionary wedge. Here, the ridges are composed of uplifted abyssal plain deposits associated with submarine fans and/or paleoslope basin deposits formed during the evolution of the accretionary wedge (Johnson et al., 2006; Torres et al., 2008). The adjoining slope basins contain the deposits from slope failure of the ridges. Both ridges and slope basins offshore Central Oregon and Vancouver Island were sampled by drilling during ODP Leg 204 and IODP Expedition 311, respectively (Figure 1). The recovered cores document the distribution and abundance of gas hydrate in these regions within a stratigraphy that is dominated by silt and sand turbidites, debris flows, and intervals of silty clay, separated by hemipelagic clay

Improving CHN measurements in carbonate‐rich marine sediments

CHN elemental analysis is a common and effective method for determining total carbon (TC), total organic carbon (TOC), inorganic carbon (IC) by difference, and total nitrogen (TN) in marine sediments. In this article, we identify and describe three problems related to CHN measurement in carbonate‐rich (\u3e30% CaCO3) marine sediments and suggest mitigation methods to obtain the most accurate measurements of TC, TOC, IC, and TN. These problems involve (1) sample size limitations, (2) incomplete carbonate digestion during in‐situ acid addition, and (3) nitrogen contamination during acid additions. Carbonate‐rich marine sediments often have low TOC and TN content and are susceptible to incomplete combustion, due to their high IC (CaCO3) content, and require careful sample size selection to optimize analysis. Incomplete digestion of carbonate prior to TOC analysis can introduce significant error into the measurement, and here we propose a testing methodology to ensure effective carbonate digestion. Atmospheric contamination of sulfurous acid, used for carbonate digestion, is also observed and modeled, and we suggest steps to minimize this contamination. Overall, our results allow for the development of sediment‐specific protocols for accurate CHN measurements in carbonate‐rich marine sediments