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doi:10.2204/iodp.proc.327.202.2015

Introduction

Marine sediments are the primary long-term reservoir for organic matter (Emerson and Hedges, 1988), and the rates of burial and oxidation of sedimentary organic matter significantly contribute to regulating Earth’s atmospheric oxygen and carbon dioxide concentrations (Holland, 1984; Holland et al., 1986; Berner, 1990). Buried particulate organic matter (POM) in the sediment is mobilized to inorganic compounds such as carbon dioxide and ammonium by aerobic and anaerobic respiration and to dissolved organic matter (DOM) by processes including microbial hydrolysis and anoxic fermentation (Laanbroek et al., 1982; Capone and Klein, 1988; Arnosti et al., 1994; Arnosti, 1995; Fenchel et al., 1998; Burdige, 2002, 2006). An imbalance between the production and consumption of DOM results in dissolved organic carbon (DOC) concentrations in the surface sediments typically an order of magnitude higher than those in the overlying seawater (e.g., Alperin et al., 1994, 1999; Burdige et al., 1999; Papadimitriou et al., 2002). As a result, the flux of DOC from marine sediments to overlying seawater is roughly equivalent to that of riverine input and is a net loss of carbon from sediments to the water column (Burdige et al., 1992, 1999).

In contrast to the numerous DOC profiles published from surficial (<1 m) marine sediments (e.g., Krom and Sholkovitz, 1977; Burdige et al., 1992, 1999; Alperin et al., 1994, 1999; Burdige and Homstead, 1994; Burdige and Zheng, 1998; Burdige, 2002, 2006), only a few studies have examined pore water DOC concentrations within profiles extending to greater sediment depths (Michaelis et al., 1982; Egeberg and Abdullah, 1990; Seifert et al., 1990; Wefer et al., 1998; Simoneit and Sparrow, 2002; Heuer et al., 2009). Although the DOC concentrations within surficial sediments generally increase exponentially with increasing depth from the seawater/sediment interface until reaching an asymptotic maximum, DOC concentrations within deep sediment pore water generally do not demonstrate such systematic variation with depth. At some locations, depth profiles of DOC concentrations are similar to concurrently measured total particulate organic carbon and inorganic carbon profiles (Simoneit and Sparrow, 2002), whereas at other locations significant increases in DOC concentrations have been observed at greater depths due to hydrothermal activity from below (Simoneit and Sparrow, 2002).

The lack of a predictable pattern in deep sediment DOC profiles suggests that the deep DOC cycle may be more complicated than that occurring within surficial sediments. It is also not clear if the impact of hydrothermal activity always results in an increase in sediment pore water DOC concentrations. The fact that DOC concentrations in warm (65°C) basaltic rock basement hydrothermal fluids are lower than those in the bottom seawater (Lang et al., 2006; Lin et al., 2012) suggests that hydrothermally heated basement fluids can be a sink for deep sedimentary DOC (Lin et al., 2012) that may cause sedimentary pore water DOC concentrations to decrease with depth.

Particulate organic matter in deep-sea sediments acts as an important energy source for a diverse array of microbial metabolisms, including aerobic and anaerobic organotrophy using nitrate or sulfate as electron acceptors (Laanbroek et al., 1982; Capone and Klein, 1988; Fenchel et al., 1998). Major electron acceptors in deep sediments, such as oxygen, nitrate, and sulfate, originate from bottom seawater, and the penetration depths of these electron acceptors vary based on their concentrations in overlying bottom seawater, the sedimentation rate, and reactivity of POM (e.g., D’Hondt et al., 2002, 2004; Hartnett and Devol, 2003; Fischer et al., 2009). On the Washington Margin and eastern flank of the Juan de Fuca Ridge in the northeastern Pacific Ocean, sediment pore water oxygen and nitrate is exhausted at a few centimeters below the seafloor (Hartnett and Devol, 2003), and sulfate is significantly depleted relative to seawater at several tens of meters below the seafloor (Elderfield et al., 1999; Wheat et al., 2013). However, below the region of sulfate depletion, sulfate concentrations increase toward the sediment/basement interface due to the supply of sulfate from basement fluids (Elderfield et al., 1999; Wheat et al., 2013). The supply of electron acceptors from the deep basement into the sediment has been suggested to support microbial metabolism within the deep-sediment biosphere (D’Hondt et al., 2004; Engelen et al., 2008). In addition, Grizzly Bare outcrop on the eastern flank of Juan de Fuca Ridge provides a conduit for bottom seawater to recharge and mix with older and more reacted (i.e., anoxic) basement fluids (Fisher et al., 2003; Hutnak et al., 2006; Wheat et al., 2013) and provides additional electron acceptors to basement fluids and subsequently into deep sediments.

Although terrigenous organic matter discharged to the ocean is generally thought to be efficiently remineralized (Keil et al., 1997; Galy et al., 2008), on the eastern flank of Juan de Fuca Ridge an unusually high amount of terrigenous-derived organic matter has accumulated in the deep sediments near the sediment/basement interface (Prahl et al., 1994; Hedges et al., 1999; Dickens et al., 2006). This unique setting provides additional value to the study of terrigenous carbon preservation and degradation. This study provides DOC and POM (carbon, nitrogen, and isotopic composition) profiles at locations where the impact of hydrothermal circulation in basaltic basement on deep sedimentary organic biogeochemistry can be investigated.

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