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doi:10.2204/iodp.proc.336.202.2014

Introduction

Oceanic crust cools with increasing distance to the ridge axis, where it is constantly reproduced. Ridge flanks (i.e., the areas between mid-ocean ridges and abyssal plains) are characterized by low-temperature seawater circulation, and it has been estimated that globally ~2 × 10¹⁶ kg of seawater circulates through oceanic crust per year, equivalent to the discharge flux of the global river system (Wheat et al., 2003). Hence, ridge flank systems play an important role in geochemical exchange between crust and seawater.

As the ridge flank crust ages, the seafloor subsides and sediment accumulates. Open circulation of seawater through permeable basement ceases when a continuous sediment layer inhibits access to seawater circulation. However, in areas of rough basement topography, such as slow-spread ridge flank crust, ponds of impermeable sediment form in deep areas that are surrounded by exposures of highly permeable basement, though which ventilation of the oceanic crust by circulating seawater is maintained.

North Pond is such a sedimented deep in which sediment thickness reaches 300 m and is bordered by >2 km high steep slopes of the surrounding rift mountains. Basement drilling and logging revealed that the crust is highly permeable to ~300 m subbasement and conducive to lateral flow of seawater (Becker et al., 2001). Heat flow measurements indicate that seawater may recharge in the southeastern part of the sediment pond and then flows laterally in a roughly south-to-north direction underneath the impermeable sediment lid (Langseth et al., 1992).

Microbial life in sediment has been assigned an important role as a carbon source in the deep sea. Marine sediment covering ridge flanks in oligotrophic parts of the oceans reveals very low rates of microbial metabolism (D’Hondt et al., 2004). Microbial activity in the North Pond area appears to differ between sandy and clayey lithologies (Picard et al., 2011). Also, it has also been noted that oxygenated seawater circulating within the ocean crust at North Pond constitutes a source of oxygen diffusing upward into the sediment pile (Ziebis et al., 2012; Orcutt et al., 2013). The supply of energy and electron acceptors for the sedimentary biosphere is hence related to both subbasement hydrology and sediment lithology, but the detailed relationships between the nature and physical properties of sediment and rates of microbial metabolism remain poorly constrained.

We present semiquantitative geochemical data obtained by X-ray fluorescence (XRF) scanning for three cores recovered during Integrated Ocean Drilling Program Expedition 336 from Sites U1382, U1383, and U1384 as well as for a core from Ocean Drilling Program Leg 174 Site 1074 (Becker et al., 2001). These data provide a cursory classification of abundance and distribution of different sediment types in the North Pond area. A comprehensive assessment of the distribution of the different sediment types in the North Pond area is required to help put results of microbiological studies into a regional lithologic context.

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