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doi:10.2204/iodp.proc.331.202.2015

Introduction

During Integrated Ocean Drilling Program (IODP) Expedition 331, several sites were drilled within the Okinawa Trough region where the Iheya North hydrothermal field is located. This report focuses on two sites located east of the main hydrothermal vent field, which is an active hydrothermal venting region (Takai et al., 2011; see the “Expedition 331 summary” chapter [Expedition 331 Scientists, 2011a]).

Site C0014 is located ~450 m east of the main vent field and is a dynamic environment with pore fluid chemistry suggestive of a hemipelagic sediment cover overlying a diagenetically and hydrothermally altered sediment package containing volcanic sediments, hydrothermally altered muds, and pumiceous sediments (Takai et al., 2011). Temperatures increase from bottom water values to ~145° at 47 mbsf with an abrupt increase to >200°C at 50 mbsf (see the “Site C0014” chapter [Expedition 331 Scientists, 2011b]). For Hole C0014G, the hole with the most continuous sampling through the sediment column and the hole for which we have the most complete data set, the upper ~20 m is rich in dissolved sulfate with a decrease to near zero at ~30 mbsf (Takai et al., 2011). Within this upper, sulfate-rich package, dissolved Mg is close to the seawater value with a decrease to values <5 mM by ~40 mbsf (Takai et al., 2011).

Site C0017 is located farther east, roughly 1.6 km from the Iheya hydrothermal vent field. This site has the deepest penetration of the holes drilled during the expedition. Similar to Site C0014, Site C0017 is primarily a site of mixed sediment types with thick hemipelagic mud and various volcaniclastic sediment types. Temperature measurements indicate that the sediment column reaches ~90°C by ~150.7 mbsf (see the “Site C0017” chapter [Expedition 331 Scientists, 2011c]). This site, however, lacks the dramatic and larger scale hydrothermal alteration seen at Site C0014. Consistent with this contention, the pore fluid chemistry of the uppermost 25–35 m is dominated by more typical hemipelagic diagenetic reactions (Takai et al., 2011). Below this depth, low-temperature reactions continue to dominate the pore fluid chemistry (Takai et al., 2011).

For both of these sites, lateral fluid flow has been inferred (Takai et al., 2011). From the perspective of this report, it is important to note that Site C0014 exhibits clear evidence for active high-temperature alteration, whereas Site C0017 does not. Both of these systems also have physical and chemical features that imply a dynamic and perhaps nonsteady-state environment where any possible fluid flow is likely to be three dimensional (Takai et al., 2011).

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