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doi:10.2204/iodp.sp.336.2010

Background

Geological setting

North Pond is an isolated, northeast-trending, ~8 km × 15 km sediment pond located on the western flank of the Mid-Atlantic Ridge (MAR) at 22°45′N and 46°05′W (Figs. F1, F2). This area exhibits normal polarity that has been interpreted as magnetic Anomaly 4 (Melson, Rabinowitz, et al., 1979), suggesting a basement age between 7.43 and 8.07 Ma (using the geomagnetic polarity timescale advised by Cande and Kent, 1995). Sediment thickness ranges up to a maximum of 300 m at the southernmost part of the pond. North Pond is bounded to the east and west by basement ridges as high as 2 km. Two holes cored using the rotary core barrel (RCB) at Site 395 (DSDP Leg 45) penetrated into the easternmost part of the sediment pond (Fig. F2; 22°45.35′N, 46°04.90′W; 4484 meters below sea level [mbsl]). A 93 m thick sediment sequence was cored in Hole 395, consisting of 89 m of foraminifer–nannofossil ooze underlain by 4 m of calcareous brown clays with manganese micronodules. Basement penetration was 91.7 m (Hole 395) and 576.5 m (Hole 395A); a reentry cone and casing to basement were installed in Hole 395A. The basement lithology at Site 395 is dominated by several units of massive and pillow lava flows, typically several tens of meters thick, that are separated by basalt- and serpentinite-bearing breccias (Bartetzko et al., 2001; Melson, Rabinowitz, et al., 1979). The serpentinite breccias result from mass wasting and contain cobbles of gabbro and serpentinized peridotite. A peridotite-gabbro complex several meters thick with brecciated contacts was cored in Hole 395 (Arai and Fujii, 1979; Melson, Rabinowitz, et al., 1979; Sinton, 1978). DSDP Leg 78B (Hyndman, Salisbury, et al., 1984), ODP Leg 109 (Detrick, Honnorez, Bryan, Juteau, et al., 1988), ODP Leg 174B (Becker, Malone, et al., 1998), and the French DIANAUT expeditions (Gable et al., 1992) returned to Hole 395A for logging operations, packer testing, and borehole fluid sampling. Temperature and flow logs indicated rapid fluid flow (~1000 L/h) down into Hole 395A (Becker et al., 1984) and low formation pressures. This flow apparently continued for many years after drilling (Becker et al., 1998; Gable et al., 1992). Despite more than two decades of recharge into and through Hole 395A, the hydrology of the North Pond system has not been significantly affected. Geothermal (temperature and heat flow) surveys indicate that recharge occurs dominantly in the southeastern part of the basin, which is consistent with basement fluid flow generally directed to the northwest (Fig. F3; Langseth et al., 1984).

Comparison of lithologic and downhole electrical resistivity logs for Hole 395A suggest a series of vertically distinct basalt flows (Bartetzko et al., 2001; Matthews et al., 1984). Each flow unit is characterized by an uphole decrease in electrical resistivity and an increase in gamma ray counts. Many of the low-resistivity intervals at the tops of the flow units correspond to recovery of cobbles or breccia, although recovery within the upper few hundred meters of basement was low. These results indicate that breccias developed between major flow units likely have a high present-day permeability. These zones also exhibit high gamma ray counts, suggesting high K and U concentrations indicative of increased oxidative alteration. The correlation between alteration chemistry and permeability indicates that the basalt flow boundaries acted as fluid conduits throughout the hydrological history of the basement at Site 395.

Downhole logging and packer results suggest that permeability at Site 395 decreases below 400 m subbasement (Hickman et al., 1984), where temperature increases. In 1998, bulk density, temperature, and spontaneous potential (SP) downhole logs were collected in Hole 395A during ODP Leg 174 (Becker, Malone, et al., 1998; Becker et al., 1998). SP logs are used in the petroleum industry to infer the locations of intervals within a borehole that receive or produce fluids. Deflections in the SP logs also correspond to the tops of individual resistivity sequences, suggesting that these thin intervals, interpreted independently on the basis of resistivity and lithologic data to have higher porosity and permeability, are indeed the most hydrologically active. The typical ratio in thicknesses of the most and least hydrologically active sections of this borehole is on the order of 1:10 to 1:100, suggesting that most of the fluid that entered the formation surrounding Hole 395A passed through a small fraction of the exposed rock (Bach et al., 2004; Fisher and Becker, 2000).

During Leg 174B, Hole 1074A was cored near the northwestern margin of North Pond (Fig. F2). Temperature and geochemical profiles are diffusive, indicating that there is no upward advection of basement fluids through the sediments, even in an area of local high heat flow (Becker, Malone, et al., 1998). This observation is consistent with the hydrologic model of Langseth et al. (1992, 1984), which indicates that fluid flow is predominantly lateral beneath all of North Pond and recharge/discharge is taking place through basement outcrops that surround the basin. Most of the seawater recharge in Hole 395A is accommodated by aquifers within the uppermost 300 m of basement. Below that depth, temperature increases (Becker et al., 1998; Fig. F2) and borehole fluid chemistry indicates significant chemical exchange with the rocks in the borehole walls (Gieskes and Magenheim, 1992; McDuff, 1984), which indicates a different hydrological regime below 300 m subseafloor that is not an artifact of drilling disturbance of the hydrological regimes.

Site survey data: seismic, bathymetric, heat flow, and sediment coring

Seismic, sediment echo sounding, bathymetry, and heat flow measurements were recorded during R/V Maria S. Merian Cruise 11/1 in February/March of 2009 (Villinger et al., 2010). A 12 kHz swath-bathymetry multibeam echo sounding system (Kongsberg EM120) was used to conduct a detailed bathymetric survey (Figs. F2, AF1). Fourteen seismic lines with spacings between <1 and 3 km were collected across North Pond using a generator-injector gun and a 100 m long, 16 channel streamer. Eleven profiles are oriented southwest–northeast and three run southeast–northwest. All heat flow measurements and sediment gravity coring were conducted on these seismic lines. The seismic two-way traveltime (TWT) was used to estimate sediment thicknesses (Table T1). At the boundaries of North Pond, severe side echoes due to the steep slopes of the bounding basement outcrops deteriorate the record. Identifying the exact basement/sediment interface can be difficult because the rough basement topography does not produce a clear reflection pattern. Migrating the seismic data improves the imaging, and accurate sediment thicknesses for existing drill holes (395A and 1074B) were calculated using a sediment velocity of 1700 m/s. The degree of difficulty in estimating sediment thickness is similar to that at the Juan de Fuca Ridge flank, where experience from ODP Leg 168 and IODP Expedition 301 shows that uncertainties are on the order of ±5 m.

Fourteen gravity cores (up to 9.5 m in length) were collected from North Pond between 4040 and 4480 mbsl and were concentrated in areas of high heat flow in the northern and northwestern part of the basin. The sedimentary sequences recovered are preliminarily interpreted to represent pelagic sedimentation of clay-sized particles interrupted by abrupt deposition of foraminifer sand layers. The presence of sharp, irregular bottom contacts and normal-graded bedding may indicate that these coarse-grained intervals are the result of gravity flows supplied from the surrounding slopes. Consistent with this interpretation, the sand layers are commonly found at the deepest parts of the basin (>4300 mbsl) and are absent in cores retrieved from the less sedimented slopes of the basin.

Microbiological studies of the cores were also conducted. Oxygen is the terminal electron acceptor in all gravity cores and hence provides the most sensitive indicator of microbial activity and fluid flow in the North Pond sediments. Dissolved oxygen penetrated all cores recovered at all coring sites. Several dissolved oxygen profiles appeared to have been affected by a deep secondary source of dissolved oxygen that caused the oxygen profiles to increase toward the base of the core. Variability in flow within the underlying basalt is hypothesized to cause these deeper increases in dissolved oxygen. This effect appears to be greatest in the northern part of the basin, which is why the sites for new CORKs are located there and not in the Site 1074 area, as proposed in the original drilling proposal (Figs. F2, F4).

The supporting site survey data for Expedition 336 are archived at the IODP Site Survey Data Bank.

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