Previous   |    Next

doi:10.2204/iodp.pr.336.2012

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 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 of Cande and Kent (1995). The sediment cover is as thick as 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. During DSDP Leg 45, two holes were cored with the rotary core barrel (RCB) at Site 395, penetrating the southeastern 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 (Melson, Rabinowitz, et al., 1979). 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 this site is dominated by several units of massive and pillow lava flows (typically several tens of meters thick) that are separated by sedimentary breccia units, which resulted from mass wasting and contain cobbles of gabbro and serpentinized peridotite (Bartetzko et al., 2001; Melson, Rabinowitz, et al., 1979). A peridotite-gabbro complex several meters thick with brecciated contacts was cored in Hole 395 (Arai and Fujii, 1978; Melson, Rabinowitz, et al., 1979; Sinton, 1978).

Several expeditions have revisited Hole 395A for logging operations, packer testing, and borehole fluid sampling, including 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). Temperature and flow logs acquired during Leg 78B indicated rapid fluid flow (~1000 L/h) into Hole 395A (Becker et al., 1984) and low formation pressures, and 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 (Langseth et al., 1984).

Comparison of lithologic and downhole electrical resistivity logs for Hole 395A suggests a series of 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 of the uppermost few hundred meters of basement was low. These results indicate that breccias developed between major flow units likely have 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 meters subbasement (msb) (Hickman et al., 1984), where temperature increases. In 1998, bulk density, temperature, and spontaneous potential (SP) downhole logs were collected in Hole 395A during Leg 174B (Becker, Malone, et al., 1998; Becker et al., 1998). The SP log is used in the petroleum industry to infer the locations of intervals in a borehole that receive or produce fluids. Deflections in the SP log 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 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 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 in the uppermost 300 m of basement. Below this depth, temperature increases (Becker et al., 1998) and borehole fluid chemistry indicates significant chemical exchange with the rocks in the borehole walls (Gieskes and Magenheim, 1992; McDuff, 1984), which indicates a hydrological regime below 300 msb that is less permeable and conducive to seawater circulation.

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 and Cruise Participants, 2010). A 12 kHz swath-bathymetry multibeam echo sounding system (Kongsberg EM120) was used to conduct a detailed bathymetric survey. 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 of these profiles are oriented southwest–northeast and three run southeast–northwest. All heat flow measurements and sediment gravity coring were conducted on these seismic lines. Seismic two-way traveltime was used to estimate sediment thickness. The seismic record is poor at the boundaries of North Pond because the steep slopes of the bounding basement outcrops caused severe side echoes. In addition, it can be difficult to identify the exact basement/sediment interface 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) can be calculated using a sediment velocity of 1700 m/s.

Fourteen gravity cores (up to 9.5 m in length) have been collected from North Pond between 4040 and 4480 mbsl in areas of high heat flow in the northern and northwestern part of the basin. Preliminary interpretation is that the recovered sedimentary sequences represent pelagic sedimentation of clay-size particles interrupted by abrupt deposition of foraminiferal 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, 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.

Oxygen is the terminal electron acceptor in all gravity cores and hence the most sensitive indicator of microbial activity and fluid flow in the North Pond sediments. Dissolved oxygen permeates all of the cores recovered at all coring sites. A number of dissolved oxygen profiles appear to be affected by a deep secondary source of dissolved oxygen, causing them to increase toward the base of the core. Flow variability in the underlying basalt is hypothesized to cause these deeper increases in dissolved oxygen. This affect appears to be greatest in the northern part of the basin, which is why the sites for new subseafloor borehole observatories (CORKs) were located there and not in the Site 1074 area, as proposed in the original drilling proposal.

Top of page   |    Previous   |    Next