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doi:10.2204/iodp.proc.336.107.2012

Introduction

Thousands of sediment ponds exist on the flanks of mid-ocean ridges where depressions in the young oceanic crust serve as catchment areas for pelagic sediments. They have been studied for more than three decades because they are the only places close to the ridge crest where sediment cores contain temporal information on climate and magmatic events of the close-by ridge. These ponds also play a major role in the exchange processes between ocean and upper young crust because they act as barriers due to their very low permeability in comparison with the upper crust (Langseth et al., 1984).

North Pond, located 140 km west of the Mid-Atlantic Ridge (Fig. F1) and 50 km south of the Kane Fracture Zone (22°–23°N, 46°W) in a water depth of ~4300 m, is probably the most investigated sediment pond in the world (Purdy et al., 1979). Its geochemistry, hydrology, and geologic setting have been studied intensively by a series of Deep Sea Drilling Program (DSDP) (Legs 45 and 78B) and Ocean Drilling Program (ODP) (Legs 109 and 174B) drill holes and seafloor observatories. The area of the roughly northeast–southwest elongated pond (Fig. F2) is relatively small, with a size of 8 km × 14 km. Crustal age at the North Pond location is 7.3 Ma (Hussong et al., 1979).

During Leg 78B, downhole temperature logs in DSDP Hole 395A showed an almost isothermal temperature profile to ~250 meters below seafloor (mbsf) (Langseth et al., 1984). Downhole pressure measurements showed subhydrostatic pressures between 90 and 150 kPa at 179 mbsf. Both observations were explained by Langseth et al. (1984) with a hydrogeological model (Fig. F3), where cold seawater enters the unsedimented basement (in the southeast) outside of the pond, warms up while flowing through the uppermost layers of oceanic basement (Layer 2A) beneath the pond, and escapes as warm water on the other side (northwest). This process results in subhydrostatic pressures in Hole 395A.

During a site survey cruise (Atlantis II 123-2, 1989) for a planned ODP drilling leg in North Pond, Langseth et al. (1992) measured seafloor heat flow within the pond. Their results show that the average heat flow at North Pond is only 20%–25% of the value calculated on the basis of a conductive lithospheric cooling model (Parsons and Sclater, 1977). Moreover, Langseth et al. (1992) found high heat flow in the northwestern part of the pond (near ODP Site 1074) and low values in the southeastern part (near Site 395). Their results are in good agreement with the data from Hussong et al. (1979) and with the suggested hydrogeological model that would also explain the overall low heat flow because this circulation transports heat out of the basement and thus increases cooling of the crust.

During Leg 174B, Hole 395A was equipped with an instrumented borehole seal, now known as a subseafloor borehole observatory (CORK) (Davis et al., 1992), and an additional hole (1074A) was drilled at the northwestern rim of the pond. A comprehensive overview of all investigations in North Pond is given by Becker et al. (2001). Oxidative weathering of oceanic crust occurs predominantly in the first 10–20 Ma of its crustal age, and the role of microbial activity in these processes is still not clear (K. Edwards, pers. comm., 2009). Therefore, a group of microbiologists and geochemists submitted a drilling proposal to the Integrated Ocean Drilling Program (IODP) (K. Edwards et al., pers. comm., 2009) to address two key questions: (1) where do deep-seated microbial communities come from, and (2) what is the nature of microbial communities hosted in young ridge flanks and what is their role in ocean crust weathering? North Pond, with its established hydrothermal flow pattern in the upper crust, is a perfect place for investigations about the magnitude and activities of basement microbial communities.

In order to better define the geometry and the geothermal regime of North Pond, a geophysical site survey cruise took place on the German research vessel R/V Maria S. Merian (MSM11/1; 17 February–12 March 2009). In addition, extensive sediment sampling with a gravity corer was undertaken to collect pore water and microbiological samples. The initially planned surveys could not be completed because the available working days were cut in half due to an accident on board. In particular, the geophysical survey of the southern part of North Pond is missing; therefore, we could not increase the heat flow coverage as originally planned. Moreover, additional seismic lines with a different source and different orientation of the profiles had to be skipped. However, sufficiently dense coverage with geophysical data enabled us to position the planned drill holes within the pond, and the collected cores allow assessing basic (bio)geochemical and microbial processes within the sediments. The goal of this report is to present all geophysical methods used and results in detail.

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