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doi:10.2204/iodp.sp.329.2010

Operational risks

There are no geologic or man-made hazards to drilling near any of the proposed sites. No hydrocarbons are expected at proposed Sites SPG-1A–SPG-11B because sediment drilling targets are <94 m thick, the flux of organic matter to the seafloor is extraordinarily low (Jahnke, 1996), the geothermal gradient is far too low to reach the thermal maturation window at any site (heat flow = 18–93 mW/m²), and the sediment temperature is <5°C at all sites and sediment depths (R. Harris, pers. comm., 2007). Additionally, the sediment is likely to be oxic throughout the entire column at proposed Sites SPG-1A–SPG-11B. At proposed Site SPG-12A, very low hydrocarbon concentrations are expected. Subseafloor respiration at this site is predominantly anaerobic, and we predict methane concentrations similar to those at Site 1231 (<2 ppm headspace concentration after 20 min at 60°C), which is characterized by similarly low rates of subseafloor respiration (D'Hondt et al., 2003). As at Site 1231, the flux of organic matter to the seafloor is relatively low at proposed Site SPG-12A (Jahnke, 1996), and the geothermal gradient is far too low to reach the thermal maturation window in the 122–130 m thick seddiment.

At all sites, we will follow the standard shipboard safety protocols of measuring methane/ethane ratios and monitoring total dissolved gas content.

Weather and sea hazards

Expedition 329 has been scheduled to take place during the early part of the Southern Hemisphere summer, when weather and sea state are calmest. Significant wave height varies considerably from summer to winter and increases greatly with latitude (Fig. F8A) (National Imagery and Mapping Agency [NIMA], 1998). The northern limit of Antarctic icebergs approaches proposed Site SPG-12A in Southern Hemisphere spring (September–November) (NIMA, 1998).

Wind force (Beaufort scale) is ~4 (11–15 kt) at proposed Sites SPG-1A–SPG-7A, 4–5 (11–20 kt) at proposed Sites SPG-9A and SPG-10A, and 5–6 (16–26 kt) at proposed Sites SPG-11B and SPG 12A (Fig. F8B) (NIMA, 1998). Particularly at the southern sites, wind force is greater in Southern Hemisphere winter than summer.

Maximum drift is higher at poleward proposed Sites SPG-9A–SPG-12A (10–15 nmi/day) than at equatorward proposed Sites SPG-1A–SPG-7A (5–10 nmi/day) (Fig. F8C) (NIMA, 1998).

High-wind tropical storms occur occasionally in the region of our most northwesterly sites (proposed Sites SPG-1A–SPG-4A) (Fig. F9) (Hurricanes, Typhoons, and Tropical Cyclones Worldwide, www.solar.ifa.hawaii.edu/Tropical). In 15 years (1994–present), 20 named storms passed through the region of proposed Sites SPG-1A–SPG-4A in the January–March window and 1 passed through the region of proposed Sites SPG-1A–4A in the November–December window. During these events, wind speed was typically 20–40 kt in this region and only very rarely exceeded 40–60 kt. Storm incidence rapidly decreases eastward along the line of drill sites; in 15 y, most high-wind storms occurred west of all the drill sites and only a couple occurred east of proposed Sites SPG-1A and SPG-2A.

Given the seasonal variation in significant wave height, wind force, and high-wind tropical storms, the ideal drilling window is November–December.

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