KNOX-02RR: drilling site survey—life in subseafloor sediments of the South Pacific Gyre

Abstract

The middle of the South Pacific Gyre is farther from continents and productive oceanic zones than any other site on Earth. To understand the nature of life in the most oxidized and food-limited subseafloor sediments, Cruise KNOX-02RR surveyed and cored sediment at ten sites throughout the South Pacific Gyre and one site at the southern margin of the gyre. Sediment at all of the South Pacific Gyre sites accumulated extraordinarily slowly for millions of years (0.1-1 m/m.y.).Shipboard scientific results indicate that South Pacific Gyre sediments contain a living community characterized by very low biomass and very low metabolic activity. At every depth in the cored South Pacific Gyre sediments, cell abundances are 2-6 orders of magnitude lower than at the same depths in all previously explored subseafloor communities. The net rate of respiration by the subseafloor sedimentary community at each gyre site is 1-3 orders of magnitude lower than the rates of subseafloor communities at previously explored sites.

Site U1368

“…The calculated spreading rate from the KNOX-02RR magnetic survey suggests the crust was accreted at an ultrafast spreading ridge with spreading halfrates of ~80-85 km/m.y. (D'Hondt et al, 2010;D'Hondt et al, 2011). ).…”

Smectite, illite, and early diagenesis in South Pacific Gyre subseafloor sediment

“…4e) are possibly caused by ammonium oxidation coupled with Fe-reduction in a process known as -feammox‖ or through the oxidation of reduced nitrogen species (Yang et al, 2012). However, because the concentrations of ammonium for both sites were below their limits of detection, the exact cause of variations in the concentrations of nitrate are not determined (D'hondt et al, 2011;Zehr et al, 2011).…”

Site U1365

“…Site U1365 (~5695 meters below sea level [mbsl]) is in the western portion of the South Pacific Gyre within a region of abyssal hill topography trending roughly east-west (085°), with relief ranging from 150 to 200 m ( Fig. F1) (D'Hondt et al, 2010;D'Hondt et al, 2011). The abyssal hill spacing is ~5-8 km with a relatively subdued fabric that has been smoothed by sedimentation.…”

“…Lithologies, biostratigraphy, and many geophysical characteristics of the target site were characterized by earlier studies of nearby Sites 595 and 596 (Menard, Natland, Jordan, Orcutt, et al, 1987) and by the 2006/2007 KNOX-02RR survey expedition (D'Hondt et al, 2011) (Figs. F1, F2, F3, F4, F5, F6).…”