Perspectives

Earth’s sea floor is created along great volcanic ridges that spread at rates of a few centimeters per year beneath every ocean basin. The volcanic and tectonic processes at these mid-ocean ridges ultimately determine the characteristics of the sea floor and the oceanic crust beneath it. These processes occur beneath several kilometers of rock and seawater, seemingly far removed from climatic variations above sea level. Thus, it is surprising that both Crowley et al. (1), in this week’s Science Express, and Tolstoy (2) found ice age periodicity in hilly topography on the flanks of ridges submerged beneath the Southern and Pacific Oceans. After its creation at the mid-ocean ridges, Earth’s sea floor forms the abyssal hills (see the figure). If we could view this terrain without its cover of water, it would appear as gentle rolling hills hundreds of meters high and spaced kilometers apart. The abyssal hills are oriented roughly par-allel to the ridges where they are formed and are carried across the ocean basins by seafloor spreading. Over the course of tens of millions of years, the abyssal hills become buried by sediments, but they are nevertheless observable in bathymetric surveys across vast expanses of sea floor (3). This coverage makes the abyssal hills the most extensive geological feature on Earth, but their remoteness means that they remain largely unexplored and poorly un-derstood. Detailed bathymetric surveys, obtained using so-nar that images a swath of sea floor beneath a ship, have only been obtained for a small portion of the sea floor. Close examination of these images informs our understanding of how the abyssal hills are created. In particular, extensional faults associated with mid-ocean ridge rifting are thought to be important (4). Such faults are observable in bathymetric surveys of newly created abyssal hills near ridge crests and are prevalent in computer models of rifting (5). Comprehensive bathymetry of the Australian-Antarctic ridge south of Tasmania, observed from a South Korean ice-breaker, allowed Crowley et al. to examine an unusually long record of abyssal hill fabric. They found that the abys-sal hills exhibit characteristic spacing of about 23,000, 41,000, and 100,000 years. Working separately but concur-rently, Tolstoy also detected 100,000-year periodicity in the abyssal hill fabric of the East Pacific Rise, a ridge west o