Honing in on the climate signal in seafloor topography

The corrugated surface of the seafloor expresses the most areally extensive landform on Earth, known as “abyssal hills”, inherited from when the oceanic crust was created at a midocean ridge spreading center (1, 2) (Fig. 1). The main process is the shifting and rotation of adjacent blocks of crust relative to one another along fault zones predominantly during periods of low magmatic activity, interspersed between times of robust magmatism and the emplacement new crust (1, 3). In the presence of the steady far-field tug of plate tectonic forces, this interplay between faulting and magmatism depends on processes influencing the time dependence of magma generation, storage, and delivery to the surface (4, 5). In PNAS, Huybers et al. (6) argue that one such process originates with the fall and rise of sea level during glacial–interglacial climate cycles. Glacial–interglacial climate fluctuations have been linked to changes in eruptive output at subaerial volcanoes through the changes in the surface loading of ice mass (7–9). There is also mounting evidence in the marine sediment record that magmatic activity at midocean ridges has correlated in time with climate cycles (10, 11). Numerical model simulations predict that changes in sea-level induced pressure variations on the seafloor are transmitted to the underlying mantle where they can lead to fluctuations in the amount of magma produced (10, 12). This can be understood when considering the fact that magmatism at midocean ridges is fed primarily by the pressure-release partial melting that occurs as (hot) mantle rises in response to the spreading of the overlying (relatively cool) lithospheric plates. This continual pressure-release melting stabilizes a layer, ∼80 km in thickness, that is right at its melting-point temperature and pressure. Although the pressure variation due to the estimated ∼100 m of sea level change is relatively small across this layer, the change occurs over geologically short timescales (10° ky to 10 ky), so the rate of pressure change can perturb the flux of decompression melting by an appreciable fraction (∼10%). The search for the hypothesized climate signal in seafloor topography has proven challenging. Goff’s (13) analysis of high-resolution, seafloor topography data collected using shipboard sonar instruments failed to show statistically significant variability in topography with seafloor age near three midocean ridges. These findings indicate that the climate signal—if present—is insufficiently coherent in time in those areas, or is below the temporal resolution availed by the data. Other studies have found undulations in seafloor topography corresponding to the dominant periods associated with Milankovitch cycles of 100, 41, and 23 ky (12, 14–16). Those findings, however, were in a few very localized areas, whereas the climate phenomenon is global and should impact many areas broadly. Furthermore, the reliance on single or few shipboard measurement profiles can be problematic (12) because local variability can cause the results to be sensitive to the choice of profile examined (17). Huybers et al. (6) provide an important advancement by examining >200 shipboard profiles in 17 different areas representing an appreciable range of seafloor spreading environments around the globe. Their results fortify prior findings (18) for the dominant global tendency of the characteristic wavelengths of abyssal hills to decrease with increasing seafloor spreading rate, a relation that opposes the positive correlation expected for climate control and, instead, reflects the first-order controls of lithosphere structure and magma flux as they differ with spreading rate (19) (Fig. 1). However, with more detailed scrutiny, Huybers et al. (6) highlight subsidiary positive trends in abyssal hill wavelengths when the data are grouped by spreading rate. At intermediate (2.3 cm/y to 3.8 cm/y) rates, abyssal hill wavelength tends to increase with spreading rate, with a bestfitting slope close to that expected for the 100-ky Milankovitch period. At fast (>3.8 cm/y) spreading rates, the bestfitting positive slope is close to that expected for the 41-ky Milankovitch period (see also ref. 20). Furthermore, seafloor ages determined from seafloor magnetic isochrons enabled the observations of topography in space to be analyzed as variations in time in the spectral domain (6) . The 100-ky period for seafloor formed at the intermediate spreading rates produced nonconclusive results, perhaps because lithosphere thickness controls on faulting are expected to produce topography at similar frequencies at intermediate spreading rates (17). In contrast, the 41-ky signal identified in the spatial domain near fastspreading midocean ridges is distinguished as a statistically significant peak in spectral power at frequencies near 1/41 ky . The second key advancement Huybers et al. (6) make is a mechanics-based explanation for causes of the observations. Computer calculations simulate the spreading of two lithospheric plates in the presence of episodes of magmatically accommodated extension, interspersed with periods of magmatic quiescence and fault-accommodated extension. For a given spreading rate, when the period spanning the full magmatic–amagmatic cycle is below a