Do sea level variations influence mid-ocean ridge magma supply? A test using crustal thickness and bathymetry data from the East Pacific Rise

Abstract

Recent studies suggest that eustatic sea level fluctuations induced by glacial cycles in the Pleistocene influence mantle-melting and volcanic eruptions at mid-ocean ridges, with models predicting variations in oceanic crustal thickness and seafloor bathymetry linked to sea level change. Analyses of seafloor bathymetry have found evidence of significant spectral energy at frequencies consistent with Milanković cycles of 1/23, 1/41, and 1/80-1/120 ka−1. However, other studies emphasize the need for crustal thickness observations to test the “sea level hypothesis”.Here we investigate the hypothesis of climate driven periodicity in mid-ocean ridge magmatism through analysis of a unique bathymetry and crustal thickness dataset derived from a 3D multi-channel seismic investigation of the East Pacific Rise from 9°42' to 57′N. Crustal thickness data spans the last ∼235 ka in age and reveals three axis-parallel zones of 200-800 m thicker crust. The amplitude and spacing of these thick crust ridges, which are most prominent on the east flank of the ridge, are consistent with predictions of sea level modulated mantle melting. Similarly spaced ridges are apparent in the longer duration (470 ka) seafloor bathymetry data. Spectral analysis of these datasets shows peaks centered near 1/80 ka−1 and locally near 1/41 ka−1 on the east flank in both bathymetry and crustal thickness. West flank spectral results show intermittent peaks near 1/100 ka−1 and 1/41 ka−1 in crustal thickness and no coherent peak frequencies in bathymetry data. We attribute differences between the east and west flank to the impacts of spatially variable asymmetric spreading and small changes in the locus of accretion. Observed half spreading rates are dominantly faster to the east with small ridge jumps transferring crust from the west flank. Lagged cross-correlations between sea level and crustal thickness indicate a maximum when the latter is lagged by ∼45 ka, which align thick crust zones with the ∼100 ka periods of lower sea level. Crustal thickness is also directly compared with seafloor bathymetry, indicating a component of compensated topography with RMS relief at the seafloor of 10 to 29% of crustal thickness variations. However, complexity inherited from variable asymmetric spreading and seafloor faulting is also apparent, and results provide new insights into how the crustal accretion filter modulates the recording of magma supply variations in the crust and in seafloor relief.While the significance of the statistical analysis of these ridge records is limited by the short duration of the available crustal thickness dataset and effects of asymmetric spreading, the novel observations of crustal thickness varying at timescales of ∼80-100 ka require a mechanism, and the sea level hypothesis provides a plausible explanation.