Geochemical evolution of the central Pacific Ocean over the past 56 Myr

Abstract

We measured the elemental concentrations of 69 sediment samples collected at Ocean Drilling Program Site 1215 and employed a suite of statistical methods and elemental ratios to quantify the lithogenic, biogenic, and authigenic/hydrogenous/hydrothermal (AHH) components. We show that the changes in their abundance record (1) northwestward migration of the Pacific Plate, (2) changes in climatic and oceanographic processes associated with the transition from the Eocene “greenhouse” into the Oligocene “icehouse,” and (3) changes in Pacific spreading rates and plate reorganization. The distributions of three separate lithogenic end‐members depend on the latitudinal positioning of the site underneath the different wind belts. Locations north of 22°N received material from Asia, which was transported to the central Pacific via the westerlies. Locations between 20° and 22°N display a silicic volcanic composition and may reflect the boundary between westerlies and northeast trade winds. Locations south of 20°N are geochemically (e.g., Cr/Ta, Sc/Th, etc.) distinct from locations north of 22°N and suggest a North American origin. Rb/Sc significantly increases from values of 2.8 g g−1 at ∼20–22 Ma to 5.7 g g−1 in the modern, recording the development of the east Asian monsoon and formation of Asian loess. During the early to mid‐Eocene (53–54 Ma), when Site 1215 was located at ∼11°–13°N and was above the carbonate compensation depth, export production increased by 40–55% and was ∼25% greater than the modern value at 9°N. The AHH component dominated from late Eocene to the late Miocene and during the late Eocene may have been driven by elevated hydrothermal activity during increased seafloor spreading.