Explosive submarine volcanism during the penultimate deglaciation: New results from the Pacific Antarctic Ridge

Abstract:

Ice sheet growth and falling sea level may promote submarine volcanism by releasing pressure on mid-ocean ridges and enhancing melt production in the upper mantle. Records of ridge flank bathymetry display Milankovitch-scale periodicities in abyssal hill spacing (1, 2) but interpretation of the bathymetric data remains controversial (3, 4). Alternative proxies are therefore necessary to determine whether sea level modulates mid-ocean ridge magmatism. Recently published sediment records suggest that hydrothermal particle flux along the East Pacific Rise varies on glacial-interglacial timescales (5). Enhanced hydrothermal activity coincides with the last two deglaciations (Terminations 1 and 2), implying that heat and carbon output from the mantle may act as a negative feedback on ice sheet size. Here we present results from a well-dated sediment core at 39ºS, 111ºW on the Pacific Antarctic Ridge (PAR) that document a 10 cm thick ash layer that coincides with Termination 2 (T2). The ash shards have 1) MORB composition, and 2) angular and curved fluidal morphologies that are typical of explosive submarine volcanism (6). The ash layer was deposited when the core site was 8 km from the PAR axis, suggesting T2 was an unusually vigorous interval of volcanism that lofted pyroclastic debris high into the water column with subsequent fallout to distal ridge flank locations. Because the ash layer is interspersed with foraminifera that document the oxygen isotopic transition from a glacial to interglacial state, it is likely the product of multiple eruptions over 10 kyr rather than a single large event that was subsequently altered through bioturbation. Additionally, the median grain size of the ash shards is 125-250 μm regardless of stratigraphic position, consistent with distal fallout from events of similar magnitude. The necessary plume height to create the T2 layer will be discussed in the context of published volcaniclastic settling rates and deep ocean current velocities for the Southeast Pacific.

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