Chimney Time Series: A Spatial-Temporal Record of Hydrothermal System Response to a Full EPR Eruption Cycle

Two well-documented volcanic eruptions of the EPR ISS (in 1991-92 and 2005-06) bracketed a full volcanic/hydrothermal cycle, and made it possible for the first time to document a complete cycle via time-series observations and sampling. Hence EPR ISS hydrothermal chimney time series samples are a unique natural seafloor experiment on how chimneys grow and petrologically encode into the geologic record a complex spatial-temporal hydrothermal response to a fast-spreading ridge volcanic cycle.

EPR chimney evolution began with formation of “proto-chimneys” (poorly-cemented mineral aggregates a few cm tall) on new basalt flows in 1991.  This was followed by subsequent growth in 1992-2005 of 7-10 m high mineral edifices encased by tubeworms and other organisms, and culminated with volcanic modifications of the chimneys that survived and petrologically recorded the 2005-06 eruption. Changes with time in chimney minerals, porosity, morphology, and substrate characteristics produced chemical and physical evolution of chimney habitats reflected in the faunal succession of organisms inhabiting the chimneys.

Before, during, and after the volcanic eruptions, dramatic changes in hydrothermal conditions were recorded by changes in chimney mineral assemblages, textures, growth features, and fossilized organisms. Changes in the redox state of vent fluids following eruptions were recorded by change of chimney minerals from lower to higher oxidation state. Changes in fluid temperatures and densities (reflecting changes in depth of hydrothermal circulation, phase separation, and near-critical fluid expansion) shifted chimney mineral stabilities/solubilities and altered hydrothermal flow dynamics in ways recorded by chimney mineral composition and growth features. Ponding, draping, and drainback of lava around standing chimneys produced previously unrecognized morphologic and petrologic features of the surviving chimneys.

Because chimney petrologic features continuously recorded hydrothermal system changes between discrete fluid sampling and vent measurements, the chimneys illuminate what transpired during time gaps in other EPR vent observations (for example, the EPR ISS chimneys record fascinating short-term oscillations in redox and in subseafloor hydrothermal fluid-seawater mixing).

Spatial complexity of the hydrothermal system was evident.  Some vents never recorded low oxidation states; some evolved from low to high oxidation states; and others flipped back and forth between these paths. While some chimneys show oscillations in seawater entrainment, others do not. Is this produced by seismicity? Or by oscillations in fluid density (i.e., buoyancy) as fluids penetrate into subcrustal regions where threshold P,T conditions are met for phase separation or fluid expansion near the critical point of seawater? How does seawater entrainment affect organisms living on and within chimneys and within the seafloor? Answers are likely to lie in linkages to other EPR ISS datasets as clues from chimneys are matched in space and time with other observations.