Observations of long‐duration episodic bottom currents in the Middle America Trench: Evidence for tidally initiated turbidity flows

Abstract

Benthic flow in the Middle America Trench off the Pacific coast of Costa Rica is examined using time series from a single‐point acoustic current meter moored 21 m above bottom at 4386 m depth at the southern end of the trench from November 2005 to April 2007. In addition to significant (∼0.1 m s−1) tidal currents, the instrument recorded a series of 12 episodic northwestward along‐trench flow events of roughly monthly duration. Event velocities often exceeded 0.25 m s−1 and were contemporary with enhanced acoustic backscatter intensity. Events ended with a rapid (<1 day) reversal to southeastward flow and reduced backscatter. Seafloor temperature records from two nearby Ocean Drilling Program (ODP) borehole observatory sites reveal that the flow events were accompanied by a steady rise in bottom water temperature. Temperatures dropped abruptly to background values at the end of each event. The event timing generally tracked the envelope of the tidal current modulation. On the basis of the November 2002 to February 2009 borehole observatory temperature records, the events had a mean duration of 40(±20) days and were separated by a between‐event interlude of 30(±25) days. Findings indicate that the episodic flows were likely rotationally modified, autosuspending turbidity currents initiated by tidal current resuspension of sediments above the shoaling trench floor to the southeast of the mooring site. Suspended particles in the turbidity currents are estimated to range from 0.0003% to 0.006% of the current by volume. Results suggest that tidally induced turbidity currents may be common to steep, well‐mixed regions of the deep ocean adjacent to sediment rich continental margins.