Seismic reflection imaging of thermohaline fine structures in the Southeast Caribbean Sea: Implications for short-term ocean circulation dynamics.

摘要

The ocean conveyor belt is a dynamic system of currents that distributes heat and matter, and is partially responsible for regulating the Earth's climate. Currents circumnavigating the world's oceans are in motion in part through thermohaline circulation (THC), a mixing process driven by density gradients. Unlike wind-driven surface currents, which affect the uppermost ~100 m of the water column and behave fairly intuitively, the underlying THC has historically been more difficult to characterize due to the complex interaction between thermohaline and surface forces that drive it. In fact, the processes that regulate a steady-state THC require both the density gradients and the downward penetration of heat ensured by turbulent mixing in the ocean's interior, which is in turn driven by tidal motions and by the winds. Unfortunately oceanographic measurements cannot distinguish between thermohaline and wind-driven currents, making the THC difficult to measure. The seismic reflection method has proven to be effective at imaging oceanic thermohaline fine structures as variations in density and acoustic velocity. This methodology, known as seismic oceanography, is used here to map the fine structure of the Caribbean Sea using legacy seismic reflection data, and to investigate how thermohaline structures evolve over time. The Caribbean Sea is an integral part of the ocean conveyor belt and a major supplier of nutrient-rich waters to the local ecosystem year round through a system of coastal upwelling. Mapping the boundaries between upwelling fronts and surrounding waters improves our understanding of the mixing processes involved in ocean circulations and helps identify the zones of the highest biological productivity in this region.;In this study I use two ~387 km-long N-S transects that extend from the northern coast of Venezuela into the Venezuelan Basin. Seismic data were acquired twice over the timespan of four days for multichannel seismic (MCS) and wide-angle refraction crustal imaging purposes (MCS and OBS profile, respectively), therefore allowing for a coincident imaging of the thermohaline structure of the Caribbean Sea along the northern coast of Venezuela. Because of the timing and geometry of acquisition, this dataset offers the additional opportunity to perform time-lapse analysis on the oceanic currents internal structure over a maximum period of four-days. Careful data processing reveals remarkably clear reflectors in the shot gathers to a depth of ~1000 m. Coincident thermoprofiles from XBT/XCTD (expendable bathythermograph) casts show a near-steady-state temperature gradient below roughly 1000 m depth, indicating a correlation between the end of the thermocline and the fading of the reflectivity. Along the seismic profile, the reflectivity can be traced throughout the entire ~387 km of MCS and OBS lines, and is characterized by several high amplitude reflectors continuous over distances of ~180 km. Structures observed along the processed, time-migrated and depth-converted profiles are interpreted as evidence of mesoscale downwelling in the form of two cold water filaments traveling northwest from upwelling foci along the Venezuelan coastline through the Leeward Antilles islands and toward the Venezuelan Basin. Subducting water masses sink to the base of the thermocline in a sigmoidal pattern over ~180 km. The initial subduction is likely the result of converging water masses with different buoyancies. Time-lapse analysis indicates that the overall reflective pattern in the Venezuelan Basin remains remarkably stable within the thermocline over ~2-4 days; the individual boundaries of the cold water filaments have apparent vertical velocities of 0-35 m/day, suggesting that filaments have a high degree of spatial and temporal variability.