Characterizing the ocean with acoustic waves

Seismic oceanography as remote sensing of the ocean structure by multichannel reflection seismic method can provide high-resolution images enabling the study of fine-scale ocean processes along large distances.

The seismic acoustic response depends on differences in ocean temperature and salinity, and the resulting seismic images track the interfaces between those thermohaline layers both laterally and in depth. The structural interpretation of observed seismic reflections provides valuable oceanographic insights to understand mixing processes and phenomena occurring at different water column depths.

Three parallel 2D multichannel seismic reflection profiles acquired by the Portuguese Task Force for the Extension of the Continental Shelf in the Madeira Abyssal Plain (MAP), profiles covering 300km and ~100km apart from each other, dating from 2006, were processed to enhance the amplitudes of the water column (Azevedo, L. et al., 2021) and analyzed jointly with conductivity-temperature-depth probes (CTDs) from 2002 and 2005 acquired by Poseidon research vessel.

The structure of the water column in this area is characterized by the intrusion of Mediterranean Outflow Waters (MOW), warmer and salty water mass expressing between the 500 and 1500 m depth, and overlaying Subarctic Intermediate Water where temperature and salinity decrease in depth. Due to the differences in temperature and salinity gradients, the MAP region is auspicious for developing double diffusion, specifically thermohaline staircases (van der Boog, C. et al., 2021). Double diffusion is shown to influence the efficiency of vertical mixing of the different water masses; it affects the vertical transport of nutrients, temperature, and salt and contributes to ocean circulation, which is intrinsically connected to the control of the earth’s climate. Nevertheless, it is still lacking information.

We detected the thermohaline staircases expression in temperature and salinity profiles plotted as a function of depth, noticing that the interfaces of mixing followed by layers of well-mixed temperature and salinity are well defined as a step structure and were validated as double diffusion by calculating the Turner angle and Density Ratio at those depths.

Simultaneously, the seismic profiles are characterized by continuous sub-horizontal reflections between the ~1200 to 2000 meters of depth. By correlating the CTD profiles with the seismic images, it is noticeable that the staircases on the vertical profiles correspond to the reflections on the seismic at the expected depths and are covering almost the entirety of seismic profiles.

Since those reflections are present in the three parallel seismic profiles, we use them to predict the lateral continuity of the step-like structures and build models of the incidence of double-diffusive thermohaline staircases in the region, contributing to the knowledge of those processes' extension and expression in the Madeira Abyssal plain.

References:

van der Boog, C. G., Dijkstra, H. A., Pietrzak, J. D., & Katsman, C. A. (2021). Double-diffusive mixing makes a small contribution to the global ocean circulation. Communications Earth & Environment, 2(1), 1-9.

Azevedo, L., Matias, L., Turco, F., Tromm, R., & Peliz, Á. (2021). Geostatistical seismic inversion for temperature and salinity in the Madeira Abyssal Plain. Frontiers in Marine Science, 8, 685007.