Tuesday, November 30, 2021


This project initiated 2012

Copyright TXu 1-795-975, TXu 1-809-958, TXu 1-823-034, TX 7-630-173 and TXu 1-852-686

 

Observations of diapycnal upwelling within a sloping submarine canyon 

Published: 26 June 2024 

Bethan L. Wynne-Cattanach, Nicole Couto, Henri F. Drake, Raffaele Ferrari, Arnaud Le Boyer, Herlé Mercier, Marie-José Messias, Xiaozhou Ruan, Carl P. Spingys, Hans van Haren, Gunnar Voet, Kurt Polzin, Alberto C. Naveira Garabato & Matthew H. Alford 

 

"The experiment site chosen was the Rockall Trough (Fig. 1), given its relatively flat, deep interior and steep sides. These characteristics enable a clear distinction between interior and boundary processes, which is essential for determining the driving forces behind the transformation of deep water. In particular, the study was conducted in a narrow, slope-incising canyon on the eastern side of the Rockall Trough. The canyon is 9-km wide at the mouth and 32-km long from the 2,900-m isobath to the tip of the eastern branch at the 1,200 m isobath. On average, the canyon walls rise 400 m above the thalweg. The canyon splits into a southwards and southeastwards branch around 54° 14.0′ N, 11° 57.1′ W. Our measurements focused on the southeastwards branch of the canyon." 

 

"Here we show vigorous near-bottom upwelling across isopycnals at a rate of the order of 100 metres per day, coupled with adiabatic exchange of near-boundary and interior fluid. These observations were made using a dye released close to the seafloor within a sloping submarine canyon, and they provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean. This supports previous suggestions that mixing at topographic features, such as canyons, leads to globally significant upwelling. The upwelling rates observed were approximately 10,000 times higher than the global average value required for approximately 30 × 106 m3 s−1 of net upwelling globally." 

 

"It has been hypothesized that upwelling may be confined to near-bottom regions within which the buoyancy flux perpendicular to the boundary must decrease to zero owing to the requirement of no flux through the boundary itself. The convergent turbulent buoyancy flux leads to upwelling near the boundary. Although the no-flux condition assumes no geothermal heat flux through the seafloor, inclusion of a geothermal heat flux reinforces upwelling at the boundary by warming deep waters from below." 

 

"Previous estimates of upwelling near the bottom were smaller in magnitude but were observed over larger spatial and temporal scales than our experiment. The estimated average upwelling value required to maintain global deep-ocean density stratification is 1 × 10−2 m d−1 (ref. 9). In a fracture-zone canyon of the Mid-Atlantic Ridge, the magnitude of the up-canyon velocity was similar to that seen here. However, the shallower bathymetric slope led to a relatively smaller upwelling velocity of 1.7 m d−1 (ref. 7). Diapycnal upwelling in the Mid-Atlantic Ridge canyon is controlled by a decrease in the volume available for mixing as the canyon narrows (hypsometry)" 
 
"Specifically, our results provide direct evidence in support of these studies’ key prediction: to overcome the substantial interior ocean downwelling implied by bottom-enhanced mixing, the net global upwelling of about 30 × 106 m3 s−1 requires the existence of much more rapid upwelling near the sloping seafloor." 

 

4.5.2 Even if we were to ignore the possible, and more likely probable, contribution of a heat flux of geothermal convection, that has already been observed and its values measured, and that is currently circulating sea water into and out of the porous and faulted slopes that form the opposing ridge structures that run parallel along the length of the 40,000-kilometer (25,000 mile) oceanic ridge system, that includes a network of volcanoes, and generates new oceanic crust at the rate of 17 km3 per year and includes the massively larger by comparison Mid-Atlantic Ridge, that which, through convective motion, could contribute a heat engine conveyor component to the bottom water overturn process. Ignoring all of that, we can still assume a sizable contribution of thermal flux increases when the mantle thermal pulses periodically increase their volcanic activity.
                                                       "Mantle thermal pulses below the Mid-Atlantic Ridge".

            "show oscillations of ,3–4 Myr superimposed on a longer-term steady increase (and undoubtedly decrease) with time.

   "The time lag between oscillations of mantle melting and crustal thickness indicates that the solid mantle is upwelling at an average rate of ,25mmyr, but this appears to vary through time."  


"They range along the VLS from 5.7% to 14.3%, with oscillations where the degree of melting appears to increase gradually and then decreases rapidly. These short-wavelength oscillations are superimposed on a long-term (.15Myr) trend of increasing (and undoubtedly decreasing) degree of melting with time"