the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2024
Effect of double diffusion processes in the deep ocean on the distribution and dynamics of particulate and dissolved matter: a case study in Tyrrhenian Sea
Abstract. This study examines CTD, ADCP and optical data collected during the PERLE-3 cruise in March 2020 between the surface and 2000 m depth over an east-west section of the Tyrrhenian Sea in the Mediterranean. The focus will be on the impact of double diffusion processes, in particular salt fingering, on the distribution and dynamics of particulate and dissolved matter. The staircases develop at the interface between the warm, salty Levantine Intermediate Water (LIW) and the colder, less salty Tyrrhenian Deep Water (TDW) in the centre of the basin with low hydrodynamic energy. The results show that thermohaline staircases formed by salt fingering significantly influence particle sedimentation and biogeochemical cycling in deep ocean environments by altering vertical flux patterns. These density steps create distinct vertical layers that act as physical barriers, slowing the descent of particles and facilitating their retention and aggregation. Retention of fine particles at density gradients promotes the formation of larger aggregates, affecting particle size distribution. The staircases also affect dissolved matter by creating pronounced concentration gradients of oxygen and nutrients, which can influence microbial activity and nutrient cycling.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3436', Anonymous Referee #1, 24 Dec 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-3436/egusphere-2024-3436-RC1-supplement.pdf
- RC2: 'Comment on egusphere-2024-3436', Anonymous Referee #2, 07 Jan 2025
- RC3: 'Comment on egusphere-2024-3436', Anonymous Referee #2, 07 Jan 2025
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RC4: 'Comment on egusphere-2024-3436', Anonymous Referee #3, 08 Jan 2025
Major comments :
I was quite enthusiast reading the abstract. After reviewing the manuscript, I liked the part about the influence of the eddy on the stratification of the Tyrrhenian Sea. However, I’m disappointed by the main focus of the study, which concerns the concept of particle retention by the occurrence of staircases. I find the figures associated with station #9 are unconvincing and inconclusive. In addition, the authors build some interpretations on the sole basis of 4 steps of a single profile (steps of sta. #9), while a significant number of other stations of that same cruise exhibit staircases. Not much is said about the other profiles, especially in terms of particle retention. Those other profiles should also be used to improve the robustness/representativity of the nitrate fluxes associated with staircases.
Note that I fully agree with the process of retention, decrease of the settling velocity, particle aggregation and consequences for the microbial community when there are strong density gradients that act more or less as a barrier: this is physical, no problem.
My criticism is that there is a lack of clear evidence of a retention and associated increase in mineralization on the sole profile presented in this study (#9 with staircases). It seems to me that there is no clear perturbation of the large scale gradients (AOU, nitrate) that are shown on Figs. 7 (and 6) for instance. At the scale of the steps of the staircases, we find the same gradient signs that are simply enhanced since a step "connects" two homogeneous regions that have been mixed by the salt-fingering process.
I expected the study to exhibit higher concentration of small scale particles, an increase in nitrate, a decrease in AOU at the base of a mixed region just above a step, or, in a step. I see no such significant anomalies, just local gradients at steps that follow the sign of the large scale gradient, with a larger amplitude induced by the adjacent convectively mixed regions, in the same way that salinity and temperature gradients are very locally enhanced between these mixed regions.
Further comments are provided below.
Detailed comments:
Fig. 4: The down- and up-casts estimates of vertical velocity are remarkably consistent on profile a, and somewhat less on profile b between 150-250 m and 500-1100 m. Is it physical and in that case what is the source of the variability ?
l. 282: “...the downward movement of water around the anticyclonic eddy”: interesting! Can this be also evidenced on a 2D-plot transect of the vertical velocity (as a supplementary subplot of Fig. 3) ?
Possibly also visible on theta plot Fig. 2A, no ?
l. 288-291 + Fig. 5c: we rather observe a mimima between 200-300 m, no ? The maxima are below. Is the zonal fragmentation of the mimina "tongue" between 200-300 m caused by the upward motion of the large reflectors at night ? Maybe a slight reformulation is needed to clarify.
l. 292 and Fig. 5d: I'm not an expert in this field. A short explanation of the reasons we expect such a distribution would be welcome. Thanks!
l. 384-385: “the presence of significant staircase structures down to 2000 m can also be influenced by mixing induced by cyclonic eddies...” : Is it the eddy that breaks the staircase structure that was in place, or the fact that the eddy was formed in a region without any staircase structure that is later advected in the middle of the Basin? Do we know where it was generated?
l. 399: “a significant reduction in the abundance of fine particles as seen by transmissometry (BAC) under each interface”: The decrease is across the interface, not under (under = the mixed layer)
l. 401 and following, Fig. 9-10 and observations: If I understand, we are looking at the variation across a step (=interface). If I look at the difference in abundance between just above a step minus just below the step, there is no clear rule, even for the two smallest size ranges on Fig. 9; for the two smallest sizes < 128 µm: first step = increase with depth, second step = decrease with depth, third step = increase, for Fig.10: decrease with depth.
I'm a little puzzled by the concept of 'retention' of small particles when 'simply' looking at Figs 9 and 10. Given the terminology used here, I would have expected to see a peak of small particle abundance within a step or immediately above the step. This does not happen. The overall abundance profile decreases with depth in the absence of staircases (Fig. 5 sta #20). In the step region, this overall profile is mixed in the convective regions associated with the salt finger. A step is not associated with an increase (retention) of small particles, it is just a strong gradient connecting two convectively mixed regions (just as the temperature over a step shows larger local gradients than it would if there were no adjacent mixed layers due to the double diffusion process)... Am I wrong in thinking that if there is retention, the time for small particles to aggregate and possibly become heavier, I should then see an increase in the abundance of small particles at the base of a mixed region above a step?
l. 447: “The vertical velocities of the current … likely to alter the settling of particles”: not sure to correctly understand the idea. You mean that a layer that is connectively mixing has down- and up-ward currents at very small scale, what can homogenize the distribution of small scale (almost neutrally buoyant) particles, preventing their deposit at the base of the convective region ??
l. 459 and following: OK for mineralization process, consumption of oxygen, release of nutrients. So what do we expect ? Increase in AOU just above a step, since a step acts as a barrier, or across a step, the time for small particles to cross it at a reduced downward velocity ? It is not clear from what is said on the process of increase mineralization.
For the first two upper steps (Fig. 11), there is an increase in AOU across the step, for the third step around 875 m, the AOU is constant or slightly decreasing, and for the fourth large step the AOU strongly decreases across the step. For the nitrate, there is some increase across the upper three steps but the large scale gradient shows an increase with depth; therefore, it is difficult to conclude that the increase in nitrate across the steps is associated with an increased mineralisation process due to particle retention by just looking at those figures.
The difficulty may be due to the presence of large scale gradients, that may mask the signal as described al l. 463. Looking at other profiles with staircases may provide more convincing evidences.
Minor comments:
Fig. 1a: add the ship route on the map. According to Fig. 3b, it seems that there is an interruption of the zonal “linear” progression of the ship around the eastern edge of the anticyclonic eddy…
For all transect figures (Fig. 2, 3c, 5, 8): add the station number on the upper abscissa.
l. 254 the typical maxima looks like more 10-15 cm/s on Figs 3bc, rather than 30 cm/s. May be the exceptional value of 30 cm/s is very locally reached along a slope, but this is not evidenced on Fig. 3.
Fig 3a: give the meaning of the arrows and provide their scale and the source of the data (gridded geostrophic currents from…) + plot the ship route on this map
l. 341: figure referencing: Fig. 8 instead of Fig. 6 ?
l. 344: repetition: “Staircases are absent under the deepest eddy (about 12° E)”, already said in the following sentence (l. 345)
l. 397: interface/step starts around 1250 m and ends near 1300 m. To avoid confusion in the wording, you may choose another term than interface if you depict a region encompassing mixed layers and steps.
l. 427: unit: “µm” instead of “m”
l. 491: “stronger than” instead of stronger that”
l. 510: extra dot..
Citation: https://doi.org/10.5194/egusphere-2024-3436-RC4
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