the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Mar 2026
Bottom currents at the Arctic Mid Oceanic Ridge: insights from deployed sensors, biological indicators, and global models
Abstract. Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements. This study addresses this challenge at DeepInsight Hill on the Mohns Ridge by combining eight months of in situ current measurements, global ocean model predictions, and novel ROV-based biological flow indicators. Our findings reveal a complex and highly variable flow regime with a striking decoupling between tidal pressure and near-bottom currents. Spectral analysis shows that the dominant semi-diurnal tidal pressure signal does not translate into coherent near-bottom currents, suggesting conversion of tidal energy into internal waves at the ridge topography. During winter, sustained directional transport occurs simultaneously in opposite directions at the summit and base of DeepInsight Hill (S-SE and N-NW respectively), accounting for most of the effective water displacement over the record. However, foliaceous sponges, whose fan-shaped morphology is known to develop perpendicular to the prevailing current, show consistent W-SW orientations at the base of the hill that do not match the long-term instrumental mean flow direction (N-NW). Instead, these orientations align with global model predictions and the direction of episodic peak currents recorded during summer, suggesting that sponge morphology records specific high-energy flow events rather than time-averaged conditions, possibly amplified by peak summer nutrient supply. Bedforms observed at the same location eight months apart show no significant change, indicating that sediment-mobilizing currents are episodic and may not recur annually. Further data and interdisciplinary research are essential to resolve the governing factors of these systems and to refine environmental impact assessments in topographically complex deep-sea settings.
- Preprint
(21634 KB) - Metadata XML
- BibTeX
- EndNote
Status: open (until 13 May 2026)
-
RC1: 'Comment on egusphere-2026-1261', Anonymous Referee #1, 31 Mar 2026
reply
-
AC1: 'Reply on RC1', Alban Souche, 13 Apr 2026
reply
Reply to Reviewer 1
We thank the reviewer for the detailed reading of our manuscript. A full point-by-point response will follow once all reviewer comments are available. In the meantime, we offer these preliminary remarks to address the core concerns raised.
Title
We acknowledge the reviewer's concern regarding the breadth of the title. We are fully open to adjusting it to better reflect the specific geographic and thematic focus of the study (e.g., focusing on DeepInsight Hill and the integration of multi-indicator data).
Analytical Rigor and Tidal Analysis
We genuinely welcome the suggestion for a more formal tidal treatment. We have now applied UTide (Codiga, 2011) harmonic analysis to both long-term mooring records and will reshape the manuscript around this quantitative framework. This more rigorous approach strengthens the analytical foundation of the manuscript, and, importantly, the results are fully consistent with our original observations and do not alter our conclusions.
- Tidal Energy Fraction: The new analysis confirms that tidal constituents account for only ~10% of total current kinetic energy (KE) at both sites (Figure A). Non-tidal variability clearly governs the local dynamics.
- Diurnal Dominance: The breakdown reveals a clear diurnal dominance (K1), accounting for up to 66% of the tidal KE (Figures B and C).
- Pressure-Current Inversion: While the semidiurnal M2 dominates the pressure signal, it contributes negligibly to current energy (Figure D), as we described in our previous analysis.To streamline the manuscript and address the concern regarding "arbitrary subsets," the short-term 2025 dataset (original Fig. 7) will be removed. The long-term records provide ample resolution to characterize these cycles.
Multi-Indicator Synthesis
The reviewer notes that biological and sediment indicators provide "anecdotal" or "binary" data. We would argue that in data-poor deep-sea environments, these proxies provide essential context for the physical records. While we will sharpen the framing to ensure these are viewed as complementary, not primary, measurements, we maintain that excluding them would weaken the study's interdisciplinary value. This synthesis provides a rare look at physical forcing in a biological context, a gap that persists in deep-sea research.
Figures and Presentation
We accept the reviewer's feedback on figure presentation, including axis scaling, unit clarity, and the use of rotary spectra. We do not, however, agree that these fixable presentation issues constitute evidence of poor scientific content. We are currently revising the figures to improve scannability and ensure that the quantitative results from the new spectral and tidal analyses are clearly conveyed. Figures A–D, given as supplement file, illustrate the updated UTide results.
-
AC1: 'Reply on RC1', Alban Souche, 13 Apr 2026
reply
-
RC2: 'Comment on egusphere-2026-1261', Anonymous Referee #2, 23 Apr 2026
reply
Review of “Bottom currents at the Arctic Mid Ocean Ridge: insights from deployed sensors, biological indicators, and global models” by Souche et. al.
General comments
This study aims to characterize near-bottom ocean currents around the DeepInsight seamount using multiple data sources, including basin-wide circulation models, in-situ deployments, and observations of the seafloor from ROV video footage. While I feel there are useful and interesting components to this study, the study falls short of achieving its aims and lacks depth. I believe that re-working this into something more biology-focused would help with presentation and dissemination of results but do not feel it is currently suited for publication.
Specific comments
The title of this work is misleading – it does not discuss bottom currents at the ridge, rather it is a detailed study of the area around DeepInsight Hill.
I find the “biological morphology as indicators of current flow and/or specific flow events” to be an interesting and potentially very useful contribution to the study of the deep ocean and interpretation of near-seafloor currents, however the current study lacks strength in its conclusions and most likely would benefit from some additional data. For example, it is not clear if sponge morphology conforms to regional, long-term flow (i.e. the model data – ln 190) or if it is related to short-term, high-energy flows (i.e. the sensor data – ln 195-196), and what effect nutrient supply has on sponge development.
Similarly the detailed measurements of the near-seafloor currents around topography (DeepInsight Hill) is a useful dataset, but the authors fail to draw meaningful comparisons or conclusions. The hydrodynamics at DeepInsight are compared with two other seamounts where tidal forces dominate (Schulz Bank and a seamount from the Azores). I can understand the comparison with the local Schulz Bank seamount, but the comparison with the Azores seems arbitrary. Incorporating results from other seamounts where eddies, gyres, and regional topography have strong influence on the bottom currents may help bring some understanding to the dominant forces at DeepInsight (e.g. [1], [2], [3], [4] )
I further find the sedimentological analysis in this paper to be under-developed and confusing. For example, the paper states that “bedforms in sediments are commonly seen over the area” (ln 100) yet only two locations are presented and discussed- and only one location is used as evidence for the static nature of the bedforms. Further the paper concludes that “the observed bedforms exhibit patterns consistent with ripple formation under strong …. short lived, high-energy events” (ln 163-165) yet provide no reasoning or referencing in support of this. I would like to see a more thorough examination of the study area, and detailed interpretation of the bedforms (with appropriate referencing).
In general, I find the referencing to be a bit shallow in this manuscript and believe it would be enhanced by a more in-depth review of the literature prior to restructuring.
Minor comments:
Ln 66: Please list the voyages in question and reference voyage reports where available.
Ln 67: I would prefer to see some quantification of “extensive” and “multiple”. How many dives? How many hours of video footage?
Ln 86: Merge brackets – (e.g. temperature, oxygen, salinity, and turbidity; Figure 2).
Ln 87: Please provide reference (e.g. Bailey et al. 2024)
Ln 90: A table may be beneficial here – summarizing short-term deployment dates, length, and location, and associated voyage.
Figures 4&5. Scales must be provided for all images.
Figure 6 (caption) and lines 139-140: please be consistent with notation. In the caption tidal cycles are noted as 12h, 24h, while in the text it is 12 h.
Figure 7. Measurements are referred to as “tide pressure” and “tidal pressure” – suggest choosing one and being consistent.
Ln 183. The sponges are found on rocky substrates or with sediment-covered substrate? Please clarify.
Figure 10&11. Unclear what these figures are contributing. A difference map may help here to highlight changes (or lack thereof). Overall, I feel these figures take up too much real estate for too small of a contribution.
Ln 266-270. It is unclear why the relative timing of marine snow bloom between Schulz Bank and DeepInsight is discussed here.
References
[1] M. McKinley, D. Sun, M. Kelly, K. G. Sabra, and A. Bracco, “Deep Mesoscale and Submesoscale Circulations Around the Atlantis II Seamount,” J. Geophys. Res. Oceans, vol. 129, no. 11, Nov. 2024, doi: 10.1029/2024JC021233.
[2] G. Chen et al., “Observed deep energetic eddies by seamount wake,” Sci. Rep., vol. 5, no. 1, p. 17416, Nov. 2015, doi: 10.1038/srep17416.
[3] R. Pollard and J. Read, “Circulation, stratification and seamounts in the Southwest Indian Ocean,” Deep Sea Research Part II: Topical Studies in Oceanography, vol. 136, pp. 36–43, Feb. 2017, doi: 10.1016/j.dsr2.2015.02.018.
[4] X. Xie et al., “Enhanced near-bottom circulation and mixing driven by the surface eddies over abyssal seamounts,” Prog. Oceanogr., vol. 208, p. 102896, Nov. 2022, doi: 10.1016/j.pocean.2022.102896.
Citation: https://doi.org/10.5194/egusphere-2026-1261-RC2 -
AC2: 'Reply on RC2', Alban Souche, 28 Apr 2026
reply
Dear Referee #2,We thank you for the careful and thorough reading of our manuscript and for the constructive specific comments. We will provide a full point-by-point response with tracked changes in our formal revision. In this initial reply, we outline our position on the key concerns raised, as the scope and framing of the study appear central to RC2's evaluation.This study is explicitly an interdisciplinary synthesis, not a standalone biological survey or sedimentological study. Its primary contribution is demonstrating, for the first time at DeepInsight Hill (Mohns Ridge), that physical, biological, and morphological proxies can be cross-validated against long-term high-resolution current records, and that they do not always agree. This disagreement is a scientific result in itself, with direct implications for how environmental monitoring proxies are interpreted in complex ridge topography. We believe that by focusing on the integration of these datasets, rather than treating each section as a standalone disciplinary study, the manuscript provides a novel and necessary contribution to the understanding of Arctic ridge systems.General Scope and Framing"While I feel there are useful and interesting components to this study, the study falls short of achieving its aims and lacks depth. I believe that re-working this into something more biology-focused would help with presentation and dissemination of results but do not feel it is currently suited for publication."We respectfully disagree that the study should be re-worked into a biology-focused paper. The biological observations are one component, not a primary subject. The primary aim of the manuscript is to characterize the near-bottom flow regime at DeepInsight Hill using complementary approaches and to evaluate their internal consistency. We observe a tendency for sponge morphological alignment to correlate with the direction of peak current velocities during the productive summer season rather than with the long-term annual mean flow, an observational result that is novel and scientifically meaningful even if not yet fully explained. We will revise the Introduction and Conclusions to make this framing explicit from the outset so the interdisciplinary scope is unambiguous.On the Title"The title of this work is misleading — it does not discuss bottom currents at the ridge, rather it is a detailed study of the area around DeepInsight Hill."We agree and accept this comment. Following the same suggestion from Reviewer 1, we will revise the title to focus explicitly on DeepInsight Hill and the integration of multi-proxy data. A possible revision would be: Near-bottom currents at DeepInsight Hill (Mohns Ridge): a multi-proxy synthesis from in situ measurements and benthic indicators.Biological Morphology as Flow Indicators"I find the 'biological morphology as indicators of current flow and/or specific flow events' to be an interesting and potentially very useful contribution to the study of the deep ocean and interpretation of near-seafloor currents, however the current study lacks strength in its conclusions and most likely would benefit from some additional data. For example, it is not clear if sponge morphology conforms to regional, long-term flow (i.e. the model data — ln 190) or if it is related to short-term, high-energy flows (i.e. the sensor data — ln 195–196), and what effect nutrient supply has on sponge development."We are glad the reviewer recognises this contribution as interesting and potentially useful. The ambiguity the reviewer identifies (whether sponge morphology reflects long-term mean flow, episodic high-energy events, or nutrient-flux windows) is precisely the scientific problem we are posing, not an oversight. We cannot resolve it fully with the current dataset, nor do we claim to. What we show is that the mean current direction does not explain the observed sponge alignment, and that the bedform morphology independently points to past episodic high-energy events whose origin remains an open question. This mismatch is a cautionary finding for environmental monitoring approaches that rely on single-proxy data.We will, however, revise the biological section to present this argument more clearly, framing the ambiguity explicitly as an open question and adding appropriate literature on sponge growth responses to flow regimes and nutrient availability to support the interpretation.Regarding additional field data: while we fully agree that expanded biological sampling would be scientifically valuable, the Arctic Mid-Oceanic Ridge is one of the most logistically demanding environments for in situ deep-sea oceanography. The data presented here represents a rare observational record from a data-poor region. We believe that clearly stated ambiguity, supported by the best available physical and biological data, is a legitimate and valuable contribution that motivates future targeted work at this site.Near-Seafloor Currents and Seamount Comparisons"Similarly the detailed measurements of the near-seafloor currents around topography (DeepInsight Hill) is a useful dataset, but the authors fail to draw meaningful comparisons or conclusions. The hydrodynamics at DeepInsight are compared with two other seamounts where tidal forces dominate (Schulz Bank and a seamount from the Azores). I can understand the comparison with the local Schulz Bank seamount, but the comparison with the Azores seems arbitrary. Incorporating results from other seamounts where eddies, gyres, and regional topography have strong influence on the bottom currents may help bring some understanding to the dominant forces at DeepInsight."We appreciate this comment and largely agree. The Azores comparison was included because it represented one of the few available published datasets analysing the transfer of tidal energy into near-bottom currents, but we acknowledge that the geographic and dynamic context differs substantially from the Mohns Ridge setting. We will either remove this comparison or substantially justify it. We welcome the references suggested by the reviewer [1–4] and will assess which seamount studies provide the most relevant dynamical comparison to DeepInsight Hill, particularly those where mesoscale eddies and regional topographic steering play dominant roles. More broadly, we accept the criticism that the discussion of comparisons and conclusions needs strengthening, and the revision will more explicitly articulate what DeepInsight Hill's flow regime has in common with, and how it differs from better-studied ridge and seamount systems.We also note that the new quantitative analysis performed in response to Reviewer 1 (UTide harmonic tidal decomposition and rotary power spectra at both sites; see AC1) directly addresses the concern about the depth of the current analysis. The new results confirm that tidal constituents account for only ~10% of total current kinetic energy at both sites, with diurnal K1 dominance in velocity and semidiurnal M2 dominance in pressure. Beyond this first-order picture, the rotary power spectra reveal a terdiurnal MK3 peak in the current records. MK3 is a compound tide generated by the nonlinear interaction between the M2 (semidiurnal) and K1 (diurnal) constituents, with a period of approximately 8.2 hours. Its presence is a recognized signature of topographic modulation of tidal flow. In the revised manuscript, we will develop this point further, situating the MK3 signature within the broader literature on topography-tide interaction, and using it as additional evidence that the local flow regime at DeepInsight Hill is shaped by the hill itself rather than by the regional tidal forcing alone.Sedimentological Analysis"I further find the sedimentological analysis in this paper to be under-developed and confusing. For example, the paper states that 'bedforms in sediments are commonly seen over the area' (ln 100) yet only two locations are presented and discussed — and only one location is used as evidence for the static nature of the bedforms. Further the paper concludes that 'the observed bedforms exhibit patterns consistent with ripple formation under strong … short lived, high-energy events' (ln 163–165) yet provide no reasoning or referencing in support of this. I would like to see a more thorough examination of the study area, and detailed interpretation of the bedforms (with appropriate referencing)."These are fair and well-targeted comments. We agree that the sedimentological section needs strengthening and will revise it as follows:- The claim that bedforms are "commonly seen over the area" (ln 100) will be grounded with a clearer description of the spatial extent of ROV coverage and the number of observations.- The bedform interpretation (ln 163–165) will be supported with appropriate references on current ripple formation criteria in deep-sea settings.- Figures 10 and 11 show the same area at two different times; their purpose is to demonstrate that the bedforms did not change between ROV visits, supporting the interpretation that the bedforms are not actively reworked by seasonal mean flows but by episodic high-energy events. This will be made explicit in the caption. We agree the two figures take up too much space and will merge them into a single compact panel.Referencing"In general, I find the referencing to be a bit shallow in this manuscript and believe it would be enhanced by a more in-depth review of the literature prior to restructuring."We accept this comment. In the formal revision, we will expand the literature context, particularly for: (i) sponge growth responses to flow and nutrient regimes, (ii) seamount hydrodynamics with dominant sub-inertial forcing, and (iii) methodological references for bedform interpretation in deep-sea settings. The specific references suggested by RC2 [1–4] will be evaluated for inclusion.Minor CommentsWe accept all minor comments and will implement the corrections in the formal revision.Kind regards,The AuthorsCitation: https://doi.org/
10.5194/egusphere-2026-1261-AC2
-
AC2: 'Reply on RC2', Alban Souche, 28 Apr 2026
reply
Viewed
| HTML | XML | Total | BibTeX | EndNote | |
|---|---|---|---|---|---|
| 241 | 91 | 20 | 352 | 10 | 15 |
- HTML: 241
- PDF: 91
- XML: 20
- Total: 352
- BibTeX: 10
- EndNote: 15
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1
Review of “Bottom currents at the Arctic Mid Oceanic Ridge: insights from deployed sensors, biological indicators, and global models” by Souche et al.
In this study, the authors show a collection of diverse ocean current data obtained over the course of several years at DeepInsight Hill at the Mohns Ridge. The idea is to combine all different indicators of ocean currents (including biological indicators, current meter records and output from a global ocean model) to obtain a holistic picture of ocean currents at this topographic feature.
In my view, the analyses fall short of this idea and appear to be rather superficial and incomplete (see below).
Unfortunately, even with more detailed and systematic analyses, I am unsure what we could learn from the collective data presented here. Instead, I would expect that more thorough analyses would result in a more targeted study (e.g. aiming at exploring the relationship between sponges and currents or focusing on dynamic processes relevant at DeepInsight Hill), substantially differing from the present manuscript. This is why I recommend to reject the manuscript at this time.
Major Concerns:
Unclear scientific scope
The title suggests general analyses of bottom currents at the Mid Oceanic Ridge. This is not done here.
According to the abstract, this study adresses the challenge that: ”Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements.”
I am unsure what the authors mean by “characterizing”, but what I see is a descriptive study presenting different (sometimes anecdotal) indicators of currents (direction and/or speed) at a specific location (all observations are in-situ in a very small area, so "across scales" can refer to time scales only). The different indicators are (loosely) compared, but I miss a clear goal, thorough evaluation and systematic analyses.
Selection of data:
The authors aim to include diverse data, but I am unsure what purpose some of the data serve:
Superficial analyses:
The nature of most data presented here means that more thorough analyses are limited to the current meter records. In my view, the authors did not take full advantage of this dataset, with both analyses and presentation lacking.
In particular, the presentation in figure 6 is incomplete, showing an arbitrary subset in a), with an unexplained scaling on the y-axis. Similarly, the method behind the calculation of spectra in b) is not explained. There are many choices which can impact on the results. It also would be more useful/common to show rotary spectra for the velocity components.
It is unclear to me, why one time series from 2025 spanning 1.5 days is used to illustrate “short-term variability” (Fig. 7). The 10min resolution from the longer time series is ample to investigate variability within tidal cycles. If it is meant to convey spatial variability, it would be necessary to know where this specific instrument was deployed and how its records differ from the others.
The authors identify an apparent discrepancy between the tidal signal in pressure and currents as “striking", but no quantitative analysis is performed that could shed light onto whether this is indeed unexpected or not. For example, a more rigorous tidal analysis could entail the identification of the dominant tidal constituents using common tools (e.g. u_tide) and compare their observed amplitude to the expected value (e.g. from tidal models, both for pressure as well as currents). Investigation into different processes such as possible bottom boundary layer attenuation or topographic trapping could further help explaining the results.
In terms of the longer term variability, much space is devoted to figures 8 and 9, but again the analysis appears rather superficial and conclusions remain largely qualitative (i.e. describing what is seen in the progressive vector diagram).
Quality of figures
More attention should be paid to the figures, their readability and what they are meant to convey. My short notes:
Figure 3: Why doesn’t this figure contain mean current arrows from the two instruments?
Figure 6: The units on the y-axes do not make sense without explicitly stating what the scaling is. What is the motivation for the time period selection in a? Spectral calculations are not explained. There are many choices with impact on the results. It would be more useful to see rotary spectra for the velocity components.
Figure 7: What is "Tide pressure"? Isn't this just pressure? Units of dBar are more common…
Figures 8,9: a,b,c are largely identical between the two figures. d,e,f,g,h are very hard to interpret. What is the radial axis of f/h? It is unclear what the message is.
Figures 10,11: What is the contribution? They are not analysed beyond stating that nothing much changes.