Elevated sources of cobalt in the Arctic Ocean

Bundy, Randelle M.; Tagliabue, Alessandro; Hawco, Nicholas J.; Morton, Peter L.; Twining, Benjamin S.; Hatta, Mariko; Noble, Abigail E.; Cape, Mattias R.; John, Seth G.; Cullen, Jay T.; Saito, Mak A.

doi:https://doi.org/10.5194/bg-17-4745-2020

Articles | Volume 17, issue 19

https://doi.org/10.5194/bg-17-4745-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/bg-17-4745-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 17, issue 19

Research article

|

01 Oct 2020

Research article |

| 01 Oct 2020

Elevated sources of cobalt in the Arctic Ocean

Randelle M. Bundy, Alessandro Tagliabue, Nicholas J. Hawco, Peter L. Morton, Benjamin S. Twining, Mariko Hatta, Abigail E. Noble, Mattias R. Cape, Seth G. John, Jay T. Cullen, and Mak A. Saito

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Final revised paper (published on 01 Oct 2020)
Preprint (discussion started on 09 Apr 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'review of bg-2020-84', Anonymous Referee #1, 16 Apr 2020
- AC1: 'Response to review', Randelle Bundy, 03 May 2020
RC2: 'Review of ‘Elevated sources of cobalt in the Arctic Ocean’ by Bundy et al.', Anonymous Referee #2, 14 May 2020
- AC2: 'Response to review', Randelle Bundy, 01 Jul 2020
RC3: 'Review of ‘Elevated sources of cobalt in the Arctic Ocean’ by Bundy et al.', Anonymous Referee #3, 27 May 2020
- AC3: 'Response to review', Randelle Bundy, 01 Jul 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (07 Jul 2020) by Peter Landschützer

AR by Randelle Bundy on behalf of the Authors (07 Jul 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (15 Jul 2020) by Peter Landschützer

RR by Anonymous Referee #3 (29 Jul 2020)

Suggestions for revision or reasons for rejection

Review of ‘Elevated sources of cobalt in the Arctic Ocean’ by Bundy et al.

The authors have done a good job on the revisions. However, I do not agree with their assessment that mixing can be ignored in the Co-P relationships. Even though stratification is strong, eventually the Pacific origin water does mix with the Atlantic origin water, otherwise we would be able to recognize Pacific water exiting through Fram Stait. Additionally, stratification is strong in summer but given that this is a region of deep water formation, very significant mixing occurs driven by winter cooling and brine rejection at other times of year. Maybe not directly along the transect, but it does strongly affect the water masses and circulation in the Arctic.
The fact that there are distinct water masses/layers going from surface to PHW to deep waters, will also affect the element profiles and thus slopes of an element-element relationship, even if there would be no mixing at all. For example, I suspect that 3 data points in surface water and 2 data point in PHW will give a different slope compared to a relationship comprising of 2 data points in PHW and 3 in underlying deeper water. It is not clear if this is accounted for in the assessment of slopes.

Specific comments:

Line 165 did not see discussion on storage in 3.2.2.

289 (Irrelevant for this paper where the correlation between Co and Mn is assessed in the surface layer with high Mn concentrations, but have to say that the detection limit of 0.55 nM for Mn is not specific for a flow injection analysis, detection limits of an order of magnitude lower (down to <0.01 nM) have been demonstrated for Mn by flow injection)

523 I insist it should be explicitly stated here Mn also has a fluvial source in the Arctic (this is indirectly noted in the next paragraph, but it should be explicitly clear for others who might want to follow a similar approach)

528 the variance explained and R2 in the associated figure do not match

4.3 I feel it should also be noted the sampling locations were quite different so spatial patterns could be important too. Notably given that the shelf is supposed to be the largest source, the dramatic variations in shelf width through the Arctic with a narrow shelf on the Canada basin side and wide shelf on the Chukchi side combined with inflow through the Bering Strait could play an important role too.

750 confusing, waters that pass through the Canadian archipelago exit via Fram strait?

770 rephrase for clarity, now it is seemingly stated that the total Zn inventory is small
compared to Zn

790 not shown but suggested

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ED: Publish subject to minor revisions (review by editor) (30 Jul 2020) by Peter Landschützer

AR by Randelle Bundy on behalf of the Authors (04 Aug 2020) Author's response Manuscript

ED: Publish as is (17 Aug 2020) by Peter Landschützer

Short summary

Cobalt (Co) is an essential nutrient for ocean microbes and is scarce in most areas of the ocean. This study measured Co concentrations in the Arctic Ocean for the first time and found that Co levels are extremely high in the surface waters of the Canadian Arctic. Although the Co primarily originates from the shelf, the high concentrations persist throughout the central Arctic. Co in the Arctic appears to be increasing over time and might be a source of Co to the North Atlantic.