The impact of land-fast ice on the distribution of terrestrial dissolved
organic matter in the Siberian Arctic shelf seas

Hölemann, Jens A.; Juhls, Bennet; Bauch, Dorothea; Janout, Markus; Koch, Boris P.; Heim, Birgit

doi:https://doi.org/10.5194/bg-2020-462

Preprints

https://doi.org/10.5194/bg-2020-462

Preprints

05 Jan 2021

Review status: this preprint is currently under review for the journal BG.

The impact of land-fast ice on the distribution of terrestrial dissolved organic matter in the Siberian Arctic shelf seas

Jens A. Hölemann¹, Bennet Juhls^2,3, Dorothea Bauch^4,5, Markus Janout¹, Boris P. Koch^1,6, and Birgit Heim³ Jens A. Hölemann et al.

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¹Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
²Department of Earth Sciences, Institute for Space Sciences, Freie Universität Berlin, Berlin, Germany
³Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Potsdam, Germany
⁴Leibniz Laboratory for Radiometric Dating and Stable Isotope Research, University of Kiel CAU, Kiel, Germany
⁵GEOMAR, Helmholtz Centre for Ocean Research, Kiel, Germany
⁶University of Applied Sciences Bremerhaven, Germany

Received: 08 Dec 2020 – Accepted for review: 28 Dec 2020 – Discussion started: 05 Jan 2021

Abstract. Remobilization of soil carbon as a result of permafrost degradation in the drainage basin of the major Siberian rivers combined with higher precipitation in a warming climate potentially increase the flux of terrestrial derived dissolved organic matter (tDOM) into the Arctic Ocean. The Laptev (LS) and East Siberian Seas (ESS) receive enormous amounts of tDOM-rich river water, which undergoes at least one freeze-melt cycle in the Siberian Arctic shelf seas. To better understand how freezing and melting affect the tDOM dynamics in the LS and ESS, we sampled sea ice, river and seawater for their dissolved organic carbon (DOC) concentration and the colored fraction of dissolved organic matter. The sampling took place in different seasons over a period of 9 years (2010–2019). Our results suggest that the main factor regulating the tDOM distribution in the LS and ESS is the mixing of marine waters with freshwater sources carrying different tDOM concentrations. Of particular importance in this context are the 211 km³ of meltwater from land-fast ice from the LS, containing ~ 0.3 Tg DOC, which in spring mixes with 245 km³ of river water from the peak spring discharge of the Lena River, carrying ~ 2.4 Tg DOC into the LS. During the ice-free season, tDOM transport on the shelves takes place in the surface mixed layer, with the direction of transport depending on the prevailing wind direction. In winter, about 1.2 Tg of brine-related DOC, which was expelled from the growing land-fast ice in the LS, is transported in the near-surface water layer into the Transpolar Drift Stream that flows from the Siberian Shelf toward Greenland. The actual water depth in which the tDOM-rich brines are transported, depends mainly on the density stratification of the LS and ESS in the preceding summer and the amount of ice produced in winter. We suspect that climate change in the Arctic will fundamentally alter the dynamics of tDOM transport in the Arctic marginal seas, which will also have consequences for the Arctic carbon cycle.

Jens A. Hölemann et al.

Status: final response (author comments only)

RC1:
'Comment on bg-2020-462', Piotr Kowalczuk, 25 Jan 2021

General opinion

Authors have presented a study based on unique data set collected during ship based and ice based expeditions between 2010-2019 in the Lena River delta and Laptev Sea. Based on this data set Authors have established a sound relationship between CDOM absorption coefficient _CDOM(350) and DOC concentration in the Laptev Sea and East Siberian Sea. Using salinity and oxygen isotopes data together with _CDOM(350), they have established water mass balance and calculated the organic carbon budget in the Lena River delta. Authors calculations revealed that ca. 15. TG of carbon is rejected from land fast ice with brine during ice formation, and is exported from Laptev Sea shelf into Transpolar Drift and to the East Siberian Sea shelf waters. Author have also concluded that melt water from mealting land fast ice in the Laptev Sea shelf dilutes the CDOM contained in the Lena River plume. The additional source of fresher water with lower CDOM content explained the mixing anomaly indicated in the CDOM vs. salinity mixing diagram.

Findings presented in the manuscript by,

The manuscript is very well written, and well edited serving as very important source of information on poorly described and quantified part of the DOM cycle in the Arctic Ocean. I found it very interesting and providing new and very relevant information. In my opinion this study deserved prompt publication after minor revision.

Detailed remarks Hölemann et al, are particularly important in the context of the current questions on the organic carbon flux exported from Arctic Ocean through Fram Strait. For this reason I recommend this paper for prompt publications.

The weak point of the manuscript is within discussion on the land fast ice annual cycle of freeze and thaw. This is especially important in the context of the CDOM. CDOM is not only rejected from the ice with brine but also its composition is significantly altered. I do recommend inclusion of findings presented in papers by Müller at al., 2011 and 2013 (Ann. Glaciol. 52 (57), 233–241; Mar. Chem. 155, 148–157), who studied the composition modification of CDOM contained in the ice during field studies and during controlled experiment. Further modification of CDOM in the ice results from biological activity in spring and summer. Studies by Granskog et al., 2015 (Ann. Glaciol. 56 (69)), Retelleti-Brogi et al., 2018 (Sci. Total Environ. 627, 802–811), and most recent by ZabÅocka et al., 2020 (Mar. Chem. 227, 103893), have documented that CDOM produced by sympagic algae leads to overall dominance of the protein-like fractions in the CDOM/FDOM composition.

I also suggest minor correction in the abstract. Specifically, phrase “ Laptev (LS) and East Siberian Seas (ESS) receive enormous amounts of tDOM rich” should be rewritten in the sentence containing a approximate quantitative information about riverine discharge.

Except suggested short paragraph in the discussion, on the alteration of the CDOM/FDOM composition due to brine rejection during freeze and biological production during spring, that should be added during revised manuscripts edits, I did not find any weak point in this presentation.

Piotr Kowalczuk

Citation: https://doi.org/10.5194/bg-2020-462-RC1
- AC1: 'Reply on RC1', Jens Hoelemann, 09 Mar 2021
  
  We would like to thank Piotr Kowalczuk for reviewing our paper and for his helpful and constructive comments and remarks. In the revised version of the paper, we will address the mentioned criticisms. We will add to the revised version a more detailed discussion of the changes in the composition of the DOM/CDOM locked in the ice. Since the fast ice in the southeastern Laptev Sea forms in the tDOM-rich Lena River plume, not only the formation of the brines is important, but also the chemical changes in the structure and degradation of the DOM in the ice are important processes that should be discussed in more detail. We are also perhaps too brief on the description of the DOM in the only ice core of signs sympagic algae. This ice core is from thinner fast ice (see paper) from the northern edge of the fast ice and was taken only a few 100 m from the polynya adjacent to the north. We will describe this ice core, which is fundamentally different from the other ice cores in its DOM properties, in more detail. However, this description is not essential for the core thesis of our paper. We would like to reiterate that in this study we sampled the mostly 2 m thick and snow-covered (~10 - 20 cm) fast ice and not the drifting pack ice. Due to the range of MI-8 helicopters, which is about 500 km when loaded (one way), the pack ice cannot be reached from land. Landing on the young ice north of the flaw lead in the Laptev Sea is impossible. At the same time, the entire marine area is closed to navigation in spring. Therefore, direct sampling of the drifting sea ice in the Laptev Sea in spring was not possible and could therefore only be discussed based on previous studies. However, our study suggests that the possibly DOM-rich meltwater of the young drifting pack ice probably plays no role in the processes we describe. At least not in the Laptev Sea.
  In the revised version of our paper we will also add to the abstract and give more precise figures for the flux input.
  
  Citation: https://doi.org/10.5194/bg-2020-462-AC1
RC2:
'Comment on bg-2020-462', Anonymous Referee #2, 08 Feb 2021

The authors present an impressive data set collected over a longer period, from a region which importance for the Arctic Ocean is substantial. This data simply needs to be published and the authors are acknoweldged for making the data publicly available.

That said, I found that the manuscript only scratches the surface, and the authors could have done a much thorough job to at least have a closer look at some aspects of the data they present. Admittedly the Siberian shelf system with runoff, sea-ice formation, sea-ice melt, complex circulation pattern is also very complex to understand, and thus also a more thorough examination of the data is warranted.

Salinity-property plots are not well equipped in such a system with multiple source waters, and the oxygen isotope data available to the authors should have been exploited more fully with the DOC and CDOM data. Also a closer look at the S275-295 for evidence of possible photodegradaton should have been done more thoroughly.

It would also be nice to see a more thorough discussion on how this data can help to explain the formation of the Arctic halocline waters.

Detailed comments are given in the attached pdfs (as a zip file).

Citation: https://doi.org/10.5194/bg-2020-462-RC2
- AC2: 'Reply on RC2', Jens Hoelemann, 09 Mar 2021
  
  We thank the referee for the constructive comments and questions. We also thank the anonymous reviewer for the positive comment on the publication of the entire data set. To "fully" analyze this dataset, which spans a 9-year period and multiple summer and winter expeditions, and present the results in this study is beyond the scope of this paper. However, we believe that the results presented in this paper give a key to any further interpretations. The main objective is to show, using measurements and calculation of budgets, that the distribution of DOM concentrations in the Laptev and East Siberian Sea is determined primarily by the mixing of three freshwater "parent water masses" (river water during the freshet, river water after the freshet, and meltwater from the fast ice) with different DOM concentrations and compositions. As far as we understand, this central hypothesis is not challenged by the reviewer. Although this paper addresses only one aspect of the topic of DOM transport and degradation in the Arctic Ocean, we believe it makes an important contribution to the scientific debate. In order to give other scientists the opportunity for further analysis regarding other aspects, we have decided to publish the entire data set in full spectral resolution. In trying to write the paper as concisely as possible, we seem to have inadequately presented important results of our analysis (such as the presentation of the ratio of the river water fraction - O18 data - to CDOM absorption). We hope that the reviewer will nevertheless believe us when we emphasize that we have thoroughly analyzed the data set.
  A detailed determination of the degradation rates of tDOM on the Siberian shelves is not the central topic of this study. Our data set also indicates that some tDOM is mineralized in the area of the Siberian shelves. However, it also indicates that degradation of tDOM cannot adequately explain the observed concentration distribution on the shelves. However, we agree with the reviewer that we should better elaborate on the key messages by including figures of river water fraction (O18 data) and CDOM absorption and discussing these data in more detail. We will do this in the revised version of the manuscript and hope to address the reviewer's main criticisms. We will also discuss the slope values (S275-295) and their significance in more detail. However, the inclusion of these results, which is certainly necessary, does not disprove the central conclusions of this paper but is further evidence of the plausibility of our hypotheses. We have summarized the response to all of the referee’s comments and questions in a separate document (see Appendix).
  The formation of the Arctic halocline is an important and interesting research topic that our group has also been working on for more than 20 years. However, the description of this physical-oceanographic process is not - and cannot be - the subject of this paper. In this context, we refer to the published studies of some of the coauthors: Bauch et al. (2009, JGR), Bauch et al. (2011, Prog in Oceanogr.), Bauch et al. (2014), Janout et al. (2017), and Janout et al. (2020). These papers, which are cited in our paper, provided an important basis for the hypotheses presented in this study.
  
  Citation: https://doi.org/10.5194/bg-2020-462-AC2

Jens A. Hölemann et al.

Data sets

Surface water dissolved organic matter (DOC, CDOM) in the Lena River Kattner, G., Juhls, B., and Heim, B. https://doi.org/10.1594/PANGAEA.898705

Colored dissolved organic matter (CDOM) measured during cruise TRANSDRIFT-XVII, Laptev Sea Hölemann, J. A., Juhls, B., and Timokhov, L. A. https://doi.org/10.1594/PANGAEA.924206

Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) measured during cruise TRANSDRIFT-XIX, Laptev Sea Hölemann, J., Koch, B. P., Juhls, B., and Timokhov, L. A. https://doi.org/10.1594/PANGAEA.924209

Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) measured during helicopter/ice camp TRANSDRIFT-XX, Laptev Sea Hölemann, J. A., Koch, B. P., Juhls, B., and Timokhov, L. A. https://doi.org/10.1594/PANGAEA.924228

Colored dissolved organic matter (CDOM) measured during cruise TRANSDRIFT-XXI, Laptev Sea Hölemann, J. A., Juhls, B., and Timokhov, L. A. https://doi.org/10.1594/PANGAEA.924203

Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) measured during cruise TRANSDRIFT-XXII Hölemann, J., Koch, B. P., Juhls, B., and Timokhov, L. A. https://doi.org/10.1594/PANGAEA.924202

Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) measured during cruise TRANSDRIFT-XXIV, Laptev Sea Hölemann, J. A., Koch, B. P., Juhls, B., and Ivanov, V. https://doi.org/10.1594/PANGAEA.924210

Colored dissolved organic matter (CDOM) and dissolved organic carbon (DOC) measured during cruise TRANSARKTIKA-2019 Leg4, Laptev Sea and East Siberian Sea Hölemann, J. A., Chetverova, A., Juhls, B., and Kusse-Tiuz, N. https://doi.org/10.1594/PANGAEA.924211

Physical oceanography, nutrients, and δ¹⁸O measured on water bottle samples in the Laptev Sea Bauch, D., Cherniavskaia, E., Novikhin, A., and Kassens, H. https://doi.org/10.1594/PANGAEA.885448

Stable oxygen isotope analysis of water samples during helicopter/ice camp TRANSDRIFT-XX Bauch, D. and Thibodeau, B. https://doi.org/10.1594/PANGAEA.924538

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Short summary

The Arctic Ocean receive enormous amounts of river water, which is rich in dissolved organic matter (tDOM) and an important component of the Arctic carbon cycle. Our analysis shows that mixing of three major freshwater sources is the main factor that regulates the distribution of tDOM concentrations in the Siberian shelf seas. In this context, the formation and melting of the land-fast ice in the Laptev Sea and the peak spring discharge of the Lena River are of particular importance.


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