The manuscript contains a lot of data and the topic of inorganic carbon dynamics in sea ice is of interest for many readers of Biogeosciences. However, the data are not properly analyzed and the discussion is mainly qualitative (although a lot of quantitative data are available), in parts misleading (to say the least). I wrote a long list with my concerns.
Abstract:
L24-26: ‘The percolation of this low pCO2 melt water into the sea ice matrix dilutes the brine resulting in a strong decrease of the in situ brine pCO2 (to 20 μatm).’
This sounds as if pCO2 is a conserved quantity (a substance) that can be mixed. However, pCO2 does not follow a linear mixing relationship. It is a strong function of temperature and depends on DIC and pH.
My suggestion: change wording when talking about pCO2 and mixing.
L525-528 ‘Therefore, if we take into account a mean uptake CO2 of -1 mmol m-2 d-1, over the minimum and maximum Arctic sea ice extend during spring and summer thaw (90 days), we derive an uptake from 7.2 to16.4 Tg of C yr-1.’
Where are these numbers from? This process requires a long time period (90 days) and large area (6 to 15 Million km2) and is still small compared to the current estimate of 70 to 200 Tg C yr-1 uptake by the Arctic Ocean (compare, Bates and Mathis, 2009, Takahashi et al., 2009) and it is not clear how much of the uptake during the melting season might be compensated by outgassing during ice formation.
L 75-77 ‘The discharge of melt water through the ice cover is proportional to the ice permeability and the hydraulic pressure gradient in the brine system.’
You may add ‘(Darcy’s law)’ at the end of the sentence.
L 104 pCO2 between 0 and 188 muatm -> pCO2 below the detection limit (GIVE VALUES OF DETECTION LIMIT HERE) instead of “0”.
L 117 ‘measurements of TA, TCO2 and pCO2 on bulk sea ice’ really means: measurements of TA, TCO2 and pCO2 of sea ice molten in the laboratory in a closed container in order to avoid gas-exchange (right?). Please specify.
Remark:
The (equilibrium) pCO2 for a water sample of a certain chemical composition (TA, TCO2, salinity) is a strong function of temperature. Thus it is important to always report pCO2 together with temperature. This applies especially when reporting pCO2 values of ‘bulk sea ice’.
Which temperatures (in-situ? temperature after melting the ice? lab temperature?) correspond to the reported pCO2? Please give this information also in the figure legend.
L 119 ‘Percolation of melt water from ponds was tracked using the isotopic ratios δD and δ18 sea ice and brine.’
A few sentences to explain the method for detection of melt water percolation with the help of isotopic compositions might be in order here. Give, for example, typical values of δD and δ18snow, sea ice, and sea water and may be hint why they are different from each other.
L 336-337: ‘Melt pond formation and subsequent percolation of melt water into the ice cover are visible in the brine system from the isotopic ratio data (Figure 3,5).’
I understand what you want to say, however, I suggest rewriting the sentence under ‘Results’.
-> Low values of δD (down to -119.2‰) and δ18 (down to -23.9‰) were observed in the upper 20 cm of the ice column. These values are much lower than typical bulk sea ice values (GIVE RANGE OF TYPICAL VALUES OF ISOTOPIC COMPOSION HERE). They can be explained by the percolation of melt water (GIVE RANGE OF TYPICAL VALUES OF ISOTOPIC COMPOSION HERE) from melt ponds into the underlying sea ice column.
“isotopic ratio data” is isotopic composition instead
Section ‘Results’: In my opinion the interesting results are not properly described. Let’s look at bulk sea ice temperature for example. The authors write ‘The mean ice temperature increased from
-2.9oC on 4 June to -1.5oC on 12 June (Figure 3). From 10 June, the temperature of the top 20 cm of the ice was slightly negative (-0.5oC to 0oC) while the rest of the ice thickness remained around
-1.5oC.’
When looking at the temperature profile I immediately recognize various other features/patterns worth mentioning as, for example, the high temperatures (above -1oC, i.e. well above the melting point of sea water with salinities > 30) at the bottom of the ice on June 12, 17, and 19, the rather unusual temperature profile with several maxima inside the ice column on 12 June, the increase in mean temperature over the observation method is not monotonic (compare data from June 8 to June 9).
“… ice was slightly negative (-0.5oC to 0oC)”
Write “… ice was between slightly negative (-0.5oC) and the melting point.” instead
The description of salinity profiles in incomplete as: ‘Bulk ice salinity ranged from 7.5 to 0 (Figure 3). The top 20 cm of the ice had salinities around 0 while the bulk salinity of the central part of the ice decreased from 7.5 to 4 during the survey.’
Salinities in the upper 15 cm are indeed quite low (below 1) on June 9 and 12, however, not ‘around 0’ for the other samples, and actually comparable to values in the central part of the ice column on June 4. The salinities near the bottom of the sea ice are worth mentioning: high values on June 4 and 9 in contrast to relative low values on and after June 17.
L 288 to 299: For the melt ponds only ranges of observed data are given The values of nTA and nTCO2 are plausible, however, cannot be calculated from the ranges given in this section. The authors should present the whole data set for the melt ponds. The maximum values for nTA and nTCO2 for the melts ponds are smaller than the maximum values for nTA and nTCO2 in the upper 20 cm of sea ice. How are the observations of melt ponds and sea ice related in time and space?
Fig. 5: The profiles of measurements in brines (Fig. 5) should be shown in the same form as in Fig. 3 to allow for comparison.
More detailed descriptions of the data are required also for the other quantities. These detailed description are necessary as foundation for the discussion that should relate the observations in a meaningful way.
5. Discussion
The discussion is largely qualitative although many quantitative data are available. One could, for example, estimate how much low-salinity water from melt ponds is necessary to explain the large changes of S, δD, and δ18 in the upper 20 cm of sea ice. Further: what are the consequences for other quantities (TA, nTA, …)?
L 328-329: ‘Over the course of our study period, the vertical temperature gradient within sea ice decreases, leading to nearly isothermal ice cover.’ This would belong under ‘Results’, however, I don’t see that this statement is supported by the data.
L 337-339 ‘The 20 cm depth on 9 and 10 June … had the same isotopic ratios as the melt ponds.’
No! First: the isotopic compositions cover large ranges in the melt ponds as well as in the upper 20 cm of the ice. Second: the ranges are not identical with more negative values in ice.
L 339-341 ‘The increase of the δ18 and δD ratios in the melt ponds observed on 19 and 21 June suggests that the contribution of sea ice melt to the melt ponds had increased.’ Please show all melt pond data.
L 347-364 The discussion on ikaite dissolution is purely qualitative and leads to nowhere.
L 388 ‘… and the dissolution of ikaite.’ This statement is not supported by the data.
Fig.9 Although some of the sea ice nTA-nTCO2 data lie close to a Delta_nTA:Delta_nTCO2=2:1 line through the sea water value, this does not necessarily speak for CaCO3 dissolution as indicated in the graph. The very high nTA-nTCO2 values occur in the upper 20 cm of the ice core (Fig. 3) where salinity values decrease to values well below 1. This decrease of S below 1 requires input of large amounts of freshwater (dilution of salinity from 5 to 1 requires addition of 4 kg of freshwater per kg of ice!). The low-salinity water observed in melt-ponds by the authors cannot do the job because its salinity was larger than 1. A possible explanation could be melt ponds with lower salinity (at an earlier state of development and/or with larger water input from snow). Such waters might show a TA:TCO2 ratio much different from sea water.
Minor points:
L 16: CO2 flux -> CO2 fluxes
L 20: increase of the ice temperature -> increase in ice temperature
L21 decrease of bulk ice salinity -> decrease in bulk ice salinity
L 28 fluxes out of the atmosphere -> fluxes from the atmosphere
L 34 please drop ‘still’
L 102 sea ice
L 213: replace ‘∂’ by `δ’
L 213: drop gap between number and per-mille sign; also L 306-307 (the same convention applies for percent) |
- Printer-friendly version
- Supplement
