|This paper is still not ready for publication. Concerning the melting ice surface of the West-Greenland ablation zone in summer, the authors argue that addition of dust causes the ice albedo to increase, so that any reduction of ice albedo by light-absorbing impurities would be due to algae. This denial of a role for dust in reducing albedo is based on several questionable arguments, which I will point out in this review. Most of these questionable arguments can be classified into one of three classes of disconnects: (a) The authors’ response to my review, saying how the paper was changed, does not correspond with what the revised paper actually shows. (b) The way figures are described in the text does not correspond to what the figures actually show. (c) References cited in support of a claim do not actually provide the claimed evidence. |
I do not doubt that algae absorb sunlight, but I do doubt the quantitative attribution of albedo change on Greenland to algal abundance. A lot of work went into this project, and I would like to see it come to more robust conclusions. It has the potential of becoming an important paper.
(1) The major comment of my first review was to point out that the imaginary part of the dust refractive index should decrease across the visible spectrum from 300 to 700 nm, not increase as shown in the first submission. In their Response to Reviewers, the authors now thank me for “pointing out the unrealistic refractive index”, but then in their revised manuscript their dust imaginary index still increases across the visible (by a factor of 2.7 from 300 to 700 nm), as shown in the new Figure 3C. [An example of Disconnect Type “a”.]
(2) I don’t want this extraordinary claim of blue dust to enter the literature on the composition of Greenland ice without further evidence. What mineral composition gives it the blue color indicated by Figure 3C? In their response to my review, the authors indicate that the mineralogy of the dust is consistent with Figure 3C, and that a paper on this topic is soon to be submitted by one of the authors (McCutcheon). A brief summary of the mineralogy in that forthcoming paper should be included in this paper; it could be cited as “unpublished data” or “manuscript in preparation”.
(3) The authors have ignored the request in my Major Comment #1, in which I asked the authors to show the computed albedo effect of the measured dust concentration; they continue to show just the computed albedo effect of arbitrary amounts of dust (100, 300, 500 ppm), and similarly for arbitrary concentrations of algae. At least a table is needed, giving measured dust and algal concentrations (ppm by mass). The numbers of cells are shown in Figure 2C, but these need to be combined with algal-cell size distributions to get the mass.
(4) Lines 272-273. Tedesco et al. (2013) is cited as indicating “a lack of red mineral phases”. In fact, Tedesco’s Figure 6a shows that both dust and algae are “red”, and that dust is redder than algae; Tedesco speculated that the goethite they found in their samples had dehydrated to hematite in the drying and heating process. Goethite is the hydrated form of hematite; it is not as absorptive as hematite but its absorption coefficient likewise decreases across the visible. It is true that goethite has a yellow appearance rather than red, but it is misleading to cite Tedesco as finding “a lack of red mineral phases”, since the present authors are using the adjective “red” here to characterize the spectral slope of reflectance, which increases toward the red for both goethite and hematite. [Disconnect type “c”]
(5) Lines 518-521. “The imaginary refractive index of the mineral dust sample (Fig 3C) . . . indicating . . . scarcity of red minerals in the bare ice.” This comment is not forthright; there is no mention of the factor-of-2.7 increase of imaginary index from 300 to 700 nm, which indicates not merely a scarcity of red minerals but actually a dominance of blue minerals. Don’t be so timid! You must highlight this strange imaginary index, and point out how it contradicts the behavior reported by Tedesco et al. 2013. [Disconnect Type “b”]
(6) Lines 539-541, discussing Figure 3B. Albedo spectra for algae “downsloping with increasing wavelength between 0.35 and 0.45 microns . . . and a gentle increase to 0.70 microns. These spectral features are consistent with our field spectra for algal ice” This is not true. The field spectrum for algal ice (Figure 2B) starts at 0.40 not 0.35, and shows albedo increasing, not downsloping, from 0.40 to 0.45 microns. And Figure 2B (field) shows a steep increase from 0.6 to 0.7 microns, whereas the dashed line in Figure 3B (model) is flat from 0.6 to 0.7. [Disconnect Type “b”]
(7) Lines 574-575. “. . . algal cells had a greater albedo-reducing effect than mineral dusts in north-west Greenland (Aoki et al. 2013).” This summary of the Aoki paper is misleading. Aoki et al. did conclude that the imaginary index of algae was larger than that of mineral dust, but did not conclude that most of the albedo reduction was due to algae. Their total impurity mass in the ice was 1127 ppm, of which 29 ppm (2.6%) was organic carbon (algae), so ~1100 ppm was dust. Their Figure 4b shows that they could explain most of the albedo reduction by 1000 ppm dust; the remainder (which looks like about 5% to me) is then attributed to algae. [Disconnect Type “c”]
(8) Lines 544-546, and Figure 2C. The authors point out that mineral dust particles can “act as substrates for the formation of low-albedo microbial-mineral aggregates”. This suggests an alternative explanation of the correlation shown in Figure 2C: The algae may be concentrated in patches of ice that have high mineral content, so the cell count then would be correlated with the albedo reduction caused by dust.
(9) Section 3.3, lines 577-610. This section analyzes the albedo trough centered at 1.02 microns, following Nolin and Dozier (2000). Nolin and Dozier found the 1.02-micron trough to be deeper for lower albedo (coarse-grained snow), whereas Supp Info 5B here shows the opposite, namely deeper trough for higher albedo. The 1.02-micron feature is therefore not useful for discussing “indirect effects of algae”. The entire section 3.3 should therefore be shortened to just the last four lines 607-610, making a single sentence starting “Algal growth is stimulated . . . “ [Disconnect Type “c”]
(10). Figure 4ABC. Half of the solar energy is at wavelengths <0.7 microns, and 80-90% (depending on cloud thickness) is at wavelengths <1.0 microns. In these figures, the peculiar wiggles in the visible region, the most energetically important part of the spectrum, are squeezed into a tiny region on the far left of the figures. These wiggles need to be discussed and explained in the text, and the figure should be redrawn, for a domain 0.3-1.5 microns instead of 0-5 microns.
line 206. “. . . they are large, far outside the domain of Mie scattering”. Mie theory is not restricted to small size-parameters. Admittedly Mie calculations do become expensive for large size-parameters.
line 247. “real refractive index”. I think you mean imaginary refractive index, or complex refractive index.
line 262. “four global average dusts from Flanner et al. (2007)”. The Flanner 2007 paper concerns black carbon; it contains no information about dust. Give the correct reference. [Disconnect Type “c”]
line 320. “CI” has not yet been defined.
line 478. Change 2B to 2D.
line 484. Change 2C to 2B.
line 488. Change 2C to 2D.
Figure 2B. The labels Hbio, Lbio, CI, SN should be defined in the figure caption.
Figure 6. This is a nice figure showing the contrast between 2016 and 2017.
Table 2 should be rearranged. Seeing the numbers from different methods side-by-side would help the reader compare them. For example, the first line (water albedo) would read “0.31, 0.08, 0.08”.
Spelling and punctuation.
line 181. Change gluteraldehyde to glutaraldehyde.
line 290. “the the”
line 983. distributed
line 1240-1245 (Table 1). “ug” is not the correct symbol for micrograms. Use the Greek lower-case mu.