The link between marine sediment records and changes in Holocene Saharan landscape: simulating the dust cycle

Egerer, Sabine; Claussen, Martin; Reick, Christian; Stanelle, Tanja

doi:https://doi.org/10.5194/cp-12-1009-2016

Articles | Volume 12, issue 4

https://doi.org/10.5194/cp-12-1009-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

https://doi.org/10.5194/cp-12-1009-2016

© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Articles | Volume 12, issue 4

Research article

| Highlight paper

|

15 Apr 2016

Research article | Highlight paper |

| 15 Apr 2016

The link between marine sediment records and changes in Holocene Saharan landscape: simulating the dust cycle

Sabine Egerer, Martin Claussen, Christian Reick, and Tanja Stanelle

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Final revised paper (published on 15 Apr 2016)
Preprint (discussion started on 10 Nov 2015)

Interactive discussion

Status: closed

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

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RC C2555: 'Referee comments on manuscript “Marine sediments records as indicator for the changes in Holocene Saharan landscape: simulating the dust cycle” by S. Egerer et al.', Anonymous Referee #1, 02 Dec 2015
- AC C2972: 'Response to Anonymous Referee 1', Sabine Egerer, 20 Jan 2016
RC C2568: 'Review on "Marine sediment records as indicator for the changes in Holocene Saharan landscape: simulating the dust cycle"', Anonymous Referee #2, 08 Dec 2015
- AC C2956: 'Response to Anonymous Referee 2', Sabine Egerer, 19 Jan 2016
  - AC C2980: 'References to 'Response to Anonymous Referee 2'', Sabine Egerer, 20 Jan 2016
RC C2583: 'Referee comments', Anonymous Referee #3, 09 Dec 2015
- AC C2961: 'Response to Anonymous Referee 3', Sabine Egerer, 19 Jan 2016
  - AC C2981: 'References to 'Response to Anonymous Referee 3'', Sabine Egerer, 20 Jan 2016
RC C2587: 'Review of Egerer et al.', Anonymous Referee #4, 09 Dec 2015
- AC C2967: 'Response to Anonymous Referee 4', Sabine Egerer, 19 Jan 2016
  - AC C2982: 'Figures and references to 'Response to Anonymous Referee 4'', Sabine Egerer, 20 Jan 2016
EC C2770: 'General comment', Denis-Didier Rousseau, 26 Dec 2015

Peer-review completion

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

ED: Reconsider after major revisions (01 Feb 2016) by Denis-Didier Rousseau

AR by Sabine Egerer on behalf of the Authors (01 Feb 2016) Author's response Manuscript

ED: Referee Nomination & Report Request started (07 Feb 2016) by Denis-Didier Rousseau

RR by Anonymous Referee #3 (13 Feb 2016)

RR by Anonymous Referee #1 (27 Feb 2016)

Suggestions for revision or reasons for rejection

The manuscript has been significantly improved, in my opinion, and it is now more clear, also with reference to my major comments on potential climate model biases and dust size distributions. I only have a few minor comments, in particular with reference to the role of vegetation, as detailed below.

Minor comments

lines 112-117: please briefly explain by which mechanisms the mmr of the modes can change

lines 145-147: clarify the relation between the uniform vegetation and and LAI (see also my comment below)

line 251: “core of” should be “core as reported in”, as McGee et al. 2013 is the original study

lines 260-270: the addition of this section satisfactorily addresses one of my major concerns, although a few clarifications may be helpful. The authors explain quite clearly how different aerosol species contribute to each mode in their model formulation, and provide explanations on how this should be interpreted when comparing with dust in this geographic setting. They also discuss the differences in comparing the size distribution of airborne dust compared to deposited dust. I think they should briefly explain why they do not directly compare modeled deposition instead of surface concentration.

line 400-404: It would be interesting to clarify how dust emissions are impacted by vegetation changes, i.e. a direct “masking” effect on dust emissions (same as lake cover would have) versus substantial indirect impacts on dust emission from precipitation (e.g. by controlling surface properties such as soil moisture). In Table 6 it is clearly shown (and further extensively discussed in the Appendix, e.g. Figure 11) that LV-6k (and L-0k-V-6k in particular) rather than AO-6k is the major contributor to precipitation changes.

Line 427: “Additionally” should be “In addition”

lines 679-680: Please explain better how your vegetation works, i.e. the relation between a prescribed vegetation (Figure 4) and changes in LAI.

Figure 6. You should specify that observations refer to dust deposition, and modeled distribution refers to the atmospheric surface mass mixing ratios.

Figure 11. It would be interesting to highlight the grid cells with statistically significant changes.

Table 4. Some of the sites listed in this table are actually not cores, but rather sediment traps. Please review and correct. This also applies to the caption of Figure 3.

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RR by Anonymous Referee #4 (07 Mar 2016)

Suggestions for revision or reasons for rejection

The revised version of this manuscript is substantially improved over the original submission. I note a few areas where relatively minor changes could substantially improve the manuscript before it is published:

-Lines 261-270, Figure 6: The authors compare the simulated grain size distributions of dust deposited at GC68 to dust deposited in the simulations of Albani et al., but it makes more sense to compare to the observed grain size distribution in the core reported by McGee et al. (2013). EM1 and EM2 (the two endmembers taken to be eolian dust) in GC68 (Fig 2 of McGee et al.) are much coarser than simulated in this model. This difference should be noted and plotted in Figure 6.

-In discussing upwelling records from the region (e.g., lines 37, 417-418), the authors should cite the new paper by Bradtmiller et al. (Changes in biological productivity along the northwest African margin over the past 20,000 years. Paleoceanography. 31 (2016).) This paper shows that the reductions in upwelling-related productivity reported by Adkins et al. (2006) from ODP658 also occur at cores ranging from 19-27˚N. These authors also favor changes in winter winds as the driver of these changes because of the coherence of the changes along the margin (in summer, upwelling stops south of ~21˚N due to southwesterly winds, and interannual/decadal-scale variability is antiphased N and S of this latitude.)

Egerer et al. note that northeasterly winds decrease in summer in their AO6kLV6k simulation, but note that winter winds don’t change very much. The authors should discuss whether the changes in meridional surface wind stresses (tau-y) in the 6ka simulations are sufficient to explain the dramatic drop in productivity all along the NW African margin during the mid-Holocene; by eye, the wind changes don’t appear large enough, but the authors could examine this quantitatively. If the changes are not large enough to explain the productivity changes, this should be mentioned as a limitation of the study and/or a question for future work.

-on a related note, somewhere in the manuscript the underestimation of high-speed wind events by global-scale GCMs should be mentioned as a limitation of the study.

-lines 144-145: I believe that the words “steppe” and “savanna” should be switched; Hoelzmann et al. (1998) suggest savanna vegetation between ~10-20˚N and steppe vegetation between 20-30˚N, and these lines say the opposite.

-line 254: what does “aitken” mean?

-line 257: need semicolon or new sentence after “dust”

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ED: Publish subject to minor revisions (review by Editor) (12 Mar 2016) by Denis-Didier Rousseau

AR by Sabine Egerer on behalf of the Authors (01 Apr 2016) Author's response Manuscript

ED: Publish as is (05 Apr 2016) by Denis-Didier Rousseau

AR by Sabine Egerer on behalf of the Authors (05 Apr 2016)

Short summary

We demonstrate for the first time the direct link between dust accumulation in marine sediment cores and Saharan land surface by simulating the mid-Holocene and pre-industrial dust cycle as a function of Saharan land surface cover and atmosphere-ocean conditions using the coupled atmosphere-aerosol model ECHAM6-HAM2.1. Mid-Holocene surface characteristics, including vegetation cover and lake surface area, are derived from proxy data and simulations.