Peer review of: The Red Sea Deep Water is a potent source of atmospheric ethane and propane (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Manuscript Review 
Title: Red Sea deep brine pools - a potent natural source of atmospheric ethane and propane 
Submitted by Bourtsoukidis et al. 

The investigations described in the submitted manuscript are comparable to the global approach of 
Dalsoren et al., published in Nature in 2018 (“Discrepancy between simulated and observed ethane 
and propane levels explained by underestimated fossil emissions”), but focus on the Arabian Peninsula 
and adjacent waters and identify a yet unconsidered geological source of NMHC into the atmosphere. 
Assuming that the current emission inventory used in their model is sufficiently well defined - it is 
mentioned that only few measurements and data are published for the region of interest - a huge 
emission of ethane and propane is indicated by comparing modelled with measured atmospheric gas 
concentrations. The calculated ethane+propane emission is comparable to the respective annual 
emissions from Iraq or UAE. 

The authors anticipate that the emission source is located at the seafloor in the Northern Red Sea. 
Two brine-filled Red Sea deeps in the area (i.e. Kebrit and Oceanographer Deep) are known from 
literature containing high concentrations of dissolved methane, ethane and propane. However, 
transport processes from brine-seawater interfaces through 1400 m of water column up to the sea 
surface are unknown. Density stratification (thermocline), inflow and outflow of water masses 
(residence times), vertical transport times, and secondary (microbial) degradation of NMHC would play 
a major role for the local flux of hydrocarbons into the atmosphere. 
State of the art onboard gas chromatography was used to measure VOC composition of air along the 
research track around the Arabian Peninsula. Established statistical approaches were used to identify 
emission sources in the area (Group/factor analyses). Atmospheric reaction and transport model EMAC 
was applied to simulate degradation and formation of trace gases and to back-trace the origin of 
emission. The measured atmospheric pollutant composition, modelled atmospheric hydrocarbon 
distribution and transport directions, and ozone and PAN formation in the area of interest is a valuable 
information by itself. 

In summary, the manuscript is very well written and the quality of measurements, modelling and 
presentation of data and interpretation is very good. The topic is of general interest for environmental 
chemists, oceanographers, geologists, climatologists etc.. 
I suggest accepting the MS for publication after considering the following concerns (major revisions). 

1. Deep-reaching eddies: 
Vertical transport of water masses by eddies could be a reasonable approach for the upper 200-300 m 
water column in the northern Red Sea, as demonstrated by published data (see submitted MS). The 
normal vertical transport from seafloor to sea surface by (turbulent) mixing is in the range of several 
100 years for a water column of about 1000 m. Thus, to convince the oceanographic community about 
a deep impact into RSDW by eddies one should discuss this a bit more in detail. E.g. eddies can 
indeed affect deep water environment down to about 4000 m with downward effective transfer rate of 
200–600 m∙day−1 (e.g. Aleynik et al. 2017). 

2. Flux calculations: 
Density differences at brine-seawater interfaces of Kebrit and Oceanographer Deeps are about 0.2 
g/cm3 per 3 m. Transport of constituents is controlled by diffusion. Concentrations of 
ethane+propane, water depths of interfaces and bathymetry are published. Thus the total area for 
emissions from brine-seawater interfaces can be estimated. Why not calculating maximum possible 
fluxes of NMHC at steady state conditions? When doing a rough calculation of e.g. methane flux across 
brine +seawater interface at Kebrit Deep, using Fick’s first law, my estimated emission is about 390 
kg CH4 per year from Kebrit Deep (see also Tab. 3 in Schmidt et al., 2003). The brine-seawater 
interface of Oceanographer Deep is of similar size and water depth, compared to Kebrit Deep. So we 
are talking about a flux of less than 1x106 g CH4 per year from these two brine pools. This is several 
orders of magnitude lower than the predicted 1.3 Tg CH4 per year emission rate from Kebrit and 
Oceanographer brines, calculated in the submitted manuscript. Obviously there is a mismatch and I 
suggest to consider additional sources for methane and NMHC from depth. 

3. Additional methane and NMHC sources: 
Additional hydrocarbon release from seafloor is possible due to fluid seepage from a partly fractured 
seafloor deposited above offshore source rocks, located between 25 and 28°N (e.g. Rudeis and Kareen 
formation). 
Upwelling of hydrocarbon-enriched bottom water derived from inflow waters originated in the Gulf of 
Suez, e.g. crude oil/gas seepage fom leaky subsea wells could be an additional source. The outflow of 
dense hydrocarbon-rich water from Gulf of Suez feeding the RSDW and subsequent upwelling in the 
Northern Red Sea is already mentioned in the manuscript. 
The upwelling concept is mainly derived from tracer studies and the respective paper should be cited 
(Jean-Baptiste et al., 2004). Consider the full range of estimated residence times for RSDW: 
~10 years (Yao and Hoteit, 2018) 
~26-60 years (Jean-Babtiste et al., 2004) 

4. Temporal variability of emissions (triggered events): 
One should think about triggered emission events e.g. by seismic events in this area. Earth quake 
(M4.4) in central graben 144 km northwest of Jeddah on June 19th, 2017 fueling the RSDW with oil-
associated gas? This was only one month before the measuring campaign started. Maybe the high 
ethane and propane anomalies in the Northern Red Sea is a temporal phenomenon? This would 
change the estimated annual flux calculation significantly. 

5. Secondary microbial oxidation 
Lines 179-181: you are citing anaerobe microbial oxidation here. However, degradation of 
hydrocarbons in the water column mainly proceeds by aerobic oxidation. So I don’t think 42 and 43 
are appropriate here. Moreover, ethane and propane are effectively oxidized in an oxygenated water 
column and bacterial degraders preferentially oxidize ethane (e.g. Valentine et al., 2010). 

Minor corrections: 
Line 496: Give full citation of reference “20” 
Legend in Fig.1-d: correct “Measuremenets” and increase symbol size (dot) 

Added references: 
Valentine, et al. (2010) Propane Respiration Jump-Starts Microbial Response to a Deep Oil Spill. 
Science 330, 208. DOI: 10.1126/science.1196830 

Aleynik et al. (2017) Impact of remotely generated eddies on plume dispersion at abyssal mining sites 
in the Pacific. Sci. Rep. 7 (1), p. 16959, 10.1038/s41598-017-16912-2 

Jean-Baptiste et al. (2004) Red Sea deep water circulation and ventilation rate deduced from the He-3 
and C-14 tracer fields. J. Mar. Syst. 48, 37–50. 

M4.4 earth quake: 
https://www.researchgate.net/publication/257786054_A_spatial_statistical_analysis_of_the_occurren
ce_of_earthquakes_along_the_Red_Sea_floor_spreading_Clusters_of_seismicity/figures 

sign. Mark Schmidt 



Reviewer #2 (Remarks to the Author): 

This paper identifies a source of ethane and propane to the atmosphere previously unrecognized and 
not included in standard emissions inventories, namely from brine pools in the Red Sea. Ethane and 
propane are important precursors of air pollution (ozone), and have been generally underestimated in 
model simulations due to an incomplete understanding and quantification of their sources. These 
results are important for improving prediction of air quality in the region, and may have increasing 
importance as the types of emissions sources in the region (such as shipping) evolve. 

The paper is well written and clearly organized. The methods are explained in sufficient detail, in 
particular the positive matrix factorization used to determine the strength of the new emissions source 
is clearly described. The figures clearly illustrate the results. 

I recommend publication of this paper and have just a few minor suggestions: 

1) I recommend plotting the ratio of model / measurement, instead of meas./model as you do in the 
main text and Supplement. The way they are plotted, I think the implication is that the measurements 
are too high, but your point really is that the model is too low. I think this would be more clearly 
shown if the measurements (truth) are the reference (model/meas.), as the scatter plots show, having 
the measurements on the x-axis. However, if there is a good argument for showing (meas./model) it 
is not necessary to change it. 

2) Ref. 20 is incomplete. 

3) Supp. Figures 14-16: Explain what the max value is exactly – maximum of daily, hourly values in 
the 3 months? 

4) It might be nice to have a figure in the main text illustrating the implications of this missing source. 
Perhaps Supp. Fig. 17 could be included in the main text? 

5) Another reference that illustrates the impact of correcting ethane emissions on PAN and O3 
distributions is Monks et al., JGR, https://doi.org/10.1029/ 2017JD028112, 2018.