Evolution of winter precipitation in the Nile-River watershed since the last glacial
Abstract. Between 11.5 and 5 ka BP, the Sahara was vegetated owing to a wet climate during the African Humid Period (AHP). However, the climatic factors sustaining the “Green Sahara” are still a matter of debate. Particularly the role of winter precipitation is poorly understood. Using the stable hydrogen isotopic composition (δD) of high molecular weight (HMW) n-alkanoic acids in a marine sediment core from the Eastern Mediterranean (EM), we provide a continuous record for winter precipitation in the Nile-River delta spanning the past 18 ka. Pairing the data with regional δD records from HMW n-alkanes, we show that HMW n-alkanoic acids constantly derive from the delta while the HMW n-alkanes also receive significant contributions from the headwaters between ~15–1 ka BP due to enhanced fluvial runoff. This enables us to reconstruct the evolution of Mediterranean (winter) and monsoonal (summer) rainfall in the Nile River watershed in parallel. Heinrich Stadial 1 (HS1) evolved in two phases with a dry spell between ~17.5–16.0 ka BP followed by wet conditions between ~16–14.5 ka BP owing to movements of the Atlantic storm track. Winter rainfall enhanced substantially between 11–6 ka BP lagging behind the intensification of the summer monsoon by ca. 3 ka. Heavy winter rainfall resulted from a southern position of the Atlantic storm track combined with elevated sea-surface temperatures in the EM reinforcing local cyclogenesis. We show that during the “Green Sahara” monsoon precipitation and Mediterranean winter rainfall were simultaneously enhanced and infer that the winter-rainfall zone extended southwards delivering moisture to the Sahara. Our findings corroborate recent hypotheses according to which southward extended winter rains were a crucial addition to the northward displacement of the summer monsoon helping to sustain a “Green Sahara”.
Vera Dorothee Meyer et al.
Status: open (until 22 Jun 2023)
- RC1: 'Comment on cp-2023-23', Anonymous Referee #1, 31 May 2023 reply
Vera Dorothee Meyer et al.
Vera Dorothee Meyer et al.
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This paper presents new analyses of the C26 and C28 alkanoic acids from a core the Levantine basin (GeoB7702-3 ). There was a previously published C-31 alkane record from this site, so analysis of these new chain lengths offers an interesting complement. More could be done with analysis of the offset between different chair lengths, though they largely reflect a very similar signal. There is a need to hone explanation of the mechanisms underlying changes in the record, but otherwise it is a very nice study.
The introduction is very clear and very well written. However, I would try to discuss more uncertainties about winter rainfall in the last glacial and mid-Holocene. See the dynamical papers linked below.
Table 1: I would actually rather see the age model figure (e.g. the pdf output from BACON) and a table like this in the Supplementary information, especially since many of the dates are already published in other sources)
3.2 Lipid extraction: do you worry about an aquatic signature with C26 and C28 at all? There is some literature suggesting that mid-chains alkanes could be produced by submerged macrophytes (see Ficken et al 2000)?
I disagree with the logic that if d13C and dD are not significantly correlated then it is not necessary to consider the impact of vegetation change. A step change in d13C can possibly mask a change in dD of precipitation in a leaf wax record if there is a shift in the dominant physiological pathway during an interval when dD of leaf wax appears relatively complacent. I think a more defensible argument to make is that the amplitude of the carbon isotope change in your record is very small, and therefore a constant epsilon/apparent fractionation would be more appropriate at this site.
Line 205: I am not sure I agree that the alkanoic acids and alkanes really show completely dissimilar patterns - if you think they do, it would be good to describe where the dissimilarities arise. To my eye, it appears that the n-acids and n-alkanes show similar trends but different amplitudes of variability.
While it is lower resolution, there is actually an n-acid leaf wax record of hydrogen isotopes from the Dead Sea that was recently published (Tierney et al 2022, Quaternary Science Reviews). It might be useful to compare your findings to that record, as it will show whether over time the isotopic signature at GeoB7702-3 more strongly reflects African records influenced by the AHP or Levantine records.
Section 5.1 and 5.2:
The idea of using modern precipitation isotopes to essentially ‘fingerprint’ the source area from different leaf wax isotopes is an interesting one. I would like to see this information incorporated into the figures a little more - you could potentially plot dD of precipitation (inferred by applying a constant epsilon to your wax records) and plot modern values of dDp from different parts of the Nile catchment on this plot.
Given the strong focus on organic matter source and provenance, it would also be nice to see changes in concentration of different leaf waxes over time plotted in one of the main text figures (at least for C26 and C28, the chain lengths measured in this study)
Again, I think more analysis is needed to clearly show that alkanoic acids vs. alkanes are picking up different signatures of winter vs. summer precipitation. Perhaps I am missing something, but Table 2 seems to suggest that the modern near-foretop values of both the C-31 alkane and the C-26 and C-28 alkanoic acids appear similar to precipitation isotopes in the Delta compared to the isotopic signature farther south?
A broader point to consider: it appears you are primarily interpreting modern precipitation isotope seasonality in terms of seasonality (e.g. summer vs. winter end members) - how does this contrast with the ‘amount effect’ often used in paleoclimate studies?
A few dynamical studies that may be useful to consider
Ludwig, P. and Hochman, A., 2022. Last glacial maximum hydro-climate and cyclone characteristics in the Levant: a regional modelling perspective. Environmental Research Letters, 17(1), p.014053.
Goldsmith, Y., Polissar, P.J., Ayalon, A., Bar-Matthews, M., DeMenocal, P.B. and Broecker, W.S., 2017. The modern and Last Glacial Maximum hydrological cycles of the Eastern Mediterranean and the Levant from a water isotope perspective. Earth and Planetary Science Letters, 457, pp.302-312.