the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Marine reservoir ages for coastal West Africa
Guillaume Soulet
Philippe Maestrati
Serge Gofas
Germain Bayon
Fabien Dewilde
Maylis Labonne
Bernard Dennielou
Franck Ferraton
Giuseppe Siani
Abstract. We measured the 14C age of pre-bomb suspension-feeding bivalves of known-age from coastal West Africa (n = 30) across a latitudinal transect extending from 33° N to 15° S. The specimens are from the collections of the Muséum National d’Histoire Naturelle (Paris, France). They were carefully chosen to ensure that the specimens were alive when collected or died not long before collection. From the 14C-dating of these know-age bivalves, we calculated the marine reservoir age (as ΔR and R values) for each specimen. ΔR values were calculated relative to the Marine20 calibration curve and the R values relative to Intcal20 or SHcal20 calibration curves. Except five outliers, the ΔR and R values were quite homogenous to a mean value of −77 ± 47 14C yrs (1sd, n = 25), and of 400 ± 59 14C yrs (1sd, n = 25), respectively. These values are typical of low latitude marine reservoir age values. Five suspension-feeding species living in five different ecological habitats were studied. For localities were different species were available, the results yielded similar results whatever the specie considered suggesting that the habitat has only a limited impact on the marine reservoir age reconstruction. We show that our measured marine reservoir ages follow the declining trend of the global marine reservoir age starting ca. 1900 AD, suggesting that marine reservoir age of coastal West Africa is driven, at least at first order, by the global carbon cycle and climate rather than by local effects. Each outlier was discussed. Sub-fossil specimens likely explain the older 14C age and thus larger marine reservoir age measured for these samples. Bucardium ringens might not a best choice for marine reservoir age reconstructions.
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Guillaume Soulet et al.
Status: final response (author comments only)
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CC1: 'Comment on gchron-2023-5', Paula Reimer, 08 Apr 2023
The presentation of marine reservoir ages for the west coast of Africa by Soulet et al. provides much needed data for calibrating radiocarbon ages of carbonates from marine organisms for this region. While the main results are not unexpected, they will give confidence to archaeologists and geoscientists using radiocarbon dates of shells or foraminifera for age models. It is interesting that two shells from near the discharge of the nutrient-rich Ogooué and Congo Rivers had lower reservoir ages than nearby shells but having been collected in the 1930s and 1940s fit the global decline in R. It would be worth mentioning that this global decline is due to fossil fuel input to the atmosphere (e.g. Druffel & Suess 1983). The manuscript is well-written and contains important details on the samples used that are often omitted in publications. I would definitely recommend publishing this paper with minor corrections.
Specific comments.
Line 41: ‘Larger ΔR values are located at high-latitudes’. I would suggest qualifying this as ‘Most larger ΔR values’ as there are low-latitude locations where ΔR values are higher due to groundwater carbonates being leached into coastal water (e.g. Hadden & Cherkinsky 2015) or where upwelling increases the ΔR values (e.g. Gulf of California, Goodfriend & Flessa 1997).
Line 501-504: ‘The fact that species living in very different ecological habitats (e.g., Senilia senilis in lagoons/semi-enclosed bays and Donax rugosus on beaches exposed to heavy surf; see also section material) show similar reservoir age values (R or ΔR) suggests that the habitat only exerts a minor influence on measured reservoir age.’ There are many examples where habitat does exert a major influence on R so please clarify that this conclusion is for the regions studied.
Line 561-562: ‘It is thus possible that these outlier samples were transported subfossil samples that died a century or more before collection date’ Two of these outlier samples were listed as having ligaments attached so it seems very unlikely that they had been dead for very long before collection.
Figure 1 caption: please specify what PG, LO, PN and Ca stand for on the map.
There are also some grammatical errors that need to be corrected by a fluent English speaker. Also the word ‘specie’ is not correct as ‘species’ is both singular and plural.
DRUFFEL, E. M. & SUESS, H. E. 1983. On the Radiocarbon Record in Banded Corals - Exchange Parameters and Net Transport of (Co2)-C-14 between Atmosphere and Surface Ocean. Journal of Geophysical Research-Oceans and Atmospheres, 88, 1271-1280.
GOODFRIEND, G. A. & FLESSA, K. W. 1997. Radiocarbon reservoir ages in the Gulf of California: Roles of upwelling and flow from the Colorado River. Radiocarbon, 39, 139-148.
HADDEN, C. S. & CHERKINSKY, A. 2015. 14C variations in pre-bomb nearshore habitats of the Florida Panhandle, USA. Radiocarbon, 57, 469-479.
Citation: https://doi.org/10.5194/gchron-2023-5-CC1 - AC1: 'Reply on CC1', Guillaume Soulet, 05 Jun 2023
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RC1: 'Review: No C cycle role in MRA around 1900 AD', Anonymous Referee #1, 12 Apr 2023
- AC2: 'Reply on RC1', Guillaume Soulet, 05 Jun 2023
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RC2: 'Comment on gchron-2023-5', Michel Fontugne, 24 Apr 2023
gchron-2023-5 Marine reservoir ages for coastal West Africa by Soulet & al.
The article by Soulet & al presents the 14C reservoir age measurements of seawater for the western coast of Africa, from Morocco to Angola. This type of data is lacking in the community and therefore deserves to be published. The calculation of reservoir ages and ∆R does not require any additional comment. Nevertheless, some points remain obscure and therefore call for further clarification Sampling. As for the results published by Ndeye (2008), a large number of samples come from the MNHN Paris collections and more particularly from the Gruvel missions. Soulet & al. classify five results as aberrant, in particular those from Guinea and the Ivory Coast which were collected by dredging (see Dautzenberg, Annales Inst. Océanogr.). This method of collection is not the most appropriate for this kind of study…. Ndeye had obtained 6 aberrant results including 4 for samples from the MNHN. The sample from Jacqueville and Ile de Roume come from the shell of B. ringens, which could encourage us to requalify this species for this type of study.
Outlying samples.
The other so-called aberrant samples are collected on coasts subject to the influence of deep water rises (upwelling) whose CID is depleted in 14C. The values obtained from Morocco to Dakar are compatible or characteristic of these upwelling zones. For comparison, the authors can refer to the reservoir ages calculated for the Peruvian upwelling (Kennett & al, 2002; Fontugne & al, 2004; Jones & al 2007, 2010; Owen, 2002, in Radiocarbon Ortlieb & al QR 2011; Etayo -Cadavid et al Geology 2013). Surprisingly, these references are absent when the authors mention the zones of deep water upwelling (see in Outlier specimens).The works cited above demonstrate the extreme variability of the values of the reservoir ages depending on the position of the upwelling cells, the intensity of the winds but also a variability during the period of life of the mollusk. The sample from Lake Ahémé (Benin) with a ∂13C of -4.76‰ seems to have been marked by a strong contribution of carbon of continental origin likely to reduce the reservoir age value. As this lake is located 10km from the coast with narrow communication with the coastal lagoons, it cannot really be considered as representative of the Atlantic Ocean. It would be better not to take it into account but the result can be published in the table S1.This sample comes from the Gruvel mission. Given the number of aberrant samples provided by this mission to the authors as well as to Ndeye (2008), it would be desirable to verify the harvesting conditions for all these samples. References. Not all bibliographic references are adequate: Stuiver et al 1986, Stuiver & Brasiunas 1993 (both in Radiocarbon) would be more justified for the definitions of R and ∆R (as mentioned later in the text). Stuiver and Pollack (1977) only specify that reservoir age corrections should not enter into the calculation of conventional 14C dates. As with ocean/atmosphere CO2 exchanges, it would be more accurate to mention the pioneering works of Revelle & Suess, 1957 and Craig, 1957 (both in Tellus). Brocker & al., 1985 and Stuiver 1980 (both in JGR) could also have been mentioned; these studies giving the distribution of 14C in the ocean and highlight the equatorial upwelling (or divergence) characterized by low 14C water. Of course, Bard et al 1988 can be added.The reference to the Black Sea, which is a lake occasionally connected to the Mediterranean Sea, is not the most judicious. A reference to the Baltic Sea (see Lougheed & al Clim. Past 2013) would reinforce this point. These remarks are not exhaustive, I think that the authors will be able to complete the bibliographical references with more precision. -Stable Carbon Isotopes and reservoir ages uncertainties. The authors indicate reproducibilities of ±0.04‰ and ±0.02‰ for the values of δ18O and δ13C, respectively. These are occasional measurements integrating a seasonal variability of 0.5 to 1‰ recorded by the growth rings of the shell (see among others Carré & al, 3P 2005, Jones & al RC, 2007, 2010. Jones' work also shows a seasonal variation of about 100 years in marine reservoir age. Jones' work also shows a seasonal variation of about 100 years in marine reservoir age. Perhaps, the authors could comment on the evolution of reservoir ages between 1890 & 1950 by reconsidering this new level of uncertainty In detail Table S1Columns F and G should simply be labeled Latitude and Longitude, the minus sign indicating S and W respectively.The authors wrote “Upwelled waters can be depleted in 14C relative to the sea surface” ; but upwelled water are always depleted in 14C. The upwellings areas are the atmosphere’s largest natural source of CO2 (see among others Takahashi & al Deep Sea Res., 2002).
To conclude, this paper needs more precisions. Nevertheless, its data are needed.
Citation: https://doi.org/10.5194/gchron-2023-5-RC2 - AC3: 'Reply on RC2', Guillaume Soulet, 05 Jun 2023
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AC4: 'Comment on gchron-2023-5', Guillaume Soulet, 05 Jun 2023
In our reponses to reviewers, we refer to the revised version of our manuscript, that we attach here for them to check our changes. This version will be checked for English by a native speaker in case our manuscript is accepted for publication.
Thanks.
Guillaume Soulet et al.
Guillaume Soulet et al.
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