Marine reservoir ages for coastal West Africa
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.
Guillaume Soulet et al.
Status: final response (author comments only)
CC1: 'Comment on gchron-2023-5', Paula Reimer, 08 Apr 2023
- AC1: 'Reply on CC1', Guillaume Soulet, 05 Jun 2023
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
RC2: 'Comment on gchron-2023-5', Michel Fontugne, 24 Apr 2023
- AC3: 'Reply on RC2', Guillaume Soulet, 05 Jun 2023
- AC4: 'Comment on gchron-2023-5', Guillaume Soulet, 05 Jun 2023
Guillaume Soulet et al.
Guillaume Soulet et al.
Viewed (geographical distribution)
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.
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.