Marine reservoir ages for coastal West Africa

. We measured the 14 C age of pre-bomb suspension-feeding bivalves of known-age from coastal West Africa across 10 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 14 C-dating of these known-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 for five outliers, the ΔR and R values were quite homogenous to a weighted 15 mean value of -72 ± 42 14 C yrs (1sd, n = 24), and of 406 ± 56 14 C yrs (1sd, n = 24), 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 where different species were available, the results yielded similar results whatever the species considered suggesting that in these locations 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, 20 suggesting that marine reservoir age of coastal West Africa is driven, at least at first order, by the atmospheric CO 2 14 C ageing due to fossil fuel burning rather than by local effects. Each outlier was discussed. Local upwelling conditions or sub-fossil specimens may explain the older 14 C age and thus larger marine reservoir age measured for these samples. Bucardium ringens might not a best choice for marine reservoir age reconstructions.

In this study we report new marine reservoir age values (n=30) based on the 14 C dating of bivalves with a known pre-bomb collection date and collected across a latitudinal transect extending from Mohammedia (Morocco, 33°N) to Moçâmedes (Angola, 15°S).Our suite of sample includes specimens from Mauritania, Senegal, Republic of Guinea, Sierra Leone, Ivory Coast, Benin, Gabon and Republic of Congo (Fig. 1, Table S1 in the Supplement).We used specimens of five different species: Senilia senilis, Bucardium ringens, Donax rugosus, Ostrea stentina and Pseudochama gryphina.We briefly discuss our results in the context of local environmental setting of the studied bivalves and regional oceanography of the Eastern Atlantic Ocean.
2 Material and methods

Material
Bivalve shells were selected from the collections of the Muséum National d'Histoire Naturelle (MNHN) (Paris, France) (Table 1).We carefully chose pre-bomb specimens of known-age and ensured that they were collected alive or very soon after death.
For example, specimens with articulated valves and exhibiting flesh remains inside the shell were clearly collected alive.For Senilia senilis, the presence of the fragile periostracum provides evidence that the specimen was collected fresh.For Bucardium ringens, remains of the hinge ligament indicate that the bivalve death occurred not long before collection.The collection date was also carefully checked.Below, we provide background information for the five different bivalve species investigated in this study.Additional information for each sample is given in section 3.1.
Senilia senilis (Linnaeus, 1758) can be found from Mauritania to northern Angola.It lives in fine sand in estuaries, creeks or lagoon with regular tidal influence from the lower intertidal zone to about 2 meters water depth.The species is tolerant to seasonal salinity changes (von Cosel and Gofas, 2019).S. senilis is a suspension feeder that lives in the top 5-10-cm layer of the sediment (Okera, 1976;Catry et al., 2017).
Bucardium ringens (Bruguière, 1789) occurs from Mauritania to southern Angola.It lives in clean fine sand and mixed sand on open coast from shallow (5-10 meters depth) to about 50 meters depth.Shells and valves are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019).B. ringens is likely a suspension feeder as typically are cardiids (Herrera et al., 2015).
Donax rugosus (Linnaeus, 1758) occurs from Mauritania to Ghana and from northern Angola to southern Angola.It lives in mixed and coarse sand in the surf zone of open beaches (von Cosel and Gofas, 2019).D. rugosus is a suspension feeder (Smith, 1971).
Ostrea stentina (Payraudeau, 1826) can be found from southern Portugal to Ghana, then from Gabon to northern Angola.It is common and occurs on various types of hard substrate such as rocks, stones, pebbles and other oysters from 1 to 30 meters depths.It can also be found in lagoons, inlets and creeks under marine condition (von Cosel and Gofas, 2019).O. stentina is a suspension feeder (Türkmen et al., 2005).
Pseudochama gryphina (Lamarck, 1819) occurs from Southern Portugal to Mauritania and from Gabon to southern Angola (von Cosel and Gofas, 2019) and lives on hard substrate such as rocks and stones in clear water offshore about 10 to 60 meters water depth.P. gryphina is a suspension feeder (Sessa et al., 2013).
A small piece (30-100 mg) of the outermost layers of each shell was cut using a Dremel TM rotary tool fitted with a cut-off wheel.We focused on the external part of the shell to ensure that we sampled and dated the most recent part (likely the last few months) of the specimen.The shell carbonate samples were then sonicated and rinsed in deionized water at least 5 times.
Samples were coarsely crushed and split into a subsample for stable isotopic analysis and a subsample for 14 C analysis.

Radiocarbon measurements
Samples were washed with dilute HNO3 (0.01M) for 15 min then rinsed to neutral pH.Then the shell carbonate was converted into CO2 following LMC14 laboratory (Laboratoire de Mesure du Radiocarbone, Saclay, France) standard phosphoric acid hydrolysis procedure (Tisnérat-Laborde et al., 2001;Dumoulin et al., 2017).The CO2 was then converted to graphite (Cottereau et al., 2007;Dumoulin et al., 2017) and analyzed for its 14 C composition by Accelerator Mass Spectrometry (AMS) using the Artémis 14 C AMS facility (Moreau et al., 2013).Results are corrected for the 13 C/ 12 C ratio as measured on the AMS (Santos et al., 2007) and are reported in the F 14 C notation (Reimer et al., 2004).F 14 C is identical to the ASN/AON metric (Stuiver and Polach, 1977), and the 14 aN notation (Mook and van der Plicht, 1999).Corresponding conventional 14 C ages reported in 14 C years Before Present (AD 1950) were calculated according to:

Marine Reservoir Age calculation
The marine reservoir age R of the selected shells is calculated according to equation (1) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), 14 Cm is the measured shell 14 C age, and 14 Catm is the 14 C age of the atmosphere.For shells picked from the northern hemisphere, 14 Catm is obtained from the IntCal20 calibration curve (Reimer et al., 2020).For shells from the southern hemisphere, we used instead the southern hemisphere calibration curve SHCal20 (Hogg et al., 2020).The uncertainty is calculated (Soulet, 2015) according to: Note that mean SHCal20 offset compared to IntCal20 is estimated to be 36 ± 27 14 C yrs.Thus, the R values calculated with IntCal20 or SHcal20 are essentially the same if one takes uncertainties into account.
The local marine reservoir offset ΔR of the selected shells is calculated according to equation (2) where t is the collection year as known from the museum records (Table S1 in the Supplement and section results), 14 Cm is the measured shell 14 C age, and 14 CMarine20 is the 14 C age of the global marine calibration curve.The uncertainty is calculated as follows: (5) Note that Reimer and Reimer (2017) do not propagate the uncertainty of Marine20 calibration curve.

Radiocarbon measurements results
The detailed description of the samples and the results are shown in Table 1 and Table S1 in the Supplement.We classified the samples by location with corresponding geographic coordinates, then by species.Code numbers "MNHN-IM-2022-xxxx" allows one to find the samples in the collections of the MNHN of Paris (France).Code numbers "SacA-xxxxx" are the radiocarbon laboratory number for the sample.

West African marine reservoir ages
The vast majority of the calculated ΔR values, with a weighted mean value of -72 ± 42 14 C yrs (1sd, n = 24), which corresponds to a weighted mean R value of 406 ± 56 14 C yrs (1sd, n = 24), are typical of low latitudes marine reservoir age values (Bard, 1988;Bard et al., 1994) (Table S1 in the Supplement, Fig. 1).Note that all averaged R and ΔR values were calculated according to the methodology recommended in the Marine Reservoir Correction Database (Reimer and Reimer, 2001; http://calib.org/marine/AverageDeltaR.html; last seen 31/05/2023).Our results agree perfectly with those already obtained (Ndeye, 2008) from the Nouadhibou-Cansado Bay area (Mauritania; Nh in Fig. 1) and the Dakar area (Senegal; Dk in Fig. 1); the only two areas that we can compare our results to.
No significant interspecific differences were observed.This is best illustrated for the localities where reservoir age values were obtained from at least two different species for the same calendar time.In the Dakar area (Senegal; Dk in Fig. 1) for years 1908-1909 AD, we present data for 5 species (Bucardium ringens, Donax rugosus, Mactra glabrata, Ostrea stentina, Senilia senilis) (Ndeye, 2008; this study) all clustering within a range of [413;546] ([min; max]) with an average ΔR value of -18 ± Lu in Fig. 1) in the 1910 AD, with two species (Donax rugosus and Senilia senilis) yielding the same reservoir age values.
This was further supported for the area of Nouadhibou-Cansado Bay (Mauritania) showing the same pattern (Ndeye, 2008; this study), although one sample out of four was likely an outlier (Bucardium ringens with # MNHN-IM-2022-4599).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 in this region.The fact that all investigated species in this study correspond to suspension feeders further implies that suspension feeders are suitable material for reservoir age reconstruction.
Unlike semi-isolated basins such as the Baltic Sea (Lougheed et al., 2013) and Black Sea (Soulet et al., 2019), where the radiocarbon system is closely linked to the local oxygen/carbon stable isotopic system respectively, the open-ocean coastal region of West Africa is characterize by the lack of any relationship between reservoir age values (R or ΔR) and stable oxygen and carbon isotope compositions (r 2 of 0.02 and 0.001, respectively), as inferred from our results.

Marine reservoir evolution over time
The local marine reservoir age were averaged over 5-yrs windows ([1886-1890 AD]-[1891-1895 AD] and so on), excluding the five outliers discussed in section 3.5.Sample with radiocarbon lab # AA-70015 (see Table S1 in the Supplement) is a single value from 1916 AD and was averaged with samples from years 1912 AD.We also calculated global marine reservoir age as the difference between the Marine20 and IntCal20 calibration curves.The evolution of the marine reservoir age of coastal West Africa (pink symbols in Fig. 2) shows a similar trend as that of the global marine reservoir age (black line in Fig. 2) with values declining steadily with time since ca.1900 AD.
The 14 C age evolution of the global ocean (Marine20 calibration curve; Heaton et al., 2020) is constructed using the global carbon cycle model BICYCLE (Köhler et al., 2006;Köhler andFischer, 2006, 2004;Köhler et al., 2005).This box model incorporates a globally averaged atmospheric box and modules of the terrestrial (7 boxes) and oceanic (10 boxes) components of the carbon cycle.It is driven by temporal changes in the boundary conditions mimicking changing climate and simulates changes in the carbon cycle including 14 C. To construct the Marine20 calibration curve, the BICYCLE model was revised to allow the atmospheric CO2 and F 14 C to be specified externally (Heaton et al., 2020).While the modelled Marine20 (global surface ocean) radiocarbon age suggest constant values between 1900 and 1950 AD, our measured marine reservoir age R indicates instead a decreasing trend during that period, as a consequence of increasing atmospheric Intcal20 radiocarbon age.This observation of decreasing trend for R in West Africa between 1900 and 1950 AD could possibly reflect atmospheric 14 CO2 ageing following enhanced fossil fuel emissions to the atmosphere by burning (e.g., Suess, 1955;Tans et al., 1979)

Marine reservoir age off equatorial Ogooué and Congo rivers
Large rivers draining equatorial Africa as the Ogooué and Congo rivers inject massive amounts of freshwater into the Atlantic Ocean (Lambert et al., 2015;Milliman and Farnsworth, 2011) leading to extensive sea surface salinity negative anomalies (Martins and Stammer, 2022).The sea surface salinity negative anomalies are associated with net primary productivity positive anomalies that are likely caused by the nutrient-rich river plumes from the Ogooué and Congo Rivers (Martins and Stammer, 2022).From a radiocarbon perspective, such net primary productivity positive anomalies should imply an increased uptake of atmospheric CO2 through intensified biological pump.As a result, the reservoir age should be lower than average.The Congo River represents the second largest supplier of dissolved organic carbon (DOC) to the global ocean with ~5% of the land to ocean DOC flux (Spencer et al., 2016;Coynel et al., 2005;Richey et al., 2022).The DOC exported by the Congo river is 14 Cmodern (Marwick et al., 2015;Spencer et al., 2012) and experiments showed that 45% of the Congo River DOC can be photomineralized by sunlight (Spencer et al., 2009;Richey et al., 2022).Dissolved inorganic carbon (DIC) released from the photomineralisation of the Congo River DOC should also be 14 C-modern.Thus, this modern DOC-derived DIC should impact the marine reservoir age towards values lower compared to average.There is a lack of available data to estimate the age and flux of dissolved CO2 discharged by the Congo river into the ocean (Richey et al., 2022).Nevertheless, the marine reservoir age value measured at Port-Gentil (Gabon) close to the Ogooué river outlet is lower than the regional weighted mean value (ΔR = -106 ± 63 14 C years, corresponding to R = 329 ± 21 14 C yrs) (PG in Fig. 1).The marine reservoir age measured in Pointe-Noire (Republic of Congo) ~150 km north of the Congo river outlet is also lower than the regional weighted mean value (ΔR = -156 ± 64 14 C yrs; R = 289 ± 20 14 C yrs) (PN in Fig. 1).These values could be interpreted as having been influenced by the Ogooué and Congo Rivers discharges.However, all other localities close to the Congo River outlet had marine reservoir age close to the regional weighted mean value (Lo, Ca and Lu, in Fig. 1).Instead the lower values observed in Port-Gentil (Gabon) and Pointe-Noire (Republic of Congo) are from years 1948 and 1937 suggesting that these lower values are in line with the declining global marine reservoir evolution linked to the atmospheric CO2 14 C ageing linked to 14 C-dead input from fossil fuel burning (Suess effect) (see section 3.3).The impact of the African equatorial rivers on the local/regional coastal marine reservoir age, if any, cannot be inferred from our results.

Outlier specimens
Mean marine reservoir age values (R and ΔR) are provided for West Africa based on our data, excluding 5 samples.These particular samples display much larger values with ΔR values ranging from 209 to 454 14 C yrs or R values ranging from 701 to 912 14 C yrs.Three specimens out of the five outlier samples correspond to Bucardium ringens specimens.We analysed 8 Bucardium ringens specimens.These 3 outlier specimens display reservoir age (R and ΔR) values that clearly disagree with neighbouring data (Nouadhibou-Cansado Bay, Loos Islands and Ivory Coast areas; Nh, LI and IC in Fig. 1).The Museum number of these specimens are MNHN-IM-2022-4597, MNHN-IM-2022-4599 and MNHN-IM-2022-4601.We do not expect that these larger values compared to those for neighbouring individuals come from the species feeding practice as they are all suspension feeders like all other investigated specimens.Similarly, we showed that the difference in the habitat in this region does not impact the species reservoir ages.Instead Bucardium ringens lives in the open coast from 5-10 meters to about 50 meters depth.Shells are commonly cast ashore on beaches but live-taken specimens are rare (von Cosel and Gofas, 2019).One of these outliers was collected at low tide (Roume Island in the Loos Islands; Republic of Guinea) and was devoid of any remain of flesh or hinge ligament.It is thus possible that this outlier sample was a transported subfossil sample that died a century or more before collection date.The two other outlier samples had small remain of the hinge ligament (Nouadhibou; Mauritania and Jacqueville; Ivory Coast).It may be possible that these samples are also subfossil specimens.In this case, the hinge ligament must have been partially preserved owing to very favourable environmental conditions (Forman et al., 2004;Huntley et al., 2021).Alternatively, these outliers are not subfossil specimens and unlike the other studied species here, the habitat may exert an influence on R and ΔR values measured in B. ringens.Finally, we cannot fully rule out that these higher values represent some sub-annual variability of up to 200 14 C in the local marine reservoir age as evidenced elsewhere (Jones et al., 2007(Jones et al., , 2010)).Although, five Bucardium ringens samples out of eight displayed reservoir age values in agreement with the neighbouring reservoir age values, this specie might not be the best suited for reservoir age reconstruction or for sediment/archaeological dating.
The two remaining outliers are Ostrea stentina specimens from the El Jadida area (Morocco; eJ in Fig. 1).The sample from El Jadida beach was a single valve looking fresh and collected from the beach (museum # MNHN-IM-2022-4609).Based on the older 14 C age of this specimen, we cannot rule out that this sample could actually be a subfossil specimen.The specimen with museum # MNHN-IM-2022-4608 collected in the Sidi Moussa lagoon (south of El Jadida) was a specimen with the articulated valves and remains of flesh still inside the shell, meaning the specimen was still alive when collected.Variations in the reservoir age could be explained by coastal upwelling that impacts some regions of the Atlantic coast of Morocco and Western Sahara (Freudenthal et al., 2001;Barton et al., 1998).Upwelled waters are depleted in 14 C relative to the sea surface potentially causing larger reservoir age values (R or ΔR) like off Portugal (Monge Soares, 1993;Monge Soares and Alveirinho Dias, 2006), California (Kennett et al., 1997), Peru (Kennett et al., 2002;Fontugne et al., 2004;Jones et al., 2007Jones et al., , 2010) ) or Southern Arabian coast (Southon et al., 2002).Conversely, upwelled waters can also be nutrient-rich causing intensified ocean CO2 uptake through enhanced primary production and biological pump (Williams and Follows, 2011), in that case, one could expect low-latitude average or decreased reservoir age values (R or ΔR).Off Morocco and Western Sahara, the second hypothesis appears most likely as coastal upwelling in this area is known to bring nutrient-rich waters to the surface ocean (Barton et al., 1998;Freudenthal et al., 2001), although to our knowledge no direct measurement of the 14 C content of coastal waters in this region has been published yet.However, according to recent studies the El Jadida area is only weakly impacted by upwelling (Lourenço et al., 2020;Cropper et al., 2014), suggesting average reservoir age values instead of larger ones.
Another explanation could be linked to the local hydrology of the Sidi Moussa lagoon.Despite the lagoon being permanently connected to the ocean, it receives waters from rainfall and resurgences that can have an impact on the salinity in the upstream section of the lagoon (Cheggour et al., 2001).As the surrounding rocks are calcareous sandstones (Manaan, 2003), one could hypothesise that freshwaters feeding the lagoon might be depleted in 14 C due to carbonate dissolution in the lagoon watershed causing a hardwater effect and thus a larger reservoir age.A last explanation could be due to an imperfect cleaning of the shell.
For Ostrea stentina, sediment can be trapped between the growing layers of the shell.If this sediment contains old detrital carbonates and was not perfectly removed before 14 C measurement, the 14 C age of the shell will appear older, and the reservoir age larger.Additional reservoir age reconstructions from this region on different species would be require to validate the larger reservoir age values reconstructed from the El Jadida area.

Conclusion
The analysis of pre-bomb suspension-feeding bivalves collected along coastal West Africa from 33°N to 15°S provides marine reservoir ages that are quite homogenous, with a mean ΔR value of -72 ± 42 14 C yrs (1sd, n = 24) and a mean R value of 406 ± 56 14 C yrs (1sd, n = 24).When including the robust dataset from Ndeye (2008), the resulting mean ΔR and R values for coastal West Africa are -54 ± 51 14 C years (1sd, n = 32) and 411 ± 61 14 C years (1sd, n = 32), respectively.We show that the marine reservoir age of coastal West Africa is mainly driven by the global carbon cycle and atmospheric 14 C changes rather that by local effects.
Our results for different species yield similar marine reservoir age values, indicating that the ecological habitat only has a second-order impact on the reservoir age reconstruction, if any.Nevertheless, we suspect that Bucardium ringens might not be best suited for marine reservoir age reconstruction as corresponding shells are typically not found alive on sample collecting sites.Additionally, ages obtained on Ostrea stentina could be possibly influenced by the presence of sediment within the growing shell layers if not fully removed after the cleaning process.
Despite these new data, large portions of the West African coast still remain to be investigated for reservoir age reconstructions, in particular off Western Sahara and Canarias Islands, Sierra Leone-Liberia, Nigeria and Namibia.

Author contribution statement
GSoulet designed the study and raised the funding.SG, PM, GSoulet and GSiani selected the specimens in the MNHN collections.GSoulet carried out the sample preparation with assistance of ML and FF.FD performed stable isotopes measurements.GSoulet performed reservoir calculations and analysed and discussed the data with GB, GSiani and SG.

Republic of Congo
Mollusc specimens reported in Boettger (1912) were collected by Hesse

Figure 1 :
Figure 1: The geographic distribution of marine reservoir age values along the West African coast.A. ΔR values.B. R values.Data shown in black are from this study.Other are selected results from previous studies discussed in the text, converted from their original format (conventional 14C ages and collection dates) to ΔR and R values using the latest calibration curves Marine20 (Heaton et al., 2020) and Intcal20 or SHcal20 (Reimer et al., 2020; Hogg et al., 2020), respectively.Data in blue are from Ndeye (2008), data in green are from (Reimer and McCormac, 2002), data in purple are from (Siani et al., 2000) and data in pink are from (Dewar et al., 2012).eJ, Nh, Dk, LI, IC, PG, Lo, PN, Ca and Lu stand for el Jadida (Morocco), Nouadhibou (Mauritania), Dakar (Senegal), Loos Islands (Republic of Guinea), Ivory Coast, Port-Gentil (Gabon), Loango (Republic of Congo), Pointe Noire (Republic of Congo), Cabinda (Angola) and Luanda (Angola).The map was drawn using Ocean Data View (Schlitzer, Reiner, Ocean Data View,