Diepkloof Rock Shelter offers an exceptional opportunity to study the onset and evolution of both Still Bay (SB) and Howiesons Poort (HP) techno-complexes. However, previous age estimates based on ...luminescence dating of burnt quartzites (Tribolo et al., 2009) and of sediments (Jacobs et al., 2008) were not in agreement. Here, we present new luminescence ages for 17 rock samples (equivalent dose estimated with a SAR-ITL protocol instead of classical MAAD-TL) as well as for 5 sediment samples (equivalent dose estimated with SAR-single grain OSL protocol) and an update of the 22 previous age estimates for burnt lithics (modified calibration and beta dose estimates). While a good agreement between the rock and sediment ages is obtained, these estimates are still significantly older than those reported by Jacobs et al. (2008). After our own analyses of the sediment from Diepkloof, it is suspected that these authors did not correctly chose the parameters for the equivalent dose determination, leading to an underestimate of the equivalent doses, and thus of the ages.
From bottom to top, the mean ages are 100 ± 10 ka for stratigraphic unit (SU) Noël and 107 ± 11 ka for SU Mark (uncharacterized Lower MSA), 100 ± 10 ka for SU Lynn-Leo (Pre-SB type Lynn), 109 ± 10 ka for SUs Kim-Larry (SB), 105 ± 10 ka for SUs Kerry-Kate and 109 ± 10 ka for SU Jess (Early HP), 89 ± 8 ka for SU Jude (MSA type Jack), 77 ± 8 ka for SU John, 85 ± 9 ka for SU Fox, 83 ± 8 ka for SU Fred and 65 ± 8 ka for SU OB5 (Intermediate HP), 52 ± 5 ka for SUs OB2-4 (Late HP).
This chronology, together with the technological analyses, greatly modifies the current chrono-cultural model regarding the SB and the HP and has important archaeological implications. Indeed, SB and HP no longer appear as short-lived techno-complexes with synchronous appearances for each and restricted to Oxygen Isotopic Stage (OIS) 4 across South Africa, as suggested by Jacobs et al. (2008, 2012). Rather, the sequence of Diepkloof supports a long chronology model with an early appearance of both SB and HP in the first half of OIS 5 and a long duration of the HP into OIS 3. These new dates imply that different technological traditions coexisted during OIS 5 and 4 in southern Africa and that SB and HP can no longer be considered as horizon markers.
► At Diepkloof, a consistent chronology is obtained for the Middle Stone Age sequence. ► It is based on TL and OSL dating of 39 burnt lithics and 5 sediment samples. ► Still Bay and Howiesons Poort appeared, earlier than previously thought, during OIS5. ► HP lasted for several thousand years, up to the beginning of OIS3. ► Various technological traditions were coexisting in southern Africa during OIS 5 and 4.
Continental margins play a central role in the composition of seawater by being an important source of trace element essentials to the functioning of the ocean ecosystems. Here, we measured ...long-lived radium isotopes (226Ra, 228Ra) along a zonal transect at 12°S (US GEOTRACES GP16) in the eastern tropical South Pacific Ocean. We used 228Ra to quantify the trace element and isotope (TEI) fluxes (DMn, DFe, and DCo) delivered from the Peruvian continental i) shelf and ii) slope. First, elevated 228Ra activities were measured in surface water over the entire transect (~8500km), evidence that the continental shelf is an important source of sediment-derived TEIs not only to coastal areas, but to central Pacific Ocean waters. Modeled 228Ra shelf fluxes combined with water column dissolved TEI/228Ra ratios were used to quantify the shelf-ocean input rates (normalized to shelf-area) for DMn (3.3×103μmolm−2y−1), DFe (1.5×103μmolm−2y−1), and DCo (1.0×102μmolm−2y−1). Second, co-occurring plumes of 228Ra, DFe, and DMn extended over 1800km from the margin at 1000–2500m depth, indicative of a continental slope sediment TEI input to the intermediate water column. The 228Ra gradient allowed us to derive an effective horizontal eddy diffusion coefficient (Kh) of 46m2s−1, which in turn permitted the calculation of slope sediment DMn (6.4μmolm−2y−1) and DFe (5.9×102μmolm−2y−1) fluxes based on their offshore concentration gradients. On the scale of the South Pacific continental margin between 0–20°S, the DMn shelf flux is approximately 2–3 orders of magnitude higher than the slope flux, while the DFe shelf/slope flux is ~3:1. Both shelf and slope sediment derived DMn was transported over a significant distance towards the ocean interior, while DFe concentration gradients were steep, consistent with longer water column residence time for DMn as compared to DFe in marine systems. These findings highlight the importance of considering the continental slope-ocean boundary in the oceanic budgets of biologically-important trace elements.
•The distribution of 226Ra is broadly consistent S. Pacific Ocean with water masses.•An extended surface 228Ra plume highlights Peru margin sediment inputs S. Pacific.•A co-occurring deep 228Ra plume from the slope indicates sediment inputs to deep water.•Radium-228 was used to derive trace element fluxes from shelf and slope sediments.•Continental margin sediments are a key source of trace elements to the open ocean.
Carbon and silicon cycles at the Earth surface are linked to long-term variations of atmospheric CO2 and oceanic primary production. In these cycles, the river-sea interface is considered a ...biogeochemical hotspot, and deltas presently receive and preserve a major fraction of riverine particles in shallow water sediments. In contrast, periods of glacial maximum lowstand were characterized by massive exports of sediments to the deep-sea via submarine canyons and accumulation in deep-sea fans. Here, we calculate present-day mass balances for organic carbon (OC) and amorphous silica (aSi) in the terminal lobe complex of the Congo River deep-sea fan as an analogue for glacial periods. We show that this lobe complex constitutes a megasink with the current accumulation of 18 and 35% of the OC and aSi river input, respectively. This increases the estimates of organic carbon burial by 19% in the South Atlantic Ocean in a zone representing less than 0.01% of the basin. These megasinks might have played a role in carbon trapping in oceanic sediments during glacial times.
•Sediments located at the termination of Congo Canyon are a megasink of organic carbon (0.35 TgC/yr) and aSi (0.11 TgSi/yr).•These sediments store in the deep-sea at 5 km depth 18 and 35% of Congo River OC and amorphous silica inputs, respectively.•OC burial in these sediments increases OC burial in the South Atlantic deep basin (>3000m) by 19% for a surface area <0.01%.•Burial efficiencies in this megasink are 85% for OC and 73% for aSi.
Radium isotopes are widely used in marine studies (eg. to trace water masses, to quantify mixing processes or to study submarine groundwater discharge). While
228Ra and
226Ra are usually measured ...using gamma spectrometry, short-lived Ra isotopes (
224Ra and
223Ra) are usually measured using a Radium Delayed Coincidence Counter (RaDeCC). Here we show that the four radium isotopes can be analyzed using gamma spectrometry. We report
226Ra,
228Ra,
224Ra,
223Ra activities measured using low-background gamma spectrometry in standard samples, in water samples collected in the vicinity of our laboratory (La Palme and Vaccarès lagoons, France) but also in seawater samples collected in the plume of the Amazon river, off French Guyana (AMANDES project). The
223Ra and
224Ra activities determined in these samples using gamma spectrometry were compared to the activities determined using RaDeCC. Activities determined using the two techniques are in good agreement. Uncertainties associated with the
224Ra activities are similar for the two techniques. RaDeCC is more sensitive for the detection of low
223Ra activities. Gamma spectrometry thus constitutes an alternate method for the determination of short-lived Ra isotopes.
Pollen, spores and dinoflagellate cysts have been analysed on three sediment cores (1.8–1.4m-long) taken from the south and middle basins of the Caspian Sea. A chronology available for one of the ...cores is based on calibrated radiocarbon dates (ca 5.5–0.8cal.kaBP). The pollen and spores assemblages indicate fluctuations between steppe and desert. In addition there are some outstanding zones with a bias introduced by strong river inflow. The dinocyst assemblages change between slightly brackish (abundance of Pyxidinopsis psilata and Spiniferites cruciformis) and more brackish (dominance of Impagidinium caspienense) conditions. During the second part of the Holocene, important flow modifications of the Uzboy River and the Volga River as well as salinity changes of the Caspian Sea, causing sea-level fluctuations, have been reconstructed. A major change is suggested at ca 4cal.kaBP with the end of a high level phase in the south basin. Amongst other hypotheses, this could be caused by the end of a late and abundant flow of the Uzboy River (now defunct), carrying to the Caspian Sea either meltwater from higher latitudes or water from the Amu-Daria. A similar, later clear phase of water inflow has also been observed from 2.1 to 1.7cal.kaBP in the south basin and probably also in the north of the middle basin.
Chronologies of lake-sediment records covering the last centuries to millennia are usually based on both short-lived radionuclides and radiocarbon dating. However, beyond the range of short-lived ...radionuclides, age model accuracy often suffers from large radiocarbon uncertainties. For high-altitude records, this issue is even more prominent as terrestrial plant fragments for radiocarbon dating are often lacking due to the sparse vegetation in such environments. In this study, we evaluate the potential of the geomagnetic field secular variations as a complementary tool to establish more robust age–depth relationships. Our palaeomagnetic study, applied to five high-altitude lakes from the western European Alps, first shows that recent unconsolidated sediments can carry stable remanent magnetization. The analysis of the magnetic parameters indicates that low-coercivity pseudo-single domain magnetite grains carry the natural magnetization. Nevertheless, the quality of palaeomagnetic secular variation records varies from one lake to another. This quality can be illustrated through the calculation of the declination/inclination maximum angular variations and their comparison to the expected value. Compared with available models, the declination variations are usually too large and the inclination too high. We discuss the validity of palaeosecular variation (PSV) of the Earth’s magnetic field regarding rock magnetism, magnetization processes and possible deformation during coring. From a magnetic point of view, the quality of data is variable, but the characteristic remanent magnetization direction is consistent at site level between neighbouring lakes and with the reference curve, suggesting that geomagnetic field secular variations are approximately recorded. Finally, we attempt to correlate the declination/inclination variations of the characteristic remanent magnetization measured in the five records to the reference geomagnetic model to provide additional chronological markers for age–depth modelling. These stratigraphic chrono-markers appear in systematic agreement with our previous chronological data and enable a reduction of dating uncertainties up to 30% when including these chrono-markers in the age–depth modelling. This agreement supports the interpretation that PSV may have been recorded more or less accurately depending on the studied lake. Therefore, coupled with a comprehensive understanding through other analysis (sedimentology, dating, geochemistry), PSV can be used to improve the age models in the more favourable cases.
Dinoflagellate cyst assemblages were analysed in four short sediment cores collected in the south Basin of the Caspian Sea for assessing environmental changes over the last few millennia. Two of ...these cores were dated by radionuclides. The sedimentation rate of one of them was very high, in the order of 20 mm per year. The interpretation of the four sequences is supported by a collection of 27 lagoonal or marine surface sediment samples. A sharp increase in the concentration of the dinocyst occurs after 1967, especially owing to Lingulodinium machaerophorum. Considering nine other cores covering parts or the whole of Holocene, it became clear that this species started to develop in the Caspian Sea only during the last three millennia. By analysing instrumental data and collating existing reconstructions of sea level changes over the last decades, we show that the main forcing for the recent increase of L. machaerophorum percentages and concentration is global climate change, especially sea surface temperature increase. Sea level fluctuations likely only have a minor impact. We argue that the recent increase in L. machaerophorum indicates that the Caspian Sea clearly is in the Anthropocene.
•Four new short sediment cores were analysed in the south basin of the Caspian Sea.•One of them displays an exceptionally high sedimentation rate of 20 mm yr−1.•The observed sea surface temperature has increased since the beginning of the 20th century.•This parallels a recent increase in the concentration of the dinocyst Lingulodinium machaerophorum.•L. machaerophorum started to develop only 3200 years ago in the Caspian Sea, contrary to the Black Sea.
The radioactivity levels in the air of the radionuclides released by the Fukushima accident were measured at the Laboratoire Souterrain de Modane, in the South-East of France, during the period 25 ...March–18 April 2011. Air-filters from the ventilation system exposed for one or two days were measured using low-background gamma-ray spectrometry. In this paper we present the activity concentrations obtained for the radionuclides 131I, 132Te, 134Cs, 137Cs, 95Nb, 95Zr, 106Ru, 140Ba/La and 103Ru. The activity concentration of 131I was of the order of 100 μBq/m3, more than 100 times higher than the activities of other fission products. The highest activities of 131I were measured as a first peak on 30 March and a second peak on 3–4 April. The activity concentrations of 134Cs and 137Cs varied from 5 to 30 μBq/m3. The highest activity concentration recorded for Cs corresponded to the same period as for 131I, with a peak on 2–3 April. The results of the radioactivity concentration levels in grass and mushrooms exposed to the air in the Modane region were also measured. Activity concentrations of 131I of about 100 mBq/m2 were found in grass.
► Air activity concentration following Fukushima accident were measured in Modane, France. ► 131I level in the air was of the order 100 μBq/m3 and 134Cs and 137Cs levels were between 5 and 30 μBq/m3. ► The highest 131I, 134Cs and 137Cs levels were on 30 March and 2–4 April 2011. ► Peak activity levels explained by meteorological conditions and consistent with simulation models.
Natural iron fertilization promotes the phytoplankton bloom that takes place on the Kerguelen Plateau in the Indian sector of the Southern Ocean. We measured
228Ra (
T
1/2=5.75
yr) and
226Ra (
T
...1/2=1602
yr) in waters above the Kerguelen Plateau in order to provide information on the water-mass pathways, which in turn could help elucidate the mechanisms controlling iron fertilization in that area.
228Ra activities are extremely low in this region, being in most cases <0.15
dpm/100
kg (<2.5×10
−18
g/kg). Station A3 (520
m depth), located on the plateau in the middle of the bloom area, also displays such low values but with higher
228Ra activities (and higher
228Ra/
226Ra ratios) in the upper 150
m. Such a pattern suggests the presence of a water mass that recently interacted with sediments and therefore has been advected onto the Kerguelen Plateau. Elevated
228Ra activities were found in shallow waters in the vicinity of Heard Island, south of the Kerguelen Plateau. Contact of the water masses with the shallow lithogenic sediments likely explains these high
228Ra activities. When combined with physical observations, these results suggest that the water mass advected onto the plateau originates from the south of the Kerguelen Plateau. This northward advection might represent a supply of dissolved and/or particulate Fe for the observed phytoplankton bloom.
Using the
228Ra profile obtained at station A3, we could estimate the vertical eddy diffusivity (
K
z
) on the Kerguelen Plateau. Our
K
z
estimate (1.5×10
−4±1.3×10
−4
m
2/s) compares well with that reported by Park et al. 2008a. Internal tides and vertical mixing over the Kerguelen Plateau. Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.027 based on CTD/LADCP data. By combining this
K
z
estimate with the gradient of dissolved Fe observed at A3 Blain, S., Sarthou, G., Laan, P., 2008. Distribution of dissolved iron during the natural iron fertilization experiment KEOPS (Kerguelen Plateau, Southern Ocean). Deep-Sea Research II, doi:10.1016/j.dsr2.2007.12.028, we calculate a vertical Fe flux of 1.0–14.3
nmol/m
2
d.
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