Emmanuel Ponzevera

Intensifying Weathering and Land Use in Iron Age Central Africa

A Price of Civilization Large expanses of rainforests in parts of Central Africa were abruptly replaced by savannas around 3000 years ago, presumably because of climate change. However, that succession occurred at a time of expansion by Bantu tribes, from near the border of present-day Cameroon and Nigeria to the south and east, in a migration that brought with it agriculture and iron-smelting technologies. Bayon et al. (p. 1219, published online 9 February; see the Perspective by Dupont) analyzed the nearby marine sedimentary record and found that chemical weathering in Central Africa also increased markedly at this time. This increase in weathering could have been caused by forest clearing by the Bantu to create arable land and to fuel their smelters, rather than climate change alone. Savannas abruptly replaced rainforests around 3000 years ago on account of both climate and human land-use changes. About 3000 years ago, a major vegetation change occurred in Central Africa, when rainforest trees were abruptly replaced by savannas. Up to this point, the consensus of the scientific community has been that the forest disturbance was caused by climate change. We show here that chemical weathering in Central Africa, reconstructed from geochemical analyses of a marine sediment core, intensified abruptly at the same period, departing substantially from the long-term weathering fluctuations related to the Late Quaternary climate. Evidence that this weathering event was also contemporaneous with the migration of Bantu-speaking farmers across Central Africa suggests that human land-use intensification at that time had already made a major impact on the rainforest.

Formation of carbonate chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

Marine sediments at ocean margins vent substantial amounts of methane1, 2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3, 4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys.

Abrupt drainage cycles of the Fennoscandian Ice Sheet

Continental ice sheets are a key component of the Earth’s climate system, but their internal dynamics need to be further studied. Since the last deglaciation, the northern Eurasian Fennoscandian Ice Sheet (FIS) has been connected to the Black Sea (BS) watershed, making this basin a suitable location to investigate former ice-sheet dynamics. Here, from a core retrieved in the BS, we combine the use of neodymium isotopes, high-resolution elemental analysis, and biomarkers to trace changes in sediment provenance and river runoff. We reveal cyclic releases of meltwater originating from Lake Disna, a proglacial lake linked to the FIS during Heinrich Stadial 1. Regional interactions within the climate–lake–FIS system, linked to changes in the availability of subglacial water, led to abrupt drainage cycles of the FIS into the BS watershed. This phenomenon raised the BS water level by ∼100 m until the sill of the Bosphorus Strait was reached, flooding the vast northwestern BS shelf and deeply affecting the hydrology and circulation of the BS and, probably, of the Marmara and Aegean Seas.

Determination of ultra-low 236U/238U isotope ratios by tandem quadrupole ICP-MS/MS

Isotope ratios of 236U/238U were measured at levels below 10−7 by single collector ICPMS with a tandem quadrupole mass separation mechanism. Peak tailing of the prominent 238U+ ion beam on the 236U+ peak was reduced to the level of ∼10−10 by use of two quadrupole mass filters. The 235UH+ interference on 236U+ was efficiently reduced to a UH+/U+ ratio of 1 × 10−8 by an ion–molecule reaction between UH+ and O2 in a collision/reaction cell placed between the two quadrupoles. The resultant detection limit for 236U/238U measurement was better than those reported by any other ICPMS study. The 236U/238U ratios, measured as 236U16O+/238U16O+, were determined in the range 10−9 to 10−7 without correction for spectral interference. Accurate measurements of 236U/238U to as low as 1 × 10−10 are projected.

Pore water geochemistry at two seismogenic areas in the Sea of Marmara

Within the Sea of Marmara, the highly active North Anatolian Fault (NAF) is responsible for major earthquakes (Mw ≥ 7), and acts as a pathway for fluid migration from deep sources to the seafloor. This work reports on pore water geochemistry from three sediment cores collected in the Gulfs of Izmit and Gemlik, along the Northern and the Middle strands of the NAF, respectively. The resulting data set shows that anaerobic oxidation of methane (AOM) is the major process responsible for sulfate depletion in the shallow sediment. In the Gulf of Gemlik, depth concentration profiles of both sulfate and alkalinity exhibit a kink-type profile. The Sulfate Methane Transition Zone (SMTZ) is located at moderate depth in the area. In the Gulf of Izmit, the low concentrations observed near the seawater-sediment interface for sulfate, calcium, strontium, and magnesium result from rapid geochemical processes, AOM, and carbonate precipitation, occurring in the uppermost part of the sedimentary column and sustained by free methane accumulation. Barite dissolution and carbonate recrystallization have also been identified at deeper depth at the easternmost basin of the Gulf of Izmit. This is supported by the profile of the strontium isotope ratios (87Sr/86Sr) as a function of depth which exhibits negative anomalies compared to the modern seawater value. The strontium isotopic signature also shows that these carbonates had precipitated during the reconnection of the Sea of Marmara with the Mediterranean Sea. Finally, a first attempt to interpret the sulfate profiles observed in the light of the seismic activity at both sites is presented. We propose the hypothesis that seismic activity in the areas is responsible for the transient sulfate profile, and that the very shallow SMTZ depths observed in the Gulf of Izmit is likely due to episodic release of significant amount of methane.

Measuring 0.01‰ to 0.1‰ isotopic variations by MC-ICPMS—testing limits for the first time with Pb δ-iCRMs

A blind comparison on Pb-isotope δ-scale measurements by MC-ICPMS of 0.01‰ to 0.1‰ level was organised, involving five laboratories. Test samples were obtained from the series of candidate ERM-3810 δ-isotopic Certified Reference Materials (δ-iCRMs), and comprise four pairs of a material with ∼natural Pb-isotopic composition (‘delta zero’ or ‘δ-0’) and the same natural Pb progressively enriched in 207Pb (with δ207Pb values certified to ∼0.1% relative uncertainty, k = 2). Participants were free to apply the measurement strategy of their choice. A result was considered ‘acceptable’ only when, simultaneously, there was agreement within stated uncertainties with the corresponding reference value and the relative uncertainty stated by the participant was < 100%. This study illustrates the high degree of difficulty inherent to these δ-scale measurements by ‘routine’ MC-ICPMS methodologies (in this case, three participants reported 55% of their results which were deemed accurate, and the other two reported none). The closer to unity the isotope ratio value the better the results became (‘acceptable’ results mostly for δ7/6‰ and δ7/8‰ measurements). This first experiment of its kind demonstrates that Pb δ-scale isotopic measurements by MC-ICPMS can be reliably carried out down to 0.05‰ levels (two participants delivered accurate results above this threshold systematically for δ7/6‰, δ7/8‰ and δ7/4‰). Below this limit, at ∼0.01‰ and ∼ 0.03‰ levels, results are no longer consistent or reproducible and appear to be susceptible to a number of effects introducing error (such as short term changes in mass discrimination) which are either not well understood, or not controlled and/or not corrected for at a sufficiently low level of uncertainty. These results also suggest that ‘routine’ methods for absolute (calibrated) Pb-isotope ratio determination by MC-ICPMS produce relative combined uncertainties on results which are unlikely to be better than 0.05‰ (k = 2).

The Amsterdam-St. Paul Plateau: A complex hot spot/DUPAL-flavored MORB interaction

The Amsterdam-St Paul (ASP) oceanic plateau results from the interaction between the ASP hot spot and the Southeast Indian ridge. A volcanic chain, named the Chain of the Dead Poets (CDP), lies to its northward tip and is related to the hot spot intraplate activity. The ASP plateau and CDP study reveals that ASP plume composition is inherited from oceanic crust and pelagic sediments recycled in the mantle through a 1.5 Ga subduction process. The ASP plateau lavas have a composition (major and trace elements and Sr-Nd-Pb-Hf isotopes) reflecting the interaction between ASP plume and the Indian MORB mantle, including some clear DUPAL input. The Indian upper mantle below ASP plateau is heterogeneous and made of a depleted mantle with lower continental crust (LCC) fragments probably delaminated during the Gondwana break-up. The lower continental crust is one of the possible reservoirs for the DUPAL anomaly origin that our data support. The range of magnitude of each end-member required in ASP plateau samples is (1) 45% to 75% of ASP plume and (2) 25% to 55% of Indian DM within 0% to a maximum of 6% of LCC layers included within. The three end-members involved (plume, upper mantle and lower continental crust) and their mixing in different proportions enhances the geochemical variability in the plateau lavas. Consequently, the apparent composition homogeneity of Amsterdam Island, an aerial summit of the plateau, may result from the presence of intermediate magmatic chambers into the plateau structure.

Geochemical Dynamics of the Natural-Gas Hydrate System in the Sea of Marmara, Offshore Turkey

Natural-gas hydrate systems are solid-state light-hydrocarbon accumulations which are encountered in the permafrost and the continental margins. They are stable under highpressure and low-temperature conditions and represent the major hydrocarbon volume on earth (Kvenvolden, 1988). Gas hydrates consist of a polycrystalline structure where a light hydrocarbon is trapped within a water lattice. The nature of the hydrocarbons is strongly related to their origin which is either microbial (also called biogenic) or thermogenic. Microbial gas-hydrate systems contain hydrocarbons produced by bacteria and archaea. There are primarily methane with a very small amount of ethane and eventually propane (Max, 2003). Others non-hydrocarbon compounds like hydrogen sulphur and carbon dioxide are also present. In the case of microbial gases, the hydrates are formed at or near the gas production area. Owing to the very high-methane content, these hydrates are commonly called methane-hydrate systems.

Rock-type control of Ni, Cr, and Co phytoavailability in ultramafic soils

Background and aimsUltramafic soils constitute an extreme environment for plants because of specific physico-chemical properties and the presence of Ni, Cr, and Co. We hypothesized that type of ultramafic parent rock depending on their origin affects the composition of soils and plants. Therefore, phytoavailability of metals would be higher in soil derived from serpentinized peridotite compared to serpentinite because of differences in susceptibility of minerals to weathering.ResultsBased on DTPA-CaCl2 extractions, we noted that soil derived from the serpentinized peridotite is characterized by a higher phytoavailability of Ni compared to soil derived from the serpentinite. On the contrary, plant species growing on soil derived from the serpentinite contain higher concentrations of metals.ConclusionsOur study suggests that the metal uptake by plants is controlled by the mineral composition of parent rocks, which results from both their original magmatic composition and later metamorphic processes. Chemical extractions show that the phytoavailability of Ni and Co is higher in soil derived from the serpentinized peridotite than the serpentinite. Surprisingly, plants growing on the soil derived from the serpentinite contain higher levels of metals compared to these from the serpentinized peridotite derived soil. This contrasting behavior is due to higher abundances of Ca and Mg, not only Ni and Co, in soil derived from the serpentinized peridotite as compared to those in the soil derived from the serpentinite. Calcium and Mg are favored by plants and preferably fill the available sites, resulting in low Ni and Co intake despite their higher abundances.