Cycles, Atmosphere, Climates

Résumé : Nitrogen is the main constituent of the Earth’s atmosphere, but its provenance in the Earth’s mantle remains uncertain. The relative contribution of primordial nitrogen inherited during the Earth’s accretion versus that subducted from the Earth’s surface is unclear1,2,3,4,5,6. Here we show that the mantle may have retained remnants of such primordial nitrogen. We use the rare 15N15N isotopologue of N2 as a new tracer of air contamination in volcanic gas effusions. By constraining air contamination in gases from Iceland, Eifel (Germany) and Yellowstone (USA), we derive estimates of mantle δ15N (the fractional difference in 15N/14N from air), N2/36Ar and N2/3He. Our results show that negative δ15N values observed in gases, previously regarded as indicating a mantle origin for nitrogen7,8,9,10, in fact represent dominantly air-derived N2 that experienced 15N/14N fractionation in hydrothermal systems. Using two-component mixing models to correct for this effect, the 15N15N data allow extrapolations that characterize mantle endmember δ15N, N2/36Ar and N2/3He values. We show that the Eifel region has slightly increased δ15N and N2/36Ar values relative to estimates for the convective mantle provided by mid-ocean-ridge basalts11, consistent with subducted nitrogen being added to the mantle source. In contrast, we find that whereas the Yellowstone plume has δ15N values substantially greater than that of the convective mantle, resembling surface components12,13,14,15, its N2/36Ar and N2/3He ratios are indistinguishable from those of the convective mantle. This observation raises the possibility that the plume hosts a primordial component. We provide a test of the subduction hypothesis with a two-box model, describing the evolution of mantle and surface nitrogen through geological time. We show that the effect of subduction on the deep nitrogen cycle may be less important than has been suggested by previous investigations. We propose instead that high mid-ocean-ridge basalt and plume δ15N values may both be dominantly primordial features.

Résumé : This paper reviews current understanding of deglaciation in North, Central and South America from the Last Glacial Maximum to the beginning of the Holocene. Together with paleoclimatic and paleoceanographic data, we compare and contrast the pace of deglaciation and the response of glaciers to major climate events. During the Global Last Glacial Maximum (GLGM, 26.5-19 ka), average temperatures decreased 4° to 8°C in the Americas, but precipitation varied strongly throughout this large region. Many glaciers in North and Central America achieved their maximum extent during the GLGM, whereas others advanced even farther during the subsequent Heinrich Stadial 1 (HS-1). Glaciers in the Andes also expanded during the GLGM, but that advance was not the largest, except on Tierra del Fuego. HS-1 (17.5-14.6 ka) was a time of general glacier thickening and advance throughout most of North and Central America, and in the tropical Andes; however, glaciers in the temperate and subpolar Andes thinned and retreated during this period. During the Bølling-Allerød interstadial (B-A, 14.6-12.9 ka), glaciers retreated throughout North and Central America and, in some cases, completely disappeared. Many glaciers advanced during the Antarctic Cold Reversal (ACR, 14.6-12.9 ka) in the tropical Andes and Patagonia. There were small advances of glaciers in North America, Central America and in northern South America (Venezuela) during the Younger Dryas (12.9-11.7 ka), but glaciers in central and southern South America retreated during this period, except on the Altiplano where advances were driven by an increase in precipitation. Taken together, we suggest that there was a climate compensation effect, or ‘seesaw’, between the hemispheres, which affected not only marine currents and atmospheric circulation, but also the behavior of glaciers. This seesaw is consistent with the opposing behavior of many glaciers in the Northern and Southern Hemispheres.

Résumé : Silicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ7Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.
Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km2/year) and CO2 consumption rate (0.65 × 1012 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14–8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91–1.5 × 1012 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.

Résumé : The application of lead isotope-based provenance analysis in Southeast Asia over the last decade has strongly suggested a central role was played by the Lao PDR in regional copper production exchange networks for approximately 1500 years. The Vilabouly Complex, in central Lao’s Savannakhet Province, has revealed major copper mining and smelting sites dated to the regional Iron Age (c. 400 BC – AD 500) and possibly Bronze Age (c.1000 – 400 BC). Metallurgical practices at the Vilabouly Complex, and indeed for all of the Lao PDR, are unknown, and the propose of this paper is to provide a comprehensive analytical study of the Vilabouly Complex metal assemblage, including 60 copper-base artefacts of multiple typologies. Cut samples of these were subjected to morpho-stylistic, metallographic (OM), elemental (XRF, SEM-EDS) and lead isotope analyses (MC-ICP-MS) in order to reconstruct the range of forms, metalworking materials, techniques (alloying, casting and post-casting treatments) used at the Vilabouly Complex. The results revealed an assemblage composed of copper, bronze and leaded bronze alloys, with a majority consistent with the lead isotopic signature for the Vilabouly Complex copper. The consistent geochemical and technological signature of the majority of artefacts strongly corroborates the extensive onsite production evidence, and fits with the burgeoning regional copper-base metals database for copper metal demand being sated in large part by Lao PDR supply.

Résumé : Rationale
High‐precision determination of magnesium (Mg) isotopes can now be routinely achieved by multi‐collector inductively coupled plasma mass spectrometry (MC‐ICP‐MS). The analytical sensitivity and instrumental mass discrimination behavior of this method are, however, sensitive to the types of sample and skimmer cones used in these measurements, so it is important that these parameters should be investigated.
Methods
Using the sample‐standard‐bracketing method in the wet‐plasma mode, four available combinations of sample and skimmer cones [Jet sample cone + H skimmer cone (Jet + H), standard sample cone + H skimmer cone (Standard + H), standard sample cone + X skimmer cone (Standard + X), and Jet sample cone + X skimmer cone (Jet + X)] were systematically investigated for peak shape, sensitivity, mass discrimination, accuracy, and precision in Mg‐isotopic ratio determination using a Neptune plus MC‐ICP mass spectrometer.
Results
The results showed that different cone combinations do not affect peak shapes but would significantly change the sensitivities for Mg‐isotopic determinations. Compared with using the Standard + H, the sensitivities of Mg‐isotopic determinations were enhanced by approximately a factor of 1.3, 1.4, and 1.9 by using the Standard + X, the Jet + H, and the Jet + X combinations, with the most stable mass discrimination behaviors obtained by the Jet + H. The instrumental mass fractionation slope for any combination of a modified cone geometry (i.e. Standard + X, Jet + X, and Jet + H) is 0.500, while it is 0.510 for the Standard + H. In addition, the mass discrimination behavior is related to Mg concentrations once the combination is set, indicating the necessity of concentration match during Mg‐isotopic determination.
Conclusions
The precision and accuracy of the Jet + H combination are better than those of the other combinations, and this is further supported by the validation of the Mg‐isotope data for four international reference materials: Cambridge‐1, NASS‐6, AGV‐2, and BHVO‐2. As the Jet + H combination also provides a high signal, this combination gives the most robust strategy for the highly precise and accurate determination of Mg isotopes.