Nathalie Vigier

Erosion timescales derived from U-decay series measurements in rivers

The relative importance of the factors influencing weathering of continental rocks has been a topic of debate for the last few decades. The principal reasons are the lack of reliable proxies for chemical weathering and the difficulty in constraining actual physical denudation rates. In this study, (234U/238U), (230Th/238U), and (226Ra/238U) were measured by TIMS and by MC–ICP–MS in the dissolved and suspended loads of rivers from the Mackenzie Basin (Northwest Territories, Canada). The data show a complementary nature between (234U/238U), (230Th/238U) in the dissolved and suspended loads while 226Ra is characterized by a more complex behavior. Modeling of fractionation of U-series nuclides in the particulate matter and the corresponding dissolved phase permits us to constrain the duration of chemical erosion for the suspended load currently sampled in the watershed (9–28±10 ka), as well as the rates of release of U-series nuclides. The results also imply significant recent changes of chemical erosion rates and underline the impact of the last glaciation on current continental fluxes of northern latitude rivers such as the Mackenzie River.

U-decay series and trace element systematics in the 1978 eruption of Ardoukoba Asal rift : timescale of magma crystallization

Abstract 230 Th– 238 U, 226 Ra– 230 Th, 231 Pa– 235 U disequilibria, Th and Sr isotopes, trace and major elements were measured in a tholeiitic basalt series erupted within a single week in November 1978 at the Ardoukoba Volcano (Asal rift, Djibouti). ( 230 Th/ 238 U) and ( 231 Pa/ 235 U) activity ratios are constant for all the lavas, within analytical uncertainties, and are equal to 1.42 and 2.20, respectively. ( 226 Ra/ 230 Th) activity ratios in the whole rocks decrease with Th content from 1.93 to 1.35. Trace elements measured in plagioclase and groundmass show that all the lavas display a similar proportion of accumulative plagioclase (∼30%). Once corrected for plagioclase accumulation, the trace element variations can be modeled as a single fractional crystallization series which requires 30% crystallization (49% plagioclase, 39% olivine and 12% clinopyroxene). However, this fractional crystallization cannot explain the whole range in ( 226 Ra/ 230 Th). 226 Ra radioactive decay must also be taken into account. Consequently, this basaltic series also represents an age series evolving from the older magma (#544) to the most recent magma (#546). To constrain magma crystallization ages from ( 226 Ra/ 230 Th) data, two end-member models are proposed. The first one corresponds to several magma batches crystallizing continuously as closed systems. The second model corresponds to a zoned chamber crystallizing continuously, and regularly replenished with fresh parental magma. The fraction of crystallized magma per unit of time is 3.3×10 −4 yr −1 for the open-system, and 3.6×10 −4 yr −1 for the closed-system. Model ages of crystallization of 1880 yr (±80 yr) and 870 yr (±30 yr) are inferred from the open- and the closed-system model, respectively.

In situ measurements of Li isotopes in foraminifera

In situ measurement of Li isotope ratios in foraminifera has been developed using a Cameca ims 1270 ion microprobe. In situ δ7Li analyses have been performed in biogenic calcite of planktonic foraminifera from various locations. Results show that for west Pacific mixed Globigerinoides and Globorotalia (22°S161°E), the isotopic variability between tests and within a single test, respectively, is not significantly greater than estimated analytical uncertainty (∼1.5‰). Mean δ7Li for several planktonic foraminifera tests corresponds to the seawater value, strongly suggesting negligible Li isotope fractionation relative to seawater, as previously inferred by Hall et al. (2005) using thermo-ionization mass spectrometer and multicollector-inductively coupled plasma-mass spectrometry techniques. Combined with scanning electron microscopy and ion microprobe imaging, micron-sized grains, enriched in lithium, silica and aluminum have been found in the foraminifera calcite matrix. A simple mixing model shows that 0.3–2 wt % of marine clays incorporated within the analyzed calcite would lower the foraminifera δ7Li value, by 3‰ to 10‰ relative to the isotopic composition of the pure calcite. By comparison, no such grains have been detected in corals. The presence of micron-sized silicate grains embedded within the foraminifera calcite is consistent with the Erez (2003) biomineralization model, involving calcite precipitation from seawater vacuoles. By contrast, coral calcium carbonate is instead precipitated from ions, which have been pumped or diffused through several membranes, impermeable to micrometric grains. Ion microprobe in situ δ7Li measurements in biogenic calcite present new methods for investigating both biomineralization processes and the past record of the ocean composition by exploring geochemical variations at a scale that is smaller in space and in time.

Abrupt response of chemical weathering to Late Quaternary hydroclimate changes in northeast Africa

Chemical weathering of silicate rocks on continents acts as a major sink for atmospheric carbon dioxide and has played an important role in the evolution of the Earth’s climate. However, the magnitude and the nature of the links between weathering and climate are still under debate. In particular, the timescale over which chemical weathering may respond to climate change is yet to be constrained at the continental scale. Here we reconstruct the relationships between rainfall and chemical weathering in northeast Africa for the last 32,000 years. Using lithium isotopes and other geochemical proxies in the clay-size fraction of a marine sediment core from the Eastern Mediterranean Sea, we show that chemical weathering in the Nile Basin fluctuated in parallel with the monsoon-related climatic evolution of northeast Africa. We also evidence strongly reduced mineral alteration during centennial-scale regional drought episodes. Our findings indicate that silicate weathering may respond as quickly as physical erosion to abrupt hydroclimate reorganization on continents. Consequently, we anticipate that the forthcoming hydrological disturbances predicted for northeast Africa may have a major impact on chemical weathering patterns and soil resources in this region.

Impact of climate change and human activity on soil landscapes over the past 12,300 years

Soils are key to ecosystems and human societies, and their critical importance requires a better understanding of how they evolve through time. However, identifying the role of natural climate change versus human activity (e.g. agriculture) on soil evolution is difficult. Here we show that for most of the past 12,300 years soil erosion and development were impacted differently by natural climate variability, as recorded by sediments deposited in Lake Dojran (Macedonia/Greece): short-lived ( < 1,000 years) climatic shifts had no effect on soil development but impacted soil erosion. This decoupling disappeared between 3,500 and 3,100 years ago, when the sedimentary record suggests an unprecedented erosion event associated with the development of agriculture in the region. Our results show unambiguously how differently soils evolved under natural climate variability (between 12,300 and 3,500 years ago) and later in response to intensifying human impact. The transition from natural to anthropogenic landscape started just before, or at, the onset of the Greek ‘Dark Ages’ (~3,200 cal yr BP). This could represent the earliest recorded sign of a negative feedback between civilization and environmental impact, where the development of agriculture impacted soil resources, which in turn resulted in a slowdown of civilization expansion.