Pascale Louvat

Present denudation rates on the island of Réunion determined by river geochemistry: Basalt weathering and mass budget between chemical and mechanical erosions

Dissolved and suspended loads of the main streams on the island of Reunion have been analysed for their major and trace element contents in order to characterise both chemical and mechanical erosion products. The chemical composition of Reunion surface waters is controlled by partial dissolution of the basaltic rocks they interact with, by atmospheric input and, in some cases, by thermal spring inflow. The elemental contributions arising from these three processes have been calculated using typical concentration ratios for each endmember. The contribution from oceanic type rains is important for Cl and Na and minor for Mg, Ca, K, and Sr. Some thermal springs join their nearest rivers in Piton des Neiges calderas and influence the sulphate, Li, and B concentrations, and to a lesser extent Ca, Sr, and Rb. For all the rivers that are not affected by the thermal springs, the contribution from basalt weathering is higher than 70% of the total dissolved solids. The chemical composition of the suspended load in Reunion rivers reflects a low weathering state for basalt in each drainage basin, implying that mechanical erosion is more important than chemical erosion. Assuming a steady-state between these two complementary erosion processes, we propose a mass budget between the dissolved and suspended erosion products and the local unweathered basalt for each of the main river catchments. Given the lack of reliable long-term measurements of river suspended load at Reunion, this mass budget enables us to calculate the suspended load concentration that is required to counterbalance the concentration of total dissolved solids due to chemical weathering. The calculated suspended load concentration is much higher than that measured which implies that equilibrium between mechanical and chemical erosion was not attained at the time of sampling. However, we suggest that in such a tropical climate, most of the suspended load will be carried during cyclonic events, these short periods of time providing about 50% of the annual amount of river water. Thus, erosional steady-state must be achieved on the scale of the annual hydrologic cycle. From the total dissolved solid concentrations induced by basalt weathering, and given the annual discharge from Reunion rivers, we infer specific chemical erosion rates of 63–170 t/km2/yr and specific atmospheric CO2 consumption rates, during basalt erosion, of 1.3–4.4 × 106 mol/km2/yr. Mechanical erosion rates deduced from the calculated suspended load concentrations range from 1200 to 9100 t/ km2/yr. Total erosion (chemical plus mechanical) give basalt denudation rates of 470–3430 mm/kyr. All these erosion rates (chemical, mechanical, total, and atmospheric CO2 consumption) are among the highest global estimates and are due to high runoff, steep slopes, active volcanics, and related active tectonics, and young basaltic lithologies, which characterise Reunion.

Riverine erosion rates on Sao Miguel volcanic island, Azores archipelago

Abstract We present here a coupled study of the dissolved and suspended loads of the rivers draining the basaltic island of Sao Miguel, in order to determine both chemical and mechanical erosion rates. Dissolved elemental concentrations of the river waters are corrected for rain inputs (mainly oceanic salts) and for geothermal water inflows. They provide specific chemical denudation rates of 26–50 t/km2/year and associated atmospheric CO2 consumption rates of 0.26–0.87×106 mol/km2/year. Suspended loads have chemical compositions similar to those of their parent rocks with slight depletions in alkaline and alkaline earths elements, which reflect their partial dissolution during rock weathering. Using the steady-state model of erosion, which assumes a relationship between chemical and mechanical erosion rates, we can establish mass budgets between the initially unaltered basaltic rocks and the dissolved and suspended erosion products in the river waters. Thereby, we calculate mechanical erosion rates of 170–500 t/km2/year. These chemical and mechanical erosion rates are very high compared to the global average, and emphasize the ease of erosion of basaltic terrains. A comparison with the erosion rates that have been previously determined for the rivers of Reunion island highlights the importance of relief, runoff and tectonics in the erosion processes.

Stable isotopes of Cu and Zn in higher plants: evidence for Cu reduction at the root surface and two conceptual models for isotopic fractionation processes

Recent reports suggest that significant fractionation of stable metal isotopes occurs during biogeochemical cycling and that the uptake into higher plants is an important process. To test isotopic fractionation of copper (Cu) and zinc (Zn) during plant uptake and constrain its controls, we grew lettuce, tomato, rice and durum wheat under controlled conditions in nutrient solutions with variable metal speciation and iron (Fe) supply. The results show that the fractionation patterns of these two micronutrients are decoupled during the transport from nutrient solution to root. In roots, we found an enrichment of the heavier isotopes for Zn, in agreement with previous studies, but an enrichment of isotopically light Cu, suggesting a reduction of Cu(II) possibly at the surfaces of the root cell plasma membranes. This observation holds for both graminaceous and nongraminaceaous species and confirms that reduction is a predominant and ubiquitous mechanism for the acquisition of Cu into plants similar to the mechanism for the acquisition of iron (Fe) by the strategy I plant species. We propose two preliminary models of isotope fractionation processes of Cu and Zn in plants with different uptake strategies.

CHEMICAL AND MECHANICAL EROSION RATES IN ICELAND AS DEDUCED FROM RIVER DISSOLVED AND SOLID MATERIAL

This study investigates dissolved, suspended loads and sands of major Icelandic rivers and determines chemical and mechanical erosion rates as well as rates of CO2 consumption by the chemical weathering. A steady state model of erosion is used to locally calculate the river suspended load fluxes needed to balance chemical weathering. The total dissolved solid concentrations range from 20 to 179 mg/kg. The highest concentrations are for spring fed rivers draining young rocks in the vicinity of active volcanoes, and the lowest for direct runoff rivers draining old Tertiary rocks. Total dissolved loads, “corrected” for atmospheric, geothermal, and magmatic inputs are used together with mean annual discharges to estimate low-temperature chemical erosion rates of 16 to 111 t/km2/yr. These rates increase with runoff but decrease with the age of the rocks. Icelandic chemical erosion rates are higher than the world average for silicate rocks, reflecting both high reactivity of the basalt and high runoff, but lower than those for other basalt-draining rivers (in Réunion, Java, Azores or Deccan). CO2 consumption rates associated to chemical denudation range between 0.18 and 2.12 106 mol/km2/yr with an average value of 0.74 106 mol/km2/yr, higher than the world average for rivers draining silicate rocks. Chemical compositions of suspended sediments and sands are similar, showing a very low weathering stage. The elements most soluble during the weathering show slightly lower concentrations in the suspended sediments. River sediment chemical compositions are assumed to reflect a mixture between 3 initially pristine rock end-members: high Mg-basalt, tholeiite and rhyolite. The most insoluble elements (REE and Th) are used to re-define the mean chemical composition of the initially unaltered rocks of each drainage basin. A mass budget between the unaltered rock of the catchments and the river dissolved and suspended loads (steady state model of erosion) is used to calculate the average annual solid load of the rivers, which range from 650 to 4300 mg/l. For some rivers there is a good agreement between calculated and measured suspended loads but for others the calculated load is much higher than the measured one. The difference stems from groundwater inputs, man-made dams and other sedimentary traps. If the relevancy of the steady state model of erosion can be questioned, the accuracy of sediment load measurements is also questionable. Pros and cons of both methods are argued. The calculated solid loads lead to very high mechanical erosion rates, ranging from 940 to 10200 t/km2/yr. Those increase with the glacier cover but decrease with the age of the catchment rocks. Icelandic mechanical erosion rates rank among the maximum reported rates, underscoring the importance of glaciers, tectonics, glassy basaltic rocks and high runoff. In association with low chemical weathering rates, these place Icelandic rivers as an end-member in the observed anti-correlation between mechanical to chemical erosion ratios and temperature for volcanic islands.

Orography-driven chemical denudation in the Lesser Antilles: Evidence for a new feed-back mechanism stabilizing atmospheric CO2

In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to large variations of runoff can be understood. Data indicate that the Lesser Antilles are characterized by extremely variable chemical weathering (40-430 t/km2/a) and CO2 consumption (300-3500.103 mol/km2/a) rates, amongst the highest found on Earth and consistent with the previous studies on the weathering of volcanic rock. A noteworthy observation is that, along the runoff gradient, concentrations of rock-derived solutes do not follow a pure dilution law and that a buffering mechanism exists stabilizing solute concentrations. As a result concentrations vary much less than runoff and chemical weathering rates are mainly controlled by runoff. Precipitation patterns in the Lesser Antilles are essentially orographic and controlled by the adiabatic decompression of the water-saturated Atlantic air masses. The production of acidity by volcanic degassing is an additional factor that modulates the runoff effect. Two main conclusions can be drawn from this study. First, chemical weathering fluxes of oceanic islands are strongly dependent upon relief repartition, which cautions the use of regional mean values to compare volcanic islands. Second, volcanic activity in the Lesser Antilles subduction arc, by creating relief, promotes high orographic precipitation and/or infiltration regimes, that in turn results in elevated chemical weathering and atmospheric CO2 consumption fluxes. This feedback mechanism, implying mainly precipitation and relief, is proposed to act in complement to the temperature-related feedback proposed by previous authors for stabilizing the atmospheric CO2 content of the atmosphere in response to volcanic CO2 degassing. This study highlights the importance of the water cycle in controlling chemical weathering of volcanic arc islands and associated CO2 consumption rates.

Geological evolution of seawater boron isotopic composition recorded in evaporites

The abundance of boron isotopes in ancient marine carbonates can be used to estimate oceanic pH that reflects atmospheric CO 2 levels. This proxy requires that the boron isotopic composition of seawater at the time the carbonate has formed is known, and thus the past changes in seawater chemistry. Here we report the boron isotopic composition of selected ancient marine halites and modern sea salts. The signal, interpreted as marine, reveals a clear evolution of the boron isotopic composition of seawater (to 8‰ variations over the Cenozoic). Comparison between this reconstructed curve and the Mg/Ca ratio reveals a high level of consistency that will help to better define oceanic geochemical cycles.

A fully automated direct injection nebulizer (d-DIHEN) for MC-ICP-MS isotope analysis: application to boron isotope ratio measurements

This work presents a fully automated setup for using direct injection nebulization as an introduction system for solution measurements by MC-ICP-MS, here applied to boron isotopes in pure boric acid solutions and natural samples. In this setup, a direct injection nebulizer (d-DIHEN) is plugged into the plasma torch without any spray chamber, and an automated 6-port valve interfaces the nebulizer and the autosampler. The advantages of a d-DIHEN for boron isotope ratio measurements are high sensitivity and short washout times, allowing for sample–standard bracketing (SSB) measurements at a higher rate than spray chambers. The measurement of boron isotopes by MC-ICP-MS at an unprecedented sub 0.1‰ repeatability level (2 standard deviation = 2SD) was achieved for pure boric acid solutions. The improved precision is allowed by a better stability of the introduction system with continuous operation of the peristaltic pump (which was manually switched off between samples before automation) and due to the possibility of multiple analyses of the same sample solution. However, such a good repeatability was not systematically obtained for boron isotopes SSB measurements of natural samples (in fine 2SD are between 0.02 and 0.5‰). Boron from natural samples has to be extracted before isotope analysis, with one to four steps depending on the sample type. Repeated analyses of boron independently separated up to ten times from the same sample lead to an external reproducibility no better than 0.2‰. Boron chemical separation from the samples prior to MC-ICP-MS analyses seems to remain the main limitation to precise measurements of boron isotope ratios.

Method for isotope ratio drift correction by internal amplifier signal synchronization in MC-ICPMS transient signals

The measurement of isotope ratios by Multi Collection Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) in transient signals often shows a drift of the isotope signal ratio during signal acquisition. This “isotopic drift” is generally related to the small and distinct time lags between the responses of the amplifiers involved in the Faraday detector configuration. In this work, a method of synchronization of transient isotope signals for a duration of a few tens of seconds is proposed in order to: (1) quantify the time lags between the amplifiers using the ratios of the raw isotope signals and (2) correct the isotope ratio drifts. The method was successfully tested on lead isotope ratio measurements obtained from two different multi-collector mass spectrometers and setups (flow injection with direct injection and gas chromatography). This approach offered a precise determination of the time lag between the different amplifier systems and an effective correction of the isotope ratio drift. The performances of the methods traditionally used for isotope ratio calculation of transient signals were also compared before and after isotope ratio drift correction.

Mass-dependent and -independent signature of Fe isotopes in magnetotactic bacteria.

An isotope record of magnetic bacteria Microorganisms have shaped Earths oceans and atmosphere over billions of years. Ancient microbes left very little direct morphological evidence of their existence in the rock record, thereby requiring geochemical clues for evidence of their activity. Amor et al. show that magnetotactic bacteria impart a distinct isotopic signature to their internal iron nanoparticles. Cultures of a modern magnetic bacterium fractionated 57Fe isotopes independent of their mass, in contrast to fractionation patterns often observed for other isotopes. Because this signature is not produced abiotically or by other iron-metabolizing bacteria, it could serve as a reliable biomarker of this ancient magnetic microbial lifestyle. Science, this issue p. 705 The iron isotope fractionation patterns of magnetotactic bacteria hint at a reliable biomarker of ancient microbes. Magnetotactic bacteria perform biomineralization of intracellular magnetite (Fe3O4) nanoparticles. Although they may be among the earliest microorganisms capable of biomineralization on Earth, identifying their activity in ancient sedimentary rocks remains challenging because of the lack of a reliable biosignature. We determined Fe isotope fractionations by the magnetotactic bacterium Magnetospirillum magneticum AMB-1. The AMB-1 strain produced magnetite strongly depleted in heavy Fe isotopes, by 1.5 to 2.5 per mil relative to the initial growth medium. Moreover, we observed mass-independent isotope fractionations in 57Fe during magnetite biomineralization but not in even Fe isotopes (54Fe, 56Fe, and 58Fe), highlighting a magnetic isotope effect. This Fe isotope anomaly provides a potential biosignature for the identification of magnetite produced by magnetotactic bacteria in the geological record.

Zn Isotope Fractionation during Sorption onto Kaolinite

In this study, we quantify zinc isotope fractionation during its sorption onto kaolinite, by performing experiments under various pH, ionic strength, and total Zn concentrations. A systematic enrichment in heavy Zn isotopes on the surface of kaolinite was measured, with Δ(66)Znadsorbed-solution ranging from 0.11‰ at low pH and low ionic strength to 0.49‰ at high pH and high ionic strength. Both the measured Zn concentration and its isotopic ratio are correctly described using a thermodynamic sorption model that considers two binding sites: external basal surfaces and edge sites. Based on this modeling approach, two distinct Zn isotopic fractionation factors were calculated: Δ(66)Znadsorbed-solution = 0.18 ± 0.06‰ for ion exchange onto basal sites, and Δ(66)Znadsorbed-solution = 0.49 ± 0.06‰ for specific complexation onto edge sites. These two distinct factors indicate that Zn isotope fractionation is dominantly controlled by the chemical composition of the solution (pH, ionic strength).