Damien Lemarchand

The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH

Ocean pH is particularly sensitive to atmospheric carbon dioxide content. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (δ11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed. This requires independent estimates of the δ11B of dissolved boron in sea water through time. The marine δ11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from δ11B measurements in 22 of the worlds main rivers), the marine boron isotope budget can be balanced. We also derive ocean δ11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of δ11B in foraminiferal carbonates, suggesting that foraminifera δ11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.

Boron isotope systematics in large rivers: implications for the marine boron budget and paleo-pH reconstruction over the Cenozoic

Abstract The chemical composition of the oceans and long-term climate changes are believed to be linked. Reconstruction of seawater pH evolution is desirable as pH may be related to atmospheric pCO2, and hence to climate evolution. Boron isotopes in oceanic carbonates have been suggested to be a proxy for oceanic paleo-pH reconstruction. Nevertheless, the calculation of paleo-pH values over geological periods requires a precise knowledge of the boron isotopic composition of the oceans when calcite precipitated. We present the systematics of boron isotopic composition of the worlds main rivers. We deduce a continental boron flux to the oceans of 38×1010 gB/year with a mean isotopic composition of +10‰. These results lead to a balanced boron budget in the oceans and allow the development of a model for the marine boron secular evolution over the past 100 Myr. It is shown that the oceanic boron cycle is mainly controlled by the boron continental discharge and the boron uptake from the oceans during low temperature alteration of oceanic crust. However, the recent important increase of the clastic sediment supply, linked to the Himalayan erosion, impacts the oceanic boron budget by enhancing significantly the boron uptake by adsorption on sediments. We predict a boron isotopic composition in the oceans lower during the Cenozoic and slightly higher during the Cretaceous than today. The modelled values for the marine boron isotopes follow the variations of boron isotopes in carbonates over the Cenozoic era provided by previous studies, suggesting that the variations of the seawater pH may not have been important on this time scale. If this is the case, it involves that buffering mechanisms occur in the oceans to maintain seawater pH at a roughly constant value against past atmospheric pCO2 variations.

An optimized procedure for boron separation and mass spectrometry analysis for river samples

An optimized procedure for the separation of boron from natural river samples and an improved mass spectrometry determination of boron isotopic ratio are presented. The chemical procedure, based on the use of the boron-specific resin Amberlite IRA 743, is especially efficient in separating boron from natural organic matter-rich samples like river waters. The properties of Amberlite IRA 743 have been investigated. The two factors important in determining the boron affinity for the resin are: the pH value and the ionic strength of the solution from which B is to be extracted. A logarithmic relationship between B partition coefficients and pH values is found. High ionic strength significantly lowers the fixation of B onto the Amberlite resin. The knowledge of the factors controlling the affinity of the resin Amberlite IRA 743 for boron enables us to design a simple and miniaturized chemical separation procedure characterized by (i) three chromatographic steps using, respectively, 50, 10 and 3 μl of resin, (ii) no evaporation step between each column, and (iii) final separation of boron from residual organic matter by sublimation of boric acid at 75 °C. Boron isotopic ratios are measured using an improved cesium metaborate technique, with graphite and mannitol. Adequate loading conditions enable us to obtain typical signal intensities of 5×10−12 A for 250 ng of boron. No in-run isotopic fractionation is observed, the external reproducibility for standards processed through the entire chemical procedure, as well as for samples, corresponds to 0.35‰ (±2σ). According to this precision, a slight, but reproducible isotopic fractionation of 0.4‰ is observed for standards processed through the entire chemical procedure whose origin is discussed, but is still unclear.

High performance automated ion chromatography separation for Ca isotope measurements in geological and biological samples

Natural mass-dependent fractionation of calcium isotopes is a promising tool for investigating Ca pathways and cycling in geological and biological materials. But since natural isotope fractionation of Ca appears to be extremely limited (∼1.25‰/amu), excellent external precision and sensitivity are needed to make full use of its potential. Here, we describe a new Ca purification procedure that consists of a high selectivity automated ionic chromatography separation protocol, which is suitable for Ca isotope measurements by mass spectrometry and applicable to multiple natural matrixes (waters, mineral and organic samples). The analytical progress in this automated technique are multiple: (1) saving time with a minimum of handling, (2) unique operating protocol whatever the nature of the sample, (3) complete separation of Ca from K, Mg and Sr, avoiding isobaric interferences which are critical during TIMS analysis, and (4) Ca separation by peak recognition optimising the full recovery of Ca even if its retention time is shifted from one sample to another. The two latter advantages ensure a Ca recovery yield close to 100%, leading to the absence of any fractionation of Ca isotopes during the chemical clean-up. Thus, this chemical separation will be of special interest for applications not compatible with the use of the double spike technique such as MC-ICP-MS and 40Ca excesses measurements. Additionally this procedure leads to a twofold improvement of the long-term repeatability of the Ca isotopes determination by TIMS (±0.11 δ44/40Ca, 2SD) as compared with a classical resin chemistry protocol and is similar to the best repeatability published so far (±0.10 δ44/40Ca, 2SD).

Biotic and abiotic experimental identification of bacterial influence on calcium isotopic signatures.

In this study, we tested experimentally the influence of plant and bacterial activities on the calcium (Ca) isotope distribution between soil solutions and plant organs. Abiotic apatite weathering experiments were performed under two different pH conditions using mineral and organic acids. Biotic experiments were performed using either apatite or Ca-enriched biotite substrates in the presence of Scots pines, inoculated or not with the rhizosphere bacterial strain Bulkholderia glathei PML1(12), or the B. glathei PML1(12) alone. For each experiment, the percolate was collected every week and analyzed for Ca concentrations and Ca isotopic ratios. No Ca isotopic fractionation was observed for the different abiotic experimental settings. This indicates that no Ca isotopic fractionation occurs during apatite dissolution, whatever the nature of the acid (mineral or organic). The main result of the biotic experiments is the 0.22 ‰ (44)Ca enrichment recorded for a solution in contact with Scots pines grown on the bacteria-free apatite substrate. In contrast, the presence of bacteria did not cause Ca isotopic fractionation of the solution collected after 14 weeks of the experiments. These preliminary results suggest that bacteria influence the Ca isotopic signatures by dissolving Ca from apatite more efficiently. Therefore, Ca isotopes might be suitable for detecting bacteria-mediated processes in soils.

Boron dissolved and dust atmospheric inputs to a forest ecosystem (Northeastern France)

Boron concentrations and isotopic compositions of atmospheric dust and dissolved depositions were monitored over a two-year period (2012-2013) in the forest ecosystem of Montiers (Northeastern France). This time series allows the determination of the boron atmospheric inputs to this forest ecosystem and contributes to refine our understanding of the sources and processes that control the boron atmospheric cycle. Mean annual dust and dissolved boron atmospheric depositions are comparable in size (13 g·ha-1·yr-1 and 16 g·ha-1·yr-1, respectively), which however show significant intra- and interannual variations. Boron isotopes in dust differ from dissolved inputs, with an annual mean value of +1 ‰ and +18 ‰ for, respectively. The notable high boron contents (190-390 μg·g-1) of the dust samples are interpreted as resulting from localized spreading of boron-rich fertilizers, thus indicating a significant local impact of regional agricultural activities. Boron isotopes in dissolved depositions show a clear seasonal trend. The absence of correlation with marine cyclic solutes contradicts a control of atmospheric boron by dissolution of seasalts. Instead, the boron data from this study are consistent with a Rayleigh-like evolution of the atmospheric gaseous boron reservoir with possible but limited anthropogenic and/or biogenic contributions.

Boron in the Weathering Environment

This chapter reviews the state of art of the use of boron isotopes to understand water-rock interaction in the Critical Zone, the thin and reactive layer at the Earth’s surface. Because boron isotopes are largely fractionated by adsorption, coprecipitation and evaporation-condensation processes, boron isotopes are well adapted to trace the main processes that convert rocks into soils and sediments on terrestrial surfaces. The difference in affinity of boron isotopes between trigonal and tetrahedral species is the main cause of isotope fractionation of boron at the Earth’s surface. Due to the competition between the speciation of boron in solution and the speciation on boron onto or into solids or gas, large isotopic variations are predicted and observed. Measured boron isotopic composition in the weathering environment varies over a considerable range of about 70‰. Precipitation, rivers and biomass are usually enriched in 11B, while a complementary depletion in 11B (enrichment in 10B) is observed in clay minerals and on organic or inorganic surfaces. At the ecosystem scale, boron appears to behave as a micronutrient with a major flux of boron associated with biological recycling. The inputs of boron to ecosystems by chemical weathering or from the atmosphere are minor. When the residence time of water in the critical zone is high, such as in groundwater systems, boron contents increase and are much more dominated by a weathering signal. Boron is mainly added to the ocean by rivers, while the most important sink of boron is adsorption on clay minerals. This makes boron a particularly good tracer of the weathering/erosion balance of terrestrial surfaces in addition to its capacity for tracing the pH and ancient seawater. A lot remains to be done to better understand the behavior of boron and boron isotopes at the Earth’s surface and on the secular evolution of boron isotopes in the ocean but our review of the available literature shows that this tracer has a great potential at a local (ecosystem) and global (ocean) scale.

Climate change: Early survival of Antarctic ice.

Analyses of boron isotopes in ancient marine carbonate sediments provide an enlightening perspective on the links between carbon dioxide and ice-cap cover at a climatically momentous time in Earths history.