Magnesium Isotope Fractionation During Arid Pedogenesis on the ...

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ScienceDirect Procedia Earth and Planetary Science 10 (2014) 243 – 248

Geochemistry of the Earth’s Surface meeting, GES-10

Magnesium isotope fractionation during arid pedogenesis on the Island of Hawaii (USA) Kyle Trostlea*, Louis Derrya, Nathalie Vigierb, Oliver Chadwickc b

a Cornell University, Department of Earth and Atmospheric Science, Ithaca, NY 14853, USA CRPG, 54501 Vandoeuvre les Nancy cedex, France/ Now at :LOV-UPMC, CNRS, Villfranche sur Mer, France c University of California Santa Barbara, Department of Geography, Santa Barbara, CA 93106, USA

Abstract We combines Mg isotopic analyses with soil characterization methods to determine Mg isotopic compositions of bulk soils, basalts, and carbonate fractions at an arid (~30 cm MAP) soil chronosequence on the Island of Hawaii. This chronosequence is developed on Pololu (350 ka) and Hawi (170 ka) lava flows. Both profiles contain pedogenic carbonates and secondary aluminosilicate and sequioxide phases. Bulk soil horizons at these sites range in 26Mg from -0.21±0.31 ‰ to -1.75±0.22 ‰ for the Hawi and -0.01±0.31 ‰ to -0.21±0.31 ‰ for the Pololu. Basalts underlying the soil profiles have average 26Mg values of 0.25±0.06 ‰. Pedogenic carbonate 26Mg values at these sites vary from -1.05±0.22‰ to -2.31±0.22‰. Integrating the soils as a whole yields bulk soil isotopic compositions of -1.35±0.16 ‰ for the Hawi and -0.12±0.12 ‰ for the Pololu. These differences in overall 26Mg between total soils may be explained by the relative abundance of Mg in carbonate; in the Hawi soil 69±11% of Mg is hosted by carbonate phases, while in the Pololu soil 16±2% of Mg is in carbonate phases. Estimates of Mg input to the soils through time, through rainfall estimates as well as rock weathering estimates provided by the immobile index element Zr, allows the calculation of the 26Mg of Mg exported from these systems. The isotopic composition Mg exported from the Hawi soil is not tightly constrained but must be significantly isotopically heavier than the basalt parent material. The Pololu soil is much better constrained and has exported Mg with a 26Mg of -0.36±0.26 ‰, close to the weighted mean value of inputs to the soil from weathering and atmospheric deposition. The evolution of the soil mineralogy and morphology during progressive development of the weathering profile result in significant changes in the isotopic composition of Mg exported through time. © 2014Published The Authors. Published Elsevier B.V. © 2014 by Elsevier B.V. by This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the Scientific Committee of GES-10. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Scientific Committee of GES-10 Keywords: Magnesium isotopes, soil, Hawaii, fractionation

* Corresponding author. Tel.: 1-215-300-5180. E-mail address: [email protected]

1878-5220 © 2014 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Scientific Committee of GES-10 doi:10.1016/j.proeps.2014.08.031

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Kyle Trostle et al. / Procedia Earth and Planetary Science 10 (2014) 243 – 248

1. Introduction The stable isotopes of magnesium can aid in understanding ecosystem-scale magnesium cycling. However, much work remains before the behaviour of magnesium isotopes in the critical zone is fully understood. Previous work shows that magnesium isotopes are fractionated in the weathering environment, and that secondary mineral formation may play a role in this fractionation1,2,3,4,5,6,7,8,. We investigate this mechanism of Mg isotope fractionation by conducting a study of two soils developed on adjacent basaltic parent materials of different ages in an arid climate regime on the island of Hawaii (USA). 1.1. Soil Sites We chose two study sites on the arid leeward coast of Kohala Volcano with comparable parent material, present-day climatic conditions, biotic factors and topography, but which varied in age to allow us to investigate the evolution of the magnesium isotopic systematics of basaltic soils through time. These sites are developed on the Pololu (350 ka) and Hawi (170 ka) lava flows, and have been studied by a number of previous workers9. The soil forming factors are nearly identical for the two sites. Both are at similar elevation and aspect with a mean annual precipitation of 35.0±0.5 cm for the Pololu site and 28.0±0.5 cm for the Hawi site, usually delivered by sporadic cyclonic storms9,10. Vegetation is dominated by non-native buffel grass (Chenchrus ciliaris) and kiawe trees (Prosopis pallida). Both soils contain a variety of secondary minerals including: Mg-rich carbonate; allophane; imogolite; halloysite; gibbsite; ferrihydrite; hematite and other trace phases. Strontium isotope data from demonstrate that the bulk of the alkaline earths in the pedogenic carbonates are derived from basalt, not precipitation16. 2. Methods 2.1 Bulk Soil and Rock Dissolutions Soil samples were sieved on a 2-mm mesh screen, and 2-mm fractions treated as separate samples. Using the relative proportions of the 2-mm fractions for each soil horizon, the overall composition of each horizon was calculated. Bulk soils and rock samples underwent the same treatment for dissolution. They were ground to a powder in a ceramic shatterbox, weighed (~0.05 g) and dissolved using HNO3, HF and HClO4. After complete dissolution this solution was split into two fractions. One aliquot underwent ion exchange column chemistry to isolate magnesium for isotope work, and the other was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES) to determine bulk composition. 2.2 Carbonate Dissolutions Carbonates were dissolved from unsieved bulk samples of each soil horizon with an ammonium acetate solution buffered to a pH of 5. Additional glacial acetic acid was gradually added to each of these samples over the course of days until effervescence was no longer observed. Samples were then centrifuged, and the supernatant removed for analysis 2.3 Isotopic Analysis Magnesium samples for isotopic analysis were purified through a series of ion exchange columns using a process modified from Bolou Bi et al. 200911. Samples were run on a Thermo


Fisher Neptune multi-collector inductively coupled plasma mass spectrometer (MCICPMS) at the Centre du Recherches Petrographiques et Geochimiques (CRPG) in Nancy, France. The samples were run using a standard-sample bracketing technique. 3. Results Basalt standards BHVO-2 and BE-N yielded mean 26Mg values and 2 values of -0.21±0.14 ‰ (-0.19±0.05 ‰)12 and -0.33±0.22 ‰ (-0.28±0.08 ‰)11 over this period of time. Basalts have measured 26Mg values that average -0.25±0.06 ‰ over both sites, and precipitation was estimated as the isotopic composition of seawater (-0.89 ‰; average BCR-403 value)11. Pedogenic carbonates from both soils vary over a wide range (1.05±0.22‰ to -2.31±0.22‰); at their lightest values they are similar in 26Mg to oceanic carbonates. 26Mg values for the 2-mm saprolite fraction range between -0.14±0.22 ‰ and -1.75±0.22 ‰ for the Hawi soil horizons, and 0.05±0.22 ‰ and 0.73±0.22 ‰ for the Pololu soil horizons. Bulk isotopic values for the Pololu tend to be heavier than the Hawi. Calculating arithmetically weighted average Mg isotope values for each horizon composed of 2-mm fractions changes the ranges slightly; the Hawi ranges from -0.21±0.31 ‰ to -1.75±0.22 ‰ and the Pololu ranges -0.01±0.31 ‰ to -0.21±0.31 ‰. Figure 1 displays the 26Mg values for all analyzed samples broken into categories by sample type, including the Mg isotopic compositions of our basaltic parent materials and seawater estimate for precipitation. 4. Discussion Figure 1 shows that most of the bulk soil and carbonate isotopic data fall outside the range of the two primary sources of Mg, basalt weathering (0.22±0.13 ‰) and precipitation (-0.89±0.1 ‰. In order to understand these fractionations, an estimate of the relative amount of input of Mg from basalt weathering and rainfall is needed. 4.1 Mg inputs to soils We can treat the Hawi and Pololu soil systems as closed systems, using estimates of Mg input from rock weathering and precipitation to calculate the amount of Mg within each soil, acting as if no export of Mg were to occur. The amount of Mg derived from basalt weathering in these soils can be estimated using mass balance constraints provided by Zr, which is an immobile index element in these arid soils13. If we assume that Mg behaves conservatively like Zr, we can calculate the amount of Mg that should be present in the soil system using Mg/Zr ratios of parent material and Zr concentrations currently within the soil. This approach does not include estimates of mechanical erosion, but this loss mechanism will be accounted for in part by the mechanical loss of Zr from soil profiles. For the Hawi soil and the Pololu soil the amount of Mg made available to the soil from rock weathering are 35±2 kg/m2 and 141±11 kg/m2, respectively. To estimate rainfall inputs of Mg, modern rainfall chemistry data from Carrillo et al. 200214 are extended over the age of the soil at rainfall rates comparable to the present. This calculation yields 3±3 kg/m2 of Mg added to the Hawi and 7±7 kg/m2 of Mg added to the Pololu. The total amount of Mg input to the Hawi and Pololu soil systems is therefore 38±4 kg/m2 and 148±13 kg/m2, respectively. With these inputs a homogeneous soil derived from basalt and rainfall mixing should approach a 26Mg of -0.24±0.22 ‰ for the Hawi system, and -0.33±0.22‰ for the Pololu soil system. 4.2 Mg within soils and calculated Mg export

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Integration of the total mass of Mg and 26Mg values present in each soil profile reveal 29±4 kg/m2 of Mg within the Hawi soil, with a 26Mg of -1.35±0.16 ‰, and 21±1 kg/m2 of Mg within the Pololu soil with an overall 26Mg of -0.12±0.12 ‰. When these numbers are combined with our estimates of Mg input to and resulting isotopic composition of the Hawi soil system (38±4 kg/m2, -0.24±0.22 ‰), and the Pololu soil system (148±13 kg/m2, -0.33±0.22 ‰) we can calculate the mass and isotopic composition of the export flux of Mg from both. We obtain an export flux of 9±6 kg/m2 Mg, with a mean isotopic composition of 3.21 ‰ for the Hawi system and 127±13 kg/m2 Mg with 26Mg = -0.36±0.26 ‰ for the Pololu system. The uncertainty associated with the mean isotopic composition of the export flux is large for the Hawi soil primarily because of the greater relative uncertainty on the mass flux, but in any case it is clear that the soil has exported isotopically heavy Mg over the course of its development. In contrast the mean isotopic composition of Mg exported from the older Pololu soil over its development is similar to that of our estimate of the flux weighted inputs from basalt and precipitation. The mineralogy and morphology of these soils can explain these trends. 1.00

0.00

Bulk Chemistry Hawi Soils by Horizon (n=5) Bulk Chemistry Pololu Soils by Horizon (n=8) Hawi Carbonates (n=5)

26Mg

-1.00

-2.00

Pololu Carbonates (n=6) Literature Limestones and Dolostones (see Figure 1) Vegetation (n=4)

-3.00

Basalts (n=6) -4.00

-5.00

Literature Oceanic Island Basalts (see Figure 1) Rainfall (seawater literature value)

Fig. 1. Overview of Mg isotope data for bulk soils, carbonates, vegetation, basaltic parent materials, and estimated rainfall along with literature comparisons. Uncertainties shown are 2.

4.3 Pedogenic carbonates vs. noncarbonates The differences between the integrated Hawi and Pololu soils can be explained in part by the relative dominance of pedogenic carbonates in each. The carbonates in the Hawi and Pololu, account for 69±11% and 16±2% of the total magnesium integrated over the soil profiles, respectively. As the carbonates are an isotopically light reservoir of Mg (Figure 1), they drive the bulk Hawi soil profile toward low 26Mg values. The Hawi soil has a relatively thin surface horizon free of carbonate dominated by kaolinite, noncrystalline alumino-silicate phases, hematite, and gibbsite. Over an additional 180 ka of development, the Pololu soil has developed thicker surficial horizons free of carbonate but dominated by the same phases as in the upper


Hawi soil horizon. This process increases the water holding capacity of the soil system, reducing flushing and increasing the residence time of soil water. These factors limit carbonate production while promoting formation of other secondary soil phases, such as halloysite15. As such, the Pololu soil has evolved to a point where the Mg budget is dominated by phases other than carbonate. Horizons without carbonates in both the Pololu and Hawi soils, dominated by kaolinite, noncrystalline silicate phases, hematite, and gibbsite, have bulk soil 26Mg values that vary from -0.01±0.31 ‰ to -0.21±0.31 ‰, reflecting the incorporation of isotopically heavy Mg into alumino-silicate and oxide secondary phases. 5. Conclusions The Hawi and Pololu soils illustrate how Mg isotopes are fractionated during basalt weathering and how Mg isotopes can be used within soil systems. The weathering process generates an alumino-silicate and oxide-rich soil that sequesters isotopically heavy Mg. Pedogenic carbonates, like their oceanic counterparts, incorporate isotopically light Mg. In the arid setting of this study, the formation of pedogenic carbonates stores isotopically depleted Mg that would otherwise be exported to groundwater and/or stream water. The influence of these carbonates varies over the lifetime of the soils, from being the dominant control on the export of Mg at the younger site to a secondary pool that influences individual horizons but is not the major control on the export budget of Mg at the older site. This is the result of the mineralogical and morphological development of the soil through time, which in turn influences soil characteristics such as water holding capacity and rates of secondary mineral formation and recrystallization. Importantly, as soil systems evolve through time, so does the isotopic composition of the Mg exported. Acknowledgements Thanks to Gregg McElwee, Aimeryc Schumacher, and Delphine Yeghicheyan for all of their technical support and efforts. This work is supported by the National Science Foundation Grant 0922070. This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1120296). References 1. Tipper, E.T., Galy, A. and Bickle, M.J., 2006. Riverine evidence for a fractionated reservoir of Ca and Mg on the continents: Implications for the oceanic Ca cycle. Earth and Planetary Science Letters, 247(3-4): 267-279. 2. Brenot, A., Cloquet, C., Vigier, N., Carignan, J. and France-Lanord, C., 2008. Magnesium isotope systematics of the lithologically varied Moselle river basin, France. Geochimica et Cosmochimica Acta, 72(20): 5070-5089. 3. Pogge von Strandmann, P.A.E. et al., 2008. The influence of weathering processes on riverine magnesium isotopes in a basaltic terrain. Earth and Planetary Science Letters, 276: 187–197. 4. Tipper, E.T., Gaillardet, J., Louvat, P., Capmas, F., White, A.F., 2010. Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California. Geochimica et Cosmochimica Acta, 74: 3883-3896. 5. Wimpenny, J., Gislason, S.R., James, R.H., Gannoun, A., Pogge von Strandmann, P.A.E, Burton, K.W., 2010. The behavior of Li and Mg isotopes during primary phase dissolution and secondary mineral formation in basalt. Geochimica et Cosmochimica Acta, 74: 5259-5279. 6. Bolou Bi, E.B., Vigier, N., Poszwa, A., Boudot, J.P., Dambrine, E., 2012. Effects of biogeochemical processes on magnesium isotope variations in a forested catchment in the Vosges Mountains (France). Geochimica et Cosmochimica Acta, 87: 341-355. 7. Opfergelt, S., Georg, R.B., Delvaux, B., Cabidoche, Y.-M., Burton, K.W., Halliday, A.N., 2012. Mechanisms of magnesium isotope fractionation in volcanic soil weathering sequences, Guadeloupe. Earth and Planetary Science Letters, 341-344: 176-185. 8. Wimpenny, J., Colla, C.A., Yin, Q.Z., Rustad, J.R., Casey, W.H., 2014. Investigating the behaviour of Mg isotopes during the formation of clay minerals. Geochimica et Cosmochimica Acta, 128: 178-194. 9. Giambelluca, T.W., Schroeder, T.A., 1998. Climate. In: Juvik S.P., Juvik, J.O. (Eds.), Atlas of Hawaii, 3rd ed. University of Hawaii Press, Honolulu, pp. 49-59.

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