Changing sources of strontium to soils and ecosystems across the Hawaiian Islands O.A. Chadwick a, ⁎, L.A. Derry b , C.R. Bern c , P.M. Vitousek d a Department of Geography, University of California, Santa Barbara, CA 93106, United States b Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, United States c U.S. Geological Survey, Denver, CO 80225, United States d Department of Biology, Stanford University, Stanford, CA 94305, United States abstract article info Article history: Received 29 June 2008 Received in revised form 14 January 2009 Accepted 19 January 2009 Editor: J. Fein Keywords: Dust Mineral aerosols Marine aerosols Tropical soil Soil development Basalt weathering Strontium isotope ratios assist ecosystem scientists in constraining the sources of alkaline earth elements, but their interpretation can be difficult because of complexities in mineral weathering and in the geographical and environmental controls on elemental additions and losses. Hawaii is a “natural laboratory” where a number of important biogeochemical variables have either limited ranges or vary in systematic ways, providing a unique opportunity to understand the impact of time, climate, and atmospheric inputs on the evolution of base cation sources to ecosystems. There are three major sources of strontium (Sr) to these ecosystems, each with distinct isotopic compositions: basalt lava, Asian dust, and rainfall. We present Sr isotope and concentration data on both bulk soil digests and NH 4 Ac extracts from soil profiles covering a wide range of environments and substrate ages. Bulk soil material from dry climates and/or young substrate ages with N 80 μg g -1 Sr retain basalt-like Sr isotopic signatures, whereas those with Sr concentrations b 80 μg g -1 can have isotope signatures that range from basalt-like values to the more radiogenic values associated with continental dust. Although both dust accumulation and lava weathering are time- and rainfall-dependent, the overall concentration of Sr drops with increasing leaching even as quartz and mica derived from continental dust sources increase to N 40% by mass. At elevated dust levels, lava-derived Sr is low and dust-derived Sr is the dominant control of 87 Sr/ 86 Sr in bulk soils; however, 87 Sr/ 86 Sr of NH 4 Ac- extractable Sr largely reflects atmospheric deposition of marine aerosol in these situations. Overall, whole- soil Sr isotope values are controlled by complex interactions between Sr provided by lava weathering but partially lost by leaching, and Sr provided by dust but held in more resistant minerals. The isotopic composition of NH 4 Ac-extractable Sr and of the biota is controlled by lava weathering and rainfall contribution of Sr with only minor contributions from radiogenic dust sources. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Over the last two decades, use of naturally occurring isotopic tracers in the rubidium–strontium system has emerged as an important means of constraining the sources and cycling of mineral nutrients in terrestrial ecosystems. The results of early studies pointed to the role of atmospheric deposition of salts and/or mineral aerosol (dust) as a major source of Sr, and by inference the other alkaline earth base cations (Ca, Mg), to forests (Graustein and Armstrong, 1983; Graustein, 1989; Aberg et al., 1990). Subsequent work has made it clear that all ecosystems derive base cations from a variety of sources whose importance varies both in time and across landscapes, climate regimes and geological substrates (Miller et al., 1993; Quade et al., 1995; Bailey et al., 1996; Bullen et al., 1997; Capo et al., 1998; Kennedy et al., 1998; Kennedy et al., 2002; Blum et al., 2002). The basic principles behind the use of Sr isotope variations as a source tracer are straightforward (Capo et al., 1998). Over geological time scales differences in the Rb/Sr ratio of source materials give rise to differences in the 87 Sr/ 86 Sr ratio of various Earth reservoirs via the β-decay of 87 Rb (λ = 1.42 × 10 -11 yr -1 ) to 87 Sr. The rate is sufficiently slow that on ecological time scales there is no significant change resulting from radioactive decay. Measurement of 87 Sr/ 86 Sr is ordinarily carried out by thermal ionization mass spectrometry, and part of the analytical procedure includes the normalization of instrumental mass fractionation to a constant 86 Sr/ 88 Sr. This has the effect of removing any natural mass fractionation effects as well, although these are likely to be small in any case. Thus on the time scale of interest for biogeochemical research, the 87 Sr/ 86 Sr ratio of a reservoir, whether rock, soil or plant material, can be considered constant and is only affected by mixing processes. In principle the sources of Sr to ecosystems can be determined by “un-mixing” the measured value into the end-member contributions, for example rock weathering and deposition of atmospheric salts. There are, however, several complicating factors. For instance, there can be strong temporal variations in the isotopic composition of Chemical Geology 267 (2009) 64–76 ⁎ Corresponding author. Tel.: +1 805 448 2564; fax: +1 805 893 8686. E-mail addresses: oac@geog.ucsb.edu (O.A. Chadwick), lad@cornell.edu (L.A. Derry), cbern@usgs.gov (C.R. Bern), Vitousek@stanford.edu (P.M. Vitousek). 0009-2541/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2009.01.009 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo