Biogeochemical weathering of serpentinites: An examination of incipient dissolution affecting serpentine soil formation Julie L. Baumeister a , Elisabeth M. Hausrath a,⇑ , Amanda A. Olsen b , Oliver Tschauner a,c , Christopher T. Adcock a , Rodney V. Metcalf a a Department of Geoscience, University of Nevada, Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV 89154, United States b School of Earth and Climate Sciences, 5790 Bryand Global Sciences Center, University of Maine, Orono, ME 04469, United States c HiPSEC, University of Nevada Las Vegas, 4505 S. Maryland Parkway, Las Vegas, NV, United States article info Article history: Available online 17 January 2015 Editorial handling by Mats Åström abstract Serpentinite rocks, high in Mg and trace elements including Ni, Cr, Cd, Co, Cu, and Mn and low in nutri- ents such as Ca, K, and P, form serpentine soils with similar chemical properties resulting in chemically extreme environments for the biota that grow upon them. The impact of parent material on soil charac- teristics is most important in young soils, and therefore the incipient weathering of serpentinite rock likely has a strong effect on the development of serpentine soils and ecosystems. Additionally, porosity generation is a crucial process in converting rock into a soil that can support vegetation. Here, the impor- tant factors affecting the incipient weathering of serpentinite rock are examined at two sites in the Klam- ath Mountains, California. Serpentinite-derived soils and serpentinite rock cores were collected in depth profiles from each sampling location. Mineral dissolution in weathered serpentinite samples, determined by scanning electron microscopy, energy dispersive spectrometry, electron microprobe analyses, and synchrotron microXRD, is consistent with the order, from most weathered to least weathered: Fe-rich pyroxene > antigorite > Mg-rich lizardite > Al-rich lizardite. These results suggest that the initial porosity formation within serpentinite rock, impacting the formation of serpentine soil on which vegetation can exist, is strongly affected both by the presence of non-serpentine primary minerals as well as the com- position of the serpentine minerals. In particular, the presence of ferrous Fe appears to contribute to greater dissolution, whereas the presence of Al within the parent rock appears to contribute to greater stability. Iron-oxidizing bacteria present at the soil–rock interface have been shown in previous studies to contribute to the transition from rock to soil, and soils and rock cores in this study were therefore tested for iron-oxidizing bacteria. The detection of biological iron oxidation in this study indicates that the early alteration of these Fe-rich minerals may be mediated by iron-oxidizing bacteria. These findings help provide insight into the incipient processes affecting serpentinite rock weathering, important to the development of extreme serpentine soils and the biota that grow on them. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Serpentine soils derived from serpentinite rocks are typically depleted in essential nutrients such as Ca, K, and P and contain high levels of Mg, Fe and trace elements such as Ni, Cr, Cd, Co, Cu, and Mn (Wildman et al., 1968; Shallari et al., 1998; Lee et al., 2001; Harris et al., 2007; Oze et al., 2007; Kram et al., 2009; McGahan et al., 2008, 2009; Rajapaksha et al., 2012, 2013; Vithanage et al., 2014). Plant communities growing above serpent- inite bedrock often develop ‘‘serpentine syndrome’’ as an adapta- tion to the stresses associated with serpentine soils (Jenny, 1980), including reduced stature, increased tolerances to elevated Mg and Ni, and more developed root systems compared to closely related plant species that grow on other soil types (Kruckeberg, 2004; Brady et al., 2005; Oze et al., 2008), as well as slower growth rates (Kram et al., 1997, 2009). The impact of the serpentinite par- ent material on the characteristics of the serpentine soils that form upon them is most important in young soils, and therefore, the ini- tial reactions in weathering serpentinite rocks are critical to the unusual ecosystems that form upon them. The first reactions affecting weathering of rock that eventually becomes soil are increasingly recognized as an important rate-lim- iting step in this process. The first mineral to dissolve within a rock has been termed the ‘‘profile-controlling mineral’’ (Brantley and White, 2009), and therefore, even if present in small http://dx.doi.org/10.1016/j.apgeochem.2015.01.002 0883-2927/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 702 895 1134. E-mail address: elisabeth.hausrath@unlv.edu (E.M. Hausrath). Applied Geochemistry 54 (2015) 74–84 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem