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1 Buss, HL., White, AF., Dessert, C., Gaillardet, J., Blum, AE., & Sak, PB. (010). Depth profiles in a tropical, volcanic critical zone observatory: Basse-Terre, Guadeloupe. In IS. Torres-Alvarado, & P. Birkle (Eds.), Water- Rock Interaction XIII (pp. 5-8). CRC Press. Peer reviewed version Link to publication record in Explore Bristol Research PDF-document University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available:

2 Depth profiles in a tropical, volcanic critical zone observatory: Basse- Terre, Guadeloupe H.L. Buss & A.F. White U.S. Geological Survey, Water Resources Discipline, Menlo Park, CA, USA C. Dessert & J. Gaillardet Institut de Physique du Globe de Paris, Paris, France A.E. Blum U.S. Geological Survey, Water Resources Discipline, Boulder, CO, USA P.B. Sak Department of Geology, Dickinson College, Carlisle, PA, USA ABSTRACT: The Bras David watershed on the French island of Basse-Terre, Guadeloupe in the Lesser Antilles is located on a late Quaternary volcaniclastic debris flow of dominantly andesitic composition. The bedrock is mantled by more than 1 m of highly leached regolith. The regolith is depleted with respect to most primary minerals and weathering is dominated by the dissolution and precipitation of clays. Mineral nutrient cations such as Mg, K, and P are largely present as impurities or adsorbed to clays and iron oxides. Surface soils (< 0.3 m depth) are enriched in feldspar, quartz, cristobalite, and solid state Fe(II), Ca, K, and Mg relative to the underlying regolith, likely reflecting atmospheric deposition, possibly related to volcanic activity. 1 INTRODUCTION The Bras David watershed is located in a rugged, humid, tropical environment with a mean annual temperature of 5 C and a mean annual precipitation of 500 mm yr -1 (Météo-France, 008). Thin soils top very thick (1+ m) regolith, which is exposed at roadcuts and excavations. The regolith appears to be highly weathered volcanic debris flows, containing rocky clasts at various stages of weathering. Volcanic flows in the immediate vicinity were dated by Ar/Ar to be 900 ka (Samper et al., 007). variety of colors and textures that were visable during sample collection. Bulk densities of augered samples are extremely low, on average 0.9 g cm -3 (Fig. 1). A large clast, with a relatively unweathered core, collected from a nearby roadcut has a bulk density of. g cm -3 in the core and 1. g cm -3 in the rind (Sak et al., Subm.), indicating that mass is lost during weathering, with little or no loss of volume. Clays, dominantly halloysite, comprise about 75 wt of the mineralogy at all depths (Table 1). Nonclays are almost entirely Fe(III)-hydroxides and quartz/cristobalite. The only distinct depth trends in METHODS 0 Vadose zone pore waters were collected approximately monthly for years from 5 cm diameter nested porous-cup suction water samplers (Soil Moisture Inc., Santa Barbara, CA) that were installed in hand-augered holes at depths from 0.15 to 1.5 m. Pore waters were filtered to 0.5 μm and analyzed by ICP-MS and ion-chromatography. A 1.5 m solid core was collected and used for bulk density measurements, quantitative mineralogy by XRD using RockJock (Eberl, 003), and bulk chemical analysis by ICP-OES (SGS, Canada) Clast Rind Clast Core 3 RESULTS Augered core samples contained a number of weathered rocky clasts of varying hardness and color. Similarly, the regolith matrix material exhibited a Density (g cm 3 ) Figure 1. Bulk density of the augered profile ( ). A clast with a weathering rind from a nearby roadcut (Sak et al., Subm.) is shown here at an arbitrary depth.

3 Table 1. Mineralogy by Quantitative XRD, in weight percent. Quartz K-spar 1 Albite Magnetite Goethite Maghemite Cristobalite Kaolinite 3 Gibbsite Halloysite Intermediate microcline. Albite var. cleavelandite 3. Disordered kaolinite mineralogy are an inverse relationship between halloysite and gibbsite. Feldspars are almost totally absent except at the surface and bottom of the augered core. This is consistent with the observation of Sak et al. (subm.) of feldspar dissolving completely at the weathering interface of the clast sampled from a nearby roadcut. Clasts recovered during augering are weathered throughout and differ from the matrix mineralogy only in the proportion of specific clays (augered clast data not shown). More feldspar (mostly microcline) is present in the upper 0.3 m than anywhere else in the profile (Table 1). Similarly, quartz and cristobalite also increase at the surface. Solid state chemical composition of the bulk regolith is dominated by Si and Al with Na and Ca near or below detection limits at most depths (Table ). However, Fe(II), Ca, K, and Mg are enriched at the surface (< 0.3 m depth) relative to the underlying regolith. Roadcut clast rind compositions are comparable to regolith compositions with the exception that the rinds are slightly enriched in P. (Sak et al, Subm; Table ). Augered clasts (data not shown) are similar in composition to the surrounding regolith. Pore waters are dominated by sea salts, as is typical in tropical island watersheds (e.g., White et al., 1998). With the exception of Si, cations show little variation in concentration over time below 1- m depth (Fig. ). Solute Si, Mg, Ca, and K concentrations generally decrease from 0- m. Only Mg shows a clear trend in the deeper part of the profile, increasing below about m depth. DISCUSSION Low densities and a dearth of primary minerals reflect the high degree of weathering of the regolith. Visible bedrock textures in the regolith including relict clasts and the replacement of primary mineral grains by clays are consistent with iso-volumetric

4 Table. Elemental Composition of the Bulk Regolith Depth m Al O 3 CaO 1 Fe O 3 FeO 1 K O MgO MnO Na O P O 5 SiO TiO Sr Zn Zr < < < < nd < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < < nd < Core nd nd nd nd Rind-A nd < nd nd nd Rind-B nd nd nd nd 1 Total iron expressed as Fe O 3 ; ferrous iron expressed as FeO nd = not determined 3 Core = un-weathered core of clast collected nearby (Sak et al., Subm.) Rind of clast (Sak et al., subm.). A and B indicate areas of high and low curvature, respectively. weathering and saprolite formation, although some volume change cannot yet be ruled out. If the core of the clast analyzed by Sak et al. (Subm., Fig. 1, Table ) is representative of the regolith parent rock, the difference in density between parent and weathered material points to a substantial loss of mass during weathering. Mass losses during weathering are often assessed by normalizing solid state concentrations to a relatively immobile element or mineral (e.g., Brimhall & Dietrich, 1987). If an element or mineral is immobile, volumetric strain can be calculated to estimate volume changes during weathering: ρ pci, p ε (1) i, w = 1 ρ wci, w where ρ p and ρ w are the dry bulk densities of the protolith and weathered material, respectively, and c is the concentration of the immobile component (i) in the protolith (p) or in the weathered material (w). An ε i,w = 0 indicates iso-volumetric weathering. In deep tropical weathering environments, Ti is often found to be relatively immobile (e.g., White et al., 1998; Sak et al., 00; Buss et al., 008). However, Equation 1 shows that in the Bras David profile, if the un-weathered clast core is representative of the parent material and if weathering is iso-volumetric, significant loss of Ti must have occurred. Alternatively, if Ti is immobile, an unrealistic expansion of volume would be required to satisfy the equation. Further research is needed to ascertain whether or not the clast is representative of the protolith. Despite easily observed heterogeneities in color and texture, the regolith profile reveals little variation in mineralogy or chemistry with depth. One exception is the inverse relationship between halloysite

5 Na Si Al Mg 1 Ca Concentration (μm) Figure. Elemental concentrations in pore waters with depth. Open symbols indicate individual samples collected over a year period. Closed symbols with lines indicate average values. Note the different concentration scale for Na and Si. K and gibbsite, which suggests that halloysite weathers to gibbsite. A lack of Mg-, K-, or P-containing minerals in the regolith suggests that these mineral nutrient cations are largely present as impurities or adsorbed to secondary clays and oxide minerals. Increasing solute Mg concentrations at depth may indicate a weathering-related input of Mg to the pore waters, likely the release of adsorbed or coprecipitated Mg from halloysite or kaolinite. Enrichment in feldspar, quartz, cristobalite, and solid state Fe(II), Ca, Mg, and K at the surface are consistent with dust deposition. These inputs may be related to recent volcanic activity on Basse-Terre and/or nearby Monserrat, providing an important source of mineral nutrients to the surface ecosystem. 5 CONCLUSIONS Despite visible heterogeneities in texture and color, the regolith displays little variation with depth in terms of mineralogy or chemistry. Mineral nutrient dynamics in the highly leached Bras David regolith are dominated by weathering of secondary minerals and dust deposition. The regolith may have lost well over 50 of the original mass, including elements often assumed to be immobile (i.e., Ti). REFERENCES Brimhall, G. & Dietrich, W.E., Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis. Geochim. Cosmoch. Acta, 51: Buss, H.L., Sak, P.B., Webb, S.M. & Brantley, S.L., 008. Weathering of the Rio Blanco quartz diorite, Luquillo Mountains, Puerto Rico: Coupling oxidation, dissolution, and fracturing. Geochim. Cosmoch. Acta, 7: Sak, P.B., Fisher, D.M., Gardner, T.W., Murphy, K. & Brantley, S.L., 00. Rates of weathering rind formation on Costa Rican basalt. Geochim. Cosmoch. Acta, 8: Sak, P.B., Navarre-Sitchler, A.K., Miller, C.E., Daniel, C.C., Lebedeva, M.I. & Brantley, S.L. Rates of formation of weathering rinds vary with clast curvature. Submitted to Chem. Geol. Samper, A., Quidelleur, X., Lahitte, P. & Mollex, D., 007. Timing of effusive volcanism and collapse events within an oceanic arc island: Basse-Terre, Guadeloupe archipelago (Lesser Antilles Arc). EPSL, 58: White, A.F., Blum, A.E., Schulz, M.S., Vivit, D.V., Stonestrom, D.A., Larsen, M., Murphy, S.F. & Eberl, D., Chemical weathering in a tropical watershed, Luquillo Mountains, Puerto Rico: I. Long-term versus short-term weathering fluxes. Geochim. Cosmoch. Acta, : 09-.