SERPENTINITE WEATHERING AND TRACE METAL MOBILITY IN DRY TROPICAL FOREST (NW COSTA RICA) AND COOL TEMPORATE GRASSLAND (SW ENGLAND)

Serpentinized ultramafic rocks on Late Pliocene wave-cut terraces in northwestern Costa Rica and southwestern England have been exposed for 3 Ma to chemical weathering in different climatic regimes. MAP and MAT on the Santa Rosa Peninsula in Costa Rica are 1500 mm and 25 C, respectively, with a 3 month rainy season and 9 month dry season. On the Lizard Peninsula in England, MAP is 1200 mm and MAT is 12 C, and seasonality is far less pronounced. Complete soil profiles from both study areas were sampled (during summer at Lizard and dry season at Santa Rosa) and analyzed by ICP-AES (using a 5-step sequential extraction for chemical analysis) and by XRD (<2 um clay fraction and <2 mm bulk fraction). Concentrations of Cr, Ni and Co are similar in serpentinite from both areas, with typical values of 3000, 2500 and 150 mg/kg, respectively.

The most abundant weathering product on the Santa Rosa peninsula is an expandable clay with XRD patterns similar to those of kaolinite/smectite, including a broad 001 peak at 1.8 – 2.0 nm. The b-axis dimension is 0.91 nm, suggesting a di-trioctahedral interstratification or physical mixture. Saponite (magnesian smectite) with a sharp 001 peak at 1.7 nm is the most common pedogenic mineral on the Lizard Peninsula. Sequential extraction and quantitative XRD indicates that Ni(II) and to a lesser extent Cr(III) are contained in the octahedral layer of these clays, with Ni values ranging up to 1.0 wt% of clay at Santa Rosa and 0.5 wt% at Lizard.

Co, Cr and Ni are concentrated in these soils by leaching of more soluble elements. At Santa Rosa, Ni in bulk soil ranges from 4670 - 8290 mg/kg, Cr from 4200 - 11800 mg/kg, and Co from 230 - 415 mg/kg. At Lizard, pedogenic enrichment of Ni, Cr and Co is less pronounced, with ranges of 1320 - 4740 (Ni), 2700 - 7320 (Cr), and 76 - 310 (Co) mg/kg bulk soil. The concentration of Ni(aq) in soil water sampled at the water table is 11 – 71 ppb at Santa Rosa and 150 – 290 ppb at Lizard; Cr(aq) in soil water is <4 ppb at both sites. In stream water, Ni(aq) concentrations are 3 – 16 ppb at Santa Rosa and 8 – 20 ppb at Lizard. Differences in concentrations of available Ni between Santa Rosa and Lizard are attributed differences in weathering intensity and soil mineralogy.