Ca Isotope Fractionation in the Hawaiian Ecosystem

Investigations of the nutrient budgets in Hawaiian soils show the sources of major cations to be weathering of volcanic rock, marine aerosols, and Asian dust inputs. Especially at deeply weathered sites older than 150 ka, soils show strong depletion of the macronutrient calcium. Most of the calcium supply in these soils is of atmospheric origin (marine aerosols and continental dust). In contrast, younger soils are mainly supplied by calcium from weathering of volcanic bedrock. Based on the results of previous studies using strontium isotopic signatures and Sr/Ca ratios (e.g. Kennedy et al. 1998, Chadwick et al. 1999, Whipkey et al. 2000, Stewart et al. 2001) we have conducted research focusing on the isotope composition of calcium as a new tool for the investigation of sources of calcium and biogeochemical processes effecting Ca isotope fractionation in the plant-soil system. The study combines δ ⁴⁴Ca with ⁸⁷Sr/⁸⁶Sr and Sr/Ca data of soils (bulk compositions and extractable Ca and Sr from soil exchange sites) and different plant species including native Ohia trees (Metrosideros polymorpha) from a soil chronosequence along the Hawaiian Island chain. The study sites differ in age of the underlying substrate from 0.3 ka to 4,100 ka, but show similar recent climate (mean annual temperature of 16 ° C) and amount of precipitation (about 2,500 mm/y). ⁴⁴Ca/⁴⁰Ca ratios were measured on a MAT262 at Stanford University, using a ⁴²Ca-⁴⁸Ca double spike, and are reported as δ ⁴⁴Ca values relative to seawater (δ ⁴⁴Ca = 0 ‰ ). Results of the extractable, plant available calcium from six soil sites show δ ⁴⁴Ca values in the range of +1.2 ‰ to -1.3 ‰ with generally more negative values related to younger soil sites where calcium is mainly derived from weathering of volcanic rocks. Bulk soil samples, however, show δ ⁴⁴Ca values between -0.1 ‰ and -2.5 ‰ , indicating differences in composition as a result of contributions from volcanic minerals, continental dust, and marine aerosols in different proportions. Leaves and wood material of different plant species have δ ⁴⁴Ca values in the range of -0.1 ‰ and -2.1 ‰ , suggesting biological fractionation of Ca isotopes during calcium uptake in plants. From our results we conclude that the pattern of Ca isotope fractionation in the Hawaiian ecosystem depend on several factors (1) the source of calcium, (2) physiological processes, and (3) soil biogeochemical processes. References Chadwick et al. (1999) Nature 397: 491-497. Kennedy et al. (1998) Geology 26: 1015-1018. Stewart et al. (2001) Geochim. Cosmochim. Acta 65: 1087-1099. Whipkey et al. (2000) Chem. Geol. 168: 37-48.