HR: 0830h
AN: B21D-0748 [PDF]
TI: Ca Isotope Fractionation in the Hawaiian Ecosystem
AU: * Wiegand, B A
EM: bwiegand@pangea.stanford.edu
AF: Department of Geological and Environmental Sciences, 367 Panama Mall
Stanford University, Stanford, CA 94305-2115
AU: * Wiegand, B A
EM: bwiegand@pangea.stanford.edu
AF: Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020
AU: Chadwick, O A
EM: oac@geog.ucsb.edu
AF: Department of Geography, Ellison Hall 3611
UCSB, Santa Barbara, CA 93101
AU: Vitousek, P M
EM: vitousek@stanford.edu
AF: Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: Department of Geological and Environmental Sciences, 367 Panama Mall
Stanford University, Stanford, CA 94305-2115
AU: Wooden, J L
EM: jwooden@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025
AB:
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 $\delta$$^{44}$Ca with $^{87}$Sr/$^{86}$Sr and Sr/Ca data
of soils (bulk compositions and extractable Ca and Sr from soil exchange sites) and different plant species including native
Ohia trees ({\it Metrosideros polymorpha}) from a soil chronosequence along the Hawaiian Island chain. The study sites differ
in age of the underlying substrate from 0.3 ka to 4,100 ka, but show similar recent climate (mean annual temperature of 16
$\deg$ C) and amount of precipitation (about 2,500 mm/y).
$^{44}$Ca/$^{40}$Ca ratios were measured on a MAT262 at Stanford University, using a $^{42}$Ca-$^{48}$Ca double spike, and
are reported as $\delta$$^{44}$Ca values relative to seawater ($\delta$$^{44}$Ca = 0 $\permil$). Results of the extractable,
plant available calcium from six soil sites show $\delta$$^{44}$Ca values in the range of +1.2 $\permil$ to -1.3 $\permil$
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 $\delta$$^{44}$Ca values between -0.1 $\permil$ and -2.5 $\permil$, indicating
differences in composition as a result of contributions from volcanic minerals, continental dust, and marine aerosols in
different proportions. Leaves and wood material of different plant species have $\delta$$^{44}$Ca values in the range of -0.1
$\permil$ and -2.1 $\permil$, 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.
DE: 1615 Biogeochemical processes (4805)
DE: 1827 Glaciology (1863)
DE: 4805 Biogeochemical cycles (1615)
DE: 4845 Nutrients and nutrient cycling
SC: Biogeosciences [B]
MN: 2003 Fall Meeting