Abstract
We report trace element/calcium ratios for modern ( Cu Ca , Mn Ca , Cd Ca , and Pb Ca ) and seventeenth-century ( Cu Ca , Mn Ca , and Pb Ca ) specimens of Pavona clavus collected in the Galapagos Islands. These data include the first reliable measurements of Cu Ca ratios in coralline aragonite. We estimate that the ratio of Cu Ca in the lattice to that in seawater (i.e., the effective distribution coefficient) is ~0.3, lower than the value of 1 observed for several other divalent elements; we estimate the effective distribution coefficient for Mn is ~ 1. Distribution coefficients in coral aragonite for 8 divalent metals are near unity despite different ionic radii and chemical speciation in seawater. Investigation of where these substituents are incorporated in the aragonite lattice is clearly warranted. In a modern Hood Island coral, quarter-annually sampled from 1964–73, Cu Ca ratios decrease twofold from the late 1960s to early 1970s. Mn Ca and Cd Ca ratios vary seasonally with upwelling and the generic El Niño. The El Nino Southern Oscillation (ENSO) events of 1965, 1969, and 1972 are marked by suppression of Cd Ca ratios by about 2.5 nmol/mol, while the Mn Ca ratio is highest during the strong 1972 ENSO. Pb Ca ratios were relatively constant throughout this period. From the Cu Ca record of a seventeenth-century Urvina Bay coral annually sampled from 1600–1725 and the estimated Cu distribution coefficient, surface seawater Cu concentrations at Galapagos during the seventeenth century were similar to present day at 0.7–1.4 nmol/kg. Estimated Pb concentrations were lower at 5–20 pmol/kg, and Mn concentrations were slightly higher at 1.6–2.8 nmol/kg.
Highlights
TRACEMETALCONCENTRATIONiSn coral skeletal aragonite have been suggested to reflect concentmtions in surface seawater (AMIEL et al, 1973; SCHNEIDERand SMITH, 1982; HOWARDand BROWN, 1984; SHEN, 1986; SHENand BOYLE, 1988)
Cu/Ca, Mn/Ca, Cd/Ca, and Pb/Ca results reported here are from two colonies of Puvona claws, one of the species studied by Shen (SHEN and BOYLE, 1987, 1988; SHENet al., 1987)
Determinations of lattice-bound trace metals Cu, Mn, Cd, and Pb in coralline aragonite were carried out using a modification of the treatment of SHEN ( 1986; SHEN and BOYLE, 1988).We have shown that Cu/Ca can be reproducibly measured in coralline aragonite, suggesting these levels may reflect Cu concentrations in seawater
Summary
Gecwhimica a Cosmcxhimica Ada Vol 54, pp. 381-394 Copyright 8 1990 Pergamon Press pk. Both the seasonally occurring generic El Nifio and occasional, longer duration ENS0 events result in warmer, more nutrient-depleted Galapagos surface waters Trace metals such as Cd, Mn, and Cu may be sensitive proxies for these circulation shifts based on their observed water column and geographic distributions which may reflect nutrient cycles (e.g., Cd) or aeolian and fluvial sources (e.g. Mn). The generic El Nifio current and ENS0 events should be recorded as maxima in Mn/Ca ratios due to the increased influence of Panama Basin waters with high Mn replacing upwelling and open ocean waters with low Mn. Cu profiles are characterized by surface minima and increasing concentrations with depth. Pb determinations serve as quality controls here on sample handling and potentially may yield information on patterns of aeolian input
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