Distribution and nature of incorporation of trace elements in modern aragonitic corals*

The objective of this study is to locate as closely as possible the sites of strontium, magnesium, sodium, and potassium in modern aragonitic corals, specifically whether these cations are adsorbed, or are substituted in the carbonate lattice or are incorporated in organic components. In addition to locating the sites of each of these four elements we wanted to find out quantitatively how much of each element occurs at each site. The experiments in this study are based on the dissolution rate of aragonite in distilled water and on the substitution of strontium and magnesium by calcium and sodium. Special attention has been given to the occurrence of strontium, magnesium, sodium, and potassium in the organic components of the corals. The main site for strontium in the corals is in the aragonite lattice. Twenty-five per cent of the total magnesium occurs in adsorbed sites and in organic compounds. The rest of the magnesium may be located in the aragonite lattice, but it is easily removed by repeated leaching or by replacement with calcium ions. Another possibility is that magnesium may occur in a dispersed mineral phase more soluble than aragonite because magnesium was released at a higher Mg to Ca ratio than is found in the solid coral; also because no local concentration of magnesium could be detected with an electron microprobe. About 12% of the total sodium is in adsorbed sites and is included in the organic compounds. The rest of the sodium might be in the lattice replacing calcium, but the low total exchange capacity is not enough to provide the needed charge balance. Another possibility is that sodium is located in a proposed mineral phase. Potassium is in adsorbed sites and incorporated in the organic compounds to an extent greater than all the other elements studied (30% of the total potassium), but again the evidence suggests that the remaining potassium is in a proposed mineral phase. Calcite is detected on the surfaces of aragonite corals after 5 months in the substitution experiment. The change of argonite to calcite took place after the inhibitor magnesium was exchanged from the surface sites and replaced by calcium. The organic compounds in corals contain small amounts of strontium, magnesium, sodium and potassium. Strontium is preferentially enriched in the organic compounds over magnesium.     

Effect of man's activities on distribution of trace elements in sub-bottom sediments of Lake George,New York*

A core from the southern mesotrophic basin of Lake George, New York, shows enrichment in the concentration of trace elements near its top. By contrast, a core from the northern oligotrophic basin shows, with the exception of manganese, an almost constant trace-element concentration throughout the core. Man's activities in the watershed surrounding the southern basin are responsible for enrichment in the trace-element concentration of newly deposited bottom sediments. Increase in trace-element concentration is not paralleled by increase in organic-carbon concentration.     

Two New Carbonate Stable-Isotope Standards

Two new carbonate isotope standards have been prepared for distribution by the IAEA and NBS. The first, TS limestone, NBS #19, will substitute for PDB, while the second carbonate, NBS #18 is depleted in ¹⁸O and ¹³C compared to the first.