Experimental Dissolution of Calcium, Magnesium, and Strontium from Holocene Biogenic Carbonates: A Model of Diagenesis: ABSTRACT

Abstract

Laboratory experiments at ordinary temperatures and pressures for periods up to 240 days on Holocene biogenic carbonate sediments showed that fresh water and seawater dissolve aragonite and Mg-calcite. The observed rates of dissolution of calcium, magnesium, and strontium indicate that these elements are incorporated in aragonite and magnesium calcite in more than one way (in lattice positions, in lattice interstices, or in inclusions). This suggests the presence of more than one mineral phase in the skeletal materials studied; these phases differ in response to solution and probably in chemical composition. The more soluble phase may influence initiation of dissolution of the material and initiation of calcite nucleation in the aragonite-to-calcite inversion process. Calcium, magnesium, and strontium are dissolved in proportions different from those in the original solid, i.e., incongruently. The experimentally established sequence of preference is, as a rule, Mg-Ca-Sr. Factors found to determine direction and degree of incongruency in dissolution include mineralogy (number, kind, and relative abundance of phases present), physiologic effects operating during lifetime of the organism, and chemical composition and volume of waters effective in dissolution. Incongruent dissolution determines presence, absence, and abundance of ions derived from solids in the diagenetic environment and, hence, ion availability for precipitation in cement and for inhibition or catalysis in inversion or cement precipitation. Magnesium and strontium contents in shell material have been used by others as temperature and salinity indicators in Holocene environments. The present study shows that because of incongruent dissolution these indicators are not dependable for paleoenvironmental analysis. End_of_Article - Last_Page 741------------