Covariation of reactive solutes in the sea

Abstract

Ideally covariant reactive solutes are defined as those that pass through oceanic cycling without fractionation. The equation for the distribution of a variable was used to conclude that a pair of such solutes must have a constant concentration ratio in any part of the ocean wherein they covary ideally. A linear relationship between their concentrations is insufficient to demonstrate ideal covariation unless the equation for the relationship also happens to have a zero intercept. The existence of a constant concentration ratio provides a test for ideality since composite profiles of concentration ratios can be compared with the vertical straight line which denotes ideal covariation. The test was applied to pairs of minor reactive solutes that were grouped into two categories: those associated with diatoms ( 226Ra, Ba, Ge and Si) and reactive metals not specifically associated with diatoms (Zn, Cd, Ni and Cu). In descending order, the covariation ranking is 226Ra-Ba > Ge-Si > Ni-Cu > Zn-Cd > 226Ra-Si=Ba-Si. Two of the important conclusions to be drawn from this ranking are: (i) 226Ra and Ba appear to be the most tightly coupled reactive solutes in the sea, probably because combinations of internal chemical and transport processes effectively counteract boundary effects on the concentration-ratio profile; and (ii) the deep Indo-Pacific shows a highly ideal 226Ra-Si covariation while the deep North Atlantic is highly non-ideal. Discrepancies in the ideality of 226Ra-Si covariation and covariation differences among the four diatom-associated solutes were further examined in a culture experiment with a neritic diatom and in box model calculations. This experiment demonstrated the previously unreported capability of a single diatom to extract all four solutes simultaneously. It also demonstrated that a diatom can produce changes in concentration ratios of the four solutes, which nearly duplicate trends between the deep and upper ocean. The box model calculations used element:Si ratios from the culture and from field samples to show the strong possibility that diatoms are important in the removal of 226Ra and Ba as well as Ge, despite the non-ideal covariation between 226Ra or Ba and Si in parts of the ocean.