Differential responses of the Mg/Ca Ratio in scleractinians to variations in Mg2+ and Ca2+ content of seawater

Abstract:

Although it is well known that the Mg²⁺/Ca²⁺ ratio of seawater has varied throughout the Phanerozoic, there is little information on whether the concentration of Ca²⁺ has changed in conjunction with Mg²⁺ or whether these two elements have changed independently. In addition while it has been documented that the Mg/Ca has varied within scleractinian corals during certain time periods, there have been no studies which have verified that changing Mg/Ca ratio in corals corresponds with a similar variation in seawater. Here we report studies using Modern corals which have been grown for an extended period of time (10 weeks) in elevated Ca²⁺and Mg²⁺ concentrations. While these studies show that the Mg²⁺/Ca²⁺ of the seawater is related to the same ratio in the coral skeleton, the slope of the relationship is dependent upon which of the two cations is altered. When the Ca²⁺ is increased, thereby decreasing the Mg²⁺/Ca²⁺ ratio in seawater, then the ratio within the skeleton decreases by ~0.7 mM/M for every 1 M/M decrease in the Mg²⁺ / Ca²⁺ ratio in the external seawater. However, when the Mg²⁺/Ca²⁺ ratio is altered by adding Mg^2+, then there is an increase is ~2 mM/M for every 1 M/M increase in Mg²⁺/Ca²⁺. Assuming that such changes can be replicated by the reverse experiment, i.e. reducing the concentration of Ca²⁺ and Mg²⁺, then such a finding offers a solution to resolving whether the Mg/Ca ratio in ancient unaltered corals is responding to a change in Mg²⁺ or a change in Ca²⁺ in seawater. For example, the Mg/Ca ratios of unaltered corals from a defined time period might only be possible if the Ca²⁺ of seawater was altered rather than Mg²⁺. The reverse might be case for other time periods. Further supporting evidence for this hypothesis will be presented by other elements which exhibited unexpected behavior in response to changing Ca²⁺ and Mg²⁺.