USING MG/CA AS A TRACER FOR “OCEAN ACIDIFICATION” IN RECENT MARINE SEDIMENTS FROM THE GREAT BARRIER REEF

Surficial carbonate sediments from the Great Barrier Reef were tested for their response to a small, 0.1 to 0.3 pH units, drop in pH. It was found that the metastable carbonates, particularly high magnesium calcite rapidly re-equilibrated in response to a lower pH in the ambient waters driven by increased CO₂ concentrations. As these carbonates undergo this re-equilibration, Mg is released into the surrounding water, even as precipitation of new lower Mg carbonate is taking place. Armed with this information we decided to test whether the drop in pH that has taken place in the last 40 years could be detected by comparing the Mg/Ca of surficial sediments in the Great Barrier Reef around the Capricorn Bunker group with sediments collected more than 40 years ago by Maxwell and his students. This group analyzed hundreds of sediment samples from around the Capricorn Bunker group reefs and the adjacent Lagoon. This surface sampling was repeated in the inter-reef plain area by Geoscience Australia several years ago as part of a larger study on inter-reef sediments.

The original sediments were analyzed by x-ray defraction (XRD) in the late 1960s we repeated the measurements using the same technique and discovered a uniformly lower Mg/Ca in the modern sediments when compared to the measurements made by the studies 40 years earlier. We found drops in Mg/Ca in every environment, in a range from 0.9 to 1.4 mole percent magnesium. This drop in magnesium concentration is consistent with a surface ocean that is slowly dropping in pH due to progressively higher partial pressure of CO₂ in the atmosphere. In those rare instances where surficial carbonate sediments have been collected during the past 50 years, this technique offers a chance to monitor the affects of dropping ocean pH.