THE EFFECTS OF PH ON PLANKTIC FORAMINIFERA: DISSOLUTION, FRAGMENTATION, AND TAPHONOMIC BIAS

The effects of ocean acidification are among the most pervasive and detrimental of any global change, and are known to have caused extinctions of both modern and prehistoric taxa. The eruption of large igneous provinces with the release of large volumes of CO₂ and SO₂ has been a major contributor to decreasing oceanic pH. A drop in surface ocean pH causes dissolution of planktic calcifiers like foraminifera and nannofossils. After deposition, the calcite tests are further exposed to pore water dissolution and recrystallization. Dissolution significantly alters the geochemistry of foraminiferal test carbonate and can affect species census data due to preferential dissolution of certain test morphologies. These factors potentially bias accurate reconstruction of paleoenvironments. A quantitative study of the effects of low surface ocean pHs on Recent and late Cretaceous planktic foraminifera was conducted to evaluate the extent of damage on different test morphologies.

Tests were suspended in filtered seawater covering a range of pH values. The pH was controlled using HCl and alkalinity was maintained using NaHCO₃. At fixed intervals foraminifera were extracted, dried and weighed to record dissolution and fragmentation, and SEM micrographs were taken to record surface textures. Results show that lower pH causes increased fragmentation and dissolution effects as recorded by calcite weight loss and signs of disintegration in the surface texture. Foraminiferal tests at seawater pH=6.8 show 4 times higher fragmentation than in the control pH=8.0 and 3 times more calcite weight loss due to dissolution. Dissolution effects are more limited for the larger and thicker complex taxa such as the globotruncanids. The extent of test fragmentation and overall dissolution is characteristic of the ambient pH after a given time. Therefore, quantification of fragmentation, dissolution, and taxonomic prevalence can constrain the pH of paleoenvironments during ocean acidification events. In addition, comparison of different sections around the globe permits the isolation of ocean acidification effects by averaging out the effects of pore water dissolution and recrystallization.