Effects of carbon dioxide sequestration on California margin deep-sea foraminiferal assemblages

Abstract

Deep-sea sequestration of CO 2 is being considered as a possible mitigation tool to decrease atmospheric CO 2 concentrations and its associated negative effects. This study investigated potential effects of liquid carbon dioxide (CO 2) injection on deep-sea foraminiferal assemblages. Foraminifera are ideal for this ecological impact investigation because of differing test composition (calcareous and non-calcareous) and thickness, and diverse epifaunal and infaunal depth preferences. The experiment was conducted on August–September 2003, at 3600 m off the coast of Monterey Bay, California, aboard the R/V Western Flyer using the ROV Tiburon. The pH of the site was monitored throughout the experiment. Sediment push-cores were collected (both from the experimental and control sites) and stained to distinguish live (stained) from dead (unstained) individuals. Effects of CO 2 injection on assemblages have been tracked both vertically (to 10 cm depth below sea floor) and horizontally (up to 10 m from CO 2 injection sites), as well as between live and dead individuals. Within corrals (containing the injected CO 2) and their underlying sediments, severe pH changes (near 4.0 units) were recorded. This compares with a record of small average reductions in ocean pH (− 0.05 units) combined with large episodic excursions (− 1.7 units) over the experimental area due to the injection of CO 2. Exposure to this gradient of low pH caused increased mortality and dissolution of calcareous forms within corrals, as far as 5 m from the injection site, and to at least 10 cm depth in the sediments. This experiment revealed several major effects of CO 2 injection on foraminiferal assemblages in surficial sediments: 1) total number of foraminifera in a sample decreases; 2) foraminiferal species richness decreases in both stained and unstained specimens; and 3) relative percentage of stained (live) forms in the remaining tests increases. Down-core trends (to 10 cm below sea floor) have revealed: 1) percent agglutinated forms decline and calcareous forms increase with depth; 2) agglutinated species richness decreases with depth; and 3) experimental core assemblages become increasingly similar with depth to those in control cores not subjected to CO 2 injection. These results imply almost complete initial mortality and dissolution in the upper 10 cm throughout the corrals following liquid CO 2 injection. Since calcareous foraminifera represent more than 50% of the total assemblages, this clearly indicates that emplacement of CO 2 will result in negative effects to diversity and survivorship of the deep-sea benthic meiofauna.