CO2 System Permeable Sediment Chemistry and Modeling of It's Behavior Under Rising temperature and Ocean Acidification

Porewater was collected from highly permeable, carbonate-rich, sandy sediments at two locations, CRIMP-2 and Ala Wai, on coral reefs on Oahu, Hawaii. Samples were collected at the sediment-water interface and from porewater wells installed at sediment depths of 2, 4, 6, 8, 12, 16, 20, 30, 40, and 60 cm. Total alkalinity and dissolved inorganic carbon were enriched, relative to the overlying water column, and ratios of TA:DIC at the two sites (0.80 and 0.93) suggest that aerobic respiration and sulfate reduction - both coupled with carbonate mineral dissolution - in the oxic and anoxic layers, respectively, are the major controls on the biogeochemistry of the porewater-sediment system. The porewater was approaching thermodynamic saturation with respect to aragonite and was found to be undersaturated with respect to all phases of magnesian calcite containing greater than 12 mol% MgCO₃. In addition to microbial controls on porewater diagenesis, transient physical events in the water column, such as swells and changing bottom current speeds, appear to exert a strong influence on the porewater chemistry due to the highly permeable and porous nature of the sediments. Profiles collected before and after swell events at each location show an apparent flushing of the porewater system, replacing low pH, high DIC interstitial waters with seawater from the overlying water column.

Using this data, along with data collected in numerous prior studies, a CO2-carbonic acid system biogeochemical box model of the barrier reef flat of Kaneohe Bay, Oahu was developed in order to determine how increasing DIC of the open ocean source waters due to rising anthropogenic CO2 emissions and ocean acidification affects the CaCO3 budget of coral reef systems. This 17-box model was forced using the Representative Concentration Pathway (RCP) scenarios that predict CO2 atmospheric concentrations and temperature anomalies out to 2100. Model outputs predict a decrease in net ecosystem carbonate production, although the reef does not reach a state of net erosion by 2100. This dual approach allows for a better understanding of how sediment porewaters, sediments, and reef frameworks will respond to anthropogenic changes over the next century and provides valuable insight into the threshold when coral reefs could switch from net accretion to net erosion.