Porewater CO2–carbonic acid system chemistry in permeable carbonate reef sands

Abstract

Porewater was collected from highly permeable, carbonate-rich, sandy sediments at two locations on Oahu, Hawai'i, CRIMP-2 on the Kaneohe Bay barrier reef and Ala Wai on a fringing coral reef nearshore to the Ala Wai canal and urban Honolulu. 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 60cm. 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 sediment–porewater system. The porewater was approaching thermodynamic saturation with respect to aragonite with increasing depth (Ω=1.2–3.5) and was found to be undersaturated with respect to all phases of magnesian calcite (Ω=0.3–0.9) containing greater than 12mol% 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. Based partially on the results of our porewater analysis, we hypothesize that future changes in surface water DIC and pH resulting from ocean acidification (OA) could have a very significant impact on the dissolution rates of metastable skeletal and abiotic carbonate phases of varying magnesian calcite compositions (Mg-calcite) and aragonite. This is especially important in sandy reef sediments like those of this study, which are greatly influenced by the overlying water column. As the carbonate mineral saturation state of the overlying water column continues to decrease due to OA, an increase in carbonate mineral dissolution is expected and the high advective rate of water exchange between the porewater of sandy sediments and the overlying water column, as observed in this study and others, along with increased rates of dissolution of metastable carbonate phases, could lead to significantly higher future rates of mass transfer of TA and DIC between the sediments and the overlying water column. This may result in a deficit of the CaCO3 balance in some reef ecosystems and a decrease in accretion rates. Analysis of our porewater work in conjuction with previous studies of the porewater chemistry of Kaneohe Bay, Oahu in both siliciclastic-rich and siliciclastic-poor carbonate sediments leads to the conclusion that the porewaters of the former are more strongly buffered with respect to pH than those of the latter due to reverse weathering reactions. Thus carbonate-rich sandy sediments of reefs with little terrestrial influence and aluminosilicate detritus may become more susceptible to calcium carbonate loss due to the enhanced environmental and microbial dissolution of carbonate substrates expected due to OA.