Density‐induced water circulations in atoll coral reefs: A numerical study

Abstract

Groundwater flow in an idealized atoll platform is simulated with CASTEM 2000, a computer code developed at the CEA (Commissariat à l'Energie Atomique). Darcy's law and a coupled solute transport equation (diffusion-dispersion-convection) is solved by the mixed-hybrid finite-element method. Inward and upward circulations of oceanic water are shown to occur. These steady-state circulations result from temperature and, to a lesser extent, salinity gradients. Inclusion of a karstified dolomitic horizon, observed at the base of the carbonate structure of certain atolls, is necessary to account for the negative vertical temperature gradient indicated by field data. The position of the platform in relation to the oceanic temperature and salinity distributions exerts a significant control on the interstitial hydraulic regime. The effect of a secondary karstic layer closer to sea surface is also modeled. In this case, calculations show that the near-surface carbonate edifice is deprived of a significant portion of the flow. Finally, an upper limit on the possible flow rate through the platform is calculated. This upper estimate is shown to be independent of the possible uncertainties on the hydraulic conductivity of the system. Corresponding upper limits on the nutrient fluxes of PO and NO are evaluated. A comparison with 3Ϫ Ϫ 4 3 current import and export data for the surface ecosystem indicates that in every case, nutrient fluxes hardly reach a few percent of the exports. Therefore, the deep oceanic nutrient pool brought to the surface by thermally driven interstitial circulation is only a minor nutrient source for the reef ecosystem.The composition and circulation of interstitial waters in the porous carbonate framework of coral reefs are central to our understanding of reef development and geological evolution. Atoll reef hydrology is also of prime importance to the management of fresh groundwater for communities on these islands.Hydrogeological studies of coral reefs were spurred by the American and French underground nuclear tests in Pacific atolls (Buddemeier and Holladay 1977;Wheatcraft and Buddemeier 1981). Cores drilled in atolls revealed that coral reefs are complex porous media with both horizontal and vertical heterogeneity over a wide range of spatial scales, including cracks, karsts, and strata of composite materials with different permeabilities. The occurrence of large dolomitized horizons also led geologists to suggest that reefs should be regarded as hydrologically active environments, as dolomitization requires the passage of large volumes of seawater in order to supply the necessary magnesium (Buddemeier and Oberdorfer 1986 AcknowledgmentsWe thank the Programme National pour les Récifs Coralliens (PNRCO) for financial support and the Laboratoire de Transferts Thermiques et Mécanique des Fluides (CEA) for providing us with computing facilities. We thank Jean-Pierre Gattuso for his fruitful remarks about the nutrient budget and Bernard Boudreau and two other anonymous reviewers for their help...

Tidal flushing and wind driven circulation of Ahe atoll lagoon (Tuamotu Archipelago, French Polynesia) from in situ observations and numerical modelling

“…If we consider the small completely closed atolls (no pass, neither any functional reef flats, such as Taiaro and Clipperton, in Leclerc et al (1999) and Charpy et al (2010)), the small atolls with no passes but with reef flats (Manihiki and Rakahanga studied by Callaghan et al (2006)), the small open but deep and compartmentalized atoll (Palmyra, Gardner et al, 2011), the large open atolls with passes and reef flats (Enewetak, Bikini, Majuro, Mururoa studied by authors cited above), the case study represented by Ahe atoll (medium size, deep atoll, semi-closed with passes and few reef flats) offers a different geomorphological setting, and not surprisingly a somewhat different scheme and set of conclusions compared to all the previous studies.…”

Controlling factors of volumetrically important marine carbonate cementation in deep slope settings

“…Here, present‐day sea surface temperature (SST) annually ranges between 24 and 27 °C. The boundary between the upper mixed and thermoclinal water masses is presently at depths of ca 100 m (Leclerc et al. , 1999).…”

“…Here, present-day sea surface temperature (SST) annually ranges between 24 and 27°C. The boundary between the upper mixed and thermoclinal water masses is presently at depths of ca 100 m (Leclerc et al, 1999). The sea water temperature at 300 m water depth is still as high as 18°C (Andreasen & Ravelo, 1997).…”

Three-dimensional flow in the Florida platform: Theoretical analysis of Kohout convection at its type locality

“…This type of convection has been invoked as a conceptual model for dolomitization in numerous modern carbonate platforms, including the Bahamas (Whitaker et al, 1994;Caspard et al, 2004), Enewetak Atoll (Saller, 1984), Mururoa (Buigues, 1997), French Polynesia (Rougerie et al, 1997), and other atolls (Leclerc et al, 1999). Numerical simulation of two-dimensional (2-D) fl ow in generalized carbonate platforms has suggested that geothermal heat fl ow can cause convective circulation to depths of several kilometers (e.g., Kohout et al, 1977;Sanford et al, 1998;Wilson et al, 2001).…”