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The Endo-Upwelling Concept : from Geothermal Convection to Reef

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The Endo-Upwelling Concept : from Geothermal Convection to Reef Coral Reefs (1993) 1219-30 0 Springer-Verlag 1993 The endo-upwelling concept: from geothermal convection to reef construction Francis Rougerie and Bruno Wauthy Département TOA, Institut Français de Recherche Scientifique pour le Développement en Coopération, Centre ORSTOM de TAHITI, B.P. 529, Tahiti, French Polynesia Accepted 1 May 1990 Abstract. Measurements of interstitial waters in deep welling” concept is defined and its role in atoll trophic wells (O-5OOm) in Mururoa and in shallow bores (0- networks is emphasized as a necessary and sufficient pro- 35 m) in Tikehau (French Polynesia) have shown high cess for reef net production and organic matter exporta- contents of dissolved nutrients and relatively low salini- tion. The endo-upwelled flow can then be viewed as a key ties, implying a deep oceanic origin. Studies of specific factor for internal diagenesis, as early ceinentation of reef thermal fields within these atolls and carbonate plat- framework and dolomitization of deep limestone. Gener- forms have led to models of deep oceanic water circula- alization of this interstitial, deep seawater circulation is tion by convection resulting from upward geothermal discussed and linked to previously described thermal con- heat flow. Cold, low-salinity, low-pH, high-CO,, nu- vection models in Florida or raised atolls. trient-rich deep oceanic water pervades the permeable atoll, where it loses density by heating related to geother- mal heat flow, subsequently rising to seep out through the outer rim. New nutrients are thus continuously pro- Introduction vided, enabling the reef-building community (algae + corals) to thrive in optimal conditions for photosynthesis A series of 75 atolls situated in the Central South Pacific and carbonate precipitation. This “geothermal endo-up- make up the Tuamotu archipelago, which is oriented Fig. 1. Drilling on the barrier reef of Tikehau Atoll (Tuamotu) (December 1988 to March 1989) by ORSTOM team led by Dr. F. Rougerie. Foreground 30-m- deep hole has been permanently fitted with a four channel polythylene hose; sampling is by peristaltic pump with 100 ml/min flow rate. Dissolved nutrients O,, pH, salinity, alkalinity are measured in the ORSTOM facility on this atoll. Backgrozind Drilling of a second hole, 3.5 cm in diameter, closer to the outer algae-coral rim of the barrier reef. These operations are performed as a tentative validation of the geothermal endo- upwelling process and are financially supported by ORSTOM (TOA Department) and’INSU/CNRS (France) Fig.2. Drilling of a 50 m hole through north Tahiti barrier reef in April 1990. This hole has been fitted with polytubes, enabling easy peristaltic pumping of interstitial water at 6 different levels Fig. 3. Atoll reef flat with the groove and spur zone. The huge productivity of the algae-coral crest is sustained by the seepage of the endo-upwelled flow, CO, and rich nutrients. Organic calcification and early cementation constitute the biological and geochemical signals of this low-energy hydrothermalism process along a southeast-northwest axis. Each atoll consists of Interstitial water properties of reefs an annular coral reef rim bearing one, or several, flat is- in the Tuamotu Archipelago lets (motus) some meters above sea level. The enclosed lagoon, often with disseminated reef patches and pin- Tikehau Atoll (15" S, 148" W) nacles, may communicate with the ocean through one or several passes. The atoll is covered on its oceanic side by Four wells, ranging in depth from 9 to 37 m, were drilled spectacular growth of reef-building corals, algae, and as- on the Holocene reef flat of Tikehau Atoll (Tuamotu) in sociated benthic populations which are constantly im- 1988 (Fig. 1). Drilling sites were close to the ocean, being pacted by waves (Fig. 3). These mechanical processes approximately 30 m from the algal ridge; elevation above transport part of the important carbonate materials pro- sea level is about 10 cm during calm weather, but during duced by the reef with a mean calcification rate of about high tides or surges, 10-50 cm of oceanic waters cover 3 kg/m2/year CaCO, and low net production (Gladfelter this platform, filling the lagoon from which excess water 1985). Thus, there is a constant exportationlsedimenta- flows out through a western pass. Polyethylene tubings tion process toward the lagoon, the finest particles reach- were inserted, each consisting of four 3-mm internal di- ing its central parts (Chevalier et al. 1969), and also along ameter tubes enclosed in a 2-cm-diameter PVC external the outer reef slope by gravitational processes. Sedimen- hose. Following insertion, coral sand was poured into the tation controls permeability: the finest sediment tends to remaining space to reduce vertical mixing of interstitial obscure reef pores, lowering permeability. Thus, the bot- water. Screening at three or four differents levels facili- tom of the lagoon may be expected to have low perme- tated sampling with peristaltic pumps powered by 12-V ability, with the exception of particular localities with batteries. The cores whose retrieval rates reached 100% thriving pinnacles and fringing reefs. In contrast, on the were sent to the University of Paris-Orsay (B. H. Purser) outer rim, wave action and oceanic flushing prevent accu- for dating and study. These cores exhibit numerous mil- mulation of fine particles. Reef plate and forereef slope limeter- to centimeter-sized vacuoles. sediments tend to be highly permeable, although lithified, Water sampling was performed with flow rates of 50- while lagoonal sediments are both fine-grained and un- 150 ml/min. Dissolved oxygen and pH were measured di- consolidated. The outer slope is largely protected by ce- rectly (under an umbrella) using specific electrodes. For ments, forming an impermeable carapace that reduces other parameters three plastic bottles of 100-ml capacity the input of surface oceanic water (Atkinson and Davies were filled for each specific depth, stored in a dark ice- 1984). box and rapidly transported to a small field laboratory in The permeability of limestone and dolomite forming the atoll village. Salinity was measured with an Autolab these atolls varies from 1 mDa to 30 mDa, while the vol- salinometer (Sydney, Australia) and nutrients with a canics of the basement have permeabilities of less than Yvon Hitachi spectrophotocolometer (Tokyo, Japan), 1 mDa. Carbonate porosity, which ranges from mini- to according the methods defined by Aminot and Chaus- megapores, averages 30%, enabling large volumes of in- sepied (1983). terstitial water to saturate the framework. Unfortu- Data from successive sampling series (December 1988 nately, relatively little research has been devoted to the to April 1989) are presented in Table 1. study of internal fluids in atolls (Oberdorfer and Budde- Excessively high content of dissolved inorganic nu- meier 1986), mainly because of logistic difficulties. trients appeared to be an important property of these in- 21 Table 1. Characteristics of interstitial waters Depth Salinity Nutrients Total (averages of values recorded wells IV in II, Alkalinity and V on the Barrier Reef, wells I and III being polluted to 10 m with fresh meteoric S 02 PO4-P NH4-N NO,-N Si0,-Si pH (Eq/m3) II water accumulating under the emerged motu). Other data from Tikehau Lagoon (m) %O 1/m3 and adjacent ocean; ri, number of data 4 35.4 1.7 0.81 0.5 3.8 5.1 7.82 2.33 6 12 35.3 1.3 0.92 0.3 3.4 6.5 7.85 2.28 12 30 35.4 1.4 0.88 0.2 5.6 8.8 7.81 2.25 12 Lagoon 0-20 36.0 5.6 0.15 0.5 0.2 0.8 8.45 2.40 10 Ocean (vicinity of the atoll) 0-50 36.0 5.4 0.05 0.1 0.1 0.7 8.42 2.43 4 500 34.8 3.6 1.o 0.1 10.0 10 8.00 2.34 4 terstitial waters: reactive silicate and inorganic nitrate within the reef mass in which the rainfall/evaporation values reached 10 times those in either lagoonal or budget can be ignored. As salinity is not altered by bio- oceanic surface waters. Dissolved inorganic phosphate in logical processes we can utilize this stable parameter as a interstitial waters is about 6 times that in lagoonal water reference against which the possible origin of this intersti- and 10 times that in the oligotrophic waters in the vast tial water can be discussed. In the open ocean around South Pacific central gyre surrounding Tuamotu Archi- atolls, as everywhere in the tropical South Pacific, salinity pelago (Wauthy 1986). The dissolved oxygen content is decreases from subsurface maxima of around 36.2%0 lower than in the open surface systems that surround the down to 600m, where Intermediate Antarctic Water reef, being somewhat above 1.O l/m3, which may be inter- reaches a minimum salinity of 34.5%0. Thus, water preted as indicating internal consumption of oxygen sampled below 400 m will have a salinity 535.5%0and within the framework, possibly by the classic mechanism relatively high nutrient content, which is the main prop- of oxidation/remineralization of detritic organic matter. erty of this deep water. As indicated in Table 1, there is However, two aspects must be considered: first, endo- close agreement between the properties of deep oceanic genic production of nutrients as important as those mea- water and near-surface interstitial waters. The question, sured in these interstitial waters would have totally ex- therefore, concerns the nature of the mechanism that hausted the original oxygen, the saturation level being would allow deep oceanic water to reach the top of the 4.5 l/m3 at 28” C; second, if nitrate was produced locally atoll.
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