METHANE CLATHRATE OUTGASSING AND ANOXIC EXPANSION IN SOUTHEAST ASIAN DEEPS DUE TO GLOBAL WARMING
PAT WILDEOffice of Naval Research Asia Office, 7-23-7 Roppongi, Minato-ku, Tokyo 106 Japan
MARY S. QUINBY-HUNTEnergy and Environment Division, Lawrence Berkeley Laboratory; Berkeley, Ca. 94720 USA
After Environmental Monitoring and Assessment v. 44, p. 149-153 (1997) Modified for the WWW May 1997, Feb. 1998
ABSTRACTNumerous marine areas in SouthEast Asia are cold and deep enough to develop stable gas hydrates of greenhouse gases methane and carbon dioxide and of reducing agents such as hydrogen sulfide. In addition many of these deeps have low oxygen values below sill depths. Warming of such waters could:
(1) destabilize existing gas hydrates (clathrates) flashing them into gas and
(2) reduce the oxygen capacity of waters below the sill depth.
Outgassing could increase the buoyancy, producing upwelling of potentially noxious deep waters into the photic zone and even to the surface where the greeenhouse gases would be added to the atmosphere. We plotted the depth (pressure) and temperatures of Southeast Asian deep basins on a clathrate phase diagram to determine their suceptibility to outgassing and upwelling using a creditable global warming scenario. In general, most of the Indonesian basins are too cold or too deep for any pre-existing clathrates to be destabilized by perceived global warming. However, the Sulu Basin of the Philippines and the Halmahera Basin in Indonesian waters have sill depths in the pycnocline shoal enough and temperatures warm enough potentially to support outgassing and upwelling of basin waters, if water temperatures were raised. The presence of gas hydrates has not been demonstrated in these deeps. Although generally associated with high latitudes, clathrates have been identified in tropical waters off Central America. Accordingly, the proximity of the SouthEast Asian deeps to land and their low oxygen content suggest that tropical plant debris could accumulate and provide sufficient organic matter to generate methane and/or hydrogen sulfide clathrates. Local fisheries initially could be affected adversely by upwelling of anoxic or near anoxic waters into the photic zone. However, in the long term, the anoxic effects would dissipate and the nutrients brought up by the upwelling could increase primary productivity. A major adverse affect would be the introduction of methane into the atmosphere, as that gas has about 20 times the warming potential of carbon dioxide.
1. INTRODUCTIONGlobal Warming scenarios predict a warming trend of 1.5-4.5 C by 2025 to 2050 and a concomitant sea level rise of 0.3-0.5 m (IPCC, 1992). The various environmental implications of such a change, sea level rise, shift in the storm tract, etc., have been discussed at length both in the scientific literature and the popular press. However, environmental consequence of a rise in air temperature such as the increase in oceanic temperatures both at the surface and at depth have not been examined fully. These increases would (1) decrease the solubility of oxygen and other gases in sea water and (2) could produce phase changes releasing greenhouse gases into seawater from marine clathrates potentially adding additional warming gases to the atmosphere through outgassing. In general, the models of global warming predict greater changes in the high latitude regions than in the tropics . Unfortunately environmentally, in the oceans, the tropical regions may be more sensitive to temperature changes than in high latitudes with respect to fisheries and human activities that depend on oxygenated seawater. This paper examines the potential consequences of global warming in the barred basins in tropical regions off SouthEast Asia.
2. SOME OCEANOGRAPHIC CONSEQUENCES OF WARMING
2.1 GasesThe solubility of many gases in sea water is inversely related to temperature (Weiss, 1970). For example, an increase of 5 degrees Celsius from 25 to 30, for sea water of normal oceanic salinity of 35 parts per thousand at the surface, will produce a reduction in solubility of 5% for carbon dioxide, 7.7 % for methane, 8 % for oxygen and 10% for hydrogen sulfide. For a small temperature range the change in solubility at the surface can be approximated linearly. Thus the decrease in solubilities of these gases is from 1% to 2% per increase of one degree Celsius. These are maximum solubility values. In the real ocean the actual concentration of oxygen is generally lower than at maximum saturation due to biological activity and oxygen demand of organic material in seawater. Below wave base and the photosynthethic maximum, the concentration of oxygen declines, modeled by the Richards (1965) equation. This reduction is due to consumption of oxygen by organic matter with the release of nutrients with depth. This process overrides the hypothetical increased solubility of oxygen with decreasing temperature at depth. The concentration reaches a minimum near the base of the pycnocline and increases to a relatively constant value with depth, although less than saturation, as the result of vertical advection of oxygen from colder and thus more oxygenated waters below the main pycnocline.
2.2 Gas Clathrates (Gas-Water-"Ices")In the ocean, gases also may be stored as ice-like solids given the proper temperature and pressure conditions (Macleod, 1982). These materials called clathrates or gas hydrates (Brown, 1962). They are analogs of "ices" discovered as early as 1810 Davy (1811). The marine clathrates are complex mixtures of the greenhouse gases (carbon dioxide and methane) or toxic gases such as hydrogen sulfide with water in an ice-like crystal structure. They are stable at tempertures above the freezing point of water and sea water but at relatively high pressures (Fig. 1). They were first described in detail in the marine environment in high latitude sediments off Alaska (Grantz and others, 1976), where incursion of warmer water flashed the clathrates into the gas phase and "blowouts" occurred in the bottom sediments which could be identified in marine geophysical surveys. Theoretically, the various gas clathrates could be formed anywhere in the ocean given the proper source concentration and temperature and pressure conditions. Based on seismic interpretations, clathrates have been postulated for lower latitude shelf areas and even tropical areas (Stoll and others, 1971; Tucholke and others, 1977, Shipley and others, 1979; White, 1979). Kvenvolden (1988a, 1988b, 1991) has explored the implications of the existence of greenhouse gas hydrates in a global warming scenario. Basically the concern is that as in Alaska, warmer water introduced over existing clathrates in lower latitudes would destabilize them and the entrapped gas would be released into the water column and eventually into the atmosphere adding to the greenhouse warming.
2.3 Affected Marine Geologic Sites (Deep-Sea silled basins)The major concern with respect to clatharate destabilization would be for regions with clathrates deeper than 250 meters (see Fig. 1) with relatively warm temperatures. In the modern ocean with an average depth of 5000 meters and a temperature of waters at that depth on the order of 5 degrees Celsius, there is relatively little area in the oceans where existing clathrates would be destablized with a warming of up to 5 degrees. In particular, below the main thermocline, at 1000 meters, the deep water is essentially isothermal and too cold and too deep for clathrates to shift into the gas phase.under any creditable global warming scenario. However, there are certain sea floor configurations, such as silled basins in the pycnocline, that are in connection with the open ocean temperture only at the sill depth. The temperature of the water in the basins is fixed and does not decline with depth. For very deep basins, the potential temperature actually rises adiabatically. SouthEast Asian contains a number of these basins, most of them in Indonesia (Table 1). Figure 1 shows the plot of the two basins, whose sill depth (pressure) and temperature lie close to the phase boundary between hydrate and gas. In the Sulu Basin, just west of the island of Mindanao in the Phillipines, the sill depth at 400 meters is actually in the gas phase so that for the upper 400 meters of the basin clatharates would probably not form. Below an absolute depth of 800 meters clarathes would be stable. [Note: due to internal waves in the thermocline changing the temperature at a given depth and the uncertainties of the position of the phase boundaries for a clathrate with a variable composition, there probably is a depth uncertainty of from 20 to 50 meters]. With a warming of 5 degrees, clathrates on the floor of the basin from from 800 meters to about 1700 meters would become unstable. In the Halmahera Basin, southeast of the island of Halmahera in the Indonesian Spice Islands, the sill depth is 700 meters and the temperature in the basin is near the phase boundary but on the hydrate side. A warming of 5 degrees could destabilize the upper 500 meters to an absolute depth of about 1200 meters in the basin. For the other basins in Table 1, it is clear that the project global warming will not bring the basins into the gas phase region.
2.4 Secondary Effects of OutgassingAs seen from the examination of the phase diagram, the vertical zones of potential outgassing are still very deep. Accordingly, to have an impact on the atmosphere the greenhouse gases must get into the atmosphere. A consequence of outgassing is to change the local density and create positive buoyancy. As the ocean is thermally density stratified except in regions of water mass formation, any change from neutral or negative to positive buoyancy will permit that packet of water to reach the surface and exchange with the atmosphere. The actually volume and composition of gases reaching the surface would be a function of the excess buoyancy and the amount of mixing with the surrounding water during the rise and the composition of the clathrate. This question can not be addressed at this time. However. waters in the pycnocline at the sill depths providing the souce of basin waters are reduced in oxygen and enriched in trace metals and nutrients (Richards, 1965, Rhodes and Morse, 1971, Emerson and others, 1983). Accordingly, the upwelled water could have a deleterious effect on fish near the surface initially and through enrichment of the surface waters in the photic produce plankton blooms. If the volume of upwelled water is great enough a potential red-tide conditions could be produced with again deleterious affects on the fish populations as the result of oxygen demand of new organic matter. Wilde and others (1984, 1990) discussed the various factors and environmental implications of deep vertical advection into the photic zone and the surface mixed layer of the ocean. Stachowitsch ( 1984) provided a dramatic description of the effects of an anoxic event in the Adriatic. Similar consequences may be expect from a deep anoxic upwelling produced by clathrate outgassing.
3. CAUTIONS AND DISCUSSIONAlthough the physical chemistry, geology, and oceanography suggest the potential for increased anoxia and destabilization of clathrates in two of the silled basins in SouthEast Asian waters, one factor needs to be proven to fully assess the environmental consequences of warming of these waters. That is, the existence of clathrates in these basins and their composition if there. However, due to the closness of these basins to tropical rainforests and the high run-off of local streams, it is likely that all of these basins contain sufficient organic debris to generate through decay sufficient carbon dioxide, methane, and hydrogen sulfide to form clathrates. The senior author personally has cored black fetid sediments from 1000 meters in the Coral Sea off the Fly River of New Guinea that looked like swamp deposits. The actual composition of the clathrates is critical in predicting the consequences. Carbon dioxide clathrates could produce additional greenhouse gases. However, carbon dioxide is relatively beign compared to methane, which has about 20 times the greenhouse warming potential as carbon dioxide. In the near term in the water column, release of very toxic hydrogen sulfide could be a real problem particularly for fish populations in addition to anoxia. The existence of marine hydrogen sulfide clathrates has been demonstrated dramatically with the toxic release of hydrogen sulfide from a clathrate core on the deck of the deep-sea drilling ship (Francis and Olivas, 1993). Considering the depth of the sills in the pycnocline, the timing of any potential destabilization will be delayed to many years after the actual surface warming. In fact, surface warming may impede the transfer of warmer waters to depth by reducing the negative buoyancy required for water mass formation. As a result of the delay, the effects of global warming may be seen even after remedial steps have been taken. Finally, the outside limit of a 5 degree Celsius air temperature for global warming was translated to a 5 degree change in seawater in Figure 1. Our knowledge of the mechanism of water mass formation is still limited. Accordingly, considering the enormous heat capacity of seawater, one would expect that the actual warming of seawater would be less than the global mean rise.
ACKNOWLEDGEMENTSWe thank K.K. Kvenvolden, of the U.S. Geological Survey, for helpful discussions on the clathrate question. N. Bray of the Scripps Institution of Oceanography provided us with helpful references and contacts for oceanographic information for the region. Experimental work on clathrates in seawater was carried out by the second author: M. S. Quinby-Hunt and colleagues at the Lawrence Berkeley Laboratory sponsored by the Department of Energy.
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TABLE ICharacteristics of SouthEast Asian Basins _________________________________________________________________ Sill Maximum Minimum Depth Depth Temperature Salinity NAME (meters) (deg. @ depth) ppt _________________________________________________________________ Sulu basin 400 5580 10.08 1225 34.49 Mindanao trench 10500 1.56 3490 .63 Talaud trough 3130 3450 Sangihe trough 2050 3820 2.40 2550 .64 Celebes basin 1400 6220 3.58 2475 .56 Morotai basin 2340 3890 1.81 2490 .65 Ternate trough 2710 3450 1.85 2761 .67 Batjan basin 2550 4810 2.06 2970 .66 Mangole basin 2710 3510 Gorontalo basin 2700 4180 2.20 2740 .63 Makassar trough 2300 2540 3.59 2133 .58 Halmahera basin 700 2039 7.76 1839 .60 Boeroe basin 1880 5319 3.02 3240 .61 Northern Banda basin 3130 5800 3.04 2990 .62 Southern Banda basin 3130 5400 3.06 2720 .60 Weber deep 3130 7440 3.07 2990 .61 Manipa basin. 3100 4360 3.10 3185 .60 Ambalaoe basin 3130 5330 3.08 3235 .61 Aroe basin 1480 3680 3.90 2240 .65 Boetoeng trough 3130 4180 Salajar trough 1350 3370 3.86 1750 .60 Flores basin 2450 5130 3.22 2480 .61 Bali basin 1590 3.58 1488 .61 Sawoe basin 2100 3470 3.39 2360 .61 Wetar basin 2400 3460 3.16 2500 .61 Timor trench 1940 3310 2.67 2254 .71 Sunda trench 7140 1.18 4230 .71 _________________________________________________________________ After Sverdrup, Johnson and Fleming (1942, p. 738) data from van Riel (1934) Figure 1: Phase Diagram for System Hydrate-Gas-Water as a function of Temperature and Pressure (Depth). Physical-chemical data after Macleod (1982, p. 2653). Hydrographic data after van Riel (1932). See Table I, this paper for addditional hydrographic data for SouthEast Asian Basina.