Introduction

Gas hydrate occurrence in the sediments of the outer continental margins is sustained in place for relatively long periods by high hydrostatic pressure and low ambient temperature. Most naturally occurring hydrate is composed of molecules of methane trapped in an ice cage of water molecules. Thus, the breakdown of hydrate in response reduced hydrostatic pressure or increased bottom-water temperature can potentially introduce significant quantities of this potent greenhouse gas in the water column and atmosphere, encouraging accelerated warming. At higher latitudes hydrate also occurs in association with permafrost at depths ranging from 130 to 2000 m. Here methane is held captive in the clathrate enclosure by frigid temperatures. An increase in the mean temperature of the higher latitudes, therefore, also has the potential to dissociate the hydrate and emit methane directly into the atmosphere.

Hydrate methane is, in turn, oxidized into water and carbon dioxide, the latter being the dominant contributor to the atmospheric greenhouse forcing. Ice-core records of the recent geological past from Greenland and Antarctica reveal that climatic warming occurs in parallel with rapid increase in atmospheric methane and carbon dioxide (Fig. 1). It has been proposed that catastrophic release of methane from hydrate sources into the atmosphere during periods of lowered sea level (and, thus, reduced hydrostatic pressure) may have been a contributing factor for rapid climate change in the past. It is also feared that escalating methane emissions from the hydrates and the permafrost may soon lead to further strengthening of the on-going global warming trend and may cause unpredictable and abrupt changes in future climatic patterns. Thus, environmental consequences of the release of methane from hydrate sources have become an important issue of societal relevance.

Figure 1. Record of 8180 (paleotemperature) changes in air bubbles trapped in ice from Greenland ice core (A) and Antarctic ice core (B). Methane fluctuations (C) in the Greenland core parallel those in temperature. Numbers 1-12 at the top represent Dansgaard/Oeschger cycles; YD= Younger Dryas stadial. (After Blunier, 2000, reproduced by permission).

Figure 1. Record of 8180 (paleotemperature) changes in air bubbles trapped in ice from Greenland ice core (A) and Antarctic ice core (B). Methane fluctuations (C) in the Greenland core parallel those in temperature. Numbers 1-12 at the top represent Dansgaard/Oeschger cycles; YD= Younger Dryas stadial. (After Blunier, 2000, reproduced by permission).

The prerequisites of high hydrostatic pressure (>5 bars) and low bottom-water temperature (< 7 °C) for the stability of gas hydrates imply that hydrates occur mostly on the continental slope and rise, below 530 m water depth in the low latitudes, and generally below 250 m depth in the high latitudes. Hydrated sediments may extend from these depths to as much as 1100 m sub-seafloor. Global estimates of methane sequestered in gas-hydrate reservoirs (both in the hydrate-stability zone and as free gas beneath it) vary widely. For example, the Arctic permafrost is estimated to hold anywhere between 7.6 and 1.8 x 104 Gt (Gigatons = 1015 grams) of methane carbon, while marine sediments are extrapolated to hold between 1.7 x 103 and 4.1 x 105 Gt of methane carbon globally (Kvenvolden, 1998). Recently, Buffet et al. (2000) have revised the estimate of total methane hydrate carbon to 1.5 x 104 Gt. They used numerical models to predict the vertical distribution of hydrate as a function of organic matter in the sediments and sedimentation rate.

These models were tested using empirical parameter values from the Blake Ridge. Comparisons between predicted and observed hydrate content, free gas below the stability zone and chlorinity profiles showed reasonable correspondence. Obtaining more precise global estimates of methane trapped in clathrate reservoirs remains one of the top priorities in gas-hydrate research. More meaningful estimates of the amount of gas sequestered in and below the hydrates are essential if we are to resolve the real quantitative impact of the methane emissions from these sources in our climate-change models, as well as in determining the efficacy of hydrates as a future energy resource.

A key unknown, especially for climatic implications, is the nature and fate of methane removal from the dissociated hydrate. How and what quantities of the gas escapes from the hydrate zone and what percentage is dissolved in the water column versus escaping into the atmosphere? Methane flux rates from the seafloor and the hydrated sediments into the water column remain difficult to quantify due to highly variable, transient and diffuse nature of the flux. Oxidation rates of methane in the water column are even more poorly understood.

In a steady state much of the methane diffusing from sediments is believed to be oxidized in the surficial sediment and the water column above (Cranston, 1996). One modeling study (Harvey and Huang, 1995) concluded that in the modeled future global-warming scenarios, the potential of methane from gas-hydrate sources, based on a variety of input assumptions, would seem small compared to the effects of C02 emissions, including the envisaged anthropogenic contributions. Thus, a clearer understanding of what happens to the significant quantities of gas that might be catastrophically released from the hydrates becomes imperative. How much of this methane makes it to the atmosphere to force additional greenhouse warming? In the abrupt climate-change scenarios it is assumed that much of this methane is emitted to the atmosphere.

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