Effects of gas hydrates on the chemical and physical properties of seawater

Abstract

Huge deposits of oceanic methane hydrate (MH) exist on the seafloor on continental margins. MH transforms into water and methane (CH 4) gas in bubbles when it dissociates in seawater. That CH 4 may be oxidized (via bacteria) into carbon dioxide (CO 2) before the bubbles rise to the sea surface or dissolve into seawater again. Similarly, nitrogen hydrate is expected to exist in seawater. The nitrogen coming out of nitrogen hydrate can be fixed and oxidized into nitrate. Analysis of the relationship among seawater temperature, “apparent oxygen utilization” (AOU) and the concentration of CO 2,, based on WOCE ocean observations, reveals some characteristics that are not part of a stoichiometry relation. Analysis of individual profiles implies that the dissociation of gas hydrates occurs according to temperature and pressure (depth) at hydrate phase boundary. A quantity, named as virtual oxygen utilization (VOU), is calculated to account for the depletion of dissolved oxygen in seawater due to biochemical oxidation processes. The results show a good agreement between AOU and VOU at depth greater than the level where hydrates dissociate. The VOU is less than AOU in the layer of seawater beneath the euphotic zone. That is attributed to the escape of CO 2 into the atmosphere. The oceanic MH provides a huge biochemical fuel source to generate heat internal to world oceans. This may explain the observed ocean warming at the intermediate depth during last several decades. The role of microbes in altering the chemical and physical properties of seawater and the course of climate change cannot be ignored.