Natural gas hydrates in sediments can never reach thermodynamic equilibrium. Every section of any hydrate-filled reservoir is unique and resides in a stationary balance that depends on many factors. Fluxes of hydrocarbons from below support formation of new hydrate, and inflow of water through fracture systems leads to hydrate dissociation. Mineral/fluid/hydrate interaction and geochemistry are some of the many other factors that determine local hydrate saturation in the pores. Even when using real sediments from coring it is impossible to reproduce in the laboratory a natural gas hydrate reservoir which has developed over geological time-scales. In this work we discuss the various stages of hydrate formation, with a focus on dynamic rate limiting processes which can lead to trapped pockets of gas and trapped liquid water inside hydrate. Heterogeneous hydrate nucleation on the interface between liquid water and the phase containing the hydrate former rapidly leads to mass transport limiting films of hydrate. These hydrate films can delay the onset of massive, and visible, hydrate growth by several hours. Heat transport in systems of liquid water and hydrate is orders of magnitude faster than mass transport. We demonstrate that a simple mass transport model is able to predict induction times for selective available experimental data for CO₂ hydrate formation and CH₄ hydrate formation. Another route to hydrate nucleation is towards mineral surfaces. CH₄ cannot adsorb directly but can get trapped in water structures as a secondary adsorption. H₂S has a significant dipole moment and can adsorb directly on mineral surfaces. The quadropole-moment in CO₂ also plays a significant role in adsorption on minerals. Hydrate that nucleates toward minerals cannot stick to the mineral surfaces so the role of these nucleation sites is to produce hydrate cores for further growth elsewhere in the system. Various ways to overcome these obstacles and create realistic hydrate saturation in laboratory sediment are also discussed.

Seafloor pockmarks of varying size occur over an area of 50,000 km² on the Chatham Rise, Canterbury Shelf and Inner Bounty Trough, New Zealand. The pockmarks are concentrated above the flat‐subducted Hikurangi Plateau. Echosounder data identify recurrent episodes of pockmark formation at ~100,000‐year frequency coinciding with Pleistocene glacial terminations. Here we show that there are structural conduits beneath the larger pockmarks through which fluids flowed upward toward the seafloor. Large negative Δ¹⁴C excursions are documented in marine sediments deposited next to these subseafloor conduits and pockmarks at the last glacial termination. Modern pore waters contain no methane, and there is no negative δ¹³C excursion at the glacial termination that would be indicative of methane or mantle‐derived carbon at the time the Δ¹⁴C excursion and pockmarks were produced. An ocean general circulation model equipped with isotope tracers is unable to simulate these large Δ¹⁴C excursions on the Chatham Rise by transport of hydrothermal carbon released from the East Pacific Rise as previous studies suggested. Here we attribute the Δ¹⁴C anomalies and pockmarks to release of ¹⁴C‐dead CO₂ and carbon‐rich fluids from subsurface reservoirs, the most likely being dissociated Mesozoic carbonates that subducted beneath the Rise during the Late Cretaceous. Because of the large number of pockmarks and duration of the Δ¹⁴C anomaly, the pockmarks may collectively represent an important source of ¹⁴C‐dead carbon to the ocean during glacial terminations.

Moderate elevated vertical methane (CH₄) flux is associated with sediment accretion and raised fluid expulsion at the Hikurangi subduction margin, located along the northeast coast of New Zealand. This focused CH₄ flux contributes to the cycling of inorganic and organic carbon in solid phase sediment and pore water. Along a 7 km offshore transect across the Porangahau Ridge, vertical CH₄ flux rates range from 11.4 mmol·m⁻²·a⁻¹ off the ridge to 82.6 mmol·m⁻²·a⁻¹ at the ridge base. Stable carbon isotope ratios (δ¹³C) in pore water and sediment were variable across the ridge suggesting close proximity of heterogeneous carbon sources. Methane stable carbon isotope ratios ranging from −107.9‰ to −60.5‰ and a C1:C2 of 3000 indicate a microbial, or biogenic, source. Near ridge, average δ¹³C for pore water and sediment inorganic carbon were ¹³C-depleted (−28.7‰ and −7.9‰, respectively) relative to all core subsamples (−19.9‰ and −2.4‰, respectively) suggesting localized anaerobic CH₄ oxidation and precipitation of authigenic carbonates. Through the transect there was low contribution from anaerobic oxidation of CH₄ to organic carbon pools; for all cores δ¹³C values of pore water dissolved organic carbon and sediment organic carbon averaged −24.4‰ and −22.1‰, respectively. Anaerobic oxidation of CH₄ contributed to pore water and sediment organic carbon near the ridge as evidenced by carbon isotope values as low as to −42.8‰ and −24.7‰, respectively. Carbon concentration and isotope analyses distinguished contributions from CH₄ and phytodetrital carbon sources across the ridge and show a low methane contribution to organic carbon.

Coastal methane hydrate deposits are globally abundant. There is a need to understand the deep sediment sourced methane energy contribution to shallow sediment carbon relative to terrestrial sources and phytoplankton. Shallow sediment and porewater samples were collected from Atwater Valley, Texas-Louisiana Shelf, Gulf of Mexico near a seafloor mound feature identified in geophysical surveys as an elevated bottom seismic reflection. Geochemical data revealed off-mound methane diffusion and active fluid advection on-mound. Gas composition (average methane/ethane ratio ~11,000) and isotope ratios of methane on the mound (average δ¹³C_CH4(g) = −71.2‰; D¹⁴C_CH4(g) = −961‰) indicate a deep sediment, microbial source. Depleted sediment organic carbon values on mound (δ¹³C_SOC = −25.8‰; D¹⁴C_SOC = −930‰) relative to off-mound (δ¹³C_SOC = −22.5‰; D¹⁴C_SOC = −629‰) suggest deep sourced ancient carbon is incorporated into shallow sediment organic matter. Porewater and sediment data indicate inorganic carbon fixed during anaerobic oxidation of methane is a dominant contributor to on-mound shallow sediment organic carbon cycling. A simple stable carbon isotope mass balance suggests carbon fixation of dissolved inorganic carbon (DIC) associated with anaerobic oxidation of hydrate-sourced CH₄ contributes up to 85% of shallow sediment organic carbon.

In June 2007 sediment cores were collected in Alaminos Canyon, Gulf of Mexico across a series of seismic data profiles indicating rapid transitions between the presence of methane hydrates and vertical gas flux. Vertical profiles of dissolved sulfate, chloride, calcium, magnesium, and dissolved inorganic carbon (DIC) concentrations in porewaters, headspace methane, and solid phase carbonate concentrations were measured at each core location to investigate the cycling of methane-derived carbon in shallow sediments overlying the hydrate bearing strata. When integrated with stable carbon isotope ratios of DIC, geochemical results suggest a significant fraction of the methane flux at this site is cycled into the inorganic carbon pool. The incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases represents a significant sink in local carbon cycling and plays a role in regulating the flux of methane to the overlying water column at Alaminos Canyon. Targeted, high-resolution geochemical characterization of the biogeochemical cycling of methane-derived carbon in shallow sediments overlying hydrate bearing strata like those in Alaminos Canyon is critical to quantifying methane flux and estimating methane hydrate distributions in gas hydrate bearing marine sediments.