Spatially heterogeneous gas hydrate dissociation on the Chile Margin: Results from Expedition 379T

Abstract:

Seismic and pore water features (e.g., gas plume, freshening) were previously interpreted to reflect gas hydrate instability on the Chile Margin (e.g., ODP Leg 202). Recent coring operations (JOIDES Resolution Exp. 379T, JR100) revisited the hydrate-rich Chile Margin and confirmed widespread, but not ubiquitous, dilution of pore water chlorinity ([Cl]) in new 120 m sediment cores. Here, we report pore water profiles of shipboard [Cl] and high-resolution oxygen isotopes (δ¹⁸O) from two sites taken within a few kilometers of ODP 1233 (41 °S, 838 m)—J1005 (41 °S, 807 m) and J1006 (41 °S, 824 m)—as a case study to test the hypothesis that [Cl] dilution reflects in situ gas hydrate dissociation.

At J1006, [Cl] decreases linearly from 550 mM at the sediment surface to ~350 mM by 120 mbsf. A subtle δ¹⁸O peak of 0.4 ‰ in the upper 50 mbsf of J1006 reflects a change in the isotopic composition of seawater during the last glacial period. Below this depth, however, δ¹⁸O continues to gradually increase to values ~1.0 ‰ towards the base of the profile. Despite its close proximity and similar depositional setting, [Cl] and δ¹⁸O at J1005 reflect the diffusive history of seawater, with glacial peaks of 570 mM and 0.4 ‰, respectively, at ~35-40 mbsf. Below this, both parameters decrease to 540 mM and -0.2 ‰, respectively, by 120 mbsf. At ODP 1233, [Cl] mirrors J1005 until 60 mbsf, at which point [Cl] reduces towards J1006 values and reaches ~475 mM at 120 mbsf. We suggest that observed differences in [Cl] and δ¹⁸O are driven by in situ gas hydrate dissociation at J1006.

By using J1005 as a “background” endmember, we constrain the influence of hydrate dissociation on J1006 pore water profiles. This assumption is supported by the observation that a gas hydrate recovered from J1006 at 80 mbsf with a δ¹⁸O = 3.02 ‰ requires formation pore water δ¹⁸O = -0.02 ‰, which is detected in J1005 at 80 mbsf. Using the hydrate δ¹⁸O and a range of hydrate saturation estimates, the smooth decrease in [Cl] and increase in δ¹⁸O at J1006 can be reconciled by in-situ dissociation of gas hydrates. However, the difference between ODP 1233, J1005, and J1006 profiles suggest that dissociation may be localized on the Chile Margin with a diminishing influence on pore water composition away from the site of dissociation.