Discovery of New Methane-bearing Hyperalkaline Springs in the Serpentinized Dun Mountain Ophiolite, New Zealand

Abstract:

The production of H₂ and CH₄ following serpentinization is considered a fundamental process for the origin/sustenance of microbial life, with several implications in astrobiology and petroleum geology. In recent years an increasing number of gas seeps or springs, with dominantly abiotic CH₄, have been discovered in land-based serpentinized peridotites worldwide. We report the discovery of a new site in the Dun Mountain Ophiolite (DMO), New Zealand. This is the second gas-bearing peridotite discovered so far in NZ, after Poison Bay which was documented in the 1990s. The DMO area is characterized by active and present-day low temperature serpentinization driven by meteoric water, a common process in ophiolites. A localised area of hyperalkaline (pH >11.6) and reduced (-243.4 mV) waters contain relatively high concentrations of dissolved calcium (43.2 ppm) aiding in carbonate precipitation. Directly above these waters flux measurements by closed chamber methods reveal CH₄ emission rates exceeding 2800 mg m^-2day^-1. The δ¹³C values of CH₄ suggest the presence of a dominant abiotic gas component (e.g. -32.7 ‰ VPDB). Additionally, an unusual clear to black gel is released from less alkaline springs elsewhere in the DMO. The origin and nature of this gel-like material is currently unknown.