Stable isotopes, 14C dating, and geochemical characteristics of carbonate nodules and sediment from an active vent field, northern Juan de Fuca Ridge, northeast Pacific

A record of early uplift events foreshadowing development of an active vent field in Middle Valley on northern Juan de Fuca Ridge is preserved in a core taken on the margin of the field by a near-surface mud fragment breccia dated 8,300 to 11,500 yr B.P. Cementation of the breccia with low-Mn, methanogenic calcite at ∼ 5°C formed a carbonate nodule layer by ∼6,800 yr B.P. Recent hydrothermal overprinting at > 6 m in the same core resulted in formation of carbonate nodules with late-stage high-Mn calcite cements, oxygen isotopic temperatures of 55–85°C, and more positive δ13C values. Reworked vent detritus suggests that surface venting began by at least 2,900 yr B.P. Localized lateral flow of hydrothermal fluid tens of cm below the seafloor is suggested by a barite-rich zone in a permeable interval bounded sharply below by relatively impervious mud clast breccia. Carbonate and barite Cementation near the seabed could eventually result in localized ponding or redirection of fluid flow in the shallow subsurface. Near the center of the active field, a barite zone originating from reaction of hydrothermal Ba with residual highly fractionated seawater sulfate (barite δ34S = +44.5‰) is found at ∼ 6 m below the seafloor.