Hydrothermal vent fields and chemosynthetic biota on the world's deepest seafloor spreading centre

Abstract

The Mid-Cayman spreading centre is an ultraslow-spreading ridge in the Caribbean Sea. Its extreme depth and geographic isolation from other mid-ocean ridges offer insights into the effects of pressure on hydrothermal venting, and the biogeography of vent fauna. Here we report the discovery of two hydrothermal vent fields on the Mid-Cayman spreading centre. The Von Damm Vent Field is located on the upper slopes of an oceanic core complex at a depth of 2,300 m. High-temperature venting in this off-axis setting suggests that the global incidence of vent fields may be underestimated. At a depth of 4,960 m on the Mid-Cayman spreading centre axis, the Beebe Vent Field emits copper-enriched fluids and a buoyant plume that rises 1,100 m, consistent with >400 °C venting from the world's deepest known hydrothermal system. At both sites, a new morphospecies of alvinocaridid shrimp dominates faunal assemblages, which exhibit similarities to those of Mid-Atlantic vents.

Figure 1. Location of the MCSC and hydrothermal vent fields.

Hydrothermal vents on the MCSC. (a) Regional location map of the MCSC. (b) Bathymetry of the MCSC, from shipboard multibeam sonar data (50-m horizontal resolution in shaded relief, against a background of GEBCO contoured bathymetry). (c) Microbathymetry of the VDVF (commemorating geochemist Karen Von Damm, 1955–2008), from Autosub6000 multibeam sonar data. (d) Microbathymetry of the BVF (named in honour of naturalist William Beebe, 1877–1962), from Autosub6000 multibeam sonar data.

Figure 2. Characteristics of the VDVF water column.

(a) Profiles of temperature and salinity above the MCSC from a CTD instrument, showing a deep isothermal water column; (b) Profiles of LSS and redox potential (Eh) signals indicating the hydrothermal plume above the VDVF (c) Autosub6000 survey of Eh anomalies at 60-m altitude above the VDVF. Eh values in these diagrams are the raw values of the platinum (Pt) electrode voltage against the silver–silver chloride (AgCl) reference electrode in a saturated potassium chloride (KCl) solution.

Figure 3. Characteristics of the BVF water column.

(a) Profiles of LSS and Eh indicating the neutrally buoyant and buoyant hydrothermal plume above the BVF. (b) Autosub6000 survey of Eh anomalies at 60-m altitude above the BVF; Eh values in these diagrams are the raw values of the Pt electrode voltage against the Ag–AgCl reference electrode in a saturated KCl solution (c) Variation of plume rise height with seafloor exit temperature in the water column of the MCSC, 1, 0.2/1.5; 2, 0.1/1.5; 3, 0.2/0.5; 4, 0.1/0.5 vent area (m⁻²)/vertical exit velocity (ms⁻¹), respectively, estimated from video observations at BVF.

Figure 4. MCSC chimneys and fauna.

(a) Peak of sulphide edifice at the VDVF, depth 2,300 m, covered by an aggregation of alvinocaridid shrimp. (b) Clear vent fluids and alvinocaridid shrimp at the VDVF. (c) High-temperature venting at the BVF, depth 4,960 m. (d) Diffuser structures ornamenting sulphide chimneys at the BVF.

Figure 5. MCSC faunal observations.

(a) Aggregation of alvinocaridid shrimp on an active chimney at the BVF. (b) Anemones and microbial mats at the BVF. (c) Aggregation of dead mussel shells on the Mount Dent OCC. (d) Empty tubes resembling those of siboglinid polychaetes on the Mount Dent OCC.