Hydrography and circulation near the crest of the East Pacific Rise between 9° and 10°N

Abstract

1ABSTRACT. Topography has a strong effect on the physical oceanography over the flanks and crests of the global mid-ocean ridge system. Here, we present an analysis of the hydrography and circulation near the crest of the East Pacific Rise (EPR) between 9 • and 10 • N, which coincides with an integrated study site (ISS) of the RIDGE2000 program. The analysis is based primarily on survey and mooring data collected during the LADDER project, which aimed to investigate oceanographic and topographic influences on larval retention and dispersal in hydrothermal vent communities. Results indicate that the yearly averaged regional mean circulation is characterized by a westward drift of 0.5-1 cm·s −1 across the EPR axis and by north-and southward flows along the western and eastern upper ridge flanks, respectively. The westward drift is part of a basin-scale zonal flow that extends across most of the Pacific ocean near 10 • N, whereas the meridional currents near the ridge crest are a topographic effect. In spite of considerable mesoscale variability, which dominates the regional circulation and dispersal on weekly to monthly time scales, quasi-synoptic surveys carried out during the mooring deployment and recovery cruises indicate subinertial circulations that are qualitatively similar to the yearly averaged flow but associated with significantly stronger velocities. Weekly averaged mooring data indicate that the anticyclonically sheared along-flank flows are associated with core speeds as high as 10 cm·s −1 and extend ≈10 km off axis and 200 m above the ridge-crest topography. Near the northern limit of the study region, the Lamont Seamount Chain rises from the western ridge flank and restricts along-EPR flow to five narrow passages, where peak velocities in excess of 20 cm·s −1 were observed. Outside the region of the ridge-crest boundary currents the density field over the EPR near 10 • N is characterized by isopycnals dipping into the ridge flanks. Directly above the EPR axis the ridge-crest boundary currents give rise to an isopycnal dome. During times of strong westward cross-EPR flow isopycnal uplift over the eastern flank causes the cross-ridge density field below the doming isopycnals to be asymmetric, with higher densities over the eastern than over the western flank.

Sulfur, sulfides, oxides and organic matter aggregated in submarine hydrothermal plumes at 9°50′N East Pacific Rise

“…Water velocities at the same depth were recorded every 20 min by an Aanderaa RCM11 current meter on LADDER mooring NA, which was located <1 km south (Fig. 1a) (Thurnherr et al, 2011).…”

The deep distributions of helium isotopes, radiocarbon, and noble gases along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)

“…This feature, however, extends further toward the EPR than the ∆ 14 C isopleths. Part of this disparity may be an artifact of contouring/data resolution (the lateral spacing of radiocarbon stations is sparser than that of silicate and δ 3 He), but it may also be a reflection of the locally complex circulation induced by the hydrothermal buoyancy injection and topography (Stommel, 1982;Hautala and Riser, 1989;Speer, 1989;Hautala and Riser, 1993;Thurnherr et al, 2011).…”

Larval Dispersal: Vent Life in the Water Column

“…Influences on the current regime are related to the shape and structure of the ridge, which can vary substantially, depending on the magmatic supply. However, intensification of along-axis and cross-axis current velocities at the ridge crest (e.g., Thomson et al, 1990;Thurnherr et al, 2011) …”