John LUPTON

Pacific Marine Environmental Laboratory

Roland WELL

Oregon State University

Our research group is studying the circulation of the deep and intermediate waters of the Pacific Ocean. Specifically, we are using noble gas tracers, especially 3He, to map the patterns of circulation and mixing. Hydrothermal venting along the axis of the global mid-ocean ridge system and from certain volcanic seamounts produces hydrothermal plumes in the deep ocean which are enriched in 3He relative to ambient ocean water. Because these plumes have localized source regions, their distribution can be used to trace patterns of circulation and mixing. This is especially true in the Pacific Ocean, where the spreading rate of the volcanic ridges is very high, resulting in a correspondingly high rate of helium injection.

Measurements of oceanic 3He from the WOCE Hydrographic Program have added greatly to our knowledge of the distribution of hydrothermal plumes in the deep Pacific, thereby providing strong constraints on the deep circulation in certain areas. Our research group has measured deep helium on several WOCE Pacific sections, including P1 (TPS-47), P3 (TPS-24), P16, P17, and P19, as well as NOAA expeditions RITS-89 and CGC-91. We are also in the process of analyzing deep helium samples from WOCE lines P13 and P18.

Although the deep helium field in the Pacific is not yet completely characterized, we have already identified several areas where the hydrothermal activity is of sufficient strength to produce intense 3He-rich plumes which clearly define the regional circulation. Some examples are as follows:

  1. Southern EPR - The most prominent example is at ~15°S on the East Pacific Rise (EPR) where the asymmetric 3He plume indicates westward flow at 2500 m depth which is somehow focussed into a jet-like plume structure. Farther south at 30°S the 3He field indicates flow in the opposite direction in which the hydrothermal effluent is carried eastward into the Chile Basin. Both of these flows over the southern EPR which are indicated by the 3He distribution are consistent with Reid's description of the deep circulation.

  2. Northeast Pacific - In the northeast Pacific, the track of the 3He signal from the Juan de Fuca Ridge indicates transport at a depth of ~2000 m in a southwesterly direction into the central north Pacific basin. This flow is roughly consistent with the sense and direction of the broad cyclonic gyre shown in Reid's steric height map.

  3. Northern EPR - The WOCE P4 section along 10°N reported by Jenkins [1996] shows a 3He plume which appears to be carried westward from the EPR crest into the Pacific basin interior. This flow is in the opposite direction to the eastward transport shown in Reid's steric height map.

  4. Loihi Seamount - The asymmetric plume emanating from Loihi Seamount on the southeastern flank of Hawaii also seems to require a very specific transport. In this case the transport is to the east at latitude 20°N at ~1100 m depth, which also disagrees with descriptions of the flow based on dynamic height calculations [Reid and Mantyla, 1978; Talley, private comm.].

Thus the steric height description of the deep flow, which rests on the assumption of a depth of no motion, agrees with the flow deep tracer field in some cases, and disagrees in others.

Inverse modelling studies are also underway with Oregon State University scientists using the helium field in the Pacific. Specifically, A. Bennett and L. Yuan are applying an inverse calculation to the south Pacific helium plume west of the EPR at 15°S, and J. Richman and Y. Spitz are undertaking a similar calculation with the north Pacific helium field.

To Abstract List

To Pacific Workshop

To WOCE Home Page

uswoce@astra.tamu.edu ÿ