Journal of Conference Abstracts

Observation and Characterization of Hydrothermal Plume Signals from 23° to 41°N on the Mid-Atlantic Ridge

Carol S. Chin College of Oceanic and Atmospheric Sciences, Oregon State University,

Corvallis, OR 97331-5503, USA

cchin@oce.orst.edu

Gary Klinkhammer College of Oceanic and Atmospheric Sciences, Oregon State University,

Corvallis, OR 97331-5503, USA

Introduction

Hydrothermal plumes are dynamic features produced by the venting of hot, chemically-rich, buoyant fluids from seafloor hydrothermal vents. As these fluids come in contact with ambient seawater, precipitates are rapidly formed and are carried up several hundred meters above the seafloor, within the hydrothermal plume. Thus, optical sensors which detect these hydrothermally derived particles can be used to prospect for sources of hydrothermal activity (Baker et al., 1985; Nelson et al., 1986/87). Furthermore, measurements of these hydrothermal plumes in the water column can be used to predict characteristics of the venting site (Coale et al., 1992; Chin et al., 1994).

Venting on the MAR

The first observations of high temperature hydrothermal vents on the Mid-Atlantic Ridge (MAR) were made in 1985 at the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08'N (Klinkhammer et al., 1986; Rona et al., 1986) and the Snake Pit (MARK, Mid-Atlantic Ridge south of Kane) site at 23°22'N (Leg 106 Scientific Party, 1986; Kong et al., 1988). Water column observations since that time have indicated that there are other active hydrothermal areas, however, the large scale of the MAR rift valley has made it difficult to pinpoint the vent sites. Until 1992, TAG and Snake Pit remained the only known sites of hydrothermal activity on the MAR. It is still commonly thought that the MAR, with its slow spreading rates (average 1.3 cm y^-1 half rate; Lattimore et al., 1974), is host to less hydrothermal activity than the faster spreading, more intensely studied ridges such as the Juan de Fuca Ridge and the East Pacific Rise (Baker and Hammond, 1992). This is based on the idea that slower spreading rates are indicative of lower magma supply rates (i.e., less magmatic activity) (Baker and Hammond, 1992). Hence, the common view holds that the MAR contributes little to the global oceanic heat and mass budgets, despite the fact that it accounts for approximately one third of the 55,000 km long global mid-ocean ridge system.

Recent Studies on the Northern MAR

Results from the deployment of the Oregon State University ZAPS (Zero-Angle Photon Spectrometer, Klinkhammer, 1994) instrument package during five cruises on the northern MAR have led to the discovery of at least three new hydrothermal venting sites, as well as seven other active segments where hydrothermal sources have yet to be pinpointed. These cruises, in chronological order, were: FAZAR (FARA Program, Aug.-Oct. 1992), KASP (Feb.-Mar. 1993), CD77 (Mar.-Apr. 1993), HEAT (Sept. 1994) and Bridget (Sept. 1994).

Hydrothermal Plume Characteristics

Because of different plume morphologies, these hydrothermal plumes are best compared by integrating the turbidity anomaly (the area given by the curve of the turbidity anomaly profile) with depth (Chin et al., 1993). In addition, because of the varying responses of our optical instruments based on particle size, the relative particle size distribution within the plume can be estimated, and these distributions compared among the different sites. Based on such analyses, the AMAR plume, detected during the FAZAR cruise, was used to predict that a much larger hydrothermal anomaly was yet to be found in the AMAR area (Chin et al., 1993). This prediction later led to the discovery of the Rainbow plume (German et al., 1996).

Apparent Venting Frequency on the Northern MAR

The observed frequency of high-temperature venting from 23° to 41°N on the MAR, a distance of approximately 1900 km, is one venting system per 150 km of ridge. This, however, is a very conservative estimate, since there are segments within this region of the MAR that have not been sampled, as well as other segments that were only sampled sparingly (one deployment of the instrument package). From these observations, we conclude that the occurrence of high-temperature hydrothermal activity on the northern MAR is more frequent than commonly thought. In fact, German et al. (1996) have recently shown evidence for 7 hydrothermal sources between 36° and 38°N, a distance of 200 km, representing a venting frequency of one site every 25 to 30 km. For the slow spreading MAR to support this frequency of venting, the ratio of on-axis to off-axis heat loss must be higher on the MAR than on faster spreading ridges. Therefore, the fluxes from these slow-spreading areas must be accounted for in the global inventory.

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