Wednesday, February 07, 2007

 

'Good vibrations' from deep-sea smokers may keep fish out of hot water (Audio)

The long-held assumption that black smokers are silent is wrong, according to recently published research led by Timothy Crone, a University of Washington doctoral student in oceanography. It's prompting scientists to wonder: Could the sound and vibrations of black smokers be the reason fish in total darkness avoid being poached by waters as hot as 750 F? And might similar sounds guide them to the smorgasbord of tube worms (Pogonophora), mussels, shrimp, snails and other fauna at vents with more temperate waters?

The research was reported online during the inaugural month of the Public Library of Sciences' interactive journal, PLoS ONE. Aimed at involving more people in science, published results are available without a subscription and can include a wealth of audio, video and other materials.

Deep-sea Black Smokers Hydrothermal Vents Audio (Evolution Research: John Latter / Jorolat)

Image Caption: The recording device that captured the sounds of black smoker venting sits here between waters that are 660 F, hot enough to poach unsuspecting fish, and cooler places lush with tube worms. It is speculated that the sound generated by hydrothermal vents may help fish navigate around such structures.

Hydrothermal vents, discovered in the 1970s, are found along volcanically active ridges where seawater seeps into the seafloor, picks up heat and minerals and then vents back into the ocean depths. The hottest and most vigorous of the vents are black smokers, so called because when the fluids they emit hit the icy cold seawater, minerals in the fluids precipitate out and it looks just like dark, billowing smoke.

Because of a paper published 15 years ago [1], it had been thought the vents were probably playing only the sounds of silence. Still a number of scientists suspected that the vents could be generating sounds, given the obvious turbulence of the flows, Crone says.

It was decided that new recordings should be attempted because Crone and other oceanographers are looking for new ways to measure vent flows, which are a source of heat and minerals in the world's oceans that scientists would like to understand better. Commonly used instruments to measure flow are often short lived when inserted in the superheated, corrosive black-smoker fluids.

How much simpler if the vents were generating some kind of sound that could be recorded and correlated to flows, Crone says.

With funding from two organizations that help take fields of research and instrumentation in new directions, the UW Royalty Research Fund and the W.M. Keck Foundation, a deep-sea digital acoustic recording system was deployed in the Main Endeavour vent field. The field is on the seafloor about 300 miles west of Seattle on the Juan de Fuca Ridge. Crone recorded 45 hours of sound at the vent scientists call "Sully" and 136 hours at the vent called "Puffer."

That's the sound of Sully you're hearing as the video runs. Crone likens the sound to the rumbling of an avalanche or a forest fire.

How loud would it be if you were sitting a foot away? (That's something you couldn't actually do because the pressure where most black smokers are found is so intense that you'd implode.) The sound level would be somewhere between conversational speech and a hairdryer, Crone says.

Four possible mechanisms might be causing - or contributing to - the noise, the researchers say. For example, the flow could be pulsating or its volume could be changing as its waters cool. Dissimilar fluids in the flow could generate noise where they mix. Or the fluids rushing through the nooks and crannies of the smoker vent itself could be creating noise.

The sounds also appear to change as flows change in reaction to such things as the Earth's tides, the authors say.

Co-authors on the paper are William Wilcock, UW professor of oceanography; Andrew Barclay, former post-doctoral researcher at the UW, now an associate research professor at Lamont-Doherty Earth Observatory, Columbia University; and Jeffrey Parsons, formerly a UW faculty member, now with Herrera Environmental Consultants.

Buried within the broad range of sounds that produce the rumbling, Crone's analysis revealed the surprise that the vents also produce resonant tones. There could be a number of things generating such tones. For example, flows along the cavities and bumps inside the vent structures may cause tones in the same way jug band members produce sound by blowing across the mouths of their jugs, causing the air inside the jug to resonate and produce a deep tone.

Both Sully and Puffer produce resonant tones at several different frequencies that we probably can't discern with all the other noise generated by the vents. But Crone has pulled examples of tones out of the racket.

"With these resonant tones, each vent within the vent field is likely to have its own unique acoustic signature," Crone says.

If so, and if fish are actually using vent sounds to navigate, then the distinctive tones might be how fish find their way back to cooler vents where the eats have been particularly good.

In that case, being on top of old smoky - all covered in sounds - would be a good thing indeed.

Listen to a deep-sea smoker

Source (Adapted): University of Washington February 5 2007

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Based on the PLoS ONE paper "The Sound Generated by Mid-Ocean Ridge Black Smoker Hydrothermal Vents"

Citation:

Crone TJ, Wilcock WS, Barclay AH, Parsons JD (2006) The Sound Generated by Mid-Ocean Ridge Black Smoker Hydrothermal Vents. PLoS ONE 1(1): e133. doi:10.1371/journal.pone.0000133

Excerpts:

Abstract

Hydrothermal flow through seafloor black smoker vents is typically turbulent and vigorous, with speeds often exceeding 1 m/s. Although theory predicts that these flows will generate sound, the prevailing view has been that black smokers are essentially silent. Here we present the first unambiguous field recordings showing that these vents radiate significant acoustic energy. The sounds contain a broadband component and narrowband tones which are indicative of resonance. The amplitude of the broadband component shows tidal modulation which is indicative of discharge rate variations related to the mechanics of tidal loading. Vent sounds will provide researchers with new ways to study flow through sulfide structures, and may provide some local organisms with behavioral or navigational cues.

Introduction

Mid-Ocean ridge hydrothermal systems support rich communities of chemosynthetic organisms and are conduits for large heat and chemical exchanges between young oceanic lithosphere and the ocean. On a global scale the time-averaged hydrothermal heat flux and many chemical fluxes are well constrained [1]. On local scales these fluxes are temporally and spatially variable [2]–[4], but the variations are poorly quantified because there are few time-series measurements of fluid flow with which to integrate temperature and chemical observations. While time-series measurements of flow have been obtained in low-temperature vents [4], [5], and point measurements have been obtained in black smokers [6], [7], no time-series measurements of black smoker flow exist. High temperatures, low pH, and mineral precipitation limit the long-term effectiveness of invasive flow measurement techniques commonly employed in these environments.

The development of a non-invasive flow measurement technique could solve this problem and enable the collection of extended time-series flow data. One proposed method [8] would use passive acoustic measurements and capitalize on the potential for fluid flow to produce sound [9]. Passive acoustic measurements near black smokers could provide flow rate information if flow-related sounds can be detected, and if a relationship between flow rate and acoustics can be established. Point measurements of flow using an invasive measurement technique [6] could be used to convert time-series measurements of acoustically-determined relative flow rates into absolute measurements.

While previous studies have noted an apparent increase in ambient noise within several hundred meters of two hydrothermal vent sites [10], [11], another study found no conclusive evidence that hydrothermal vents generate sound [8]. In this report we present the first detailed description of the localized sound generation by two mid-ocean ridge black smoker hydrothermal vents. We discuss the likely sound source mechanisms that operate to produce both broadband and narrowband signals. We then discuss the tidal variations observed in one record, which we argue is related to tidal forces affecting fluid circulation within the hydrothermal system. We conclude with speculation on the biological implications of black smoker sound production.

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[1] See "The sound field near hydrothermal vents on Axial Seamount, Juan de Fuca Ridge"

Citation: Little, S. A., K. D. Stolzenbach, and G. M. Purdy (1990), The sound field near hydrothermal vents on Axial Seamount, Juan de Fuca Ridge, J. Geophys. Res., 95(B8), 12,927–12,945.

High-quality acoustic noise measurements were obtained by two hydrophones located 3 m and 40 m from an active hydrothermal vent on Axial Seamount, Juan de Fuca Ridge, in an effort to determine the feasibility of monitoring hydrothermal vent activity through flow noise generation. Most of the measured noise field could be attributed to ambient ocean noise sources of microseisms, distant shipping, and weather, punctuated by local ships and biological sources. Long-period, low-velocity, water/rock interface waves were detected with high amplitudes which rapidly decayed with distance from the seafloor. Detection of vent signals was hampered by unexpected spatial nonstationary due to the shadowing effects of the caldera wall. No continuous vent signals were deemed significant based on a criterion of 90% probability of detection and 5% probability of false alarm. However, a small signal near 40 Hz, with a power level of 10-4 Pa2/Hz was noticed on two records taken within 3 m of the Inferno black smoker. The frequency of this signal is consistent with predictions, and the power level suggests the occurrence of jet noise amplification due to convected density inhomogeneities.

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Related post: "Volcanoes of the deep"

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