Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

Earth and Planetary Science Letters, 81 (1986/87) 245-252 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 245 [21 Real-tim...

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Earth and Planetary Science Letters, 81 (1986/87) 245-252 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

245

[21

Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge Terry A. Nelsen 1, Gary P. K l i n k h a m m e r 2, John H. Trefry 3 and Robert P. Trocine 3 I NOAA-AOML-OCD, 4301 Rickenbacker Causeway Miami, FL 33149 (U.S.A.) 2 Department of Earth, Atmospheric and Planetary Scienees, MIT, Cambridge, MA 02139 (U.S.A.) 3 Department of Oceanography and Ocean Engineering, FIT, Melbourne, FL 32901 (U.S.A.) Received May 19, i986; revised version received October 10, 1986 As part of the 1984-1985 N O A A VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved M n helped lead to the discovery of high-temperature black smokers. The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved M n (r = 0.88; n = 78) throughout a large concentration range (0.2-31.0 nmol/kg). These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.

1. Introduction Although slow-spreading oceanic ridges comprise more than half of the 55,000 km long global ridge system, most recent studies of ridge crest hydrothermal processes have tended to focus on fast-spreading centers. Thus, our knowledge of the effects of hydrothermal activity on the oceans at slow spreading ridges remains incomplete. Our program was designed to determine the existence and nature of hydrothermal activity along representative portions of the Mid-Atlantic Ridge (MAR). Site selection for our stations was based on previously collected data indicating the locations of magnetic anomalies where basalt was believed to have been altered by past or present hydrothermal activity [1,2]. Our investigations were conducted on the N O A A ship "Researcher" during August, 1984 along the Mid-Atlantic Ridge between 26 ° and l l ° N and again in July, 1985 at the TAG hydrothermal field (Fig. 1). Results from the August, 1984 cruise confirmed 0012-821X/87/$03.50

© 1987 Elsevier Science Publishers B.V.

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246 active hydrothermal venting at at least five sites along a 1700 km segment of the MAR, based on elevated concentrations of dissolved Mn [3] as well as of particulate Fe and Cu [4]. During the 1984 cruise, a strong relationship became apparent between shipboard measurements of dissolved Mn and profiles of turbidity measured by a nephelometer. Sampling strategies were thereafter established to enable quantitative comparison of this relationship at all subsequent stations. Nephelometers have proven valuable in oceanographic research for determining the relative distribution of total suspended matter (TSM) in the water column [5,6]. Traditional uses have included mapping the distribution of TSM in ocean basins [5] as well as inferring the influence of oceanic currents on particle distributions [6]. Previous use of optical methods at hydrothermal sites have shown qualitative agreement between transmissometer response and particulate Mn [7]. This paper presents the qualitative and quantitative aspects of the response of an optical system (nephelometer) to TSM, particulate Fe and dissolved Mn in the water column and specifically in dispersed hydrothermal plumes. 2. Methods Water column measurements were made with a Neil Brown Mark II CTD system equipped with rosette-mounted 10- and 30-liter Go-Flo Niskin bottles and a SeaMarTec model 6000AR autoranging nephelometer. The nephelometer has a 90 ° scattering angle with illumination provided by a xenon bulb with flash duration of 5 microseconds and a repetition rate of 5 flashes/second. In addition to real-time graphical and digital display of CTD and nephelometer output on a shipboard CRT, data from both were tape recorded for later analysis. The digitized voltage output of the n e p h e l o m e t e r was expressed as a r b i t r a r y nephelometer units (" nephels") on a relative 0-100 scale. During deployment, CTD and nephelometer data was acquired and recorded at a rate of 15 scans/minute. The remote firing of a rosettemounted sampling bottle required about 15 seconds thus yielding approximately 225 data points during bottle firing. Even though bottles were closed when the rosette system was stationary,

during the 1984 cruise the exact data collected during the 15 second bottle firing could not be distinguished from data collected at that sample depth before and after that firing. Improvements in the CTD software package for the 1985 cruise allowed identification of the data collected during the firing of rosette-mounted sampling bottles. Subsequent data listing allowed direct comparison of digital nephel values, recorded during the firing of each water sample bottle, with the gravimetrically measured TSM, particulate Fe and dissolved Mn concentrations from those bottles. Once on deck, water samples were immediately drawn and processed for dissolved Mn measurements (1 liter) and for filtration of particles (30 liters). Analysis for total reactive Mn (TRM) followed the method of Klinkhammer et al. [8] which has a detection limit of 0.05 n m o l / k g and a precision of + 10%. The TRM concentrations represent dissolved Mn values and are expressed in nanomoles of Mn per kilogram of seawater (nmol/kg). The methods for total suspended matter (TSM) and particulate metals are given by Trefry et al. [4]. The TSM values are expressed in micrograms of suspended particulate matter per liter of seawater (/1g/l). Samples for scanning electron microscope-energy dispersive analysis (SEM-EDA) of characteristic secondary X-rays were pressure filtered directly from the Go-Flo bottles through 0.4/~m pore-size, 25 mm diameter Nuclepore filters held in Teflon-lined stainless steel filter holders. All filters were loaded and unloaded from their holders in a class-100 laminar-flow hood. Each filtered sample was washed with approximately 30 ml of pH-adjusted (pH - 8) distilled, deionized, particle free ( - 0.2 /zm filtered) water to remove sea salts without dissolving any particulate Mn. Thirty filtered samples were collected on the 1984 cruise and 108 on the 1985 cruise. Qualitative visual analysis and semiquantitative elemental analysis of the filtered SPM was accomplished with an ISI model DS-130 SEM equipped with a T r a c o r N o r t h e r n model TN-2000 E D A / d a t a reduction system. The latter allowed subtraction of both background and sample coating spectra (Pd and minor contaminants) from the elemental spectra of the sample.

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3. Results

3.1. Quantitative relationships between nephelometer, TSM and TRM values To quantify the relationships between the nephelometer output and measured TSM and TRM, digital nephelometer values, recorded during the firing of sample bottles, whether o n u p - or downcasts, were averaged and plotted against TSM and TRM. Because of the limitations of identifying only the data collected during bottle firing, as noted above, quantitative relationships developed herein are based only on 1985 data. Averaged nephel values were calculated from printouts of recorded data (n - 200 data points/each plotted nephel value). Fig. 2A shows this comparison for

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3.2. Water column distributions of nephelometer and TRM values Quantifying the direct relationship between nephels and TRM with statistics can only provide an indirect appreciation for the utility of this relationship. The individual cast-by-cast relationships, however, are equally instructive. Fig. 3 presents data from 2 typical casts, from a group of 24 collected at the TAG station (Fig. 1) during the 1985 cruise. The nephelometer trace in Fig. 3A shows a

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Fig. 2. Correlation statistics and least squares plots for digital nephelometer values (nephels) versus simultaneously sampled (A) total suspended matter (TSM) and (B) total reactive Mn (TRM). See text for details.

T A G (Fig. 1) showing plots of nephelometer response and measured high concentration TRM for (A) cast 1, surface to 10 m above bottom (3747 m), (B) expanded 3000-3737 m region of cast 1, (C) cast 18, surface to 10 m above bottom (3912 m) and (D) expanded 3000-3902 m region of cast 18.

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248 strong near-surface signal between the surface and 300 m due to scattering from biogenic sources with saturation by daylight causing anomalous detector response near the surface. A midwater low continues down to 3300 m with an elevated signal from 3300 to 3600 m and a return to background from 3600 m to within 10 m of the b o t t o m (3747 m). An expanded view of the lower scattering region (Fig. 3B) shows a relatively symmetrical scattering layer, approximately 300 m thick, with a m a x i m u m increase of about 40 nephel units above the midwater background value. With real-time shipboard display of these data to guide upcast sampling, depths were chosen to include, as well as to bracket, this feature. Seen in these figures are the plots of sampled depths and the T R M values from samples taken on the upcast. The strong spatial relationship between nephels and T R M is obvious and corroborates the strong statistical evidence presented in Fig. 2B. In a second example from the T A G station (Fig. 3C, D) the scattering layer thickness ( - 400 m), and magnitude ( - 7 0 nephel units above background) are greater than the prior example and the profile is substantially more complex. Nevertheless, the same strong spatial and proportional relationship exists between nephels and T R M with proportional increases in T R M relative to nephels. This relationship held for all 24 casts at the T A G area in which nephelometer and T R M data were taken (Fig. 2B). These data comprise a T R M range from mid-Atlantic background values of - 0 . 2 n m o l / k g to 31 n m o l / k g in dispersed hydrothermal plumes. It could be argued that the above relationships are valid only for the T A G site a n d / o r for highly elevated T R M values such as those found over this vent field. Data collected from other sites during the 1984 M A R survey cruise demonstrate that this relationship holds even in very dispersed plumes. The T R M from these stations has been shown to be of hydrothermal origin [3] but of significantly lower concentrations than in the T A G area. Fig. 4A shows the expanded lower portion of the n e p h e l o m e t e r / T R M data from the cast at station 4 (Fig. 1). The hydrothermal enrichment at this station did not comprise a single peak but rather a diffuse region from about 3500 m to 20 m above the b o t t o m (4489 m) where sampling terminated. The largest change is a simple step

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Fig. 4. Examples of data from the Mid-Atlantic Ridge showing qualitative correlation between plots of nephelometer response and measured low concentration TRM for (A) 3000-4469 m (20 m above bottom) region at station 4 (Fig. 1) and (B) - 2100-3059 m (20 m above bottom) region at station 9 (Fig. 1). increase just below 3500 m for nephels as well as for TRM, with a gradual decrease in both from there to the bottom. At this station the observed step in nephels, i.e., from nephel units - 2 1 . 5 to 23.5, corresponded to a T R M change of 0.80 n m o l / k g . In the second example a more complex nephelometer trace was found at station 9 (Fig. 1), about 1000 km south of station 4. At this station nephel maxima, at - 2700 and - 2850 m, bracket a relative minimum at - 2780 m (Fig. 4B). Guided by real-time shipboard readout, bracketing and sampling of these three features allowed additional evaluation of the response of the nephelometer to anticipated variations in TRM. As seen in Fig. 4B, the nephel trace has defined a subtle variation in T R M with resolution as small as 0.04 n m o l / k g (i.e., 1.70-1.66). Although at the precision limit of our shipboard T R M measurements, resolution of this magnitude, with corroborating nephelometer data, was not limited to this station but occurred on other casts as well, thus reinforcing the validity of such data for low concentration anomalies. The correlation between nephels and T R M is yet to be resolved. As noted above, sample acquisition during the cruises included recovery of TSM for metal analyses. The results of these analyses, for the 1984 cruise, show that elevated levels of particulate Fe were found in hydrothermal plume samples [4] located by nephelometry which were also shown to have elevated T R M values [3]. Specifically, the particulate Fe values

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increase from an average of 1.9 _+ 0.9%, in ambient (non-plume) deep waters, to an average of 6.0% and a maximum of 15.8% of the TSM, in T R M enriched water. For all T S M / T R M co-samples (n = 34), a strong correlation (r = 0.88) exists between particulate Fe and TRM. As such, this

relationship between particulate Fe and T R M provides the unifying link between nephel anomalies and T R M anomalies in that nephelometer detection of particulate Fe is coincident with T R M due to the co-venting and co-dispersal of these two species.

Fig. 5. Scanning electron microscope images of (A) hydrothermal plume suspended particulate matter from 3324 m, cast 18, at station T A G (Fig. 3D) showing colloidal-sized material (white scale bar = 1 g m ) and (B) similar results from a lower concentration plume at station 9 from 2779 m (Fig. 4B) showing material essentially identical to that at station TAG.

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3.3. The nature of SPM from zones of anomalous nephelometer signals To evaluate the physical as well as the chemical nature of the TSM sampled at depths which coincided with anomalous nephelometer signals, filtered samples were examined using SEM-EDA analysis. Fig. 5 shows the results of a SEM analysis of two typical samples collected in zones of elevated nephels and TRM. The SEM image of Fig. 5A shows the suspended particulate matter collected at station TAG, cast 18, in the zone of elevated nephels and T R M at 3324 m (Fig. 3D). As can be seen from this image, a considerable amount of very finely divided material blankets the filter, with all particles in the submicron range. An EDA analysis of this material reveals only a strong iron signal (assumed to be present as an oxide). Although not directly measured for this sample, TSM was estimated to be 45 /~g/1 based on the relationship in Fig. 2A. The second example is from a lower nephel, T R M and TSM (17 /~g/1 measured) value sample (Fig. 4B). It is clear from Fig. 5B that the vast majority of material, as at T A G (Fig. 5A), is very finely divided particles in the submicron range. As before, EDA detected only iron for this material. These two samples exemplify the nature of all particles analyzed from our stations near hydrothermally active sites ov the Mid-Atlantic Ridge (Fig. 1) and corroborate earlier studies on particle composition [4]. The nature of this material is in sharp contrast to coarser, aggregated, organic-rich detrital material normally found in this environment [8]. The finely divided Fe-rich material observed on any of our filters was present in p r o p o r t i o n to the n e p h e l / T R M anomaly. When no anomaly was present, as in waters exhibiting n e p h e l / T R M background values, the Fe-rich matter was not observed. However, for casts which encountered hydrothermal plumes, the Fe-rich particulate matter quantitatively increased and decreased, as clearly shown by the relative n e p h e l / T R M changes shown in the above examples (i.e., Figs. 3D and 5A versus 4B and 5B). 4. D i s c u s s i o n

As seen in the statistical (Fig. 2) and graphical (Figs. 3 and 4) data presented above, the nephelometer clearly provides in-situ detection of

dispersed hydrothermal plumes and associated TRM. The utility o f the detection of these plumes with nephelometry is several fold. First, as seen in Figs. 3 and 4, in water depths of up to 4 km, the hydrothermal signal, at our study sites, is rarely more than 400 m thick thus occupying at most 10% of the water column. At some stations on the MAR, not shown as examples herein, the plume was only - 100 m thick resulting in a hydrothermal signal in < 3% of the water column and as such easily missed by unguided discrete sampling methods. Moreover, refinement of plume depth, thickness, structure and magnitude as provided by nephelometry can supply critical information as to plume geometry, extent and source direction. The latter proved valid during the 1985 cruise when the nephelometer provided data critical in closing range on, and subsequently sampling from, hydrothermal black smoker plumes in a sampling area of approximately 36 km 2 [9]. From the T R M data set collected simultaneously with the nephelometer data in this study area, Klinkhammer et al. [10] have demonstrated, from chemical arguments, that the scale length of the dispersion (the distance at which the T R M anomaly would be diluted to background levels) is about 20 km for the sources in this hydrothermal field. As such, the scale length of the dispersion for nephelometer observations should be of similar magnitude base on the limits of detection for this method described above (Fig. 4). It must be kept in mind however that these distance estimates are for the major trajectory path of the plume. When sampling off of this path, even within a few kilometers of the source, signals can drop off to background levels, as was experienced in 1985 at the T A G hydrothermal field [10]. Finally, resolution of hydrothermal plume structure, such as seen in Fig. 3D, can give insights into plume dynamics not seen from, or resolved in this detail by, discrete sampling methods. For example, initial observation of the nephel plume, shown in Fig. 3D, suggested that hydrothermal particulate matter might be accumulating on oceanic density interfaces in a manner similar to non-hydrothermal TSM observed elsewhere [11]. However, scrutiny of the CTD data collected simultaneously with the nephelometer data for this depth interval showed isopycnal (e.g., sigma-t = 27.867 + 0.001) conditions. Although no abrupt

251 t h e r m a l discontinuities were associated with the p l u m e b o u n d a r i e s in this s t u d y area, h y d r o t h e r m a l h e a t i n g a s s o c i a t e d with these p l u m e s was evident as b r o a d a n o m a l i e s of p o t e n t i a l .temperature along the p l u m e path. A discussion of thermal signals associated with the M n a n d nephel anomalies, for the 1985 cruise, in the T A G area, is p r e s e n t e d elsewhere [10]. W i t h o u t a d d i t i o n a l data, however, o n e can o n l y speculate that the p l u m e structure seen in Fig. 3D m a y reflect either p l u m e turbulence, the i n t e r l a y e r i n g of several plumes, or both. If the f o r m e r is true, then high resolution n e p h e l o m e t e r d a t a m a y be of interest to workers modeling-plume dynamics.

5. Summary and conclusions D a t a from two cruises to the M i d - A t l a n t i c R i d g e have shown that w a t e r c o l u m n n e p h e l o m e try can p r o v i d e reliable, real-time in-situ d e t e c t i o n of dispersed h y d r o t h e r m a l plumes. O u r observations have d e m o n s t r a t e d that: (1) A strong positive relationship ( r = 0.88) exists b e t w e e n n e p h e l o m e t e r values (nephels) a n d a t r a d i t i o n a l tracer of oceanic h y d r o t h e r m a l activity, dissolved M n ( T R M ) . (2) This r e l a t i o n s h i p is valid for T R M concentration ranges from background (-0.2 n m o l / k g ) to at least 31 n m o l / k g . (3) T h e seemingly a n o m a l o u s correlation between an optical signal a n d a dissolved chemical species was shown to be due to p a r t i c u l a t e F e c o - v e n t e d a n d c o - d i s p e r s e d with the T R M . F r o m these o b s e r v a t i o n s we d e m o n s t r a t e d that the use of an i n - s i t u o p t i c a l i n s t r u m e n t s ( n e p h e l o m e t e r ) with real-time s h i p b o a r d d a t a disp l a y can p r o v i d e researchers with: (1) The ability to p r e d i c t in situ c o n c e n t r a t i o n s of T S M a n d T R M . (2) A real-time s a m p l i n g guide to o p t i m i z e s a m p l i n g strategy. (3) The ability to define the detail structure of the p l u m e n o t feasible with discrete s a m p l i n g methods. (4) The g u i d a n c e to define the g e o m e t r y a n d lateral extent of h y d r o t h e r m a l plumes. In s u m m a r y , the m e t h o d s d e s c r i b e d herein can p r o v i d e the researcher with a r a p i d a n d accurate survey tool p o t e n t i a l l y c a p a b l e of aiding in the locating of venting sources as well as p r o v i d i n g

d e t a i l e d i n f o r m a t i o n on internal p l u m e structure of p o t e n t i a l value to p l u m e m o d e l i n g efforts.

Acknowledgements The V E N T S p r o g r a m of N O A A p r o v i d e d funding for the a b o v e - m e n t i o n e d cruises as well as some of the analytical e q u i p m e n t a n d i n s t r u m e n tation. The officers a n d crew of the N O A A ship " R e s e a r c h e r " p r o v i d e d the s e a m a n s h i p critical to the success of these cruises. Peter A. R o n a p r o v i d e d Fig. 1; M a r g a r e t G o r e a u p r o v i d e d expert assistance in S E M - E D A work; a n d Beth W a r d , D e n n i s Sweeney, K i m K e i t h a n d M i k e M i n t o n p r o v i d e d v a l u a b l e assistance in d a t a acquisition a n d reduction. A d d i t i o n a l thanks are due to H a r r y Elderfield for stimulating conversations a n d to M e r v i n G r e a v e s for a b r o a d s p e c t r u m of assistance. The a u t h o r s wish to t h a n k Pierre E. Biscaye a n d an a n o n y m o u s reviewer for constructive criticism which h e l p e d i m p r o v e this manuscript.

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252 T. Nelsen, Manganese geochemistry near high temperature vents in the Mid-Atlantic Ridge rift valley, Earth Planet. Sci. Lett. 80, 230-240, 1986. 11 D.E. Drake, R.L. Kolpack and P.J. Fisher, Sediment transport on the Santa Barbara-Oxnard shelf, Santa Barbara

Channel, California, in: Shelf, Sediment Transport Process and Pattern, D.J.P. Swith, D.B. Duane and O.H. Pilkey, eds., pp. 307-331, Dowden, Hutchinson and Ross, Inc., 1972.