Dissolution rates of calcium carbonate in the deep ocean; an in-situ experiment in the North Atlantic Ocean

Earth and Planetary Science Letters, 40 (1978) 287-300 © Elsevier Scientific Publishing Company, Amsterdam - Printed in the Netherlands

287

[6]

DISSOLUTION RATES OF CALCIUM CARBONATE IN THE DEEP OCEAN; AN IN-SITU EXPERIMENT IN THE NORTH ATLANTIC OCEAN SUSUMU HONJO and JONATHAN EREZ Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543 (U.S.A.)

Received January 17, 1978 Revised version received March 9, 1978

An in-situ water circulator (ISWAC)was developed to allow accurate measurement of dissolution rates of various carbonate particles with minimal stagnation and mechanical weight loss. Three ISWACpackages were deployed at 3600, 4800 and 5518 m for 79 days in the Sargasso Sea (Northwest Atlantic). Weight losses for different particles during 79 days at 5518 m were as follows: pteropods and synthetic aragonite, 72.8%; reagent calcite, 57.5%; foraminifera, 23-36%; coccoliths, 11.3-24%, and diatoms, 12%. These weight losses are 2.5-7.5 times higher than those reported in earlier in-situ experiments. Normalization of weight losses with respect to BET specific surface area for different calcite particles yielded specific dissolution rates that differed by more than 2 orders of magnitude. Bleached biogenic particles dissolve significantly faster than non-bleached although their surface area is identical. We suggest that the BET surface area does not represent the reactive surface area available for dissolution, especially in biogenic calcite particles. Coatings, probably of organic matter, may reduce the reactive surface area and thus retard dissolution rates. The existence of a chemical lysocline in the Northwest Atlantic was confirmed. However, it seems that different kinds of particles have different lysoclines. The origin of the lysocline cannot be attributed to water flow or to the thermodynamic transition from supersaturation to undersaturation (I2 -- 1). It seems to be a kinetic phenomenon. A simple model comparing the complete dissolution time and the residence time of a particle on the sedimentwater interface suggests that coccoliths can be preserved in the sediments of the deep Northwest Atlantic below the CCD, in good agreement with SEM observations.

1. Introduction Large portions of the oceanic water column are undersaturated with respect to calcite and aragonite [ 1,2]. The manifestation of this undersaturation on the sea floor is a narrow transition zone separating CaCO3-rich sediments above from CaCO3-poor sediments below (e.g. [ 3 - 6 ] ) . This transition zone is called the calcite compensation depth (CCD) and is believed to represent a narrow depth range where rate of dissolution equals rate of supply, causing accumulation rates of CaCOa to be very low. The CCD can be represented as a sub-horizontal surface occupying a depth of 4 . 5 - 5 . 5 km in the major ocean basins [7]. Contribution No. 4076 of the Woods Hole Oceanographic Institution.

In most places the CCD does not coincide with the depth at which seawater becomes undersaturated with respect to calcite. This depth is considerably shallower than the CCD in both the Atlantic and the Pacific [1,2]. Furthermore, below 2 km the calcite saturation factor, ~2 [8], decreases close to linearly with depth [1,2] while percent carbonate and the degree of preservation of planktonic foraminifera decrease at a much faster rate [ 9 - 1 1 ]. The position of the CCD is determined by the rate at which CaCO3 is supplied to the ocean floor, dilution by noncarbonate sediments and the rate at which CaCO3 dissolves. Calcium carbonate production by the plankton (mainly coccoliths, planktonic foraminifera tests, and pteropod shells) by far exceeds the supply of calcium and carbonate by rivers [12]. Therefore, most of the CaCO3 produced must dissolve if the chemi-

288 cal composition of seawater is to remain constant [9, 12,13]. Dissolution of carbonates in the ocean also regulates the pH of seawater (at least over an oceanic mixing time scale). Thus changes in pH caused by CO2 added to the atmosphere by combustion of fossil fuels will be buffered by CaCO3 dissolution in the ocean [ 14]. Accurate dissolution rates of carbonates are therefore essential for any attempt to understand CaCOa-related phenomena in the oceans. Estimates of dissolution rates of carbonate sediments based on accumulation rates and preservation patterns of deep-sea cores [6,13,15,16], are limited in their accuracy because they are averaged over millions of years and depend on inexact age determinations and assigned sediment densities. A more direct approach for estimating dissolution rates was used by Morse and Bemer [17] in laboratory experiments using artificial seawater at various degrees of undersaturation under controlled conditions. These rates, however, cannot be directly applied to deep-sea sediments because of different pressures and temperatures which exist inthe deep ocean, the role of inhibiting agents such as phosphate ions and organic compounds and possible other unknown factors. In-situ experiments, such as those initiated by Peterson [18] and Berger [19,20] and recently continued by Milliman [21,22], appear to be the best way to measure actual rates of dissolution. In these experiments CaCO3 particles are suspended in cages or permeable containers at different depths in the water column for a few months. Dissolution rates are estimated from the weight loss during that period. Perhaps the most important result of these experiments is that at a certain depth level ( 4 - 5 km), the rate of dissolution shows a sharp increase with depth (Fig. 1). This depth is above the CCD. It coincides with the foraminiferal lysocline as defined by Berger [20] and is referred to as the "chemical" lysocline [17]. In their laboratory experiments, Morse and Berner [17], found a sharpt increase in calcite dissolution rate at critical ApH [8] values (0.14--0.16) which are roughly equivalent to ~ = 0.5. They further showed that Peterson and Berger's lysocline also coincides with g~ value of 0.5, in good agreement with their laboratory experiments. Although the shape of the curves in laboratory and field experiments is similar (Fig. 1), the actual rates of dissolution in the

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Fig. 1. After Morse and Berner [17]. Peterson [18] and Berger [19] in-situ experiments versus laboratory experiments of Morse and Berner [17]. Note the orders of magnitude difference in the absolute dissolution rates.

field are about one or two orders of magnitude lower than those measured in the laboratory [ 17,23]. This difference in rates is observed for reagent and optical calcite as well as for deep-sea calcareous sediments. Possible mechanical weight loss of small fragments through the net or cloth container in the in-situ experiments make this difference even larger. One explanation for this discrepancy is that circulation of seawater in the experimental containers was restricted. As dissolution proceeded, ~2 value within the container became significantly higher than ~2 of the am-

289 bient seawater. As a result lower dissolution rates were measured. In the following discussion this phenomenon will be referred to as "stagnation". Rates of dissolution of coccoliths which are major CaCO3 producers in the ocean, have never been measured directly because they cannot be retained in the net containers. Thus, rates of dissolution measured so far may not be accurate enough to allow realistic modeling of the CaCO3 system in the ocean. The origin of the lysocline is under debate. Morse and Berner [17] suggested that it is a kinetic phenomenon. Edmond [ 1,24] suggested that the lysoo cline observed by Peterson and Berger is due to increased flow of deep water. Broecker and Takahashi [25] suggest that the foraminiferal lysocline is thermodynamically controlled, and that it represents the transition from saturation to undersaturation (~2 = 1). Using advanced technology we have attempted to measure in-situ rates of CaCO3 dissolution in the deep ocean. The main purpose of our experiment was to obtain more accurate dissolution rates for a variety of biogenic and synthetic carbonates and hopefully to elucidate the origin of lysocline.

2. Method and material

2.1. Instrumentation and field operation The significance of the ISWAC II (In Situ Water Circulation Mark II) system (Fig. 2) is that finegrained samples can be exposed to in-situ water for several months with minimal mechanical loss and without experiencing stagnation. A sample is placed in a plastic cylinder 3.3 cm long and 2.4 cm diameter (ID). Both ends of the cylinder are protected by a Nuclepore filter with a pore size of 0.6 or 0.4 #m. Seawater in the chamber is replaced every 20 minutes by pumping in-situ water through the chamber at a rate of 0.6 cm 3 per minute, a rate that will prevent stagnation even at very high rates of dissolution. Sixteen stainless steel bellows pumps pull the water through 16 chambers simultaneously. Water is sucked through a 3-m 2 Nuclepore pre-filter of 0.41 /~m that is finely pleated to form a cylindrical strainer. Water then enters the sample chambers and exhausted from the system away from the intake hose. A mechanical counter registers the number of

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Fig. 2. Diagram to explain the function of the In-Situ Water Circulator (ISWAC).An ISWACMark II which was used in this research consists of 16 bellow pumps, 16 reaction chambers, 81 check valves and a pre-filter. An ISWACII Mark weighed 650 lb in air.

pump-strokes and this number can be converted to the volume of water pumped through the sample chambers [26]. Three ISWAC II instrument packages were deployed at depths of 3598 m, 4799 m and 5518 m along the taut line of a sediment trap mooring [27] in the Sargasso Sea at the Parflux S station (32°22.0'N; 55°00.8'W; water depth 5581 m). The samples were exposed to in-situ seawater for 79 days beginning October 20, 1976. Surface water usually contains biogenic and man-made organic matter, and it is possible that the rate of dissolution might be retarded by the formation of an organic film on the specimen surface [28]. To avoid such contamination the interior volume (including chambers, tubing, pumps and prefilter) was filled with artificial seawater, and pumping began three hours after the anchor was set in position.

290

2.2. Percent weight loss measurement The average sample weight used in this experiment was 300 mg for 3598 m, 200 mg for 4799 m and 150 mg for 5518 m, As soon as the ISWAC II's were retrieved, the sample chambers were flushed by pH 9 ammoniated distilled water (diatom samples were flushed separately with pH 4 distilled water). To ensure total removal of sea salt the chambers were rinsed until no AgC1 precipitation could be detected in the effluent. Chambers were emptied carefully into evaporating dishes and dried. Weighing was repeated three times before and after the experiment in a humidity-controlled room. The average standard deviation o f replicate weighings was -+0.12% o f the final sample weight. Few samples in the shallow and intermediate pumps showed a very small weight gain that was mainly due to incomplete removal o f sea salt. Unfortunately, duplicate experiments o f reagent calcite in the lowest and intermediate pumps failed to provide meaningful information because of mechanical rupture o f the protecting filters. Scanning electron microscope (SEM) observation showed that the calcite crystals in these chambers dissolved very little. It is suspected that the filters were blocked and ruptured upon recovery due to

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2.3. Materials One silica (opal) and twelve carbonate samples were used in this experiment, as listed below: (1) Reagent calcite: samples supplied from several manufacturers were examined under the scanning electron microscope and b y X-ray diffraction spectrometry. We chose Allied Chemicals' Code 1504, batch Y102 on the basis o f its consistent completeness o f crystalline habit (Plate 1, a). According to the manufacturer, the calcite crystals were precipitated and were not crushed. Crystals between 10 and 53 gtm were obtained by wet-sieving through a nylon sieve in pH 9, distilled water. (2) Calcite crystals: euhedral crystals 1 - 0 . 7 mm diameter were prepared by T. Takahashi, Lamont. Doherty Geological Observatory. The sample was obtained b y crushing large Iceland spar crystals. The fragments were annealed by heat treatment at 700°C for two days in CO2 atmosphere in order to remove possible strain (Plate 1, c). (3) Synthetic aragonite: a sample containing less

PLATE 1 gZ ARAGONITE .50(~1 CALCITE,O. ,70 (I) CD,LCITE,~ 1,0 [2i

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the water expansion within the chamber. Aragonite duplicates, however, agreed well with each other (Fig. 3).

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WEIGHT LOSS (%)

Fig. 3. Percent weight loss of different particles dissolved in three ISWAC systems at 3600, 4800 and 5518 m for 79 days in the western North Atlantic. Different particles have different rates of dissolution. Bleached foraminifera and coccoliths dissolve faster than the non-bleached ones.

Scanning electron micrographs of carbonate particles before and after dissolution and bottom sediment sample from the Parflux S station. The length of the scale bar is 5 #m, except for (h), (j) and (k) where thicker bars represent 3 #m. (a) Part of large calcite crystal (Island spar) before dissolution. (b) Same as (a) after 2.2% weight loss. (c) Reagent calcite before dissolution. (d) Same as (c) after 57.5% weight loss. Note the fragmentation of the crystals. (e) Synthetic aragonite before dissolution. (f) Same as (e) after 3.9% weight loss. (g) Coccoliths from laboratory cultured C. neohelis before dissolution. (h) Same as (g) after 14% weight loss. (i) Sediment sample from the Parflux S station (5550 m), showing the presence of dissolved coccoliths and diatoms. The size fraction photographed was concentrated by centrifugation and settling. (j) Coccoliths from laboratory culture E. huxleyi before dissolution. (k) Same as (j) after 24% weight loss. Note the thinning of the rays and the development of dissolution cavities on the lower plate. (1) Pteropod surface before dissolution. (m) Same as (1) after 72.8% weight loss.

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292 than 0.1% calcite by weight was prepared in the laboratory by J. Morse, Rosenstiel School of Marine and Atmospheric Science. A 10-53-/am sample was obtained by sieving (Plate 1, e). (4) Foraminifera assemblage: a sample from core Chain 115, 74PC, 5 - 8 cm, collected in the Rio Grande Rise at 2190 m was selected because of its good preservation, indicated by the existence of pteropods. The fraction between 63 and 1000/am that was used in the experiment consisted entirely of foraminifera, and contained at least 20 species. (5) Bleached foraminifera assemblage: a sample identical to the previous one was treated with 2.5% NaOC1 solution for 2 hours. (6) GlobigerinoMes sacculifer: monospecific sample was prepared by handpicking individuals larger than 250/am selected from core Atlantis II 131, 10PC, 0 - 8 cm, collected in the Vema fracture zone, 3711 m depth. (7) Globigerina bulloides: monospecific sample was prepared by handpicking individuals larger than 250/am from core Eltanin 45-74PC, 15-40 cm, collected at 47°36.1 'S and 114°26.4'E, 3744 m depth. (8) Globerina pachyderma: monospecific sample was prepared by handpicking individuals larger than 149/am from core Eltanin 47-3PC, 127-140 cm, collected at 62°23.1 ' S and 80°47.3'E, 2736 m depth. The Eltanin samples were supplied by J. Kennett and D. Cassidy, University of Rhode Island. A brief ultrasonic treatment was applied to all the foraminifera samples to remove coccoliths and clay particles that were attached to the skeletons. (9) Emilliania huxleyi: monospecific coccolith population that was cultured in our laboratory by M. Goreau from a stock that was isolated from the central Sargasso Sea in Guillard F/10 culture medium. Three-liter batch culture without air bubbling under illumination of 500 lumens at 18°C, provided optimum growth rate as well as minimum defective coccoliths. Coccospheres were cropped near the termination of their logarithmic growth stage (usually after 7 days) by continuous centrifuging. Then the coccoliths were separated from protoplasmatic material by a succession of centrifuging and gravity sieving of the crop in distilled water of pH 9. Finally, the purified coccolith samples were checked under SEM for the complete depletion of organic cell fragments (Plate 1, j).

(10) Bleached E. huxleyi: a part of the E. huxleyi monospecific sample was immersed in 2.5% NaOC1 solution for 24 hours. (11) Cruciplacolithus neohelis: was isolated from the pelagic North Atlantic by Dr. A. B6 in 1968 and continuously cultured at Woods Hole Oceanographic Institution. Culture and refining procedure was similar to the procedure for E. huxleyi except this species was cultured at 22°C (Plate 1, g). Using high-resolution scanning electron microscopy the morphology of the coccoliths of both species were compared to live specimens collected from the Sargasso Sea and to specimens from deep-sea sediment. We found no systematic difference between our laboratory-produced coccoliths and natural specimens [29]. (12) Pteropod assemblage: a sample of pteropodrich layers in core Chain 115, 80PC, 18-22 cm and 3 3 - 3 7 cm, collected on the Rio Grande Rise at a depth of 2293 m, was selected. It was wet-sieved through 831-/am screen to remove coccoliths and foraminifera, and contained at least five species of pteropods. These pteropods were more opaque than live specimens (Plate 1,1). (13) Diatoms, Coscinnoidiscus sp.: were prepared by C. Stein, Harvard University. This diatom was cultured in a 20-liter batch of silica-enriched Guillard F/2 medium. After continous centrifuging a small amount of local surface seawater was added to the concentrate and allowed to stand at room temperature for 10 days to encourage biodegradation of organic constituents. The diatoms were then bleached in 2.5% NaOC1 solution for 2 hours and cleaned by centrifuge in dilute HC1.

2.4. BET surface area measurement The sample was dried at 60°C overnight and purged in helium gas flow for an hour at 75°C. Surface areas of samples were measured by the Brunauer, Emmett and Teller (BET) method. A slope (S) and intercept (1) were obtained from three sets of plots of P/Po vs. (P~/P - 1) -1 , where P is the partial pressure and Po is the saturated pressure of the adsorbate (N2). A Quantasorb adsorption meter was used for the measurement. The surface area (St) of the sample was computed as follows: St = Xn~Acs/Ma where Xm = (S +/)-1, N is Avogadro's number (6.023 x 1023),Ma is the mo-

293 lecular weight o f adsorbate, and A cs is the cross section o f the adsorbate (m2). The specific surface area was then obtained by dividing S t by the sample weight (g). The Sterling FT-G carbon black surface area standard was used for calibration. The following is a comparison with other published data:

Morse and de Kanel [30] (Kr BET) Sjtiberg [311 (N2BET) This paper

Reagent calcite (m2/g)

Aragonite (m2/g)

0.554 0.551 0.37 0.358 +0.004 (lo)

1.40 1.39 .1.529 -+ 0.009 (lo)

These differences are probably caused by differences in grain size o f the different production batches of these materials.

3. Results 3.1. Measured dissolution rates

No weight loss was detected for the calcite samples at 3600 m and 4800 m. Aragonite samples showed significant dissolution at those depths (Table 1, Fig. 3). All samples deployed at 5518 m were significantly dissolved (Table 2). The percent weight loss varied widely among samples. The pteropod and synthetic aragonite samples dissolved most (72.8% and 72.2% respectively). Reagent calcite lost 57.5%; the foraminiferal assemblage lost 30.4%; individual species of foraminifera ranged from 23 to 36%; and cultured coccoliths lost only 11.3-14.5%. The bleached E. h u x l e y i coccoliths dissolved twice as much as the unbleached sample. The difference in weight loss between the bleached and unbleached foraminiferal assemblages was much smaller than for coccoliths (30% and 36% respectively), probably because the foraminifera were bleached for a much shorter period. The depth prof'de (Fig. 3) is based on three points and thus is not intended to represent an accurate dissolution profde. Nevertheless, the CaCO3 lysocline is evident in this North Atlantic profde. Our observations confirm earlier reports o f the existence o f chemical lysocline in the ocean [ 1 8 - 2 1 ]. The reagent calcite tysocline is sharper and shallower than the foram-

TABLE 1 Weight loss (%) of samples during 79 days at 3598 m, 4799 m, and 5518 m depth Depth of ISWAC II Flow volume (1)

3598 m 76.1

1. Reagent calcite 2. Large calcite crystals 3. Synthetic aragonite 4. Foraminifera assemblage 5. Bleached foraminifera assemblage

0 0 3.9 0

6. G. sacculifera 7. G. bulloides 8. G. pachyderma 9. E. huxleyi 10. Bleached E. huxleyi 11. C neohelis

5518 m 88.2

0 19 0

57.5 2.2 72.2 * 30.4

0

0

36.5

0 0 0

0 0 0

36.3 23,0 31.1

0 0

0 0

11.3

0 2.4 6.2

12. Pteropod assemblage 13. Diatom, Coscinnoidiscus sp.

4799 m 71.5

24.0 14.5 72.8 * 12.0

0 45 6.9

* Note that these rates are probably lower than the actual rates due to stagnation in the experimental chambers.

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WE/GHT LOSS (%) Fig. 4. Percent weight loss vs. percent increase in specific surface area before and after dissolution for different biogenic calcite particles. The sharp increase in surface area between 20% and 30% weight loss may contribute to the sharpness of the lysocline.

294 iniferal lysocline; similarly the lysocline found by Peterson [ 18] for the optical calcite was approximately 1 km shallower than Berger's planktonic foraminiferal lysocline [19] (see also Fig. 1). The coccolith lysocline is considerably deeper than the planktonic foraminiferal lysocline. Specific dissolution rates and surface area problems will be discussed below. The diatom samples dissolved at all depths (Table 1). Weight loss during 79 days at 5518 m was 12% while at 4800 m and 3600 m the weight loss was 6.9% and 6.2% respectively. In the 5518-m experiment the specific surface area increased by ~170% during dissolution. It is hard to compare the results we obtained with those reported by Berger [32] for the following reasons: Berger [32] used radiolaria that were obtained from sediments while we used laboratory-cultured diatoms. The radiolaria were treated with acid and hydrogen peroxide while the diatoms were exposed to bacterial biodegradation for 10 days, then bleached in NaOC1 solution and finally washed with dilute HCI. Dissolved silica concentrations in the deep Pacific where Berger's experiment was carried out is ~130 /lmoles/kg [32], while in the Atlantic Parflux S station it is ~40-50/lmoles/kg [33]. In Berger's experiment rate of dissolution at 500 m, where dissolved silica concentrations are "~40-50/lmoles/kg, is roughly 0.13%/day. Our average dissolution rate for the three depths is 0.1 l%/day. Yet even this comparison is not valid because the temperature at 500 m in the Pacific is around IO°C while at the deep Atlantic it is around 2.5°C, and higher temperature enhances dissolution of biogenic silica [34]. We cannot explain the increase in dissolution rates with depth (Table 1, Fig. 3). This is surprising especially because dissolved silica concentration increases over this depth range roughly from 30 to 50//moles/ kg [33].

4. Discussion

4.1. Degree o f saturation and dissolution rate

The degree of saturation in the Northwest Atlantic with respect to calcite and aragonite has been deter-

mined in earlier studies [ 1,2,21 ]. Because different authors use different dissociation constants and different analytical techniques, significant differences in ~2 values in the same area and depth exists (Fig. 3). Our data shed some light on this problem. No calcite weight loss was detected at 3600 m and 4800 m. Nevertheless, SEM observations show that dissolution takes place at the latter depth indicating that I2 < 1. Milliman's experiment [22], which lasted much longer than ours, also showed significant weight loss at 4800 m while at 3600 m the weight loss was within the analytical error. Therefore, it seems that the transition from supersaturation to undersaturation (~2 = 1) for calcite is somewhere between 3600 m and 4800 m. This supports the f2 values calculated by Takahashi [2] who obtained the saturation level at about 4000 m using the K'sp for calcite reported by Ingle et al. [35]. Regardless of which set of ~2 values is chosen, the close to linear decrease of ~2 with depth is accompanied by a sharp increase (lysocline) in the dissolution rate (Fig. 3). This seems to support a high-order or exponential kinetics for CaCO3 dissolution [17,36]. In the ISWAC experiment we did not test Edmond's [1 ] suggestion that water flow may increase dissolution of CaCO3 and SiO2 on the sea floor. However, the water column or the chemical lysocline we observe cannot be explained hydrodynamically [1,24] because the dissolution chambers were completely protected from ambient turbulence or water flow. Even if Takahashi's [2] higher estimates for f2 are used, the lysocline seems to coincide with ~ value below 0.8 (Fig. 3). This does not support the thermodynamic origin of the lysocline [25]. A kinetic origin for the lysocline as suggested by Morse and Berner [17] is compatible with our data. 4.2. Specific dissolution rates

One might expect that the specific dissolution rate (weight loss/specific surface area/unit time) should be equal for all calcite particles. Yet the specific dissolution rates we measured for different kinds of particles differed by more than two orders of magnitude. The coccoliths which have the largest specific surface area showed the lowest weight loss, while reagent calcite that has very small specific surface area shows the highest weight loss. Foraminifera, with intermediate specific surface area, show higher weight

Reagent calcite Large calcite crystals Synthetic aragonite ** Foraminifera assemblage Bleached foraminifera assemblage 57.5 2.2 72.2 30.4 36.5 36.3 23.0 31.1 11.3 24.0 14.5 72.8 12.0

Weight loss in 79 days (%) 266 10 333 140 169 168 106 144 52 111 67 336 55

Weight loss per year (%) 137 3591 109 260 216 218 343 254 699 329 545 108 658

Complete dissolution time (days)

0.36 0.013 1.53 1.51 1.50 3.44 2.67 1.41 10.4 10.4 9.86 2.17 6.50

Initial SSA * (m 2 g - l )

3.33 5.18 8.19 3.72 1.88 11.5 12.3 3.09 17.5

Final SSA * (m 2 g - l )

* SSA = specific surface area. ** Note that these rates are probably lower than the actual rates due to stagnation in the experimental chambers.

12. Pteropod assemblage * * 13. Diatom, Coscinnoidiscus sp.

6. G. sacculifera 7. G. bulloides 8. G. paehyderma 9. E. huxleyi 10. Bleached E. huxleyi 11. C. neohelis

1. 2. 3. 4. 5.

Ma~rial

Summaryofmeasurements;ISWACIlex~rimentat5518 m

TABLE2

0.74 0.78 0.22 0.09 0.11 0.05 0.04 0.10 0.005 0.011 0.007 0.15 0.009

Sp. dissolution rate (init. SSA *) (mg cm -2 yr -1 )

0.042 0.033 0.021 0.029 0.076 0.005 0.009 0.11 0.003

Sp. dissolution rate (I'mal SSA *) (mg can-2 y r - 1)

121 245 138 40 34 10 18 42 169

SSA * increase (%)

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296 loss than coccoliths, but lower weight loss than the reagent calcite (Table 2). It is possible that these unexpected results were caused by stagnation within the experimental chambers. Each chamber contained roughly 150 mg of CaCO3. Thus the chambers containing coccoliths, foraminifera, and reagent calcite contained CaCO3 samples with surface areas of 1.5 m 2, 0.25 m 2 and 0.05 m 2 respectively. Assuming that the dissolution rate of calcite was proportional to the surface area of the samples, and that water exchange was too slow, stagnation would occur first in the coccolith chamber and last in the reagent calcite chamber. However, we believe that stagnation did not occur in the chambers and that the large differences in dissolution rate are real. The pumping rate of the ISWAC II was 0.05 1/hr per chamber and flushing time was 20 minutes. The weight loss of the coccolith samples was approximately 10% in 79 days. Thus, during 20 minutes the average alkafinity and E CO 2 increase in the chamber is 3.5 x 10 -a meq/kg and 1.7 x 10 -3 mmol/kg, respectively. This will increase ~2 value in the chamber by roughly 1.5% compared to the ambient seawater, and hence should not effect the dissolution rate appreciably. In contrast to the biogenic samples, the specific dissolution rate for different kinds of non-biogenic calcite are remarkably similar. Specific surface area of reagent calcite is 0.358 m2/g and it lost 57.5% of its weight; the large calcite crystals have specific surface area of 0.013 m2/g and it lost 2.2% of its weight. Thus their specific dissolution rates are 0.74 mg cm -2 yr -~ and 0.78 mg cm -2 yr -1, respectively. We conclude that although the BET method accurately measures the total surface area of carbonate particles, this measurement does not represent the reactive surface area available for the dissolution in biogenic carbonates. It is possible that biogenic particles are partially protected by organic coatings that may retard their rate of dissolution. This explanation is supported by our observation that bleached coccoliths dissolved twice as much as the untreated coccoliths although their initial specific surface area was the same. Similarly, mildly bleached foraminifera dissolved significantly faster than the untreated ones. Specific dissolution rates at 5518 m of non-biogenic calcite and aragonite are 0.74-0.78 and 0.22 mg CaCO3 cm -2 yr -1 respectively. The degree of satura-

tion for calcite and aragonite at this depth are 0.73 and ~0.51 respectively (using the value of Takahashi [2] for calcite and the aragonite/calcite solubility ratio of Berner [45]). These data indicate in contrast to experimental data by Morse (personal communication), that calcite dissolves faster than aragonite. In this particular depth (5518 m) we believe that stagnation in the experimental chambers of the aragonite sample did occur, and the rates of dissolution we report for non-biogenic aragonite and pteropods are probably lower than the true rates. The alkalinity and ECO2 build up in the chamber for synthetic aragonite are ~2.6 x 10 -2 meq/kg and 1.3 x 10 -2 mmole/kg respectively. This will increase ~ within the chamber by roughly 12%, hence ~2 for aragonite would be around 0.57. If aragonite dissolution obeys high-order kinetics (Keir, personal communication), our reported rate for aragonite can be considerably lower than the actual rate. This may explain why the specific dissolution rate we observe for synthetic aragonite is lower than that for calcite. Bearing in mind the limitation of the BET surface area estimate for biogenic calcites, we observe that for every sample measured, an increase in the specific surface area was experienced after dissolution (Table 2). Fig. 4 shows the relationship between weight loss and specific surface area increase for different kinds of biogenic calcite. When weight loss is above 25% the specific surface area increased very rapidly, suggesting that increase in surface area itself becomes an important factor in determining the dissolution rates as dissolution proceeds. This self-perpetuating process, superimposed on other surface chemistry factors that control the kinetics of dissolution [37], may help to explain the sharpness of the lysocline.

4.3. Comparison of previous in-situ experiments Using Takahashi's [2] ~2 values for the Atlantic and the Pacific it is possible to compare our results with those of Peterson [18] and Berger [19]. In the Sargasso Sea ~2 value at 5500 m is 0.7. A similar value of g2 is achieved at 4000 m in the Horizon Guyot area where Peterson and Berger did their experiments. In the Pacific, dissolution rate at 4000 m for calcite spheres was approximately 0.2 mg cm -2 yr -I [18]. The BET surface area of our large calcite crystals was approximately two times larger than the geometric

297 surface measured using the SEM. If Peterson's geometric surface area was also underestimated by a factor of 2, the dissolution rate at 4000 m in the Pacific was approximately 0.1 mg cm -2 yr -1. This rate is slower than our rate for synthetic calcite and calcite crystals by a factc.r of 7.5. Peterson's calcite spheres were well exposed to seawater and it is unlikely that the rate have been retarded by stagnation. The difference might be due to the different types of calcite used and the different treatments prior to the experiments. It is also possible that Peterson's calcite sphere became coated by organic film as the mooring array passed through the organic-rich surface water, and this could have retarded the rate of dissolution. The weight loss of the foraminiferal assemblage used by Berger [19] at 4000 m was 2.5 times slower than the loss of our foraminiferal assemblage. Berger [19] observed fragmentation of the foraminifera tests during dissolution. If some fragments were lost through the 63-tam nylon gauze used by Berger, the difference is even larger. Milliman [22] conducted his experiments near our stations and his results provide a more directly comparable data set. The dissolution rates he measured for calcite (foraminifera) and aragonite (ooids) were I/4 and 1/5, respectively, of our rates. It was shown by Ben Yaakov and Kaplan [38] that a relatively short time (~15 minutes) is needed for interstitial seawater to approach equilibrium with C a C O 3 in a small container. If water exchange between the containers and ambient seawater was restricted by the gauze or cloth in Milliman's and Berger's experiments, g2 value within the container could have significantly increased and lower dissolution rates were observed. This problem seems to have been more severe in Milliman's experiment where C a C O 3 particles were packed together and the volumetric ratio of water to particles was very low.

4.4. Complete dissolution time Recent sediment trap experiments have shown that biogenic carbonates reach the floor of the deep Atlantic without undergoing significant dissolution [27]. Thus, most of the dissolution must take place on the ocean floor before particles are buried [37, 40]. The time required for complete dissolution of a particle can be calculated from the dissolution rates measured during our experiment (Table 2). When the

accumulation rate is known, the time a particle spends on the sediment-water interface before burial can be estimated for different size particles. It is assumed that once a particle is buried, its dissolution rate is strongly retarded [41]. If the time required for complete dissolution is longer than the time a particle spends on the sediment-water interface it has a good chance of being preserved in the geological record. Obviously, other factors like bioturbation, resuspension, and changes in accumulation rates, including those caused by dissolution, must be taken into account in more elaborated models of carbonate dissolution at the water-sediment interface. Yet, application of this model to the data we collected reveals useful information. A sedimentation rate of 3 mm/1000 yr (0.008 /am/day) is a conservative estimate for the central Sargasso Sea where the ISWAC was deployed. The average size of a pteropod is above 2000 ~m and the corresponding time before burial is more than 240,000 days. Its complete dissolution time is 108 days. Thus, no pteropods should be found in the sediments at the Parflux S station. The time before burial of foraminifera with an average diameter of 150 ~m is more than 18,000 days, and their complete dissolution time is 260 days; hence, there is only a very small chance that foraminifera will last long enough on the interface to be buried. The time before burial of coccoliths and small diatoms with an average height of 5/~m is 600 days, while their complete dissolution time is about 700 days. Thus, coccoliths and small diatoms are expected to be found in the bottom sediments of the deep Northwest Atlantic. A surface sediment sample was collected using a box corer at the Parflux S station at 5550 m. The sample consists of fine red clay containing approximately 10% CaCO3 by weight [27]. No pteropods or foraminifera were found, the carbonate fraction is mostly coccoliths, and small diatoms are also found. Both coccoliths and diatoms are partially dissolved (Plate 1, i). This is expected because their burial time is shorter than their complete dissolution time only by 100 days. These observations are in good agreement with the predictions of the simple model presented above. Other workers have previously observed that coccoliths are more dissolution resistant than foraminifera [42-44]. We suggest that this is caused by their lower dissolution rate as well as their

298 smaller size and its associated shorter residence time on the sediment-water interface before burial.

5. Summary and conclusions (1) An in-situ water circulator (ISWAC) was developed to allow accurate measurement of dissolution rates of various carbonates without stagnation and with minimum mechanical loss. Three ISWAC packages were successfully deployed for 79 days in the Sargasso Sea. (2) Dissolution rates for pteropod shells, planktonic foraminifera tests and coccoliths, and for synthetic aragonite and calcite were measured. Weight loss during the experiment ranged from 2.2 to 72.8% at 5500 m. These rates are 4 - 5 times greater than those reported for earlier in-situ experiments, suggesting that the results of previous workers were influenced by increase in ~2 value within the experimental containers or by coating of organic matter that retarded dissolution [28]. (3) The existence of a lysocline in the Northwest Atlantic was confirmed. However, different particles seem to have different lysoclines. The coccolith lysocline is deeper than the foraminifera lysocline. Pteropods have the shallowest lysocline. This implies that different particles have different compensation depths and the generally referred to CCD can be divided into pteropod, foraminiferal and coccolith compensation depths. (4) The origin of the lysocline cannot be attributed to water flow or to the thermodynamic transition from supersaturation to undersaturation (fZ = 1). It seems to be a kinetic phenomenon. (5) The degree of saturation for calcite in the North Atlantic calculated by Takahashi [2] based on Ingle's et al. [35] K~p for calcite, appears to be most consistent with our data. (6) An attempt to normalize observed dissolution rates with respect to BET surface area shows that specific dissolution rates for different kinds of calcite particles differ by more than two orders of magnitude. The surface area measured b~' BET method apparently does not represent the surface area available for dissolution. We suggest that some coating (probably organic) on biogenic calcites may retard dissolution and cause these large differences in spe-

cific dissolution rate. Bleached coccoliths and foraminifera dissolve significantly faster than non-bleached ones, although their BET surface area is identical, supporting this suggestion. (7) A simple model comparing the complete dissolution time and the time before burial on the sediment-water interface for different CaCO3 particles is presented. The model predicts that coccoliths and diatoms can be preserved in the sediments of the deep Atlantic ocean while foraminifera and pteropods should completely dissolve. Surface sediment sample in the Sargasso Sea showed good agreement with these predictions.

Acknowledgements

We thank John Connell (WHOI) and Peter Fontecchio (Metal Bellows Company, Sharon, MA 02067) for their cooperation in developing, manufacturing and testing of ISWAC Mark I and II since 1975. A number of colleagues including Robert Berner, Taro Takahashi, John Morse, James Kennett, Margaret Goreau and Carol Stein, provided their valuable sampies for this experiment. Margaret Goreau contributed this research with the most careful electron microscopic observation of samples. We are grateful to Taro Takahashi, John Milliman, Robin Keir, Peter Brewer, Tom Goreau, Marcia Rottman and Derek Spencer for their valuable suggestions and constructive criticism. Deployment and recovery of the ISWAC II's in the southern Sargasso Sea were performed by the expert hands of Moored Array Program, Physical Oceanography Department of our Institution. We thank the Captain, Officers and the men of the R.V. "Knorr", particularly Chief Engineer Walter Huckabee, for their cooperation during the deployment and recovery cruise. This investigation was made possible through financial support by the Oceanography Section of the National Science Foundation under grant OCE7602122. References 1 J.M. Edmond, On the dissolution of carbonate and silicate in the deep ocean, Deep-SeaReg. 21 (1974) 455-

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