A comparison of aggregate profiles with sediment trap fluxes

Deep-Sea Re~ear~h Vol ~9, No I1/12, pp 1S17-I834 1902

[)I080J149/92 $'~ 00 + 0 I~) © 1992 Pergamon Pres', Ltd

Printed m Great Britain

RAPID RESPONSE PAPER

A comparison of aggregate profiles with sediment trap fluxes I. D WALSH* and W. D GARDNER~ (Received 20 February 1991, m revised form 15 November 1991, accepted 9 December 1991) Abstract--Previous work (b.~ us and others) has shown that sediment trap fluxes do not correlate well with the total particulate mass concentration as determined with a transmlssometer Sediment traps are thought to collect the settling particles in the marine snow size range (d > 0 5 mm) Cameras ha,~e been developed to quantitatively image particles in the marine snow size range but a correlanon between measured flux in sediment traps and large-particle camera (LPC) profiles has not been established In this stud)', LPC total particulate volume data are correlated with fluxes measured in sediment traps, indicating that sediment traps sample the large aggregate size range and that the flux is proportional to the concentration and size distribution of large aggregates Partitioning of the major components of the bulk chemistry indicates that rebound aggregates (particles which do not undergo an appreciable change In their bulk chemistry before resuspension from the seafloor) contribute to aggregate nephelold layers and increase measured trap fluxes The bulk chemical composition of material from the deepest sediment traps indicates that downslope advection as well as cross-slope advcctlon and subsequent settling may be an important pathway for blogenlc material to the deep ocean

INTRODUCTION

THE flUX of pamcles through the water column reflects the biological, chemical and physical processes that act to produce, mtroduce, degrade and redistribute pamcles. Theoretically, the flux of particles could be calculated from the concentration (number per volume) of pamcles if the size distribution, size to mass relanonship and setthng velocity of all the particles were known. For many years the pamcle flux was assumed to be slow and steady because the observed particles in the ocean were very small and hence settled very slowly. However, while the total particle mass concentration is dominated by the numerically abundant small particles, flux ~s a function of both mass and setthng veloctty Consequently, the particle flux is dominated by the rare large particles that have high settling rates (McCAVE, 1975). Sampling these large particles for abundance and size distribution is difficult because the traditional methods of determining particle concentration, such as filtration and hght transmission, do not sample a large enough volume of water to obtain statistically meaningful data. There are also technical problems in sample handling. For example,

~Department of Oceanography, Texas A & M University, College Station. TX 77843. U S A 1817

1818

I D WALSHand W D GARDNER

while large particles can be collected in water bottles, they can settle quickly (in minutes) below the bottle spigots (GARDNER, 1977) or break up during extraction from the bottle (GIBBS and KONWAR, 1983) Filtering large volumes of water to sample the large particles has been successful in characterizing the chemistry and biology of large particles In the upper water column, but estimates of flux based on the size distribution of filtered large particles have not shown a strong correlation with sediment trap fluxes (BISHOPetal., 1980; BISHOP et a l , 1986). Since measuring the concentration of large particles has proven difficult, and few in situ measurements of particle settling velocity have been made (ALLDREDGE and GOTSCHALK, 1988), esnmates of particle flux have largely relied on the use of sediment traps to measure directly the flux of pamcles However, while sediment traps allow for the quantification and composmonal analysis of the particle flux, sediment traps necessarily integrate the flux over periods of time longer than the natural events that they sample (e.g. phytoplankton blooms and benthic storms). Additionally, collection of flux data with sediment traps is economically limited, resulting in few, if any, replicate measurements and a pauoty of data over vertical and lateral scales. Nevertheless, sediment traps have been used to demonstrate dissolution and degradation of biogenic components of the particle flux with depth (WAKEHAMet al , 1984; WALSH et al., 1988a), increases in flux with depth due to biological actwlty (KARLand KNAUER, 1984; KARLetal., 1984; WALSHetal., 1988a), and a substantial flux that is derived from resuspended material (GARDNER et a l , 1984, 1985; BAKER and HICKEY, 1986, MONACOet al ; 1987 RICHARDSONand HOLLISTER,1987, WALSHet al., 1988b: BISCAYEet al., 1988; GARDNER, 1989). Clearly, sediment traps are useful instruments, but because of the lack of data density in both time and space it is difficult at best to integrate sediment trap data into the coupled physical and biogeochemlcal models necessary to understand the ocean as a system To reach the goal of bringing flux data into biophysical models will require techniques that can characterize particle flux rapidly, repeatedly and accurately Measurements of particle concentration by filtration (HONJO et a l , 1982a,b, GARDNER and RICHARDSON, 1992) and beam attenuation (GARDNER, 1989) In conjunction with sediment trap measurements of flux have failed to show a correlation between particle mass concentration and particle flux. While filtration and beam attenuation may adequately sample the total particle mass concentration, they do not distinguish between the small, slowly settling particles that dominate mass concentration and the rare large particles with high settling velocities that dominate the mass flux Camera systems have been developed to characterize milhmeter-size particle distributions in the water column (EISMA et a l , 1983; KRANCK,1984: HONJO et a l , 1984: ASPER, 1987; GARDNERand WALSH, 1990). It IS conjectured that the mdhmeter-slze class range of particles, thought to be composed primarily of aggregates ("marine snow") may dominate the total mass flux because of their abundance and high settling rates (AsPER, 1987) A few efforts have been made to demonstrate an aggregate concentration and flux relationship. A comparison of aggregate camera profiles and sediment trap fluxes in the Panama Basin indicated that the flux was not uniquely controlled by the abundance of aggregates (AsPER et a l , 1992). In a previous paper (GARDNERand WALSH, 1990) sediment trap data was not available for comparison and the mass flux due to aggregate setthng was calculated from aggregate abundance profiles measured using a large-particle camera system While the range of calculated flux appeared reasonable, the calculation neglected the displacement mass of the particles, resulting in lnvahd flux estimates For this paper, the only

Aggregate profiles and sediment trap fluxes 97

96

-I"

30

95

94

93

1819 92

v------

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0

50

Kilometers

TEXAS 29 _-~" . . . . . . . .

t - 40m " ~ ' - ' ' -

zt00a 28 / /I1

27

I

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O 26

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f ~

r "J 20OOm

~

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~

2000 m },-"-x~f "

~

CATSTIX Mooring location

Fig 1 Locationof the Project CATSTIX mooring on the continental slope off Texas Symbols show locations of instrument profiles made during the pre-deployment cruise (diamonds) and mooring deployment cruise (triangles) Open symbols mdmate the sites of CTD/transmlssometer profiles Filled d~amonds indicate the sites where both CTD/transmlssometer profiles and largeparticle camera profiles were made

assumption we make is that the particles are spherical m comparing the measured trap fluxes and the LPC data. ProJect C A T S T I X ( C A m e r a , Transmissometer, Sediment T r a p Integration eXperiment) was designed to compare three in situ particle m e a s u r e m e n t methodologies to see if the limitations of each method could be overcome by integrating the data from each into a combined methodology that can be used at the ttme and space scales of biological and physical modeling. During Project C A T S T I X a sediment trap mooring was deployed on the continental slope of the northwest Gulf of Mexico (Fig. 1). CTD/transmissometer and large particle camera (LPC) profiles were made at the mooring site prior to the mooring deployment and upon recovery. This p a p e r presents the comparison of particle fluxes measured with the sediment trap mooring with the fluxes calculated from the LPC profiles. To our knowledge, these are the first reported m o o r e d sediment trap data from the Gulf of Mexico and the first report of a correlation between measured sediment trap fluxes and particle size distributions measured in situ METHODS A sediment trap mooring was deployed on the continental slope of the northwest Gulf of Mexico at 26°56 77'N, 94°46.08'W in 1450 m of water on 4 D e c e m b e r 1987 and recovered

1820

I D WALSHand W D GARDNER

28 January 1988. Six cylinder traps were attached to the mooring at depths of 336,636,936, 1236, 1352 and 1404 m. The traps have an external aspect ratio greater than 3' 1 with an opening of 731 cm 2, and were fitted with a 5 cm thick square plastic baffle (1 cm cell diameter) mounted flush to the opening of the trap A sliding tongue door at the bottom of a funnel within the trap was triggered to close by an OIS timer (GARDNER etal., 1984). The timers were set to close the traps on 12 January 1988, 38.66 days after deployment, to isolate the sample prior to recovery The sample cups were poisoned during deployment with sodium azlde to prevent bacterial decay. Upon recovery, all timers appeared to have fired and all doors were closed. All sample cups had visible sample and no odor of decomposition was detected Though there was no independent confirmation of closure, subsequent retesting of the timers in the lab indicated that all six had operated at the pre-set time. The sediment trap samples were frozen for transport ashore After thawing, each of the sediment trap samples was wet sieved through a 1 m m Nylon screen The > 1 mm fraction was removed and stored. No detailed analysis has been made of this fraction of the sample, but it was estimated by v~sual inspection during processing that < 5 % of the total sample was > 1 m m and consisted mostly of whole or fragments of pteropod tests The < 1 mm fraction was wet split into four fractions using a turntable splitter Due to operator error, an unknown amount of the sample from the 337 m trap was lost during splitting, and no data from that trap are reported Two 1/4 splits from each of the five remaining samples were recomblned into pre-welghed centrifuge tubes and centrifuged at 15 krpm for 100 mxn The supernatant was drawn off and the tubes weighed. After the tubes were frozen and freeze dried, they were reweighed to measure the water loss. The samples were removed from the centrifuge tubes and ground to a powder in an agate mortar. Ground samples were placed into pre-welghed Petrl dishes and rewelghed. The empty centrifuge tubes were also reweighed to estimate the remaining sample on the wall and as a double check on the Petrl dish weight The remaining two splits were refrozen and stored Subsequently they were thawed, and one split (1/4 of sample) was turther spht into quarters. One 1/16th split was retained for pigment analysis. One 1/16th spht was refrozen and stored. The other two 1/16th splits were combined with the remaining 1/4 split and processed as above The concentration of aluminum in the samples was measured by neutron activation analysis (BooTnE and JAMES, 1985). The concentrations of organic carbon and nitrogen were measured from acidified samples using a Carlo Erba N A 1500 carbon analyzer (VERARDO et al., 1990). Calcium carbonate concentration measurements were made with a gasometrlc technique on a pressure rig following the method of JONES and KAITERIS(1983) using a differential pressure gauge instead of a vacuum gauge. Precision for the carbonate analysis is better than 0.5% by weight The bulk flux and concentration data were corrected for salt concentration in the freeze-dried sample by difference in weight before and after freeze-drying multiphed by the salinity of the water. Salinity m the supernatant was measured with an American Optical Corporation hand refractometer. CTD/transmissometer profiles were made at the mooring site on 22 N o v e m b e r 1987, 4 D e c e m b e r 1987, and 28 January 1988. Large-particle camera (LPC) profiles were made immediately following the C T D casts on 22 N o v e m b e r 1987 and 28 January 1988 Total particle volume profiles were calculated from the large-particle camera (LPC) particle abundance and circular diameter data, assuming spherical particles. A nine-point running average was used to smooth the data for comparison with the measured sediment trap fluxes (Fig. 2) The LPC used was a Lobslger Deep-Slope camera with a wide-angle (90 °)

1821

Aggregate profiles and sediment trap fluxes

lens. Illumination was from a 150 Ws strobe. The strobe head was masked with aluminum tape to emit light from a 2 cm x I cm hole directed toward a Fresnel lens. The Fresnel lens collimates the light normal to the camera field of view where an area approximately 35 cm x 25 cm was photographed. The light slab thickness was controlled by baffles to yield an illuminated volume 9.5 cm deep (8.3 1) The field of view mcluded a wire with centimeter marks for scale. Because the camera has a wide-angle lens there is a difference in the apparent size of a particle dependent on its distance from the camera. This difference is less than a factor of 2, and was minimized by placing the cahbration wire m the center of the hght slab and relying on randomness to average out the error The camera has a button panel for lnsertmg multiple photographic sequences, mcludlng delays, wtth data (date, time, photo number) recorded on each frame. The firing rate and lowering rate yielded photographs at about 6 m intervals. A pmger on the C T D and camera allowed bottom approaches to within 5-10 m Frames m the upper water column where sunlight was visible on the negative were excluded from the analysts because of potential ambiguity as to the water volume sampled (i e. particles outside of the strobe illummated volume may have been tlluminated by sunlight). Images from the camera were analysed &rectly from the film negatives using a video camera mput to a Kontron IBAS-2 image analysis system running IPS release 4 2 software for data capture The area analysed in each frame corresponded to approximately 51 Each tmage was analysed for the total n u m b e r of particles and their maximum, minimum and equivalent circular diameters. The particles were binned into six size ranges based on the equivalent circular diameter (0.53-1, 1-1 5, 1.5-2, 2-2 5, 2.5-3 and >3 mm). Particle volume was calculated assuming sphertcaty and diameters equal to 0.75 m m for the smallest size range, the means of the four middle ranges, and as a conservattve estimate, the minimum size (3 m m ) of the unbounded largest size bin Total Aggregate Volume

cm 31-1 0

0I

0.002 0.004 0.006 0.008 I

I

o

D E P T H

I

2

o

I

0 I

0.0l I

I

0.02 I

I

0.03 I

I

ov. 198

o

(m)

F~g 2

Total aggregate volume data from the pre- and post-deployment profiles The nine-point smoothed data (sohd hnes) were used for comparison with the se&ment trap data

1822

I D WALSHand W D GARDNER

T r a n s m i s s o m e t e r d a t a w e r e c o l l e c t e d using a 660 n m Sea T e c h 25 cm p a t h l e n g t h b e a m t r a n s m i s s o m e t e r c o n n e c t e d to a N e i l B r o w n M a r k I I I C T D a n d N i s k i n b o t t l e r o s e t t e o r a Sea B i r d S B E - 9 C T D . T r a n s m i s s o m e t e r d a t a r e d u c t i o n was a c c o m p l i s h e d by c o r r e c t i n g for d e c a y o f the L E D u s m g the v o l t a g e a l g o r i t h m s u p p l i e d by Sea T e c h a n d a v e r a g e air c a l i b r a t i o n a n d b l o c k e d values for each crmse. P e r c e n t t r a n s m i s s i o n was c o n v e r t e d to a total b e a m a t t e n u a t i o n coefficient (c) using the e q u a t i o n :

Vc,JVm~t = % T =

e -cz,

(1)

w h e r e Veal is the i n s t r u m e n t v o l t a g e o u t p u t c o r r e c t e d with the Sea T e c h a l g o r i t h m , Vi.st is t h e m a x i m u m v o l t a g e of the t r a n s m l s s o m e t e r ' s L E D (5 V ) , % T is the p e r cent of t r a n s m i s s i o n , c is the b e a m a t t e n u a t i o n coefficient (with units of m - l ) , a n d z is the p a t h l e n g t h of t h e i n s t r u m e n t (in m e t e r s ) . T h e b e a m a t t e n u a t i o n c o e f f i o e n t m n a t u r a l s e a w a t e r is a result of the s u m m a t m n o f the b e a m a t t e n u a t i o n coefficients for s e a w a t e r (Cw), " y e l l o w m a t t e r " (Cy), a n d p a r t i c l e s (%) (PAK et al., 1988); C = Cw q- Cy Jr- Cp

(2)

T h e c o n t r i b u t i o n o f Cy at 660 n m is a s s u m e d c o n s t a n t and negligible (BRICAUD et a l , 1981). C a l i b r a t i o n o f the t r a n s m i s s o m e t e r with filtratmn p a r t i c l e c o n c e n t r a t i o n d a t a was perf o r m e d on a s u b s e q u e n t cruise in the study a r e a (WALsH, 1990) and Cp was c a l c u l a t e d by using a l i n e a r fit to the filtration c o n c e n t r a t i o n versus the b e a m a t t e n u a t i o n m i n u s the m i n i m u m value of c f r o m the c r m s e , then a d j u s t i n g the r e l a t i o n s h i p to the origin

RESULTS T r a p fluxes a n d L P C d a t a w e r e statistically c o r r e l a t e d using a l e a s t - s q u a r e s fit to a h n e a r r e g r e s s i o n . T h e n m e - p o i n t s m o o t h e d d a t a o f the t o t a l n u m b e r and t o t a l v o l u m e of particles

•

MASS 30 FLUX 20

•

•

•

btg cm-2 d-l 10

0

i

0

I

I

I

I

2 4 Total Number of Pamcles #1-1

,

0

~

',

'

I

I

I

0.002 0.004 Total Volume of Particles cm-31-1

Fig 3 Correlauon of the measured trap fluxes with the total number and total volume of partmles imaged using the LPC The LPC data are the nine-point smoothed data from the 22 November 1987 profile (diamonds) and the 28 January 1988 profile (squares) The LPC profiles are "snapshots", while the sediment trap fluxes are mtegratmns over the 38 days the traps were open See Table 1 for correlaUon staUstms

1823

Aggregate profiles and sedament trap fluxes

M a s s Flux ~ g c m "2 d -1 ) 0 0

25 . . . .

I

50 . . . .

I

75 . . . .

I

100 . . . .

I

125

....

500 D E P T H

(m) 1000 22 Nov. 1987 LPC flux --

•

28 Jan. 1988 LPC flux 4 Dec.-16 Jan. 1988 Sediment trap flux

1500 Fig 4 Calculated LPC flux profiles at the mooring site from profiles taken on 22 N o v e m b e r 1987 and 28 January 1988 compared to the measured fluxes from the sediment traps that sampled from 4 D e c e m b e r 1987 to 12 January 1988 LPC flux profiles are calculated using the nine-point smoothed total volume of particles data and the hnear correlation between the 28 January 1988 total volume data and sediment trap data (Table 1) The mooring bottom depth is 1450 m, the LPC profiles were taken to w~thln 5-10 m above the bottom

from the pre- and post-trap deployment profiles were compared (Fig. 3 and Table 1). The correlation was significant for both total number (f-test, P = 0.013) and total volume (P = 0.002) from the 28 January 1988 profile, with the total volume data accounting for more of the variance than the total number. The correlations between the trap data and the

Table 1 Lmear correlatzons between trap and L P C data The values from whtch calculations m tht+ paper were made are zn bold

LPC date

Slope

Flux as a function of total n u m b e r of particles 22 Nov 1987 15 98 28 Jan 1988 7 02

Intercept

R

- 5 14 - 3 01

0 570 0 951

Flux as a function of total parucle volume 22 Nov 1987

13817

-5

0 844

28 Jan. 1988

6992

-2

0.985

1824

I D WALSH and W D GARDNER

Cp (m-1) 0 00

0.02

0.04

0.06

0.08

0.1

D e

P t

500

h

m

e t e

1000

m

4 Dec. 1987

r

S

28 Jan. 1988

\

Fig 5

22 Nov. 1987

--

18 Oct. 1988

1500 Profiles of pamcle beam attenuanon (Cp) profiles at the CATSTIX mooring site over a period of 1 y

predeployment profile (22 N o v e m b e r 1987) were not significant at the 0.05 level The LPC profiles were converted into flux profiles using the total volume relationship from the 28 January 1988 profile (Fig. 4). The pattern of trap fluxes, increasing to 936 m, decreasing to 1236 m and increasing again to a m a x i m u m near the bottom is reflected in the LPC calculated flux from the 28 January 1988 profile (Fig. 4). The 22 N o v e m b e r profile did not record the four distinct immediate aggregate nepheloid layers at 800,900, 1050 and 1150 m seen in the 28 January profile (Fig. 4). Both profiles recorded benthic aggregate nephelold layers below 1400 m, though the thickness of the benthic aggregate nepheloid layer on 28 January 1988 is open to interpretation. The intermediate aggregate nepheloid layers were not reflected in b e a m attenuation profiles taken in conjunction with the LPC profiles (Fig 5). The b e a m attenuation profiles demonstrate a decreasing or constant particle mass concentration from 600 to 1400 m. The benthic aggregate nephelold layers are reflected in the increase in the b e a m attenuation below 1400 m on 22 N o v e m b e r and 28 January. C o m p o n e n t fluxes of the bulk flux show a similar pattern to the bulk flux (Fig. 6). Ratios of the biogenic components and bulk flux to the aluminum flux generally decrease with depth below the 636 m trap. The aluminum flux is used as a tracer of the refractory clay c o m p o n e n t of the bulk flux (GARDNER et a l , 1985; WALSH et al., 1988a). Assuming that the

Aggregate profiles and se&ment trap fluxes

Mass Flux

AI Flux

CaCO 3 Flux

1825 Corg Flux

650" D e P t h

850

1 n

1050 m e I e r 1250 s

1450

1})

I

20

3;

015

I

1

I

15

;

1'0

115

015

I

10

I 15

(gg cm-2d-1) (gg cm-2d-1) (gg cm-2d-1) (gg cm-2d 1) Fig 6 Bulkand major component fluxes measured in se&ment traps from 22 November 1987to 12January 1988on the continental slope of the northwest Gulf of MexLco Water depth was 1450m Fdled symbols are fluxes Open symbols are the fluxes ratloed to the AI flux

ratio of each biogenic c o m p o n e n t to the clay content of a particle is fixed at the time of particle formation, the biogenlc to aluminum ratio of the material in traps below the depth of particle formation will reflect the dissolution and degradation losses of the biogenlc components during settling plus inputs of biogenlc and clay components from secondary sources such as resuspension of sediments or fresh material (WALSH et al., 1988a). The resuspension of fresh material as a process distinct from the resuspension of older, bloturbated sediments is termed rebound (WALSH et al., 1988b). In this study the first trap below the depth of particle formation (the assumed primary flux trap) is the 636 m trap. While this trap was probably influenced by resuspension and rebound of material from the slope, comparing the flux measured in the traps at greater depths to the 636 m trap is useful in understanding the processes affecting the particle flux between 636 m and the bottom.

DISCUSSION A major objective of this study is to determine whether an LPC system can be used to estimate the mass flux through the water column If an L P C system does supply the data necessary to calculate flux, then an LPC system used in conjunction with sediment traps will be a cost-effective way of measuring variations in flux over time and spatial scales appropriate to understanding the impact of biogeochemical and physical processes on mass flux. Regardless of the assumptions made in calculating flux from the LPC data, the flux comparison is problematic in that the L P C system gives a snapshot representative of the water column at the time it is taken, and traps provide a time-integrated flux at a single depth over the time they are open. Other than the two profiles presented here there are no data to estimate the temporal variability of LPC profiles at this site. In the P a n a m a Basin

1826

I D WALSHand W D GARDNER

the shape of the vertical profiles of aggregate abundance at a single site appeared stable over a year, but with at least a factor of 2 variability (AsvER et al., 1992). In this study traps integrated the settling flux over the 38.7 days they were open, while the LPC profiles were taken 2 months apart. Furthermore, the LPC profiles were taken 12 days before and 16 days after the time period the traps sampled, so the question is how appropriate it is to compare a pair of profiles representing conditions 2 months apart with data integrated over a part of the period between the profiles The appropriateness of comparing LPC and sediment trap data probably will be judged adequately only in the future when there are sufficient simultaneous data on the temporal variability of LPC profiles, trap fluxes and in sltu measurements of aggregate settling velocities The fact that there is a significant linear correlation between the LPC and trap data (Fig. 3) indicates that the effort is worthwhile at least, and that further work in this direction needs to be done. At the least, it is clear that the LPC profiles are much better predictors of sediment trap fluxes than transmlssometer profiles. The correlation between the 28 January 1988 LPC data and the sediment trap data and the lack of a correlation using the 22 November 1987 data may be fortuitous or it suggests that the 28 January 1988 LPC profile reflects the water column conditions during the period the sediment traps sampled Based on a comparison of the beam attenuahon profiles taken at the mooring site, it appears that changes in the general circulation pattern in the northwest Gulf of Mexico occurred between the 22 November 1987 profiles and the 4 December 1987 sediment trap deployment. Mesoscale circulation patterns in the northwest Gulf of Mexico affect the distribution of small particles (sampled by transmlssometers) and aggregates (WALsR, 1990). When the high velocity currents of mesoscale features encounter the slope, particles are resuspended and transported cross-slope during advection. This results in higher concentrations of particles further across the slope in areas associated with the circulation than at comparable positions relative to the shelfbreak that are not under the influence of the mesoscale circulation. The mooring site may have been influenced by shifts in the mesoscale circulation during the deployment period such that the pre-deployment LPC profile is not indicative of the aggregate distribution prevailing during the trap sampling period. At the time of the pre-deployment LPC profile (22 November 1987) a cyclonic cold-core feature lay to the east of the mooring site and an anti-cyclonic warm-core ring lay to the south (SAIC, 1989). At the time of trap deployment (4 December 1987) the 15° isotherm had risen with respect to its depth on 22 November 1987, which probably signaled a westward shift of the cold core feature. Comparing the particle beam attenuation profiles of 22 November 1987 and 4 December 1987 shows an increase m particle concentration below 100 m (Fig. 5), probably reflecting increased horizontal transport of particles resuspended from the slope to the north and advected southward on the western edge of the cold-core feature. By the time of the mooring recovery (28 January 1988), the 15° isotherm was below the depth measured on 22 November 1987, probably reflecting northward progression of the warm-core ring, and thus a changing of the current regime at the site from southward transport to eastward transport. While more extensive hydrographic data are not available during the deployment period, the study area was almost fully occupied by a warm-core feature in October 1988 (SAIC, 1989). Comparing Cp profiles at the mooring site from 22 November 1987 to 18 October 1988 shows that the profiles from the day of the deployment (4 December 1987) and day of recovery (28 January 1988) are more similar to each other

Aggregate profiles and sediment trap fluxes

Table 2

1827

Parttuontng model parameters Value (pg/pg)

Parameter [AI],

o 06

[c],

o oJ

(CfAI).,r (C/AI),. 936 m trap [CaCO3] `

0 0 0 6 7

(CaCO~/Al)dr (CaCO~/AI),,r 936 m trap

80 85 30 90 00

[C], and [GaOl3] , from LIN and MORSE (1991) [AI]. from TREFRY (1977), adjusted for higher [CaCO3], as compared to the cores analysed in TREFRY (1977)

and to the 18 October 1988 profile than to the 22 November 1987 profile (Fig. 5) Note that 4 December 1987 profile has one less significant digit than the other profiles. All four profiles were performed with the same transmissometer and calibration routines (WALSH, 1990) Since the 22 November 1987 profile indicates clearer water than the other profiles, a dirty lens can be ruled out as the cause of the difference. If the temporal variation in particle concentration at least partly reflects changes in the current regime and particle sources, then the beam attenuation data suggest that the mooring site was influenced by cross-slope advective transport to a greater extent during the mooring period than at the time of the pre-deployment LPC profile. Alternatively, the difference in beam attenuation FLUX 0 500

( a g cm-2d -I )

02

04

06

08

1

12

I

I

I

I

I

I

65O D e

P t

h

850

1 n

m 1050 e t e r

s 1250

1450 • C o r g m [ ] C o r g p O C o r g ar 0 C o r g s Fig 7 Results of the p a r t m o n m g model described in the text for the measured organic carbon flux Corg m IS the measured flux. Corg p is the primary flux, Corg ar lS the aggregate rebound flux and C,,rg , is the resuspended sediment flux

1828

I D WALSH and W D GARDNER

and large particle aggregate profiles might simply be due to higher productivity at the site after 4 December 1987 as compared to 22 November 1987. However, the presence of intermediate aggregate nepheloid layers in the 28 January 1988 LPC profile indicates the influence of horizontal advection, as discussed below. In either case, considering the increased particle load between the 22 November 1987 and 4 December 1987 and the similarity between the 4 December 1987 and 28 January 1988 Cp profiles (at least below the surface layer) the post-deployment LPC flux profile (28 January 1988) probably reflects the aggregate abundance and flux prevailing during the deployment period to a greater extent than the pre-deployment (22 November 1987) profile. Assuming that to be the case, the more important question is the degree to which changes in the vertical structure of the sediment trap flux can be related to the LPC profile In particular, what is the cause of the four intermediate aggregate nephelold layers seen between 700 and 1100 m in the 28 January 1988 profile, and are the aggregate nephelold layers related to the differences in measured fluxes between the 636,936 and 1236 m traps'? The most likely cause for the increase in flux between the 636 and 936 m traps is horizontal advection of rebound aggregates from the slope, which also explains the intermediate aggregate nepheloid layers seen in the 28 January 1988 LPC flux profile between 700 and 1100 m This interpretation means that the total flux of material trapped at 936 m consists of the primary flux of particles from above plus a horizontal flux that increases the concentration and flux of aggregates for some period of time in the water column between the 636 and 936 m levels The horizontal flux must have a high rate of lateral advection considering that the 1236 m trap recorded a flux only slightly greater than

FLUX 500

I

05 I

(I.tg cm-2d -1 ) I

1 I

I

15 I

I

65O D e

P t

h

850 •

1 n

m

1050.

e t e r

s

1250

1450 [,A]m

rlAlp

OAlar

OAIs

]

Fig 8. Results of the partlonlng model described m the text for the measured aluminum flux AI m IS the measured flux, Alp is the primary flux, Al~r ts the aggregate rebound flux and AI, is the resuspended sediment flux

1829

A g g r e g a t e p r o f i l e s a n d s e d i m e n t t r a p fluxes

(gg

FLUX -1 500

0 I

2 I

I

cm-2d-l)

6

4 I

8

I

I

I

10 I

I

12 I

650 D e

p t h

850

1

1050 m e t e

r

1250

1450 111 CaCO~ m [] CaCO~ p

CaCO 3 ar

CaCO3 s + CaCO3 r

O

Fig 9 Results ot the partlonmg model described in the text for the measured calcium carbonate flux CaCO ~m lS the measured flux, CaCO~ p is the primary flux, CaCO 3a r lS the aggregate rebound flux and CaCO~, is the resuspended sediment flux and CaCO~ r is the residual flux, 1 e the flux of C a C O 3 that is either o~er or underestimated by the partitioning model m e a s u r e d in the 636 m trap. T h u s the a g g r e g a t e s m u s t be a d v e c t e d s e a w a r d of t h e m o o r i n g site b e f o r e t h e y have t i m e to settle to the level of the 1236 m t r a p T h e collection of p a r t i c l e s f r o m a s e c o n d a r y source by the 936 m t r a p a n d o n l y m i n i m a l l y in the 1236 m t r a p ts s u p p o r t e d by c h e m i c a l analysis of the t r a p m a t e r i a l . A p a r t i t i o n i n g m o d e l t h a t divides the m e a s u r e d fluxes into p r i m a r y , a g g r e g a t e r e b o u n d and r e s u s p e n d e d s e d i m e n t c o m p o n e n t s was used to e s t i m a t e the c o n t r i b u t i o n of s e c o n d a r y s o u r c e s to the m e a s u r e d fluxes (WALSH et a l . , 1988b. note that we n o w use the t e r m a g g r e g a t e r e b o u n d r a t h e r t h a n p a r t i c l e r e b o u n d ) T h e m o d e l a s s u m e s that the t h r e e c o m p o n e n t s a c c o u n t for all of t h e m e a s u r e d fluxes a n d that the c o m p o s i t i o n of the c o m p o n e n t s is k n o w n . W h i l e m a n y a s s u m p t i o n s as to the c o m p o s i t i o n of the s e c o n d a r y sources are r e q u i r e d for the m o d e l , the results for this set of d a t a are quite conclusive. T h a t is, the excess flux m e a s u r e d in the t r a p s b e l o w the 636 m t r a p (i.e. the m e a s u r e d flux m i n u s the p r i m a r y flux) has a c o m p o s i t i o n m u c h m o r e similar to the p r i m a r y flux t h a n to the s e d i m e n t s . T h e m o d e l simply is a w a y of q u a n t i f y i n g the c o m p o s i t i o n a l b r e a k d o w n . T h e 636 m t r a p was d e f i n e d as the p r i m a r y flux t r a p , a n d the p r i m a r y fluxes o f Corg a n d C a C O 3 w e r e e x t r a p o l a t e d with d e p t h a s s u m i n g a f i r s t - o r d e r r e l a t i o n s h i p , thus: p,

,

= Poe

-kz

,

(3)

w h e r e P~ is the m e a s u r e d flux o f c o m p o n e n t t in the p r i m a r y flux t r a p , P' is the e x t r a p o l a t e d flux of c o m p o n e n t t, k is the r e a c t i o n rate c o n s t a n t for t, a n d z is the d i f f e r e n c e in d e p t h b e t w e e n the p r i m a r y flux t r a p a n d the d e p t h o f e x t r a p o l a t i o n . T h e d i s s o l u t i o n a n d d e g r a d a t i o n r a t e c o n s t a n t s used were c a l c u l a t e d f r o m u p p e r w a t e r c o l u m n s e d i m e n t t r a p s

1830

I D WALSH and W D GARDNER

in the Pacific (k for Corg = 6.43 x 10 -4 m - 1 k for C a C O 3 = 3.8 x 10 -4 m -1) (WALSHet al., 1988a). These values are empirical and based on the bulk chemistry of the particle flux and therefore are not directly dependent on water column chemistry, obviating the need to gauge the effect of microenvlronments within the settling particles along with other factors influencing the kinetics (WALSH et a I , 1988a). The assumed rate constants are a potential source of error in estimating the vertical flux However, even if the degradation and dissolution rates were set equal to zero, the model would yield similar numerical results and conclusions as to the particle sources. The primary A1 flux was assumed constant with depth, which assumes that the biogenic to clay ratio of a particular particle is fixed at the time of particle formation, i.e. there is no net aggregation of clays to the particle as it settles The measured fluxes not accounted for by the extrapolated primary flux were assumed to be derived from resuspended sediment and aggregate rebound The contributions from each of these sources were calculated by assuming that all the Al and organic carbon can be accounted for by the sum of the contributions from the primary flux, resuspended sediment, and aggregate rebound Organic carbon was used for the second equation because of its low concentration in the surface sediments and high concentration in the settling particles. For AI: (4)

A1 m = Alp + {[AI]~ x S} + Alar, CaCO 3 / C org 500

7 I

8 i

I

Corg/N

9 t

I

i

I

7 I

)

8 I

t

9 )

I

650 • D e

P I

h

850 •

m

1050"

e t

e r

s

1250.

1450

Fig 10 Ratios by weight of CaCO 3 to Corg and Corg to N for the sediment trap samples Filled symbols are the measured ratios The heavy line in the CaCO3 to Corg plot is the primary flux ratio extrapolated assuming degradation and dissolution at rates discussed in the text The open symbols are the ratios after adjustment by removing the contributions of CaCO-~ and Corg from the flux of resuspended sediment as calculated from the partitioning model Deviations of the trap material to the right of the extrapolated CaCO 3 to Corg primary flux ratio (the heavy line), and increases in the Corg/N ratio suggests lateral input of "old" resuspended material Similarly, decreases in the C~,rg/N ratio, the CaCO3/Corg ratio remaining close to the extrapolated primary flux ratio, and increases In the bulk flux (see Fig 6) suggests lateral input of "fresh" rebound aggregates

Aggregate profiles and se&ment trap fluxes

1831

where A1m is the measured flux of aluminum in •g cm -2 day -1, Alp is the primary flux of aluminum in/xg cm -2 day- 1 [A1]s is the concentration of A1 in the surface sediments in pg 1~g-1 estimated from the data in TREFRY(1977), adjusted for the higher CaCO3 percentage in the core measured by LIN and MORSE (1991) than measured in the cores used by TREFRY (1977). SlS the bulk flux of the sediment component in~tg cm-2 d a y - i . The contribution of A1 from resuspended sediment is given by the term {[AI]~ x S}, and Alar is the flux of A1 contributed by aggregate rebound in ~tg cm -2 day- 1 For organic carbon' Corg m = Corg p -I- {[Corg]~ X S} + {(Corg/Al)a r × Aldr},

(5)

where Corg m is the measured flux of organic carbon in/~g cm -~ day i Corg p is the primary flux of organic carbon in pg cm 2 d a y - 1 extrapolated to the depth of the particular trap using equation (1), [Corg]~is the concentration of organic carbon m the surface sediments in/~g ktg-1 estimated from the surface sediment sample of a core on the slope of the northwest Gulf of Mexico (LIN and MORSE, 1991), and (Corg/A1)ar is the mass ratio of organic carbon to A1 in the aggregate particles. The value of (Corg/Al)ar is assumed equal to the ratio in the primary flux extrapolated using equation (3) to the depth of the sediment surface (1450 m) The contribution of Corg from resuspended sediment is given by the term {[Corg]s X S}. The contribution of Corg from aggregate rebound lS given by the term {(Corg/Al)~r x Alar}. As only S and Al~r are unknown in equations (4) and (5), the equations can be solved simultaneously. For CaCO 3 the partitioning equation is: C a C O 3 m = C a C O 3 p + {[CaCO3]~ x S} + {(CaCO3/AI)~ r x Alar } + C a C O 3 r ,

(6)

where CaCO 3 m is the measured flux of CaCO3 in ~tg cm-2 day - 1 CaCO3 p is the primary flux of CaCO3 m ktg cm -2 day -1 extrapolated to the depth of the particular trap using equation (3), [CaCO3]~ is the concentration of CaCO 3 in the surface sediment in Bg/~g- 1 estimated from the surface sediment sample of a core on the slope of the northwest Gulf of Mexico (LIN and MORSE, 1991), S is the bulk sediment flux in ktg cm -2 day-1 determined from equations (4) and (5), (CaCO3/Al)dr is the ratio of CaCO3 to A1 in the aggregate rebound particles. The value of (CaCO3/AI)~r is assumed equal to the ratio in the primary flux extrapolated using equation (1) to the depth of the sediment surface (1450 m). Alar IS the flux of A1 In ~tg cm -2 day 1 contributed by aggregate rebound determined using equations (4) and (5), and CaCO3 r is the residual flux of CaCO 3 in ktg cm-2 d a y - x needed to balance the equation. The contribution of CaCO 3 from resuspended sediment is given by the term {[CaCO3]s x S}. The contribution of CaCO 3 from aggregate rebound is given by the term {(CaCO3/A1)a r × Alar}. The results of this partitioning model (parameter values are listed in Table 2) indicate that the 1236 m trap is dominated by the primary flux based on the small fraction of the measured fluxes of Corg (Fig. 7)~ A1 (Fig. 8) and CaCO3 (Fig. 9) contributed by the aggregate rebound and resuspended sediment components. This result supports the interpretation of the LPC profile as recording the primary flux of aggregates and that the intermediate nepheloid layers seen between 700 and 1100 m did not substantially impact the flux measured in the 1236 m trap. In contrast, the bottom two traps have aggregate rebound fluxes equal to or greater than the primary fluxes of Co~g(Fig. 7), AI (Fig. 8) and CaCO3 (Fig. 9) accounted for by the aggregate rebound component of the model, and

1832

I D WAzsHandW D G~RDNER

significant (approximately 10%) contributions from resuspended se&ments This result is supported by the LPC and beam attenuation profiles from 28 January 1988 which show nepheloid layers of both aggregates (Fig. 2) and small particles (Fig. 5) within at least the bottom 100 m. The partitioning of the 936 m trap is more problematic. Using a primary flux Corg/A1 ratio extrapolated to 1450 m for the aggregate rebound ratio (0.80) overestimates the aluminum flux, resulting in negative fluxes. By iteration, the ratio was adjusted to 0 85, a round number that gives a positive solution to the equations The CaCO3/AI ratio for the aggregate rebound particles was adjusted to the equivalent primary flux ratio (Table 2) For comparison, the Corg/Al ratio extrapolated from the primary flux to 936 m is 0.93. The adjusted Corg/A1 ratio (0 85) still overestimated the C a C O 3 flux (Fig 9), probably due to overestimating the CaCO3/AI ratio of the rebound aggregates However, the important result is that the excess material (above the primary flux) captured in the 936 m trap must be relatively fresh, in the sense that the blogenic to A1 ratios of the excess material are close to the ratios of the primary flux. This interpretation is supported by the fact that the CaCO3/Corg ratio measured in the 936 m trap is essentially equal to the ratio extrapolated from the 636 m trap, and the C,,rg/N ratio decreased between the 636 and 936 m traps (Fig 10). Degradation of nitrogen-rich compounds (mostly protein) should proceed faster than the overall rate for organic carbon, therefore the Corg/N ratio should increase with time, and assuming vertical settling, with depth. Since the CaCO3/Corg and Corg/N ratios in the 936 m trap are equal to or less than would be expected from degradation, the measured flux in the 936 m trap must include a secondary source of material that is at least as fresh as primary flux material If this source is rebound aggregates, it lmphes that the rebound aggregates have degraded over a period of time that is not significantly longer than the time it took for the primary material to settle to 936 m If the ultimate source of both the primary and rebound material is the euphoric zone, then the shortest path length is the vertical, and the path length of the rebound particles must be longer. Therefore, in addition to their downward velocity due to gravitational settling, the rebound aggregates must be advected downwards during some part of their journey to shorten their travel time. Such a mechanism has been inferred to lead to faster transport of organic rich particles to the deep water column on the continental slope south of New England than would be expected due to passive settling (BISCAVE et al., 1988) In that study the process of advection of particles down the slope was inferred to be the cause of the decrease in the Corg/N ratio with depth (rather than the expected increase) measured in sediment traps (BIsCAYE et a l , 1988) Downslope flows of 1-5 cm s -1 were observed (BUTMAN, 1988) during the time the BiscavE et al. (1988) sediment traps were collecting particles. While there are no current meter data from this study, the similarity of setting and results suggests that a downslope flow may have contributed to the flux measured in the 936 m trap A similar process is indicated for the bottom two traps in the Gulf of Mexico as the Corg/N ratio in these traps is essentially the same as that measured in the 636 m trap (Fig. 10). Also, adlustlng t h e CaCO3/Corg ratio measured In the bottom two traps by subtraction of the flux contributed by resuspended sediments yields a ratio closer to that extrapolated from the 636 m trap using equation (3) (Fig. 10). Thus, for the traps for which the aggregate rebound component is a substantial fraction of the total measured flux the aggregate rebound material was found to be compositionally fresh. This indicates at the least that the rebound process is quite rapid (1 e. the aggregates do not spend appreciable

Aggregate profiles and sediment trap fluxes

1833

time on the sediment surface) and possibly that they are advected downward in the slope enwronment. Ac knowledgements--We would like to thank the crew, captain and science parties of the R V Gvre cruises we pamclpated on. especially the chief scientists Drs John Morse, Douglas Blggs, John Wormuth and Eric Powell, and the Texas A&M University Department of Oceanography's Technical Support Ser'.lces Group for their help m acqmrmg the data We thank the Geochemical and Environmental Rese,uch Group ot the Departnlent ot Oceanograph} ol Texas A&M Umverslty lor donating the mooring cable Special thanks go to Dr Paul Boothe ol Texas A&M who performed the neutron activation analysis and Dr David Murray of Brov, n Umverslty who perlormed the C/N/CaCO~ analysis We thank Tom Stephens o1 the Texas A&M Unlxcr,,lty Electron Mlcro,,.opy Center tor patiently assisting m the use o1 the Center's nnage ploeessmg system Dr Vernon Aspcr and .in anonymous reviewer offered valuable comments on the manuscript Thl', v, ork was funded partially b 3 the Office ol Na,,al Re,.earch Contract N(III0-14-87-K-01(12 and the Texas Sea Grant College Program (Walsh)

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