A model for interpreting continental-shelf hydrographic processes from the stable isotope and cadmium: Calcium profiles of scallop shells

Palaeogeography, Palaeoclimatology, Palaeoecology, 64 (1988): 123 140 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

123

A MODEL FOR INTERPRETING CONTINENTAL-SHELF HYDROGRAPHIC PROCESSES FROM THE STABLE ISOTOPE AND CADMIUM:CALCIUM PROFILES OF SCALLOP SHELLS D A V I D E. K R A N T Z 1, A N D R E W T. K R O N I C K 1"3 a n d D O U G L A S F. W I L L I A M S " 1Marine Science Program, University of South Carolina; Columbia, SC 29208 (U.S.A.) 2Department of Geological Sciences, and Belle W. Baruch Institute for Marine Biology and Coastal Research, University of South Carolina, Columbia, SC 29208 (U.S.A.) (Received October 1, 1987)

Abstract Krantz, D. E., Kronick, A. T. and Williams, D. F., 1988. A model for interpreting eontinental-shelf hydrographic processes from the stable isotope and cadmium:calcium profiles of scallop shells. Palaeogeogr., Palaeoclimatol. Palaeoecol., 64:123 140. Stable oxygen and carbon isotope, and cadmium:calcium profiles from the shells of scallops collected from the outer continental shelf of the South Atlantic Bight and the Virginia Bight appear to trace the hydrographic processes of the two regions. A model is developed to distinguish two principal modes of nutrient and trace metal input to the outer shelf from the biogeochemical profiles of the shell. The oxygen isotope profiles record bottom-water temperature changes caused by seasonal temperature cycles and water-mass movements. Nutrient input and subsequent phytoplankton blooms are recorded in the carbon isotope and Cd:Ca profiles. The shell profiles from the South Atlantic Bight record the dominant effect of upwelling caused by Gulf Stream meandering. The shell profiles from the Virginia Bight: show the influence of spring river runoff and summer water-column stratification. Biogeochemical profiles in mollusk shells are potentially important tools for studying the dynamics of both modern and ancient continental-shelf upwelling systems.

Introduction R e c e n t s t u d i e s of t h e g e o c h e m i s t r y of cadmium in the ocean have provided insights into o c e a n o g r a p h i c p r o c e s s e s r e l a t e d to p r i m a r y productivity and ocean circulation (Bruland, 1980; B o y l e et al., 1981; K n a u e r a n d M a r t i n , 1981; W a l l a c e et al., 1983; a m o n g o t h e r s ) . F o r a m i n i f e r a ihave b e e n s h o w n to i n c o r p o r a t e c a d m i u m i n t o t h e c a l c i t e t e s t s i n p r o p o r t i o n to cadmium concentrations in deep-ocean bottom

3present address: Department of Environmental Sciences, University of Massachusetts, Boston Harbor Campus, Boston, MA 02125 (U.S.A.). 0031-0182/88/$03.50

w a t e r ( H e s t e r a n d B o y l e , 1982). L i m i t e d evidence suggests that mollusks similarly incorporate c a d m i u m into shell c a r b o n a t e in proport i o n to a m b i e n t s e a w a t e r c o n c e n t r a t i o n s ( S t u r e s s o n , 1978; C a r r i k e r et al., 1980). A s s u m ing a relationship between shell Cd:Ca values a n d s e a w a t e r Cd c o n c e n t r a t i o n s , m o l l u s k s h e l l s s h o u l d c o n t a i n a r e c o r d of h y d r o g r a p h i c c o n d i t i o n s d u r i n g t h e life of t h e o r g a n i s m . T h i s a p p r o a c h h a s b e e n u s e d s u c c e s s f u l l y to correl a t e Cd c o n c e n t r a t i o n s i n t h e 2 0 - y e a r s k e l e t a l r e c o r d of a c o r a l f r o m t h e G a l a p a g o s I s l a n d s w i t h f l u c t u a t i o n s i n u p w e l l i n g c a u s e d b y t h e E1 Nifio-Southern O s c i l l a t i o n p h e n o m e n o n (Shen, 1986). O u r s t u d y i n v e s t i g a t e s t h e u t i l i t y of

;t~ 1988 Elsevier Science Publishers B.V.

124

Cd:Ca ratios in conjunction with stable oxygen and carbon isotope records from scallop shells to detect changes in primary productivity, seasonal hydrographic conditions and circulation in continental shelf areas. The dissolved and particulate forms of cadmium have been used extensively as biogeochemical tracers to identify water masses on the continental shelf and slope, and to trace mixing processes (Windom and Smith, 1972; Trefry and Presley, 1976; Wallace et al., 1983; Windom and Smith, 1985). The biogeochemical cycle of cadmium is primarily controlled by phytoplankton uptake at the surface and release at depth during decomposition. The vertical distribution of cadmium in the ocean is characterized by a surface depletion and a deep-water maximum, mimicing the distribution of the labile nutrients phosphate and nitrate (Boyle et al., 1976; Martin et al., 1976; Wallace et al., 1977; Bruland et al., 1978; Bruland, 1980; Yeats and Campbell, 1983). Cadmium concentrations are elevated in areas of upwelling and depleted in oligotrophic regions. Anthropogenic sources may affect cadmium concentrations locally or regionally (Turekian, 1977; Forstner and Wittman, 1981; Simpson, 1981). The general distribution of dissolved cadmium on the continental s h e l f is controlled by the relative inputs from two major sources, riverine flux and transported slope waters, and one minor source, atmospheric flux (Windom and Smith, 1985). The stable oxygen isotopic composition of mollusk shell carbonate is controlled by temperature and the isotopic composition of seawater, with little effect from physiological factors (Epstein et al., 1951, 1953; Horibe and Oba, 1972). On the outer continental shelf, the salinity and isotopic composition of seawater remain relatively constant throughout the year (Fig.l) (Lee and Atkinson, 1983; Atkinson et al., 1983). Therefore, the primary control on the ~ s O of shell carbonate is the variation of bottom-water temperature (Erlenkeuser and Wefer, 1981; Jones et al., 1983; Krantz et al., 1984). On the outer shelf, water temperature varies over two time scales: seasonal temper-

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ature cycles, and transitory temperature fluctuations caused by water-mass movements. The incremental shell sampling technique used in this study allows the identification of seasonal cycles as well as the more transitory variations which may relate to water-mass movements. The carbon isotopic composition of biogenic carbonates is a more complex system than that of oxygen isotopes because of the interaction with both physical-chemical and biological processes. Killingley and Berger (1979) and Arthur et al. (1983) proposed models which emphasized the control of phytoplankton productivity and water-mass movement on the 513C of mollusk shell carbonate. Phytoplankton fractionate CO 2 during photosynthesis producing organic matter which is isotopically very light (approximately -20%0). Isotopically light CO 2 is released from this organic matter during decomposition at depth causing a 13C depletion in the bottom-water XCO 2 (Kroopnick, 1974a, b, 1980). Seasonal changes in the 513C of seawater bicarbonate of continental shelf areas may therefore be controlled by hydrographic processes such as stratification

125

and upwelling, and the resulting patterns of p h y t o p l a n k t o n productivity. The (~13C of the seawater bicarbonate reservoir is also influenced by the input of terrestrial organic m at t er and dissolved carbon species (Mook and Vogel, 1968; Mook, 1971; Fritz and Poplawski, 1974). Additionally, physiological effects and food source may affect mollusk shell 5~3C values (Jones et al., 1986; T a n a k a et al., 1986; Krantz et al., 1987). Methods Living specimens of the calico scallop, Argopecten gibbus, were collected in J u n e 1984, from a water depth of 63m at two stations approximately 150km east of Savannah, Georgia (32~1.14'N, 79~23'W and 32°1.09'N, 7922'W). Isotope, trace metal and growth increment analyses were performed on three of these specimens (Kronick, 1986). Two specimens of the Atlantic sea scallop, Placopecten magellanicus, collected from 57 m water depth off the Virginia coast (37°15'N, 74°45'W) were used to complement the A. gibbus data. Detailed isotope analyses, growth increment and growth rate interpretations for these specimens have been discussed previously (Krantz et al., 1984, 1987). Only trace element analyses were performed on the P. magellanicus specimens as part of this study. Sampling of shells for stable isotope analyses followed standard procedures (Wefer and Killingley. 1980; Er lenke us er and Wefer, 1981; Williams et al., 1982). After grinding away the periostracum, carbonate powder samples were sequentially drilled from the outer shell layer along the axis of growth. The CO2 gas produced by reacting individual sample powders in purified phosphoric acid at 60°C was analyzed on a VG SIRA-24 isotope ratio mass spectrometer. The ~ 8 0 and 5~3C values are expressed in the standard delta (5) not at i on relative to the Pee Dee Belemnite (PDB) reference scale (Epstein et al., 1953). Overall analytical precision of both oxygen and carbon isotope data was +0.13%o s.d. For the trace element analyses, the shell

exterior was ground down to remove any fouling or encrusting organisms and to expose fresh calcite. The shells were washed in distilled water to remove any dust, and then sequentially sampled similar to the isotope samples. Powder samples for analysis averaged 7 mg, ranging between 4 and 17 mg. There was no evidence of contamination or anomalously higher Cd concentrations in lower weight samples. All wet chemical preparations of standards and samples were performed in a laminated flow system. The powdered carbonate samples were completely dissolved in a matrix of 0.2N double distilled nitric acid which was prepared using ultrapure distilled water (E. A. Boyle and G. Shen, pers. comm., 1984). A reducing/complexing pretreatment with hydrazine/citrate (Boyle, 1981; Hester and Boyle, 1982) was abandoned because a large percentage of the powdered CaCO 3 dissolved in the cleaning solution. Skipping this pretreatment step increases the risk of contamination: however, calcite removed from within the shell should have little contamination from surface coatings of cations such as Fe and Mn. Cadmium concentrations were determined with a Perkin Elmer 3030 Atomic Absorption Spectrophotometer with a graphite furnace accessory. Calcium concentrations were obtained using the flame atomic absorption accessory of the same Perkin Elmer instrument. Results are expressed as Cd:Ca ratios (ttmol Cd mol 1 Ca). Working standard curves were generated prior to each analytical run and following every ten samples using reagent blanks and a set of Cd standards prepared in a matrix solution of 0.2M CaCO 3. Procedural blanks were run through the entire dissolution procedure and analyzed with the samples. The mean Cd:Ca ratio for the procedural blanks was 0.0006+_0.0003 pmol mol ~ (1 s.d.). Replicate analyses of carbonate powder samples were run c o n c u r r e n t l y and on alternate days. The analytical precision for trace element analyses was _+0.004 gmol mol 1 (1 s.d.) based on 23 replicated samples. Although overall precision was acceptable, individual data points which lie above the general trend of the shell profile and have not been replicated

126

should be considered with caution because of the possibility of contamination.

Argopecten gibbus AG-2 - 1 , 0 - (a)

Results oo o o

Stable isotope profiles of Argopecten gibbus specimens Variations in the oxygen isotope profiles of the A. gibbus specimens (Figs.2a, 3a and 4a) are interpreted as being controlled primarily by seasonal temperature changes. "Light" or more negative 51sO values in the shell profiles indicate warm water temperatures during summer, and "heavy" or more positive values indicate cold water temperatures during winter. The effect of regional salinity changes on the shell isotopic composition is probably minimal since salinities on the South Atlantic Bight outer shelf remain relatively constant and above 36%0 throughout the year (Fig.l) (Lee and Atkinson, 1983). Specimen AG2 has a shell height of 36 mm and an oxygen isotope record with only one major cycle (Fig.2a). The ~lsO record has a range of 2.43%0, from -0.40%0 to +2.03%0. Using the calcite paleotemperature equation (Epstein et al., 1953), the temperature range recorded in the shell calcite of AG2 is approximately ll°C. The observed temperature range for continental-shelf bottom waters of the SAB at 50-70 m is approximately 13°C (Atkinson et al., 1983; Lee and Atkinson, 1983; unpublished data, NOAA-NODC, 1972-1978; Fig.l). Summer (warm) conditions are represented between 10-15 mm shell height in the profile by isotopically light values (Fig.2a). The transition to winter (cold) conditions occurs at approximately 18-20 mm shell height. The nature of this cycle indicates that specimen AG2 is approximately one year old and was spawned the year prior to collection. No powder samples were obtained from the umbo to 10 mm shell height because the outer shell layer was too thin to avoid contamination from the inner layer during sample drilling. The carbon isotope record of specimen AG2 has a variation of 0,96%0, ranging from

om 2.0

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Fig.2. Stable oxygen isotope (a), stable carbon isotope (b), and cadmium:calcium (c) shell profiles for the Argopecten gibbus specimen AG-2 which was collected alive from the South Atlantic Bight (SAB). The horizontal axis for all three graphs indicates shell height in millimeters from 0 mm at the umbo to 36 mm at the ventral margin. Each point plotted on the profile represents one carbonate powder sample drilled from the shell. Approximate seasons are designated on the horizontal axis of the 5'sO profile. "Winter" generally includes the months from J a n u a r y through May when bottom-water temperatures are relatively cold; "summer" includes the months from June through November or December. The positions of growth or disturbance lines which are visible on the shell exterior are indicated by the solid arrows along the horizontal axis.

127 -0.09%0 to + 0.87%o (Fig.2b). A large p o r t i o n of the r e c o r d (from 8 to 28 mm) shows v e r y little v a r i a t i o n . A general t r e n d t o w a r d s d e p l e t i o n o c c u r s after 28 mm shell height, w h i c h represents calcification d u r i n g late w i n t e r and spring a c c o r d i n g to the ~ s O profile. Specimen AG1A has a shell h e i g h t of 38 mm and also exhibits only one m a j o r ~lsO cycle (Fig.3a). A 2.08%o r a n g e b e t w e e n m i n i m u m and m a x i m u m v a l u e s of - 0 . 5 4 and + 1.54%o translates into an e s t i m a t e d isotopic t e m p e r a t u r e

A~'gopecte~ gibbus AG-1A -1,0.

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r a n g e of a p p r o x i m a t e l y 10.4~C. The single seasonal cycle suggests t h a t specimen AG1A is the same age as AG2 and was spawned the y e a r prior to collection. T h e shift in the 1so r e c o r d of A G I A t o w a r d s h e a v i e r v a l u e s and w i n t e r conditions o c c u r s at a p p r o x i m a t e l y 20 22 mm shell height, i n d i c a t i n g t h a t this specimen grew slightly faster t h a n specimen AG2. The 513C profile of specimen AG1A shows several a b r u p t n e g a t i v e shifts with v a r y i n g amplitudes at 13, 17 and 22 mm shell height. Specimen AG1 is larger, with a shell h e i g h t of 45 mm, and has c o n s i d e r a b l y more complex isotope profiles t h a n the o t h e r two specimens. The 51sO profile appears to h a v e two m a j o r cycles which c o r r e s p o n d to two years of shell g r o w t h (Fig.4a). The first w i n t e r in the profile begins at a p p r o x i m a t e l y 28 mm and the second at 38 mm shell height. The values in the first cycle r a n g e from - 1 . 0 2 to + 1.24%o, and those in the second cycle from - 0 . 3 9 to + 2.15%o. The t e m p e r a t u r e r a n g e s c a l c u l a t e d from these values are 8°C and 10"C respectively. A significant positive t r e n d in the 51sO profile b e t w e e n 14 and 22 mm p r o b a b l y r e p r e s e n t s an e x t e n d e d cooling event, but not w i n t e r conditions. The c a r b o n isotope profile of specimen AG] shows t h r e e cycles which peak with n e g a t i v e v a l u e s (Fig.4b). The first n e g a t i v e peak at 13 mm is followed by an a b r u p t positive shift w h i c h c o r r e s p o n d s to a similar t r e n d in the 51sO profile. A r e l a t i v e l y complete cycle bet w e e n positive v a l u e s at 15 mm and 28 mm has a n e g a t i v e peak at 23mm. This cycle is followed by a g r a d u a l t r e n d towards n e g a t i v e values n e a r the v e n t r a l margin.

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Cd:Ca profiles of A r g o p e c t e n gibbus specimens

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Shell Height (rnm) Fig.3.

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oxygen

and

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isotope

(a,

b),

and

cadmium:calcium (c) profiles for the A. gibbus specimen AG-1A collected from the SAB. Shell height at the ventral margin is 38 mm. Growth or disturbance lines are indicated by the solid arrows on the horizontal axis.

T h e Cd:Ca profiles of the t h r e e A. gibbus specimens are p r e s e n t e d in Figs.2c, 3c and 4c. T h e r a n g e s of the Cd:Ca v a l u e s are 0.003 0.015 pmol m o l - 1 for specimen AG2 (Fig.2c), 0.004-0.031 pmol mo1-1 for specimen A G I A (Fig.3c), and 0.006-0.035 pmol m o l - l for specimen AG1 (Fig.4c). C a r r i k e r et al. (1980) rep o r t e d a m e a n Cd:Ca v a l u e of 0.37 pmol mol 1 for the prismatic l a y e r of the shell from

128

Argopecte~ giblyus AG-1 -1.0

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3b Height (ram)

Fig.4. Stable oxygen and c a r b o n isotope (a, b), and cadmium:calcium (c) profiles for t h e A. gibbus specimen AG-1 collected from the SAB. Shell h e i g h t at the v e n t r a l m a r g i n is 45 mm. G r o w t h or d i s t u r b a n c e lines are indicated by t h e solid a r r o w s on t h e h o r i z o n t a l axis.

specimens of Crassostrea virginica which were grown in a tidal creek. This mean value is approximately one order of magnitude greater than values in the present study. The difference might represent the greater concentration of Cd in the seawater of the estuarine environment relative to the outer shelf (Sharp et al., 1982). The one year old specimens AG2 and AG1A appear to be recording very similar Cd:Ca trends (Figs.2c and 3c). The Cd:Ca concentra-

tions in both shells vary between approximately 0.005 and 0.015 #mol mol-1, with the exception of increased cadmium concentrations at 15 mm in the profile of specimen AG1A (Fig.3c). The profile of specimen AG2 also shows declining Cd concentrations from 15 to 2 0 m m (Fig.2c) but lacks the preceding peak observed in the profile of AG1A. The ~13C profile of specimen AG2 also does not contain the negative values shown between 10 and 18 mm in the profile of specimen AG1A, indicating that part of the seasonal record was not sampled from the shell of AG2. Similarly, we assume t h a t the corresponding Cd:Ca peak was not sampled from specimen AG2 and would probably appear at 10-12 mm. The second year of growth of specimen AG1 coincides with the records of specimens AG2 and AG1A. The Cd:Ca profile of specimen AG1 from approximately 34mm to the ventral margin seems to represent the equivalent record as in specimens AG2 and AGIA although the profile of AG1 is compressed by slower growth (Fig.4c). The remainder of the Cd:Ca profile of specimen AG1 includes a trend of declining Cd concentrations from 13 to 20 mm and a transient spike at 25 mm. As with the other two specimens, the "baseline" Cd:Ca values fall in the range of 0.005-0.015 #mol mol

- 1

Growth lines of Argopecten gibbus specimens Growth and disturbance lines on the exteriors of the three A. gibbus shells lend support to the shell age interpretations made from the 51sO records. No internal growth lines were observed in the sectioned halves of the specimens. Specimens AG2 and AG1A have "daily" growth increments along the shell exterior with a frequency of 4-6 increments per millimeter except near the margin where the frequency increases to 10-15 increments/mm. Both specimens have five disturbance lines indicated by major breaks in the repetitive record of the daily increments (Figs.2 and 3). The locations of these disturbance lines on the two specimens are very similar relative to their

129

shell heights, but are slightly offset because of minor differences in growth rate. Specimen AG1 shows "daily" external growth increments at a frequency of 4 6 increments/mm prior to 25 mm shell height. From 25 to 30 mm shell height the frequency increases to 7 10 increments/mm and the last 15 mm of the shell has a frequency of 11-15 increments/mm. The increase in the number of increments/mm indicates a decrease in the growth rate during the second year. Specimen AG1 has eight disturbance lines (Fig.4), almost twice the number of the other two specimens, which supports the i n t e r p r e t a t i o n of the 6~sO profile as representing two years of growth. Although the 6~80 profiles were used to determine the season in which growth or disturbance lines formed, no single line was found to represent a true annual increment in any of the shells.

Placopecten raagellaniczts PMI@ -1.0 ( 0 )

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Stable isotope profiles of Placopecten magellanicus specimens

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Oxygen and carbon isotope data for P. magellanicus specimens PM10 and PM26 are presented in Figs.5 and 6. A detailed discussion of these stable isotope records and the interpretations of growth rates and external line formation are reported in Krantz et al. (1984) and Krantz et al. (1987). As with the A. gibbus specimens, the cycles in the 6~sO profiles are interpreted as yearly cycles controlled by seasonal hydrographic conditions. Two major cycles are present in the 6~so profile of specimen PM10 (Fig.5a), indicating two years of growth and the beginning of a third. Oxygen isotope values have a range of 2.5%o, corresponding to temperatures of 6.5 14.5~'C. Specimen PM26 has four largeamplitude cycles in its oxygen isotope record (Fig.ba) with a 2.7%0 range. Estimated isotopic temperatures were from 5.7 to 16.5°C, consistent with the record of specimen PM1O and very close to observed seasonal bottom-water temperatures (Nickerson and Mountain, 1983). The last year of growth for specimen PM26 is compressed and the heavy ~ s o values charac-

S~mmer 50 60

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~ 0.040

0.030 ~5 0.020 C) 0.010 0.000

.

.

40

.

.

50

.

.

. 60

.

Shell Height (mrn)

Fig.5. Stable oxygen and carbon isotope (a, b) and cadmium:calcium (c) profilesfor the Placopectenmagellanicus specimen PM10 collected from the Virginia Bight (VB). Shell height at the ventral margin is 75 mm. Notice that the ~13Cand Cd:Ca scales differ between the A. gibbus and P. magellanicus specimens. teristic of the winter are t r u n c a t e d (Fig.ba). This is interpreted to represent cessation of shell growth during the winter and generally slower growth related to the onset of sexual m a t u r i t y (Krantz et al., 1984). The 5~sO profiles of both P. magellanicus specimens are offset in the positive direction from the profiles of the A. gibbus specimens as a result of the cooler water temperatures of the Virginia Bight.

130

Placopecten magellanicus PM26 -1.0 -

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(a)

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100 110 20 130

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(C)

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8 (.3

0.020

Cd: Ca profiles of Placopecten magellanicus specimens The Cd:Ca concentrations in the shell of specimen PM10 range from 0.009 to 0.037 gmol mo1-1 (Fig.5c). The Cd:Ca profile represents the second year of growth according to the 61sO interpretation (Krantz et al., 1984). The highest Cd:Ca values in the profile of specimen PM10 occur during the late winter or spring. The Cd:Ca profile of specimen PM26 represents all but the first year of growth and appears to show a trend similar to the Cd:Ca profile of the equivalent year in PM10. Relatively higher Cd:Ca values occur in the late winter or spring (Fig.6c). The lowest Cd concentrations are the same as those of specimen PM10 at 0.009 /~mol mol -~, but the maximum values are much higher at 0.064/~mol mol - 1. Both the P. magellanicus specimens are enriched in cadmium relative to the A. gibbus specimens. This enrichment might represent the higher seawater cadmium concentrations in the Middle Atlantic Bight compared with the South Atlantic Bight (Simpson, 1981). Alternatively, it could indicate a differential fractionation of Cd by the two species during shell production.

0.010 0.000

' lb ' 2b ' 5b ' 4b ' 5'0 ' sb ' 7'o ' 8'0 ' 9'o '15o'11o'1~o'1~o Shell Height (ram)

Fig.6. S t a b l e o x y g e n a n d c a r b o n i s o t o p e (a, b), a n d c a d m i u m : c a l c i u m (e) p r o f i l e s for t h e Plaeopeeten magellanicus s p e c i m e n P M 2 6 c o l l e c t e d f r o m t h e V i r g i n i a B i g h t (VB). S h e l l h e i g h t a t t h e v e n t r a l m a r g i n is 125 ram.

The carbon isotope records from PM10 and PM26 vary generally between + 1.0 and + 2.5%0 and have similar cycles for the equivalent years of growth (Figs.5b and 6b). Isotopically light (more negative) 513C peaks are associated with the summer or fall using the &1sO record to determine season. The 613C profiles were interpreted as primarily recording changes in the &~3C of the seawater bicarbonate as controlled by phytoplankton productivity on the continental shelf (Krantz et al., 1987).

Discussion We present here a model for the interpretation of combined isotope and Cd:Ca profiles from mollusk shells. However, several caveats must preface the discussion. It has not been sufficiently demonstrated by a controlled study that the Cd:Ca ratios of mollusk shell carbonate accurately reflect seawater Cd concentrations under normal environmental conditions. Our assumption of such a relationship is based on analogy with foraminifera (Hester and Boyle, 1982), corals (Shen, 1986), and limited data from mollusks (Sturesson, 1978; Carriker et al., 1980). With this working assumption, the temporal resolution of water-mass events as recorded in the shell isotopic and trace metal composition is on the order of three or four weeks per sample. Presently, only one study

131

(Sakamoto-Arnold et al., 1987) reports changes in Cd and n u t r i e n t concent r a t i ons of a parcel of seawater, a warm-core ring in this case, with a temporal resolution of one m ont h or better. Because of this lack of supporting data, the following interpretations of the shell profiles are largely speculative, but are based on known processes which control the biogeochemistry of the three parameters. Shell car bona t e 51sO values of mollusks living on the out er shelf are controlled primarily by water temperature (Epstein et al., 1953; Jones et al., 1983; Krantz et al., 1984). Shell 513C values are related to water mass movement and the a l t e rn a tio n between removal of 12C from the seawater XCO 2 pool by p h y t o p l a n k t o n productivity and release of 1zc during decomposition (Kroopnick, 1980; A r t h u r et al., 1983; Krantz et al., 1987). Cd:Ca ratios in seawater reflect water mass movements and removal of Cd by p h y t o p l a n k t o n (Boyle et al., 1976; Bruland, 1980; Wallace et al., 1983). In order to reasonably interpret the shell profiles from this study, it is first necessary to consider the hydrographic and biological processes which affect the distribution of ~sO, 13C and Cd on the outer continental slbelf. The specimens for this study were collected from two regions of the eastern United States outer continental shelf which have significantly different oceanographic characteristics: the South Atlantic Bight (SAB) and the Virginia Bight (VB).

Hydrography of the South Atlantic Bight The South Atlantic Bight is the continental shelf region from Cape Hatteras, N or t h Carolina, to Cape Canaveral, Florida. The Gulf Stream acts as an effective hydrographic boundary to the east. The SAB is divided into three distinct bathymetric zones based on physical characteristics of the water masses, p h y t o p l a n k t o n productivity and n u t r i e n t concentrations: the inner-shelf or nearshore/estuarine zone (< 20 m water depth), the mid-shelf zone (20 40m) and the outer-shelf zone (40 100 m) (Menzel et al., 1981). Salinity fronts

form barriers to o n s h o r e - o f f s h o r e t ransport and significantly limit material exchange between the rivers and the outer-shelf region (Hanson et al., 1981; Menzel et al., 1981; Yoder et al., 1981). The Gulf Stream controls circulation processes on the outer shelf of the SAB. Onshore meanders of the Gulf Stream bring warm, nutrient-depleted waters up onto the shelf (Atkinson, 1977). Alternately, offshore meanders cause the upwelling of cold, nutrient-rich slope water, which at times may intrude across half of the shelf (Atkinson, 1977). A related phenomenon occurs when cold-core eddies or spinoffs from the Gulf Stream impact the outer shelf. Doming of isotherms along the shelf break during these events is caused by' the t ransport of slope water onto the shelf (Atkinson, 1985). Three modes of intrusion have been recognized: overriding, interleaving and bottom-water intrusion (Atkinson, 1977). The mode of intrusion is somewhat seasonal in that it depends upon the relative density of the shelf, slope, and Gulf Stream water masses. Overriding, or the surface intrusion of warm Gulf Stream water, occurs most frequently during the winter, and bottom intrusions of cold slope water occur during the summer (Atkinson, 1977). Thermal stratification of SAB outer-shelf waters is most intense from J u n e t hrough September, and is followed by abrupt mixing in late October (Atkinson et al., 1983: Lee and Atkinson, 1983). Three major water masses i nt eract on the SAB outer shelf: shelf, slope, and Gulf Stream surface waters (Atkinson, 1985). Low salinity, high turbidity water is restricted to the inner and middle shelf. The water of the outer shelf is a mixture of warm Gulf Stream surface water and cold upwelled slope water. The Gulf Stream water is notably depleted of nutrients, particulate matter, and dissolved metals (Wallace et al., 1983). The slope water is typically 10 15"C colder t han the Gulf Stream water and is characterized by high nutrient, high dissolved metal and low dissolved oxygen concentrations (Atkinson, 1985). The n u t r i e n t and trace metal concent rat i ons of the Mope water

132 are very similar to water at 500m in the Sargasso Sea, and the water mass has been identified as Western North Atlantic Water (WNAW; also referred to as Central Atlantic Water) (Windom and Smith, 1985). The distribution of dissolved cadmium on the SAB shelf is controlled by water mass movement and mixing. The maximum concentration of dissolved Cd (up to 0.25 nmol 1-1) generally occurs at 30%o salinity on the inner shelf (Windom and Smith, 1972, 1985). River flux to the inner shelf is an important source of Cd but there is no significant anthropogenic input of Cd to the SAB outer shelf (Windom and Smith, 1985). The surface waters of the Gulf Stream are depleted of Cd (< 0.003 nmol 1-1) as a result of biological vertical removal (Wallace et al., 1983). Beneath the Gulf Stream, Cd concentrations increase rapidly below 100 m (0.03-0.09 nmol 1-1) to a maximum approaching 0.30 nmol 1-1 at 500m in the core of the slope water (Wallace et al., 1983). The turnover or regeneration time for the outer shelf water (0.14-0.59 year; Atkinson, 1985) is shorter t h a n the biological scavenging time for Cd (0.5-1.0 year; K n a u e r and Martin, 1981; Bruland and Franks, 1983; Wallace et al., 1983). Therefore, actual dissolved cadmium concentrations on the SAB outer shelf will reflect the relative inputs of the principal water masses through time.

Hydrography of the Virginia Bight The Virginia Bight is a section of the Middle Atlantic Bight (MAB) extending from Cape Henlopen, Delaware, to Cape Hatteras, North Carolina. Average temperature and salinity conditions of the study area are illustrated in Fig.1. Exchange between the estuaries and shelf waters of the VB is greater than the SAB because of greater riverine input (Bumpus, 1973; Beardsley et al., 1976). Freshwater runoff and warming in the late spring and summer cause intense stratification (Beardsley et al., 1976; Fischer, 1980). Stratification is destroyed in the fall by surface cooling and storm mixing. The Gulf Stream also influences the outer

shelf of the VB, but the effect is less direct than in the SAB because the axis of the Gulf Stream veers to the east at Cape Hatteras. The Gulf Stream axis is usually offshore in the summer, but may move closer to the shelf break in the winter (Stefansson et al., 1971). A distinct slope water mass is present in the VB. As in the SAB, the shelf water-slope water boundary migrates, largely in response to the position of the Gulf Stream (Wright, 1976; Halliwell and Mooers, 1979). The water masses of the VB include shelf, slope and Gulf Stream waters similar to the SAB. The VB shelf water differs from the SAB in that it includes considerably more river input and mixing from a fourth water mass, the cold pool (Bumpus, 1973; Han and Niedrauer, 1981; Houghton et al., 1982). The cold pool is a mass of extremely cold (frequently below 6°C), nutrient-rich, oxygen-depleted shelf water which remains on the outer shelf of the New York Bight through most of the summer. Because of shore-parallel circulation towards the south in the MAB, cold pool water which has mixed with resident shelf water can be identified in the VB (Church et al., 1984). The Middle Atlantic Bight has generally higher concentrations of dissolved Cd than the South Atlantic Bight, largely because of significant anthropogenic input (Simpson, 1981). Particulate Cd from sewage and industrial effluent is effectively removed by flocculation and sedimentation in estuaries (Klinkhammer and Bender, 1981; Sharp et al., 1982; Yeats and Bewers, 1983). However, there is still a net transport of dissolved Cd to the MAB, yielding 0.4-0.5 nmol 1 1 at 30%0 salinity on the inner shelf (Sharp et al., 1982). Bruland and Franks (1983) documented conservative mixing between the MAB outer shelf and the ocean with Cd concentrations of 0.20 and 0.002 nmol 1-1 respectively. This conservative mixing trend again implies that biological removal of Cd from shelf waters does not operate as quickly as water mass movement and mixing processes. Estimated flushing times for the VB shelf are 1-6 months (Stefansson et al., 1971).

133

Interpretation of A r g o p e c t e n gibbus shell profiles The (5~sO, (5~3C and Cd:Ca profiles of the t h r e e A. gibbus specimens are i n t e r n a l l y consistent (Figs.2 4). F o r the (51sO records, the lightest s u m m e r v a l u e s fall b e t w e e n - 0 . 5 and -0.7%o, and the h e a v i e s t w i n t e r values bet w e e n 1.5 and 1.8%o. The (51sO values for the v e n t r a l m a r g i n samples from each shell are close to + 1.5%o. These v a l u e s are r e a s o n a b l e c o n s i d e r i n g the early J u n e c o l l e c t i o n date preceded any significant seasonal w a r m i n g of outer-shelf waters. S e a s o n a l t e m p e r a t u r e r a n g e s of 10 or 1 1 C c a l c u l a t e d from the shell (5~sO v a l u e s c o r r e s p o n d well with the a c t u a l w a t e r t e m p e r a t u r e r a n g e of 13°C. T h e slight d i s c r e p a n c y b e t w e e n the r a n g e s p r o b a b l y relates to slower shell g r o w t h d u r i n g the w i n t e r as has been d o c u m e n t e d in o t h e r studies (Wefer and Killingley, 1980; J o n e s et al., 1983; K r a n t z et al., 1984). T h e (5~3C and Cd:Ca profiles for specimens AG1A and A O ~ I are c h a r a c t e r i z e d by cyclic trends p u n c t u a t e d by several a b r u p t peaks which a p p e a r to be c o r r e l a t i v e in some cases (Figs.3 and 4). The profiles of specimen AG2 (Fig.2) do not r e c o r d a complete y e a r of growth, and h e n c e some e x t r e m e (513C and Cd:Ca v a l u e s are not r e c o r d e d as in the o t h e r shells. The v a r i a t i o n s in both the c a r b o n isotope and cadmium d a t a are on the same o r d e r of m a g n i t u d e as m e a s u r e d v a r i a t i o n s in t h e i r r e s p e c t i v e s e a w a t e r p a r a m e t e r s . F o r example, the Cd:Ca r a t i o s from the shells a p p e a r to h a v e a " b a s e l i n e " b e t w e e n 0.005 and 0.010 /~mol mol 1 M a x i m u m values from spikes in the Cd:Ca profile a p p r o a c h 0.040 #mol mol ~. In comparison, s e a w a t e r dissolved Cd c o n c e n t r a tions on the SAB o u t e r shelf v a r y from 0.03 to 0.30 nmol 1 ~ (Wallace et al., 1983; W i n d o m and Smith, 1985). U n f o r t u n a t e l y , few d a t a are available for the (513C c o m p o s i t i o n of shelfw a t e r b i c a r b o n a t e . G E O S E C S d a t a for the open Atlantic: O c e a n show a surface enrichm e n t of 13C on the order o f l . 0 1.5%o r e l a t i v e to w a t e r below 500 m (Kroopnick, 1980). The t h r e e b i o g e o c h e m i c a l profiles of speci-

men A G I A r e c o r d one complete seasonal cycle (Fig.3). The (51sO profile t r a c e s the seasonal t e m p e r a t u r e c h a n g e s from light v a l u e s in the s u m m e r to h e a v y v a l u e s in the winter. S u m m e r and late s u m m e r are r e p r e s e n t e d by the shell samples b e t w e e n 10 and 20mm. W i n t e r t h r o u g h late spring is r e p r e s e n t e d by the r e m a i n d e r of the shell (20-40 mm). The (513C profile of specimen A G I A shows a general t r e n d of light v a l u e s in the s u m m e r and h e a v y v a l u e s in w i n t e r (Fig.3b). This p a t t e r n m a y r e l a t e to t h e r m a l s t r a t i f i c a t i o n and buildup of isotopically light CO 2 from oxidized o r g a n i c m a t t e r below the t h e r m o c l i n e d u r i n g the s u m m e r ( A r t h u r et al., 1983). F r o m late fall t h r o u g h spring, the w a t e r c o l u m n of the SAB is well mixed, and Gulf S t r e a m s u r f a c e - w a t e r e x c u r s i o n s o n t o the shelf are more common. B o t h of these factors tend to i n c r e a s e the (5'3C of the b o t t o m waters. A b r u p t (513C c h a n g e s observed in specimen A G I A probably r e l a t e to the i n j e c t i o n of n u t r i e n t - r i c h upwelled w a t e r and s u b s e q u e n t p h y t o p l a n k t o n blooms. Phytop l a n k t o n blooms can r e m o v e most. of the n i t r a t e and p h o s p h a t e from an upwelled parcel of w a t e r w i t h i n 5 7 days ( H o f m a n n et al., 1980; Yoder et al., 1983; Yoder, 1985). P r e s u m a b l y an equally rapid r e m o v a l of '2C can o c c u r d u r i n g periods of e n h a n c e d p r o d u c t i v i t y following an upwelling e v e n t a l t h o u g h d a t a are not presently available. C h a n g e s in b o t t o m - w a t e r (513C can be affected d i r e c t l y by p h y t o p l a n k t o n blooms w i t h i n a n u t r i e n t - r i c h i n t r u s i o n because of the v e r y deep photic zone on the o u t e r shelf ( C h u r c h et al., 1984; Yoder, 1985). Cold slope w a t e r i n t r u s i o n s onto the SAB shelf would d r a m a t i c a l l y i n c r e a s e both the n u t r i e n t and dissolved Cd c o n c e n t r a t i o n s . E l e v a t e d Cd:Ca ratios at a p p r o x i m a t e l y 15 mm on the shell profile of specimen A G I A (Fig.3c) a p p e a r to be r e c o r d i n g a late s u m m e r upwelling event. C o n c u r r e n t with the Cd:Ca e v e n t is a significant e n r i c h m e n t in (5tsO c o r r e s p o n d i n g to a drop in t e m p e r a t u r e of 3 or 4 C. The e n r i c h m e n t in the (513C profile immediately p r e c e d i n g the Cd:Ca e v e n t at 15 mm may have been caused by rapid r e m o v a l of' '2C by p h y t o p l a n k t o n d u r i n g the same event. Intense

134 stratification during this late-summer event might have enhanced the water-column 513C changes by essentially restricting the changes to water below the thermocline. The decline in the Cd:Ca ratios and more positive 513C values after 15 mm shell height appear to trace the history of the water mass following the upwelling event. If Cd is removed from the water column more slowly than the nutrients, the Cd:Ca ratios will remain high until water-mass movement, diffusion or gradual depletion by phytoplankton reduce the dissolved Cd concentration. In contrast, oxidation of phytoplankton organic matter would reintroduce 12C into the seawater bicarbonate pool soon after the bloom deteriorates. It should be emphasized t h a t the 513C record is only an indirect measure of the dynamics of the phytoplankton bloom and n u t r i e n t concentrations. The hypothesis that the removal of Cd and nutrients occurs at differential rates in an upwelled parcel of water requires testing by frequent sampling following an upwelling event. Under steady-state conditions, Cd and phosphate concentrations are linearly related in open-ocean systems (Boyle et al., 1976; Bruland et al., 1978; Bruland, 1980; Yeats and Campbell, 1983). Sakamoto-Arnold et al. (1987) documented with monthly sampling that Cd is removed from a warm-core ring (non-steadystate conditions) in molar ratios with the nutrients phosphate and nitrate. However, the warm-core ring represents the converse case to continental shelf upwelling in t h a t lateral or vertical diffusion and advection inject nutrients and trace metals into a nutrient-depleted core. The linear relationship between the removal of Cd and nutrients may or may not hold for the high nutrient concentrations and changes in the phytoplankton assemblage (Yoder, 1985) associated with upwelling. It is currently not known whether Cd is actively taken up by phytoplankton cells, similar to nutrients, or whether it. is passively removed, possibly by adsorption to protoplasm surfaces. The mechanisms and rates of Cd versus nutrient removals, and the relationship to seawater

513C changes, in non-steady-state systems need to be more rigorously quantified. Although specimen AG2 is the same age as specimen AG1A, the early growth of AG2 was much slower (Fig.2a), and the beginning of the shell profile at 10 mm represents a time later than t h a t of AG1A. Consequently, the peaks at both 513C and Cd:Ca shown in the profiles of specimen AG1A do not appear in the profile of AG2 (Fig.2). Similarly, light 51so values in the p~ofile of specimen AG2 represent only the very end of the summer. The 51so profile of specimen AG1 shows two annual cycles which are separated by a winter at 28mm shell height (Fig.4a). Based on a comparison of all three biogeochemical profiles, the shell record of specimen AG1 from 35 mm to the ventral margin represents the same period of time as the records of specimens AG1A and AG2. The 51so values at 35 mm in the profile of specimen AG1 represent late summer, similar to the values at 10 mm for the other two specimens. Both the Cd:Ca and 13C profiles in all three specimens show similar trends and reasonably similar values. Discrepancies in the 513C values between AG1 and AG1A in particular might relate to differences in growth rate. For specimen AG1, slow shell growth during the second year compresses the profile and reduces the temporal resolution. The profiles of specimen AG1 provide good evidence for an extended slope-water intrusion event during its first year of growth. A schematic model of the response of the three biogeochemical profiles to the upwelling event is presented in Fig.7. A very distinct positive 3180 anomaly of almost 1.5%o between 12 and 22 mm represents a cooling of approximately 7°C. Coincident with the 3180 anomaly is an extremely large change in 513C of 2.0%o. These enrichments might represent both a flushing of the outer shelf with new water and an extreme depletion of lac by phytoplankton uptake. The trend towards negative 513C values between 15 and 24 mm would then represent the gradual regeneration of 12C as organic matter is oxidized. The elevated Cd:Ca ratio at 14 mm shell height and the gradual depletion between

135 eral, a positive 5180 anomaly representing cold water would be expected to coincide with a Cd spike caused by upwelling. Very low Cd concentrations and positive 513C values during the winter are consistent with mixed conditions, which would flush the bottom water, and with the increased tendency of the Cd- and ~2Cdepleted Gulf Stream to meander onto the outer shelf.

SOUTH ATLANTIC BIGHT Upwell[ng event 0 , 0

~

/

o.o-

/ x~]' /

13 -

Bottom water temperatures depressed

23 •

l- 2.0

Phytopiankton bloom, 120 uptake /%

Foil mixing 0

//

"1

S~',

'1 / ' -10

of 12C OC4O A~ l

0.020 -

~Gradual Transient removai ~ upwelling ~-L~xreor~Cd ,'11 event

~pwelhng event, \\~ nutrient ond trace ", metal [nDut gOfO •

~..~.

I

dun Sep

i Oct-Dec

aan-Mmy

Season

Fig.7. Schematic representation of the influenceof hydrographic processes on the biogeochemical profiles of a mollusk specimen from the SAB. Data are from the first year of growth of!specimen AG-1.

14 and 22mm also supports this scenario. Following an injection of nutrient- and Cdenriched slope water, Cd is removed gradually by mixing, diffusion and phytoplankton uptake. The shell profiles of specimen AG1 between 25 and 30mm represent the transition into winter (Fig.4a). Relatively abrupt enrichments in both the 613C and 5180 profiles appear to correspond to cooling and water-column mixing during autumn and winter. Cadmium also attains very low concentrations at this time. One transient spike in Cd at 25ram might represent a brief upwelling event; however, because this point was not replicated, it could also indicate sample contamination. In gen-

Interpretation of Placopecten magellanicus shell profiles The hydrography of the Virginia Bight is affected more by river input and less by the Gulf Stream than the South Atlantic Bight. The character of the biogeochemical profiles of the two Placopecten magellanicus specimens appears to reflect this hydrographic difference. Instead of recording repeated upwelling events as in the A. gibbus from the SAB, the biogeochemical profiles of the P. magellanicus specimens from the VB seem to be most influenced by spring river runoff, summer water-column stratification, and autumn destratification and mixing. As with the A. gibbus specimens, the 51sO profiles of the P. magellanicus specimens are primarily controlled by seasonal temperature cycles (Figs.5a and 6a). The measured shell 51sO values fall very close to those predicted from hydrographic data (Krantz et al., 1984). Specimen PM10 shows two complete years of growth with late-summer peaks in the 5180 profile at 15 and 55 mm shell height (Fig.5a). Specimen PM26 has four years of growth and summers occur at 14, 52, 95 and ]18mm (Fig.6a). The 513C profiles of the two specimens show a general cyclicity with heavier values occurring more frequently in the winter and early spring, and lighter values occurring in the summer and early fall (Figs.5b and 6b). The Cd:Ca profiles also show a seasonality which is very similar to the 513C record. The highest Cd:Ca ratios occur consistently in the spring, with minor peaks occurring throughout the year (Figs.5c and 6c). In all but two cases, an increase in the Cd:Ca ratio in the shell profile

[36

VIRGINIA BIGHT

1°1

Maximum bottom-water

temperature

0,0

?

:¢

1.0

Reduced so!inity

2.0 3.0 0,0

S ring phytoplankton

-~

Fall mixing

Gradual release

1.0

LII

of 120

0,070 0.060 Gradual removal of Cd

0,050 0.040 0,030 0.020 0.010 0,000

Spring river runoff, nutrient and trace metal input

I Joo-Mo.I

Apr-Jun

.n-May

Season

Fig.8. Schematic representation of the influence of hydrographic processes on the biogeochemical profiles of a mollusk specimen from the VB. Data are from the second year of growth of specimen PM10.

corresponds with a positive shift in 513C. As with the SAB A. gibbus specimens, this correlation between the Cd:Ca and 513C responses strongly suggests common causal processes. The biogeochemical patterns of specimens PM10 and PM26 are interpreted as recording the injection of water rich in nutrients and trace metals from increased river input during the spring. The data from the second year of growth in specimen PM10 are presented in Fig.8 as a model of this process. Freshwater input from rivers is sufficient to cause a 2%0 drop in salinity (Fig.l) and corresponding light 51sO values in the shell profile (for the relationship between salinity and seawater 51sO, refer to: Epstein and Mayeda, 1953; Fairbanks, 1982). Shell 5180 values calculated

with a constant salinity of 35%0 are plotted as a dashed line in the 51sO profile (Fig.8). The shaded area therefore represents the effect of decreased salinity in the spring caused by increased runoff. Phytoplankton blooms on the shelf during the spring remove nutrients, ~2C and Cd from the water column. Stratification intensifies in the late spring and summer, and 12C is released below the thermocline during decomposition of phytoplankton organic matter (Arthur et al., 1983). As in the model for the Cd records from the SAB, we assume that Cd regeneration proceeds slower than removal by phytoplankton, and the Cd:Ca profile will trace continually declining concentrations unless new Cd is brought in by water-mass movement. Destratification and mixing in October or November flushes the bottom waters of the VB, and the 513C values become more positive. The exact timing and magnitude of these events will vary from year to year depending upon weather conditions and the effect of major storms. Even so, the general pattern appears to be consistent as evidenced by the longer record from specimen PM26 (Fig.6).

Comparison of Virginia and South Atlantic Bight specimens The differences between the biogeochemical profiles of the mollusks from the two regions appear to be consistent with the known differences in the hydrographic processes which dominate the Virginia and South Atlantic Bights. As discussed above, upwelling processes in the SAB (Fig.7), and seasonal river runoff and thermal stratification in the VB (Fig.8), control the pattern of the Cd:Ca, 513C and 51sO profiles from the scallop shells. Other, more general differences exist in the data. Cadmium concentrations in the VB specimens, especially PM26, are higher than those of the SAB specimens. These elevated concentrations are probably the result of the increased influence of river influx and greater anthropogenic input of Cd to the Middle Atlantic Bight. The 51sO values of the P. magellanicus specimens are approximately 1.0%o

137 heavier than those of A. gibbus. On average, temperatures in the SAB are 10°C warmer and salinities 2%o higher than the VB throughout the year (Fig.l). The temperature difference, if taken alone, translates into 2%0 heavier ~ s O values for P. magellanicus. However, this effect is offset by 1%o lighter 5~sO values caused by the lower salinity of the VB; the net effect thus being 1%o heavier 5~80 values for the VB. The 5t3C values for the SAB specimens are generally 1.5%0 lighter than those of the VB. Regional differences in three environmental factors might explain these lighter 513 C values: water temperature, upwelled slope water, and sedimentary organic matter. The 10°C difference in mean temperature between the SAB and VB might influence the shell 5~3C as well as the 5xso. Temperature dependence for the fractionation of ~3C between seawater bicarbonate and biogenic carbonate has been suggested in previous work, however, the results have often been contradictory. McCrea (1950) discussed theoretical reasons why 313C should decrease with increasing temperature, similar to the effect on 51sO. In apparent contradiction to these arguments, Emrich et al. (1970) determined a +0.035%o°C-~ temperature dependent fractionation of ~3C during the inorganic precipitation of a mixture of calcite and aragonite. Applying the Emrich et al. (1970) relationship to the scallop data from this study would result in expected 513C values for the SAB being 0.35%0 heavier than the VB; the opposite trend is actually observed. Grossman and Ku (1986) present evidence for a -0.11 to 0.13%o'~C ~ temperature dependent fractionation of 13C in aragonitic benthic foraminifera and mollusks, but no significant fractionation in calcitic benthic foraminifera. If the 1.5%o offset between the 513C values of the calcitic scallops from the SAB and VB is solely an effect of the 10°C temperature difference, the resulting _0.15%ooc 1 value is reasonably consistent with those of Grossman and Ku (1986). However, the scallop data do not necessarily support the contention t h a t temperature directly controls 5~3C. All three -

shells from the SAB (Figs.2-4) show significant trends in which 613C becomes lighter while ~180 remains heavy. Alternatively, the profiles of specimen AG1 show a series of positively covarying fluctuations between 10 and 30 mm shell height (Fig.4). The fluctuations in 513C are greater than those of the 3180 profile and would imply a temperature dependent fractionation of 13C of approximately 0.3%oOC 1, which seems unreasonable. The two specimens from the VB also frequently show a positive covariance between ~180 and 513C (Figs.5 and 6), but the two signals are slightly out of phase and there are cases of distinct negative covariance. These data are certainly not conclusive but they do strongly suggest that while temperature may have some effect on the 5~3C of calcitic mollusk shells, other factors must exert considerable influence. In this case, the 513C differences probably relate to the distinctly different hydrographic regimes of the SAB and VB. More prevalent upwelling in the SAB relative to the VB might be a significant cause for the ~13C difference. The repeated introduction of upwelled slope water onto the shelf should result in a depletion in the 5~3C of the dissolved inorganic carbon. Greater primary productivity caused by upwelling in the SAB may also contribute to the difference in shell 6~3C values from the two regions. The dissolved bicarbonate in pore waters of sediments with a high percentage of organic carbon is depleted of ~3C because of the generation of isotopically light CO 2 (Presley and Kaplan, 1970; Claypool and Threlkeld, 1983; McCorkle et al., 1985). Scallops typically scoop out a depression in the surface of the sediment and live semi-infaunally. Therefore, at least some of the bicarbonate incorporated into the shell might be from isotopically light pore water. If the SAB has a significantly greater percentage of sedimentary organic carbon than the VB because of the upwelling, the A. gibbus specimens would be expected to have more negative 6~3C values. The incorporation of Cd from the pore waters is also highly likely, and this process should be considered in future studies.

138 The interpretive model presented in this paper addresses the p r o b a b l e r e s p o n s e of the s t a b l e o x y g e n a n d c a r b o n i s o t o p e , a n d cadm i u m : c a l c i u m c h e m i s t r y o f s c a l l o p s h e l l calc i t e to h y d r o g r a p h i c p r o c e s s e s o c c u r r i n g o n t h e o u t e r c o n t i n e n t a l shelf. T h e r e s o l u t i o n of h y d r o g r a p h i c e v e n t s is l a r g e l y t i e d to t h e s e r i a l s a m p l i n g t e c h n i q u e a n d t h e u s e o f t h r e e complementary biogeochemical tracers. Water-column isotope and trace element data collected o n a t e m p o r a l s c a l e of d a y s o r w e e k s d o c u m e n t ing an upwelling event and the subsequent r e m o v a l o f n u t r i e n t s a n d c a d m i u m w o u l d be n e c e s s a r y to t e s t t h e p r o p o s e d m o d e l . I f t h i s m o d e l c o u l d be v e r i f i e d i n m o d e r n s y s t e m s , t h e t e c h n i q u e c o u l d b e u s e d to i n v e s t i g a t e presumed upwelling systems in the geologic past b y u s i n g w e l l p r e s e r v e d fossil s p e c i m e n s .

Acknowledgements W e w o u l d l i k e to e x p r e s s o u r a p p r e c i a t i o n to E l i z a b e t h Blood, N o r i m i t s u W a t a b e a n d Dougl a s J o n e s for h e l p f u l c o m m e n t s a n d c o n s t r u c t i v e c r i t i c i s m s d u r i n g v a r i o u s p h a s e s of t h e p r o j e c t . W e t h a n k G o r d o n W a l l a c e for s h a r i n g and discussing cadmium data from the South A t l a n t i c Bight. The i n s t r u c t i o n s and suggest i o n s provided by E d w a r d Boyle a n d G l e n S h e n w e r e i n v a l u a b l e to s e t t i n g u p for t r a c e e l e m e n t analyses. E d w a r d Boyle, W i l l a r d M o o r e a n d Douglas Jones made comments on an earlier draft which improved the final manuscript. We a r e a l s o g r a t e f u l to G l e n n U l r i c h , w h o m a d e i t p o s s i b l e for A T K to p a r t i c i p a t e o n a r e s e a r c h c r u i s e o f t h e R~ V Delaware H. T h i s p r o j e c t w a s s u p p o r t e d by funds from the M a r i n e Science P r o g r a m a n d t h e Office of t h e G r a d u a t e S c h o o l of t h e U n i v e r s i t y Of S o u t h C a r o l i n a , a n d b y N a t i o n a l Science F o u n d a t i o n G r a n t EAR8517207. T h i s is C o n t r i b u t i o n N u m b e r 699 of t h e B e l l e W. B a r u c h I n s t i t u t e .

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