GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 1, PAGES 21-24, JANUARY 1, 1997

Carbon-13 variations in the dissolved inorganic carbon in estuarine waters

William M. Sackett, Toedsit Netratanawong, and M. Elizabeth Holmes Department of Marine Science, University of South Florida, St. Petersburg

Abstract. The stable carbon isotope composition of dissolved inorganic carbon [DIC] was measured in Tampa Bay and Florida Bay. The dependence of isotopic composition was evaluated in terms of atmospheric CO 2 exchange, carbon exchange between fresh water and seawater (i.e. salinity) and DIC derived from the reaction between calcium carbonate and

organically derived COo. The extent of organic carbon oxidation and the

magnitude of organic carbon loading [pollution] in an estuary have implications for variations in the 8•3C of DIC and its use as an indicator of the relative amounts

of land and marine derived organic and inorganic carbon in paleogeographic studies.

Introduction

Stable carbon isotope compositions have proven extremely useful in many geochemical studies for process, origin, maturation and other determinations on naturally occurring organic and solid inorganic materials. Relatively little use has been made of •3C variations in the dissolved inorganic carbon [DIC] in estuarine studies. In this report we review several early case studies and report on several new studies on the Tampa and Florida Bay estuaries.

Experimental

All water samples were collected and sealed in screw cap glass bottles and refrigerated at about 10øC to inhibit bacterial decomposition of organic ma•ter and were kept in the dark to minimize photosynthesis. In the field, samples were kept on ice and in the dark. Bacterially derived CO 2 [6•3C=ca.-25 o/oo vs PDB] would alter, significantly, the indigenous 6•C of DIC [6•3C ranging from +1 to about -13 o/oo]. Photosynthesis would preferentially remove carbon-12 and cause an enrichment of carbon-13 in the DIC. Within a period of about one month, samples were analyzed for 1. chlorinity (titration with standard silver nitrate using sodium fluorescein indicator) and 2. 6•3C (reaction of 60 ml sample with 85% H3PO 4 in a partially evacua:cd side armed reaction vessel). Evacuation was limited so as to minimize loss of dissolved COe [ca. 2% at ph=•l. Complete loss of this

Copyright 1997 by the American Geophysical Union.

IDaper number 96GL03694. 0094-8534/97/96GL-03694505.00

21

2% of CO 2 would have only a minimal effect on the 613C value. After turning the reaction vessel and thus reacting the acid with DIC to produce CO2, product gases were passed through traps cocked with a liquid nitrogen-isopropanol slush [T=-80øC] to remove water vapor. The CO 2 was then frozen into a sample bulb with liquid nitrogen and sequesterd for isotope ratio mass spectrometer analysis. The reaction vessel was removed and shaken and the CO 2 extraction was repeated at least two times. Replicate analyses agreed to within 0.2 o/oo.

The sample gases were measured relative to a working standard with 6•3C=-0.15 o/oo versus PDB. Data are reported relative to PDB• Chlorinities were converted to salinities by multiplying by the universal factor, 1.805.

Early Studies

Before 1966 there were only a few sporadic values of DIC 6•3C found in the literalt, re. In 1966, however, Sackett and Moore [1966] reported 6•3C versus chlorinity relationships for the Hudson, Potomac and Mississippi River estuaries.

These data, given in Figure 1, all show 6n3C end member compositions between -9 and -11 at 0 chlorinity and about 0 at C1 = 18 o/oo. For Lower New York Bay, and for two samples in Mississippi Sound, points fall off the general trends. Apparently, rather saline samples at these two locations have equilibrated with organically derived CO2, suggesting an extraordinary source of biological oxidation. New York Bay x eceives the sewage outfall from New York City, the pres•a.med source of the organically derived CO 2.

In 1991, Mook and Tan summarized subsequent 8•aC data for various European rivers (Mook 1970), the Mackenzie River (Hitchon and Krouse, 1972) and the Amazon River (Longinelli and Edmonds 1983). The data all seem to be explained by the conceptual model given below (Figure 2). Mook [1970] also reported seasonal variations in t•3C which increases in the summers due to preferential uptake of C-12 by photosynthetic activity and returns to normal as a result of heavy rainfall and groundwater discharge in the fall.

Model To Explain •'13C Variations

The 6•3C model is based on the classical weathering reaction [CaCO 3 + CO 2 + H•O = 2HCO 3- + Ca 2+].

22 SACKETT ET AL.: CAi•-•ON -13 VARIATIONS IN 14S'I'UAPdNE WATERS

-lO

I ! I I [ I I i I I I I I I I I I-? I

0 5 10 15

Chlorinity (%o)

,, Surface Water Atlantic Ocean 12/13/63 o Lower New York Bay 12/13/63 ß Hudson River 4/14/64 o Potomac River 4/23/64

a, Mississippi Sound 6/5/64 zx Hudson River 7/29/64

Figure 1. Isotopic composition of DIC versus chlorinity for the Hudson River and Potomac River estuaries and Mississippi Sound (after Sackett and Moore, 1966).

CaCO 3 has a 6•3C of about zero [a typical value for marine limestones] and organically derived CO 2 has a value of about -26 o/oo [typical for C 3 land plants and the CO 2 derived from their oxidation]. For organic sediments in the Tampa Bay estua:'y and Mississippi Sound, fresh water end members have 613C values of-26 or -27 o/oo which fit this model, (Sackett and Thompson 1963 and; Sackett et al 1986). T•:,• weathering reaction thus produces HCO 3- with a 613C of about -13 [one carbon from each reactanti. On exchanging with atmospheric CO 2 [6•3C:-8 o/oo] the composition of bicarbonate gradually shifts to an equilibrium seawater value of near zero. In some restricted environments, the bicarbonate may also undergo exchange with additional organically derived CO 2 [6•3C :- 26] and shift towards -18 o/oo, (a value appropriate to equilibrium between gaseous CO2, with 6•3C=-26 o/oo and bicarbonate). If CO 2 is derived from oxidation of methane, the bicarbonate may become even lighter with values of about -30 o/oo [6•3C - CH 4 and deriveJ CO2 less than -60 o/oo reacting with CaCO 3 with 613C of about 0]. As suggested below these processes may be responsible for the nonlinearity in plots of salinity versus 6•3C [DIC] observed for Tampa Bay.

In an estuary, ideally, one would .see the mixing of two end members, fresh water and open ocean water with 6•3C values of- 13 and 0, respectively and a linear relationship between chlorinity or salinity and 6•3C. This is seen to be generally true for Tampa and Florida Bays.

Exchange Between CO 2 and DIC

The rate of exchange between gaseous CO 2 and dissolved bicarbonate is very important in understanding carbon isotope variations. In order to get a rough estimate of this exchange rate, two pan experiments were

CaCO3 (LIMESTONE, 8 ~ 0) +

H20 + ii2 (ORGANIC DERIVED, 5 -- -26) ' '- 5- -13 • 5--18

WiTH ATMOSPHERICE COa WITH ORGANIC DERIVED

5- -8 CO2, 5 - -26

Figure 2. Model for the change in DIC due to the weathering of limestone by organically derived CO2 and the exchange with atmospheric CO2.

run; one with a bicarbonate [613C .... 19.7 o/oo versus PDB], at a concentration close to that of the ocean and most fresh waters [2 X 103M], and another with the same bicarbonate concentration and a carbonic

anhydrase concentration of 5 mg/liter, probably much higher than in natural environments. Solutions in the pans had a volume of 8.6 liters and a surface area of 1808 cm 2. Since they were exposed to the outdoor atmosphere, there was usually a gentle breeze during the course of the experiments. Carbonic anhydrase which is commercially available is an enzy:ne produced by phytoplankton and other natural processes and has been shown to catalyze the exchange between gaseous CO 2 and DIC [Forster et al, 1969].

More recently Paneth and O'Leary [1985] showed in a laboratory study that the catalytic action of carbonic anhydrase has a carbon isotope effect of about 10 o/oo. Presumably, the effect would be much different [probably smaller] under the variable temperatures, salinities and anhydrase concentrations of natural environments.

As shown in Figure 3, the exchange rate is much faster in the pan with carbonic anhydrase (It 1/2 ca. 1 day) than without carbonic anhydrase (7 days). These experiments show the potential importance of gaseous exchange to 613C valves in natural waters. ".•

0.0 • [ i O • 'O. '

-5.0

-lO.O

45.0

-20.0,

-25.00

I i O' I o

O [] []

O Pan 1, with enzyme [] Pan 2. without enzyme

, I ,,, I , I I , I I ,I

50 100 150 200 250 300 350 400

Time (hours)

Figure 3. Change in/•3C of Die with time due to exchange with atmospheric CO2 in the presence and absence of carbonic anhydrase.

SACKETT lœ1' AI... CARB()N-13 VAR1ATI()NS IN I'S'I'IJARINt:: WA'I'I•RS 23

Results and Discussion 5 Field Study- Tampa Bay, March 1990

I •3C = -8.4556 + 0.11784 salinity ,'2 = 0.473; Sept. data % Samples were all collected from the shore in a one

day period and stored and preserved as given above. A 0 I- I A •eml•rData plot of salinity [S = 1.805 x C1] versus the/;•3C of the DIC ] i-1 May Data _

is shown in Figure 4. Linear regression analysis gives the I' equation 6•3C = -10.41+ 0.240 S with r2=0.812. The rather poor correlation coefficient is not due to a. -58• o• 0 - experimental variability but probably due to the variety • •ck•. •"• - [] of fresh water sources into Tampa Bay and inefficient • .- o end member mixing in some instances which allowed greater exchange with atmospheric and/or organically -10 derived CO2. The-10.4 intercept at zero salinity is

heavier than predicted by the model [-13 o/oo]. This f,,,,,,,,, •,,,,,,,,, I,,,,,,,,, •,,,, ..... I .... , .... could be due to factors such as Ca plant material less enriched in carbon-12 and/or exchange with atmospheric CO 2.

Field Study Tampa Bay, January 1993

These samples were taken from a boat during an east-west transect across Tampa Bay. These data, plotted in Figure 4, are represented by filled squares. The 1990 sampling has a better coverage for all salinites but a rather poor correlation coefficient (r 2 =0.812). The 1993 samples do not have as good a coverage with respect to salinity but have a much better correlation, r 2 = 0.945. The zero salinity intercepts are nearly identical, (-10.41 and -10.64).

813C = -10.087 + 0.19951 salinity R'2 = 0.609; May data -15

0 10 20 30 40 50 Salinity (%o)

Figure 5. 513C of DIC versus salinity of waters of the Florida Bay estuary in 1991 and 1992.

one of the authors, MEH, did an extensive sampling of Florida Bay water for three different periods in 1991 and 1992. 6•3C versus salinity relationships from this work are given in Figure 5. These relationships show a pattern similar to that shown for the Hudson, Potomac and Mississippi studies described above.

Field Study- Florida Bay, 1991 & 1992

As part of a MS thesis research to determine if fluorescent humic materials are responsible for the fluorescent banding in coral growing in the Florida Keys,

-2

-4

-10

-12

-14

Tampa Bay O March, 1990 ( ) I January, 1993 (__.) O

O

O

o o o

8 •3 C = -10.41(:L-0.79) + 0.239(:L-O.039)salinity, rZ=0.812

C = -10.64(:L-0.43) + 0.254(:t-0.019)salinity, r*'=0.945

0 5 10 15 20 25 30 35 40 Salinity (%0)

Figure 4. 613C of DIC versus salinity of waters of the Tampa Bay estuary in 1990 and 1993.

Effect of Varying Total CO 2 Concentration on (513C [DIC]

Although not apparent in the field studies given above, the concentration of total dissolved inorganic carbon in the fresh water end member determines the

shape of the/•3C vs salinity gradient for coastal waters. Consider, for ex, ample, the three cases where the fresh water [5•C=-13o/oo] has 1, 2, and 3 mM total CO 2 is mixed with seawater [6•aC =0] with 2 mM total CO 2. The predicted trends are shown in Figure 6. As most fresh waters have total CO2 similar to or somewhat less than sea water, most natural gradients tend to'be linear or concave downward.

Implications

Gradients for 6•dC-[DIC] vs salinity or chlorinity for all five studied estuaries indicate that the 6•3C of plankton will show isotopic compositions of photosynthesized organic carbon which vary with sample location in an estuary. These gradients with end member DIC compositions of + 1 and -13 are important to consider in other estuarine studies such as by Qian et al [1996] on stable carbon isotope variations of organic matter in estuaries. Light inorganic carbon should be seen in the composition of offshore carbonate deposits at times of major freshwater flooding of ceastal waters.

24 SACKETT lit AL.' CARBON -13 VAI•dA'I'IONS IN ES'I'UARINI5 WATERS

0

, I, i i i ,, i , i

ß I mM sumCO2 rw + 2 mM sumCO2 sw -I- 2 mM sumCO2 rw + 2 mM sumCO2 sw ß 3 mM sumCO2 rw + 2 mM sumCO2 sw

-15 - , 0 1•) "1 ' I ' ' I ' I I' 20 30

Salinity (%0)

Figure 6. The effect of different rivefine concentrations of total CO2 on mixing with sea water in an estuary; changes in 6•3C of DIC.

Conclusions

The measured gradients for 6•3C of D!C versus salinity or chlorinity in five estuaries indicates that the observed isotopic variations in dissolved inorganic carbon can be explained by the mixing of freshwater and saltwater endmembers and by interaction of limestone with organically derived CO 2 in the watershed and subsequent isotopic exchange of the DIC with atmospheric CO2 and organically derived CO•.

Acknowledgements .,

The authors wish to thank Mr. Xuewu Liu for his

help in running the gaseous exchange experiments, Mr. Chad Edmisten for his help with the graphic, Dr. Robert Byrne for his editoral assistance and Ms. Ingrid Williams

for preparing the camera ready copy.

References

Forester, 1L F., Edsal, J. T., Otis, A. B. and Roughton, F.J.W. eds. 1968, CO2: Chemical, Biochemical, and Physiological Aspects, NASA Syrup. Hayerford College, Aug. 20-21, 1968, NASA SP-188.

Hitchon, B. and Krouse, H. R. [1972] Hydrogeochemistry of the surface waters of the Mackenzie River drainage basin Canada- III, stable isotopes of oxygen, carbon and sulfur. Geochim. et Cosmochim. Acta 36, 1337-57.

Holmes, M. E. [1992] Fluorescence and stable isotopes in near shore waters of South Florida and their relation to fluorescent

banding in the coral, Montastrea annularis, MS thesis, Department of Marine Science, U. South Florida.

Longinelli, A. and Edmond, J. M. [19.83] Isotope geochemistry of the Amazon basin: a reconnmssance. J. Geophys. Res. 88, 3703-!7.

Mook, W. G. [1970] Stable carbon and oxygen isotopes of natural waters in the Netherlands, In: Proceedings IAEA Conference on Isotopes in Hydrology, Vien•a. pp 163-90.

Mook, W. G. and Tan, F. C. [1991] Stable carbon isotopes in rivers and estuaries in Biogeochemist•y of Major World Rivers [eds. E. T. Degens, S. Kempe, J. E. Richey] Scope 42 J. Wiley & Sons.

Paneth, P. and O'Leary, M. [1985] Carbon isotope effect on dehydration of bicarbonate ion catalyzed by carbonic andyrase, Biochemistry, 24, 5143-5147.

Qian, Y., Kennicutt, M. C., Svalberg, J., Macko, S. A., Bidigare, IL IL and Walker, J. [1996] Suspended particulate organic matter [SPOM] in Gulf of Mexico estuaries: compound-specific isotope analysis and plant pigment compostion, Org. Geochem [In Press].

Sackett, W. M. and Thompson, IL IL [1963] isotopic organic carbon composition of recent continenlal derived clastic sediments of the eastern Gulf of Mexico. Bull. AAPG 47, 525-528.

Sackett, W. M. and Moore, W. S. [1966] Isotopic variations of dissolved inorganic carbon, Chem. Geol. 1, 323-328.

Sackett, W., Netratanawong, T. and Holmes, M. [1991] Stable carbon and oxygen isotope variations of the Tampa Bay estuary, in Tampa Basis 2, 137-142 [eds Treat, S. F. and Clark, P. A.]

Sackett, W. M., Brooks, G., Conkright, M., Doyle, L. and Yarbro, L. [1980] Stable Isotopic compositions of sedimentary organic carbon in Tampa Bay, Florida, U.S.A.: implications for evaluating oil contamination. Appl. Geochem. 1, 131-137.

(Received February 23, 1996; revised October 28, 1996; accepted October 31, 1996.)