a __

Nuclear Instruments and Methods in Physics Research B 123 (1997) 443-446

__

!!I!!

b!lOW B ELSEVIER

Beam Interactions with Materials & Atoms

Radiocarbon in marine dissolved organic carbon (DOC)

M. Le Clercq a. * , J. Van der Plicht a, H.A.J. Meijer a, H.J.W. De Baar b

’ Center for Isotope Research, Nyenborgh 4, 9747 AG Groningen, Netherlands b Netherlands Institute of Sea Research (NIOZ), Texel. Netherlands

Abstract

Dissolved Organic Carbon (DOC) plays an important role in the ecoIogy and carbon cycle in the ocean. Analytical problems with concentration and isotope ratio measurements have hindered its study. We have constructed a new analytical method based on supercritical oxidation for the determination of ‘3 and ‘46 of DOC. Initial results from the North Sea show

that a large fraction of DOC is refractive and no proof for terrestrial influence.

1. Introduction

Dissolved organic carbon is defined as all the organic carbon present in seawater that passes a 0.2 Frn filter. It is a mixture of simple substances such as sugars, fatty acids

and alkanes, and of complex polymeric molecules [ 1,2]. These have a wide range of molecular weights and are often referred to as “humic-acids”. They can be present as truly dissolved molecules, as colloids or as viruses [3-51. Although DOC concentrations are very low (about 50 p,M below 1000 m), the total amount roughly equals that of

atmospheric CO,. Biological processes such as the decay of plankton,

material loss in grazing by zooplankton and the execration

of extra-cellular material produce DOC. A part of this material is very unstable and breaks down to CO, in times ranging from hours to a year, partly by bacteria which use

this DOC as a food source. [6]. The refractive part of DOC can circulate through the oceans on time scales of thou- sands of years [7]. The mechanisms of formation and

removal of the refractive DOC are not known [8]. Together with the formation and decay of sinking

organic particles, DOC can remove nutrients and carbon

from the surface of the ocean (the so-called biological pump). The relative contribution of particulate and dis- solved transport is still unresolved [8,9].

Measuring DOC concentrations in seawater is very

difficult [lo]. The method most applied is the High Tem- perature Catalytic Oxidation (HTCO). Here a sample of seawater is acidified, the CO, is stripped from the water and 200 ~1 are injected into a quartz furnace. Next the resulting amount of CO, is measured. Other oxidation

l Corresponding author. Fax: + 3 l-50-363-4738; email:

leclercq@phys.rug.nl.

methods use UV light, wet oxidation or freeze drying followed by sealed-tube combustion. Contamination during sampling or oxidation is a problem since the concentration of DOC is very low, and organic carbon is omnipresent. Dry combustion methods suffer most from this problem. [ 1 I]. The second problem is the chemical stability of DOC.

It is unclear whether the UV and wet chemical oxidation oxidize all the DOC. The formation of chlorine could interfere with the oxidation [ 121.

For accurate measurements of carbon isotopic ratios, the sample volume has ,to be 500 ml. This renders 300 p,g carbon, enough for an AMS ‘“C measurement. Making the HTCO system suitable for such a large volume is techni- cally very difficult. UV systems are suitable for large sample volumes and have been used for ‘“C measurements [7]. In view of the confusion concerning the measurement of DOC, we decided to construct a new method, suitable for large sample volumes [ 131. This paper describes the current status of the project. The first results from a set of North Sea surface samples are shown.

2. Experimental methods

The destruction of waste waters in the chemical indus- try often requires the complete oxidation of very refractive, biologically not degradable, organic material. A solution to this problem is an oxidation system in the supercritical

phase. This method can destruct very stable compounds such as chlorophenol [ 141. We have employed this tech- nique to construct a DOC oxidation system (Fig. 1). A more detailed description of it will be published later.

One liter of seawater is acidified with 200 ~1 of sulfuric acid and placed in a glass bulb. The CO, is removed by stripping for 30 min in vacuum. The sample is

0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved

PII SOI 68-583X(96)00721 -5 X11. ENVlRONMENTAL/PALEOCLIMATIC STUDIES

444 M. Le Clercq et al./Nucl. Instr. and Meth. in Phys. Res. B 123 (1997) 443-446

Fig. 1. A schematic overview of the supercritical oxidation sys-

tem.

then saturated with high purity oxygen. A HPLC pump pressurizes the sample to 350 bar. Next it flows through a

ceramic tube at 650°C where all organic material is oxi- dized. The elevated pressure prevents the crystallization of salts from the seawater [ 151.

The construction of this reactor was the main obstacle in this method. Seawater at high pressures and tempera- tures is an extremely corrosive medium. Even very corro- sion-resistant materials, such as titanium, Hastalloy@ and steal coated with gold or enamel are resistant. Alumina is the only material that can withstand the conditions in our oxidation reactor. However construction for high pressure, high temperature alumina reactors were not known from literature. We have developed a good working system,

described in detail in [ 161. A metal tube. surrounds the alumina tube. Two clamps

connect the tubes to the HPLC pump and the capillary tubing. The seawater flows through the alumina tube from left to right. An equal flow of distilled water is flowing between the alumina tube and the steel surrounding tube in the opposite direction as the seawater. It heats the seawater on the left before the oven and cools it after the oven so the O-rings remain at room temperature. The pressure of the distilled water is 345 bar; the seawater is at 350 bars. With this construction there is little mechanical stress on the brittle alumina tube and no danger of contamination from the water flow.

A length of PEEK capillary tubing reduces the pressure of the sample. Seawater and resulting CO, are collected in a vacuum-drawn glass bulb. After 5 h of pumping enough water has passed the reactor for r4C measurement by AMS.

The next step is stripping the CO, from the sample in vacuum. Water, is cryogenically removed by a dry ice/ethanol trap and the CO, is collected in a liquid air trap. A Cu oven at 600°C removes any N,O. Sulphur compounds do not cause any problems as these are oxi-

dized and the resulting sulfuric acid is not stripped from the water. The CO, pressure is measured in a calibrated volume, and renders the original DOC concentration.

The CO, collected in this way is used for a ‘3s mea- surement on one of the Groningen isotope ratio mass

spectrometers. Next the CO, is converted to graphite for a

‘“C measurement on the Groningen AMS facility, [17,18]. The samples reported here were taken with the C.T.D.

system of the RV Pelagia. They were filtered over glass

fibre prefilters and regenerated cellulose (cut-off 0.2 FM) to remove particulate material. The particulate inorganic

carbon (PIG) was isolated by acidifying the glass fibre filters in a vacuum system. The same vacuum system was

used to isolate the dissolved inorganic carbon (DIG) [ 191. In addition, tritium activities were measured in the Gronin-

gen low level counting facility, according to Ref. [20].

3. Results and discussion

To test the supercritical oxidation as described above,

we have oxidized, among others, tannic acid, phtalic acid, and urea and found 100% recovery. The ‘3s values repro- duced within 0.5%0. The system blank is comparable to the HTCO method ( + 0.1 mg C/l) and is constant, allowing correction.

Measurements were performed with six samples from the North Sea. The sample locations, shown in Fig. 2, varied widely in terrestrial influence. Station 1 (German Bight) was near the estuary of the river Elbe. The salinity here was low, 29 PSU (practical salinity units) and biologi- cal activity high, coloring the water green. Stations 2, 3 and 6 were located further from the shore with intermedi- ate terrestrial influence with salinity at 33 to 34 PSU. Stations 4 and 5 were in the open sea, resembling Atlantic

Fig. 2. The sample positions in the North Sea.

M. Le Clercq et oL/Nucl. Instr. and Meth. in Phys. Res. B 123 (1997) 443-446 44.5

Ocean conditions. DOC concentrations decreased from 200 PM in the German Bight to 80-90 FM in the other

stations.

The results for the carbon isotopic ratio measurements are shown in Fig. 3. The results are shown for PIC, DIC

and DOC. The top of Fig. 3 shows the ‘36,+ as measured on one the Groningen isotope ratio mass spectrometers; the bottom of Fig. 3 shows the ‘“C value, expressed in ‘44, i.e.

corrected to a I38 of -25%0.

Concerning the ‘3s results, we note that the range for the PIC fraction in general compares well with the mea-

surements done before [Zl]. There is no trend in the I38 of

the DOC that reflects the decreasing terrestrial input. The ‘$ values themselves do not rule out that part of the DOC

has a terrestrial source as these are between the normal

marine ( - 19%0) and normal terrestrial ( - 25%0) values for organic carbon. Pyrolysis GC-MS analysis of DOC iso-

lated from the same water samples with ultrafiltration also did not show terrestrial markers such as lignin, even in the

German Bight station. The change in DOC composition was limited to the carbohydrates [22].

Concerning the 14C results, we note that the low values for the particulate inorganic fraction indicate a large contri- bution from resuspended bottom material and from eroded

terrestrial material, but not from locally produced material. This effect correlates strongly with the terrestrial influence and is known from literature [21]. Relative high DIC 14n values were found in the German Bight. This corresponds to a high tritium content (Fig. 4). Therefore this is proba-

* . . . n . n

. l l .

Fig. 3. The “‘6 and ‘46 values of the North Sea samples: (+) ‘%

DIG; ( n ) 13S PIG; (0) ‘$ DOC; (0) ‘% DIC; (0) ‘46 PIC; (0)

‘*d DOC.

+ 20 - ! i

3.

10

+

" 5 L_. L .J.-_--_. i

100 120 140 160 160 200 220

14ADlC (G)

Fig. 4. The tritium content as a function of the DIC ‘“d.

bly caused by input from nuclear powerplants. The precipi- tation in the Netherlands and Northern Germany is cur- rently about 15 TU [23]. A combination of high 14C and

tritium is known from nuclear activities [24,25]. There is roughly 200%0 difference in ‘“d between the

DIC and DOC. This shows that a substantial part (at least

20%, assuming an infinite age) of the DOC must be very refractive. This refractive fraction could originate from eroding terrestrial material or from undegradable marine

material. Measurements in open ocean water with UV oxidation showed 300%0 difference between DIC and DOC [7]. As the biological productivity in the North Sea is

higher than in open ocean water, any refractive marine material will be more diluted with freshly produced mate- rial, leading to a smaller 14C difference between DIC and DOC. A study with XAD isolated DOC from the Irish Sea

[25] did not show such a clear correlation between the ‘“C in DIC and DOC, but there the effect of anthropogenic ‘“C was much stronger.

4. Summary

We have developed a new reliable method for measur- ing isotope ratios in marine DOC. This will enable us to shed new light on the dynamics of the DOC pool. As a new oxidation method, it is useful for intercomparison with the other DOC concentration methods. Some of the first results from the North Sea samples show that a major fraction of DOC is refractive.

References

ill

121

E.K. Duursma and R. Dawson, eds., in: Marine Organic Chemistry, Evolution, Composition Interactions and Chem- istry of Organic Matter in Seawater (Elsevier, Amsterdam,

1981).

P.J. Williams, in: Chemical Oceanography, eds. J.P. Riley and C. Skirrow (Academic Press, London, 1975) pp. 301-

363.

XII. ENVIRONMENTAL/PALEOCLIMATIC STUDIES

446 M. Le Clercq et al./Nucl. lnstr. and Meth. in Phys. Res. B 123 (1997) 443-446

[3] R. Benner, J.D. Pakulski. M. McCarthy, J.I. Hedges and PG.

Hatcher, Science 255 (1992) 1561.

[4] J.H. Sharp, Limnol. Oceanogr. 18 ( 1973) 44 1.

[5] G. Cauwet, G. Oceanol. Acta 1 (1978) 99.

[6] D.L. Kirchman, Y. Suzuki, C. Garside and ,H.W. Ducklow,

Nature 352 (1991) 612.

[7] E.M.R. Dmffel. P.M. Williams, J.E. Bauer and J.R. Ertel, J.

Geophys. Res. 97 (1992) 15639.

[8] J.R. Toggweiler. in: Productivity of the Oceans: Present and

Past, eds W.H. Berger, V.S. Smetacek and G. Wefer (Wiley,

Chichester, 1989) pp. 65-83.

[9] C.A. Carlson, H.W. Ducklow and A.F. Michaels, Nature 37 1

(1994) 405.

[lOI P.J. Williams, P.J. LeB, J. Bauer, R. Benner, A. Miller, B.

Nomnan, Y. Suzuki, P. Wangersky and M. McCarthy, Mar.

Chem. 41 (1993) 11.

[Ill P.J. Wangersky, Mar. Chem. 41 (1993) 61.

[12] C. Lee and S.M. Henrichs, Mar. Chem. 41 (1993) 105.

[I31 M. Le Clercq, PhD thesis, University of Groningen, in

preparation.

[ 141 H.H. Yang and C.A. Eckert, Ind. Eng. Chem. Res. 27 (1988)

2009.

[15] S. Sourirajan S. and G.C. Kennedy, Am. J. Phys. 260 (1962)

115.

[I61 M. Le Clercq, AIChE J. 42 (1996) 1798.

[ 171 A.T. Aerts Bijma, H.A.J. Meijer and J. van der Plicht, these

Proceedings (AMS-7). Nucl. Instr. and Meth. B. 123 (1997)

221.

I181 S. Wijma and J. Van der Plicht, these Proceedings CAMS-7).

Nucl. Instr. and Meth. B 123 (1997) 87.

[I91 W.G. Mook, in: Prcc. IAEA Conf. on Isotope Hydrology,

Vienna (1970) pp. 163-190.

[20] H.G. ijstlund and E. Werner, Tritium in the Physical and

Biological Sciences (IAEA, Vienna, 1962) p. 1302.

[21] W. Salomons, PhD thesis, Groningen University (1973).

1221 Van Heemst et al., to be published.

(231 IAEA Technical Reports series, Environmental Isotope Data

no. 10, World survey of Isotope Concentrations in Precipita-

tion (1988-91).

[24] D.J. Groeneveld, Thesis, University of Groningen (1977).

[25] G.T. Cook, F.H. Begg. P. Naysmith, E.M. Scott and M.

McCartney, Radiocarbon 37 (1995) 459.