CIESM Congress 2001, Monaco, article 0165

Rapp. Comm. int. Mer Médit., 36,2001

165

Introduction

The activity of

C in the dissolved inorganic carbon in water

(

CDIC) is frequently used as a dating tool for groundwater (1),

oceanic water masses (2) and marine pore water (3). It was recognized

that 14CDIC activity is dependent not only on the decay of

C but

also on diagenetic and transport processes (1,4-5). The effect of trans-

port processes (diffusion, advection) was demonstrated only in theo-

retical studies (4-5).

Our study used field measurements to quantify the effect of the

major biogeochemical processes, diffusion, advection and decay on

the

CDIC activity in water.This was achieved by measurements of

major ions and stable and radioactive carbon isotopes in SE

Mediterranean deep-sea sediment and pore water.This area serves as

an ideal natural laboratory to investigate

C processes due to its sole

well-defined seawater end-member, relatively homogeneous sediment,

steady state conditions and lack of significant advection.

Methods and Materials

Three cores, 25, 40 and 230 cm long were collected in June and

September 1996, and in April 1999 at ca. 1500 m water depth in the

SE Mediterranean. The cores were immediately sectioned and cen-

trifuged under argon atmosphere to avoid contamination with atmos-

pheric CO

. Pore water was sampled for DIC, total alkalinity (A

C of DIC (

DIC

and major ion (Cl

, Br

, SO

42-

, Na

, Ca

) analyses. Sediments were analyzed for stable and

radioactive carbon isotopes in inorganic (SIC) and organic (SOC) frac-

tions (

SIC

SOC

SIC

SOC

,) and also in foraminifera

skeletons (

SKT

). Sediments from the long core were also sampled

every 5 or less cm for

O high-resolution chronostratigraphy.

Results and Discussion

The main results of this study are that the

SOC

SIC

and

SKT

apparent ages at each depth were similar to each other and to

the independent Ť

agesť and therefore represent the real age of

the layer.The sediment ages varied from recent (top of the cores) to

ca. 30 ky b.p. (bottom of the long core), covering most of the dynam-

ic range of the

C method (ca. 5 half lives) (Fig. 1). These ages were

markedly older than the apparent ages of the pore water DIC, calcu-

lated directly from the radioactive age equation (Fig. 1).

Major ions and stable carbon isotope profiles proved that this

marked apparent rejuvenation of the pore water was caused by a

downward diffusive ?ux of ~90 atoms of

DIC

ˇm

-2

ˇs

-1

from the

overlying bottom water into the sediment. A simple algorithm is sug-

gested to correct for the effect of diffusion (6). The

DIC

?ux is

opposite in direction to the DIC ?ux out of the sediment.

It is suggested here that the

DIC

?ux may severely interefere

with age determination of small submarine Ťbrine lakesť formed in the

deep Mediterranean. In addition, the

C excess should show up in

authigenic carbonate phases precipitating within the sediment and

hence, may serve as a tool to identify the extent and duration of authi-

genic carbonate precipitation.

References

1. Mook W.G., 1980. p. 49-74. InP. Frits and J. C. Fontes (eds.),

Handbook of environmental isotope geochemistry I.

2. Broecker W.S. and Peng T.H., 1982. Traces in the Sea, Eldigio, New

York.

3. Bauer J.E., Reimers C.E., Druffel E.R.M. and Williams P.M., 1995.

Isotopic constraints on carbon exchange between deep ocean sediments

and sea water.Nature,373: 686-689.

4.Walker G.R. and Cook P.G., 1991. The importance of considering

diffusion when using carbon-14 to estimate groundwater recharge to a

confined aquifer.J. Hydrology, 128: 41-48.

5. Sanford W.E., 1997. Correction for diffusion in carbon-14 dating of

groundwater.Groundwater,35 (2): 357-361.

6. Sivan O., Herut B., YechieliY., and Lazar B., 2001. Radiocarbon

Dating of porewater correction for diffusion and diagenetic processes.

Radiocarbon,Accepted.

THE EFFECT OF DIFFUSION ON THE CONTENT OF RADIOCARBON IN PORE WATER

OF DEEP MEDITERRANEAN SEDIMENTS

Sivan O.

1,2

*, Lazar B.

,YechieliY.

and Herut B.

Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, Israel - sivan@vms.huji.ac.il, boazl@vms.huji.ac.il

Geological Survey of Israel, Jerusalem, Israel - yechi@mail.gsi.gov.il

Israel Oceanogrphic and Limnologic Research, Haifa, Israel - barak@ocean.org.il

Abstract

We present here field evidence from the deep SE Mediterranean sediments for significant

C excess in total dissolved inorganic carbon

(DIC) of pore water down to depths of more than 2 meter. It is concluded that this enrichment is caused by diffusion of

CDIC across the

water-sediment interface. The

CDIC ?ux distorts the

C ages of pore water, and may also severely interfere with age determination of

small oceanic water masses. In addition, the

C excess can serve as a tool to identify the extant and duration of authigenic carbonate

precipitation.We suggest a simple algorithm to correct for the effect of diffusion on the

C ages of pore water.

Keywords:

C; DIC ages;

C diffusion; Pore water; Deep SE Mediterranean.

Fig. 1: Radiocarbon and

O apparent ages as a function of

depth in the long core.

O ages are based on the global events found in

O depth pro-

file of the planktonic Foraminifera Globigerinoides ruber (insert in

the upper right). The identified horizons in this profile are marked as

S S1 sapropel, YD - Younger Dryas, LGM - Last Glacial Maximum.

C apparent ages were calculated for the DIC in pore water, for the

inorganic and the organic sediment (SIC and SOC respectively),

and for the carbonate skeletons (SKT). The

C sediment's ages of

SIC, SOC and SKT and the

O ages are similar to each other at

each depth. Their profile shows a constancy down to ~5 cm due to

bioturbation.Asedimentation rate of 4.5 cm kyr

-1

(the slope of the

best fit line) was calculated for the 5 to 75 cm depth interval and 9.0