PB-210 RADIOMETRIC DATING OF ESTUARINE SEDIMENTSFROM THE EASTERN COAST OF THAILAND

VORAVIT CHEEVAPORNa AND PHAITHOON MOKKONGPAlba Department of Aquatic Science, Burapha University, Chonburi 20131, Thailand.

b Institute of Marine Science, Burapha Universil}', Chonburi 20131, Thailand.

ABSTRACT

Eighteen sediment cores were taken from the Bang Pakong and Chantaburi River estuaries. The

excess Pb-210 concentration profiles obtained were used to calculate annual sedimentation rates and

thus the age profiles of the cores. The dating model assumes a constant flux of Pb-21O frolll the decar

of atmospheric Rn-222, followed by adsorption on suspended particulates which carry the Pb-210 to theestuarine sediments at a constant rate. The results of investigation showed higher values of sedimentatiOll

rate of Bang Pakong River mouth than those from Chantaburi River mouth. This is probably due to

dramatic decrease of the mangrove forests in the area and as a result of diagenetic remobilization of Pb-

210 in the deeper sections of the sedimentary column.

.INTRODUCTION

The estuarine sediments are a complex mixture of materials introduced through rivers,shoreline erosion, in situ biological and chemical processes and from human sources. Thesesediments thus store in them information about these sources, in particular the temporalvariations in their rates of supply. There is a strong need to obtain data on these variations,especially those due to man. Documentation of such changes requires a chronological studyof the sediment core, as well as knowledge of the distribution of relevant chemical species insediments.

In the past few decades many applications of natural and man-made radionuclides tothe determination of sediment accumulation rates have been documeotedl.2,a,4. These methodshave provided useful information in some cases, but in many others they have met with onlylimited success, since post-depositional processes (such as biological and physical mixing,erosion) confounded the record. These processes tend to alter the sequence of events stored inthe sediment column or even erase them, making the geochronological studies difficult. Inspite of thIS, in the last few years it has been possible to understand the effects these variousprocesses have on radionuclide distribution in the sediment column, and to establish thechronology of sediment accumulation and other sedimentary processes in estuaries. Thisstudy investigated the sediment-accumulation rates in the estuarine sediments from the easterncoast of Thailand. Such information may enable a reasonable estimate of the background leveland changes in input of some pollutants over an extended period of time5,6.

MATERIALS AND METHODS

Pb-210 is a naturally occurring radioisotope of the U-238 family (Fig. 1). Its closestparent with a significant half-life is radon 222 (Rn-222, half-life: 3.8 days; Fig.1)

Fig. 1. Decay scheme of Ra-226 and its daughters. These isotopes belong to the U-2:i8 series!.

which being a noble gas, escapes into the atmosphere from surface soil layers andprovides lead-210 with an unexpected mobility. This mobility creates an "excess" of lead-210(Fig.2) compared to those expected from secular equilibrium with radium-226 (Ra-226, half-life: 1622 years; Fig.1) which forms the basis of the Iead-210 dating method. Lead-21O isintroduced into the estuarine environment through atmospheric precipitation, terrestrial run-off and i/1 situ production from radium-226 in the water column. Studies of the behavior oflead-210 in continental waters] and its budget in shallow coastal areas show that atmosphericprecipitation is its dominant pathway to estuaries. Lead-210 once introduced in the estuarineand near coastal water is removed quickly to sediments by adsorption/scavenging processes5( Fig.2).

The mean residence time of lead-210 in the Long Island Sound waters was reported byBeninger et al. to be only a few months8. The fast removal of lead-210 from coastal waters hasincited efforts to use it as a dating tool in these areas. Initial tests for the lead-210 datingmethod were made on sediments of the Santa Barbar~ Basin2. In addition this method wasproven successful in the dating of marine deposits of the continental margins~ and estuarineregions9.

This dating method is based on the assumptions of constant flux of lead-210 to thesediments over the sampling interval and insignificant mobility of lead-210 in the sediment