Sediment Sedimentation rate values in the Manila Bay and Thailand Gulf using 210Pb method

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Che Abd Rahim MohamedMarine Ecosystem Research Centre (EKOMAR)Faculty of Science & TechnologyUniversiti Kebangsaan Malaysia43600 Bangi, SelangorMalaysia

E-mail: carmohd@ukm.my

Topics

Introduction
Chronology
Sampling & Analytical procedure
Results & Discussion
Major Past, Current & Future Research Plans

Introduction

Why Date Marine Sediments?Usually dated with two goals in mind:

(1) to establish the timing of past environmental change, and
(2) to determine the accumulation of materials (e.g. sediment, pollutants, micro-fossils) in the reservoirs.

Radioactive classified into two group as;

1. Natural – cosmogenic and primodial

cosmogenic – 26Al, 10Be, 7Be, etc
primodial – 238U series, 232Th series

2. Artificial or man-made – 137Cs, 241Am

The natural Uranium-Thoriumdecayseries

geochemistry

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Last modified: 20.2.2001

Dating method

Half-life

Dating range

Application

230Thexcess

75,200 yr

< 300,000 yr

Deep sea sedimentation rates, Mn nodule formation rate

231Paexcess

34,300 yr

< 150,000 yr

Deep sea sedimentation rates, Mn nodule formation rate

226Ra

1,602 yr

10,000 yr

Marine and terrestrial

210Pbexcess

22.3 yr

100 yr

Sedimentation rate in lakes, estuaries and coastal marine environment

228Thexcess

1.913 yr

10 yr

Rapid sedimentation rates in shallow waters

234Thexcess

24.1 days

100 days

Rapid sedimentation rates in shallow waters

Summary of U-Th decay series dating methods

Why using short half-life radionuclide?

Deposition of sediment at the coastal water, lake and estuary areas was controlled by;

a. bioturbation

b. tidal cycles

c. bottom currents

d. human activity

e. suspended solid input

Why using 210Pb methods?

Lead-210 Chronology

- Studies for human impacts (e.g. pollution, eutrophication, erosion), which typically focus on the last 100-200 years, the dating method of choice is 210Pb.

Lead-210 is a naturally occurring radioisotope in the 238U-decay series formed by decay of 226Ra, and the subsequent evasion of the intermediary 222Rn (an inert gas) from the earth's surface.
Radon-222 decays through a series of short-lived daughters to 210Pb which is stripped from the atmosphere in precipitation and accumulates in lake sediments and wetlands where it decays away with a half-life of 22 years.
Cores are typically dated by analyzing a series of stratigraphic levels from the core surface to a depth where unsupported 210Pb is no longer measurable (roughly 5-8 half-lives).
From the resulting 210Pb profile, dates are calculated according one of several mathematical models that make assumptions regarding the accumulation of 210Pb and sediment at the core site.

The model assumed that;

a. the flux of excess nuclides to the sediment-water interface is constant

b. the sedimentation rate is constant at all times.

c. there is no post-depositional migration of the radionuclide within the sediments.

d. the activity of daughter supported by parents in the sediments is independent with depth.

Calculation

Sedimentation rate from vertical profile of excess nuclide in core can be calculated from the relation;

Az = Aoe(-t) ------- 1

where,

Az is the activity of excess study nuclide (dpm/g) in the sediment at any depth Z (cm) from the sediment-water interface

Ao is the activity of excess study nuclide (dpm/g) in the freshly deposited at depth Z = 0 (sediment-water interface)

t is time (year) or t = Z/S

S is the sedimentation rate (cm/yr)

210Pb is decay constant/half-life or 0.0318/yr

Az = Aoe(-)(Z/S) --------- 2

Analytical procedure

Generally, analytical procedure for uranium-thorium nuclides in sediments, suspended solid (TSS), seawater, freshwater and organisms are combination with acidification, precipitation, purification and electrodeposition.
The analytical methods using anion or cation are well established [i.e, Koide & Bruland, 1975; McCabe et al., 1979; Harada & Tsunogai, 1985; Mohamed et al., 1996 & etc.,].