PertanikaJ. Trap. Agric. Sci. 26(1): 65 - 71 (2003) ISSN: 1511-3701© Universiti Putra Malaysia Press

The Detennination of Accretion Rate in Setiu Mangrove, Malaysia:Thorium-230 Versus Artificial Horizontal Marker Method

KAMARUZZAMAN, B. Y., MOHD LOKMAN, H.,SULONG, 1. & HASRIZAL, S.

Mangrove Research Unit (MARU),Institute of Oceanography,

Kolej Universiti Sains & Teknologi Malaysia,21030 Kuala Terengganu, Terengganu,

Malaysia

Keywords: 230Thexcess

, artificial horizon marker, mangroves

ABSTRAK

Dalam kajian ini, penentuan kadar sedimentasi telah dibuat dengan menggunakan kaedah 230Th danpenanda tiruan mendatar. Dengan menggunakan kaedah 230Thexcess' purata sedimentasi sebanyak 0.62~es:n thn­1 adalah diperolehi. Kadar sedimentasi ini adalah setanding dengan kaedah penanda tiruan mendatar yangmemperolehi sebanyak 0.61 cm thn-1

• Kaedah 230Thexcess adalah didapati lebih baik dan tepat bagi penentuankadar sedimentasi. Penyediaan sampel adalah juga ringkas, malah prosedur penyediaannya adalah cepat danmudah bagi penentuan kadar sedimentasi di hutan paya bakau. Sekiranya kadar sedimentasi melalui sampelteras diambil kira, kadar sedimentasi pada kedalaman 100 cm adalah dianggarkan termendap pada 163 tahundahulu.

ABSTRACT

In this study, determination of sediment accretion rate using the 230Th and the artificial horizon marker methodin Setiu mangrove were done. Applying the 230Thexcess method, an average accretion rate of 0.62 cm yr1 wasobtained. This is comparable to that of an artificial horizon marker method giving an average of 0.61 cm yr1•

The 230Th method provides a rapid and simple method of evaluating 230Th accumulation histories insediment t:;.;s. Sample preparation is also significantly simplified, thus providi:;ta relatively quick and easymethod for the determination of the accretion rate in mangrove areas. Assuming that the accretion rate values areaccurate, this may imply that the sediments in the upper 100 cm were deposited during the last 163 years.

INTRODUCTION

Mangrove forests are a buffer zone between thecoast and the ocean. One of their presumedimportant functions is to provide a mechanismfor trapping sediment. In terms of their biologicaland chemical aspects, mangrove forests are ahighly productive source of organic matter, fromwhich there is a net outwelling of energy thatsupports the complex estuarine and nearshorefood web. Geologists, on the other hand, viewmangroves as sediment sinks, characterized bylong-term import of sediments as indicated bythe substantial accretion of recent sediments,which underlie mangrove forests and adjacentcoastal plains (Wolanski et al. 1992). Physically,by virtue of being in the intertidal areas, they

can act as recorders of environmental changesvia sedimentological characteristics and in thepreservation of spores and pollens.

Despite the acceptance that mangroveecosystems are important sinks for sediments,few studies have addressed sediment accretionin this environment. Several authors (Goldbergand Koide 1962; Koide et al. 1972) have studiedsome aspects of the sedimentology of mangrovesand quote different sedimentation rates, whichis probably a reflection of the non-representativesampling techniques employed. Spenceley (1982)and Shahbuddin et al. (1998) have introduced asimple method for measuring accretion bysimulating pneumatophores using rods and stakesand an artificial horizon marker method,

KAMARUZZAMAN, B. Y., MOHD LOKMAN, H., SULONG, I. & HASRIZAL, S.

respectively. Although long-term accretion ratesusing radionuclides have been well-documented(Sharma et aL 1987; Lynch et al. 1989; Anderson1982) publications on the use of this approachhave been few (Kamaruzzaman et al. 2000;Shahbuddin et al. 1998) and limited in themangrove ecosystems. In the present study, weuse thorium (230 Th) and an artificial horizonmarker method to establish sediment accretionrates.

The measurement of 230Th concentrationsin sediments provides one method of developingaccretion histories. 230Th is formed at a constantrate in the water column from the decay of 238Uand is rapidly scavenged and incorporated intothe underlying sediments. 230Th is a valuabletracer of the processes whereby reactive elementsare scavenged from seawater and produced at aconstant rate throughout the oceans. Followingits production in seawater, 230Th is rapidlyhydrolyzed and subsequently removed tosediments on a time scale of a few decades inthe deep ocean and weeks to months in surfacewater. Their excess 230Th (i.e., the amount inexcess of that expected from secular equilibriumwith 238U present in mineral latices) has beenwidely used to date sediment horizons andestimate average accretion rates (Goldberg andKoide 1962; Ku 1976; Scholten et al. 1990). Forthe artificial horizon marker method, sedimentaccretion was based on the vertical basis only.Vertical sediment accretion refers to the verticalthickness of sediment gained in a certain areafor a certain period and can either be positive,denoting accretion and growth, or negative,denoting erosion.

MATERIALS AND METHODS

Sampling

The Setiu mangrove of the study area is locatedon the South China Sea coast of PeninsularMalaysia and about 60 km northwest of KualaTerengganu, the capital state of Terengganu(Fig. 1). The Setiu mangrove is a unique area asit covers many ecosystems such as estuaries,intertidal areas and lagoons. The lagoon'secosystem is semi-enclosed with limited and poortidal flushings and has a total water surface areaof about 880 ha. This study area is of primaryoceanographic interest since it is one of thelargest estuaries of the Terengganu coast intowhich two river systems flow, i.e., the Setiu andChalok rivers. These areas are areas of diverse

ecosystems, with utilizable natural resources, avast array of biological diversity and coastal andriverine fishing activities. The Setiu area lies inthe wet tropics where high rainfall (averaging400 mm) is recorded during the monsoon season.In this study, four transect lines (TR1, TR2, TR3and TR4) were set up inside the mangrove(Fig. 1), where the total of 54 sampling stationswere fIXed along the transects. Two 150 cmsediment cores were also collected with aD-section core sampler from the mangroveforests. The cores were cut into segments ofapproximately 5 cm interval, labelled and storedfrozen until analysis in the laboratory.

Determination of Sediment Accretion Using ArtificialHarizon Marker Method

For the determination of the sediment accretion,the methodology applied in this work is basedon measuring the thickness of a vertical sedimentsection divided by the time span necessary for itsdeposition. For the purpose of this study, a slabof perspex (9 cm x 9 cm x 1.5 mm) was plantedat each sampling point and acts as a markerlevel. As a stabilization measure, the disturbedsediment above the markers were left for amonth before initial readings of marker leveldepths were recorded. As another me~ure toaid stabilization of sediments above the markerlevels, 5 holes were drilled in each perspex slabto enable water to pass through from the surfaceto the bottom. The thickness of sediments abovethe markers was then measured on a monthlybasis for a 12 month period. The thickness ofsediment at every sampling point was determinedby taking the average of six readings per marker.Accuracy of reading is approximately ± 2 mm.The average thickness of the sediment for themonth in study is then subtracted by the thicknessof sediment obtained from the previous month.A positive value indicates accretion while anegative value indicates erosion.

Analytical Method for 232Th and 230Th

The sediment samples were digested and theanalyses for total Th (232Th) following thepublished methodologies with some modifications(Noriki et al. 1980; Sen Gupta and Bertrand1995; Kamaruzzaman 1999). An inductively­coupled plasma mass spectrometer (ICP-MS) wasused for the quick and precise determination ofTh in the digested sediment. In brief, thedigestion method involved heating of 50 mg of

66 PERTANlKAJ. TRap. AGRIC. SCI. VOL. 26 NO.1, 2003

THE DETERMINATION OF ACCRETION RATE IN SETIU MANGROVE, MAlAYSIA

N

rTR4

SOUl'tlCHINASEA

Fig. 1: Location of transects (TRl, TR2, TRJ and TR4) and cores (.) in themangrove forests of Setiu mangrove, Terengganu, Malaysia

a fmely powdered sample in a sealed teflonvessel in a mixture with a mixed acid solution(1.5 mL) of concentrated HF, HNOs and HCI.The Teflon vessel was kept at 150°C for 3 - 5 h.After cooling, a mixed solution of boric acid andEDTA (3 mL) was added, and the vessel wasagain heated at 150°C for at least 5 h. Aftercooling to room temperature, the content of thevessel was transferred into a 10 mL polypropylenetest tube and was diluted to10 mL with deionized water. A clear solutionwith no residue should be obtained at this stage.A laboratory standard material of mangrovesediment was also subjected to the sameprocedure. The relative 1 a value of replicatedeterminations of a sample was less than 3%and the results obtained for HA, coincided withcertified values within a difference of ± 3%.

The analytical method of 2soTh for thedetermination of sedimentation rate in thesample was carried out according to thepublished method (Tsunogai and Yamada 1979;Harada and Tsunogai 1985; Kamaruzzaman1999) with some modifications. The methodinvolved heating 1 - 2 g of dried sediment anddigesting it with a mixture of solution ofconcentrated HF, H Os and Hei. The solutioncontaining Th was heated to make the solution

clear before being treated with anion and cationexchange resins for the separation andpurification. The effluent containing Th wasthen heated to dryness and finally dissolved in5% HNOs' The concentration of 2soTh was thenmeasured with a fast and sensitive ICP-MS. Theprecision assessed by the replicate analyses wasless than 3%. The accuracy was also examinedby analyzing in duplicate a Canadian CertifiedReference Materials Project standard (DL-la)and the results coincided with the certified valueswithin a difference of ± 3%.

RESULTS AND DISCUSSIONAccretion Rates

This study spanned 11 times from January 2001to January 2002 and a thickness at all samplingpoints along each transet was recorded duringlow tides. Sampling dates and times werepredetermined based on the yearly predictedtidal data table produced by the HydrographicDepartment of the Royal Malaysian Navy. Fig. 2shows that the summary of the average accretionrate obtained was 0.61 cm yrl. The highestaverage accretion rates are found in the periodfrom March to July corresponding to the non­monsoon season (Fig. 2). There is a positiveaccretion rate for all months (Table 1). The

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 26 NO.1, 2003 67

KAMARUZZAMAN, B. Y, MOHD LOKMAN, H., SULONG, I. & HASRIZAL, S.

r 1.4

r'j ~ 1.2

I: L1 §i i 0.8 i

0.6 i0.4 ~

0.2

SOOT

e: t~ 600,·

~ 500i

~ 400 {'..Q300

j 200 'n r~ ,'j I"

100 " : i J I ,!, I ' I 11 1 •o ·r-L.... ..L i ~ - + - :. ~_J __.......... ..l.-. ..... 1 + 0

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Month. . ~_. . . i

Fig. 2: Average of monthly mean rainfall (D) with average monthly accretionrate (.) at Setiu mangrove

TABLE 1Monthly accretion rate for TRl, TR2, TR3 and TR4 giving

an averaged yearly accretion rate of 0.61 cm/yr

Month Accretion Rate (em)

FebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember

0.620.990.570.460.401.310.890.650.550.450.36

Average cm/yr 0.61

where 230Th and 232Th are the measuredLOtaI total

concentrations of 230Th and 232Th, respectively,and 234U and 1230 are the concentrations of 234U(of which radioactivity is assumed to be 1.1times the 238U concentration) and the decay

highest accretion rate was observed in the monthofJuly (1.31 cm month·I ) and lowest in December(0.36 cm month·I ). The much lower accretionrate during monsoon seasons (Nov - Jan) maybe due to the higher energy of water movementfrom the upper stream, which may carry the finesediment to the seas.

For the second method using thorium,230Th was used to determine the accretionrates ex~f the study areas (Scholten et al. 1994;Suman and Bacon 1989; Mangini and Stoffers1990). The amounts of 230Th

excessare calculated

using the following equation:

constant of 230Th (9.24 x 10-6 yr), respectively.The second term on the right hand side of theequation (232Th

LOtal) is necessary in order to

subtract the lithogenic fraction and the assumedcoefficient, 0.8, which is a mean activity ratio of230Th/232Th for the lithogenic fraction as reportedby Anderson (1982). The third term (234U(1 ­exp{- A230t}) is for the correction of 230Thproduced from 234U in the sediments, which isnecessary because 230Th is produced fromauthigenic U contained in the sediment.

The determination of average sedimentationrate is based on the assumption that the 230Th

excess

is incorporated into the sediments with a constantrate (Ku and Broecker 1966; Osmond 1979).The values of 230Th

excessderived from Equation 1,

are as in Table 2. If this assumption is correct,the radioactivity of 230Th in sediment corewhich decreases exponen~iillywith depth, andthe sedimentation rates can be calculated fromthe following equation:

230Thexcess

230Thtotal

- (0.8 X 232Thtotal

)

_234U (l-exp{-1230t}) (1)

S=_A23°/b (2)

68 PERTANIKAJ. TROP. AGRIC. SCI. VOL. 26 O. 1,2003

THE DETERMINATION OF ACCRETION RATE IN SETIU MANGROVE, MALAYSIA

TABLE 2The values 23O'fh from both cores derived from the ICP-MS determination

for sa;Pies from Setiu mangrove, Terengganu, Malaysia

Depth (em) CORE 12~O'fhexcess (dpm)

CORE 22~O'fhexcess (dpm)

5152535455565758595

0.9212354n.dn.d

0.811268420.80461206

1.00912553160.840165840.791154620.811211360.64565212

0.0999891n.d

0.1756432560.17549332

n.d0.05433326

0.154021403n.d

0.0975659560.087338325

(n.d: not detectable)

where b is a gradient of the 'best-fit' curve in theplot of logarithmic concentrations of 230Thexcessagainst depth (cm). As shown in Fig. 3, accretionrates for both cores were calculated to be 0.61cm yrl and 0.62 cm yr\ respectively.

Our results for both methods, i.e. 230Th andthe artificial horizon marker method, are quiteconsistent giving an average accretion rate ofabout 0.61 cm yrl. However, the 230Thexcess methodprovides a rapid and simple method of evaluating230Th accumulation histories in sedimentcores~xcSample preparation is also significantlysimplified, thus providing a relatively quick andeasy method for the determination of theaceretion rate in mangrove areas. Even thoughour values for both the methods are somewhat

higher, it is comparable to accretion ratesreported at other intertidal areas (Bird 1971;Spenceley 1982; Shahbuddin et al. 1998).

Our higher values can be explained by thegeographical position of our study area, wheretheir location is located inside the estuary, closeto the mouth, providing it with 2 sedimentsources, fluvial and tidal. Greater water dischargefrom the river also brings much more suspendedsediment to be trapped by the mangrove roots.Assuming that the sedimentation rate values areaccurate, this may imply that the sediments inthe upper 100 cm were deposited during the last163 years. The high average accretion rates inthe Setiu mangroves indicate that this mangroveis still prograding and in an immature stage.

1: r---------- ------------------------I

20 j •30 !

I •C 40 I! I::T 50 i0- I..a. 60 i

i •70 ii

80 1

90 Is'" 0.61 cm/yr

100 :.---- -- - - ----.---.------ -

•

S '" 0.62 cmJyr

230Thex (dpm)

1.0 1.5 2.00.0 0.5

0

10

20

30

40:?~ 50'02- 60

70

80

90

100

(b)0.5 1.0

230Thexcess (dpm)

0.0(a)

Fig. 3: (a) 230THexass

versus depth for core 1 and (b) 230THexcess versus depth for core2 with sedimentation rate, S = 0.61 cm/yr and 0.62 em/yr, respectively

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 26 NO.1, 2003 69

KAMARUZZAMAN, B. Y, MOHD LOKMAN, H., SULONG, I. & HASRIZAL, S.

This finding suggests that the mangrove forestsare not just passive colonizers of mud banks, butactively capture mud to create their ownenvironments. Mangroves are thus an importantsink for the fme sediment from rivers and coastalwaters. Sediment supplied to the mangrove areamight undergo the common process where theyare suspended and carried back and forth,deposited and eroded before they fmally settleeither permanently or for a certain time period.The opportunity for sediment to settle or bedeposited is greatly influenced by the frequencyand duration of flooding (Cahoon and Reed1995) .

ACKNOWLEDGEMENTS

This research was conducted with joint fundingfrom the Malaysian Ministry of Science,Technology and Environment, under theIntensified Research for Priority Areas (IRPA)project numbers 51513 and 51447 and a grantfrom the Malaysian Toray foundation (MTSF)project number 63905. The authors wish toexpress their gratitude to the MARU researchteams for their invaluable assistance andhospitality throughout the sampling period.

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(Received: 4 July 2002)(Accepted: 19 December 2002)

PERTANIKAJ. TROP. AGRIC. SCI. VOL. 26 NO.1, 2003 71