1.0 INTRODUCTION:

Mangroves are highly dynamic ecosystems and their growth and decline often reflect the changing conditions of the coastal environment. The Mangals play a special role as a nursery habitat for juvenile fish and prawns, Kathiresan, K., (2010) and Krishna Mohan (2008). In most of the developing countries, priority of necessity is given to food production and socio-economic development, which are often at the expense of biodiversity conservation (Adeel and Caroline, 2002). Moreover, many countries have decided to use mangroves in the treatment of sewage effluents Ashokkumar et al (2009). Geologists, on the other hand, view mangroves as sediment sinks, characterized by long-term import of sediments as indicated by the substantial accretion of recent sediments, which underlie mangrove forests and adjacent coastal plains Wolanski, (1992). Despite the acceptance that mangrove ecosystems are important sinks for sediments, few studies have addressed sediment accretion in this environment.

Several authors Goldberg et al (1962), Koide et al (1972), have studied some aspects of the sedimentology of mangroves and quote different sedimentation rates, which is probably a reflection of the non-representative sampling techniques employed. Spencely (1982) and Shahbuddin et al. (1998) have introduced a simple method for measuring accretion by simulating pneumatophores using rods and stakes and an artificial horizon marker method, respectively. Meanwhile, longterm accretion rates using radionuclides have been well documented Sharma et al. (1987), Anderson(1982), but use of this approach has been not many published Shahbuddin et al. (1998) , Kamaruzzaman et al. (2000), and limited in the mangrove ecosystems. In the present study, we use ²¹⁰Pb to estimate sediment accretion rates. The concentration of ²¹⁰Pb within sediment is governed by processes such as source input from overlying waters through sedimentation, in situ production and radioactive decay. Many scientists have successfully demonstrated ²¹⁰Pb as a sensitive tracer for understanding the geochemical changes which have occurred during the century in different ecosystems, including coastal marine sediment, due to industrialization. ²¹⁰Pb has proved to be a valuable tracer of sediment mixing and accumulation in a variety of environments Koide, et al (1972), Sharma et al. (1987), Lynch al (1989), Carpenter, et al (1981), Crusius et al (1991). ²¹⁰Pb are naturally occurring radionuclides of the ²³⁸U decay series with a 22.3 year. ²¹⁰Pb is used to examine sediment processes on a 100 year time scale. ²¹⁰Pb is supplied by its effective parent ²²⁶Ra in seawater and from ²²²Rn in the atmosphere. The atmospheric source is produced as ²²²Rn, a short-lived (3.8 days) intermediate daughter of ²²⁶Ra, escapes from the earth’s crust, and decay to ²¹⁰Pb and is deposited back to the ground. In most shallow water environment such as mangrove ecosystem, atmospheric input is the major source. In view of the importance of the mangrove to various aspects of the environment, researches on the sediment accretion as well as the sediment age were carried out.

2.0 MATERIALS AND METHOD:

2.1 Study site:

Geographically Bhavanapadu (18⁰ 32’ N and 84⁰ 17’ E) is a panchayat village of Santhabommali Mandal of Srikakulam district, Andhra Pradesh and a coastal village where marine and estuarine fishing is the main occupation of the inhabitants. Around this mangrove forest was numerous such as Eucalyptus plantations, Casuarinas plantations and human settlements, some of which provide the main sources of sediments into the rivers and mangroves. Boat activities were carried out daily by fishermen for fishing and ecotourism purposes. (Figure I.). The dominant species of mangrove tree found are Avicennia sp. Rhizopora sp., and Xylocarpus sp. The mangroves which are relatively undisturbed have been gazetted as a mangrove reserve forest by the Bhavanapadu (BP) Forestry Department, Srikakulam District, Andhra Pradesh State, East Coast of India. The tide floods the area twice daily as it is semi-diurnal with a mean range of 1.8 m. In this study, a 20 cm sediment core from Bhavanapadu Estuary and Meghavaram mangrove were collected with a D-section core sampler from this area (Fig. 1) and was cut into segments of approximately 5 cm interval, labelled and stored in acid cleaned bottle for analysis.

Fig. 1: Sampling location where 2 cores were collected at Bhavanapadu (BP) and Meghavaram (MV) mangrove forest, North Coastal Andhra Pradesh

2.2 Analytical methods for²¹⁰Pb: The samples were cut into segment approximately 2 cm interval using plastic knife and was transferred to Petri disc. The samples were dried in an oven at 60°C to constants weights. To facilitate geochemical analysis the sample were ground into a fine powder using a pestle and mortar. The process of ²¹⁰Pb excess dating method is divided into two process, digestion process and Po platting process Carpenter, et al (1981), and Carpenter et al. (1982). The excess ²¹⁰Pb in a different core was measured by the same procedure describes by Carpenter, et al (1981), and Carpenter et al. (1982). Activities of ²¹⁰Po along with the ²⁰⁸Po tracer were measured by Alpha Spectrometry on a silicon surface barrier detector connected to a multi channel analyzer.

2.3 Data analysis: Total ²¹⁰Pb activity was determined indirectly by the measurement of its alpha-emitting grand daughter nuclide, ²¹⁰Po Zuo,(1991). Measurement of ratio ²¹⁰Po and ²⁰⁹Po activity will provide an adequate figure of supported ²¹⁰Pb, as these two elements are assumed to be in equilibrium Gale, et al (1995). Subtracting of supported ²¹⁰Pb from total ²¹⁰Pb will determine unsupported ²¹⁰Pb. The activity of ²¹⁰Pb is obtained by the formula below:

Activity ²¹⁰Po (²¹⁰Pb) = A (dpm/g)

X ²⁰⁹P_o (24.74 dP_m / g)

Where:

A = Ao e^-λt (equal to the accumulative residual unsupported ²¹⁰Pb below the sediment age of t)

Ao =A/e^-λt(equal to the total unsupported Pb in the sediment column)

λ = Ln2/t1/2 =0.639 (decay constant of ²¹⁰Pb)

T1/2 =Half-life (22.3 years)

t =Depth (cm)/sedimentation rate in years

Inventories (I) of ²¹⁰Pb (unsupported) are expressed in dpm-2 and were calculated according to Turekian et al (1961):

I = ΣA_Iihi

Where:

A_i = The ²¹⁰Pb_xs (dpm g^-1)

_I = The bulk density interval i (g cm^-3)

h = The thickness of the interval (cm)

And finally the sedimentation rate can be calculated using the formula:

A = A_oe^-λt

A = A_oe^-^λ (x/S)^t

ln A = -(l/S)x+ln Ao

Where:

A = Activity of excess ²¹⁰Pb in the sediment at any depth

Ao = Activity of excess ²¹⁰Pb in the freshly deposited sediment at depth = 0

(The sediment-water interface)

S = Sedimentation rate in cm year^-1

l = Radioactive decay constant (0.0311/year)

t = Time in year

3.0 RESULTS AND DISCUSSION:

Figure 2 shows the activities of ²¹⁰Pbxs were generally decreasing exponentially with depth. Some higher value of ²¹⁰Pb activity on the surface might be caused by the settling particles which were basically derived from the atmosphere and land or from the older sediment issued from the resuspension process Miralles, et al (2005). It is clear that ²¹⁰Pb increases monotonically with depth, suggesting of the particulate scavenging Chung, et al (2004). The determination of average sedimentation rate is based on the assumption that the ²¹⁰Pbxs is incorporated into the sediments with a constant rate Chung, et al (2004). As shown in Fig. 2, applying the formula as above, the sedimentation rate of Bhavanapadu (BP) and Meghavaram mangrove forest were estimated to 1.11 and 0.94 cm year^-1, respectively. The sedimentation rate was estimated by selecting the ‘best curve’ from the establishment ²¹⁰Pb (dpm g^-1) distribution with depth. The p>0.01 calculated from the statistical test, ANOVA two factor prove that the sedimentation rate among the study area are not significantly different. In this study, Meghavaram mangrove which is situated about 2 km from river mouth had the lowest accumulation rate while the highest sediment accumulation rate occurs at Bhavanapadu mangrove which is situated near the river mouth. Rivers are believed to be the dominant source of sediment to the Bhavanapadu (BP) mangrove forest.

Bhavanapadu (BP) Estuarine Mangrove Forest

Meghavaram Mangrove Forest

Fig. 2: ²¹⁰Pb_excess distribution with depth in Bhavanapadu (BP) mangrove forest

In Bhavanapadu(BP) mangrove forest, the sedimentation rate obtained is consistent with the result obtained by Shabuddin et al.(1998), ranging from 0.64-1.46 cm year^-1 with artificial horizontal marker method but is slightly higher compared with Kamaruzzaman et al.(2001) and Kamaruzzaman et al.(2001), 0.68 cm year^-1 with²³⁰Th_ex /²³²and ²³⁰Th_ex/²³²Th method. In comparison with other studies in BP mangrove, sedimentation rate at BP mangrove is slightly higher than Meghavaram mangrove0.65 cm year^-1 Kamaruzzaman et al.(2003), and Meghavaram mangrove, 0.84 cm year^-1 Kamaruzzaman et al.(2002), but lower compared to BP mangrove, 1.08 cm year^-1 using ²³⁰Th_excess method. From the sedimentation rates, ²¹⁰Pb dating technique was used to determine the sediments age for the major estuarine mangrove forest in BP region. Sediment age can be determined by dividing the sediment depth with sedimentation rate. The lower the sedimentation rate indicates the older age of sediment. Assuming that the sedimentation rate values are accurate, this implies that the sediments in the upper 100 cm at Bhavanapadu, Meghavaram mangrove forest were deposited during the past several years. From the ANOVA test, the sediment age does not differ significantly among the study areas with p<0.05. Briefly, Meghavaram mangrove which had the lowest sedimentation rate had the oldest mangrove with 106.6 years while the youngest mangrove, 90.2 years, occurred at Meghavaram mangrove which had the highest sedimentation rate.

4.0 CONCLUSION:

The sedimentation rates of Bhavanapadu (BP) and Meghavaram mangrove forest, based on the ²¹⁰Pb dating techniques were shown to be similar to the known local sea-level rise rate. The immobility of Pb in sediments was confirmed with stable ²¹⁰Pb at the mangrove. It is suggested that the stable ²¹⁰Pb is a powerful tracer in mangrove sediment studies on the determination of changing sedimentation rates, post depositional mobilization and historical Pb sources.

5.0 ACKNOWLEDGEMENT:

This work was carried out with financial assistance provided by the New Delhi MOEF Project (No. 22/26/2004-CSC (M) dtd. 24-2-2005). We are grateful to the commission for the grant-in-aid. Facilities at the Department of Environmental Sciences, Andhra University and, local village peoples of Bhavanapadu were utilized and we are thankful to the authorities’ concerned.

6.0 REFERENCES:

1. Adeel, Z. and Caroline, K. (Ed) (2002) Conserving Our Coastal Environment: A Summary of Unu’s Research on Sustainable Management of the Coastal Hydrosphere in the Adia Pacific Region. United Nations University, Tokyo, Japan.

2. Anderson, R.F., (1982). Concentration, vertical flux and remineralization of particulate uranium in seawater. Geochim. Cosmochim. Acta, 46: 1293-1299.

3. Ashokkumar, S., Mayavu P. Sampathkuma P., Manivasagam P. And Rajaram, G. (2009). Seasonal Distribution of Heavy Metals in the Mullipallam Creek of Muthupettai Mangroves (Southeast Coast of India) , American-Eurasian Journal of Scientific Research 4 (4): 308- 312.

4. Carpenter, R., J.T. Bennett and M.L. Peterson, (1981). 210Pb activities in and fluxes to sediments of the Washington continental slope and shelf. Geochim. Cosmochim. Acta, 45: 1155-1172.

5. Carpenter, R., M.L. Peterson and J.T. Bennett, (1982). Derived sediment accumulation and mixing rates for the greater Puget Sound region. Mar Geol., 48: 135-140.

6. Chung, Y., H.C. Chang and G.W. Hung, (2004). Particulate flux and ²¹⁰Pb determined on the sediment trap and core samples from the northern South China Sea. Elsevier Science Ltd. Continental Shelf, 24: 673-691.

7. Crusius, J. and R.F. Anderson, (1991). Immobility of ²¹⁰Pb in Black Sea sediments. Geochim. Cosmochim. Acta, 55: 327-333.

8. Gale, S.J., R.J. Haworth and P.C. Pisanu, (1995). The ²¹⁰Pb chronology of the late Holocene deposition in eastern Australian lake basin. Q. Sci. Rev., 14: 395-408.

9. Goldberg, E.D. and M. Koide, (1962). Geochonological studies of deep sea sediments by he ionium-thorium method. Geochim. Cosmochim. Acta, 26: 417-450.

10. Kamaruzzaman, B. Y., Antotina, A., Airiza, Z., Syalindran, S. and Ong, M. C., (2007). The Geochemical Profile of Mn, Co, Cu, and Fe in Kerteh Mangrove Forest, Terengganu. The Malaysian Journal of Analytical Sciences. 11(2), 336-339.

11. Kamaruzzaman, B.Y., (1999) Geochemistry of the marine sediments; its paleoceanographic significance. Ph.D. Thesis, Hokkaido University, Japan.

12. Kamaruzzaman, B.Y., H. Mohd Lokman, I. Sulong, B.T. Jamil and S. Hasrizal, (2003).Sediment accretion and trace metal concentrations in the surface sediment of Kuantan mangrove forest, Malaysia. Chem. Res. Commun., 16: 2-13.

13. Kamaruzzaman, B.Y., H. Suhaimi, E.K. Teh, H.F. Leong, K.H. Soon and K.Y. Chong, (2001). The determination of ²³⁰Th in the sediments: Sedimentation in the mangrove frests of Pulau Sekeping, Kemaman, Terengganu. J. Ultra Sci. Phys. Sci., 13 (2): 239-245.

14. Kamaruzzaman, B.Y., H. Suhaimi, N.A.M. Shazili, H.F. Leong, S. Hasrizal and N. Tahir, (2004). Sediment accretion and trace metal concentrations in the surface sediments of Matang mangrove forest, Malaysia. Malaysian J. Anal. Sci., 8 (1): 105-111

15. Kamaruzzaman, B.Y., H.F. Leong, H.M. Lokman, I. Sulong and K.Y. Chong, (2002). The determination of ²³⁰Th in the sediments: Sediment accretion in the mangrove forests of Tanjung Piai, Johore, Malaysia. Sains Malaysiana 31: 49-56.

16. Kamaruzzaman, B.Y., K.H. Soon, M.L. Husain, I. Sulong and E.K. Teh, (2001). The sedimentary process in the mangrove forests of Pulau Che Wan Dagang, Kemaman, Terengganu. J. Biosci., 12 (1): 75-82.

17. Kamaruzzaman, K.Y., H. Suhaimi, E.K. Teh, H.F. Leong, K.H. Soon and K.Y. Chong, (2000). The determination of 230Th in the sediments: Sedimentation in the mangrove Forests of Pulau Sekeping, Kemaman, Terengganu. J. Ultra Sci. Phys. Sci., 13 (2): 239-245.

18. Koide, M.A. Soutar and E.D. Goldberg.(1972). Marine geochronology with ²¹⁰Pb. Earth Planet. Sci. Lett., 14: 442-446.

19. Krishna Mohan, G.V. (2008). “Studies on the Ecorestoration of Bhavanapadu Mangroves of Srikakulam District of Andhra Pradesh”. Ph.D. Thesis, Andhra University, Visakhapatnam.

20. Kathiresan, K., (2010).Mangrove Ecosystems. Distribution of Mangroves. In: Kathiresan, K. and Ajmal Khan, S. (Ed.) “International Training Course on Coastal Biodiversity in Mangrove Ecosystems”, Annamalai University, Parangipettai, India. pp: 436-462.

21. Lynch, J.C., J.R. Meriwether, B.A. McKee, F. Vera-Herrera and R.R. Twinlley,(1989). Recent accretion in mangrove ecosystems based on 137Cs and ²¹⁰Pb. Estuaries, 4: 284-299.

22. Miralles, J., O. Radakovitch and C. Aloisi, (2005). ²¹⁰Pb sedimentation rate from the Northwestern Mediterranean margin. Elsevier Science. Mar. Geol., 216: 155-167.

23. Shahbuddin, S., H. Mohd Lokman, Y. Rosnan and T. Asano, (1998). Sediment accretion and variability of sedimentological characteristics of a tropical estuarine mangrove: Kemaman, terengganu, Malaysia. Mangroves and Salt Marshes, 55:1-8.

24. Sharma, R., L.R. Gardner, W.S. Moore and M.S. Bollinger, (1987). Sedimentation and bioturbation in a salt marsh as revealed by ²¹⁰Pb, 137Cs and 7Be studies. Limnol. Oceanogr., 32: 313-326.

25. Spenceley, A.P.,(1982). Sedimentation patterns in a mangal on Magnetic Island near Townville, North Queensland, Australia. Singapore J. Trop. Geogr.,.3: 100-107.

26. Turekian, K.K. and K.K. Wadepohl,(1961). Distribution of the elements in some major units of the earth’s crust. Bull. Geol. Soc. Am., 72: 175-192.

27. Wolanski, E., Y. Mazda and P. Ridd,(1992). Mangrove Hydrodynamics. In: Tropical Mangrove Ecosystems, Robertson, A.I. and D.M. Alongi (Eds.). American Geophysical Union, Washington D.C., pp: 436-462.

28. Zuo, Z., (1991). Dynamic behaviour of ²¹⁰Pb, ²¹⁰Po and 137Cs in Coastal and Environments, Geboren Te NanJing, China.

Received on 24.05.2011 Modified on 01.06.2011

Asian J. Research Chem. 4(7): July, 2011; Page 1137-1140