*For correspondence. (e-mail: meloth@ncaor.org)
Association of trace elements with various geochemical phases in the Indian sector of Southern Ocean during past 22,000 years and its palaeoceanographic implications
K. V. Sruthi, Meloth Thamban*, M. C. Manoj and C. M. Laluraj
National Centre for Antarctic and Ocean Research, Headland Sada, Goa 403 804, India
Adopting sequential extraction procedures, Ba, Cu, Mn, Ni, V and Zn concentrations were determined in the moderately reducible, organically bound, carbonate- associated and adsorbable fractions within a sediment core collected from the Indian sector of the Southern Ocean. The elemental abundances were studied with reference to the Last Glacial Maximum (LGM), degla- ciation and Holocene periods. The study showed, with the exception of Ba, elemental abundances in the following order: moderately reducible > organically bound > carbonate-associated > adsorbable fractions.
Ba showed high affinity to organically bound and car- bonate-associated fractions. Ba concentration revealed large variability (200–1400 ppm) within the carbonate- associated fraction and is related to the reduced carbonate productivity during LGM and increased carbonate productivity during deglaciation as well as Holocene intervals. The relative increase in the con- centration of V, Mn and Ni in the reducible fraction suggests enhanced suboxic conditions during LGM.
The decreased concentrations of V, Mn and Ni during the deglaciation and late Holocene indicate oxygen- ated conditions. The suboxic conditions during LGM could be attributed to reduced ventilation resulting from reduced strength of the global thermohaline circulation at this time interval.
Keywords: Last Glacial Maximum, redox condition, sequential extraction, Southern Ocean, trace element.
PROCESSES influencing the recycling and burial of ele- ments in marine sediments affect the geochemical cycle of oceans on different timescales. Major processes con- trolling the concentration of elements in marine sediments are detrital influx, biological productivity, authigenic precipitation as well as the submarine hydrothermal activity1. Sedimentary proxy records have been effec- tively used in the past towards the reconstruction of sea surface temperature, productivity and environmental changes on millennial to centennial timescales2. The Southern Ocean is unique among the world’s oceans due to its circumpolar connectivity and close interactions with
other oceans3. The biogeochemical cycles in the Southern Ocean have a large impact on global climate change, as it is a region of intermediate and deep water mass forma- tion, apart from being a High Nutrient Low Chlorophyll (HNLC) region. It plays an important role in controlling the atmospheric CO2 concentration. Geochemical cycling of trace metals, particularly iron, also influences the bioavailability of nutrients as well as productivity in the Southern Ocean4. Transportation and distribution of trace metals in the Southern Ocean is controlled by processes like atmospheric fallout, oceanic upwelling, advection through currents, melting of ice as well as export from the euphotic zone by settling and remineralization5. The Southern Ocean played a major role in the modula- tion of CO2 concentration during the glacial–interglacial periods and its relation to natural climate variability2. It has been reported that the concentration of CO2 in the atmosphere during the last glacial period was approxi- mately 80 ppm lower than in the pre-industrial times4. The mechanisms attributed to such low CO2 levels include: (i) an increase in the strength of biological pump of the Southern Ocean as a result of changes in the supply or utilization of nutrients or light3; (ii) an increase in the alkalinity of the Southern Ocean6 and (iii) inhibiting gas exchange between the deep sea and the atmosphere in the Southern Ocean either by increase of Antarctic sea ice7 or midwater stratification8. Enhanced CO2 during the degla- ciation and interglacial periods has been explained by the increased wind-driven upwelling and deep water ventila- tion during these periods2.
The Southern Ocean palaeoceanography is mainly con- trolled by the past fluctuations in oceanic fronts and changes in important water masses such as advancement of Antarctic Bottom Water (AABW) and strong currents related to the Antarctic Circumpolar Current (ACC)9. These water masses play an important role in the global climate system as reservoirs of heat, freshwater and dis- solved gases and act as a damping mechanism on varia- tions in the global climate10. The southern westerlies are the most dominant atmospheric circulation in the Southern Ocean and are considered to have significantly changed positions in the past, associated with shifting of frontal regimes2. The westerlies are also the carriers of aeolian dust transported to the open ocean, which fertilizes the water column by trace nutrients like iron and is related to the changes in atmospheric CO2 content in respect of relative positions of the westerlies and ACC.
A few studies were conducted in the Indian sector of the Southern Ocean sediments with a focus on the trace metal geochemistry. Several workers highlighted the role of trace metals in the Southern Ocean processes4,11,12. The mechanisms by which these trace metals participate in marine biogeochemical cycles are still not well under- stood13,14. In the Southern Ocean biogeochemical cycling, Fe is considered to be a limiting factor for the biological productivity that could directly influence the atmospheric
CO2 concentration4,15. Distribution of Zn, Ni, Cu, Ba, Cd and U is found to correlate with major nutrients implying that common biogeochemical processes control their oce- anic distribution12. Zn is known to be incorporated into diatom frustules and hence co-varies with Si16. Ni and Cu concentrations provide indirect evidence for controls on biological uptake and regeneration cycles17. The correla- tion of Cd and P concentrations in the ocean corresponds well with the marine biological productivity13. The varia- tions in redox-sensitive elements such as Mn, Cd and U in marine sediments have been used to understand the past oceanic oxygen levels at the sediment–water inter- faces18,19. The Southern Ocean sediments are ideally suited to decipher the regional and global changes in trace metal contribution and its palaeoceanographic signi- ficance. The present study using a sediment core from the Indian sector of the Southern Ocean investigates the changes in various leachable fractions of trace metals and the processes involved in their distribution during the last glacial to interglacial period.
The Indian sector of the Southern Ocean is characteri- zed by strong and complex physico-chemical and biological characteristics due to the confluence in latitu- dinally restricted zone of subtropical, subantarctic and polar frontal regimes (Figure 1). During the 200th expedi- tion of ORV Sagar Kanya, a gravity core (SK200/22a) was collected from a water depth of 2,300 m within the sub- Antarctic zone of the Indian sector of the Southern Ocean at 43°42′S lat. and 45°04′E long. (Figure 1). The sedi- mentological and rock magnetic variations in the sedi- ment core have been reported earlier20,21. The ages of the
Figure 1. Study area, oceanic fronts and location of sediment core SK200/22a. Major frontal systems depicted are: Subtropical front (STF), Sub-Antarctic front (SAF) and the Polar front (PF).
sediment at different depth intervals of the core were determined by 14C dating of handpicked planktic fora- miniferal species, Globigerina bulloides and Neoglobo- quadrina pachyderma, using an accelerated mass spectrometer (AMS). The radiocarbon ages were cor- rected and converted to calendar years using the CALIB 5.0.2 program22. For calibration, Δ–R correction value of 800 years was used for the core location23. Accordingly, the top 185 cm of the core was used for the present study.
This segment of the core represents the past 22,000 years (22 ka), including the LGM and the present interglacial period. The sedimentation rates of the core vary from 4 to 10 cm/ka and are highest during the glacial interval (Fig- ure 2). Subsamples of the core at 4 cm interval were sub- jected to sequential leaching experiments following the protocol by Tessier et al.24. The various fractions (adsorbable, carbonate-associated, moderately reducible, organically bound) were sequentially extracted using 40 mg of sediment. The exchangeable/adsorbable fraction was obtained by treating the sample with ammonium ace- tate for 15 min at room temperature. The carbonate frac- tion was obtained by treating the residue with 1 M sodium acetate at room temperature. The moderately re- ducible fraction (bound to Fe–Mn oxides) was obtained by treating the above residue with 0.25 M hydroxyl ammonium hydrochloride for 6 h at 90°C. The organi- cally bound fraction was obtained by extracting the resi- due with 0.01 M nitric acid and hydrogen peroxide at 85°C for 3 h. The leaching procedures were verified by repeating the extractions in duplicate splits of selected samples. The concentrations V, Mn, Ni, Cu, Zn, Ba from each fraction were determined using ICP-MS (Thermo Elemental X 7 Series) equipped with a collision cell.
Continuous calibration with standard and blanks was per- formed during measurements. Blank levels for different elements were within 0.5 ppm. As no certified standard
Figure 2. AMS 14C dates and sedimentation rates for the core SK200/22a.
Table 1. Average trace element concentration in different geochemical phases for different periods (in ppm)
Time-period Ba Cu Mn Ni V Zn
Adsorbable/exchangeable Interglacial 41.1 0.75 2.4 0.16 0.90 1.4
Early deglacial 59.2 0.66 1.0 0.12 0.61 0.75
LGM 44.2 0.69 0.79 0.43 0.61 0.85
Average concentration 48.2 0.7 1.4 0.24 0.71 1
Carbonate-associated Interglacial 925.3 2.9 4.3 0.88 0.40 4.5
Early deglacial 697.3 1.3 2.0 1.6 0.98 3.7
LGM 540.5 0.84 1.3 1.3 0.87 2.4
Average concentration 721.0 1.7 2.5 1.3 0.75 3.5
Moderately reducible Interglacial 128.5 15.3 56.0 2.9 6.4 21.8
Early deglacial 95.4 13.2 19.1 2.5 6.3 13.9
LGM 166.4 23.6 5.7 9.8 17.9
Average concentration 130.1 13.3 32.9 3.7 7.5 17.9
Organically bound Interglacial 364.8 2.4 5.0 1.9 1.0 9.4
Early deglacial 279.2 1.3 3.3 1.4 2.3 3.3
LGM 260.9 0.79 2.0 0.94 2.1 2.7
Average concentration 301.6 1.5 3.4 1.4 1.8 5.1
Figure 3. Down core variation of Ba, Cu, Mn, Ni, V and Zn distribution in adsorbable fraction.
reference materials are available for the analysis of sequentially extracted leaches, only the precision and re- producibility could be estimated. Rhodium was employed as an internal standard for better precision in ICP-MS measurements. The reproducibility of all trace element values determined by duplicate analyses was better than 6%.
Distribution of trace elements in four different geo- chemical fractions is shown in Table 1 and Figures 3–6.
The order of elemental distribution in different fractions is as follows: moderately reducible fraction > organically bound fraction > carbonate-associated fraction > adsor- bable/exchangeable fraction, except for Ba (Table 1). Ba shows significant association with carbonate and organi- cally bound fraction. We discuss the down core geo- chemical variations with reference to three time-periods:
glacial (22–18 ka), early deglacial (18–12 ka) and inter- glacial (1–12 ka; Figures 3–6). The redox-sensitive
Figure 4. Down core variation of Ba, Cu, Mn, Ni, Zn and V distribution in carbonate-associated fraction.
elements like V and Ni show higher concentrations dur- ing the glacial periods, while Mn shows a higher concen- tration during the interglacial period in all the four fractions (Table 1 and Figures 3–6). Ba shows a signifi- cantly higher concentration during the interglacial period in the carbonate-associated fraction. Within the organi- cally bound fraction, Ba, Cu and Zn showed an increase in concentration during glacial period (Figures 4 and 6).
In the organically bound fraction, Ba shows a positive correlation with Cu (r = 0.46, n = 49) and Zn (r = 0.59, n = 49).
Barium is the dominant element in the adsorbable frac- tion (Figure 3), possibly due to its greater ability to form surface complexes25. Ba, Cu and Ni were higher during the interglacial period, whereas V was lowest during this period. The increased values of Ba, Cu and Ni are syn- chronous with the enhanced Mn concentration, pointing to the scavenging capability of the Fe–Mn oxides26. Ba is also higher in the carbonate-associated fraction (Figure 4) which indicates its affinity towards the carbonate spe- cies27. Gradual increase of Ba in the carbonate fraction from early deglaciation to Holocene indicates increase in productivity with increase in sea surface warming. There are studies showing increased export production occur- ring either at glacial/interglacial transitions or during interglacial intervals28. The low concentrations of Ba, Cu and Zn (Table 1) in the carbonate fraction during the glacial period may also indicate the higher alkalinity of the Southern Ocean6. It has been established that the ap- parent reductions in the North Atlantic Deep Water (NADW) during LGM lead to an increase in deep-water
nutrients and dissolution of carbonate sediments in the Southern Ocean29.
The redox-sensitive elements, in general, are higher in the moderately reducible fraction (Figure 5) due to the high adsorption capacity of the Fe–Mn oxides associated with this fraction. Dominant elements (V, Cu, Ba) in this fraction are adsorbed on the Fe–Mn oxides and are ther- modynamically unstable under anoxic conditions24. V, Cu and Ba concentrations in this fraction are higher during LGM compared to those in the subsequent deglaciation period, with V remaining uniformly low during the deglacial and Holocene intervals. Previous studies have indicated that V is highly redox-sensitive, with enhanced preservation under reduced environments19,30. Relatively depleted V and enhanced Mn concentration in this frac- tion during Holocene support increase of oxygen in the bottom water of the Southern Ocean during the period.
The variations of Cu and Zn imitate that of Ba, suggest- ing dominant control of biological productivity and related biogeochemical processes on their distribution.
The organically bound fraction (Figure 6) is dominated by Ba (125–450 ppm) followed by Cu (1–10 ppm) and Zn (10–30 ppm). Higher concentration of Ba in this fraction could be due to its association with marine phytoplank- ton, which incorporates Ba during metabolic uptake and is removed from the water column by adsorption on sink- ing organic particlesand its release during decay31–33. The biological uptake of Zn and Cu as well as their regenera- tion cycle is well established14,15. Barium is moderately correlated with Cu (r = 0.46, n = 49) and Zn (r = 0.59, n = 49). Therefore, the temporal variations of Ba, Cu, Ni
Figure 5. Down core variation of trace elemental distribution in moderately reducible fraction.
Figure 6. Down core variation of trace elemental distribution in organically bound fraction.
and Zn support a significant increase in biological pro- ductivity during the early deglaciation and decreasing productivity towards late glacial to Holocene boundary.
The concentration of Ba, Cu and Zn increases again dur- ing the Holocene (Figure 6). Several geochemical proxy records from the Southern Ocean indicate increased
productivity towards the end of LGM and the beginning of subsequent deglaciation7,34–36.
Concentrations of redox-sensitive elements such as V and Ni are high during LGM (Table 1 and Figure 5), sug- gesting the suboxic bottom water conditions during LGM.
Under reducing conditions, V (V) is reduced to V (IV) and forms vanadyl ions (VO2–), related hydroxyl species and insoluble hydroxides30. Such a condition is supported by the reported reduction in dissolved oxygen in the deep water during LGM, possibly associated with a reduced deep circulation through the North Atlantic29,36. The Holocene is characterized by enhanced concentration of Mn, suggesting more oxidizing condition during late Holocene. Such a condition supports strengthened deep- water ventilation during the Holocene37,38.
The present study of selected trace element species during the past 22 ka reveals that the Ba, Cu and Zn are largely associated with biological sources, whereas V, Mn and Ni are associated with oxic/suboxic conditions at the sediment–water interface. The concentration profile of Ba in the carbonate-associated fraction indicates reduced carbonate productivity during the glacial period and its increase during deglaciation to Holocene transi- tion. The speciation studies shows that the biologically controlled trace elements contribute more towards the organically bound and carbonate-associated fractions, whereas the redox-sensitive elements are higher in the moderately reducible and adsorbable fractions. The gla- cial–interglacial variations of the redox-sensitive ele- ments support a reduced ventilation of the deep-water column due to a cessation of reduced strength of the global thermohaline circulation during LGM and its sub- sequent strengthening during the Holocene.
1. Pattan, J. and Pearce, N. J. G., Bottom water oxygenation history in southeastern Arabian Sea during the past 140 ka. Results from redox-sensitive elements. Palaeogeogr., Palaeoclimatol., Palaeoecol., 2009, 280, 396–405.
2. Anderson, R. F., Ali, S., Bradtmiller, L. I., Nielsen, S. H. H., Flei- sher, M. Q., Anderson, B. E. and Burckle, L. H., Wind driven up- welling in the Southern Ocean and the deglacial rise in atmospheric CO2. Science, 2009, 323, 1443–1448.
3. Anderson, R. F. and Smith Jr, W. O., The US Southern Ocean Joint Global Ocean Flux Study: volume two. Deep Sea Res. Part II, 2001, 48, 3883–3889.
4. Martin, J. H., Glacial–interglacial CO2 change: the iron hypothe- sis. Paleoceanography, 1990, 5, 1–13.
5. Lai, X., Norisuye, K., Mikata, M., Minami, T., Bowie, A. R. and Sohrin, Y., Spatial and temporal distribution of Fe, Ni, Cu and Pb along 140°E in the Southern Ocean during austral summer 2001/02. Mar. Chem., 2008, 111, 171–183.
6. Rickaby, R. E. M., Elderfield, H., Roberts, N., Hillenbrand, C. D.
and Mackensen, A., Evidence for elevated alkalinity in the glacial Southern Ocean. Paleoceanography, 2010, 25, PA1209.
7. Elderfeld, H. and Rickerby, R. E. M., Oceanic Cd/P and nutrient utilisation in the glacial Southern Ocean. Nature, 2000, 405, 305–
310.
8. Toggweiler, J. R., Variation of atmospheric CO2 by ventilation of the ocean’s deepest water. Paleoceanography, 1999, 14, 571–588.
9. Mackensen, A., Changing Southern Ocean paleocirculation and effects on global climate. Antarct. Sci., 2004, 16, 369–386.
10. Tomczak, M. and Liefrink, S., Interannual variations of watermass volumes in the Southern Ocean. J. Atmos. Sci., 2005, 10, 31–42.
11. Löscher, B. M., Relationships among Ni, Cu, Zn, and the major nutrients in the Southern Ocean. Mar. Chem., 1999, 67, 67–102.
12. Frew, R., Bowie, A., Croot, A. and Pickmere, S., Macronutrient and trace-metal geochemistry of an in situ iron-induced Southern Ocean bloom. Deep Sea Res. Part II, 2001, 48, 2467–2481.
13. Nolting, R. F. and De baar, H. J. W., Behaviour of nickel, copper, zinc and cadmium in the upper 300 m of a transect in the Southern Ocean (57°–62°S, 49°W). Mar. Chem., 1994, 45, 222–242.
14. Hutchins, D. A. and Bruland, K. W., Fe, Zn, Mn, and N transfer between size classes in a coastal phytoplankton community: trace metal and major nutrient recycling compared. J. Mar. Res., 1995, 53, 297–313.
15. Kumar, N., Anderson, R. F., Mortlock, R. A., Froelich, P. N., Kubik, P., Dittrich-hannen, B. and Suter, M., Increased biological productivity and export production in the glacial Southern Ocean.
Nature, 1995, 378, 675–680.
16. Ellwood, M. J. and Hunter, K. A., Determination of the Zn/Si ratio in diatom opal: a method for the separation, cleaning and dissolu- tion of diatoms. Mar. Chem., 1999, 66, 149–160.
17. Sunda, W. G., Trace metal/phytoplankton interactions in the sea.
In Chemistry of Aquatic Systems: Local and Global Perspectives (eds Bidoglio, G. and Stumm, W.), ECSC, EEC, EAEC, Brussels, Luxemburg, 1994, pp. 213–247.
18. Rosenthal, Y., Boyle, E. A., Labeyrie, L. and Oppo, D., Glacial enrichments of authigenic Cd and U in sub-Antarctic sediments: a climatic control on the element’s oceanic budget? Paleoceanogra- phy., 1995, 10, 395–413.
19. Morford, J. L. and Emerson, S., The geochemistry of redox sensi- tive trace metals in sediments. Geochim. Cosmochim. Acta, 1999, 63, 1735–1750.
20. Thamban, M. et al., Changes in the source and transport mecha- nism of terrigenous input to the Indian sector of Southern Ocean during the late Quaternary and its palaeoceanographic implica- tions. J. Earth Syst. Sci., 2005, 114, 443–452.
21. Manoj, M. C., Thamban, M., Basavaiah, N. and Mohan, R., Evi- dence for climatic and oceanographic controls on terrigenous sedi- ment supply to the Indian Ocean sector of the Southern Ocean over the past 63,000 years. Geo-Mar. Lett., 2012, 32, 251–265.
22. Stuiver, M., Reimer, P. J. and Reimer, R. W., CALIB 5.02 (pro- gram and documentation), 2005; http://www.calib.qua.ac.uk/
23. Bard, E., Correction of accelerator mass spectrometry 14C ages measured in planktonic foraminifera: paleoceanographic implica- tions. Paleoceanography, 1988, 3, 635–645.
24. Tessier, A., Campbell, P. G. C. and Bisson, M., Sequential extrac- tion procedure for the speciation of particulate trace metals. Anal.
Chem., 1979, 51, 844–851.
25. Cole, T., Bidoglio, G., Soupioni, M., O’Gorman, M. and Gibson, N., Diffusion mechanisms of multiple strontium species in clay.
Geochim. Cosmochim. Acta, 2000, 64, 385–396.
26. Dong, D. M., Yang, F., Li, Y., Hua, X. Y., Lü, X. J. and Zhang, J. J., Adsorption of Pb, Cd to Fe, Mn oxides in natural freshwater surface coatings developed in different seasons. J. Environ. Sci.
(China), 2005, 17, 30–36.
27. Schnetger, B., Brumsck, H. J., Schale, H., Hinrichs, J. and Dittert, L., Geochemical characteristics of deep-sea sediments from the Arabian Sea: a high resolution study. Deep Sea Res. Part II, 2000, 47, 2735–2768.
28. Latimer, J. C. and Filippelli, G. M., Glacial/interglacial producti- vity in the subantarctic Southern Ocean: ODP Site 1089 vs Site 1171. American Geophysical Union, Fall Meeting, 2001, abstr.
#PP52B-04.
29. Howard, W. R. and Prell, W. L., Late Quaternary CaCO3 produc- tion and preservation in the Southern Ocean: implications for
*For correspondence. (e-mail: biswajeet_thakur@yahoo.co.in) oceanic and atmospheric carbon cycling. Paleoceanography,
1994, 9, 453–482.
30. Algeo, T. J. and Maynard, J. B., Can marine anoxic events draw down the trace element inventory of seawater? Geology, 2004, 32, 1057–1060.
31. Dehairs, F., Lambert, C. E., Chesselet, R. and Risler, N., The bio- logical production of marine suspended barite and the barium cycle in the western Mediterranean Sea. Biogeochemistry, 1987, 4, 119–139.
32. Dymond, J., Suess, E. and Lyle, M., Barium in deep-sea sediment:
a geochemical indicator of paleoproductivity. Paleoceanography, 1992, 7, 163–181.
33. Stamatakis, M. G. and Hein, J. R., Origin of barite in tertiary marine sedimentary rocks from Lefkas Island. Greece Econ. Geol., 1993, 88, 91–103.
34. Anderson, R. F., Kumar, N., Mortlock, R. A., Froelich, P. N., Kubik, P., Dittrich-hannen, B. and Suter, M., Late Quaternary changes in productivity of the Southern Ocean. J. Mar. Syst., 1998, 17, 497–514.
35. Bareillie, G., Labracherie, M., Bertrand, P., Labeyrie, L., Lavaux, G. and Dignan, M., Glacial–interglacial changes in the accumula- tion rates of major biogenic components in Southern Indian Ocean sediments. J. Mar. Syst., 1998, 17, 527–539.
36. Rutberg, R. L., Hemming, S. R. and Goldstein, S. L., Reduced North Atlantic deep water flux to the glacial Southern Ocean inferred from neodymium isotope ratios. Nature, 2000, 405, 935–
938.
37. Skinner, L. C., Fallon, S., Waelbroeck, C., Michel, E. and Barker, S., Ventillation of the deep Southern Ocean and deglacial CO2
rise. Science, 2010, 328, 1147–1151.
38. Knorr, G. and Lohmann, G., Southern Ocean origin for the resumption of Atlantic thermohaline circulation during deglacia- tion. Nature, 2003, 424, 532–536.
ACKNOWLEDGEMENTS. We thank the Ministry of Earth Science (MoES) and the Director, National Centre for Antarctic and Ocean Research (NCAOR), Goa for support. We also thank Dr Rahul Mohan (NCAOR) and Dr M. Sudhakar (MoES) for their support in sample retrieval, and B. L. Redkar for technical support with ICP-MS. K.V.S.
thanks Dr N. C. Kumar, Department of Chemical Oceanography, Cochin University of Science and Technology, Kochi for the guidance and encouragement. We thank to the anonymous reviewer for construc- tive and useful suggestions. This is NCAOR contribution number 37/2012.
Received 3 October 2011; revised accepted 3 September 2012
Primary productivity and organic matter distribution during SW and NE monsoon from Alleppey mudbanks, Kerala, India
Biswajeet Thakur*, Vandana Prasad and Rahul Garg
Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow 226 007, India
Formation of mudbank during summer monsoon and its dissipation during winter is a characteristic feature of the southwest coast of Kerala, South India. Both southwest (SW) and northeast (NE) monsoons play an important role in the overall run-off-related changes and sedimentation pattern in this region that governs the primary productivity in the region. The present study is an attempt to assess primary productivity and precipitation-induced run-off-related changes using biotic proxies (diatoms and palynofacies) during NE and SW monsoons, from the surface sediments of the Alleppey mudbanks along Kerala coast. ANOVA test was performed on diatoms and palynofacies to assess their significance during both monsoonal periods. The study provides a significant insight into the monsoon- controlled palynofacies distribution behaviour pattern that attests to the concept of precipitation-induced high run-off during SW monsoon being the governing factor in the formation of mudbanks in this region.
Keywords: Diatoms, monsoons, mudbank, palynofa- cies, primary productivity.
THE state of Kerala, situated on the southwest coast of the Indian subcontinent, is bestowed with 41 westerly-flowing rivers and estuarine complexes with a coastline extending for more than 560 km. The formation of mudbanks during the southwest (SW) monsoon period is a unique pheno- menon that is exhibited at certain locations along the coastal region (Malabar coast) of Kerala1,2 (Figure 1).
The Alleppey region along the Malabar coast experiences formation of mudbanks during the SW monsoon period3. Mudbanks are patches of calm, turbid water with heavy suspended load of clay-size particles less than 1 μm in size2,4. The historical record of Alleppey mudbanks is known since 1885, the formation of which is an annual phenomenon during June to August5. The geographical extent of the mudbanks and their nature and formation based on sediment characteristics have been discussed in detail, demonstrating fine deposition of silt and mud6–9. The phenomenon of coastal upwelling for mudbank forma- tion is regarded to be one of the major factors along the Malabar coast, Kerala10. Mudbank formation by run-off- induced activity and coastal upwelling during monsoon
