Climate-induced Late-Holocene ecological changes in Pichavaram estuary, India

ORIGINAL ARTICLE

Climate-induced Late-Holocene ecological changes inPichavaram estuary, IndiaJyoti Srivastava1, Anjum Farooqui1 & Shaik M. Hussain2

1 Birbal Sahni Institute of Palaeobotany, Lucknow, India

2 A.C. College Campus, University of Madras, Chennai, India

Keywords

Climate; Late Holocene; Pichavaram; salinity;

thecamoebians.

Correspondence

Jyoti Srivastava, Birbal Sahni Institute of

Palaeobotany, 53 University Road, Lucknow

226007, U.P., India.

E-mail: [email protected]

Accepted: 11 December 2012

doi: 10.1111/maec.12048

Abstract

Variation in sedimentology as well as freshwater and marine palynomorphs has

been studied in ecological perspective in two 2.5- and 5-m deep sediment cores

deposited since 3440 and 3630 cal BP, respectively in the central part of Pich-

avaram mangrove wetland, Cauvery River delta. The palynological and sedi-

mentological results of the sediments reveal a monsoonal circulation and a

climatic shift from warm and humid with strengthened monsoon (3630–3190 cal BP) to dry and arid (~2750–760 cal BP). Since the last millennium

(~760 cal BP), Pichavaram estuary has been influenced by a similar cyclicity

but with a less wet and humid climate due to weakened monsoon conditions.

These ecological changes in turn affect the relative sea level rise and fall which

is reflected by the variability/extinction of freshwater and marine pal-

ynomorphs. The estuary remained an active water channel between ~3630 and

2750 cal BP, responding to the strengthened monsoon, during which the fresh-

water algal remains with thecamoebians and marine dinoflagellate cysts and

foraminiferal linings both dominated with a ratio of 1.5 for marine/freshwater

forms. After this period, since ~2750 cal BP there has been a dominance of

marine forms with a ratio of 4.5 for marine/freshwater forms, indicating fluvi-

o-marine sediment deposition and suggesting the recent landward intrusion of

seawater during weakened monsoon conditions. Freshwater thecamoebians are

vulnerable to the salinity >3 in the aqueous soil solution of estuarine sediment,

and therefore serve as an excellent proxy for monitoring salinity gradient along

with short-term high resolution palaeoecological fluctuations induced by

climate and relative sea-level changes in an estuarine ecosystem.

Introduction

Estuaries are subjected to frequent salinity changes

depending on the interplay of tidal influx and the fresh-

water input from land. This blending of marine and fresh

water ushers in a change in flora and fauna assemblage

composition and behaviour, suggesting that some agents

such as tidal amplitudes and fluvial discharge can be qual-

itatively monitored through biotic features in estuaries

(Bonetti & Eichler 1997). The river systems of east coast

of India are essentially monsoon-driven and as such the

sediments embedded in their deltas are considered excel-

lent repositories of past monsoon events to which the

biotic forms have responded. The composing organisms,

spores/pollen, thecamoebians, foraminifera and dinofla-

gellates are useful hydrodynamic bioindicators because

their microfaunal composition demonstrate environmen-

tal characteristics such as sedimentological features, cli-

mate and ecological changes (Ellison 1995; Patterson &

Kumar 2002; Lahr et al. 2006). Those studies have been

focused on higher latitudes, whereas few records are from

the tropical zone (Roe & Patterson 2006). There are

474 Marine Ecology 34 (2013) 474–483 ª 2013 Blackwell Verlag GmbH

Marine Ecology. ISSN 0173-9565

records of intermittent relative sea-level rise and fall dur-

ing Late Holocene from Pulicat lagoon and other contem-

porary sites along the east coast (Bannerjee 2000;

Farooqui & Vaz 2000).

Stratigraphic changes in the palynological assemblages

in sediments responding to ecological changes during

Late Holocene induced by external factors such as relative

sea level and climate have been studied from Pichavaram

wetland. Palynomorphs such as the freshwater algal mat-

ter, fungal remains, dinoflagellate cysts, thecamoebians,

foraminifera and loricated tintinid remains are often

completely overlooked while studying the palynological

assemblages retrieved from the Quaternary sediments of

India. Nevertheless, the paucity of spores and pollen

grains in sediments make it necessary to look for alterna-

tive forms for interpretation of palaeoenvironmental con-

ditions. Thus such biotic forms constitute an important

component of the palynological data for palaeoecologial

information. Based on the respective occurrence of terres-

trial and marine palynomorphs, it is possible to assess the

freshwater or marine character of sediments, and thus to

reconstruct climatic changes in the past along with the

relative sea level. In the present study, a combination of

freshwater algal matter, fungal remains, dinoflagellate

cyst, thecamoebian, foraminifera and loricated tintinid

assemblages and abiotic parameters such as sediment tex-

ture and salinity were examined for sediment cores col-

lected from a swampy region and from an exposed land

section of the Pichavaram estuary to identify a deposi-

tional zonation representing hydrodynamic and ecological

patterns that could be applicable to paleoenvironmental

analysis in the Southeast coast of India.

Study area

The Pichavaram mangrove ecosystem (latitude 11º25′ Nand longitude 79º47′ E) is a shallow estuarine complex

sandwiched between two prominent estuaries, Vellar estu-

ary in the north and Coleroon estuary in the south, with a

total area of 1100 ha. The complex has 15 islets ranging in

size from 10 m2 to 2 km2 separated by intricate waterways,

which connect the Vellar estuary in the north and the

Coleroon estuary in the south (Ramanathan 1997). The

Coleroon estuary part is largely dominated by mangroves,

whereas the Vellar estuary is dominated by mud-flats.

Tidal water enters the Pichavaram mangrove through a

small direct connection with the Bay of Bengal at Chinna-

vaikal and estuarine water finds its way through the two

adjacent river systems (Fig. 1). Site P2 is located in the

central part of Pichavaram estuary and is mainly inhabited

by Avicennia officinalis, Avicennia marina and Suaeda sp.,

with fringes of Rhizophora sp. along the backwater channel.

Site T2 is towards the south of Pichavaram close to the

village TS Pettai and is dominated by salt-tolerant Suaeda

sp. with A. marina and A. officinalis occupying the land-

ward zones.

Material and Methods

Two sediment cores, P2 and T2, measuring 2.5 and 5 m

depth, respectively, were obtained using a hand-operated

augur cum piston corer (Eijelkamp, the Netherlands).

Immediately after collection, P2 and T2 cores were sub-

sampled at 2- and 5-cm intervals, respectively. The samples

were stored in air-tight polythene bags without any preser-

vative. In the laboratory, sediment colour was identified

using a Munsell colour chart (Munsell and Farnum, 2004)

and texture was analysed on the basis of the percentage of

sand in the sediment according to the soil density method

(USDA 1992). Salinity was measured in 10 g of an air-

dried soil sample dissolved in 100 ml of deionized water.

Prior to measuring salinity, the aqueous soil solution was

kept overnight after rigorous shaking for an hour. The

samples were homogenized for 30 min before measuring

the salinity using Orion-5 star (Thermo-Orion, Scientific

Equipment, USA) at standardized 25 °C temperature.

For the palynological and thecamoebian study, 10 g of

an air-dried soil sample was treated with 10% potassium

hydroxide (KOH) on a sand bath for 5 min and sieved

through 150-mesh. The filtrate was then acetolysed fol-

lowing Erdtman (1943). The samples were then passed

through 600-mesh and the residue (>10 lm) was col-

lected for palynological slides referring to standard litera-

ture (Ogden & Hedley 1980; Patterson & Kumar 2002).

The palynological spectra represent the percentage of total

300 counts of macrophyte pollen, freshwater algal spores,

fungal remains, thecamoebians, foraminifera and dinofla-

gellate cysts in homogenized 10 ml acetolysed sample.

The ratio of marine and freshwater forms was calculated

to determine the variability in these marine/freshwater

indicators during climatic fluctuations.

Results

Radiocarbon dates and age-depth models

The age of the oldest sediment in core P2 (�250 cm) is

3440 cal BP, at �138 cm the age of the sediment is

2430 cal BP, and at �65 cm the sediment is 590 cal BP

old. Similarly, the age of the oldest sediment in core T2

(�500 cm) is 3630 cal BP, at �85 cm the sediment is

2750 cal BP, and at �25 cm the sediment dates back to

760 cal BP. Plots of these calibrated dates against depth

are presented for both the sites with age-depth models

proposed to provide means for estimating sample ages

and sedimentation rates (SR) for the sequences.

Marine Ecology 34 (2013) 474–483 ª 2013 Blackwell Verlag GmbH 475

Srivastava, Farooqui & Hussain Ecological changes in Pichavaram estuary, India

The gradient of segments of the age-model for core P2

is shown in Fig. 2A. The lowermost segment (P2s-1a)

contains two dates with the deposition of heterogeneous

sediment. Estimated SR for the initial phase of infilling

prior to 2430 cal BP is higher (0.11 cm�year�1). After c.

2430 cal BP, estimated SR decreases to 0.03 cm�year�1

(P2s-1b), and again rises to 0.11 cm�year�1 for the per-

iod from c. 590 cal BP until present (P2s-2). As the

radiocarbon ages fall some distance from the sediment

zone boundaries, changes in SR occurring at zone

boundaries cannot be determined. However, the broad

contrast in SR is demonstrated between sediment zones

P2s-1a and P2s-2 (high average SR) and P2s-1b (reduced

SR).

As with core T2, estimated SR for the first phase of

infilling prior to c. 2750 cal BP is high (0.47 cm�year�1).

For the period between c. 2750 and 760 cal BP, esti-

mated SR falls to 0.03 cm�year�1. After 760 cal BP, esti-

mated SR remains reduced, suggesting long-term average

SR until present, around 0.03 cm�year�1. Lithostrati-

graphical zones are plotted next to the age model

(Fig. 2B). Zones T2s-1 and T2s-2 are characterized by

overall high average SR of around 0.5 cm�year�1. How-

ever, given that changes in lithological characteristics are

likely to be allied with the changes in deposition time,

sedimentation rates across the heterogeneous zones are

unlikely to have been uniform. Rather, sedimentation is

likely to have been episodic, alternating between periods

of comparatively slow accumulation of clay sediments

and rapid to instantaneous deposition of sands. Never-

theless, the total deposition time for T2s-1 and T2s-2

overall appears to have been short, on the order of

800 years.

Fig. 1. Map showing the study area in Pichavaram Mangrove (India).

The core locations are in the estuarine part of the study area from

the backwater channel. P2 is named for Pichavaram estuary as it is

retrieved from the central part of the estuary. Core T2 is named for

Tspettai because it is close to the village Tspettai towards the South

of Pichavaram.

A

B

Fig. 2. (A) Age-depth model, P2, based on linear interpolation. Solid

dots show the median calibrated ages. (B) Age-depth model, T2,

based on linear interpolation. Solid dots show the median calibrated

ages.


Ecological changes in Pichavaram estuary, India Srivastava, Farooqui & Hussain

Sedimentology and salinity

Core P2

The following lithological zones are distinguished from

base to the top:

P2s-1a: 250–120 cm: This zone contains predominantly

fine-grained sandy clay sediments with intermittent bands

of sand (5Y4/2). The zone shows a heterogeneous

sediment composition with medium to fine sandy layers

interspersed in a sandy clay composition which is pre-

dominantly fine-grained with a greater proportion of silt/

clay and fine sand.

P2s-1b: 120–85 cm: This zone contains sediment domi-

nated by sand (5Y4/2) with a relatively low percentage of

finer fractions of the sediment (silt and clay).

P2s-2: 85–0 cm: The uppermost zone is a clay-rich soil

(5Y2/2) with a homogeneous composition.

Salinity (P2): Values for salinity gradient are presented in

Fig. 3. Zone P2s-1a constitutes fine sand and clay with a

dominance of clay showing higher salinity with an aver-

age of 3.0 maximum going up to 4.4 in the intermittent

clayey layer and a minimum of 1.9. The values of salinity

in zone P2s-1b comprised sand as the main fraction with

an average salinity of 2.2 with a maximum of 2.4 and a

minimum of 2.1. Zone P2s-2 was dominated by fine clay

sediment fractions as compared with the other zones and

shows an average salinity of 4.0 with a maximum of 5.4–4.3 and a minimum of 2.1–3.6.

Core T2

The following lithological zones are distinguished from

base to the top:

T2s-1: 500–215 cm: Basal deposits of T2 sedimentary fill

are composed of sandy sediment (5Y4/2) with intermit-

tent bands of clay (5Y4/2). The zone shows a heteroge-

neous sediment composition with medium to find sand

Fig. 3. Diagram showing the radiocarbon dates, textural composition, salinity gradient for cores P2 and T2 from Pichavaram, India. The

lithostratigraphical zones are delimited on the basis of results of sediment analysis. The zones are prefixed with the site code and the letter’s’ to

distinguish them from independent biostratigraphical zones defined on the basis palynomorph data.



A

B

Fig. 4. (A) Palynological diagram showing the percentage count of the most frequent freshwater and marine forms and the total marine/

freshwater ratio calculated for Core P2 of Pichavaram Estuary, India This figure also includes three radiocarbon dates. (B) Palynological diagram

showing the percentage count of the most frequent freshwater and marine forms and the total marin/freshwater ratio calculated for Core T2 of

Pichavaram Estuary, India This figure also includes three radiocarbon dates.



interspersed by thin to broad bands of fine grained

clay.

T2s-2: 215–0 cm: Zone T2s-2 contains predominantly fine-

grained compact clay (5Y3/2) sediment overlying the

fluctuating sand zone. A thin band of sand and clay is

noted at 20 cm from the surface.

Salinity (T2): Values for the salinity gradient are presented

in Fig. 3. The topmost zone T2s-2 constituting fine clay

sediment fractions shows the highest salinity status with

an average of 3.6, a maximum value going up to 10.4

and a minimum of 1.9. The values of salinity in zone

T2s-2 constituting fine sand with intermittent clay bands

show intermediate salinity with an average of 1.8, maxi-

mum going up to 4.2 in the intermittent clayey layer and

a minimum of 0.8.

Palynomorph assemblage

On the basis of palynomorph succession, four climatic

phases are identified.

Core P2

Phase P2-I (250–200 cm): Macrophytes – freshwater algal

remains – thecamoebian assemblage – dinoflagellate cyst –

foraminiferal linings: Signatures of both freshwater and

marine forms mark this phase constituting freshwater

algae Botryococcus, Coelastrum, Pediastrum boryanum,

Pediastrum simplex, macrophytes such as Eichhornia and

Typha, thecamoebians such as Cyclopyxis, Nebela and

Arcella along with marine dinoflagellate cysts such as Spi-

niferites and Operculodinium, Scolecodont and brackish

water foraminiferal chitinous linings of Ammonia and

Cornuspira (Plate 1). The marine/freshwater forms ratio

for the present phase is calculated as 1.3 with an overall

abundance of both groups of palynomorphs (Fig. 4A).

Phase P2-II (200–75 cm): Freshwater algal remains –

thecamoebian assemblage – dinoflagellate cyst – foraminiferal

linings: This phase is characterized by overall reduced val-

ues for freshwater algae such as Pediastrum boryanum

and Pediastrum simplex and a relatively good percentage

of thecamoebians such as Arcella, Cyclopyxis and Nebela,

along with the abundance of marine forms such as dino-

flagellate cysts, and foraminiferal linings such as Ammo-

nia and Cornuspira. The ratio of marine and freshwater

form curve shows high to low peaks throughout the

phase, with an average ratio of 2.3 due to higher pro-

portion of foraminiferal linings, with Cornuspira and

dinoflagellate cysts in the uppermost section of the

phase.

Phase P2-III (75–25 cm): Marine foraminiferal linings: Phase

P2-III shows the dominance of marine forms with mainly

foraminiferal linings of Ammonia and Cornuspira along

with Scolecodont fragments with a reduction in the fresh-

water thecamoebians Cyclopyxis, Arcella and Nebela. The

ratio of marine and freshwater forms towards the marine

side also goes up to 7.0, increasing the prominence of

these forms.

Phase P2-IV (25–0 cm): Thecamoebian – marine foraminiferal

linings – dinoflagellate cyst: A low abundance of freshwater

algae Pediastrum boryanum and thecamoebians such as

Cyclopyxis and Nebela along with a good percentage of chi-

tinous linings of Ammonia and dinoflagellate cyst has been

recorded. The ratio of marine/freshwater forms is calculated

as 3.5, showing the abundance of marine indicators along

with a lower proportion of freshwater forms.

Core T2

Phase T2-I (500–280 cm): Macrophytes – freshwater algae –

thecamoebians – marine dinoflagellate cysts – foraminiferal

linings: The presence of macrophyte Typha, the abundance

of freshwater algae such as Botryococcus, Pediastrum borya-

num and Pediastrum simplex, and the dominance of thec-

amoebians such as Cyclopyxis and Arcella along with a

relatively low percentage of marine dinoflagellate cysts and

a good percentage of foraminiferal lining of Ammonia is

marked in this phase. Fungal spores, VAM fungi and Tet-

raploa are also well represented. The ratio of marine and

freshwater indicators comes out to 1.1, with an equal pro-

portion of both marine and freshwater forms (Fig. 4B).

Phase T2-II (280–100 cm): Macrophytes – freshwater taxa –

thecamoebians – marine dinoflagellate cysts – foraminiferal

lining assemblage: Phase T2-II is characterized by the pres-

ence of macrophytes such as Eichhornia, Typha and algae

such as Pediastrum boryanum, Pediastrum simplex and a

dominance of thecamoebians such as Arcella, Cyclopyxis

and Nebela along with the presence of dinoflagellate cysts

such as Spiniferites, Operculodinium and foraminiferal lin-

ings such as Ammonia. The ratio of marine/freshwater is

1.6, indicating a slightly higher abundance of marine

forms.

Phase T2-III (100–30 cm): Thecamoebians – marine

foraminiferal linings: Phase T2-III displays evidence of

dinoflagellate cysts, Spiniferites and Operculodinium and

foraminiferal linings such as Ammonia along with Scolec-

odont fragments, with a decline in thecamoebians such as

Cyclopyxis and Arcella. The marine/freshwater forms is

calculated as 5.4, indicating the prominence of marine

forms.



Plate 1. 1.Typha; 2. Myriophyllum; 3. Nebela; 4. Cyclopyxis intermedia; 5. Botryococcus; 6, 7. Pediastrum boryanum; 8. Pediastrum simplex; 9,

13. Centropyxis aculeata; 10. Arcella spp.; 11, 14. Arcella artocrea; 12.Centropyxis corona; 15. Cornuspira; 16, 17. Foraminiferal linings; 18, 19.

Spiniferites; 20. Operculodinium; 21, 22. Tintinid lorica remains.



Phase T2-IV (30–0 cm): Thecamoebian – marine foraminiferal

linings – dinoflagellate cysts: A relatively low percentage of

the freshwater palynomorphs compared with Phase T2-I

such as Pediastrum boryanum and thecamoebians such as

Cyclopyxis, Nebela and Arcella in the surface and sub-

surface sediment has been encountered. On the other

hand, an abundance of estuarine to marginal marine

palynomorphs such as dinoflagellate cysts, foraminiferal

linings such as Ammonia, Scolecodont fragments and

Tintinids has been recorded. The ratio of marine and

freshwater forms goes down to 2.3 due to an overall

dominance of marine forms with a low abundance of

freshwater indicators.

Discussion

Sediment analysis along with palynology has been under-

taken in the present study with a view to the character-

ization of the hydrodynamic environment of deposition.

The interpretation of textural data was only possible

within the framework of a general understanding of nat-

ure of sedimentation in the estuarine environment. Sedi-

ment transport within an estuary depends on several

factors, including river discharge, tidal circulation pat-

terns and salinity (Dyer 1979). Salt in the soil is best

washed away by freshwater but clay and silt are relatively

impermeable, hence the filtration process called leaching

is slow. In some relatively dry zones, salt already accumu-

lates on the surface and becomes crystallized. As a result

the salinity problems may persist for a long period unless

checked and measures are taken to remove the salts by

flushing or leaching (Sparks 1995). Palynomorph data for

freshwater algal forms, thecamoebians, foraminiferal lin-

ings and dinoflagellates also support important observa-

tions for the interpretation of hydrological condition of

the estuary. The dinoflagellates observed in the sample

suggest at least some tidal influence during the period of

infilling (Jennings et al. 1993). However, it is recognized

that biological indicators of brackish or marine condi-

tions may occur well upstream of sedimentary tidal influ-

ence, relating to periods of reduced river discharge (Frey

& Howard 1986). Pediastrum is considered an indicator

of freshwater conditions (Round 1965). The freshwater

protozoans that form agglutinated or autogenous tests are

the thecamoebians. They are also called Arcellaceans

because their tests resist dissolution in low pH environ-

ments, unlike those of other freshwater organisms such as

molluscs and ostracods (McCarthy et al. 1995; Dalby

et al. 2000). The mechanism of encystment enables these

organisms to populate dry areas and remain dormant

until water brings them back to active life. They are read-

ily transported over a long distance by a variety of agents.

Hence, they are found in freshwater to slightly brackish

environments, including freshwater lakes, estuarine envi-

ronment, salt and freshwater marshes, soil peat, moss

under tree bark, ponds and standing water (Medioli &

Scott 1983; Patterson et al. 1985). Foraminiferal linings

generally categorize hypo-saline lagoons and estuaries,

which may also be found in intertidal and shallow water

in coastal areas. Their frequent occurrence in palynologi-

cal preparations suggests an exclusively marine condition.

The radiocarbon data, sedimentological and palynolog-

ical records from the present study reveal four climatic

and ecological phases in Pichavaram estuary since Late

Holocene. First, a stabilized estuarine ecosystem as Phase

I (~3630–3190 cal BP), showing mixed conditions in the

water column with a relatively high fluvial energy due to

more freshwater input to the estuary, indicating strength-

ened monsoon conditions as evidenced by a higher sedi-

mentation rate, heterogeneous sediment composition, and

the presence of both freshwater (macrophyte, algae, thec-

amoebians) and marine forms (foraminifera and dinofla-

gellates). The freshwater and marine forms encountered

in this study include macrophyte pollen such as Typha

and Eichhornia, freshwater algal remains such as Botryo-

coccus and Pediastrum, freshwater thecamoebians such as

Cyclopyxis and Arcella, which reveal a lacustrine environ-

ment and are indicators of hydrological changes (fresh

water input). In contrast, dinoflagellate cysts (Spiniferites

and Operculodinium) and foraminiferal linings such as

Ammonia and Cornuspira in sediment indicate an estua-

rine to marginal marine depositional environment and

are an excellent marker for marine incursion during

lower hydrological input. There are also records of maxi-

mum winter precipitation in Northwestern India during

the Indus Culture period of 5000–3500 years BP (Singh

et al. 1974; Vishnu-Mittre & Sharma 1975).

In Phase II, an unstable depositional environment is

evident from the heterogeneous sediment constituting

medium to fine sand embedded with occasional bands of

fine sticky clay, perhaps due to occasional high tidal

influx and freshwater input from land during the

strengthened monsoon conditions in the early part of

Late Holocene (~3190 years BP). Variation in the salinity

gradient for Phase I and II also reflects this heterogeneous

sediment composition, showing finer clay particles with

greater affinity for salts in comparison with their coarser

counterparts. The clay particles are flexible and plastic

because of their lattice-like design and have a large sur-

face area which is chemically reactive, attracting and

holding positively charged nutrient ions (Hiller 2003).

Wide fluctuations in the dominance of freshwater thec-

amoebians, mainly Cyclopyxis and marine forms such as

dinoflagellate cysts and foraminiferal linings, along with

the salinity suggest climatic amelioration from warm and

humid to dry and arid (3190–2430 years BP). Records



using mixed proxies from Asia and Africa show a severe

and lasting drought during Middle-Late Holocene

(Kaniewski et al. 2008). A steady weakening of the Asian

summer monsoon between 3700 cal years BP and

1500 cal years BP has been reported from low and middle

latitudes in India and China (Selvaraj et al. 2008; Liu

et al. 2009), and is generally understood to be a response

to decreasing summer insolation (Overpeck et al. 1996).

The dominance of marine forms such as foraminif-

era and dinoflagellates with a decline in freshwater

thecamoebians in Phase III (2430–760 years BP) indicates

marine incursion with low freshwater input from land,

supporting a dry and arid climate due to weakened mon-

soon conditions. Low salinity recorded in the present

phase suggests deposition of a coarser sediment fraction

with a high proportion of sand with larger inter-particle

pore sizes causing vertical translocation of salts to deeper

layers. The study of lacustrine sediments of Southwestern

Ghats, India, also explains the reduction in rainfall and

arid climatic conditions in the beginning of Late

Holocene which slowly ameliorated to warm and humid

conditions until 780 years BP (Bera & Farooqui 2000).

The climate in Phase IV (since ~760 years BP) was less

warm and humid than Phase I due to weakened mon-

soons and comparatively stable low energy intertidal estu-

arine setting, favouring the deposition of fine grain

sediment and leading to an increase in salinity. In the

beginning of the last millennium (Phase IV) the climate

ameliorated from dry and arid to warm and humid but

with weakened monsoon conditions, which is evidenced

by a moderate percentage of freshwater thecamoebians

along with the estuarine to marginal marine pal-

ynomorphs such as dinoflagellate cysts of Spiniferites and

foraminiferal linings of Ammonia. This indicates a lower

hydrological input during the recent decades leading to

the deposition of fine grained clay sediment with higher

accumulation of salts. The precipitation of salt through

capillary action during dry months in exposed land is

common in the coastal wetlands (Farooqui 2010). These

changes drastically affected the diversity and dominance

of freshwater thecamoebians, which serve as a suitable

proxy for monitoring the magnitude of salinity fluctua-

tions in the river mouth.

Conclusion

On the basis of proxy records, two major phases of cli-

mate and ecological changes are inferred. The ratio of

marine and freshwater forms is low between the third

and second millennia (Phase I), as indicated by a high

percentage of thecamoebian and algal forms, which

suggests strengthened monsoon conditions. A higher

ratio of marine and freshwater forms since the last

millennium (Phase II) indicates seawater incursion due

to low freshwater input from land as evidenced by a

decline in freshwater thecamoebians, which reveals weak-

ened monsoon conditions. The freshwater thecamoebians

serve as an excellent proxy for monitoring salinity gradi-

ent in coastal wetlands. The recent changes in the

strength of freshwater flux leading to landward intrusion

of seawater could be attributed to the geomorphological

and hydrological changes in the estuary influenced by

climatic deterioration.

Acknowledgements

The authors are grateful to Director BSIP for granting

permission and providing necessary facilities to accom-

plish this work. J.S. thanks the Department of Science

and Technology, New Delhi for providing financial

support as Junior Research Fellowship in a DST spon-

sored Project No. SR/S4/ES-264/2007.

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Climate-induced Late-Holocene ecological changes in Pichavaram estuary, India

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