Palynological investtigation from a deep core at the coastal area of the red river delta, Vietnam

show that the region supported a Fagaceac-Coniferous, especially Quercus, Pinus và Castanopsis, similar to contemporary vegetation dcscribcd in Vietnam and southeast Chin[r]

VNU Journal of Science, Earth Scienccs 25 (2009) 192-203

area o f the Red River Delta, V ie tn a m

Nguyen Thuy Duong*

H anoi U niversity o f Science, VNU, 334 Nguyen Trai, H anoi, Vietnam

Received 15 November 2009

A b stract Palynological s tu d y in a d eep core (69.5 m depth) in the Coastal area o f th e Red R iver

D e lta, V ie tn a m p ro v id e s c v id e n c e s o n v eg etation ch ange in th e re g io n a l a re a in a b o u t 12000 years

p a le o e n v iro n m e n ta l c h a n g e s in th e R ed R iv er D elta, V ie tn a m d u rin g th e H o lo c e n e T h cse d ata

show that the region supported a Fagaceac-Coniferous, especially Quercus, Pinus và Castanopsis,

similar to contemporary vegetation dcscribcd in Vietnam and southeast China Tropical broadlcaí

forest dominated al the time right after 12.000 BP At the same time, Scyphiphora hydrophyllaceae

is th e d o m in a n t vvood sp e c ie s in the b ack m an^rovc ĩorcst at th e stu d ic d area A íte r this tim c, thc

c l i m a t e b e c a m e w a r m e r a l t h o u g h t h e r e w e r e s e v e r a l p e r i o d s w i t h c o l d e r c l i m a t e i n b e t w e e n

Keyxvords: p aly n o lo g y , p aleo e co lo g y , R ed R iver, pollen, spore, p aleo c n v iro n m en t

1 In tr o d u c tio n

W h e re a s th e L ate Q u a te m a ry vegctation al

h is to ry o f la rg e p a rts o f th e w o rld is íầirly vvell

d o c u m e n te d an d p aly n o lo g ica l stu d ies in the

R ed R iv e r D elta are n u m e ro u s, little attention

h a s b e e n p a id to th e H o lo cen e vegetation

h is to ry o f N o rth -V ie tn a m [1-3] M ost o f the

a v a ila b le p a ly n o lo g ic a l stu d ie s in V ietnam w ere

fo c u se d o n c h a ra c te riz e the sed im en tary strata

in re g io n a l g eo lo g ical m ap p in g [4] and in

a rc h a e o lo g y [5, 6] U sin g p o lle n reco rd s and

o th e r m ic ro íb s s il reco rd s for H olocene

p a la e o g e o g ra p h y re c o n stru c tio n , sea level

ch an g e , an d c lim a te h isto ry [7-12] w as still

lim ite d a n d o n ly re a c h e d a lo w tem poral

reso lu tio n R e c e n tly , Li e t al [13] p resen te d a

* E - m a i l : n t d u o n g g e o @ g m a i l c o m

detaileđ an d w ell-d a ted reco n stru ctio n o f

H o lo cen e c lim a te c h an g e in the R ed R iver delta M o re reco rd s are n e e d to ad v an c e our stan d in g o f v e g e ta tio n su ccessio n T h is research aim s to re v e a l th e v e g e ta tio n d ev elo p m en t and

p a le o e n v iro n m e n ta l c h a n g e s in the R ed R iver

D elta, V ie tn a m d u rin g H olo cen e O ur reco n stru c tio n s, w h ich are p rim a rily b a se d on pollen a n a ly se s, are in te n d e d to co n trib u te to

u n d crstan d in g e c o lo g ic a l pro cesses

2 P h y sica l c o n d itio n s o f th e stud icd area

2.1 G eolog ical setting

T h e V ie tn a m e s e c a tc h m e n t o f the R ed R iver

D elta o c c u p ie s a p p ro x im a te ly 10,000 k m 2 and consists o f a 5 ,6 0 0 k m 2 large d clta plain

N T D uon^ / V N l l Ịoiirnnì o f Science, Enrth Sàeiices 25 (2009) 192-203 193

b o rd ered by a m o u n tain o u s re g io n c o m p o sc d o f

P recam b rian cry stallin e ro ck s a n d P alcozoic to

M esozo ic lim estones [14] T h e m o st im p orlant

m o u n tain ridge, H oang L icn S o n , is a south-

e a stc m ex tcnsion o f th e Ilim a la y a s ex ten d in g

írom Y unnan (C h in a) into n o rth c m V ietnam

and ío n n s a v ast a rc a o f u p la n d s o f 1600-2000

m hig h vvith p cak s re a c h in g 2 5 0 0 -3 0 0 0 m abo ve sea level (a.s.l) (fig 2.1), in clu d in g thc h igh est

m o un tains o f m a in la n d S o u th e a st A sia (F an Si Pan, 3143 m , an d L an g K ung, 2913 m)

1 0 3 0 0 1 0 4 0 0 1 0 5 X 0 1 0 6 X 0 1 0 7 0 0 1 0 8 0 0

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Fig 1 Location o f the studying core

D e e p bo reho le d ala sh o w th a t the

Q u a te m a ry sed im en ts o fte n o v e rla y th e older

sed im e n ts un co m fo rm ably T h e y c o v e rs a large

part o f the stud y a re a b u t th e ir th ick n ess

in c re a s e s -fro m th e e d g e to th e c e n trc o f the

della T h e L ate P leisto cen e s e d im e n ts arc

allu v ial, m arine o r a llu v ia l-m a rin e T h e late

P leisto cen e sed im en ts a rc c o lo rtu l m o tlc y and

sh o w sig n s o f lo n g -tim e w e a th c rin g T h is

w ea th c rc d su rĩace is q u ite c o m m o n o v e r a largc

arca o f th e R ed R iver D elta fro m I la n o i to the

C o a s ta l zone T h e su ria c e s e d im e n ts a re m ainly

H o lo cen e and m ain ly c o m p o se d o f san d , sitlty

clay, clayey silt T h e y are g e n e ra lly d iv id e d into

tw o stratig rap h ic al u nits: th e H ai H ung

ío ư n a tio n ( Q :1 :) and the T h ai B in h (Q :3)

tb rm ation [14]

2.2‘Climate

T h e stu dy arca has a Irop ical evcrvvet

m onsoon c lim ate w ith a h o t (31-3 3° C ) and rainy su m m e r fro m M ay to S c p te m b e r an d a

c o ld e r (15 - 18“ C ) dry vvinter fro m N o v cm b er

to M arch In sp rin g an d au lu m , c lim a te is m ore

co m íb rtab lc b u t th c se se a so n s are sh o rt In Ju ly and A u gu st rain shovvcrs arc o f 5 0 0 -6 0 0 m m per

m o n th cau se th e riv e r level to rise u p to 2 -3 m

194 N.T D uong / V N U Ịournaỉ o f Science, Earth Sciences 25 (2009) 192-203

h ig h e r than the land su rfa c e b e c a u se o f dykes

In w in te r averagc rain fa ll is a b o u t 3 0 -5 0 m m per

m onth In the m o u n ta in a re a , lem p cratu re

d ep en ds on elevation

2.3 Vegetation

B ecause o f its c o m p le x a b io tic co nd itio ns,

the N o rthern part o f V ie tn a m has a very diverse

vegetation T h is d iv e rs ity is n o t only

attributable to the la rg e n u m b e r o f endem ic

species, but also to th e fact th at th e a rc a is a

m eeting p o in t o f the ílo ra s o f C h in a , the

H im alaya, an d M alay sia T h e h ig h la n d s o f the

H oan g Lien Son rid g e form th e so u th -easte m

part o f the S ik an g -Y u n n an ílo ris tic P ro v in ce o f

th e H olarctic ílo ristic k in g d o m T h is P ro vin ce

co n stitu tes a very im p o rta n t b o u n d a ry a re a o f

the H olarctic and P a le o tro p ic realm s w ith

num ero us tro p ical, su b tro p ic a l a n d tem p erate

co nn ection s A s a t th e sa m e tim e local

en d em ism is very h ig h , th c S ik an g -Y u n n an

A oristic P rov ince h a s o n e o f th e ric h e st and

specific in d ig en o u s ílo ra s o f m a in la n d A sia

[15]

T h e R ed R iv cr c a tc h m e n t area can be

su bd iv id ed into th e fo llo w in g altitu d in al

vegetation zo n es [15]:

T h e tro p ic a l lo w la n d (0-1 OOm a s.l.) is

cu ltivated e v ery w h ere T h e v e g c ta tio n co n sists

o f m ain ly h crbs an d sh ru b s AU zo n es h ig h cr

than 100 m a.s.l are c alled th e u p la n d s,

T h e tro p ic a l m id ìa n d (1 0 0 -7 0 0 m a s.l.) w as

fo im erly ch ara cterized b y a p rim a ry v eg etatio n

o f closeđ trop ical e v e rg rc e n b ro ad -leav ed

ío rests on hills a n d m o u n ta in ío o th ills At

present, this forest ty p e is c o m p le te ly ex tin ct

and rep laccd b y a g ric u ltu ra l íìeld s and

seco nd ary p la n t c o m m u n itic s , in c lu d in g h ig h ly

dcg rad ed o pcn tb rests, b a m b o o sta n d s, shrubs,

and grasslan ds [16]

In th e S u b tr o p ic a ỉ s u b m o n ta n e b elt (700-

1600m ) ann ual m e an te m p eratu re is a b o u t 15°c -2 0 °c , the tc m p c ra tu rc o f th e co ld est m onth below 1 5 °c, the m in im u m e v e n b e lo w 0 ° c

d u ring som e days T h e v eg etatio n is dom inated

by F agaceac, L au raceae, T h e a c e a e , U lm aceae,

M agnoliaceac, Ju g lan d ac eae, a n d R osaceae

K eteleeria d a v id ia n a (P in ace ae) is p rcsen t [16],

\vhereas D ip tcro carp accac, E ryth ro p h ỉeu m /o r d ii (L e g u m in o sa c ), an d L a g erstro em ia tom entosa (L y th ra c e a e ), w h ich are characteric

for lo w er altitu d es, arc lcss co m m o n O n flat arcas an d on g en tle slopes, esp ecially o f lo w and m iddlc e le v a tio n , w arm th lo ving, broad- leavcd spccies like C a sía n o p sis sp.,

C innam om um sp , D ip te ro ca rp u s retusus,

H opea m o ỉissim a , M a d h u c a p a sq u ieri, Syzygium sp., V atica sp an d \varm th loving

L ithocarpu s and Q u ercu s sp c c ie s form th e 35"

45 m high íìrst stratu m A lo n g the rid g e to p s and on thc u p p c r p arts o f m o u n tain slopes, especially on th e d rier, stc c p c r slo pes, thc

co n iíero u s D a c ry c a rp u s im b ric a tu s ap p ca rs as a

co-dom inant trec, vvhcrcas a lo n g the d ry steep sum m its o f rid g e s th is sp ecies m ay even bc

m ono-dom inant A c c o m p a n y in g sp ecies o f the second stratu m (2 5 -3 0 m h ig h ) arc the \varm th-

loving, b ro a d -lc a v c d trces A r ío c a rp u s sp.,

C anarium sp., C ry p to c a ry a sp., D iỉlen ia sp.,

E b erh a rd íia a u ra ta , E ỉa e o c a rp u s tonkinensis,

E laeocarpus sp p , G iro n n iera sub a eq u a lis, Litsea sp., M ic h e lia spp., N ep h eỉiu m m illiferu m t

X a n th o p h ylỉu m u ro p h y llu m , an d a large p alm

L ivistona c h in en sis [ 17].

In th e T em p era te s u b m o n ta n e b e lt (16 00 -

2400m ) an nu al m c an te m p e ra tu re is ab o u t 10- 15°c, th e m ean te m p eratu re o f the co ldest

m onth b elo w 1 0 °c, and the m inim um tem perature o ltcn b elo w 0°c S pecies o f vvarm tem perate te m p eratu re co n d itio n s ap pear

including AlnuSy B e ta la , A c e r , an d C a rpinu s

T hey are founđ to g c th c r vvith th e g y m no sp erm s

N T D uong / V N U Ịoum al o f Science, Earth Sciences 25 (2009) 192-203 195

C ryp to m eria, an d P o kien a [16] O n gentle

slop es, esp ecially in th e lo w an d m iddle parts o f

m oun tain slopes, b ro ad -leav ed ev ergreen and

dccid u o u s trees o ccu r, such as A rch id en d ro n ,

C rỵptocarya , E b e rh a rd tia , E xb u c kla n d ia ,

L ith o ca rp u s, Litseci, M a g n o lia , M a n g lieíia ,

M ich e lia , R elid ero d en d ro n , R lìoipíelea, S c h im a ,

S ym p lo co s etc., ac c o m p a n ie d b y the conifer

F okien ia h o d g in si on d rie r, steep slo p es [17].

T he T em perate m o n ta n e b e lt (2400m ) has a

sn o w co ver in vvinter N e x t to sh ru b species o f

F agaceae and E ricacea e, g y m n o sp erm s o f cold

tem p erate clim ate o c c u r lik e /1Ố/&S p in d ro x v and

D etbrestation an d degradation have

destroyed m ost p rim ary ỉo rests o f the H oang

Lien Son area and fa r beyond and botanical

investigations in this reg io n are urgently required

for the organization o f conscrvatio n areas to

protect the rem aining p rim ary plan t com m unitics

that have w orld vvide sig n iíican ce as centers o f

plan t diversity [15]

3 M a te r ia ls a n d m c th o d s

T h e study ing co re is lo cated in th e B alat

m o u th at 1 06°33’4 8 ” E , 2 0 ° 1 9 W ’N (fig 1)

T h e c o re is 6 9 5m d ep th , m a in ly co n sists o f

clay an d silty c la y (d etail d escrip tio n in fig 2)

6 0 sam p les o f th e co re in th e in terval o f 50

cm w as co llected 0.5 cm 3 fro m eac h sam ple

w as ex tra ctcd for p o lle n analysis S am ple

p rcp aration íbllovved S ta n d a r d m eth o d s [18] and

in clud ed ad dition o f a k n o w n am o u n t o f

L yco pod iu m clav atu m sp o res to calculate

pollcn co n cen tratio n s [18], trea tm e n t w ith HC1

and K O H , siev in g o v e r a 120 ụxn an d a 7-8 ịim

sieve, treatm en t w ith H F , aceto ly sis (7 m in),

and m o u n tin g in Silicon oil

P o llen and sp o res \v ere id en tified w ith and

n am ed aftc r [4], H u ang [19], W an g et al [20,

21], T h an ik aim o n i [2 2 ] a n d Y u lo n g Z hang et al [23] an d c h e c k e d w ith a m o d cm refcrence collectio n o f so m e ty p ic a l rep rcsen tativ c s o f the

V ietn am ese flo ra an d vvith the reference collectio n o f th e In stitu te o f B otany and

L andscape E co lo g y , G re ifs w a ld U niversity Pollen ty p e s a re in the tex t displayed in SMALL CAPITALS in o rd e r to differen tiate them clearly from ta x a [2 4 ] T h e u n id c n tiíìed pollen grains co n sist o f c o rro d e d or o thenvise đam ag ed g rain s a n d th o s e g ra in s for vvhich no

m a tc h in g t>pe coulcỉ be íb u n d in the litcrature

o r in the re íe rc n c e c o llc c tio n In general, the

n u m b e r o f u n id c n tifie d p o lle n ty p cs is low

T he rcsu lts o f th e a n a ly se s are presented as relativ e v alu es (in w h ic h the p o llen frequencies are cx p ressed a s p c rc e n ta g e s o f a pollensum ) and as c o n c e n tra tio n s v alu es (g rain s/c m 3)

T o avoid (e x tra ) lo c a l o verrepresentation for the b o reh o le s a m p le s , a p o llen su m o f typcs attribu table to tax a th a t arc cu rren tly absent from the lo w la n d s a n d re stric te d to areas over

100 m a.s.l w a s c h o s e n A lso the pollen types attrib u tab le to ta x a th a t c u rre n tly o ften grovv alo n g riv e r b a n k s a re c x c lu d e d from the

p o llen su m b e c a u s e a lso th e y m ay show

o v errep resen tatio n G e n e ra lly , ab o u t four slides

p er sed im en t s p e c tru m w e re an aly sed to reach a pollen sum o f 8 0 - 2 0 0 grains Pollen

p ercc n tag e c a lc u la tio n s w e re carried o u t \vith the Excel p ro g ra m ; th e diagram s are

co n stru cted w ith T ilia g ra p h an d T ilia v iew [25, 26] T h e d ia g ra m s d iv id e d into site pollen

zo nes, b ased o n th e c h a n g e s in th e curv es o f

d o m in an t o r c h a ra c te ris tic p o lle n types Pollen

ty p es w ere a s s ig n e d to e c o lo g ic a l groups using

th e T l o r a o f V ie t N a m ’ [2 7 , 2 8 ]; th e ‘V ietnam forest tre e s ’ [2 9 ]; th e ‘S o m e b asic ch aracters o f

V ie tn a m ílo ra ’ [30]; th e ‘F lo r a M a le sia n a ’ [31];

th e ‘M an g ro v e E c o lo g y ’ [32]

C I 4 d a t i n g

O nly o n e s a m p le s o f w o o d an d o th e r plant rem ain s c o llected at th e d c p th o f 62 m w ere I4C

196 N.T D uong / V N U Ịournaì o f Science, Earth Sciences 25 (2009) 292-203

A M S d ated at th e L eib nitz L ab o rato ry o f Kiel

U niv ersity (P rof Dr P ie t G ro o tes) and

calibrated w ith C A L IB rev 4.3 [33] F or the

calibration o f thc carb o n a tes a rcgional

co rrection (A R ) vvas ap p lied o f -25 ± 20 years

follo\ving S o u th o n c t al [34] T h e agcs are

ex p rcsseđ as cal y r B P (B P = 1957) The calib rated re su lt o f th e sam p le sh o w e d thc date

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B r o u n s iliy c la y in íx cd u iih fin c sand a n d g r c c n ish g i c y d a y

c o n la in in g p lan l h u m u s and Shell S o m c p c a l in Ihe lo w e r parl

B ro w n silcy d a y (rích in pout) rnixcd w iih g r cc n ish g r cy c la y Inycrs w ith plant rcm ains nnd latcn tc

r.*> C o a rsc s j n d w ỉth ilu n s h c lls m ix c d w iih Ihin la v c r s o l c la y c y silt

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PleiS tocene sedim ent

Fig 2 Sedimenlary and envừonmental charactcristics o f NP core (reĩerenccd from Grothe St [35])

4 T h e p ollen d iagram

AU to g eth er, 175 sp ecies and genera

b elo n g in g to 90 lam ilies \vere id e n tiíìcd and

catcg o rized into cc o -stra tig ra p h ic a l groups (T ab lc 2)

T h e N am p h u c o re c an be su b d iv id e d into 8

p o llcn zo n es (fig 3)

,V.T Duong / V N U Ịournaỉ of Science, Larth Sciences 25 (2009) 792-203 197

Z o n e N P 1 ( 6 9 3 5 m - 6 4 2 5 m , s p e c tra 1-4) is

c h a ra c te riz e d b y h ig h p e rc e n ta g e s o f PlNUS

QUERCUS a n d CASTANOPSIS TYPE in th e u p p c r

p a rt o f th is z o n e ( tìg 3 ) B e s id e s , v a lu e s o f

CYPERACEAE, GRAMINEAE a n d Sa l ix a re very

h ig h (fig 4 ) M a n g ro v e ty p e s a n d p o lle n ty p es

r e s tn c te d to th e lovvlands a n d m id la n d s su c h as

CASUARiNA, PSIDIUM, a n d VlTEX a re a b s e n t in

th is z o n e (tìg 4)

7.one N P 2 ( 6 4 2 5 m - 6 0 2 5 m , s p e c tra 5 -9 ) is

c h a ra c te riz e d b y th e p r e s e n c e a n d o íte n

d o m in a n c e o f PTEROCARYA Ul m u s a ls o

shovvs h ig h v a lu c s (fig 4 ) Al n u s (fig 3) ju s t

a p p e a rs in th is z o n e a n d s h o w s , ju s t like

CaRPINUS, h ig h v a lu e s in ih c u p p c r p a rt o f th is

z o n e T h e p c rc e n ta g e s o f PlNUS, Qu e r c u s,

CASTANOPSIS TYPE, a n d Sa l i x a re (m u c h )

lo w e r th a n in z o n e N P 1 Cy c aS (fig 3 ) a ls o

s h o w s h ig h v a lu e s in th is z o n e Gr a m in e a e

a n d CYPERACEAE (fíg 4 ) s h o w lo w e r v a lu e s

th a n in N P 1 S p o r e ty p e s a re v e ry d o m in a n t,

e s p e c ia lly Po l y p o d ĩACEAE ( fig 4 ) th a t re a c h e s

its h ig h e s t v a lu e s in th is z o n e

Z o n e N P 3 (6 0 2 5 m - 5 6 6 m , spectra 10-13)

is ch ara cterized b y th e d o m in a n c e o f back

m an g ro v c p o lle n ty p cs ìn c lu d in g SCYPHIPHORA

HYDROPHYLLACEAE an d PHOENIX (ílg 4)

S om e m a n g ro v e í o r e s t ty p e s are also

sp o rađ ica lly found in th is zo n e HELVVINGIA

an d ACER đ isp lay h igh frcq u en cies, w h ereas

QUERCUS is also very vvell-reprcsented in this

zo n e (fig 3) V alu es o f PlNUS are slig h tly

h ig h e r than in N P 2 , w h e rc a s CASTANOPSIS

TYPE and CARPINUS are n o t m uch different

L o w lan d ty p e s in clu d in g G y m n o s p o r i a and

TRIANTHENUM a ls o s h o w h ig h c r v a lu e s th a n in

zo n e N P2 T h e v alu es o f GRAMINEAE are

h ig h e r than p re v io u sly (fĩg 3)

Z o n e N P 4 ( 5 6 6 m ^ 7 0 m , s p e c tra 1 4 -2 1 ) is

c h a ra c te riz e d b y th e a b s e n c e o f SCYPHIPHORA

HYDROPHYLLACEAE a n d PHOENĨX (fig 3)

M a n g ro v e p o lle n ty p e s a ls o a re a b s e n t in this

z o n e T h e p e rc e n ta g e o f PlNUS c o n tin u e s to ris e PODOCARPUS a n d ALNUS a re o n ly

p re v a ilin g in th e lo w e s t p a rt a n d a re rare ly

io u n d in th e re st o f th e 7.0n e (tìg 3) TRIANTHENUM (fig 3 ), HELWINGIA, a n d ACER (fig 3 ) a rc le s s íre q u e n t Ca s t a n o p s is TYPE, QUERCUS, a n d Ca r p i n u s d o n o t c h a n g e m u ch

c o m p a re d to N P 3 (fig 3)

Z o n e N P 5 (4 7 0 -4 0 5m , s p e c tra 2 1 -2 8 ) is

c h a ra c te riz e d b y a r e o c c u ư e n c e o f m an g ro v e

p o lle n ty p e s in c lu tlin g ACANTHUS,

Br u g u i e r a, Ce r i o p s t a g a l, and

Rh i z o p h o r a, a n d a ls o o f b a c k m an g ro v e

p o lle n ty p e s in c lu d in g EXOECAR1A and ACROSTICHUM a lth o u g h w ith low v a lu e s (fig

4 ) PlNUS s h o w s m u c h lo w c r v a lu e s , QUERCUS

a g ra d u a l d e c re a s e ILEX, ALNUS, MACARANGA, a n d MaLLOTUS shovv h ig h er

v a lu e s th a n in th e p r e v io u s z o n e (fig 3) GRAMINEAE g ra d u a lly d c c re a s e s (fig 4 ) T h e

n u m b e r o f p o lle n ty p e s is m u c h h ig h e r th a n in

o th e r z o n e s (fig 3)

Z o n e N P 6 (4 0 5 m -2 8 0 m , sp e c tra 2 8 -4 6 ) is

c h a ra c te riz e d b y th e p re s e n c e o f m a n y m an g ro v e

fo re st ty p e s lik e BRUGUIERA, SONNERATIA, and

Rh i z o p h o r a, a n d a ls o o f b a c k m a n g ro v e ty p es lik e Ex e o c a r ia a n d Ac r o s t ic h u m, a n d o f CHENOPODIACEAE O n ly BRUGU1ERA shovvs a

h ig h a n d s ta b le p re s e n c e , o th e r m an g ro v e ty p cs

a re ra re ly fo u n d T h e b a c k m an g ro v e ty p es show

h ig h e r v a lu e s th a n th e m a n g ro v e ty p e s (fig 4) PlNUS a n d ALNUS sh o w h ig h cr, QllERCUS and CASTANOPSIS TYPE lo w e r v a lu e s than in N P 5

Ca r p in u s re m a in s a p p ro x im ate ly the sa m e (fig 3) Gr a m in e a e sh o w s a grad u al in crease (fig 4)

Z o n e N P 7 (2 8 0 m -1 6 7 m , s p e c tra 4 7 -5 4 ) is

c h a ra c te riz e d b y th e V irtual a b s e n c e o f

m a n g ro v e fo re s t ty p e s T h e y a rc o n ly rare ly

fo u n d in th e lo w e r p a rt o f th e z o n e

Ac r o s t i c h u m s h o w s a m u c h lo w e r v a lu e than

in N P 6 (fig 4 ) QUERCUS a n d CASTANOPSIS TYPE (fig 3 ) a n d PTEROCARYA a n d ULMUS (fig

4 ) s h o w g e n e ra lly h ig h e r v a lu e s th a n in z o n e

198 N.T Duong / V N U Ịournnl o f Science, Enrlh Sciences 25 <2009) 192-203

N P 6 A ls o Cy p e r a c e a e a n d Gr a m i n e a e are

to u n d w ith h ig h e r v a lu e s th a n b e fo rc

Z o n e N P 8 (1 6 7 m -3 3 m , sp cctra 55-57) is

c h a ra c te riz e d b y a re la tiv e ly hig h v alue o f

Br u g u i e r a in th e u p p e r p a rt o f th e z o n e (fig

3) PiNUS s h o w s h ig h v a lu e s , a s d o , ACER,

Ca r im n u s, a n d T a x u s (fig 3) T h e p o llc n typc div ersity in th is zo n e is lo w cr th an in the other zones M any t> p es a re no lo n g er p re sc n t and the pollen co n ce n tratio n is v ery low S p o re types

a re m o re c o m m o n , e s p e c ia lly Cy a t h e a, MlCROLEPIA, a n d ATHYRIACEAE

ư)

I>.

i

ị

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Fig 3 Rclative pollen diagram o f pollcnsum !>pcs (Sellectecd curvcs only)

5 D iscu ssỉon

T h e h ig h p c rc e n ta g e s o f PlNUS,

Ca s t a n o p s i s TYPE, a n d Qu e r c u s (a ll p rcse n t-

d a y m o u n ta in o u s t> p e s), Sa l i x, POACEAE, a n d

CYPERACEAE, th e a b s e n c e o f m a n g ro v c ty p e s

an d the lo w n u m b e rs o f p o lle n ty p c s in zon e

N P1 sh o w th at th e c n v iro n m e n t m u st ha ve been

te rre strial L o w lan d p o llen ty p es are absent

fro m th is zo n c, in d icatin g th at th e clim ate vvas

c o ld e r th an to d a y an d te m p erate vegetation

p rc v a ile d in th c area T h e h ig h p crcc n tag cs o f

Po a c e a e a n d C y p e r a c e a e in th e a s s e m b la g e s

an d the a b sc n c c o f p o llen o f p la n ts o f standing

\vater ind icate a fre sh w a te r m a rsh en viro nm cn t [36]

Pt e r o c a r y a is th e d o m in a n t p o lle n ty p c in

z o n c N P 2 P le r o c m y a is a n ativ e sp ecies to

n o rth cm V ietn am , w h ere it is vvidely distrib u tcd from 50 to lOOOm a.s.l., csp e c ia lly on alluvial sed im cn ts alo n g riv e rs and stream s A lso

U lm u s sh o w s h ig h v alu cs in this zone AIso

ư lm u s often g ro w s alorm s trc a m an d riv er

banks o r in v alley s in lim esto n e m ou ntain rang es [29] T h e high v alu es o f b oth typcs

in d icate thai th e se sp e c ic s vvcrc grovving closc

to sam p lin g site and that thc sam p lin g site at that tim e m ay h a v e b ccn a riv e r llo o d p lain T he

N T Dĩiong / V N U Ịoum aỉ o f Science, Earth Sciences 25 (2009) 192-203 199

very low v alu e o f Sc y p h ip h o r a

HYDROPMYLLACEAE in this z o n e su g g ests that

the area vvas v cry rarcly in ílu en ce d by tide

T h e highcr d iv e rsity o f p o llen ty p es shovvs a

vvarmer clim ate, a co n clu sio n su p p o rted b y the

p rescn cc o f m an y p o llen ty p es th at are

attributable to tropical an d su btro pical taxa,

Ca m p a n u m o e r a, Sp a t h o d e a, Ziz v p h u s,

He d e r a, Ja s m in u m l a n c e l a r iu m,

FaGOPYRUM, and CALYSTEGIA A lso th e high

v alu cs o f sp ore ty p es, esp ccially

Po l v p o d ỉa c e a e, in the se d im e n t p o in t at a

h ig h e r tem p erature [37]

Z o n e N P 3 is ch ara cterized b y the

d o m in an cc o f Sc y p h ip h o r a

h y d ro p h ylla cea e is a b ack m a n g ro v e species

Ihat is cu rren tly com m o n in th e land w ard zones

o f the m ang ro vc th a t are in freq u e n tly íloodcd

b y tid e [32] W e w ill call th is ty pe o f

cn v iro n m en t a “h igh tidal flat”

A lso P h o e n ix (cf PHOENIX p o llen in this

zo n e) is a b ack m ang ro ve typ e th at is p resen t on

the h ig h tidal Hat T h e ra re o ccu rren ce o f

BRUGUIERA and RHIZOPHORA in th is zone

sh o w s that B ru g u ie ra and R h izo p h o ra w ere no

co m p o n en ts o f th e m a n g ro v e forest T h eir

p o lle n vvas p ro b ab ly w a sh e d in b y th e tid es

from m an g ro v e forests c lo se r to the sea

G ram in c ae seem to be a c o m p o n e n t o f the local

v eg etatio n also h ere, as th e high v alu es o f

G r a m i n e a e illustrate V a lu e s o f p o lle n types

a ttrib u tab le to tax a o f n o n -m arin e

e n v iro n m en ts, in clu d in g QUERCUS,

HELWINGIA, an d ARTEMISIA are quite high in

th is zo n e, w h ich m e an s that the in ílu en ce o f

riv er vvater w as still largc W e m ay c o n clu d e that the sam p lin g site belo n g cd to the hig h tidal flat en v iro n m en t vvith back m a n g ro v e ío rest

do m in ated b y S c y p h ip h o ra h y d ro p h y ỉla c e a e

an d P hoetùx.

T h e d ev elo p m en t o f b ack m a n g ro v e ío rest and th e presen ce o f som e p o lle n ty p es o f tropical taxa, su ch as DOXANTHA and

Gy m nOSPORIA in th is zo n e in d icate an incrcased d ev elo p m en t o f tro pical v cg etatio n in

th e area

m a n g ro v e p o llen ty p c s arc a b sen t in z o n e N P 4

su g g estin g a co m p lete ch an ge o f en v iro n m en ta l conditions T h e a b sen ce o f m a n g ro v e p o llen

m ay be d u e to e ith e r a c h a n g e to te rre strial o r a chan ge to m arin e co n d itio n s In th e íìrst case,

th e sea level m u st d ro p s tro n g ly a n d rap id ly lead in g to a co m p lete ch an ge in v e g etatio n T h e

p o llen signal, h o w ev er, d o cs n o t s u p p o rt such sccnario W ith a reg ressio n , the d ep o sitio n o f

lo v v lan d p o l l e n t y p c s m u s t i n c r e a s e in t h e p o l l e n

assem blages In fact, the lovvland p o lle n type

T r i a n t h e n u m shovvs a stro n g decrease, vvhereas an o th er lo w la n d p o llen ty p e DODONEA

is ab sen t in this zone T h e seco n d case,

im plying a rap id ris e o f the sca lev el, w ould

p r e v e n t m a n g r o v e d e v e lo p m e n t in t h e C o astal

zo n e area w hen th e sam p lin g a rc a w a s rap id ly

co v ere d b y salt w a te r and th e tidal actio n to ok place íu rth er in lan d , a ffectin g a larg e p a rt o f the low land T h is m ay be th e reaso n fo r th e m uch lovver v alu es o f lo w lan d p o lle n ty p e s In

ad d itio n , th e v alu es o f m o u n tain forest types,

in clu d in g PlNUS, Q u e r c u s , an d C a s t a n o p s i s TYPE are h ig h e r th a n in th e p re v io u s zone

2 0 0 N T Duong / VN U Ịoum aỉ o f Science, Earth Sciences 25 (2009) 192-203

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M angrove p o llen typcs start to be found in

z o n e N P 5 su g g cstin g an in itiation o f delta

developm cnt, vvhich h ap p en s vvhcn thc sea level

rises m ore slo w ly [38] lcad in g to the stronger

and m ore ex ten siv c d ev elo p m en t o f m angrove

forest T h e lo w p crc c n ta g c o f m a n g ro v e ío rest

ty p es shovv that thc cn v iro n m cn t w as not

optim al for p o llcn d ep osition

W ith thc d ccrc ase o f tcm pcrate elem cnts,

tropical m id lan d an d lovvland p o llen types

( T r is t a n ia , D r y p e t e s , M a c a r a n g a ,

MaLLOTUS and ELAEOCARPUS) are m ore

com m on in zone 5 A lth ou gh th c ir p crcen tag cs

are lo\v thcy sh o w an e x p a n sio n o f the tropical

flora in thc reg io n a l veg etation

T h e rath er high v alu es o f the m a n g ro v e type

BRUGUIERA and th e b ack m an gro ve types

(ACROSTỈCHUM) in z o n e N P 6 sh o w that the

m angrove tb rcst d e v e lo p e d stron gly at thc

Coastal z o n e a r e a in t h e co n d itio n o f a h i g h

stand o f thc sca, as this is th e m o st su itab le tim e

for m ang rovc d cv clo p m cn t T h e increased

values o f these ty p e s p ro b ab ly p o in t a t m ore

erosion fro m thc Coastal low land as a resu lt o f

the co astlin e m igratin g scavvard T h is sh o w s a

c h a n g e in s e d i m e n ta r y c o n d i t i o n f r o m e s tu a r in e

to delta T h e fin c-g ra in cd scđ im cn ts are

characteristic for a p ro d elta e n v iro n m en t [39]

T h e ap pcarance o f p ro d clta sed im cn t m arks the

b e g in n in g o f a reg ressio n T h e increase o f

m agrovc ty p e s at th e d cp th o f 35m su g g ests the

cnv iron m en t c h a n g in g fro m m orc scavvard to

m ore land w ard an d a c h a n g e íro m a p ro d clta to

a dclta front en v iro n m cn t

T he low p rescn cc o f b ack m an grov e ty p es

and the h ig h e r v alu es o f fresh w ater and

riverine types ind icatc a s tro n g c r in llu c n c c o f

the riv e r in z o n e N P 7 T h is also m ay be caused

b y a rapid d ccre asc in sea lcvel th at led to a

co ntin uo us ch an g e o f th e c o astlin e an d

pro vid ed in su fficien t tim e for an extensiv e

dev elo p m en t o f m a n g ro v e forcst T h e p resen ce

o f ACROSTICHUM, a b ack m angrove type,

sh o w s an cn v iro n m cn t a ffected by thc sea The silt m ixed vvith finc sand scd im cn t o f zone N P7

an d the p ro d clta en v iro n m cn t o f zone N P6 suggests a d e lta fro n t slo p e env iron m en t for

zo n e N P7

T h e low p o lle n co n ce n tratio n and diversity shovv a rapid clastic scdim cn tatio n in zone

N P 8 , vvhich is ch ara ctcristic for the prevailing sandy sed im cn ts (th at w ere larg cly lost during sam pling) T h e p rcscn cc o f BRUGUIERA in the

u p p cr part o f th e zo n e ind icates the

d ev elo p m en t o f m ang rov c lo rcst around the sam p ling site an d th at thc site in this period

b elo n g ed to th e C oastal zo n e P ossibly the sam p ling site w as part o f a sandy b ar in the delta front p la tfo rm arca T h e high values o f pollen ty p e s ascrib cd to tem perate taxa

in clud ing C a r p i n u s , A c e r , T a x o d ỉ a c e a e ,

an d also PlNUS, sug g ests a slig h tly cooler clim ate

6 C o n c lu sỉo n

T h e stud y o f pollcn an d sp o res from the N P

c o re in th e C o a s ta l zo n e arca o f the Red R iver

D elta, p ro v id e s a rc c o rd o f vegetation and clim ate c h an g e c o v e rin g ca 12,000 years B p (B efo re P rescn t) B ascd on th is study results,

v eg etatio n d e v e lo p m c n t h istory, environm ent and sea level c h a n g e w a s reconstructed

B efo re 12.000 B p (at a b o u t 14.000 B.P), sed im en tary e n v iro n m c n t o f thc study area w as

b e lo n g to fresh vvatcr m a rsh co n d itio n \vith the stro n g d e v e lo p m e n t o f frcsh vvater plant like

G ram in a e, C y p e ra c c a e an d S a ỉix in a colder

clim ate co n d itio n th an today A t a b o u t 12.000

B p , th e e n v iro n m e n t vvas transfo rm ed to riv erin e co n d itio n s ra rc ly in ílu en ce d b y tide

w ith th e d o m in a n t o f riv erin e taxa like

P ỉerocarya, ư lm u s, S a ỉix in a vvamer clim ate