Distribution of mangrove species during Holocene

I. INTRODUCTION

The mangrove ecosystem is an important ecotone zone of land and ocean interaction in coastal zones, as a buffer area between land and ocean. In the Red River Delta, their presence and distribution in the past, in relation with land evolution affected by natural processes (e.g. sea-level changes) as well as human activities, are poorly understood.

The presence and distribution of mangrove flora are controlled by the frequency and duration of tide, climate, topography and salinity. The floristic composition and distribution of true mangroves or their principal components are controlled by deposition, climate and tidal flood.

Palynologists have been using mangroves as criteria in the study on the process of deposition under anaerobic conditions [5]. The discovery of sediments containing mangrove fossils has not only provided supporting evidences to the sea-level oscillation, but also to the related change of the floristic composition and distribution of the mangroves. Ward (1988) carried out a palynological study in Kosrae Island, Micronesia, and Fujimoto et al. (1996) stated that the critical value of mangrove peat accumulation with the sea-level rise is 2-10 mm/yr. If the forecasted sea-level rise exceeds this value, all mangrove forests will move landward and decrease rapidly.

II. METHODS

Field investigations were conducted on the tidal flats of Giao Thủy District, Nam Định Province. They include 40 hand-drilled holes of 9 cross-sections, 6 holes drilled by machine (shallowest borehole reaches 30 m; the deepest - 68 m). In total, 210 samples were taken for palynological analyses, including 68 samples from hand-drilled holes and 142 samples from machine-drilled holes.

III. RESULTS

On the basis of results of palynological analysis of the samples from cores of deep-drilling and shallow-drilling holes, on percentage content, specific composition and specific groups in the pollen spectrum, the Holocene spore and pollen complex can be divided into four following assemblages.

- Pollen Zone 1 (Pz1)

This assemblage is established on the basis of analytic results of the samples from the depth of 47.6 - 42.6 m (LK.4GT); 53.7 - 48 m (LK.6GT); 55 - 42.5 m (LK.56ND); 47.4 - 49.5 m (LK.2GT). The average spore content is about 20-25% or even higher and reaches 28% at 44.6 m (LK.2GT) or 30% at 40 m (LK.56ND). The spore composition consists mainly of Polypodium, Pteris, Osmunda, Cyathea, etc. There is almost no evidence of mangrove in this assemblage except some forms with very small quantity, such as Avicennia, Sonneratia at the depth of 45.4 m (LK.2GT) and 49 m (LK.6GT).

The back-mangrove occupies 8-12% in some samples or absent in some others (48.6 m at LK.6GT; 51 m at LK.56ND). Frequent forms are Acanthus, Acrostichum, Hibiscus, Cyperus, and the lesser are Aegiceras, Palmae, Phoenix, Sueda, Excoecaria. They occur in pollen spectrum as much as 2%, while fresh-water plants occupy a dominant part, up to 65-75% in average, even 80% in some special samples (48.6 m LK.6GT). The most abundant forms are of allochthonous origin, coming from mountainous areas, such as Tsuga, Cedrus, Castanae, Quercus, Myrica. The autochthonous representatives usually are non-arboreal or brushwood forms, such as: Compositae, Graminae, Morus, Rubia, Nyphar, etc.. A remarkable feature is that, in the autochthonous pollen group the content of back-mangroves is much lesser than that of non-mangroves.

The diatoms determined in the samples from LK.2GT and LK.56ND consist of a fresh, brackish community, including Cymbella sp., Diploneis ovalis, Coscinodiscus lacustris, Synedra affinis, Cyclotella caspia, Thalassiosira eccentrica.

This means that continental factors dominated over the coastal ones and it characterizes for flood tidal plain or estuarine environment.

- Pollen zone 2 (Pz2)

This assemblage comprises the samples of the depth from 31.2 to 20.7 m (LK.1GT), 38.1 - 18.5 m (LK.2GT), 38.6 - 20.7 m (LK.6), 32.9 - 19.7 (LK.3GT), 36.7 - 20.5 m (LK.4GT), 29.2 - 17.3 m (LK.5GT) and from 40.5 - 21 m (LK.56ND). In comparison with the Pz1, the pollen content is slightly higher and is about 20-30% in average in pollen spectrum, the highest reaches 32% (32.5 m at LK.56ND). There is mostly not change in pollen composition.

The mangrove pollen in this assemblage increases strongly in amount as well as in specific composition. It is about 5-12% in pollen spectrum, rarely up to 17% (20.7 m at LK.6GT) and frequently represented by Avicennia, Rhizophora, Sonneratia (about 4% in pollen spectrum) seldom Kandelia, Bruguiera (1-2%). This assemblage is rather rich in back mangrove (15-25% in average and up to 30-35% in some special cases, such as LK.3GT, LK.5GT). The frequent forms are Acrostichum, Acanthus, Cyperus, Hibiscus that occur in almost all samples (up to 7-8% and 2-5% in average), and the rarer forms are Aegiceras, Sueda, Xylocapus, Palmae, Phoenix, Ipomoea, Cylodon, Lumnizera etc.. Especially, in this assemblage occurs Nypa fructicans (LK.6GT, LK.2GT). At present, this species occurs only in costal areas of the Mekong Delta.

In comparison with the Pz1, in this assemblage the non-mangrove pollen decreases and the main representatives are allochthonous that are widely developed in mountainous areas in temperate climate zone, such as Quercus, Castanea, Juglans, Corylus, Pinus, Cedrus, Tsuga. The autochthonous pollens hardly occur with, usually, Compositae, Rubia, Graminae, etc.. The non-mangrove content is about 50-60% in the spectrum.

In core samples from boreholes LK.2GT, LK.56ND, an abundant foraminifera complex occurs that consists principally of Ammonia beccari, A. annectens, A. japonica, Quinqueloculina seminulina, Q. vulgaris, Elphidium advenum, E. hispidulum, Spiroloculina eximia, Sp. sp., Pseudorotalia menhardi, Nonion sp. and Cibicides sp..

At the same sites, there is also a brackish diatom complex determined mainly by the presence of Coscinodiscus radiatus, C. nodulifer, C. lacustris, Thalassiosira eccentrica, T. nitzschioides, Nitzschia sigma, Grammatophora oceanica, Cyclotella stylorum, C. caspia, Achnanthes brevipes, Cocconeis sublittoralis, Diploneis bombus, Actinocyclus ehrenbergii, Paralia sulcata with some very rare fresh-water representatives, such as Aulacosira granulata, Stephanodiscus astrea, Synedra affinis.

- Pollen zone 3 (Pz3)

This assemblage is established from samples at the depth of 19-11 m (LK.1GT), 17.4 - 12.1 m (LK.2GT), 18 - 11.4 m (LK.3GT), 19.3 - 10.3 m (LK.4GT), 14 - 10.9 m (LK.5GT), 19.3 - 12.3 m (LK.6GT) and 20 - 10.5 m (LK.56ND). The pollen content of this assemblage increases strongly in comparison with Pz2, reaching 30-37% in average and seldom up to 40 % in some samples (14.7 m at LK.3GT, 16.5 m at LK.56ND). The forms, such as Polypodium, Sphagnum, Pteris increase in some spectrum, even up to 5-7% (LK.3GT, LK.5GT, LK.1GT).

In comparison with the Pz2, there is no differences on both mangrove representatives and amount. Concerning back-mangroves, the situation remains the same as with the mangroves except in LK.1GT, LK.3GT, where the percentage content is slightly higher reaching 30-35% or even to 39-40% as in samples from 11.4 m of LK.3GT, or 18.2 m of LK.1GT. This assemblage seems to be free from Heriticra, while Nypa fructicans occurs in some samples from LK.2GT, LK.3GT at the depth of 16.4 m and 12.9 m, respectively.

The fresh-water plants decrease in this assemblage, occupying only 30-40% in average, however there exist some samples where they reach 50% (LK.3GT, LK.5GT). The allochthonous plants remain the same, that are developed in temperate mountainous regions, such as: Quercus, Castanea, Carpinus, Pinus, Tsuga. In the pollen spectrum, Pinus is dominant component upon the others (3-7%); the algae and brush plant also increase in amount and in composition. Besides, the frequent representatives of fresh marshes have been found, such as Nyphar and Sapindus.

The diatom complex consists mainly of fresh-brackish representatives: Amphora sp., Diploneis sp., Cocconeis placentula, Paralia sulcata, Synedra affinis, S. ulna, etc. In the samples of the upper part from LK.2GT, LK.56ND (14-19 m), there are some brackish-saline community: Achnanthes brevipes, Diploneis bumbus, Nitzschia cocconeiformis, Trachineis aspera. Foraminiferas are found in the upper part of these two boreholes, but very poor in quatity and specific composition, such as: Ammonia beccari, A. annectens, Milliolinella cf. subrotunda, Quinqueloculina sp., Pseudorotalia menhardi, Nonion sp..

- Pollen zone 4 (Pz4)

This is the youngest assemblage established in the upper part of the machine- and hand-drilled holes in Ba Lạt tidal flood plain. The thickness of the sediments containing this assemblage varies from 8 to 11 m for the machine-drilled holes, while the hand-drilled holes have not reached the lower part of the sediments containing the assemblage. In comparison with the Pz3, there is not significant difference in composition, but the amount of the mangrove pollens decreases significantly, occupying usually 5-7%, except at the lower part of some hand-drilled holes it can attain 9-11% (LK.T2-T1; LK.T1-T2). The frequent forms in the spectrum are Avicennia and Rhizophora, but they do not exceed 2-3% in maximum.

The back-mangrove pollens also decrease in comparison with Pz3, about 15-22 % in average. They frequently include Acrostichum, Acanthus, Cyperus, Aegiceras, etc. In this assemblage there is not evidence of Nypa fructicans, evidently it means that this species just is restrictedly distributed at the river mouth in coastal zone of Bắc Bộ during a short time in Middle Holocene.

In comparison with that of Pz3, fresh-water plants in this assemblage has a big amount of 45-55% in average with the dominance of allochthonous pollen species and in the spectrum, Pinus is less frequent, but the conifers are more abundant, especially Tsuga, Taxus, Taxodium, Sequoia.

The diatom complex characterizes the fresh-brackish water environment with similar composition and amount as that in previous assemblages.

Foraminiferas are rarely found in some hand drilling holes (KT1-T2; KT4-T2) and represented by Quinqueloculina sp., Ammonia japonica, Elphidium sp..

IV. DISCUSSION

In Holocene, the studied region is mainly influenced by fluvial and marine activities, whose role in various time might be alternatively dominant. This problem is reflected by determined organic and inorganic matters. Based on results of isotopic datation of 12,340 ± 115 yrs BP obtained from C¹⁴ method on the sample taken at the depth of 59.4 m in LK.6GT, we could only interpret that the Pleistocene/Holocene boundary is situated at the depth of 56 m in the LK.6GT borehole. This boundary well corresponds to that at the depth of 56 m in LK.56ND borehole. All the rest of boreholes with the depth less than 48 m do not reach this boundary.

At present, the Pleistocene/Holocene boundary is considered as 11,500 yrs BP. It is adopted at the 32^nd International Geological Congress in 2004. The boundaries between the other Holocene subdivisions are as follows:

- The Early / Middle Holocene boundary: 6000 yrs BP;

- The Middle / Late Holocene boundary: 3000 yrs BP.

Due to the lack of C¹⁴ data, the division and dating applied for the studied sediments are done just on the basis of paleontological and lithological studies and partly on the assumption from the rate of sediment deposition measured at Ba Lạt river mouth during many years of 4.5 - 5 mm/yr in average, reported by Đinh Văn Thuận et al. [6].

- Early Holocene

At the beginning of the Holocene, the studied region is strongly influenced by fluvial activities; the marine influence is restricted due to the lower sea level in the region in comparison with the present level [6]. The vegetation cover had continental characters, the brackish- and salt-tolerant forms occupied a small ratio. This period corresponds to the Pz1 assemblage, where fresh-water diatoms are dominant components, such as Cymbella sp., Diploneis ovalis,… with some brackish components, such as Coscinodiscus lacustris, Cyclotella caspia, etc.

It can be said that some salt-tolerant forms begun to appear after a long time due to the regression. The sedimentary environment was typically estuary and flood tidal delta. The autochthonous pollens are mainly found with non-arboreal forms, such as Acanthus, Acrostichum, Graminae, Compositae and the arboreal pollens are of allochthonous origin coming from the northwest mountainous region of Việt Nam characterizing the temperate subtropical climate.

- Early-Middle Holocene

This period was characterized by the strongest Flandrian transgression, which followed the late glaciation. The whole studied region was influenced by the activity of the sea. This period corresponds to Pz2. The shoreline changed in large area and reached the recent situation of Hà Nội and Hà Đông cities [5]. In this period, the mangrove was rapidly developed in quantity and specicific composition as well. Typical forms, such as Avicennia, Rhizophora, Bruguiera, etc. occurred with medium quantity, whereas the back-mangroves seemed to be rather abundant. Especially at that time, the Nypa fructicans was met in some samples. This form recently is just distributed under the 11 parallel, and has not been recognized in North Việt Nam in previous research work even in the older than Holocene sediments. Its presence can be explained by two following ways:

- It was dispersed from the south northwards by sea-currents with a restricted distribution;

- Before Holocene, it might probably be so poorly developed in the north that the previous works did not discover yet.

One can say that Early-Middle Holocene is the period of strongest development of the mangrove vegetation in North Việt Nam, not only in Holocene, but also in Quaternary (1.6 million years). However, in comparison with the development of mangrove in the Mekong delta and adjacent areas, where in the pollen spectrum it occupies up to 80-90%, in the north it is much less developed (less than 50% in the spectrum) [5]. In this period, typical salt diatoms had a dominant role together with brackish forms. Foraminiferas occur also with a very abundant assemblage, characterizing pro-deltaic and delta-front sediments.

In combination with the presence of allochthonous pollen forms, the climate in the studied region can be determined as subtropical-tropical humid.

- Middle-Late Holocene

This period corresponds to Pz3 and was characterized by the displacement of sea-shoreline which coincides approximately with the recent one. The mangrove at this time got the similar composition with that of the previous period, however there was a little decrease in quantity. For Nypa fructicans, it occurred rarely in some samples from LK.2GT, LK.3GT. The percentage content of fern spores increases in comparing with the two previous periods. This reflects a depositional environment of strong dynamic streams and a more humid climate [10].

Foraminiferas occur only in the upper part of the sections together with saline diatoms, while brackish- and fresh-water algae - in the lower part. The sediments got a better granular sorting in the upper part (So = 1.4-2.3) than in the lower one (So = 1.7-2.6).

So, it is shown that, at the beginning of the period the coastal, marshy and flood plain depositional environments dominated. But the later stage was characterized by the delta-front and sub-aqueous delta plain environment.

- Late Holocene

In this period, the mangrove (including back-mangrove) flora remarkably decreased. In almost all pollen spectrum the mangrove occupied just only 15-20%, except in some samples from the lower part (about 30%), if excluding the spectrum of surface samples that is influenced by the present vegetation cover having human impact (forest plantation). There is completely not Nypa fructicans in this period, probably due to unsuitable climate of the north for its development. There are also rare foraminiferas. The brackish- and brackish-fresh-water diatoms dominated; the granular sorting of sediments is poor (So = 2.2-2.8).

The coastal marshy and tidal flat depositional environment dominated with some delta-front factors in the lower part of the section.

CONCLUSIONS

The following conclusions can be drawn from analytic results:

1) There are probably four successive mangrove developing stages: Early Holocene, Early-Middle Holocene, Middle-Late Holocene and Late Holocene, corresponding to the Pz1, Pz2, Pz3, and Pz4 palynological assemblages, respectively.

2) Despite of the differences in percentage content of pollen and specific composition, all these assemblages can be related to the recent vegetation cover in the studied region, except the Nypa fructicans species, that was restrictedly developed in Pz2 (Middle Holocene); a further research on this question should be needed.

3) The mangrove development and distribution is directly influenced by the sea transgression and regression during Holocene. After the strongest transgression in Middle Holocene up to now, it still exist some smaller transgressions intercalated with regressions.

4) The pollen assemblages reflect also the evolution of the depositional environment from tidal flood plain or estuarine during Early Holocene to delta front, pro-delta in Early-Middle Holocene to delta front and delta plain in Middle-Late Holocene. It passes to Late Holocene since 2,000 yrs with different origin types, but all related to tidal flat, such as coastal-marshy or estuarine ones.

5) The obtained pollen assemblages show the subtropical and humid tropical climate during Holocene.

Acknowledgements. The author would like to express his deep thanks to the Basic Reasearch Program (2007-2008), the Project “Holocene eco-stratigraphy in large deltas of Việt Nam” and the Netherlands Foundation for the Advancement of Tropical Research for their support in the implementation of this paper.

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