A Holocene Pollen Record of Vegetation and Coastal Environmental Changes in the Coastal Swamp Forest at Batulicin, South Kalimantan, Indonesia

 

Eko Yulianto ^1, *

A.T. Rahardjo ²

Dardji Noeradi ²

D.A. Siregar ³

K.Hirakawa ⁴

 

¹ Research Center for Geotechnology, Indonesian Institute of Sciences; Jl. Sangkuriang Bandung 40135, Indonesia

² Department of Geology, Institute of Technology Bandung; Jl. Ganesha 10 Bandung 40132, Indonesia

³ Geological Research and Development Center; Jl. Diponegoro 57 Bandung, Indonesia

⁴ Laboratory of Geoecology, Graduate School of Environmental Earth Science, Hokkaido University; Kita-ku, Kita 10 Nishi 5, Sapporo-Japan

 

 

Abstract

 

A pollen analysis of a coastal peat swamp core represented 9.1 kyrs BP from Batulicin, South Kalimantan, Indonesia shows that mangrove forest has been established since early Holocene with the main element Rhizophora. However the vegetation particularly mangrove forest has changed several times responding to Holocene environmental changes. The highest value of Rhizophora at ca. 8.2 kyr BP indicates early Holocene sea-level drop and implies sea-level at ca. –9 m. Subsequently mangrove forest severely disrupted at ca. 6.4 kyr BP due to rapid sea level rise prior the Holocene Maximum. However it quickly reverted and getting flourished following lower rate sea-level rise or subsequent sea-level drop from ca. 6.0 kyr BP and persisted to ca. 1.0 kyr BP. From ca. 6.0 kyr BP the environmental setting around the site may have gradually shifted landward from the mangrove forest to peat swamp forest due to higher precipitation and intensive progradation. Human influence is recognized from ca. 1.6 kyr BP.

 

Keywords: Kalimantan; Palynology; Holocene; Sea-level; Environmental changes.

 

 

 

1.     Introduction

 

Pollen records of Holocene environmental change in the Indonesia region have been mainly focused on submontane/montane environments (Hope, 1976; Walker & Flenley, 1979; Polhaupessy, 1981; Morley, 1982; Flenley, 1984; Newsome & Flenley, 1988; Stuijts, 1990; Maloney, 1993; Hope & Tulip, 1994). Pollen analysis on Upper Pleistocene marine sediment core from Australasia, on the other hand, reveals good correlations between abundance of mangrove elements and the d¹⁸O curve (van der Kaars, 1991). Because mangrove ecosystems are sensitive to relative sea-level rise due to permanent inundation, salinization and coastal erosion (Pernetta, 1993; Snedaker, 1993; Field, 1994), it is thought Holocene pollen record particularly mangrove elements could exhibit the Holocene fluctuation of sea level in some extent. Studies of mangrove mortality on several continents proved that these ecosystems are so specialized that any minor variations in their hydrological regimes causes noticeable mortality (Breen and Hill, 1969; Blasco, 1984; Jimenez et al., 1985).

This paper presents a pollen study on coastal peat swamp core at Batulicin, South Kalimantan, Indonesia to reveal characteristics of pollen assemblages in tropical coastal peat sediment in relation to Holocene environmental change.

 

2.  Site description

 

Study area is located in southeastern Kalimantan, separated from Makassar Strait by Laut Island (Fig. 1), where the modern devastated coastal swamp forest that developed in an area with marginal seasonal climate covers Quaternary sediment. The sediment is underlain by the Tertiary Warukin Formation, Tanjung Formation and Berai Formation (Rustandi & Sanyoto, 1995). Some small rivers originating from the surrounded hills flow on the quaternary sediment to Laut Strait.

 

3.  Method

 

Pollen analysis was carried out in the Quaternary Laboratory, Geological Research and Development Center, Bandung, Indonesia. One cm slices of the core were taken at 50 cm intervals of the 10 m core. Two cm³ was extracted from each slice for pollen analysis. Two samples were taken from 5.40 – 5.45 m and 9.80 – 9.85 m depth for radiocarbon dating.

Each sample was initially treated with 10% KOH before mixed-acid treatment (HCl + HNO₃ with 1:1 proportion). The supernatant was then treated with heated 10% KOH before sieving to remove larger fragments. The remaining organics were then separated from mineral matter using heavy liquid, ZnCl₂ before 40% HF treatment to remove silica and dissolution of cellulose substances by 1-minute acetolysis. The remaining organic was then washed using distilled water and submerges with glycerol for 30 minutes. After decantation some drops of glycerin jelly were put in and mixed before being mounted on microscope slides.

Pollen counts were undertaken on a Seiz Microscope at the magnification of X 400. Initial identification was verified using X 100 oil immersion objective, giving a magnification of X 1000. All the pollen grains present in the sample were counted. Pollen and spore frequencies were calculated on a sum of total pollen and presented in a pollen diagram (Fig. 2). 

 

 4.      Stratigraphy and C-14 Dating

 

The Batulicin 10 m core consists of three units: peat (0.0 – 2.1 m), wood-bearing peat (2.1 – 5.0 m), and clay intercalating sand with mollusk shell fragments (Fig. 2). An erosional surface appears to be in the horizon between the uppermost peat unit and underlying wood bearing-peat unit. Radiocarbon dating at 5.40 – 5.45 m and 9.80 – 9.85 m depth yielded consecutively 5140 ± 180 yr BP and 8830 ± 280 yr BP.

 

5.  The Pollen Diagram

 

Excluding samples at 650 and 998 cm that consecutively bearing 193 and 113 count respectively, total pollen reaches more than 200 counts or mostly more than 400 counts in every sample. Spores are significantly represented with frequencies 10 - 34% in all samples. The assemblages are composed of submontane/montane, lowland/peatland, mangrove and grassland elements. Sixty-six of 77 taxa were identified and 11 taxa remain unknowns. Most taxa have virtual representations and only 10 taxa prominently present in the core including ferns i.e. Rhizophora, Avicennia, Elaeocarpus, Macaranga/Mallotus, Castanopsis/Lithocarpus, Quercus, Engelhardia, Aglaia, Pasania, Aspleniaceae, and Polypodiaceae. Variation between mangrove and non-mangrove frequencies seems to be consistent with variation between mangrove, lowland/peatland and submontane/montane as well as variation between pollen and spores (Fig. 2). Grassland elements that represented by Graminae, Cyperaceae and Compositae sporadically present in virtual quantities. Frequency variation of taxa in Batulicin Pollen diagram allows recognizing 4 zones.

 

5.1. Zone 1 (10.00 – 8.25 m; ca. 9.1 – 7.5 kyr BP)

 

Zone 1 is characterized by high representation of Mangrove and low representation of submontane/montane. Lowland/peatland declines from 51 to about 30% upward. Mangrove shows high value as well as Rhizophora. Mangrove and Rhizophora in subzone 1b is the highest in the whole core. Elaeocarpus shows high values as well. Macaranga is high in subzone 1a, but significantly decline in subzone 1b. Quercus, Asplenium and Acrostichum are low in subzone 1a and slightly increase in subzone 1b. Montane elements are of low values, only represented by Castanopsis/Lithocarpus and Quercus.

 

5.2. Zone 2 (8.25 – 5.75 m; ca. 7.5 – 5.2 kyr BP)

 

Relatively high values of Elaeocarpus distinguish this zone from zone 1 and 3. Mangrove frequency is widely fluctuated while lowland/peatland shows high value. Diversity of submontane/montane elements inclined and represented not only by Fagaceae but also Podocarpaceae such as Podocarpus imbricatus, Podocarpus, Phyllocladus and Dacrydium. However they are in virtual values. Although Castanopsis/Lithocarpus is high in subzone 2a, 2b and 2c, it significantly decreased in subzone 2d. In subzone 2a Rhizophora shows relatively low value, notably increased in subzone 2b, drop to 4% in subzone 2c and back to increase in subzone 2d.  Macaranga presents in relatively high values, Asplenium suddenly incline to 32% in subzone 2a, and subsequently decrease to less than 10%. Aglaia briefly shows high values in subzone 2c and in the beginning of subzone 2d while Pasania shows prominent values in subzone 2b and 2c.

 

5.3. Zone 3 (5.75 – 1.75 m; ca. 5.2 – 1.6 kyr BP)

 

In this zone, frequencies of Elaeocarpus, Castanopsis/Lithocarpus, Quercus, Pasania, Acrostichum and Polypodiaceae significantly inclined from the previous zone. Avicennia and Asplenium show somewhat increased values while Palaquium, Durio, Canthium, Lycopodium and Stenochlaena areolaris present more prominent. This trend is obvious and even continues to lower part of zone 4 where Elaeocarpus, Avicennia and Blumeodendron tend to somewhat incline while Quercus, Castanopsis/Lithocarpus, Rhizophora and Polypodiaceae tend to decline upward. However intervention shown by brief increases of Quercus and Castanopsis/Lithocarpus values occurred at ca. 3.6 kyr BP. Sudden increase of Rhizophora and Elaeocarpus is also recognized at ca. 1.4 kyr BP in zone 4. Spore shows high representation in this zone. Hibiscus, Nypa and Oleaceae were present which are absent in the previous zone.

 

5.4. Zone 4 (1.75 – 0.00 m; ca. 1.6 kyr BP – present)

 

Zone 4 is characterized by lower values of montane/submontane elements such as Castanopsis/Lithocarpus and Quercus. Some lowland/peatland forest and fern elements such as Engelhardia, Elaeocarpus, Lycopodium, Stenochlaena areolaris and Polypodiaceae also show lower values. As mentioned before Rhizophora seem to continue a decline trend from zone 3, and reached the lowest value of 13% at ca. 1.0 kyr BP. It reverted, however, in subzone 4a and significantly inclined to 50% in subzone 4b. Those two subzones are well differentiated by Rhizophora, Avicennia and Macaranga values.

 

6.     Interpretation and discussion

 

6.1 Chronology

 

     The C-14 dating results imply that the deposition rate was relatively constant to be 0.1 cm/yr during Holocene at Batulicin. Accordingly unit 2 and 1 began to deposit respectively ca. 5.0 kyr BP and ca. 2.0 kyr BP. Meanwhile sample interval (50 cm) should represent ca. 500 yrs time interval. Taking into account the occurrence of erosional surface at 2.1 m, it is more likely that the deposition rate of the upper part of the core could be more rapid than that of the lower one. Therefore unit 1 was possibly deposited slightly after ca. 2.0 kyr BP.

 

6.2.     Vegetation and environmental reconstruction

 

Significant representation of Rhizophora and mangrove in pollen assemblages of Batulicin core indicate strong influences of local elements. High diversity with mostly low values indicates that the taxa are mostly extra local elements. High values of mangrove elements particularly Rhizophora and Avicennia in the whole core show that mangrove forest has continuously occupied the site or nearby since early Holocene. Mixed open lowland/peatland forest dominated by Macaranga/Mallotus and Elaeocarpus grew behind the mangrove forest. Fagaceous forest with some dominant elements of Castanopsis/Lithocarpus and Quercus occupied the higher altitude. Ferns also remarkably grew in those forests. Those taxa seem to be important elements in surrounding forests and their fluctuation may reflect forest dynamics.

The earliest period indicated by zone 1 may be dated back to ca. 7500 years BP, when Elaeocarpus remarkably grew in open lowland/peatland forest among the Macaranga/Mallotus behind the mangrove forest. Castanopsis/Lithocarpus and Quercus seem to be important elements in the submontane/montane forest. Increase of Rhizophora which reached the peak at 900 cm (ca. 8.2 kyr BP) couples with reduce of Macaranga, slight increases of Castanopsis/Lithocarpus and Quercus, remain high value of Elaeocarpus and increase of Asplenium may indicate that the site has shifted within the mangrove forest meanwhile a more close-canopied lowland forest with dominance of Elaeocarpus occurred nearby.  It has probably been caused by sea-level drop and slightly wetter climate. Assuming no significant hiatus occurs in the core, the sea-level may stand ca. - 9 m when subzone 1b deposited at ca. 7.5 - 8.5 kyr BP. We interpret that highest peak of Rhizophora and mangrove in the subzone 1b possibly relate to 8.2 kyr BP cold event. Radiocarbon dataset at Great Barrier Reef, Australia reported a fall of sea-level ca. –17 m at ca. 8.2 kyr BP. That had happened after a brief stillstand or peak at ca. –11 m at 8.5 kyr BP (Larcombe et al., 1995). An evidence of sea-level at ca. –12 m at ca. 8.0 kyr BP have also been reported from Malacca Strait (Geyh et al., 1979).

Significant decrease of Elaeocarpus, incline of Macaranga and Aglaia and slight increase of Grassland at 800 to 550 cm (ca. 7.5 to 5.0 kyr BP) suggest more open lowland forest. Engelhardia became an important element in the lowland/peatland forest. A possible explanation for a brief smallest representation of Rhizophora around 6.4 kyr BP might suggest a sea-level rise. Considering the sea-level has been risen 20 m at Holocene maximum (height from the lowest to the highest of neighboring stillstands) and the planar slope of the site, mangrove forest would not have significantly shifted landward to yield notably differences of Rhizopora values in pollen assemblages due to palynofacies changes. Therefore mangrove forest should have suffered severe disruption against rapid sea level-rise prior Holocene maximum that reported beyond 25 mm/yr around 8.0 kyr BP (Tooley, 1978; Ters, 1987; Chappell & Polach, 1991; Eisenhauer et al., 1993). That is somewhat reasonably rate to disrupt the mangrove forests by permanent inundation, salinization and/or coastal erosion. We interpret that the lowest value of Rhizophora at sample 700 cm is the analogue of the peak of sea-level rise occurred at ca. 6.4 kyr BP and correlative with the Holocene maximum.

Mangrove quickly reverted at 650 cm (ca. 6.0 kyr BP) possibly due to the decreased sea-level rise and subsequent sea-level drop after Holocene Maximum. It achieved optimum growth at 600 cm (ca. 5.5 kyr BP). Afterward vegetation within Batulicin area appears to have been relatively stable through the second-half Holocene. More close-canopied forest behind the Rhizophora-Avicennia mangrove forest characterized by dense Elaeocarpus, Engelhardia, Pasania and Macaranga/Mallotus may occur in the lowland/peatland forest. Fagaceae forest seems to be more prominent in the submontane/montane area. Quercus has a greater representation to become equal to Castanopsis/Lithocarpus. Increases of spore elements, particularly Polypodiaceae show a greater abundance of ferns in the forest. In common those may indicate stable wet condition and high precipitation in this period, though slight oscillation of some elements still occurred. Somewhat wetter condition might have prevailed as shown by relatively high representation of Fagaceae and ferns overlapped with relatively low representation of Macaranga at 400 cm (ca. 3.6 kyr BP) and 200 cm (ca. 1.8 kyr BP). More consistent representation of salt less-tolerant elements such as Nypa and Durio and continues decrease of Rhizophora imply landward shift of the site.

Increases of Avicennia and decreases of Engelhardia, Castanopsis/Lithocarpus, Quercus and Polypodiaceae from 150 cm (ca. 1.6 kyr BP) should indicate forest disturbance. Increase of Avicennia may ascribe the rapid progradation due to higher sediment accumulation in land-sea interface area. It may relate to more intensive anthropogenic forest clearance. Decline of lowland/peatland and submontane/montane elements supports evidence of forest disturbance. It is noteworthy here that some states were established in 14 century in Banjarmasin (less than 150 km to the west from the site) and in Samarinda (about 300 km to the north from the site) ca. 400 A.D. (Wortmann, 1971). Low representation of Macaranga in the topmost sample is also possibly related to the modern extensive landuse. Significant increase of Rhizophora in that sample is unclear. Lower influx of lowland/peatland and montane/submontane elements due to land-clearing may ascribe to this in some extent.

 

6.3. Peat formation

 

Tropical peat development from different region in Indonesian shows obvious similarity. First growth phase of peat is represented from ca. 5.0 kyr BP which overlies marine sediment. In Sundaland region, peat growth initiated from ca. 5.0 – 6.0 yrs BP (Morley, 1981). Therefore inclines of Elaeocarpus, submontane/montane and fern elements coinciding with slight drop of Macaranga in zone 3 may indicate high precipitation in the site that was likely to grow peat. Further upward, incline in values of salt less-tolerant elements in zone 3 possibly indicate changes in environmental setting from mangrove forest to peat swamp or peat swamp forest. A visible erosional surface at 2.1 m separating the topmost peat unit from the underlying wood-bearing peat unit possibly related to the initiation of second phase of peat growth. This erosional surface may occur as the result of late Holocene delta development. An analogue of this stratigraphic feature has been reported from estuarine Holocene plains in Sarawak consists of a basal marine clay deposited beneath mangrove forests, overlain by woody peat formed beneath peat swamp forest (Liechti et al., 1960; Wolfenden, 1960; Wilford, 1961).

 

7.  Conclusion

 

At Batulicin, mangrove forest has been established since early Holocene in which Rhizophora has been the main element. However the vegetational landscape particularly mangrove forest has been changed several times responding to Holocene environmental changes. The markedly highest value of Rhizophora which indicates early Holocene sea-level drop at ca.8.2 kyr BP implies sea-level position at ca. –9 m. Subsequently mangrove forest severely disrupted due to rapid sea level rise and reached the lowest representation at ca. 6.4 kyr BP which correlative with the Holocene maximum. The climate was wetter and the mangrove forest was getting flourished from ca. 6.0 kyr BP and persisted to ca. 1.0 kyr BP. This period was likely to be the period of peat growth in the area. Meanwhile environmental setting around the site may have gradually shifted landward from mangrove forest to peat swamp forest due to higher precipitation and intensive progradation. Increase of Rhizophora values since ca. 1.0 kyr BP to the present is obscure but may indicate somewhat sea-level rise coupled with extensive forest disturbance due to human cause.

 

Acknowledgements

 

We would like to thank the Geological Research and Development Center, Bandung, Indonesia for allowing to study the core (ST-3) and to perform the pollen preparation and determination at Quaternary Laboratory.

 

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