Reconstruction of Quaternary Vegetation and Climate of Borobudur Lake

Reconstruction of quaternary vegetation and climate of Borobudur Lake

Sandy Hardian Susanto Herho [email protected] July 10, 2018

1 Introduction

1.1 Background Borobudur is the largest Buddhist Temple in the world which belongs to the seven wonders of the world. Borobudur Temple was built on the surface of a lake that dried in the period 760 to 830 AD [Murwanto et al., 2004]. Borobudur is located in Central Java. A region that is vulnerable to natural disasters, such as volcano eruptions, landslides, and droughts. The climate phenomenon in Borobudur aﬀect the dynamics of vegetation in this region [Murwanto, 2011]. Palynology analysis is an important eﬀort in increasing the understanding and predictability of the climate phenomenon, one of which is rainfall (Zhao et al, 2007). In this thesis, palynology analysis will be conducted in the former Borobudur Lake area, which covers a period of 20,000 years (22,000 to 660 BP). This study aims to maximize the potential use of pollen to see the intensity of rainfall and vegetation during the active time of Borobudur Lake.

1.2 Problem Statement Based on the background of problems description above, the use of palynology method in reconstructing climate and vegetation in Borobudur is still not optimal, due to the absence of spatial and temporal studies in this area. Therefore, in order to reconstruct rainfall and vegetation distribution in Borobudur, the problem statement in this thesis is palynological analysis with regard to vegetation dynamics and rainfall during the evolution of the Borobudur Lake.

1.3 Objective Based on the above problem statement, the purpose of this study is to understand vegetation and climate dynamics that using palynology data in Borobudur Lake for 22,000 years.

1.4 Scope and Limitations • The study area is limited to the area of Borobudur Lake (Figure 1). • Parameters that are considered in this study are the climate rainfall and vegetation distribution data. • The study timeframe is within the evolutionary range of the Borobudur Lake, which is 22,000 to 660 BP [Murwanto et al., 2004].

1 Figure 1: The study area [Murwanto et al., 2004]

2 Literature Reviews

2.1 Regional Geology According to the Central Java physiographic zone [Van Bemmelen, 1949], the Borobudur region lies in the central zone of depression. Its position lies between the Menoreh Mountains range on the south side and the Quaternary mountains on the north side. Borobudur region occupies a plain landform that has an area of ± 20 km2 with height between 225 to 240 meters above sea level (m.a.s.l.). The geological map of Borobudur and its surrounding areas is shown in Figure 2. The Quaternary volcanic ranges that conﬁnes the Borobudur Plain on the West-North- East side are Mounts Sumbing (3,135 m.a.s.l.); Sundoro (2,271 m.a.s.l.); Tidar (505 m.a.s.l.); Merbabu (3,142 m.a.s.l.); and Merapi (2,911 m.a.s.l.).

Figure 2: Geological map of Borobudur and surrounding areas [Murwanto, 2011]

The Borobudur temple was built on an isolated hill located in the middle of the plains morphology. The plains morphology is bounded by the Menoreh Mountains extending east-west with varying altitudes between 500 and 1,000 m.a.s.l. On the Borobudur Plain, several tributaries ﬂowing down the slopes of the Menoreh Mountains, these rivers include the Progo River; the Elo River; the Pabelan River; the Tangsi River; and the Sileng River. In the Borobudur Plain, the tributaries converge along the Progo River, reaching to the south side of the Borobudur Plain

2 obstructed by the Menoreh Mountain cliffs, so that it is deflected to the southeast which then empties into the Indian Ocean. The geological structure of the Borobudur region is strongly influenced by the Plio-Pleistocene orogeny characterized by folding and faulting structures [Murwanto et al., 2004]. Through direct observation in the field, Murwanto [2011] observed some drag fault structure; fault scrap; and fault breccia. Fault planes and joints are filled with ore minerals and also the source of salt water and swamp gas passing through this fault planes . The folding structures in the Borobudur region caused the old Andesite Formation to be folded [Van Bemmelen, 1949]. In this formation, the rock layers have a slope ranging from 23 to 27 to the south. The structure is a homoclinal structure located in the southern part of Borobudur region, in Menoreh Mountains [Murwanto, 2011].

According to Murwanto et al. [2004], the Borobudur region has a ﬂat topography formed from swamps or lakes in the past. This lake was formed before the Holocene. Borobudur Lake is the region with the lowest topography in the Magelang Plain [Murwanto, 2011]. Consequently, the entire upper stream of the Quaternary volcanic ranges, as well as from the Menoreh Mountains will ﬂow centrally to Borobudur Lake. Borobudur Lake serves as a temporary base level erosion before it empties into the Indian Ocean [Murwanto et al., 2004].

2.2 Borobudur Depositional Environment Borobudur and its surroundings area is largely composed of siltstone, sandstone, and pebbly sandstone intercalations [Murwanto et al., 2004]. These rock units are covered by the Quater- nary volcanic deposits in the eastern, northern, and western sections, as well as the Tertiary volcanic deposits in the southern region (Figure 2). Sandy claystone according to Murwanto et al. [2004] is the main lithological character of lacustrine sediment because it contains many pollen fossils that come from swamp ecosystem which was deposited in Borobudur Basin.This lacustrine sediment is present at the base of Progo River, Elo River, and Sileng River val- ley. Above this sandy claystone layer, there are brownish gray lapilli-tuﬀ deposits that contain many of pumice fragments with the thickness more than 10 meters. This volcaniclastic deposits was allegedly derived from the eruption of the Quaternary volcano in the northern part of the Borobudur Plain [Murwanto, 2011].

Another evidence suggests that a lake once formed in the Borobudur is the discovery of pollen that indicates the formation of a lacustrine depositional environment. The analytical results of pollen and spore that contained in the upper black claystone deposits show some types of pollen originating from swamp ecosystem plants, such as Commelina; Cyperaceae; Eleocharis; Nymphaea stellata; Polygonum berbatum; and Ranunculus blumei [Backer and Van Den Brink, 1963 in Murwanto et al, 2004]. Based on the ﬁndings of Murwanto et al. [2004], many Pteri- dophyta spores are most commonly thought to originate from upstream areas but are carried by wind and river ﬂow to Borobudur Lake, thus depositing with black clay (Figure 3). These results prove that the Borobudur Plain in the past is a lake environment.

According Murwanto et al. [2004], Borobudur Lake in the past was an open lake environment obtained from the comparison interpretation of aboreal percentage that is greater than unboreal pollen. This open condition on Borobudur Lake may be aﬀected by Quaternary volcanic eruptions [Murwanto, 2011]. The eﬀect is apparent through the abundance of volcanic material deposited along with black claystone. This young volcanic activity resulted in changes in the lake environment becoming increasingly shallow and narrow.

3 Figure 3: Borehole logs in the Sileng and Elo Rivers[Murwanto et al., 2004]

2.3 Formation Process of Borobudur Basin Process of forming the Borobudur Lake basin is closely related to the tectonic dynamics in Java Island. Tectonic subduction process in Java formed an Oligo-Miocene volcanic arc known as the Old Andesite Formation [Van Bemmelen, 1949]. Sedimentary basin that formed at this time was back-arc basin, namely Kendeng Basin and Serayu Basin in Central Java. At the Miocene, the old volcano complex begins its inactive phase. Then the volcano complex in the shallow marines was overgrown by coral reefs, while the complex that was located in the deep marine environment was overlayed by sand to clay sized clastic limestone and marl intercalations, forming what is currently known as the Jonggrangan Formation (Early Miocene), and the Sentolo Formation (Early Miocene to Late Miocene) [Dolinger and de Ruiter, 1975 in Murwanto, 2011].

The continuous tectonic process leads to an increase in the compression force so that the Kulon Progo Volcano complex and its sediments deposited on it (Jonggrangan and Sentolo Forma- tion) undergo a process of folding, lifting, and faulting followed by magmatic activity forming the Peniron Formation. These processes formed lava dome landscapes around Kaliangkrik and Salaman, as well as the hills of Mount Gendol and Gunung Sari in southeast Muntilan.

At the beginning of the Quaternary epoch, strike-slip; upthrust ; shear joint; and release joint structures was formed at the peak of the compression force, undergoes a stretching process so that gravitational forces play a greater role in the formation of the landscape. This process leads to the formation of graben, normal fault, and normal stratiﬁed fault structures. In the central part of Java, the evidence occurred in the northern part of the Kulon Progo Dome, where the northern block undergoes a process of drowning the southern block to form a steep wall extending 20 km along the east-west direction [Murwanto, 2011]. The drowned northern block appears above sea level as isolated islands. The process of drowning the northern part of the Kulon Progo Dome or Menoreh Mountains that occurs in the Early Pleistocene marks the beginning of the formation of the Basin of Borobudur Lake. The drowning process occurs on the eastern side of the Menoreh Mountains and the western side of the Southern Mountains. The process contributes to the formation of Bantul Graben. This graben structure separates the Southern Mountains from the Menoreh Mountains. This separation occurs on the eastern side of the Menoreh Mountains, by two main fault faults, namely Serang Fault and Progo Fault, and on the western side of the Southern Mountains by Opak Fault and Oyo Fault. Bantul Graben in the Middle Pleistocene until the Late Pleistocene acts as a medium between the Borobudur Basin and the Indian Ocean interactions. Through Bantul Graben, seawater from the Indian Ocean moves to ﬁll the Quaternary basins located at the north of Menoreh Mountains, which are Borobudur Basin and Banyuasin Basin.

4 The relationship between the Borobudur Basin and the Java Sea in the north has been blocked by the hills formed in Pliocene. In the middle of the Quaternary, the relationship between the Java Sea and Borobudur basin was completely cut off as a consequence of the Kendeng Basin and the Serayu Basin undergoing the process of faulting and lifting followed by a magmatic activity. The orogenesis process is still ongoing until now, thus forming the ranges of Kendeng Mountains and North Serayu Mountains. This magmatic activity begins with the formation of Mount Tidar in Magelang, Mount Puser in Secang, Mount Condong in Windusari, and Mount Bibi in Boyolali. Then followed by the formation of Mount Andong, Mount Gilipetung, and Mount Telomoyo. In the late Pleistocene to recent, there is a growth of young volcanoes such as Merbabu Volcano, Sumbing Volcano, Sindoro Volcano, and Merapi Volcano. Along with the development of this young volcanoes, Borobudur Basin becomes increasingly shallow and narrow. This is evidenced by the activity of ancient Merapi at about 40,000 - 20,000 years ago that left traces in Plawangan hill and Turgo hill [Berthomimier, 1990 in Murwanto, 2011]. Murwanto [2011] explains that most of the Merapi eruption products in that period were deposited in the southern and southwestern valleys, then eruption products was carried by the river as fluvio-volcanic and laharic sediments, deposited massively in the southeastern part of the Borobudur Basin thus closed Bantul Graben. This event closes the relationship between the Borobudur Basin and the Indian Ocean and transforms the Borobudur environment from the lagoon into a lake at the end of the Pleistocene ( 22,000 years ago) [Murwanto, 2011]. The environmental traces of this lake are recorded through brownish black clay sediments which containing high organic carbon and freshwater plant pollen. Two to three centuries after the construction of Borobudur temple (precisely between the 11th and 13th centuries), a tectonic earthquake occurred, followed by a volcanic eruption of Quaternary volcanoes that filled up Borobudur Lake up to 10 meters [Murwanto et al., 2004] These events changed the landscape of Borobudur from the basin to a plain which known as Southern Kedu Plain.

2.4 Evolution of Borobudur Lake Murwanto [2011] suggest that he existence of the Borobudur lake is supported by several evidence. Murwanto [2011] points out the existence of black claystone outcrop along the Sileng River; on the Elo River from the northwest of Mendut Temple to near its estuary on the Progo River; at several locations on the Progo River, which is to the eastern side of Pawon Temple and to the north of the Teluk Village. This opinion is also supported by geoelectric mapping data which suggests that there is a very widespread of clay at a depth of 200 m.a.s.l beneath the Borobudur Current soil, as well as sediment variations; soil; pollen and spores both on the surface and subsurface support the evidence of Borobudur Lake existence [Murwanto et al, 2004, Murwanto, 2011]. Based on the existence of these pieces of evidence, Murwanto [2011] undertook the reconstruction of Borobudur Lake paleogeography and divided the evolution of Borobudur Lake into three stages (Figure 4).

The evolution of Borobudur Lake in the ﬁrst stage (Figure 4a), is characterized by a lake extending in the west and north directions, and limited spreading in the eastern part due to Merapi activity, while the spread to the south direction is blocked by the Old Andesite Forma- tion which the outcrop is now located at the eastern side of Pawon Temple. The Old Andesite Formation acted as a continuous horst to the eastern hills of Borobudur, which at that time was a landmass. In the second evolutionary stage (Figure 4b), there was a constriction of the lake when compared to the previous stage. Borobudur Lake in the second stage extends in the northwest-southeast direction and there are small islands east of Borobudur Temple. In this second stage, environmental changes occur in the northern part of Borobudur temple, which is sometimes occasionally dried, this is evidenced by the results of paleosoil drilling in the Elo River. Nevertheless, Sileng River is still inundated at this stage. T In the third stage (Figure

5 4c), there was a process of narrowing, and the lake is divided into two parts. The Environment around the River Sileng remains a swamp (closed lake), while the Elo River environmental changes from land to a swamp.

Figure 4: The evolution stages of Borobudur Lake, a. 22,000 to 19,000 years ago; b. 4310 years ago; c. 660 years ago [Murwanto, 2011]

6 3 Study Design

3.1 Material and Methods 3.1.1 Data The data that used in this thesis consists of:

• Recent pollen data on the surface of the Borobudur Plain.

• Pollen data which obtained from drilling to a depth of 50 m at Sileng River.

3.2 Methods 3.2.1 Surface Pollen and Sediment Core Sampling The ﬁrst step of this study is collecting some recent sediment samples on the surface along a certain traverse in the former Borobudur Basin region. This step is needed to ascertain whether the condition of recent pollen can represent the spread of recent vegetation in the Borobudur Plains. Then, a 50-meter sediment core was taken at Sileng River by using a piston corer with a diameter of 6 cm [Zhao et al., 2007]. The sediment core will then be stored in a closed PVC pipe to carry as a laboratory analysis material.

3.2.2 Loss-On-Ignition (LOI) Analysis and Carbon Dating LOI analysis of the sediment core is carried out at a temperature of 500 C to calculate the organic material content in the sediment, and at 1000 C to measure its carbonate content [Dean, 1974]. Several pollen samples in the sediment core will be used for carbon dating with Accelerator Mass Spectrometry (AMS) in one of the laboratories. All dates in this dating process are calibrated into units of years before present (BP = 1950 AD) using the calibration dataset of IntCal04 on CALIB Rev. program. 5.0.1 [Reimer et al., 2004]. The age-dept model used will be based on the third polynomial curve after 6,000 BP, and linear interpolation before 6,000 BP [Zhao et al., 2007].

3.2.3 Analysis of pollen samples The pollen subsample from the sediment core will be taken as much as 2 cm3 at a given interval. The treatment performed in this subsample uses a modified acetolysis procedure [Fægri et al., 1989] involving subsample treatment using HCl; NaOH; HF; handling of acetolysis; and fine sieving to filter out clay size particles. The resulting concentrate is then combined with glycerol gel. Each pollen sample obtained will be calculated using a light microscope with x400 magnification for the usual sample, and x1000 magnification for critical appears sample. The pollen identification process will use Faegri and Iversen (1989) as a reference. The pollen percentage will be calculated by using the total number of selected pollen. The pollen abundance diagrams will be plotted using the TGView 2.0 program [Zhao et al., 2007].

3.2.4 Multivariate Statistical Analysis Twelve taxa pollen with a percentage greater than 2% in each sample will be used for Principal Component Analysis (PCA) using CONOCO software [Ter Braak, 1988]. This is done because the percentage of pollen data is a closed composition data, and often we are constrained by the addition method, therefore it takes a logarithmic transformation of the data [Wilks, 2011]. The PCA log-contrast that used is an alternative form of PCA using logarithmic transformation, which serves to concentrate each sample and the types of pollen (using double centering) that

7 will represent the ecological distance between each sample. This is done while considering the abundance of each species simultaneously and the comparison of species composition diﬀerences in the sample.

3.3 Expected Result This research is expected to found vegetation and climate reconstruction in Borobudur Lake during the period 22,000 - 660 BP.

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