The Mid Miocene Climatic Optimum ( MMCO ) Indication at Low Latitude Sediment Case Study : The Miocene Cibulakan Formation , Bogor Basin

Global climatic event on Middle Miocene triggered by geology activity is called by MidMiocene Climatic Optimum (MMCO). This event was widely distributed and associated with increasing temperature and CO2 content in the atmosphere. The effect of MMCO was widely known the mid-latitude region, but still limited information in low latitude sediments. This study try to perform the effect of MMCO at Cibulakan Formation in which deposited in the low latitude basin, Bogor Basin. Fifty eights samples from Cileungsi River were taken at Cibulakan Formation and quantitative nannoplankton analysis was carried out for this study. Nannoplankton shows the sensitive response with sea surface temperature changes. Increasing of total population nannoplankton indicates the rising of temperature and dropping temperature is marked by decreasing population. The effect of sea surface temperature changes relates with salinity changes as the effect of evaporation. Helicosphaera carteri and Umbilicosphaera jafari were counted to know the salinity trend at Cibulakan Formation. Sea surfaces temperature changes was observed on Early Miocene which was influenced by small scale Early Miocene glaciation and active tectonic during this period. Warming temperature taken place on Middle Miocene as the effect of warm and open sea during Mid Miocene Climatic Optimum. Afterwards, hot temperature continued on Late Miocene triggered by global increasing temperature at Pacific Ocean and widely distribution of clean water at North West Java Basin.


Introduction
Global climatic event during Middle Miocene showed increasing of global temperature called by Mid-Miocene Climatic Optimum (MMCO) [1]; [2]. Increasing of temperature in MMCO period related with geology activity which followed by increasing of CO 2 content in the atmosphere. The impact of Mid Miocene Climatic Optimum was widely distributed and associated by 6 0 C temperature warming in the midlatitude region [3]. Moreover, Antarctic vegetation in the MMCO reported that average temperature during summer period showed 11 0 C warmer than today and annual sea surface temperature ranging between 11.5 0 C.
The increasing of temperature during MMCO performs the relationship with the changing of nannoplankton population. Nannoplankton shows the sensitivity response with the increasing of temperature. It can be performed by the number of species diversity and population of nannoplankton significantly increased in MMCO period, when the surface temperature rose from 5 0 C to 8 0 C [4]. Not only increasing of nannoplankton population, but also MMCO evidence made the coccolithophores evolved rapidly and several species showed high diversification [4].
Limited information of MMCO evidence in the tropic evidence is a major challenge to solve. Limitation of geochemical data and quantitative microfossil microfossil analysis in continue section of Middle Miocene Sediments caused the impact of MMCO in equator area, especially in Indonesia is not solving. This study aim to know the impact of MMCO with in Early -Middle Miocene Cibulakan Formation by nannoplankton population changes.

Geology Setting
The research area is situated in Cileungsi River, Bogor,West Java as a part of North West Java Basin [5] ( Figure 1 A.). The research area is situated in Cileungsi River, Bogor, as a part of North West Java Basin [5] (Figure 1A.). This basin was formed by the collision of the Eurasian Plate with the Indian Australian Plate during Late Cretaceous to Early Eocene [5]; [6]; [7]; [8]. On Early -Late Miocene, Cibulakan Formation was deposited in Bogor Basin in back arc setting [5] ( Figure 1B). The back arc setting is a place with stable environment and the high sediment supplies from continental crust that triggers the domination of siliciclastic sediment. Depositional environment changes in the back arc setting is controlled by sea level changes both regional and eustasy. Drowning phase during this age resulted deepening upward sequence and revealed the transition -shallow marine environment.
On Early -Late Miocene, Cibulakan Formation was deposited infill of Bogor Basin in a back arc setting [5]( Figure 1B). Generally, Cibulakan Formation is represented by interbedded of claystone and sandstone, and minor limestone as intercalation [9]; [10]. This formation has conformity contact with Parigi Formation in the upper part, and unconformity contact with Jatibarang Formation in the lower part [5]. Moreover, Cibulakan Formation has interfingering contact with deep water Jatiluhur Formation [11].
Transgressive phase was taken place during Cibulakan Formation deposition. Sea level rise during Early Miocene drowned the Jatibarang Formation and changed the environment from terrestrial and volcanic deposit to transition deposit [9]. At the bottom part, The Cibulakan Formation was deposited in paralic environment and near with active delta progradation [12]. Trangressive phase continued into middle part of Cibulakan Formation and showed gradually changes of paralic environment to shallow clean water environment. At the upper part, offshore bar sediments occupied which was characterized by claystone to bioturbated silty claystone and closed by calcarenite limestone [11].

Method
Measuring section and samples collecting were performed during fieldwork in the Cileungsi River, Bogor, West Java. The outcrop condition is continue, represent the sedimentology dynamic, and cover the short term of nannoplankton ecological changes. Five kilometers traverse was described to know the vertical succession of sediment profile ( Figure 2). Sampling was focused on the claystone and limestone of Cibulakan Formation and total samples were fifty eight samples.
Preparation was employed by quick smear slides method [13]. Samples were crushed and the powder smeared in the cover glass. After that, the powders were draped by canada balsam and covered by cover glass. To keep the original composition of sediment, no material was added during preparation. For the observation, quantitative methods were examined which has been accomplished by the commonly-used Field of View (FOV) method [13]. Nannoplankton observation was performed in Micropaleontology Laboratory, Department of Geology, Institut Teknologi Bandung using polarization microscope Nikon Alphashot YS2-H.

Result and Discussion
Overall, Cibulakan Formation in the research area was indicated formed in the offshore environment. The indication can be observed by thick offshore shale deposit and capped by bioclastic limestone. The vertical section showed Cibulakan Formation, from bottom to top, can be divided into three sequence which bordered by bioclastic limestone, from bottom to top namely by Sequence I, Sequence II, and Sequence III.
Sequence I is characterized by 350 meters thickness of claystone followed by 500 interlamination of thin sandstone and claystone, and wackestone to packestone limestone at the top of sequence ( Figure 3).
Limestone has many biota fragments, consist coral and foraminifera, which indicates as transgressive carbonates at the offshore. intensive than limestone in Sequence I ( Figure 4). The different character with Sequence I, intensive limestone at Sequence II, is interpreted as the product of sea level rise, which is marked by intensive limestone as the cap of clastic sediment. Sequence III shows interbedded of claystone and sandstone as the representation of regressive sediment ( Figure 5).   Helicosphaera vederii -Sphenolithus heteromorphus zone Concurrent zone is bordered by FAD (First Appearance Datum) of Helicosphaera vederii and extinction of Sphenolithus heteromorphus. This zone occupy from RBK -41 to RBK -39 which is equal with NN -5 [15], Middle Miocene around 13.53 mya [16].  As sea surface temperatures and paleosalinity indicator, nannoplankton fluctuation was adopted to know the dynamic of environment changes. The total of nannoplankton was carried out as sea surface temperature where rising temperature is followed by blooming nannoplankton and dropping temperature is marked by decreasing population. Another parameter which is revealed besides sea surface temperature is salinity.

Sphenolithus heteromorphus -Discoaster challengeri zone
Temperature has close relationship with salinity condition due to high temperature associated by high salinity or hypersaline condition. On the contrary, low temperature related with low salinity or hyposaline environment. Salinity change is observed by comparison between Helicosphaera carteri and Umbilicosphaera jafari. Helicosphaera carteri has description ellipsoid coccolith, flange end in wings, two narrow pores in central area ( Figure 8A and 8B). Increasing population of Helicoshapera carteri represents low salinity and brackish environment [17]; [18]. The similar result is revealed by Santoso et al. [19], which analyzed the population of Helicosphaera carteri on Late Miocene -Pliocene Sediments in North East Java Basin, Indonesia, and this species increases in low salinity condition. Umblicosphaera jafari represents high salinity environment (>35 ppt) [17]. Umbilicosphaera jafari is marked by small circular species of coccolith, narrow central-area, and wide distal shield with complex suture (Figure 8C and 8D). As the result, sea surface temperature changes had been occurred from Early to Late Miocene at Cibulakan Formation. Fluctuation of temperature in cooling phase was observed on Early Miocene showed the minimum number of nannoplankton, as result of cooling phase on Early Miocene. Subsequently, sea surface temperatures became warming and followed by rising number of nannoplankton and continued into Middle Miocene. Maximum temperature was founded on Late Miocene which indicated by nannoplankton blooming (Figure 9).
On Early Miocene (NN3 or older zone), fluctuation nannoplankton population was observed as the effect of rapid environment changes. Fluctuation environment had happened due to fluctuation temperature on Early Miocene. The temperature fluctuation correlates with active tectonic related with volcanic sediment in North West Java Basin [20] and global cooling and climatic transition events on Early Miocene [21].
Locally at North West Java Basin, new subduction trend formed at Southern Java, south of North West Java Basin [6]; [20], which deposited volcanic debris Jatiluhur Formation which interfingering with Early Miocene Cibulakan Formation [11]. On Late Miocene (NN8 -NN9), blooming nannoplankton continued and reached the peak of population.
Increasing temperature around 4 0 C at Pacific Ocean [23] triggered the increasing population of nannoplankton. Hence, warm and shallow marine at North West Java Basin [20]; [24] supported suitable As the effect of sea surface temperature changes, salinity changes have been detected during deposition of Cibulakan Formation (Figure 9). High salinities are affected by hot sea surface temperatures which caused high evaporation. Otherwise, low salinities are performed as the effect of low evaporation and decreasing of sea surface temperature.
On the Early Miocene (NN3 or older zone), salinity changes rapidly fluctuated and showed unstable environment. Helicosphaera carteri dominates two starting samples which taken at the base of Early The suitable and stable environment continued to Late Miocene (NN8 -NN9). High salinity condition performed and was marked by Umbilicosphaera jafari had bloom condition, but Helicosphaera carteri drastically decreased. Blooming condition of Umbilicosphaera jafari was controlled by high salinity condition, shallow environment, restricted area, and near shore environment during this age [17].