Quaternary palaeoenvironments of the Kallang River Basin, Singapore
Chua, Stephen Chong Wei
Date of Issue2019-06-06
Asian School of the Environment
Earth Observatory of Singapore
The Quaternary period represents the last ~2.6 Ma a period when global climate was marked by a series of more than 50 glacial–interglacial climate cycles. Palaeoenvironmental records from this period enable us to reconstruct past sea level change, climate change, and associated environmental response, which can be used to better predict and prepare for future environmental change. Unfortunately, there remains a paucity of such records in the Sunda shelf region where at least 450 million people face environmental risks associated with future climate change. The Quaternary stratigraphy of the inner Sunda shelf and much of the coastal areas in Southeast Asia is poorly understood. Developing a detailed framework for the Quaternary evolution of geological terranes is important as many coastal megacities are built on such coastal-marine sequences formed predominantly by palaeoenvironmental change over the last 2.6 million years. A detailed record will also shed light on sea level, climate and coastal change during the Quaternary, of which we know little compared to other parts of the world. Singapore lies near the core of Sundaland which was largely exposed during the penultimate and last glacial maximums. It is considered to exhibit relative tectonic stability, and the meso-tidal conditions coupled with relatively low-energy wave and wind regime result in reliable, relatively undisturbed sedimentary archives recording palaeoenvironmental change. In my thesis, I report on the Quaternary stratigraphy, sea level, and coastal change, of the Kallang River Basin (KRB) based on high-resolution sedimentological and geochemical analysis of a ~38.5 m sediment core (~50m below mean sea level) sediment core (MSBH01B), constrained by 23 14C AMS dates, and augmented by an extensive collection of borehole data within the KRB. The dataset allows an improved understanding of the stratigraphy of this critical area which contain Singapore’s downtown area, National Stadium and numerous commercial, retail and residential buildings. First, I created fence diagrams comprising 14 cross-sections spanning the KRB as well as the first 3D geological model with chronology constrained by radiocarbon and OSL ages. My new geological model reveals a more complex geology than currently known, as well as late Quaternary morphological and hydrological changes in the basin, with strong implications for geotechnical and engineering work in the area. Second, I produced 4 new sea level index points (SLIPs) obtained from basal peats from MSBH01B, recalibrated existing SLIPs, and used a Bayesian modelling approach to produce an extended, statistically-robust sea level history for Singapore. This new record reveals a period of rapid sea level rise (>15 mm/yr) from 9.5 ka BP before a slowdown at ~9 ka BP, and a 2nd slowdown between 8 ka and 6 ka BP at ~4 mm/yr. The revised sea level record shows a minor inflection at ~7.5 ka BP and no unequivocal evidence for notable meltwater pulses observed at 8.2 ka and 7.5 ka elsewhere. Third, I analysed sediment core MSBH01B using sedimentological and geochemical techniques at high-resolution (cm-scale) to produce a coastal evolution model for Singapore during the early-mid Holocene (~9.5 ka - 7.3 ka BP). A mangrove coast existed from ~9.5 ka – 9.2 ka BP becoming locally extinct within 300 years as estuarine conditions set in from ~9.2 ka– 8.8 ka BP. Prodelta muds were deposited from ~8.8 ka to 8.25 ka BP coincident with an increase in subtidal calcareous fauna, succeeded by delta front sediments deposited from ~8.25 ka - 7.8 ka BP. A period between 8.5 ka and 8 ka BP with markedly higher precipitation and weathering rates coupled with a dip associated with monsoon weakening is a possible local expression of the 8.2 ka climate event. Finally, seaward progradation of coarse, shelly deltaic sediment occurred from 7.8 ka - 7.3 ka BP, coeval with global delta initiation. This thesis improves our understanding of Singapore’s late Quaternary stratigraphy through a high-resolution geological model of the Kallang River Basin. The thesis also contributes new knowledge about early-mid Holocene sea level and Holocene environmental change in Singapore and the region.