(Source: Sasin Tipchai/Pixabay)

Sea-level rise is a hot topic today in Singapore. Prime Minister Lee Hsien Loong, in his 2019 National Day Rally Speech, spoke at length about how vulnerable our island state is to 21^st-century projections. However, sea-level rise is not a recent phenomenon and neither are the extreme impacts it has had on Singapore’s landscape.

Here’s another interesting fact – Singapore was not always an island. During past ‘ice ages’ where most of the world’s water was locked at the Poles, the sea was found hundreds of kilometres further away from where it is today. The clues pointing to what happened to Singapore thousands of years ago as a result of sea-level changes can be found right under our feet.

EOS Research Fellow Dr Stephen Chua, who is lead author of the paper, at the coring site in Marina South (Source: Stephen Chua/Earth Observatory of Singapore)

A team from NTU’s Earth Observatory of Singapore (EOS), the British Geological Survey (BGS), James Cook University (Cairns), and the Building and Construction Authority of Singapore (BCA) produced the first high-resolution 3D geological model for the Kallang River Basin (KRB) in southern Singapore. The KRB is an important area in Singapore occupied by major financial and commercial buildings (i.e. Singapore Central Business District), key infrastructure (e.g. Singapore Sports Hub) and dense residential areas and amenities (e.g. East Coast Park).

Composite figure providing geographical context. A) Regional map of Southeast Asia with red rectangle showing the location of Singapore. Dashed line marks the estimated maximum extent of Sundaland based on modern bathymetric data [(produced by Ocean Data View (Schlitzer, 2002)]. B) Map showing the basic geological units of Singapore based on the current nomenclature (DSTA, 2009)

We selected 161 boreholes, out of more than 4,000, to create 14 cross-sections and a 3D geological model. The dataset is augmented with a new ~38.5 m-long sediment core obtained from Marina South, and used together with a previous record from Geylang (Bird et al., 2010), to provide the age (through radiocarbon and Optically-stimulated Luminescence dating) and stratigraphic reference. This model revealed the different layers of sediments laid down due mainly to sea-level change in the Kallang River Basin from 125,000 years ago to the present day.

The geology of Singapore in the KRB is more complex than represented in the current stratigraphic framework. This study, published online in the Journal of the Asian Earth Sciences, proposed a new and more comprehensive framework and nomenclature for future geotechnical work in Singapore.

3-dimensional model (exploded) showing the Quaternary (last 2.6 million years) geological units found in the Kallang River Basin (Source: Stephen Chua/Earth Observatory of Singapore)

During the Pleistocene, river deposits were deposited in the KRB. As sea levels rose and sea waters reached the coastal areas of modern Singapore ~125,000 years ago, nearshore deposits such as tidal sands and mangrove muds were deposited as thin layers up to 40 m deep. This was followed by soft marine muds as sea levels continued rising to maximum heights as high as 6 m during the same period. Subsequently, as sea levels fell from ~120,000 years ago to their lowest levels at ~20,000 years ago, weathering and drying of the exposed marine muds produced a stiff light grey clay unit at depths of ~20 m.

Sea levels started to rise again from ~18,000 years ago, and reached Singapore shores ~9,500 years ago when nearshore sandy and peaty units were deposited above the ‘stiff clay’. As sea levels continued to rise, a younger unit of blue-grey marine muds was deposited from 9,200 to as late as 1,200 years ago. Sea levels were postulated to reach as high as 3 m above modern levels at ~6,000 years ago, and organic-rich deposits thought to be from the mangroves were found as far inland as MacPherson Road. A thin unit of sandy sediments was deposited during the last 5,000 years, tracking the falling sea level to modern levels.

Understanding the geology of Singapore is critical not just because of current and future geoengineering projects like the Deep Tunnelling Sewerage System (DTSS) in such complex environments, but it also holds crucial information in understanding the geological response of Singapore to future environmental change due to sea-level rise. And for the security of our future, we should continue to unravel the mysteries of our geological past.