**2. Sediment as geo-marker for monitoring study**

 Marine sediments (**Figure 3**), including materials originating from the terrestrial inputs, as well as atmospheric deposition and autogenetic matter from the ocean itself, preserve a continuous record of regional and even global environmental changes, which can be employed in metal pollution evolution [25, 26]. Because of its unique characteristic, sediment always is considered as mirror of sedimentary environmental changes, which can reflect the biological, geodynamic, and geochemical processes of former conditions [27, 28]. On the other side, environmental changes are not only driven by natural forces but also by anthropogenic effects by human [29]. Some studies had concluded that the anthropogenic impacts on the environment have led to eutrophication process in coastal zone and offshore and the interaction of the natural force and human activities has exerted great effects on the whole environmental system[30].

 Sediments can pick up metals due to several chemical process and normally will settle down in marine aquatic environment. Because of this characteristic, sediment can act as an appropriate indicator to monitor the metal pollution. In aquatic environment, these pollutants are originated from natural and anthropogenic sources in the same manner [31]; thus, scientists have difficulty to identify and classify the origin of these pollutants in the environment. Therefore, to overcome these obstacles, several scientists were using sediment fraction and characterized them into several sizes to normalize the metal concentration [31, 32]. The rationale applying this approach is normally metals are associated with fine-grain fraction because this fraction has larger surface area and higher cation exchange capacity that can enhance metal adsorption [33]. These fine sediments such as silt and clay with size less than 63 μm (**Figure 3**) are categorized as the most geochemically active fraction in the sediment. With this characteristic, this fraction is suitable to determine the potential pollution in the sediment (**Figure 4**).

Because of their large adsorption capabilities, fine-grain sediments represent a major repository for metals and a record of the temporal changes in contamination. Thus, they can be used for historical reconstruction. Although metals can occur naturally in marine environment due to their presence in local rocks, it is difficult to differentiate whether the source of the metals comes from anthropogenic or natural sources. Therefore, for better understanding about the metal behavior and distribution, it is important to distinguish between metals released from natural processes and those anthropogenic mainly introduced by human activities.

Marine sediments play a key role in the geochemical and biological processes of an estuarine ecosystem. In particular, these sediments act as sinks for toxic metals that enter the estuary. This sediment characteristic can regulate the concentration of these minerals and compounds in the water column [34]. Marine sediment also plays a very important role in the physicochemical and ecological dynamics of metals in marine aquatic ecosystems. The physicochemical nature of sediment-bound metals is important in the bioaccumulation of aquatic organisms such as fishes and shellfish.

#### **Figure 4.**

*Fine-grain sediments have high surface area-to-grain size ratio which can accumulate more metals in the sediment. Photo by Ong Meng Chuan.* 

#### **Figure 5.**

*Sediment core collected from mangrove ecosystem to study the metal proxy and sediment accumulation rate. Photo by Ong Meng Chuan.* 

Sediment quality has been recognized as an important and sensitive indicator or geo-marker of environmental pollution by various scientists [35, 36] since sediments can act as an important sink for various pollutants, such as metals that had been discharged into the environment [37, 38]. Besides acting as pollution indicator, sediments are also important in the remobilization process of contaminants in aquatic environment under favorable conditions through the interaction process between waste column and surface sediments. Due to this process, scientists had developed several comprehensive methods to identify and assess the sediment contamination mainly to protect the marine aquatic organisms [39].

Over the last few decades, the study of sediment cores has shown to be an excellent tool for establishing the effect of anthropogenic and natural processes on depositional environments. Meanwhile, sediment cores (**Figure 5**) can provide chronologies of contaminant concentrations and a record of the changes in concentration of chemical indicators in the environment. During the early 1960s, sediment profiles from depositional areas were used to trace human activity, witnessed by anthropogenic contamination like phosphorus [40], and later in the 1970s, it was possible to distinguish radioactive isotope inputs due to nuclear tests. Metal accumulation rates in sediment cores can reflect variations in metal inputs in a given system over long periods of time. Hence, the study of sediments core provides historical record of various influences on the aquatic system by indicating both natural background levels and the man-induced accumulation of metals over an extended period of time. In addition, the dating of sediment cores using radioactive traces like 210Pb [41] permitted the precise quantification of the history of the inputs in a system [42].
