Rare Earth Elements in the Water Column of Sungai Balok, Pahang, Malaysia as Monsoon Event Proxies

Sampling of surface water at seven stations along the Sungai Balok, Pahang was conducted from 2013 to 2015 to investigate the distribution of dissolved rare earth elements (REE) in river systems. The whole concentration of ΣREE in the dissolved phase recorded during this study ranged from 368 to 9121 pmol∙L −1 with a mean of 2328 ± 1442 pmol∙L −1 that was dominantly influ-enced by the concentration of Ce ranging from 84 to 3237 pmol∙L −1 . Similarly, the large ranged value of La/Yb N (0.69 - 11.57) might be due to the fluctuating rainfall events during samplings as well as input from lithogenic sources that suggests the influence of monsoon events. The highly significant statistical correlation of Al and Fe (R 2 = 0.65; p < 0.01) also suggests the resuspension and mixing of REEs in the water column. However, the lower ratio of Y/Ho < 55 might be due to the large volume of freshwater input especially during the Northeast monsoon (November to March). Therefore, the highest inventories of Ce were during 15 th January 2014 and 1 st November 2014 with 586.5 pmol∙cm −2 and 643.4 pmol∙cm −2 , accordingly. Subsequently, results showed an increasing flux of Ce occurring in the dissolved phase from November 2013 to January 2014 and November 2014 to January 2015, with 39.14 nmol∙cm −2 ∙yr −1 and 59.78 nmol∙cm −2 ∙yr −1 , respectively.

the dissolved phase of tropical rivers usually corresponds to salinity, carbonate complex and biological processes [4].
The monsoon period of the southern South China Sea is always changing and not fixed every year. Therefore, this study is important to examine the presence of monsoon effects over sampling periods with the climatological data of wind flow direction and rainfall distribution. The distribution of REEs in dissolved phases had been observed in various environments such as global rivers [5], biogeochemical cycles in Tokyo Bay [6], granite sources [7], and most recently published in the Bay of Bengal by Yu et al. (2017) [8]. Dissolved REEs in rainwater have been discussed based more on industrial effects e.g., mining and atmospheric pollutants during wet deposition within acidic rainwater [9]. However, the rainfall aspect in tropical river systems and changing monsoons have not been well discussed by the previous researchers and in this study the authors have selected the Sungai Balok, Pahang, Malaysia as a pilot study area to elucidate REEs with rainfall events.
The upstream of Sungai Balok is in the hilly mountain near the granite belt [10] therefore, the river receives large amounts of dissolved and particulate loads due to heavy rainfall events especially during the Northeast monsoon. The increase in river discharge also brings higher terrestrial inputs as well as erodes the mangrove forest in the river system. The idea of establishing selected REE proxies i.e., La/Yb and Y/Ho was conducted previously in studies on dissolved REE in rainwater but overlooked the abundance of La compared Ce and the positive anomaly of Ce and Gd. This however, was not used to directly determine any REE proxies for studying the monsoon changes. Since La/Yb N was used by Thompson et al. (2013) [11], and Takahashi and Noriki (2007) [6] in the estuaries of several major rivers around the world and in Tokyo Bay, respectively, it was deduced to have the potential to be the REE proxy in Sungai Balok for the determination of the monsoon due to rainfall seasonality [12]. The terrigenous detritus also contains lithogenic sources including organic and inorganic components through the weathering in estuarine environments. Meanwhile, previous studies on higher dissolved REE concentrations from lithogenic organic sources were investigated in Tokyo Bay [6] as well as Bay of Bengal [13]. Hence, the objective of this study is to investigate the distribution of REEs during various sampling periods and to determine the suitable proxy of lithogenic sources using a species of REEs.
The estuarine and river systems are surrounded by mangrove forests and undergo semi-diurnal tides. Surface water was collected using a clean water sampler at seven stations of the river during nine sampling trips from 2013 to 2015 (Table 1; Figure 1). In the laboratory, water samples were filtered immediately using 0.45 μm membrane filter paper and acidified to pH 2. The filtrate was in the dissolved form while the samples remaining on the filter paper were considered suspended particles samples. Dissolved samples were then stored in acid washed polyethylene bottle [7] with 50 ml of 4% HNO 3 and kept in a cool box prior to further analysis in the laboratory. Water quality parameter i.e., salinity, was also recorded using the calibrated water quality multi-parameter AAQ-1183H manufactured by Alec Electronics Co. Ltd.  The published analytical procedure was followed with the pre-concentration method using Chelex-100 resin [14]. While the trace elements such as Al, Mn and Fe were also analyzed using the published protocol of Bourg (1983) [15]. The Standard Reference Material (SRM) was also prepared by spiking multi-REE elements (10 ppm) and standard solution of trace elements (10 ppm) into artificial seawater and running them as actual samples to observe their recovery yield (90% -95%). The concentration levels of biological silicate (Biogenic SiO 2 ) were determined using the yellow silicomolybdate techniques Coradin et al., (2004) [16].
The results of REEs have been normalized with the Post Archean Australian Average Sedimentary rock or Shale (PAAS), which is the nearest continental crust for further discussion [7]. According to Daud and Mohamed (2016) [1], the geological setting of Sungai Balok is enriched with sedimentary rock or clay along basaltic rock, which is similar to PAAS. Generally the REEs members will divide into two groups as light rare earth element (LREE) which in the periodic table is from lanthanum (La) to Europium (Eu) and heavy rare earth element (HREE) is from gadolinium (Gd) to lutetium (Lu). Subsequently, the estimated flux of REE in Sungai Balok was calculated using the following published Equation (1) [17].    Table 2). These results were probably due to the high rainfall events [14]. Dissolution of Ce in January 2014 was observed to be statistically higher due to the higher monthly rainfall distribution (r = 0.328; p < 0.05) with 277 mm (Figure 2). The same results were obtained after the flooding disaster in December 2013 due to heavy rainfall (>1800 mm) as well as in November 2014 when the rainfall was about 288 mm. Hence these incidents have increased the REE contents in the water column.

Rare Earth Element in Dissolved Phase
There were no significant differences and distributions of REE contents among the stations from upstream to downstream (p > 0.05) except between sampling periods (p < 0.05) because the depth of the water column is very shallow e.g., less than 5 m during high tide which might have caused active resuspension. According to Shynu et al. (2011) [3] the resuspension activities of dissolved REEs in rivers might be from surface sediment to water column as well as re-suspended in subsurface waters from the river runoff. The resuspension and mixing process of REEs in the water column was also observed based on the significant statistical correlation between Al and Fe (R 2 = 0.65; p < 0.01) that was discussed previously in Bourg (1983) [15]. Regarding the concentration of Fe and Al that showed opposite trends to concentration of dissolved REEs, the species in Table 2 might be desorbed from particles or resuspended from surface sediment.
This may have been caused by active river discharge transportation from the upper stream after incorporation of heavy rainfall and sources from groundwater discharge [12].
The sampling sites were divided into three zones which were river, estuarine and coastal (     The same scenarios were also observed by the other elements e.g., Ce, Nd, Eu and Ho in the coastal zone especially during monsoon events because of the abundant inorganic carbon in seawater during semidiurnal tides and the tidal pump phenomenon [20]. The river zones occupied by station 2 and station 3 have a water depth of less than 5 m along the sand bar. Hence, these stations experienced higher deposition of sediments and particles from the sand bar ( Figure   1; Table 1). However, the fluctuating rainfall events affected the concentrations of suspended particulate matter in surface river discharges (Figure 3(a)).
According Agaki (2013) [21], it was indicated that the silicate or opal has the potential as an REE scavenger in the seawater column. Notwithstanding the presence of abundant biogenic SiO 2 also has a linear relationship with organic matter or organic carbon in the estuarine or marine environment. The distribution of silicate originated from terrestrial sources and detrital minerals from lithogenous and hydrogenous phases [22]. Sungai Balok is surrounded by a mangrove habitat and studies of estuarine fluxes by using REE proxies around the world found that the tropical river flux of trace metals in the dissolved phase have a high possibility of being from rainfall runoff and submarine groundwater discharge [23].  Figure 4(b) with a range value of lower than 6 also indicated more natural freshwater from the hinterland to the study location [24], and the high positive anomaly value (>5) of Pr in Figure 4(d) was due to regenerated particle matter from surface sediments and nutrients [25].

Terrestrial Input Proxies
Sungai Balok drains towards the southern South China Sea (sSCS) and the effect of salt water intrusions is higher compared to the west coast of Peninsular Malaysia i.e., the Straits of Malacca [26]. The dissolved REEs have a strong affinity to particulate REEs and reactive particles in the marine environment [1]. Terrestrial runoff occurring during heavy rainfall may have brought an abundance of terrigenous detrital matter and particulate organic matter into the river and ocean environments. The negative statistical correlation was observed between particles and rainwater, which means particles such as dust, particulate matter and volatile gases were scavenged during wet deposition [27].
Ce species is dominant in certain conditions due to abundance among the REEs and in the environment it can be applied to determine the oxidation and reduction stage of cerium either Ce/Ce* < 1 and Ce/Ce* > 1 by Ce 4+ and Ce 3+ , respectively. The adsorption of dissolved REEs will be scavenged to surface sediments, and the negative Ce anomaly (Ce/Ce*) along the stations ranging from 0.2 to 1.33 showed that oxidation process of stage IV occurred at Sungai Balok.
Researchers Nozaki et al. (1997) [28] and Sholkovitz et al. (1999) [2] also explained the adsorption process could be detected by process of Ce oxidation in the marine environment. However, De Baar (1983) [29] noted that in the presence of oxygen, the seabed regeneration can reduce the Ce (IV) to Ce (III) especially in the river and terrestrial area. Sungai Balok also showed that the positive Ce/Ce* was abundant at the stations in the river and estuarine zones (i.e., ST3 to ST7) ranging from 1.0 to 1.33. The highest was recorded at ST7 in November 2013 and might be due to the weathering process as shown by lowest ratio value of Y/Ho (9.247 ± 4.026) and high contents of Mn (3.543 ± 2.74 μmol•L −1 ) compared to other sampling periods. According to Sholkovitz (1992) [4], the increased Mn oxide acts as the catalyst for supplying oxygen demand in surface  [14]). The Eu anomaly has the ability to evaluate the lithogenic input through the ratio of Y/Ho [30], where the ratio Y/Ho < 55 in the study suggests a lot of freshwater input in the Sungai Balok river system ( Figure 5

Rare Earth Elements Budget into the Southern South China Sea
Sungai Balok falls into the southern South China Sea zone [14] and the amount of REEs discharged are still not well documented. Due to the abundance of Ce among REE members, it has been selected for the calculation of river fluxes. The flux of REEs was estimated using formula (1), while an inventory could be developed by formula (2) adopted from the published journal [28]. The inventory and flux of dissolved REEs were estimated using the Ce element in Table 4 and Table 5. By consideration of the residence time of Ce [33] where the value of Ce is 80 years while Nd is 270 years. Due to the unstable nature of Ce, it is not the main choice for the geochemistry scientists to determine the residence time in  (2); The value of L is converted to cm 3 and m is converted to cm, where 1L is equals to 1000 cm 3 and 1m is equals to 100 cm. Note: Period was divided to two seasons which were during monsoon from November to January while after monsoon from April until July of sampling periods. Average Ce is obtained from average concentration within four months. The conversion factors 1/N of sampling periods are obtained by calculated the days of sampling period time to value 86400 of second per days. Fluxes is obtained from Equation (1) after entering the value of conversion factor, average daily river discharge and dissolved Ce concentration. Journal of Environmental Protection marine, coastal and ocean areas and mostly use the Nd as a proxy. However, from this finding the dissolved REE river flux was identified by measuring the accumulation duration over the sampling period conducted in Table 5, but not the remaining dissolved REE or resident at the sampling point.

REE
Inventory C D = × (2) where, the C is the concentration of dissolved REE (pmol•L −1 ) in the water column, D is the depth of water column (m). Therefore, an inventory could also be produced by multiplication of the C which is concentration of dissolved REE (pmol•L −1 ) in the water column with D which is the depth of water column (m).
The accumulation of dissolved REEs could be represented by the dissolved Nd and Eu fluxes with the overall domination during the Northeast monsoon which was also supported by the previous findings in November 1996 [34] and in August 1997 [13]. Although Nd fluxes were widely used to measure the river and  [2] where Eu followed the same behavior of Ce.
According to Sholkovits and Szymczak (2000) [37], the Indonesian archipelagos, Papua New Guinea (PNG), Peninsular Malaysia and Borneo were believed to supply more than 20% of global sediment input into the sea. The island weathering activities explained that the terrestrial sources draining into rivers and the huge river discharges of more than 2 × 10 15 m 3 •s −1 fold was due to storms and inundation [2]. The PNG and Indonesia archipelagoes were also fed by sources

Conclusion
The fluctuations of monthly rainfall events directly control the concentration and distribution of REEs in the dissolved phases at sampling locations especially during the Northeast monsoon with the dominant element demonstrated by Ce.
The ratio of La/Yb N in the dissolved phase of REEs was also found to be a suitable proxy for evaluating lithogenic sources during rainfall events. The rainwater runoff along lithogenic sources from terrestrial into the river found Y/Ho < 55 that also indicates a large volume of freshwater discharge into the river. The strong significant correlation between Al and Fe also supports the lithogenic sources being actively re-suspended in the water column during water mixing due to the rainfall events. Therefore, the river inventories and fluxes could be estimated through the dominated element of Ce, which is related to the high rainfall distribution during the Northeast monsoon. REEs also have the ability to indicate the changes of monsoon seasons and origins of lithogenic sources in rivers through Ce fluxes and La/Yb N ratio.