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Mercury cycle

Content: It is a very brief article. It would have benefited with more details. The sources of mercury are given clearly. However, the cycling of mercury within the Earth system demands more explanation. Certain factual statements regarding processes involving mercury are given but they lack a cohesive structure. Explaining the interrelationships between each component of the cycle would greatly benefit the reader.

Tone: The article presents factual statements which are cited from scientific papers published in good reputed journals. The author has paid great attention to just presenting the facts. There is no opinion or any bias.

Sources: Sufficient number of citations have been made in the article. Most of the references used are from reputed journals, and their valid doi’s are given. Citations are used appropriately throughout the text when claims about the sources of mercury are made, or when describing about relevant processes and mechanisms. Recent review articles (Lyman et al. 2020, and Ma et al. 2020) could be used to update the cycle.

Figure: The article lacks a figure depicting the described processes. It would have been very helpful to the reader if a figure had been provided. One such figure with fluxes can be found in Mason et al. (2012).

References Lyman, S.N., Cheng, I., Gratz, L.E., Weiss-Penzias, P. and Zhang, L., 2020. An updated review of atmospheric mercury. Science of The Total Environment, 707, p.135575.

Ma, M., Du, H. and Wang, D., 2019. A new perspective is required to understand the role of forest ecosystems in global mercury cycle: A review. Bulletin of environmental contamination and toxicology, 102(5), pp.650-656.

Mason, R.P., Choi, A.L., Fitzgerald, W.F., Hammerschmidt, C.R., Lamborg, C.H., Soerensen, A.L. and Sunderland, E.M., 2012. Mercury biogeochemical cycling in the ocean and policy implications. Environmental research, 119, pp.101-117.

Iron cycle

Content: The article is detailed, structured, and coherent. All the processes have been given categorically and explained in detail. Citations are given wherever necessary. All the facts presented are substantiated by citations. The article could however use some rearrangement in structure. There is a brief paragraph about iron related processes in the ancient Earth. The evolution of iron cycle with time could be included in both the oceanic and terrestrial ecosystems. That could provide a complete picture to the reader.

Tone: The article has a neutral tone throughout its length. All claims are substantiated by scientific reasoning; cause and effects are explained clearly. There are no biased statements.

Sources: The article relies heavily on citations, and in fact almost all of the references are scientific journal articles.

Figure: The entire iron cycle has been depicted in the figure, and the fluxes associated with each mechanism have also been given. The figure is of high quality and well presented. The legend given is detailed and self-explanatory.

Silica cycle

Content: Compared to the mercury and iron cycle articles, this article is better presented and well thought out. The cycle has been divided into two main components i.e., terrestrial and oceanic, each with its own sources and sinks. This gives a clear idea to the reader on the processes involved in the cycle and direction of the said processes. The article is well balanced with scientific data and jargon in such a way that it is not overwhelming to the reader.

Tone: The article is well written without any bias. There is no indication on the emphasis of the author’s view in the text.

Sources: All the sources cited are from scientific literature, and they have been appropriately used in support of a claim or a factual statement.

Figure: It is of high quality and clearly bring out the processes involved in the cycle. Fluxes are also mentioned and the figure caption is self-explanatory. However, the figure is constructed with simple 2D art. The figure could use an update with embellishments to make it more appealing to the reader.

Lead Cycle



Lead is a toxic element to life (Patterson, 1965). But, because it is malleable, ductile, dense, and corrosion resistant, it has been used in a wide range of commercial and industrial applications. Its use in such applications has been known for a long time (Shotyk and Le Roux, 2005). Hence, anthropogenic effect has a profound influence on the natural lead cycle. Also, natural cycling of lead, in terms of a quantitative measure, is low compared to other elements like Carbon, Oxygen, and Nitrogen. This is because there are few biological mechanisms that can contribute to the lead cycle, the reason being that lead is toxic to life.

Sources of Lead

Lead is one of the most abundant heavy trace elements (Wampler 1972). The cosmic abundance of lead is high. It is formed by the radioactive decay of Uranium and Thorium. It is found in crustal rocks; the most predominant form being Galena (PbS) (Cullen and McAlister, 2017). It is the most frequently used and economical ore source for lead extraction (Shotyk and Le Roux, 2005). Other minerals include anglesite (PbSO4), cerrussite (PbCO3), and pyromorphite (Pb4(PbCl)(PO4)3). Lead can substitute potassium in feldspar and mica since its ionic radius (132 pm) is close to that of potassium (133 pm). It is also found to substitute in other silicate minerals (Shotyk and Le Roux, 2005). Natural sources of lead to the atmosphere include three major pathways: wind borne dust, volcanic outgassing, and forest fires [Nriagu and Pacyna (1988), Pacyna and Pacyna (2001)]. The contribution from each of these sources is shown in the figure. It is estimated that the global contribution of lead from natural sources to the atmosphere is approx. 12,000 Mg/yr. Lead transport from continental runoff as a result of natural weathering to oceans is estimated to be approx. 295,000 Mg/yr [Nriagu (1989), Nriagu and Pacyna (1988), Pacyna and Pacyna (2001)]. Anthropogenic activities have resulted in much higher lead outputs. The majority of the lead contribution comes from non-ferrous metal manufacturing plants, mining and smelting of ores, stationary and mobile fossil fuel combustion platforms, and lead batteries [Nriagu and Pacyna (1988), Pacyna and Pacyna (2001)]. The contribution from these sources is significantly higher which can be observed from the figure. The estimated fluxes of lead mobilization during the 80’s are 180 Mg/yr and 565,000 Mg/yr for natural and anthropogenic sources (Nriagu 1990a). But the lead emissions dropped to 120,000 Mg/yr from anthropogenic emissions during the 90’s. This was an effect of the regulation passed against the use of leaded gasoline (Nriagu 1990b). However, the decreasing trend has not continued to the present day. The global production in lead has seen a steady rise in the 21st century.

Lead in atmosphere

Anthropogenic emissions are the dominant source of Pb to the atmosphere. This is quite clear from the available data [Nriagu 1990a, Nriagu and Pacyna (1988), Nriagu 1996, Pacyna and Pacyna (2001), Rauch and Pacyna (2009)]. Ice core records show increased Pb deposition [Boutron and Patterson (1986), Boutron and Patterson (1987), Murozumi, Chow, and Patterson (1969), Matsumoto and Hinkley (2001)]. This could be because of fossil fuel, mining, and industrial emissions because these activities produce very fine micron-sized particles which can be transported as aerosols. Emissions from natural sources tend to be larger in size, and hence their transport would be comparatively limited (Cullen and McAlister, 2017). Wet deposition is the significant process that removes Pb from the atmosphere to the surface ocean. Almost approx. 80% of the Pb gets removed from the atmosphere by this process (Duce et al., 1991). Precipitation leads to solubilization of aerosols and washout of particulates.

Lead in soils

Chemical and physical weathering of rocks mobilize Pb in soils. However, this mobility is dependant on many factors such as chemical speciation, redox potential, pH, presence of ligands in the system [Bradl (2004), Degryse et al. (2009)]. Mobile and reactive Pb can react to form oxides or carbonates. Pb can also co-precipitate with other minerals by being occluded through surface adsorption and complexation [Bradl (2004), Degryse et al. (2009)]. Organic matter also plays an important role in mediating Pb availability mainly through complexation and coordination [Pinheiro et al. (1999, 2000), Strawn and Sparks (2000)]. Generally, however, Pb is relatively less mobile compared to other elements in the environment (Nelson and Campbell, 1991).

Lead in freshwater

The significant factor controlling Pb in freshwater is its chemical speciation (Filella et al. 1995). Pb has a higher affinity to be present as lead carbonate complex in aqueous solution. It can form complexes with sulphate and hydroxyl groups as well [Morel and Hering (1993), Vuceta and Morgan (1978), Hem and Durum (1973)]. The degree of complexation is affected by pH, presence of organic matter and ligands. Adsorption of Pb onto suspended particles is the other factor that controls Pb mobility. The distribution coefficient is used to calculate the dissolved and suspended Pb concentration.

Lead in oceans

Pb concentrations in oceans is dependant on the wet deposition, and the concentration of Pb present in atmosphere [Boyle et al. (1986), Boyle et al. (2005), Wu and Boyle (1997), Weiss et al. (2003), Shen and Boyle (1988), Helmers et al. (1990),Helmers et al. (1993)]. Due to the variation in Pb levels in atmosphere, even the water samples from oceans have shown similar variation [Boyle et al. (2014), Gallon et al. (2011), Wu and Boyle (1997)]. Periodic measurements of Pb concentrations in the Atlantic ocean from 1980 to 2011 have revealed a decreasing trend in Pb concentration with around 160 pmol/kg in 1980 to 20 pmol/kg in 2011. Also, oceanic depth profile shows decreasing Pb concentration with increasing depth (Boyle et al., 2014). Other factors that can affect Pb concentration are the same as mentioned for freshwater systems i.e., pH, presence of organic matter, ligands, and suspended particles. The main sink for Pb is burial in marine sediments. This occurs when Pb co-precipitates or is adsorbed on to the surface of other minerals.

Biological processes

Pb being toxic, there are not many known metabolic pathways for Pb transformation. However, phytoplankton are known to internalize Pb through surface adsorption of the metal (Fisher et al., 1988). This Pb is associated with structural components of cells (Fisher et al., 1983). Lipophilic organic Pb complexes can passively diffuse into phytoplankton and it can also be adsorbed onto cell walls [Phinney and Bruland (1994), Phinney and Bruland (1997), Sánchez-Marín et al. (2010)]. Hence, these mechanisms act as biological sinks for Pb. Another important point to note is that although the plankton may be consumed by other organisms and the Pb is transferred, biomagnification does not occur [Suedel et al. (1994), Reinfelder et al. (1998)].

Detection of Lead

Lead is present in trace amounts. Hence, mass spectroscopy is used to determine lead concentrations, since it is one of the few tests that can determine trace concentrations of a metal with a high degree of accuracy (Weiss, Kylander, and Reuer, 2007). This is in fact, one of the major contributions made by Patterson at Caltech (Patterson and Settle, 1976). Lead has 4 stable isotopes: 204Pb, 206Pb, 207Pb, and 208Pb. 204Pb is not radiogenic however the other three isotopes are radiogenic. They are formed by the decay of Uranium and Thorium. The relative abundance of each isotope is given below in the table (Dickin, 1995).

Uses of Lead

Ancient use of Pb: In construction, coinage, glass manufacture, water pipes, cosmetics, and mining (Nriagu 1983). Modern use of Pb: Paints, water pipes, lead solder, food storage cans, ammunition, radiation shielding equipment, alloys, batteries, and leaded gasoline (Shotyk and Le Roux, 2005). However, lead use in most of these cases has been regulated.

Impacts on human health

Lead is primarily adsorbed via respiration and ingestion. It is circulated throughout the body by blood. Children are more prone to lead exposure because they absorb more of ingested Pb from gastrointestinal tracts (Lidsky and Schneider, 2004). Lead affects the developing brain and the nervous system. It has severely damaging and irreversible effects on brain functions. This happens because lead enters the brain cells and damages mitochondria. It can replace Ca2+ and Zn2+ in binding sites of proteins; this alters the structure and function of proteins (Godwin 2001).

Lead Cycle
The lead cycle is the biogeochemical cycle of lead through the atmosphere, lithosphere, biosphere, and hydrosphere, which has been profoundly influenced by anthropogenic activities.

Natural lead sources

Lead is one of the most abundant heavy trace elements and is formed by the radioactive decay of uranium and thorium. In crustal rocks, it is most often present as the lead sulfide mineral galena which is used for economical ore source for lead extraction. Natural sources of lead to the atmosphere include wind borne dust, volcanic outgassing, and forest fires. Chemical and physical rock weathering mobilize lead in soils. Mobilized lead can react to form oxides or carbonates, and lead can also co-precipitate with other minerals by being occluded through surface adsorption and complexation. Generally, lead is relatively less mobile compared to other elements in the environment. Lead emissions from natural sources tend to be larger in size, and hence their transport is limited.

Anthropogenic lead cycle

Since antiquity, lead has been used in a wide range of commercial and industrial applications due to its malleable, ductile, dense, and corrosion resistant properties, including construction, coinage, glass manufacture, water pipes, cosmetics, and mining. Today, lead is used in paints, water pipes, lead solder, food storage cans, ammunition, radiation shielding equipment, alloys, batteries, and leaded gasoline. Lead use in most of these cases has been regulated due to lead's high toxicity.

Anthropogenic activities have greatly accelerated lead mobilization to the environment. The majority of anthropogenic lead comes from non-ferrous metal manufacturing plants, mining and smelting of ores, stationary and mobile fossil fuel combustion platforms, and lead batteries. These activities produce very fine micron-sized particles that can be transported as aerosols. Anthropogenic lead fluxes decreased from the 1980s to the 2000s as a result of global regulation and outlawing of leaded gasoline. However, the decreasing trend has not continued to the present day. The global production in lead has seen a steady rise in the 21st century [need a reference].

Lead accumulation in the ocean

Wet deposition removes lead from the atmosphere to the surface ocean. Precipitation leads to solubilization of aerosols and washout of particulates. Pb concentrations in the oceans is dependent on wet deposition and the concentration of Pb present in atmosphere. Periodic measurements of Pb concentrations in the Atlantic ocean from 1980 to 2011 revealed a decreasing trend in Pb concentration with around 160 pM in 1980 to 20 pM in 2011. Oceanic depth profiles shows decreasing Pb concentration with increasing depth. The main sink for lead is burial in marine sediments.