One of the most important

One of the most important ways that Mo can be concentrated and fractionated from Cu is through the oxidation–reduction cycle during chemical weathering on the surface of the Earth. During weathering, molybdenum is readily oxidized to form water-soluble MoO42− in surface environments (Sun et al., 2015b), after the second major elevation of atmospheric oxygen in the Earth\'s history that occurred ∼550 Myr ago (Kump, 2008; Scott et al., 2008; Tribovillard et al., 2006). Water-soluble MoO42− is transported to oceans and lakes, and then, through interaction with anoxic sediments it precipitates and is retained to form Mo sulfides/complexes (Fig. 2). As an extreme example, Mo contents could reach weight percent levels in Mo-rich black shales formed immediately after the Snowball Earth in south China (Jiang et al., 2009; Mao et al., 2002; Wille et al., 2008). In general, reduced water bodies with large catchment areas and high chemical weathering rates are favorable places for the formation of Mo enriched sediments, due to anoxic environments and Mo-rich surface water. Given that chemical weathering is far more intense in tropical regions (Ma et al., 2007), and that developing orogenic belts are usually much more unstable and thus more easily eroded, developing orogens in tropical or sub-tropical regions should be the best suppliers of MoO42− to surface waters. In addition, copious amounts of vegetation can act as reducing agents in waters. Such a scenario, however, operates on a local scale, and cannot readily explain the gigantic Cenozoic Mo deposit belt along the western coasts of the American continents.
Oceanic anoxic events Nine major OAEs are recorded in the Jurassic and the Cretaceous, including those of the early Toarcian (∼183 Ma), early Aptian (∼120 Ma), early Albian (∼111 Ma), and Cenomanian–Turonian (∼93 Ma) (Alberdi-Genolet and Tocco, 1999; Jenkyns, 2010; McElwain et al., 2005; Scopelliti et al., 2006; Suan et al., 2010; Turgeon and Brumsack, 2006; Turgeon and Creaser, 2008; Wang et al., 2001; Wilson and Norris, 2001; Zou et al., 2005). Currently available data suggest that the major forcing conditions behind OAEs were abrupt increases in global temperature (Suan et al., 2010; Wilson and Norris, 2001), induced by rapid ingrowth of CO2 and/or methane into the liothyronine sodium (Hesselbo et al., 2000; Jenkyns, 2010). Methane is a more effective greenhouse gas than CO2 (Yang et al., 2013; Yu et al., 2007) and is eventually oxidized to CO2 in the atmosphere. Both global warming and high atmospheric CO2 are favorable for chemical weathering (Jenkyns, 2010). Meanwhile, elevated temperatures and CO2 in the Phanerozoic may have led to blooming of plants and consequently rapid evolution of the whole biosphere. This in turn resulted in high oxygen levels in the atmosphere due to photosynthesis, followed by low temperatures as the greenhouse conditions eased, and then most importantly, the extreme conditions of the OAEs. The release of greenhouse gases, e.g., CO2 from volcanic eruptions and/or methane from buried biomass, starts another warm-cool-OAE cycle. Therefore, high CO2 warm and high O2 cool periods usually appeared successively before OAEs (Suan et al., 2010). For example, a pronounced (∼5 °C) cooling event was discovered in the Late Pliensbachian, which is followed by a remarkable ∼7–10 °C seawater warming, then a second cooling event, and finally the OAE in the early Toarcian (Suan et al., 2010). Highly fluctuating climate and hydrological cycles could dramatically enhance the chemical weathering, which, consequently, leads to release of Mo. During high oxygen periods before the OAEs, the released Mo was intensively oxidized to form water soluble MoO42−, which would result in enhanced MoO42− discharge to oceans and lakes. Given that Mo cannot be retained by sediments in oxic or even suboxic bottom waters (Turgeon and Brumsack, 2006), the Mo concentration in sea waters rose to much higher levels during warm high-oxygen periods than would occur under more temperate conditions. Subsequently, Mo was precipitated by interaction with organic-rich sediment during reducing conditions associated with the OAEs. Black shales and other organic-rich sediments formed during OAEs in the Jurassic and the Cretaceous are widely distributed in the Pacific (Robinson et al., 2004; Takashima et al., 2010), the Atlantic (Berrocoso et al., 2008; Lipinski et al., 2003) and the Tethys Ocean (McArthur et al., 2008; Wang et al., 2001). Molybdenum concentrations in anoxic sediments are up to several hundred times higher than those in the continental crust (Rimmer, 2004; Scott et al., 2008; Sun et al., 2015b) (Fig. 2).