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Imbalances between metal supply and demand, real or anticipated, have inspired the concept of metal criticality. We here characterize the criticality of 62 metals and metalloids in a 3D "criticality space" consisting of supply risk, environmental implications, and vulnerability to supply restriction. Contributing factors that lead to extreme values include high geopolitical concentration of primary production, lack of available suitable substitutes, and political instability. The results show that the limitations for many metals important in emerging electronics (e.g., gallium and selenium) are largely those related to supply risk; those of platinum group metals, gold, and mercury, to environmental implications; and steel alloying elements (e.g., chromium and niobium) as well as elements used in high-temperature alloys (e.g., tungsten and molybdenum), to vulnerability to supply restriction. The metals of most concern tend to be those available largely or entirely as byproducts, used in small quantities for highly specialized applications, and possessing no effective substitutes.

International audience; An 18 million year record of the Ca isotopic composition (d44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. d44/42Ca in this record averages +0.37 ± 0.05 (1r SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their d44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in d44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (d44/42Caw) and for isotope fractionation associated with the production of carbonate sediments (Dsed) results in unrealistically large variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in d44/42Caw of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in Dsed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in d44/42Caw and Dsed have been established, modeling the Ca2+ content of seawater from Ca isotope curves should be approached cautiously.

The ecological success of marine diatoms comes despite their unusual additional requirement for silicon, a nutrient that often limits their growth in the ocean. There may be, however, some physiological and ecological advantages to silicon limitation. A model of nitrogen and silicon metabolism in diatoms (Flynn et al. 1997; Flynn and Martin- Jezequel 2000) suggests that silicon-starved diatoms will recover more quickly than nitrogen-starved (but otherwise identical) diatoms on the resupply of nutrients. In culture, following 24 h of stationary phase induced by nutrient exhaustion, silicon-starved Thalassiosira weissflogii displayed maximum growth rates within a few hours of the readdition of silicon. But 37 h elapsed before nitrate-starved cultures began dividing again and nitrate-starved cultures did not reattain maximum growth rates over the period investigated. The model suggests that, in terms of number, cells descended from the nitrate starved will never catch up to cells descended from the silicon starved if both populations are simultaneously supplied with a pulse of nutrients. The longer the lag between nutrient supply and the resumption of growth in the nitrate-starved cells, the greater the proportion of the supplied nutrients that ends up in the descendants of the silicon starved. In the model run, the silicon-starved cells acquired 75% of the added nutrients and the nitrate starved acquired only 25%. Silicon limitation may thus be a mechanism by which diatoms ensure they respond more quickly than other phytoplankton to the upwelling of nutrients into the euphotic zone. Diatoms are astoundingly successful phytoplankton de- spite somewhat uniquely requiring silicon for growth in an ocean largely depleted of silicic acid. Silicon limitation of diatom growth in the ocean has been observed in locations ranging from high-nutrient, upwelling zones in the eastern equatorial Pacific (Dugdale et al. 1995) to the Mississippi River plume (Nelson and Dortch 1996) and the oligotrophic Sargasso Sea (Brzezinski and Nelson 1996). Despite this widespread potential for silicon limitation, diatoms manage to account for 75% of the primary production occurring in high-nutrient and coastal regions of the ocean and as much as 40% of the total annual marine primary production. Be- cause marine primary production constitutes about half of the total of global primary production, marine diatoms carry out 20% of the primary production that occurs on Earth each year.