A nacre protein forms mesoscale hydrogels that "hijack" the biomineralization process within a seawater environment

This journal is © 2016 The Royal Society of Chemistry.We examined the mineralization performance of a nacre protein, AP7, within seawater mineralization assays that form aragonite and magnesium calcite. Under these conditions AP7 forms hydrogel particles that vary in size and complexity depending upon ionic conditions. These hydrogels "hijack" the mineralization process by limiting nucleation in bulk solution and promoting nucleation within the hydrogels

The long-term impact of magnesium in seawater on foraminiferal mineralogy: Mechanism and consequences

Foraminifera are unicellular protists, primarily known for their calcium carbonate shells that provide an extensive fossil record. This record, ranging from Cambrian to present shows both major shifts and gradual changes in the relative occurrence of taxa producing different polymorphs of carbonate. Here we present evidence for coupling between shifts in calcite- versus aragonite-producing species and periods with, respectively, low and high seawater Mg/Ca throughout the Phanerozoic. During periods when seawater Mg/Ca is <2 mol/mol, low-Mg calcite-producing species dominate the foraminiferal community. Vice versa, high-Mg calcite- and aragonite-producing species are more abundant during periods with relatively high seawater Mg/Ca. This alteration in dominance of the phase precipitated is due to selective recovery of groups producing the favorable polymorph after shifts from calcite to aragonite seas. In addition, relatively high extinction rates of species producing the mineral phase not favored by the seawater Mg/Ca of that time may be responsible for this alteration. These results imply that the current high seawater Mg/Ca will, in the long term, favor prevalence of high-Mg and aragonite-producing foraminifera over calcite-producing taxa, possibly shifting the balance toward a community in which calcite production is less dominant

Large variations of C-13 values in stalagmites from southeastern China during historical times: implications for anthropogenic deforestation

Variations in speleothem C-13 values can reflect changes in overlying surface vegetation, which, over historical time scales, may represent the influence of human activities. Here, we examined C-13 variations in two stalagmites growing for the last 2200 years in Shennong Cave, Jiangxi Province, SE China. The two C-13 records corroborate well one another and show a prominent 6 parts per thousand enrichment of the C-13 values from AD 700 to 1100. The isotopic equilibrium for modern calcite and negative correlation between O-18 and C-13 values along the growth axis suggest that the influences of kinetic fractionation are negligible. Varied correlations between Mg/Ca and Sr/Ca ratios and divergent changes between C-13 values and Mg/Ca and Sr/Ca ratios from AD 700 to 1100 reveal that the prior calcite precipitation (PCP) and water-rock interaction did not dominate the increase of C-13 values. It is plausible that the obvious C-13 variation was largely influenced by the changes in vegetation cover overlying the cave. Our C-13 results, together with the records of climate and human activity from historical documentary records, suggest that: (i) prior to AD 700, small fluctuations in relatively light C-13 values reflect the presence of lush forest coverage above the cave, which was minimally disturbed by human activities; (ii) during AD 700-1100, the drastic increase in C-13 values indicates persistent and massive deforestation associated with large-scale immigration into northern Jiangxi after the Rebellion of An &amp; Shi (AD 755-763) in the Tang Dynasty and the subsequent development of agriculture and economic activity; and (iii) since AD 1100, fluctuations in relatively high C-13 values suggest that local vegetation during the last millennium has been sparse. Since the Rebellion of An &amp; Shi, southeastern China was progressively developed, coincident with deforestation and vegetation deterioration caused by human disturbance in the form of deforestation and cultivation.</p