Bacterially produced calcium phosphate nanobiominerals: sorption capacity, site preferences, and stability of captured radionuclides.

A Serratia sp. bacterium manufactures amorphous calcium phosphate nanominerals (BHAP); this material has shown increased sorption capacity for divalent radionuclide capture. When heat-treated (≥450 °C) the cell biomass is removed and the biominerals are transformed to hydroxyapatite (HAP). Using a multimethod approach, we have elucidated both the site preferences and stability of analogue radionuclide incorporation for Sr, Co, Eu, and U. Strontium incorporates within the bulk amorphous inorganic phase of BHAP; however, once temperature modified to crystalline HAP, bonding was consistent with Sr substitution at the Ca(1) and/or Ca(2) sites. Cobalt incorporation occurs within the bulk inorganic amorphous phase of BHAP and within the amorphous grain boundaries of HAP. Europium (an analogue for trivalent actinides) substituted at the Ca(2) and/or the Ca(3) position of tricalcium phosphate, a known component of HAP grain boundaries. Uranium was surface complexed with no secondary minerals detected. With multiple sites for targeted radionuclide incorporation, high loadings, and good stability against remobilization, BHAP is shown to be a potential material for the remediation of aqueous radionuclide in groundwater.

Global climate-driven trade-offs between the water retention and cooling benefits of urban greening.

Urban greening can potentially help mitigate heat-related mortality and flooding facing the >4 billion urban population worldwide. However, the geographical variation of the relative combined hydrological and thermal performance benefits of such interventions are unknown. Here we quantify globally, using a hydrological model, how climate-driven trade-offs exist between hydrological retention and cooling potential of urban greening such as green roofs and parks. Using a Budyko framework, we show that water retention generally increases with aridity in water-limited environments, while cooling potential favors energy-limited climates. Our models suggest that common urban greening strategies cannot yield high performance simultaneously for addressing both urban heat-island and urban flooding problems in most cities globally. Irrigation, if sustainable, may enhance cooling while maintaining retention performance in more arid locations. Increased precipitation variability with climate change may reduce performance of thinner green-infrastructure more quickly compared to greened areas with thicker soils and root systems. Our results provide a conceptual framework and first-order quantitative guide for urban development, renewal and policymaking.

Modelling the role of groundwater hydro-refugia in East African hominin evolution and dispersal.

Water is a fundamental resource, yet its spatiotemporal availability in East Africa is poorly understood. This is the area where most hominin first occurrences are located, and consequently the potential role of water in hominin evolution and dispersal remains unresolved. Here, we show that hundreds of springs currently distributed across East Africa could function as persistent groundwater hydro-refugia through orbital-scale climate cycles. Groundwater buffers climate variability according to spatially variable groundwater response times determined by geology and topography. Using an agent-based model, grounded on the present day landscape, we show that groundwater availability would have been critical to supporting isolated networks of hydro-refugia during dry periods when potable surface water was scarce. This may have facilitated unexpected variations in isolation and dispersal of hominin populations in the past. Our results therefore provide a new environmental framework in which to understand how patterns of taxonomic diversity in hominins may have developed.

Evaporative cooling of speleothem drip water.

This study describes the first use of concurrent high-precision temperature and drip rate monitoring to explore what controls the temperature of speleothem forming drip water. Two contrasting sites, one with fast transient and one with slow constant dripping, in a temperate semi-arid location (Wellington, NSW, Australia), exhibit drip water temperatures which deviate significantly from the cave air temperature. We confirm the hypothesis that evaporative cooling is the dominant, but so far unattributed, control causing significant disequilibrium between drip water and host rock/air temperatures. The amount of cooling is dependent on the drip rate, relative humidity and ventilation. Our results have implications for the interpretation of temperature-sensitive, speleothem climate proxies such as δ(18)O, cave microecology and the use of heat as a tracer in karst. Understanding the processes controlling the temperature of speleothem-forming cave drip waters is vital for assessing the reliability of such deposits as archives of climate change.