Dust provides ecosystem-sustaining nutrients to landscapes underlain by intensively weathered soils. Here we show that dust may also be crucial in montane forest ecosystems, dominating nutrient budgets despite continuous replacement of depleted soils with fresh bedrock via erosion. Strontium and neodymium isotopes in modern dust show that Asian sources contribute 18–45% of dust deposition across our Sierra Nevada, California study sites. The remaining dust originates regionally from the nearby Central Valley. Measured dust fluxes are greater than or equal to modern erosional outputs from hillslopes to channels, and account for 10–20% of estimated millennial-average inputs of bedrock P. Our results demonstrate that exogenic dust can drive the evolution of nutrient budgets in montane ecosystems, with implications for predicting forest response to changes in climate and land use.

No abstract

Isotopic characterization of aerosol mineral particles (atmospheric dust) of varying sizes is essential in classifying source areas, and for determining the source of dust deposited over oceans and icesheets. However, the effect of atmospheric transport on radiogenic isotopic compositions is not well constrained, making provenance interpretation difficult. In order to investigate the isotopic variability of 176 Hf/ 177 Hf, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd we analyzed eight airborne dust samples in two size fractions collected along a cross-Atlantic transect through a dust storm originating in the Sahara in late 2002. Past measurements of 176 Hf/ 177 Hf, 87 Sr/ 86 Sr and 143 Nd/ 144 Nd of dust have focused primarily on coarse sized particles (<30 µm), whereas far field deposition is primarily finer particles (<2 µm). Strontium or neodymium isotopic sorting based on distance is not evident in our dataset; however, the combined isotopic ratios of the dust collected suggests a Saharan origin. Hafnium isotopic compositions show an east to west trend towards more radiogenic compositions across the Atlantic, suggesting grain and mineral sorting during dust transport along the ∼4000 km transect. Transport models with variable dust particle diameter and wind speed demonstrate that the Hf isotopic compositions can be explained by the loss of the high-density mineral zircon during transport of dust from the source area. Modeling of this "zircon effect" in the Hf isotopic composition of marine, terrestrial and glacial dust deposits can reveal additional information concerning dust transport and sources in the geologic past.

No abstract

The Upper Fremont Glacier (UFG), Wyoming, is one of the few continental glaciers in the contiguous United States known to preserve environmental and climate records spanning recent centuries. A pair of ice cores taken from UFG have been studied extensively to document changes in climate and industrial pollution (most notably, mid-19th century increases in mercury pollution). Fundamental to these studies is the chronology used to map ice-core depth to age. Here, we present a revised chronology for the UFG ice cores based on new measurements and using a novel dating approach of synchronizing continuous water isotope measurements to a nearby tree-ring chronology. While consistent with the few unambiguous age controls underpinning the previous UFG chronologies, the new interpretation suggests a very different time scale for the UFG cores with changes of up to 80 years. Mercury increases previously associated with the mid-19th century Gold Rush now coincide with early-20th century industrial emissions, aligning the UFG record with other North American mercury records from ice and lake sediment cores. Additionally, new UFG records of industrial pollutants parallel changes documented in ice cores from southern Greenland, further validating the new UFG chronologies while documenting the extent of late 19th and early 20th century pollution in remote North America.

Recent declines in ice shelf and sea ice extent experienced in polar regions highlight the importance of evaluating variations in local weather patterns in response to climate change. Airborne mineral particles (dust) transported through the atmosphere and deposited on ice sheets and glaciers in Antarctica and Greenland can provide a robust set of tools for resolving the evolution of climatic

No abstract

Magmatic differentiation produces positive correlations between  49 Ti and SiO2. The equilibrium Ti isotope fractionation factors of Ti-bearing minerals are essential for understanding the mechanisms driving this isotopic fractionation. We present ab initio derived mean force constants of Ti-bearing minerals (barium orthotitanate, potassium titanium oxide, fresnoite, diopside, geikielite, karooite, titanite, pseudobrookite, anatase, and titanium oxide) based on density functional theory (DFT) to calculate equilibrium isotopic fractionation factors. We find that the main driver for Ti isotopic fractionation is its coordination, with 4-, 5-, and 6-fold coordinated Ti characterized by mean force constants of 547, 462, and 310 N/m respectively. The coordination number of Ti in silicate melts is thought to be lower than in minerals, driving magmas towards higher  49 Ti values by fractional crystallization. The mineral-melt fractionation factors allow modeling of the observed Ti isotope trends in tholeiitic and calc-alkaline rocks. Our model results indicate that to first order, the steeper  49 Ti trend observed in tholeiitic vs. calc-alkaline magmas is most likely due to enhanced removal of Ti into sequestered minerals at low SiO2 concentration in tholeiitic series compared to calc-alkaline series. The  49 Ti-SiO2 differentiation trends however depend on Ti coordination in the melt and the strengths of Ti bonds in diverse Fe,Ti-oxides, which are still uncertain. Our resultsshow that Ti isotopes can be used to reconstruct the crystallization history and identify the magmatic series parentage of magmas that otherwise lack context, but further work is needed to identify the drivers behind Ti isotopic fractionation in igneous rocks.