Li and U Isotopes as a Potential Tool for Monitoring Active Layer Deepening in Permafrost Dominated Catchments

Permafrost in the Arctic is decreasing in extent and the depth of the seasonally thawed layer, the active layer, is increasing. Increased exposure to water is increasing fluxes of organic and inorganic solutes with potential impacts for the global carbon cycle and downstream ecosystems. Understanding the relationship between solute release and active layer depth will be critical for modeling environmental impact, especially in inaccessible regions where there is a lack of data. In this study, we focus on the potential for the isotopes of lithium (Li) and uranium (U) to track active layer extent in two permafrost-dominated catchments in Svalbard: one glaciated and one unglaciated. These isotope systems can be measured to a much higher precision than concentration measurements and act as sensitive tracers of environmental change. The extent of Li isotope fractionation provides information on the balance between dissolution of primary phases and formation of secondary phases, such as clay minerals and oxides. The U activity ratio provides information on water-rock interaction times and physical properties. We observe contrasting behavior between the two catchments. The highest U activity ratios and Li isotope values (those most distinct from bedrock) are observed in summer in the unglaciated catchment, when the active layer depth is expected to be at its maximum extent, whereas negligible seasonal variation and the lowest values are observed in the glaciated catchment. We therefore propose that the extent of solute acquisition is directly linked to the active layer depth, which is restricted in the glaciated catchment due to a layer of “dead ice” underneath the glacial outwash plain, and could therefore provide a valuable tool to assess changes in active layer depth at catchment scales.This project was funded by a Swiss National Science Foundation fellowship for prospective researchers (PBEZP2-137335), a Marie Curie Intra-European Fellowship (PIEF-GA-2012-331501), and NERC Standard Grant NE/M001865/1. Fieldwork was supported by an Arctic Field Grant (219165/E10, The Research Council of Norway)

Ice sheets as a missing source of silica to the polar oceans

Ice sheets play a more important role in the global silicon cycle than previously appreciated. Input of dissolved and amorphous particulate silica into natural waters stimulates the growth of diatoms. Here we measure dissolved and amorphous silica in Greenland Ice Sheet meltwaters and icebergs, demonstrating the potential for high ice sheet export. Our dissolved and amorphous silica flux is 0.20 (0.06-0.79) Tmol year(-1), ∼50% of the input from Arctic rivers. Amorphous silica comprises >95% of this flux and is highly soluble in sea water, as indicated by a significant increase in dissolved silica across a fjord salinity gradient. Retreating palaeo ice sheets were therefore likely responsible for high dissolved and amorphous silica fluxes into the ocean during the last deglaciation, reaching values of ∼5.5 Tmol year(-1), similar to the estimated export from palaeo rivers. These elevated silica fluxes may explain high diatom productivity observed during the last glacial-interglacial period

Decoupling of dissolved and bedrock neodymium isotopes during sedimentary cycling

The radiogenic neodymium isotope ratio 143Nd/144Nd (expressed as εNd) has been applied to examine seawater elemental budgets, sedimentary provenance, oceanic water mass source and circulation, large-scale geochemical cycling, and continental crust growth rates. These applications are underpinned by the assumption that during sedimentary processing the parent/daughter (samarium/neodymium) ratio is conservative during low temperature fluid related processes. In this study, we report εNd data from two streams draining sedimentary formations in the Arctic archipelago of Svalbard. The εNd value of the dissolved load is offset from stream suspended sediment samples by up to 5.5 epsilon units. We demonstrate that dissolved load εNd is controlled by the dissolution of labile phases present in the catchment rocks which are isotopically distinct from the silicate residue and account for up to 12% Nd in the bulk sediment. This study highlights 1) the potential for incongruent release of Nd isotopes to seawater from rocks and sediments, with implications for the isotopic composition of seawater and 2) the large-scale decoupling between a rapidly exchanging labile reservoir and a silicate-bound reservoir during sediment recycling.This project was funded by a Swiss National Science Foundation fellowship for prospective researchers (PBEZP2-137335), a Marie Curie Intra-European Fellowship (PIEF-GA-2012-331501), and NERC Standard Grant NE/M001865/1. Fieldwork was supported by an Arctic Field Grant (219165/E10, The Research Council of Norway)

Dust outpaces bedrock in nutrient supply to montane forest ecosystems.

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

Dust outpaces bedrock in nutrient supply to montane forest ecosystems.

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

Paleoclimates: what do we learn from deep ice cores?

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