ALBERT

Description: Large Arctic rivers are key locations for nitrogen processing, which controls the supply of this limiting nutrient to the Arctic Ocean. In a warming Arctic, longer ice-free periods increase riverine productivity and modulate nitrogen consumption and delivery to the ocean. In this study, the annual variability of nitrate concentrations at the Lena River outlet (Samoylov station) was investigated. Significantly higher nitrate concentrations in water were observed sub-ice (winter) than in the open water (summer), and the higher nitrate concentrations follow phases of colder air temperature at the Lena catchment scale (ERA5 reanalysis data). We hypothesize that colder phases result in thicker river ice leading to darker under-ice conditions preferred by nitrifying microbial communities, thereby inducing increasing sub-ice nitrification. We tested this hypothesis using silicon isotopes known to fractionate upon freezing. The high nitrate concentrations in the winter are associated with heavier silicon isotope compositions in river water. This can be explained by the supersaturation and precipitation of amorphous silica preferentially incorporating the lighter silicon isotopes, leaving the water isotopically heavier. Supersaturation of amorphous silica can result from thicker ice formation upon colder air temperature at catchment scale. The silicon isotope data support phases of thicker ice formation, and indirectly support darker sub-ice conditions at the river base creating pulses of increasing nitrification. Our hypothesis is also supported by a change in the value of an index for dissolved organic carbon aromaticity (SUVA) during the colder phases: this suggests that conditions favour the decomposition of dissolved organic matter during periods of thicker river ice. Air temperature, nitrate concentration, silicon isotopes and SUVA are supporting evidence for pulses of sub-ice microbial activity in the river during winter. It follows that decreasing ice cover duration throughout the catchment is likely to decrease winter nitrate fluxes from the Lena River to the Arctic Ocean.

Description: Yellowstone Plateau Volcanic Field, USA: Ge concentrations measured by inductively coupled plasma mass spectrometry (ICP-MS), Si concentrations measured by inductively coupled plasma optical emission spectrometry (ICP-OES), Ge/Si ratio, Si isotope compositions (d30Si and standard deviation SD) measured by Multicollector ICP-MS, concentrations in Ca, Na, Mg, K measured by ICP-OES, and concentrations in SO4 and Cl measured by ion chromatography in thermal waters, major rivers draining the Yellowstone Plateau Volcanic Field, and creeks flowing into Yellowstone Lake.

Description: The sedimentary data on two Yellowstone Lake sediment cores from 2016 - YL16-2C and YL16-5A, which include total organic carbon (TOC), total inorganic carbon (TIC), biogenic silica (BSi) concentrations, total nitrogen (TN) concentrations and carbon-nitrogen ratios (C/N) of the bulk sediment thought the Holocene. Furhter, stable silicon isotopes, and Ge/Si ratios of single endemitic diatom species Stephanodiscus yellowstonesis are included. Additionally, there are complete elemental composition data aquired using X-ray fluorescence (XRF) with an ITRAX and core logger data, including magnetic susceptibility measured ad LacCore Facility, Minneapolis, USA. Data on Yellowstone Lake water, Yellowstone Lake triburates and hydrothermal vents from two fields - West Thumb and Steavenson Island- include dissolved silicon, stable silicon isotopes and Ge/Si ratios sampled over period 2016-2018. All data were sampled to constrain the lake's silicon dynamics though out the Holocene and to identify sources of dissolved silicon in the lake. The sediment core analyzes of TOC, TN and C/N were measured on 5 to 10 mg of freeze-dried sediment using elemental analyzer COSTECH ECS4010 at Lund University, Sweden. The biogenic silica concentrations were determined using weak alkaline extraction by Conley and Schleske (2002, doi:10.1007/0-306-47668-1_14) using 30 mg of freeze-dried sediment reacting with 40 ml of 0.1M Na2CO3 for 5 hours. The DSi concentration in the extracted aliquot was measured using the molybdate-blue method (Strickland and Parsons, 1972) using Smarrtchem 200, AMS System discrete analyzer. Stable silicon isotopes were performed on leached cleaned diatoms (using NaOH) cation-exchange column cleaned (Georg et al., 2006; doi:10.1016/j.chemgeo.2006.06.006) samples using MC-ICP-MS at Vegacenter, Stockholm. Water samples for DSi and stable Si isotopes were sampled in HDPE acid-washed 125ml Nalgene bottles and filtered directly in the field through a 0.45μm Sterivex filter and further acidified using HCl to pH 2. DSi was analyzed by molybdate-blue method (Strickland and Parsons, 1972 (https://epic.awi.de/id/eprint/39262/1/Strickland-Parsons_1972.pdf)) using Smarrtchem 200, AMS System discrete analyzer. Stable silicon isotopes were performed on anion and cation-exchange column cleaned (Georg et al., 2006; doi:10.1016/j.chemgeo.2006.06.006, Gaspard et al. 2021; doi:10.1029/2021gc009904) samples using MC-ICP-MS at Vegacenter, Stockholm.