Iron isotope transformations in the meromictic Lake Cadagno

Abstract

The iron isotope composition of sedimentary deposits is a key tool for tracking changes in the biogeochemistry and redox conditions of modern and geologically ancient aquatic systems. The use of iron isotopes to reconstruct oxic, anoxic and redox conditions is based on iron isotope fractionation associated with the iron(II)-iron(III) redox reaction and the formation of various iron oxy, hydroxy and sulfur species. However, the degree of iron isotope fractionation varies within the sedimentary record, and the processes leading to isotope fractionation within aquatic systems also vary. Here we investigate iron isotope fractionation within the water column of Lake Cadagno, Switzerland. Lake Cadagno is a 21 m deep alpine meromictic lake that is permanently stratified. It has two chemically distinct water layers: an oxic mixolimnion separated by a narrow chemocline from an anoxic monimolimnion. The chemocline is located across a narrow band between 10 and 13.5 m and is defined by steep chemical gradients in dissolved oxygen, redox potential, nutrients (nitrite + nitrate, ammonia), dissolved and particulate trace metals (iron and manganese) and sulfur species. Iron isotope determination ( 56 Fe/ 54 Fe ratio; expressed as δ 56 Fe) of both dissolved (δ 56 Fedissolved) and particulate (δ 56 Feparticulate) iron reveals a sharp transition within the chemocline with a heavy δ 56 Fedissolved value (+0.75 ‰) at 11.5 m and light δ 56 Fedissolved value (-0.61 ‰) below at 12 m.The large shift in δ 56 Fedissolved occurs where anoxygenic phototrophic bacteria become abundant.Modelling of the dissolved iron isotope fractionation within the chemocline produced fractionation factors of -0.60 ‰ (kinetic model) and -1.52 ‰ (equilibrium model) assuming a two-step process involving iron oxidation followed by precipitation. Changes in the isotope composition of particulate iron with depth are subtler to that of dissolved iron, with slightly lighter values (-0.18 ‰) above the chemocline and slightly heavier values (+0.13 ‰) below. The production of reduced iron(II) within and below the chemocline is likely coupled to dissimilatory iron(III) reduction and dissolution reactions associated with sinking iron(III) oxy and hydroxy species. These processes lead to a peak in dissolved iron concentrations at 12.5 m. The decline in dissolved iron concentration below this depth and its change in isotope composition is consistent with the formation of iron sulphide species, e.g. mackinawite, under mildly euxinic conditions. Overall our results indicate that variations in δ 56 Fedissolved likely involve a combination of biotic and abiotic processes with biological mixing enhancing the rate at which iron(II) is transferred across the chemocline. The observed isotope transformation of dissolved iron across the chemocline of Lake Cadagno may make it a reasonable analogue to past shallow-water systems with mild euxinia.

Bacterial, phytoplankton, and viral dynamics of meromictic Lake Cadagno offer insights into the Proterozoic ocean microbial loop

“…This study was conducted in Lake Cadagno (21 m deep), a high alpine meromictic lake situated at 1921 m above sea level in the Southern Alps of Switzerland (54). Water sampling and characterization of physical (turbidity, temperature, oxygen, conductivity), biological (Chl a, phycocyanin) and chemical (H 2 S, NH 4 + ) parameters of the water column were performed as previously described (11). Water was sampled on the 28 (Day 1) and 29 (Day 2)…”

“…Purple and green sulfur bacteria (PSB and GSB, respectively) are amongst the hallmark bloom-forming microbes that are frequently observed in the chemocline of Lake Cadagno (9, 10). Within the chemocline, carbon, nitrogen, sulfur, and iron cycling are integrated through metabolic activities of the anoxygenic photosynthetic PSB and GSB (11, 12, 13, 14). Two PSB, Thiodictyon syntrophicum and Chromatium okenii , have been identified as the greatest contributors to primary production in Lake Cadagno (10, 15, 14).…”

Microbial loop of a Proterozoic ocean analogue

“…This study was conducted in Lake Cadagno (21 m deep), a high alpine meromictic lake situated at 1921 m above sea level in the Southern Alps of Switzerland 57 . Water sampling and characterization of physical (turbidity, temperature, oxygen, conductivity), biological (Chl a , phycocyanin) and chemical (H 2 S, NH 4 + ) parameters of the water column were performed as previously described 16 . Water was sampled on the 28 (Day 1) and 29 (Day 2) August 2017 using a double diaphragm teflon pump (Almatech PSG Germany Gmbh) connected to acid-washed low-density polyethylene (LDPE) line deployed from a platform that was anchored above the deepest part of the Lake Cadagno (46.55087° N, 8.71152° E).…”

“…This study was conducted in Lake Cadagno ( ) parameters of the water column were performed as previously described 16 . Water was sampled on the 28 (Day indicator of phytoplankton (including eukaryotic autotrophs and cyanobacteria) biomass 57 , and phycocyanin (µg/L), a pigment characteristic of cyanobacteria 58 .…”

“…Purple and green sulfur bacteria (PSB and GSB, respectively) are amongst the hallmark bloom-forming microbes that are frequently observed in the chemocline of Lake Cadagno 11,12 . Within the chemocline, carbon, nitrogen, sulfur, and iron cycling are integrated through metabolic activities of the anoxygenic photosynthetic purple sulfur and green sulfur bacteria [13][14][15][16] . Two purple sulfur bacteria, Thiodictyon syntrophicum and Chromatium okenii, have been identified as the greatest contributors to primary production in Lake Cadagno 12,14,17 .…”