B Proemse

Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction

The latest Permian extinction (LPE), ca. 252 Ma, represents the most severe extinction event in Earths history. The cause is still debated, but widespread marine anoxic to euxinic (H2S rich) conditions, from deep to shallow water environments, are commonly suggested. As a proxy for marine oxygen levels, we analyzed δ98/95Mo of two LPE sections that represent a gradient in water depth on the northwest margin of Pangea. Results from deep-water slope environments show a large shift in δ98/95Mo values from −2.02・to +2.23・at the extinction horizon, consistent with onset of euxinic conditions. In contrast, sub-storm wave base shelf environments show little change in the molybdenum isotopic composition (−1.34・to +0.05・, indicating ongoing oxic conditions across the LPE. These results indicate that areas of the continental shelf of northwest Pangea underwent mass extinction under oxic conditions throughout the LPE event, and that shallow-water anoxia was therefore not a global phenomenon.

A multi-isotope approach for estimating industrial contributions to atmospheric nitrogen deposition in the Athabasca oil sands region in Alberta, Canada

Industrial nitrogen (N) emissions in the Athabasca oil sands region (AOSR), Alberta, Canada, affect nitrate (NO3) and ammonium (NH4) deposition rates in close vicinity of industrial emitters. NO3-N and NH4-N open field and throughfall deposition rates were determined at various sites between 3 km and 113 km distance to the main oil sand operations between May 2008 and May 2009. NO3 and NH4 were analyzed for δ(15)N-NO3, δ(18)O-NO3, Δ(17)O-NO3 and δ(15)N-NH4. Marked differences in the δ(18)O and Δ(17)O values between industrial emissions and background deposition allowed for the estimation of minimum industrial contributions to atmospheric NO3 deposition. δ(15)N-NH4 values also allowed for estimates of industrial contributions to atmospheric NH4 deposition. Results revealed that particularly sites within ~30 km radius from the main oil sands developments are significantly affected by industrial contributions to atmospheric NO3 and NH4 deposition.

Deep groundwater circulation through the High Arctic cryosphere forms Mars-like gullies

We report here the discovery of the northernmost known perennial spring, located in the polar desert of the Canadian High Arctic (average precipitation 75.5 mm/yr; average annual air temperature –19.7 °C). The high-discharge spring (∼520 L/s) has also anomalously high temperatures (9.0 °C), despite occurring in a region of low geothermal gradient and thick (>400 m) permafrost. Active erosion at the spring outlet forms gullies with alcove-channel-apron morphology, remarkably similar to archetypal gullies observed on mid-latitude regions of Mars. Geochemical and isotopic data show a meteoric origin for the waters, demonstrating that deep circulating groundwater systems can form active connections through the cryosphere to the subsurface, even in the absence of thermal anomalies. This discovery challenges current understanding of high-latitude permafrost hydrology.

Stromatolites on the rise in peat-bound karstic wetlands

Stromatolites are the oldest evidence for life on Earth, but modern living examples are rare and predominantly occur in shallow marine or (hyper-) saline lacustrine environments, subject to exotic physico-chemical conditions. Here we report the discovery of living freshwater stromatolites in cool-temperate karstic wetlands in the Giblin River catchment of the UNESCO-listed Tasmanian Wilderness World Heritage Area, Australia. These stromatolites colonize the slopes of karstic spring mounds which create mildly alkaline (pH of 7.0-7.9) enclaves within an otherwise uniformly acidic organosol terrain. The freshwater emerging from the springs is Ca-HCO3 dominated and water temperatures show no evidence of geothermal heating. Using 16 S rRNA gene clone library analysis we revealed that the bacterial community is dominated by Cyanobacteria, Alphaproteobacteria and an unusually high proportion of Chloroflexi, followed by Armatimonadetes and Planctomycetes, and is therefore unique compared to other living examples. Macroinvertebrates are sparse and snails in particular are disadvantaged by the development of debilitating accumulations of carbonate on their shells, corroborating evidence that stromatolites flourish under conditions where predation by metazoans is suppressed. Our findings constitute a novel habitat for stromatolites because cool-temperate freshwater wetlands are not a conventional stromatolite niche, suggesting that stromatolites may be more common than previously thought.

Iron cycling in the anoxic cryo-ecosystem of Antarctic Lake Vida

Iron redox cycling in metal-rich, hypersaline, anoxic brines plays a central role in the biogeochemical evolution of life on Earth, and similar brines with the potential to harbor life are thought to exist elsewhere in the solar system. To investigate iron biogeochemical cycling in a terrestrial analog we determined the iron redox chemistry and isotopic signatures in the cryoencapsulated liquid brines found in frozen Lake Vida, East Antarctica. We used both in situ voltammetry and the spectrophotometric ferrozine method to determine iron speciation in Lake Vida brine (LVBr). Our results show that iron speciation in the anoxic LVBr was, unexpectedly, not free Fe(II). Iron isotope analysis revealed highly depleted values of −2.5‰ for the ferric iron of LVBr that are similar to iron isotopic signatures of Fe(II) produced by dissimilatory iron reduction. The presence of Fe(III) in LVBr therefore indicates dynamic iron redox cycling beyond iron reduction. Furthermore, extremely low δ18O–SO42− values (−9.7‰) support microbial iron-sulfur cycling reactions. In combination with evidence for chemodenitrification resulting in iron oxidation, we conclude that coupled abiotic and biotic redox reactions are driving the iron cycle in Lake Vida brine. Our findings challenge the current state of knowledge of anoxic brine chemistry and may serve as an analogue for icy brines found in the outer reaches of the solar system.

Historical black carbon deposition in the Canadian High Arctic: a 190-year long ice-core record from Devon Island

Black carbon aerosol (BC), which is emitted from natural and anthropogenic sources (e.g., wildfires, coal burning), can contribute to magnify climate warming at high latitudes by darkening snowand ice-covered surfaces, and subsequently lowering their albedo. Therefore, modeling the atmospheric transport and deposition of BC to the Arctic is important, and historical archives of BC accumulation in polar ice can help to validate such modeling efforts. Here we present a > 250-year ice-core record of refractory BC (rBC) deposition on Devon ice cap, Canada, spanning the years from 1735 to 1992. This is the first such record ever developed from the Canadian Arctic. The estimated mean deposition flux of rBC on Devon ice cap for 1963–1990 is 0.2 mg m−2 a−1, which is at the low end of estimates from Greenland ice cores obtained using the same analytical method (∼ 0.1–4 mg m−2 a−1). The Devon ice cap rBC record also differs from the Greenland records in that it shows only a modest increase in rBC deposition during the 20th century. In the Greenland records a pronounced rise in rBC is observed from the 1880s to the 1910s, which is largely attributed to midlatitude coal burning emissions. The deposition of contaminants such as sulfate and lead increased on Devon ice cap in the 20th century but no concomitant rise in rBC is recorded in the ice. Part of the difference with Greenland could be due to local factors such as melt– freeze cycles on Devon ice cap that may limit the detection sensitivity of rBC analyses in melt-impacted core samples, and wind scouring of winter snow at the coring site. Air back-trajectory analyses also suggest that Devon ice cap receives BC from more distant North American and Eurasian sources than Greenland, and aerosol mixing and removal during long-range transport over the Arctic Ocean likely masks some of the specific BC source–receptor relationships. Findings from this study suggest that there could be a large variability in BC aerosol deposition across the Arctic region arising from different transport patterns. This variability needs to be accounted for when estimating the large-scale albedo lowering effect of BC deposition on Arctic snow/ice.