1 2 It has long been suggested that hydrothermal systems might have provided habitats for the origin 3 and evolution of early life on Earth, and possibly other planets such as Mars. In this contribution 4 we show that most impact events that result in the formation of complex impact craters (i.e., >2-5 4 and >5-10 km diameter on Earth and Mars, respectively) are potentially capable of generating 6 a hydrothermal system. Consideration of the impact cratering record on Earth suggests that the 7 presence of an impact crater lake is critical for determining the longevity and size of the 8 hydrothermal system. We show that there are six main locations within and around impact 9 craters on Earth where impact-generated hydrothermal deposits can form: 1) crater-fill impact 10 melt rocks and melt-bearing breccias; 2) interior of central uplifts; 3) outer margin of central 11 uplifts; 4) impact ejecta deposits; 5) crater rim region; and 6) post-impact crater lake sediments. 12We suggest that these six locations are applicable to Mars as well. Evidence for impact-13 generated hydrothermal alteration ranges from discrete vugs and veins to pervasive alteration 14 depending on the setting and nature of the system. A variety of hydrothermal minerals have been 15 documented in terrestrial impact structures and these can be grouped into three broad categories: 16(1) hydrothermally-altered target-rock assemblages; (2) primary hydrothermal minerals 17 precipitated from solutions; and (3) secondary assemblages formed by the alteration of primary 18 hydrothermal minerals. Target lithology and the origin of the hydrothermal fluids strongly 19 influences the hydrothermal mineral assemblages formed in these post-impact hydrothermal 20systems. There is a growing body of evidence for impact-generated hydrothermal activity on 21 Mars; although further detailed studies using high-resolution imagery and multispectral 22 information are required. Such studies have only been done in detail for a handful of Martian 23 4 craters. The best example so far is from Toro Crater (Marzo et al., 2010). We also present new 1 evidence for impact-generated hydrothermal deposits within an unnamed ~32-km diameter crater 2 ~ 350 km away from Toro and within the larger Holden Crater. Synthesizing observations of 3 impact craters on Earth and Mars, we suggest that if there was life on Mars early in its history, 4 then hydrothermal deposits associated with impact craters may provide the best, and most 5 numerous, opportunities for finding preserved evidence for life on Mars. Moreover, 6hydrothermally altered and precipitated rocks can provide nutrients and habitats for life long 7 after hydrothermal activity has ceased. 8 5 1

Blooms of Zygnematophycean "glacier algae" lower the bare ice albedo of the Greenland Ice Sheet (GrIS), amplifying summer energy absorption at the ice surface and enhancing meltwater runoff from the largest cryospheric contributor to contemporary sea-level rise. Here, we provide a step change in current understanding of algal-driven ice sheet darkening through quantification of the photophysiological mechanisms that allow glacier algae to thrive on and darken the bare ice surface. Significant secondary phenolic pigmentation (11 times the cellular content of chlorophyll a) enables glacier algae to tolerate extreme irradiance (up to ∼4,000 μmol photons·m −2 ·s −1 ) while simultaneously repurposing captured ultraviolet and short-wave radiation for melt generation. Total cellular energy absorption is increased 50-fold by phenolic pigmentation, while glacier algal chloroplasts positioned beneath shading pigments remain low-light-adapted (E k ∼46 μmol photons·m −2 ·s −1 ) and dependent upon typical nonphotochemical quenching mechanisms for photoregulation. On the GrIS, glacier algae direct only ∼1 to 2.4% of incident energy to photochemistry versus 48 to 65% to ice surface melting, contributing an additional ∼1.86 cm water equivalent surface melt per day in patches of high algal abundance (∼10 4 cells·mL −1 ). At the regional scale, surface darkening is driven by the direct and indirect impacts of glacier algae on ice albedo, with a significant negative relationship between broadband albedo (Moderate Resolution Imaging Spectroradiometer [MODIS]) and glacier algal biomass (R 2 = 0.75, n = 149), indicating that up to 75% of the variability in albedo across the southwestern GrIS may be attributable to the presence of glacier algae.Greenland Ice Sheet | glacier algae | photophysiology | melt | cryosphere

Abstract. Melting of the Greenland Ice Sheet (GrIS) is the largest single contributor to eustatic sea level and is amplified by the growth of pigmented algae on the ice surface, which increases solar radiation absorption. This biological albedo-reducing effect and its impact upon sea level rise has not previously been quantified. Here, we combine field spectroscopy with a radiative-transfer model, supervised classification of unmanned aerial vehicle (UAV) and satellite remote-sensing data, and runoff modelling to calculate biologically driven ice surface ablation. We demonstrate that algal growth led to an additional 4.4–6.0 Gt of runoff from bare ice in the south-western sector of the GrIS in summer 2017, representing 10 %–13 % of the total. In localized patches with high biomass accumulation, algae accelerated melting by up to 26.15±3.77 % (standard error, SE). The year 2017 was a high-albedo year, so we also extended our analysis to the particularly low-albedo 2016 melt season. The runoff from the south-western bare-ice zone attributed to algae was much higher in 2016 at 8.8–12.2 Gt, although the proportion of the total runoff contributed by algae was similar at 9 %–13 %. Across a 10 000 km2 area around our field site, algae covered similar proportions of the exposed bare ice zone in both years (57.99 % in 2016 and 58.89 % in 2017), but more of the algal ice was classed as “high biomass” in 2016 (8.35 %) than 2017 (2.54 %). This interannual comparison demonstrates a positive feedback where more widespread, higher-biomass algal blooms are expected to form in high-melt years where the winter snowpack retreats further and earlier, providing a larger area for bloom development and also enhancing the provision of nutrients and liquid water liberated from melting ice. Our analysis confirms the importance of this biological albedo feedback and that its omission from predictive models leads to the systematic underestimation of Greenland's future sea level contribution, especially because both the bare-ice zones available for algal colonization and the length of the biological growth season are set to expand in the future.

This study formulates a comprehensive depositional model for hydromagnesite-magnesite playas. Mineralogical, isotopic and hydrogeochemical data are coupled with electron microscopy and field observations of the hydromagnesite-magnesite playas near Atlin, British Columbia, Canada. Four surface environments are recognized: wetlands, grasslands, localized mounds (metrescale) and amalgamated mounds composed primarily of hydromagnesite [Mg 5 (CO 3 ) 4 (OH) 2 Á4H 2 O], which are interpreted to represent stages in playa genesis. Water chemistry, precipitation kinetics and depositional environment are primary controls on sediment mineralogy. At depth (average % 2 m), CaMg-carbonate sediments overlay early Holocene glaciolacustrine sediments indicating deposition within a lake post-deglaciation. This mineralogical change corresponds to a shift from siliciclastic to chemical carbonate deposition as the supply of fresh surface water (for example, glacier meltwater) ceased and was replaced by alkaline groundwater. Weathering of ultramafic bedrock in the region produces Mg-HCO 3 groundwater that concentrates by evaporation upon discharging into closed basins, occupied by the playas. An uppermost unit of Mg-carbonate sediments (hydromagnesite mounds) overlies the Ca-Mg-carbonate sediments. This second mineralogical shift corresponds to a change in the depositional environment from subaqueous to subaerial, occurring once sediments 'emerged' from the water surface. Capillary action and evaporation draw Mg-HCO 3 water up towards the ground surface, precipitating Mg-carbonate minerals. Evaporation at the water table causes precipitation of lansfordite [MgCO 3 Á5H 2 O] which partially cements pre-existing sediments forming a hardpan. As carbonate deposition continues, the weight of the overlying sediments causes compaction and minor lateral movement of the mounds leading to amalgamation of localized mounds. Radiocarbon dating of buried vegetation at the Ca-Mg-carbonate boundary indicates that there has been ca 8000 years of continuous Mg-carbonate deposition at a rate of 0Á4 mm yr À1 . The depositional model accounts for the many sedimentological, mineralogical and geochemical processes that occur in the four surface environments; elucidating past and present carbonate deposition.

Abstract. One of the primary controls upon the melting of the Greenland Ice Sheet (GrIS) is albedo, a measure of how much solar radiation that hits a surface is reflected without being absorbed. Lower-albedo snow and ice surfaces therefore warm more quickly. There is a major difference in the albedo of snow-covered versus bare-ice surfaces, but observations also show that there is substantial spatio-temporal variability of up to ∼0.4 in bare-ice albedo. Variability in bare-ice albedo has been attributed to a number of processes including the accumulation of light-absorbing impurities (LAIs) and the changing physical properties of the near-surface ice. However, the combined impact of these processes upon albedo remains poorly constrained. Here we use field observations to show that pigmented glacier algae are ubiquitous and cause surface darkening both within and outside the south-west GrIS “dark zone” but that other factors including modification of the ice surface by algal bloom presence, surface topography and weathering crust state are also important in determining patterns of daily albedo variability. We further use observations from an unmanned aerial system (UAS) to examine the scale gap in albedo between ground versus remotely sensed measurements made by Sentinel-2 (S-2) and MODIS. S-2 observations provide a highly conservative estimate of algal bloom presence because algal blooms occur in patches much smaller than the ground resolution of S-2 data. Nevertheless, the bare-ice albedo distribution at the scale of 20 m×20 m S-2 pixels is generally unimodal and unskewed. Conversely, bare-ice surfaces have a left-skewed albedo distribution at MODIS MOD10A1 scales. Thus, when MOD10A1 observations are used as input to energy balance modelling, meltwater production can be underestimated by ∼2 %. Our study highlights that (1) the impact of the weathering crust state is of similar importance to the direct darkening role of light-absorbing impurities upon ice albedo and (2) there is a spatial-scale dependency in albedo measurement which reduces detection of real changes at coarser resolutions.

Ultramafic and mafic mine tailings are a valuable feedstock for carbon mineralization that should be used to offset carbon emissions generated by the mining industry. Although passive carbonation is occurring at the abandoned Clinton Creek asbestos mine, and the active Diavik diamond and Mount Keith nickel mines, there remains untapped potential for sequestering CO 2 within these mine wastes. There is the potential to accelerate carbonation to create economically viable, large-scale CO 2 fixation technologies that can operate at near-surface temperature and atmospheric pressure. We review several relevant acceleration strategies including: bioleaching of magnesium silicates; increasing the supply of CO 2 via heterotrophic oxidation of waste organics; and biologically induced carbonate precipitation, as well as enhancing passive carbonation through tailings management practices and use of CO 2 point sources. Scenarios for pilot scale projects are proposed with the aim of moving towards carbon-neutral mines. A financial incentive is necessary to encourage the development of these strategies. We recommend the use of a dynamic real options pricing approach, instead of traditional discounted cash-flow approaches, because it reflects the inherent value in managerial OPEN ACCESS Minerals 2014, 4 400 flexibility to adapt and capitalize on favorable future opportunities in the highly volatile carbon market.

A cyanobacteria dominated consortium collected from an alkaline wetland located near Atlin, British Columbia, Canada accelerated the precipitation of platy hydromagnesite [Mg5(CO3)4(OH)2·4H2O] in a linear flow-through experimental model wetland. The concentration of magnesium decreased rapidly within 2 m of the inflow point of the 10-m-long (∼1.5 m(2)) bioreactor. The change in water chemistry was monitored over two months along the length of the channel. Carbonate mineralization was associated with extra-cellular polymeric substances in the nutrient-rich upstream portion of the bioreactor, while the lower part of the system, which lacked essential nutrients, did not exhibit any hydromagnesite precipitation. A mass balance calculation using the water chemistry data produced a carbon sequestration rate of 33.34 t of C/ha per year. Amendment of the nutrient deficiency would intuitively allow for increased carbonation activity. Optimization of this process will have application as a sustainable mining practice by mediating magnesium carbonate precipitation in ultramafic mine tailings storage facilities.

The Arctic is being disproportionally affected by climate change compared with other geographic locations, and is currently experiencing unprecedented melt rates. The Greenland Ice Sheet (GrIS) can be regarded as the largest supraglacial ecosystem on Earth, and ice algae are the dominant primary producers on bare ice surfaces throughout the course of a melt season. Ice-algal-derived pigments cause a darkening of the ice surface, which in turn decreases albedo and increases melt rates. The important role of ice algae in changing melt rates has only recently been recognized, and we currently know little about their community compositions and functions. Here, we present the first analysis of ice algal communities across a 100 km transect on the GrIS by high-throughput sequencing and subsequent oligotyping of the most abundant taxa. Our data reveal an extremely low algal diversity with Ancylonema nordenskiöldii and a Mesotaenium species being by far the dominant taxa at all sites. We employed an oligotyping approach and revealed a hidden diversity not detectable by conventional clustering of operational taxonomic units and taxonomic classification. Oligotypes of the dominant taxa exhibit a site-specific distribution, which may be linked to differences in temperatures and subsequently the extent of the melting. Our results help to better understand the distribution patterns of ice algal communities that play a crucial role in the GrIS ecosystem.