ALBERT

Description: Impact of icebergs on net primary productivity in the Southern Ocean Shuang-Ye Wu and Shugui Hou The Cryosphere, 11, 707-722, doi:10.5194/tc-11-707-2017, 2017 The primary productivity in the Southern Ocean (SO) is limited by the amount of iron available for biological activities. Recent studies show that icebergs could be a main source of iron to the SO. Based on remote sensing data, our study shows that iceberg presence is associated with elevated levels of ocean productivity, particularly in iron-deficient regions. This impact could serve as a negative feedback to the climate system.

Description: Bromine, iodine and sodium in surface snow along the 2013 Talos Dome–GV7 traverse (northern Victoria Land, East Antarctica) Niccolò Maffezzoli, Andrea Spolaor, Carlo Barbante, Michele Bertò, Massimo Frezzotti, and Paul Vallelonga The Cryosphere, 11, 693-705, doi:10.5194/tc-11-693-2017, 2017 Sea ice is a crucial parameter within Earth's climate system. Understanding its dynamics and its response to other climatic variables is therefore of primary importance in view of a warming climate and sea ice decline. In this work we investigate some features of a chemical parameter in ice cores, bromine enrichment, which is linked to sea ice and can therefore be used to reconstruct sea ice in the past.

Description: Application of a two-step approach for mapping ice thickness to various glacier types on Svalbard Johannes Jakob Fürst, Fabien Gillet-Chaulet, Toby J. Benham, Julian A. Dowdeswell, Mariusz Grabiec, Francisco Navarro, Rickard Pettersson, Geir Moholdt, Christopher Nuth, Björn Sass, Kjetil Aas, Xavier Fettweis, Charlotte Lang, Thorsten Seehaus, and Matthias Braun The Cryosphere Discuss., doi:10.5194/tc-2017-30,2017 Manuscript under review for TC (discussion: open, 0 comments) For the large majority of glaciers and ice caps, there is no information on the thickness of the ice cover. Any attempt to predict glacier demise under climatic warming and to estimate the future contribution to sea-level rise is limited as long as the glacier thickness is not well constrained. Here, we present a two-step mass-conservation approach for mapping ice thickness. Measurements are naturally reproduced. The reliability is readily assessible from a complementary map of error estimates.

Description: Ice-shelf damming in the glacial Arctic Ocean: dynamical regimes of a basin-covering kilometre thick ice shelf Johan Nilsson, Martin Jakobsson, Chris Borstad, Nina Kirchner, Göran Björk, Raymond T. Pierrehumbert, and Christian Stranne The Cryosphere Discuss., doi:10.5194/tc-2017-37,2017 Manuscript under review for TC (discussion: open, 0 comments) Recent data suggest that a one-kilometre thick ice shelf extended over the glacial Arctic Ocean during MIS 6, about 140 000 years ago. Here, we theoretically analyse the development and equilibrium features of such an ice shelf. The ice shelf was effectively dammed by the Fram Strait and the mean ice-shelf thickness was controlled primarily by the horizontally-integrated mass balance. Our results can aid to resolve some outstanding questions of the state of the glacial Arctic Ocean.

Description: Oceanic dimethyl sulfide (DMS) is of interest due to its critical influence on atmospheric sulfur compounds in the marine atmosphere and its hypothesized significant role in global climate. High resolution shipboard underway measurements of surface seawater DMS and the partial pressure of carbon dioxide ( p CO 2 ) were conducted in the Atlantic Ocean and Indian Ocean sectors of the Southern Ocean (SO), the southeast Indian Ocean and the northwest Pacific Ocean from February – April 2014 during the 30th Chinese Antarctic Research Expedition. The SO, particularly in the region south of 58 °S, had the highest mean surface seawater DMS concentration of 4.1 ± 8.3 nM (ranged from 0.1 to 73.2 nM) and lowest mean seawater p CO 2 level of 337 ± 50 μatm (ranged from 221 to 411 μatm) over the entire cruise. Significant variations of surface seawater DMS and p CO 2 in the seasonal ice zone (SIZ) of SO were observed, which are mainly controlled by biological process and sea ice activity. We found a significant negative relationship between DMS and p CO 2 in the SO SIZ using 0.1 degree resolution, [DMS] seawater = - 0.160 [ p CO 2 ] seawater + 61.8 (r 2 = 0.594, n = 924, p 〈 0.001). We anticipate that the relationship may possibly be utilized to reconstruct the surface seawater DMS climatology in the SO SIZ. Further studies are necessary to improve the universality of this approach.

Description: No abstract is available for this article.

Description: s Reducing parameter uncertainty of process-based terrestrial ecosystem models (TEMs) is one of the primary targets for accurately estimating carbon budgets and predicting ecosystem responses to climate change. However, parameters in TEMs are rarely constrained by observations from Chinese forest ecosystems, which are important carbon sink over the Northern Hemispheric land. In this study, eddy-covariance data from 6 forest sites in China are used to optimize parameters of the ORCHIDEE (ORganizing Carbon and Hydrology In Dymanics EcosystEms) TEM. The model-data assimilation through parameter optimization largely reduces the prior model errors and improves the simulated seasonal cycle and summer diurnal cycle of net ecosystem exchange (NEE), latent heat (LE) fluxes as well as gross primary production (GPP) and ecosystem respiration. Climate change experiments based on the optimized model are deployed to indicate that forest net primary production (NPP) is suppressed in response to warming in the southern China but stimulated in the northeastern China. Altered precipitation has an asymmetric impact on forest NPP at sites in water-limited regions, with the optimization-induced reduction in response of NPP to precipitation decline being as large as 61% at a deciduous broadleaf forest site. We find that seasonal optimization alters forest carbon cycle responses to environmental change, with the parameter optimization consistently reducing the simulated positive response of heterotrophic respiration to warming. Evaluations from independent observations suggest that improving model structure still matters most for long term carbon stock and its changes, in particular nutrient- and age-related changes of photosynthetic rates, carbon allocation and tree mortality.

Description: Tropical methane sources are an important part of the global methane budget and include natural wetlands, rice agriculture, biomass burning, ruminants, fossil fuels and waste. δ 13 C CH4 can provide strong constraints on methane source apportionment. For example, tropical wetlands in this study give δ 13 C CH4 values between -61.5 ±2.9 ‰ and -53.0 ±0.4 ‰ and in general are more enriched in 13 C than temperate and boreal wetlands. However, thus far, relatively few measurements of δ 13 C CH4 in methane-enriched air have been made in the tropics. In this study samples have been collected from tropical wetland, rice, ruminant, and biomass burning emissions to the atmosphere. Regional isotopic signatures vary greatly as different processes and source material affect methane signatures. Measurements were made to determine bulk source inputs to the atmosphere, rather than to study individual processes. These measurements provide inputs for regional methane budget models, to constrain emissions with better source apportionment.

Description: Burial of biogenic silica (bSi total ) in high sedimentation rate continental margins remains highly uncertain. Cosmogenic 32 Si (t 1/2 ~140y) can be used to trace the fates of bSi total post deposition, including as opal (bSi opal ) and diagenetically altered opal (bSi altered ); the latter dominantly authigenic clay (bSi clay ). To determine the magnitude and form of bSi total storage in coastal sediments, conventional operational leaches targeting bSi opal and bSi altered (including bSi clay ) were modified for large scale samples necessary for measurement of 32 Si. 32 Si activity was used to estimate total biogenic silica burial (bSi total = bSi opal +bSi altered ) in several depositional settings: Gulf of Papua, Gulf of Mexico, Long Island Sound, and in the previously studied Amazon-Guianas deltaic system. In subtropical and temperate regions, 32 Si was detected in both traditional biogenic silica leaches (bSi opal ) and residual authigenic clays. Traditional bSi opal and modified operational leaches designed to target the most reactive authigenic silicates (~bSi altered ) consistently underestimate authigenic clay formation (bSi clay ) and thus the magnitude of bSi total burial in temperate coastal zones and subtropical deltas by 2–4 fold. In tropical deltas, 32 Si activities in the residua≳l fraction after removal of bSi opal demonstrate rapid and almost complete alteration of initial bSi opal to new forms, most likely bSi clay . Globally, 4.5–4.9 Tmol/y Si may be trapped in marine nearshore deposits as rapidly formed clay (bSi clay ), ≳100% of the “missing silica sink” in the marine silica budget.

Description: The implications of a mesoscale eddy for relevant properties of the Southern Ocean carbon cycle is examined with in situ observations. We explored carbon properties inside a large (~190 km diameter) cyclonic eddy that detached from the Subantarctic Front (SAF) south of Tasmania in March 2016. Based on remote sensing, the eddy was present for ~2 months in the Subantarctic Zone (SAZ), an important region of oceanic carbon dioxide (CO 2 ) uptake throughout the annual cycle and carbon subduction (i.e., where mode and intermediate waters form), before it was re-absorbed into the SAF. The eddy was sampled during the middle of its life, 1 month after it spawned. Comparatively, the eddy was ~3°C colder, 0.5 PSU fresher and less biologically productive than surrounding SAZ waters. The eddy was also richer in dissolved inorganic carbon (DIC) and had lower saturation states of aragonite and calcite than the surrounding SAZ waters. As a consequence, it was a strong source of CO 2 to the atmosphere (with fluxes up to +25 mmol C m -2 d -1 ). Compared to the SAF waters, from which it originated, DIC concentration in the eddy was ~20 μmol kg -1 lower, indicating lateral mixing, small-scale recirculation or eddy stirring with lower-DIC SAZ waters by the time the eddy was observed. As they are commonly spawned from the Antarctic Circumpolar Current (ACC), and as 50% of them decay in the SAZ (the rest being re-absorbed by the SAF-N), these types of eddies may represent a significant south-north transport pathway for carbon across the ACC and may alter the carbon properties of SAZ waters.

Description: No abstract is available for this article.

Description: We investigated the biogeochemical cycling of dissolved zinc (Zn) in the western and central subarctic North Pacific during the GEOTRACES GP 02 cruise. The relationship between dissolved Zn and silicate in the subarctic North Pacific plotted as a concave curve. Values of Zn* were strongly positive in the intermediate waters (26.6–27.5 σ θ ) of both the western and the central subarctic North Pacific. There was a distinct kink in the relationship between dissolved Zn and soluble reactive phosphorus (SRP) at the transition from shallow to intermediate water, which is similar to what has been reported for other open oceans. The high Zn:SRP ratio and high Zn* in the intermediate water suggests that intermediate water masses play an important role in the decoupling of dissolved Zn and silicate in the subarctic North Pacific, which implies that the biogeochemical processes that control dissolved Zn and silicate in the intermediate water are different from those in other oceanic regions.

Description: The Eastern Tropical Pacific (ETP) hosts two of the world's three Oxygen Deficient Zones (ODZs), large bodies of suboxic water that are subject to high rates of water column denitrification (WCD). In the mean, these two ODZs are responsible for about 15 to 40% of all fixed N loss in the ocean, but little is known about how this loss varies in time. Here, we use a hindcast simulation with the ocean component of the NCAR Community Earth System Model over the period 1948 to 2009 to show that the El Niño-Southern Oscillation (ENSO) drives large variations in the rates of WCD in this region. During mature La Niña (El Niño) conditions, peak denitrification rates are up to 70% higher (lower) than the mean rates. This large variability is the result of wind-driven changes in circulation and isopycnal structure concurrently modifying the thermocline distribution of O 2 and organic matter export in such a way that WCD is strongly amplified. During average La Niña (El Niño) conditions, the overall changes in ODZ structure and primarily the shoaling (deepening) of the upper boundary of both ODZs by 40 to 100 m explains 50% of the changes in WCD in the North Pacific and 94% in the South Pacific. Such a large variability of WCD in the ETP has strong implications for the assessments of trends, the balance of the marine N-cycle and the emission of the greenhouse gas N 2 O.

Description: Reanalysis of a 10-year record (2004–2013) of seasonal mass balances at Langenferner/Vedretta Lunga, Ortler Alps, Italy Stephan Peter Galos, Christoph Klug, Fabien Maussion, Federico Covi, Lindsey Nicholson, Lorenzo Rieg, Wolfgang Gurgiser, Thomas Mölg, and Georg Kaser The Cryosphere, 11, 1417-1439, https://doi.org/10.5194/tc-11-1417-2017, 2017 Records of glacier mass balance represent important data in climate science and their uncertainties affect calculations of sea level rise and other societally relevant environmental projections. In order to reduce and quantify uncertainties in mass balance series obtained by direct glaciological measurements, we present a detailed reanalysis workflow which was applied to the 10-year record (2004 to 2013) of seasonal mass balance of Langenferner, a small glacier in the European Eastern Alps. The approach involves a methodological homogenization of available point values and the creation of pseudo-observations of point mass balance for years and locations without measurements by the application of a process-based model constrained by snow line observations. We examine the uncertainties related to the extrapolation of point data using a variety of methods and consequently present a more rigorous uncertainty assessment than is usually reported in the literature. Results reveal that the reanalyzed balance record considerably differs from the original one mainly for the first half of the observation period. For annual balances these misfits reach the order of  〉 300 kg m −2 and could primarily be attributed to a lack of measurements in the upper glacier part and to the use of outdated glacier outlines. For winter balances respective differences are smaller (up to 233 kg m −2 ) and they originate primarily from methodological inhomogeneities in the original series. Remaining random uncertainties in the reanalyzed series are mainly determined by the extrapolation of point data to the glacier scale and are on the order of ±79 kg m −2 for annual and ±52 kg m −2 for winter balances with values for single years/seasons reaching ±136 kg m −2 . A comparison of the glaciological results to those obtained by the geodetic method for the period 2005 to 2013 based on airborne laser-scanning data reveals that no significant bias of the reanalyzed record is detectable.

Description: To investigate the controls on N 2 fixation and the role of the Atlantic in the ocean fixed nitrogen (N) budget, Atlantic N 2 fixation is calculated by combining meridional nitrate fluxes across World Ocean Circulation Experiment sections with observed nitrate 15 N/ 14 N differences between northward- and southward-transported nitrate. N 2 fixation inputs of 27.1±4.3 Tg N/yr and 3.0±0.5 Tg N/yr are estimated north of 11°S and 24°N, respectively. That is, ~90% of the N 2 fixation in the Atlantic north of 11°S occurs south of 24°N, in a region with upwelling that imports phosphorus (P) in excess of N relative to phytoplankton requirements. This suggests that, under the modern iron-rich conditions of the equatorial and North Atlantic, N 2 fixation occurs predominantly in response to P-bearing, N-poor conditions. We estimate a N 2 fixation rate of 30.5±4.9 Tg N/yr north of 30°S, implying only 3 Tg N/yr between 30° and 11°S, despite evidence of P-bearing, N-poor surface waters in this region as well; this is consistent with iron limitation of N 2 fixation in the South Atlantic. Since the ocean flows through the Atlantic surface in 〈2500 years, similar to the residence time of oceanic fixed N, Atlantic N 2 fixation can stabilize the N-to-P ratio of the global ocean. However, the calculated rate of Atlantic N 2 fixation is a small fraction of global ocean estimates for either N 2 fixation or fixed N loss. This suggests that, in the modern ocean, an approximate balance between N loss and N 2 fixation is achieved within the combined Indian and Pacific basins.

Description: Methane (CH 4 ) is a potent greenhouse gas that is both produced and consumed in soils by microbially mediated processes sensitive to soil redox. We evaluated the classical conceptual model of peatland CH 4 dynamics—in which the water-table position determines the vertical distribution of methanogenesis and methanotrophy—versus an emerging model in which methanogenesis and methanotrophy can both occur throughout the soil profile due to spatially heterogeneous redox and anaerobic CH 4 oxidation. We simultaneously measured gross CH 4 production and oxidation in situ across a microtopographical gradient in a drained temperate peatland and ex situ along the soil profile, giving us novel insight into the component fluxes of landscape-level net CH 4 fluxes. Net CH 4 fluxes varied among landforms (p〈 0.001), ranging from 180.3 ± 81.2 mg-C m -2 d -1 in drainage ditches to -0.7 ± 1.2 mg-C m -2 d -1 in the highest landform. Contrary to prediction by the classical conceptual model, variability in methanogenesis alone drove the landscape-level net CH 4 flux patterns. Consistent with the emerging model, freshly-collected soils from above the water table produced CH 4 within anaerobic microsites. Even in soil from beneath the water table, gross CH 4 production was best predicted by the methanogenic fraction of carbon mineralization, an index of highly reducing microsites. We measured low rates of anaerobic CH 4 oxidation, which may have been limited by relatively low in situ CH 4 concentrations in the hummock/hollow soil profile. Our study revealed complex CH 4 dynamics better represented by the emerging heterogeneous conceptual model than the classical model based on redox strata.

Description: The biological carbon pump (BCP) transports organic carbon from the surface to the ocean's interior via sinking particles, vertically migrating organisms, and passive transport of organic matter by advection and diffusion. While many studies have quantified sinking particles, the magnitude of passive transport remains poorly constrained. In the Southern Ocean weak thermal stratification, strong vertical gradients in particulate organic matter, and weak vertical nitrate gradients suggest that passive transport from the euphotic zone may be particularly important. We compile data from seasonal time-series at a coastal site near Palmer Station, annual regional cruises in the Western Antarctic Peninsula (WAP), cruises throughout the broader Southern Ocean, and SOCCOM autonomous profiling floats to estimate spatial and temporal patterns in vertical gradients of nitrate, particulate nitrogen (PN), and dissolved organic carbon (DOC). Under a steady-state approximation, the ratio of ∂PN/∂z to ∂NO 3 - /∂z suggests that passive transport of PN may be responsible for removing 46% (37%-58%) of the nitrate introduced into the surface ocean of the WAP (with DOM contributing an additional 3-6%) and for 23% (19%-28%) of the BCP in the broader Southern Ocean. A simple model parameterized with in situ nitrate, PN, and primary production data suggested that passive transport was responsible for 54% of the magnitude of the BCP in the WAP. Our results highlight the potential importance of passive transport (by advection and diffusion) of organic matter in the Southern Ocean, but should only be considered indicative of high passive transport (rather than conclusive evidence) due to our steady-state assumptions.

Description: Models for polythermal ice sheets and glaciers Ian J. Hewitt and Christian Schoof The Cryosphere, 11, 541-551, doi:10.5194/tc-11-541-2017, 2017 Many glaciers contain ice both below and at the melting temperature. Predicting the evolution of temperature and water content in such ice masses is important because they exert a strong control on the flow of the ice. We present two new models to calculate these quantities, demonstrate a number of example numerical calculations, and compare the results with existing methods. The novelty of the new methods is the inclusion of gravity-driven water transport within the ice.

Description: The phytoplankton bloom in the Ross Sea is the largest in spatial extent and one of the most productive in Antarctica, yet the fate of the summer bloom remains poorly understood. Here we present carbon system data from the first biogeochemical process cruise to be conducted in both the western and central Ross Sea during late summer (February-March 2013). Using one-dimensional carbon budgets, we found evidence for substantial positive net community production (425 ± 204 mmol C m -2 d -1 ) during the late summer in Terra Nova Bay (TNB) of the western Ross Sea, which was rapidly exported to below 200 m. In addition, seasonally integrated carbon export was higher in diatom-dominated TNB (7.3 ± 0.9 mol C m -2 ) compared to the Phaeocystis antarctica -dominated central Ross Sea (3.4 ± 0.8 mol C m -2 ). Substantial late summer productivity and export may be a widespread phenomenon in Antarctic coastal regions that is not accounted for in regional carbon models.

Description: On the mean, ocean carbon uptake is linked to biological productivity, but how biological variability impacts carbon uptake is poorly quantified. Our ability to diagnose past change, understand present variability and predict the future state of the global carbon cycle requires improving mechanistic understanding in this area. Here, we make use of co-located pCO 2 and temperature data, a merged surface ocean color product, and physical fields from an ocean state estimate to assess relationships between surface ocean biology and the carbon cycle on seasonal, monthly anomaly, and interannual timescales over the period 1998-2014. Using a correlation analysis on spatial scales from local to basin-scale biomes, we identify the timescales on which ocean productivity could be directly modifying ocean carbon uptake. On seasonal timescales outside of the equatorial Pacific, biome-scale correlations are negative between chlorophyll and pCO 2 . Though this relationship is pervasive, the underlying mechanisms vary across timescales and biomes. Consistent with previous findings, biological activity is a significant driver of pCO 2 seasonality only in the subpolar biomes. For monthly anomalies acting on top of the mean seasonality, productivity and pCO 2 changes are significantly correlated in the subpolar North Pacific and Southern Ocean. Only in the Southern Ocean are correlations consistent with a dominant role for biology in the surface ocean carbon cycle on all timescales.

Description: Experimental observation of transient δ 18 O interaction between snow and advective airflow under various temperature gradient conditions Pirmin P. Ebner, Hans Christian Steen-Larsen, Barbara Stenni, Martin Schneebeli, and Aldo Steinfeld The Cryosphere Discuss., doi:10.5194/tc-2017-16,2017 Manuscript under review for TC (discussion: open, 0 comments) Stable water isotopes (δ 18 O) obtained from snow and ice samples from polar regions are used to reconstruct past climate variability. We present an experimental study on the effect on the snow isotopic composition by airflow through a snowpack in controlled laboratory conditions. The disequilibrium between snow and vapor isotopes changed the isotopic content of the snow. These measurements suggest that the history of metamorphism affects the snow isotopic composition.

Description: Distinguishing between old and modern permafrost sources 1 with compound-specific δ 2 H analysis Jorien E. Vonk, Tommaso Tesi, Lisa Bröder, Henry Holmstrand, Gustaf Hugelius, August Andersson, Oleg Dudarev, Igor Semiletov, and Örjan Gustafsson The Cryosphere Discuss., doi:10.5194/tc-2017-17,2017 Manuscript under review for TC (discussion: open, 0 comments) Pleistocene ice complex permafrost deposits contain roughly a quarter of the organic carbon (OC) stored in permafrost terrain. When permafrost thaws, its OC is remobilized into the (aquatic) environment where it is available for degradation, transport or burial. Aquatic or coastal environments contain sedimentary reservoirs that can serve as archives of past climatic change. As permafrost thaw is increasing throughout the Arctic, these reservoirs are important locations to assess the fate of remobilized permafrost OC. We here present compound-specific deuterium (δ 2 H) analysis on leaf waxes as a tool to distinguish between OC released from thawing Pleistocene permafrost (Ice Complex Deposits; ICD) and from thawing Holocene permafrost (from near-surface soils). Bulk geochemistry (%OC, δ 13 C, %total nitrogen; TN) was analyzed as well as the concentrations and δ 2 H signatures of long-chain n -alkanes (C 21 to C 33 ) and mid/long-chain n -alkanoic acids (C 16 to C 30 ) extracted from both ICD-PF samples ( n  = 9) and modern vegetation/O-horizon (Topsoil-PF) samples ( n  = 9) from across the northeast Siberian Arctic. Results show that these Topsoil-PF samples have higher %OC, higher OC/TN values, and more depleted δ 13 C-OC values than ICD-PF samples, suggesting that these former samples trace a fresher soil and/or vegetation source. Median concentrations of high-molecular weight n -alkanes (sum of C 25 -C 27 -C 29 -C 31 ) were 210 ± 350 µg/gOC (median ± IQR) for Topsoil-PF and 250 ± 81 µg/gOC for ICD-PF samples. Long-chain n -alkanoic acids (sum of C 22 -C 24 -C 26 -C 28 ) were more abundant than long-chain n -alkanes, both in Topsoil-PF samples (4700 ± 3400 µg/gOC) and in ICD samples (6630 ± 3500 µg/gOC). Whereas the two investigated sources differ on the bulk geochemical level, they are, however, virtually indistinguishable when using leaf wax concentrations and ratios. However, on the molecular-isotope level, leaf wax biomarker δ 2 H values are statistically different between Topsoil-PF and ICD-PF. The mean δ 2 H value of C 29 n -alkane was −246 ± 13 ‰ (mean ± stdev) for Topsoil-PF and −280 ± 12 ‰ for ICD-PF, whereas the C31 n -alkane was −247 ± 23 ‰ for Topsoil-PF and −297 ± 15 ‰ for ICD-PF. The C 28 n -alkanoic acid δ 2 H value was −220 ± 15 ‰ for Topsoil-PF and −267 ± 16 ‰ for ICD-PF. With a dynamic isotopic range (difference between two sources) of 34 to 50 ‰, the isotopic fingerprints of individual, abundant, biomarker molecules from leaf waxes can thus serve as end-members to distinguish between these two sources. We tested this molecular δ 2 H tracer along with another source-distinguishing approach, dual-carbon (δ 13 C-δ 14 C) isotope composition of bulk OC, for a surface sediment transect in the Laptev Sea. Results show that general offshore patterns along the shelf-slope transect are similar, but the source apportionment between the approaches vary, which may highlight the advantages of either. The δ 2 H molecular approach has the advantage that it circumvents uncertainties related to a marine end-member, yet the δ 13 C-δ 14 C approach has the advantage that it represents the bulk OC fraction thereby avoiding issues related to the molecular-bulk upscaling challenge. This study indicates that the application of δ 2 H leaf wax values has potential to serve as a complementary quantitative measure of the source and differential fate of OC thawed out from different permafrost compartments.

Description: Constraining the continental silicon cycle is a key requirement in attempts to understand both nutrient fluxes to the ocean and linkages between silicon and carbon cycling over different timescales. Silicon isotope data of dissolved silica (δ 30 Si DSi ) are presented here from Lake Baikal and its catchment in central Siberia. As well as being the world's oldest and voluminous lake, Lake Baikal lies within the seventh largest drainage basin in the world and exports significant amounts of freshwater into the Arctic Ocean. Data from river waters accounting for c. 92% of annual river inflow to the lake suggest no seasonal alteration or anthropogenic impact on river δ 30 Si DSi composition. The absence of a change in δ 30 Si DSi within the Selenga Delta, through which 62% of riverine flow passes, suggest a net balance between biogenic uptake and dissolution in this system. A key feature of this study is the use of δ 30 Si DSi to examine seasonal and spatial variations in DSi utilisation and export across the lake. Using an open system model against deep water δ 30 Si DSi values from the lake, we estimate that 20-24% of DSi entering Lake Baikal is exported into the sediment record. Whilst highlighting the impact that lakes may have upon the sequestration of continental DSi, mixed layer δ 30 Si DSi values from 2003 and 2013 show significant spatial variability in the magnitude of spring bloom nutrient utilisation with lower rates in the north relative to south basin.

Description: Snowfall in the Alps: Evaluation and projections based on the EURO-CORDEX regional climate models Prisco Frei, Sven Kotlarski, Mark A. Liniger, and Christoph Schär The Cryosphere Discuss., doi:10.5194/tc-2017-7,2017 Manuscript under review for TC (discussion: open, 0 comments) Snowfall is central to Alpine environments, and its future changes will be associated with pronounced impacts. We here assess future snowfall changes in the European Alps based on an ensemble of state-of-the-art regional climate model experiments and on two different greenhouse gas emission scenarios. The results reveal pronounced changes in the Alpine snowfall climate with considerable snowfall reductions at low and mid elevations, but also snowfall increases at high elevations in mid-winter.

Description: Wave-ice interactions in the neXtSIM sea-ice model Timothy D. Williams, Pierre Rampal, and Sylvain Bouillon The Cryosphere Discuss., doi:10.5194/tc-2017-24,2017 Manuscript under review for TC (discussion: open, 0 comments) As the Arctic sea ice extent drops, there is more ship traffic seeking to take advantage of this, and with it a need for better wave and sea ice forecasts. One aspect of this is the location of the sea ice edge. The waves here can be quite large, but they die away as they travel into the ice. This causes momentum to be transferred from the waves to the ice, causing ice drift. However, our study found that the effect of the wind drag had more impact on the ice edge position than the waves.

Description: Cryostratigraphy, sedimentology, and the late Quaternary evolution of the Zackenberg River delta, northeast Greenland Graham L. Gilbert, Stefanie Cable, Christine Thiel, Hanne H. Christiansen, and Bo Elberling The Cryosphere, 11, 1265-1282, doi:10.5194/tc-11-1265-2017, 2017 We reconstruct the Holocene development of the Zackenberg River delta (northeast Greenland) using a combination of sedimentology, cryostratigraphy, and geochronology. We distinguish four major depositional environments and identify three cryofacies. We apply the principles of cryostratigraphy to infer the aggradational history of permafrost. This paper contains an archive of ground ice in epigenetic permafrost in northeast Greenland.

Description: Recent dynamic changes on Fleming Glacier after the disintegration of Wordie Ice Shelf, Antarctic Peninsula Peter Friedl, Thorsten C. Seehaus, Anja Wendt, Matthias H. Braun, and Kathrin Höppner The Cryosphere Discuss., doi:10.5194/tc-2017-91,2017 Manuscript under review for TC (discussion: open, 0 comments) Fleming Glacier is the biggest tributary glacier of the former Wordie Ice Shelf (south-western Antarctic Peninsula). Radar satellite data and airborne ice elevation measurements show that the glacier accelerated by ~ 30 % from 2007–2011 and that tinning of the glacier increased by ~ 60–70 %. This was a response to an ungrounding of the glacier tongue from the glacier bed between 2008 and 2011, which was likely crucially triggered by increased basal melt at the grounding line due to ocean warming.

Description: Evaluation of different methods to model near-surface turbulent fluxes for an alpine glacier in the Cariboo Mountains, BC, Canada Valentina Radić, Brian Menounos, Joseph Shea, Noel Fitzpatrick, Mekdes A. Tessema, and Stephen J. Déry The Cryosphere Discuss., doi:10.5194/tc-2017-80,2017 Manuscript under review for TC (discussion: open, 0 comments) Our overall goal is to improve the numerical modelling of glacier melt in order to better predict the future of glaciers in Western Canada and worldwide. Most commonly used models rely on simplifications of processes that dictate melting at a glacier surface, in particular turbulent processes of heat exchange. We compared modelled against directly measured turbulent heat fluxes at a valley glacier in British Columbia, Canada, and found that more improvements are needed in all the tested models.

Description: Calibrated cryo-cell UV-LA-ICPMS elemental concentrations from the NGRIP ice core reveal abrupt, sub-annual variability in dust across the GI-21.2 interstadial period Damiano Della Lunga, Wolfgang Müller, Sune Olander Rasmussen, Anders Svensson, and Paul Vallelonga The Cryosphere, 11, 1297-1309, https://doi.org/10.5194/tc-11-1297-2017, 2017 In our study we combined the wealth of information provided by Greenland ice cores with an ultra-high-resolution technique well known in geoscience (laser ablation). Our set-up was developed and applied to investigate the variability in concentration of ions across a rapid climatic change from the oldest part of the last glaciation, showing that concentrations drop abruptly from cold to warm periods, representing a shift in atmospheric transport that happens even faster than previously thought.

Description: Evaluation of different methods to model near-surface turbulent fluxes for an alpine glacier in the Cariboo Mountains, BC, Canada Valentina Radić, Brian Menounos, Joseph Shea, Noel Fitzpatrick, Mekdes A. Tessema, and Stephen J. Déry The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-80,2017 Manuscript under review for TC (discussion: open, 0 comments) Our overall goal is to improve the numerical modelling of glacier melt in order to better predict the future of glaciers in Western Canada and worldwide. Most commonly used models rely on simplifications of processes that dictate melting at a glacier surface, in particular turbulent processes of heat exchange. We compared modelled against directly measured turbulent heat fluxes at a valley glacier in British Columbia, Canada, and found that more improvements are needed in all the tested models.

Description: Recent dynamic changes on Fleming Glacier after the disintegration of Wordie Ice Shelf, Antarctic Peninsula Peter Friedl, Thorsten C. Seehaus, Anja Wendt, Matthias H. Braun, and Kathrin Höppner The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-91,2017 Manuscript under review for TC (discussion: open, 0 comments) Fleming Glacier is the biggest tributary glacier of the former Wordie Ice Shelf (south-western Antarctic Peninsula). Radar satellite data and airborne ice elevation measurements show that the glacier accelerated by ~ 30 % from 2007–2011 and that tinning of the glacier increased by ~ 60–70 %. This was a response to an ungrounding of the glacier tongue from the glacier bed between 2008 and 2011, which was likely crucially triggered by increased basal melt at the grounding line due to ocean warming.

Description: Cryostratigraphy, sedimentology, and the late Quaternary evolution of the Zackenberg River delta, northeast Greenland Graham L. Gilbert, Stefanie Cable, Christine Thiel, Hanne H. Christiansen, and Bo Elberling The Cryosphere, 11, 1265-1282, https://doi.org/10.5194/tc-11-1265-2017, 2017 We reconstruct the Holocene development of the Zackenberg River delta (northeast Greenland) using a combination of sedimentology, cryostratigraphy, and geochronology. We distinguish four major depositional environments and identify three cryofacies. We apply the principles of cryostratigraphy to infer the aggradational history of permafrost. This paper contains an archive of ground ice in epigenetic permafrost in northeast Greenland.

Description: Iceberg calving of Thwaites Glacier, West Antarctica: full-Stokes modeling combined with linear elastic fracture mechanics Hongju Yu, Eric Rignot, Mathieu Morlighem, and Helene Seroussi The Cryosphere, 11, 1283-1296, https://doi.org/10.5194/tc-11-1283-2017, 2017 We combine 2-D ice flow model with linear elastic fracture mechanics (LEFM) to model the calving behavior of Thwaites Glacier, West Antarctica. We find the combination of full-Stokes (FS) model and LEFM produces crevasses that are consistent with observations. We also find that calving is enhanced with pre-existing surface crevasses, shorter ice shelves or undercut at the ice shelf front. We conclude that the FS/LEFM combination is capable of constraining crevasse formation and iceberg calving.

Description: Characterizing permafrost soil active layer dynamics and sensitivity to landscape spatial heterogeneity in Alaska Yonghong Yi, John S. Kimball, Richard Chen, Mahta Moghaddam, Rolf H. Reichle, Umakant Mishra, Donatella Zona, and Walter C. Oechel The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-87,2017 Manuscript under review for TC (discussion: open, 0 comments) An important feature of the Arctic is large spatial heterogeneity in active layer conditions, which is generally poorly represented by global models. In this study, we developed a spatially integrated modelling and analysis framework combining field observations, local scale (~ 50 m) active layer thickness (ALT) and soil moisture maps derived from airborne low frequency (L + P-band) radar measurements, and global satellite environmental observations to investigate the ALT sensitivity to recent climate trends and landscape heterogeneity in Alaska. Model simulated ALT results show good correspondence with in-situ measurements in higher permafrost probability (PP ≥ 70 %) areas (n = 33, R = 0.60, mean bias = 1.58 cm, RMSE = 20.32 cm). The model results also reveal widespread ALT deepening since 2001, with smaller ALT increases in northern Alaska (mean trend = 0.32 ± 1.18 cm yr -1 ) and much larger increases (〉 3 cm yr -1 ) across interior and southern Alaska. The positive ALT trend coincides with regional warming and a longer snow-free season (R = 0.60 ± 0.32). Uncertainty in the spatial and vertical distribution of soil organic carbon (SOC) was found to be the most important factor affecting model ALT accuracy. Potential improvements in characterizing SOC heterogeneity, including better spatial sampling of soil conditions and advances in remote sensing of SOC and soil moisture, will enable more accurate predictions of permafrost active layer conditions.

Description: Centuries of intense surface melt on Larsen C Ice Shelf Suzanne Bevan, Adrian Luckman, Bryn Hubbard, Bernd Kulessa, David Ashmore, Peter Kuipers Munneke, Martin O'Leary, Adam Booth, Heidi Sevestre, and Daniel McGrath The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-81,2017 Manuscript under review for TC (discussion: open, 0 comments) Five 90 m boreholes drilled into an Antarctic Peninsula ice shelf show units of ice that are denser than expected and must have formed from refrozen surface melt which has been buried and transported downstream. We used surface flow speeds and snow accumulation rates to work out where and when these units formed. Results show that as well as recent surface melt, a period of strong melt occurred during the 18th century. Surface melt is thought to be a factor in causing recent ice shelf break up.

Description: Modelling debris transport within glaciers by advection in a full-Stokes ice flow model Anna Wirbel, Alexander Helmut Jarosch, and Lindsey Nicholson The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-92,2017 Manuscript under review for TC (discussion: open, 0 comments) As debris cover affects the melt water production and behaviour of glaciers it is important to understand how, and over what timescales, it forms. Here we develop an advanced 3D numerical model that describes transport of sediment through a glacier to the point where it emerges at the surface. The numerical performance of the model is satisfactory and it reproduces debris structures observed within real-world glaciers, thereby offering a useful tool for future studies of debris-covered glaciers.

Description: Wave-induced stress and breaking of sea ice in a coupled hydrodynamic–discrete-element wave–ice model Agnieszka Herman The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-95,2017 Manuscript under review for TC (discussion: open, 0 comments) It is often assumed that ocean waves break sea ice into floes with sizes depending on wavelength. The results of this modeling study (in agreement with some earlier observations and models) suggest that this is not the case and instead the sizes of ice floes produced by waves depend only on ice thickness and mechanical properties. This may have important consequences for predicting sea ice response to oceanic and atmospheric forcing in regions where sea ice is influenced by waves.

Description: The modelled liquid water balance of the Greenland Ice Sheet Christian R. Steger, Carleen H. Reijmer, and Michiel R. van den Broeke The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-88,2017 Manuscript under review for TC (discussion: open, 0 comments) Mass loss from the Greenland Ice Sheet, which contributes to sea level rise, is currently dominated by surface melt and runoff. The relation between these two variables is rather uncertain due to the firn layer’s potential to buffer melt in solid (refreezing) or liquid (firn aquifer) form. To address this uncertainty, we analyse output of a numerical firn model run over 1960–2014. Results show a spatially variable response of the ice sheet to increasing melt and an upward migration of aquifers.

Description: Stable accumulation patterns around Dome C, East Antarctica, over the last glacial cycle Marie G.~P. Cavitte, Frédéric Parrenin, Catherine Ritz, Duncan A. Young, Donald D. Blankenship, Massimo Frezzotti, and Jason L. Roberts The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-71,2017 Manuscript under review for TC (discussion: open, 0 comments) We reconstruct the pattern of surface accumulation in the region around Dome C, East Antarctica, through the last glacial cycle. We use internal isochrones interpreted from ice-penetrating radar surveys and a 1D pseudo-steady ice flow model to invert for both time-averaged accumulation rates and paleoaccumulation rates. We observer that surface accumulation patterns are stable through the last 128 kyrs, this suggests a stable position of the dome throughout the last glacial cycle.

Description: Glaciological settings and recent mass balance of the Blåskimen Island in Dronning Maud Land, Antarctica Vikram Goel, Joel Brown, and Kenichi Matsuoka The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-61,2017 Manuscript under review for TC (discussion: open, 0 comments) Ice rises are locally-grounded features surrounded by ice shelves. They help stabilize the Antarctic Ice Sheet and in turn are affected by ice-sheet evolution. However, details of these influences depend on the glaciological settings of the ice rises. We report first detailed ground-based investigations from Blåskimen Island ice rise in East Antarctica. We found that the ice rise is at least ~ 600 year old and has been thickening by ~ 0.2 m per year over the past decade.

Description: Self-affine subglacial roughness: consequences for radar scattering and basal water discrimination in northern Greenland Thomas M. Jordan, Michael A. Cooper, Dustin M. Schroeder, Christopher N. Williams, John D. Paden, Martin J. Siegert, and Jonathan L. Bamber The Cryosphere, 11, 1247-1264, https://doi.org/10.5194/tc-11-1247-2017, 2017 Using radio-echo sounding data from northern Greenland, we demonstrate that subglacial roughness exhibits self-affine (fractal) scaling behaviour. This enables us to assess topographic control upon the bed-echo waveform, and explain the spatial distribution of the degree of scattering (specular and diffuse reflections). Via comparison with a prediction for the basal thermal state (thawed and frozen regions of the bed) we discuss the consequences of our study for basal water discrimination.

Description: Spatial and temporal variability of water-filled crevasse hydrologic states along the shear margins of Jakobshavn Isbrae, Greenland Casey A. Joseph and Derrick J. Lampkin The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-86,2017 Manuscript under review for TC (discussion: open, 0 comments) This work characterizes the spatial and temporal variability in the hydrologic state (filled or drained) of seven crevasse groups on Jakobshavn Isbræ from 2000–2015 using several optical satellite platforms. Meltwater from these crevasse groups can be transported to the bedrock and accelerate mass loss. The frequency of multi-drainage events were observed to increase over the study period. Multi-drain events may be related to temperature, terminus front location, and strain rate.

Description: A new approach to estimate ice dynamic rates using satellite observations in East Antarctica Bianca Kallenberg, Paul Tregoning, Janosch Fabian Hoffmann, Rhys Hawkins, Anthony Purcell, and Sébastien Allgeyer The Cryosphere, 11, 1235-1245, https://doi.org/10.5194/tc-11-1235-2017, 2017 Mass balance changes of the Antarctic ice sheet are of significant interest due to its sensitivity to climatic changes and its contribution to changes in global sea level. While regional climate models successfully estimate mass input due to snowfall, it remains difficult to estimate the amount of mass loss due to ice dynamic processes. It has often been assumed that changes in ice dynamic rates only need to be considered when assessing long-term ice sheet mass balance; however, 2 decades of satellite altimetry observations reveal that the Antarctic ice sheet changes unexpectedly and much more dynamically than previously expected. Despite available estimates on ice dynamic rates obtained from radar altimetry, information about ice sheet changes due to changes in the ice dynamics are still limited, especially in East Antarctica. Without understanding ice dynamic rates, it is not possible to properly assess changes in ice sheet mass balance and surface elevation or to develop ice sheet models. In this study we investigate the possibility of estimating ice sheet changes due to ice dynamic rates by removing modelled rates of surface mass balance, firn compaction, and bedrock uplift from satellite altimetry and gravity observations. With similar rates of ice discharge acquired from two different satellite missions we show that it is possible to obtain an approximation of the rate of change due to ice dynamics by combining altimetry and gravity observations. Thus, surface elevation changes due to surface mass balance, firn compaction, and ice dynamic rates can be modelled and correlated with observed elevation changes from satellite altimetry.

Description: Dark ice dynamics of the south-west Greenland Ice Sheet Andrew J. Tedstone, Jonathan L. Bamber, Joseph M. Cook, Christopher J. Williamson, Xavier Fettweis, Andrew J. Hodson, and Martyn Tranter The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-79,2017 Manuscript under review for TC (discussion: open, 0 comments) The bare-ice albedo of the south-west Greenland ice sheet varies dramatically between melt seasons. Darkening by inorganic particulates, cryoconite hole processes and ice algae can occur, but the reasons for inter-annual albedo variability are unclear. We use satellite imagery to examine dark ice dynamics, and climate model outputs to find likely climatological controls. Melt-out particulates best explain the spatial extent of dark ice, but the darkening itself is likely due to ice algae growth.

Description: Methanesulfonic acid (MSA) migration in polar ice: Data synthesis and theory Matthew Osman, Sarah B. Das, Olivier Marchal, and Matthew J. Evans The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-84,2017 Manuscript under review for TC (discussion: open, 0 comments) We combine a synthesis of 22 ice core records and a model of soluble impurity transport to investigate the enigmatic, post-depositional migration of methanesulfonic acid (MSA) in polar ice. Our findings suggest that migration may be universal across coastal regions of Greenland and Antarctica, though is mitigated at sites with higher accumulation and (or) lower impurity content. Records exhibiting severe migration may still be useful for inferring decadal and lower-frequency climate variability.

Description: Optical properties of laboratory grown sea ice doped with light absorbing impurities (black carbon) Amelia A. Marks, Maxim L. Lamare, and Martin D. King The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-76,2017 Manuscript under review for TC (discussion: open, 0 comments) Arctic sea ice extent is declining rapidly. Prediction of sea ice trends rely on sea ice models that need to be evaluated with real data. A realistic sea ice environment is created in a laboratory by the Royal Holloway sea ice simulator and is used to show a sea ice model can replicate measured properties of sea ice e.g. reflectance. Black carbon, a component of soot found in atmospheric pollution, is also experimentally shown to reduce the reflectance of sea ice, which could exacerbate melting.

Description: Exceptional retreat of Novaya Zemlya's marine-terminating outlet glaciers between 2000 and 2013 J. Rachel Carr, Heather Bell, Rebecca Killick, and Tom Holt The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-32,2017 Manuscript under review for TC (discussion: open, 0 comments) Glaciers on Novaya Zemlya (NVZ) retreated rapidly between 2000 and 2013. This was far faster than the previous 25 years, but retreat then slowed from 2013 onward. This may result from changes in broad-scale climatic patterns. Glaciers ending in lakes retreated at a similar rate to those ending in the ocean and retreat rates were very consistent between glaciers, which contrasts with previous studies.

Description: Properties of black carbon and other insoluble light-absorbing particles in seasonal snow of northwestern China Wei Pu, Xin Wang, Hailun Wei, Yue Zhou, Jinsen Shi, Zhiyuan Hu, Hongchun Jin, and Quanliang Chen The Cryosphere, 11, 1213-1233, https://doi.org/10.5194/tc-11-1213-2017, 2017 We conducted a large field campaign to collect snow samples in Xinjiang. We measured insoluble light-absorbing particles with estimated black carbon concentrations of 10–150 ng g-1 . We found a probable shift in emission sources with the progression of winter and dominated contributions of BC and OC to light absorption. A PMF model indicated an optimal three-factor/source solution that included industrial pollution, biomass burning, and soil dust.

Description: Improving gridded snow water equivalent products in British Columbia, Canada: multi-source data fusion by neural network models Andrew Snauffer, William Hsieh, Alex Cannon, and Markus Schnorbus The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-56,2017 Manuscript under review for TC (discussion: open, 0 comments) Estimating winter snowpack throughout British Columbia is challenging due to the complex terrain, thick forests and high snow accumulations present. This paper describes a way to make better snow estimates by combining publicly available data using machine learning, a branch of artificial intelligence research. These improved estimates will help water resources managers better plan for changes in rivers and lakes fed by springtime snowmelt and will aid other research that supports such planning.

Description: Hypsometric amplification and routing moderation of Greenland ice sheet meltwater release Dirk van As, Andreas Bech Mikkelsen, Morten Holtegaard Nielsen, Jason E. Box, Lillemor Claesson Liljedahl, Katrin Lindbäck, Lincoln Pitcher, and Bent Hasholt The Cryosphere, 11, 1371-1386, https://doi.org/10.5194/tc-11-1371-2017, 2017 The Greenland ice sheet melts faster in a warmer climate. The ice sheet is flatter at high elevation, therefore atmospheric warming increases the melt area exponentially. For current climate conditions, we find that the ice sheet shape amplifies the total meltwater generation by roughly 60 %. Meltwater is not stored underneath the ice sheet, as previously found, but it does take multiple days for it to pass through the seasonally developing subglacial drainage channels, moderating discharge.

Description: In this study we used fluorescence excitation and emission spectroscopy (EEMs), hydrographic data and a self-organizing map (SOM) analysis to assess the spatial distribution of labile and refractory FDOM for the Chukchi and Beaufort Seas at the time of a massive under-ice phytoplankton bloom during early summer 2011. Biogeochemical properties were assessed through decomposition of water property classes and sample classification that employed a SOM neural network-based analysis which classified ten clusters from 269 samples and 17 variables. The terrestrial, humic-like component FDOM (ArC1, 4.98±1.54 QSU) and protein-like component FDOM (ArC3, 1.63±0.88 QSU) were found to have elevated fluorescence in the LPML (salinity ~29.56±0.76). In the LPML water mass, the observed contribution of meteoric water fraction was 17%, relative to a 12% contribution from the sea-ice melt fraction. The labile ArC3-protein-like component (2.01±1.92 QSU) was also observed to be elevated in the PWW mass, where the under-ice algal bloom was observed (~40-50 m). We interpreted these relationships to indicate that the accumulation and variable distribution of the protein-like component on the shelf could be influenced directly by sea-ice melt, transport and mixing processes; and indirectly by the in situ algal bloom and microbial activity. ArC5, corresponding to what is commonly considered marine humic FDOM indicated a bimodal distribution with high values both in the freshest and saltiest waters. The association of ArC5 with deep, dense salty water is consistent with this component as refractory humic-like FDOM whereas our evidence of a terrestrial origin challenges this classic paradigm for this component.

Description: Numerical Earth System Models are generic tools used to extrapolate present climate conditions into a warming future and to explore geo-engineering options. Most of the current-generation models feature a simple pelagic biogeochemical model component that is embedded into a three dimensional general circulation ocean model. The dynamics of these biogeochemical model components is essentially controlled by so-called model parameters most of which are poorly known. Here we explore the feasibility to estimate these parameters in a full-fledged three dimensional Earth System Model by minimizing the misfit to noisy observations. The focus is on parameter identifiability. Based on earlier studies, we illustrate problems in determining a unique estimate of those parameters, that prescribe the limiting effect of nutrient and light-depleted conditions on carbon assimilation by autotrophic phytoplankton. Our results showcase that for typical models and evaluation metrics no meaningful “best” unique parameter set exists. We find very different parameter sets which are, on the one hand, equally consistent with our (synthetic) historical observations while, on the other hand, they propose strikingly differing projections into a warming climate.

Description: The presence, magnitude, and even direction of long-term trends in phytoplankton abundance over the past few decades is still debated in the literature, primarily due to differences in the data sets and methodologies used. Recent work has suggested that the satellite chlorophyll record is not yet long enough to distinguish climate change trends from natural variability, despite the high density of coverage in both space and time. Previous work has typically focused on using linear models to determine the presence of trends, where each grid cell is considered independently from its neighbors. However, trends can be more thoroughly evaluated using a spatially resolved approach. Here a Bayesian hierarchical spatio-temporal model is fitted to quantify trends in ocean chlorophyll from September 1997 to December 2013. The approach used in this study explicitly accounts for the dependence between neighboring grid cells, which allows us to estimate trend by ‘borrowing strength’ from the spatial correlation. By way of comparison, a model without spatial correlation is also fitted. This results in a notable loss of accuracy in model fit. Additionally, we find an order of magnitude smaller global trend, and larger uncertainty, when using the spatio-temporal model: -0.023 ± 0.12 % yr -1 as opposed to -0.38 ± 0.045 % yr -1 when the spatial correlation is not taken into account. The improvement in accuracy of trend estimates, and the more complete account of their uncertainty emphasizes the solution that space-time modeling offers for studying global long-term change.

Description: Water flow in the active layer along an arctic slope – An investigation based on a field campaign and model simulations Sebastian F. Zastruzny, Bo Elberling, Lars Nielsen, and Karsten H. Jensen The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-97,2017 Manuscript under review for TC (discussion: open, 0 comments) The hydrological regime in the arctic will change as the climate conditions change and the release and associated transport of nutrients will also be affected. In this study we analyze water flow and tracer transport along a sloping transect in Disko Island in Greenland. The results suggest that the movement of dissolved nitrogen compounds such as nitrate, being released along the slope in consequence of permafrost thawing, can quickly influence nitrogen cycling at the end of the slope.

Description: Inter-comparison of snow depth retrievals over Arctic sea ice from radar data acquired by Operation IceBridge Ron Kwok, Nathan T. Kurtz, Ludovic Brucker, Alvaro Ivanoff, Thomas Newman, Sinead L. Farrell, Joshua King, Stephen Howell, Melinda A. Webster, John Paden, Carl Leuschen, Joseph A. Macgregor, Jacqueline Richter-Menge, Jeremy Harbeck, and Mark Tschudi The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-103,2017 Manuscript under review for TC (discussion: open, 0 comments) Since 2009, the ultra-wideband snow-radar on Operation IceBridge has acquired data in annual campaigns conducted during the Arctic and Antarctic springs. Existing snow depth retrieval algorithms differ in the way the air-snow and snow-ice interfaces are detected and localized in the radar returns, and in how the system limitations are addressed. Here, we assess five retrieval algorithms by comparisons with field measurements, ground-based campaigns, and analysed fields of snow depth.

Description: Brief communication: The global signature of post-1900 land ice wastage on vertical land motion Riccardo E. M. Riva, Thomas Frederikse, Matt A. King, Ben Marzeion, and Michiel R. van den Broeke The Cryosphere, 11, 1327-1332, https://doi.org/10.5194/tc-11-1327-2017, 2017 The reduction of ice masses stored on land has made an important contribution to sea-level rise over the last century, as well as changed the Earth's shape. We model the solid-earth response to ice mass changes and find significant vertical deformation signals over large continental areas. We show how deformation rates have varied strongly throughout the last century, which affects the interpretation and extrapolation of recent observations of vertical land motion and sea-level change.

Description: Accurate representation of the remineralization of sinking organic matter is crucial for reliable projections of the marine carbon cycle. Both water temperature and oxygen concentration are thought to influence remineralization rates, but limited data constraints have caused disagreement concerning the degree of these influences. We analyse a compilation of POC flux measurements from 19 globally distributed sites. Our results indicate that the attenuation of the flux of particulate organic matter depends on temperature with a Q 10 between 1.5 and 2.01, and on oxygen described by a half saturation constant between 4 and 12 μ mol/L. We assess the impact of the temperature and oxygen dependence in the biogeochemistry model COBALT, coupled to GFDL's Earth System Model ESM2M. The new remineralization parameterization results in shallower remineralization in the low latitudes but deeper remineralization in the high latitudes, redistributing POC flux towards the poles. It also decreases the volume of the oxygen minimum zones, partly addressing a long-standing bias in global climate models. Extrapolating temperature-dependent remineralization rates to the surface (i.e., beyond the depth range of POC flux data) resulted in rapid recycling and excessive surface nutrients. Surface nutrients could be ameliorated by reducing near surface rates in a manner consistent with bacterial colonization, suggesting the need for improved remineralization constraints within the euphotic zone. The temperature and oxygen dependence cause an additional 10% decrease in global POC flux at 500m depth, but no significant change in global POC flux at 2000m under the RCP8.5 future projection.

Description: Snow farming: Conserving snow over the summer season Thomas Grünewald, Michael Lehning, and Fabian Wolfsperger The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-93,2017 Manuscript under review for TC (discussion: open, 0 comments) Snowfarming is the conservation of snow during summer. Large snow piles are covered with a saw-dust insulation layer, reducing melt and guaranteeing a specific amount of snow available in autumn independent of the weather conditions. Snow volumes changes of two heaps were monitored showing that about 1/3 of the snow was lost. Model simulations confirmed the large effect of the insulation on energy balance and melt. The model can now be used as a tool to examine future snowfarming projects.

Description: Sonar gas flux estimation by bubble insonification: application to methane bubble flux from seep areas in the outer Laptev Sea Ira Leifer, Denis Chernykh, Natalia Shakhova, and Igor Semiletov The Cryosphere, 11, 1333-1350, https://doi.org/10.5194/tc-11-1333-2017, 2017 Vast Arctic methane deposits may alter global climate and require remote sensing (RS) to map. Sonar has great promise, but quantitative inversion based on theory is challenged by multiple bubble acoustical scattering in plumes. We demonstrate use of a real-world in situ bubble plume calibration using a bubble model to correct for differences in the calibration and seep plumes. Spatial seep sonar maps were then used to improve understanding of subsurface geologic controls.

Description: Wind-driven snow conditions control the occurrence of contemporary marginal mountain permafrost in the Chic-Choc Mountains, south-eastern Canada: a case study from Mont Jacques-Cartier Gautier Davesne, Daniel Fortier, Florent Domine, and James T. Gray The Cryosphere, 11, 1351-1370, https://doi.org/10.5194/tc-11-1351-2017, 2017 This study presents data from Mont Jacques-Cartier, the highest summit in the Appalachians of south-eastern Canada, to demonstrate that the occurrence of contemporary permafrost body is associated with a very thin and wind-packed winter snow cover which brings local azonal topo-climatic conditions on the dome-shaped summit. This study is an important preliminary step in modelling the regional spatial distribution of permafrost on the highest summits in eastern North America.

Description: Measuring snow water equivalent from common offset GPR records through migration velocity analysis James St. Clair and W. Steven Holbrook The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-90,2017 Manuscript under review for TC (discussion: open, 0 comments) We investigate the performance of a semi-automated algorithm for measuring snow water equivalent (SWE) from common-offset ground-penetrating-radar (GPR) data. The algorithm automatically estimates SWE from a selected portion of the data and the user is only required for quality control. GPR derived SWE estimates are similar to manual measurements indicating that the method is reliable. Our results will hopefully make GPR a more attractive tool for monitoring SWE in mountain watersheds.

Description: No abstract is available for this article.

Description: Impacts of freshwater changes on Antarctic sea ice in an eddy-permitting sea-ice–ocean model Verena Haid, Doroteaciro Iovino, and Simona Masina The Cryosphere, 11, 1387-1402, https://doi.org/10.5194/tc-11-1387-2017, 2017 Since the Antarctic sea ice extent shows a recent increase, we investigate the sea ice response to changed amount and distribution of surface freshwater addition in the Southern Ocean with the ocean–sea ice model NEMO/LIM2. We find that freshwater addition within the range of current estimates increases the ice extent, but higher amounts could have an opposing effect. The freshwater distribution is of great influence on the ice dynamics and the ice thickness is strongly influenced by it.

Description: In situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March-May 2013. We used observations of pico-, nano-, and microphytoplankton to evaluate community structure and diversity, and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O 2 m -2 d -1 , gross primary production (GPP) between 40 and 100 mmol O 2 m -2 d -1 , and NCP/GPP, a measure of export efficiency, ranging from 0.1-0.2 (0.05-0.1 in carbon units). However, notable exceptions to this basin scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of DOC export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better-studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.

Description: Livestock systems play a key role in global sustainability challenges like food security and climate change, yet, many unknowns and large uncertainties prevail. We present a systematic, spatially explicit assessment of uncertainties related to grazing intensity (GI), a key metric for assessing ecological impacts of grazing, by combining existing datasets on a) grazing feed intake, b) the spatial distribution of livestock, c) the extent of grazing land, and d) its net primary productivity (NPP). An analysis of the resulting 96 maps implies that on average 15% of the grazing land NPP is consumed by livestock. GI is low in most of worlds grazing lands but hotspots of very high GI prevail in 1% of the total grazing area. The agreement between GI maps is good on one fifth of the world's grazing area, while on the remainder it is low to very low. Largest uncertainties are found in global drylands and where grazing land bears trees (e.g., the Amazon basin or the Taiga belt). In some regions like India or Western Europe massive uncertainties even result in GI 〉 100% estimates. Our sensitivity analysis indicates that the input-data for NPP, animal distribution and grazing area contribute about equally to the total variability in GI maps, while grazing feed intake is a less critical variable. We argue that a general improvement in quality of the available global level datasets is a precondition for improving the understanding of the role of livestock systems in the context of global environmental change or food security.

Description: New respiration (R new , of freshly fixated carbon) and old respiration (R old , of storage carbon) were estimated for different regions of the global surface ocean using published data on simultaneous measurements of: 1) primary productivity using 14 C ( 14 PP); 2) gross primary productivity (GPP) based on 18 O or O 2 ; and 3) net community productivity (NCP) using O 2 . The ratio R new / GPP in 24 h incubations was typically between 0.1 and 0.3 regardless of depth and geographical area, demonstrating that values were almost constant regardless of large variations in temperature (0 to 27 °C), irradiance (surface to ~100 m deep), nutrients (nutrient rich and poor waters), and community composition (diatoms, flagellates, etc). As such, between 10 and 30% of primary production in the surface ocean is respired in less than 24 h, and most respiration (between 55 and 75%) was of older carbon. R new was most likely associated with autotrophs, with minor contribution from heterotrophic bacteria. Patterns were less clear for R old . Short 14 C incubations are less affected by respiratory losses. Global oceanic GPP is estimated to be between 70 and 145 Gt C y -1 .

Description: Concentrations of dissolved organic carbon (DOC) from terrestrial sources have been increasing in freshwaters across large parts of the boreal region. According to results from large scale field and detailed laboratory studies such a DOC increase could potentially stimulate carbon dioxide (CO 2 ) production, subsequently increasing the partial pressure of CO 2 ( p CO 2 ) in freshwaters. However, the response of p CO 2 to the presently observed long-term increase in DOC in freshwaters is still unknown. Here we tested whether the commonly found spatial DOC- p CO 2 relationship is also valid on a temporal scale. Analysing time series of water chemical data from 71 lakes, 30 streams and four river mouths distributed across all of Sweden over a 17 year period, we observed significant DOC concentration increases in 39 lakes, 15 streams and four river mouths. Significant p CO 2 increases were however only observed in six of these 58 waters, indicating that long-term DOC increases in Swedish waters are disconnected from temporal p CO 2 trends. We suggest that the uncoupling of trends in DOC concentration and p CO 2 are a result of increased surface water runoff. When surface water runoff increases, there is likely less CO 2 relative to DOC imported from soils into waters due to a changed balance between surface and groundwater flow. Additionally, increased surface water runoff causes faster water flushing through the landscape giving less time for in situ CO 2 production in freshwaters. We conclude that p CO 2 is presently not following DOC concentration trends, which has important implications for modelling future CO 2 emissions from boreal waters.

Description: While the number of surface ocean CO 2 partial pressure (pCO 2 ) measurements has soared the recent decades, the Southern Ocean remains undersampled. Williams et al . [2017] now present pCO 2 estimates based on data from pH-sensor equipped Bio-Argo floats, which have been measuring in the Southern Ocean since 2014. The authors demonstrate the utility of these data for understanding the carbon cycle in this region, which has a large influence on the distribution of CO 2 between the ocean and atmosphere. Biogeochemical sensors deployed on autonomous platforms hold the potential to shape our view of the ocean carbon cycle in the coming decades.

Description: Nitrogen-fixing micro-organisms (diazotrophs) provide biologically available nitrogen to plankton communities and thereby greatly influence the productivity in many marine regions. Various cyanobacterial groups have traditionally been considered the major oceanic diazotrophs, but later non-cyanobacterial and presumably heterotrophic diazotrophs were also found to be widespread and potentially important in nitrogen fixation. However, the distribution and activity of different diazotroph groups is still poorly constrained for most oceanic ecosystems. Here, we examined diazotroph community structure and activity along a 7,500-km south-north transect between the central equatorial Pacific and the Bering Sea. Nitrogen fixation contributed up to 84% of new production in the upper waters of the subtropical gyre, where the diazotroph community included the gammaproteobacterium γ-24774A11 and highly active cyanobacterial phylotypes (〉50% of total nifH transcript abundance). Nitrogen fixation was sometimes detectable down to 150 m depth and extended horizontally to the edge of the gyre at around 35°N. Nitrogen fixation was even detected far north on the Bering Sea shelf. In the Alaskan Coastal Waters on the Bering Sea shelf, low nitrate together with high dissolved iron concentrations seemed to foster diazotroph growth, including a prominent role of UCYN-A2, which was abundant near the surface (1.2 × 10 5 nifH gene copies L -1 ). Our study provides evidence for nitrogen fixation in the Bering Sea and suggests a clear contrast in the composition of diazotrophs between the tropical/subtropical gyre and the separate waters in the cold northern regions of the North Pacific.

Description: Brief communication: Estimation of hydraulic properties of active layers using ground-penetrating radar (GPR) and 2D inverse hydrological modeling Xicai Pan, Stefan Jaumann, Jiabao Zhang, and Kurt Roth The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-77,2017 Manuscript under review for TC (discussion: open, 0 comments) This study proposes a new method for estimating hydraulic properties of active layers using ground-penetrating radar (GPR) and 2D inverse hydrological modeling. This method creatively turns over the adverse features of undulating frost table for 1D inverse estimation of hydraulic parameters to assets for 2D inverse estimation. Its advantages include non-destructive observations, a bigger scale of the soil hydraulic properties and efficiency for permafrost studies.

Description: Numerical modelling of convective heat transport by air flow in permafrost talus slopes Jonas Wicky and Christian Hauck The Cryosphere, 11, 1311-1325, https://doi.org/10.5194/tc-11-1311-2017, 2017 Talus slopes are a widespread geomorphic feature, which may show permafrost conditions even at low elevation due to cold microclimates induced by a gravity-driven internal air circulation. We show for the first time a numerical simulation of this internal air circulation of a field-scale talus slope. Results indicate that convective heat transfer leads to a pronounced ground cooling in the lower part of the talus slope favoring the persistence of permafrost.

Description: A multi-layer physically-based snowpack model simulating direct and indirect radiative impacts of light-absorbing impurities in snow Francois Tuzet, Marie Dumont, Matthieu Lafaysse, Ghislain Picard, Laurent Arnaud, Didier Voisin, Yves Lejeune, Luc Charrois, Pierre Nabat, and Morin Samuel The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-94,2017 Manuscript under review for TC (discussion: open, 0 comments) Light absorbing impurities deposited on snow, such as soot or dust, strongly modify its evolution. We implemented impurities deposition and evolution in a detailed snowpack model, thereby expanding the reach of such models into addressing the subtle interplays between snow physics and impurities optical properties. Model results were evaluated based on innovative field observations at an alpine site. This allows future investigations in the fields of climate, hydrology and avalanche prediction.

Description: A statistical fracture model for Antarctic glaciers Veronika Emetc, Paul Tregoning, and Malcolm Sambridge The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-98,2017 Manuscript under review for TC (discussion: open, 0 comments) We developed a statistics-based method to identify zones of Antarctic ice shelves that are likely to fracture, with an average accuracy of 77 % when compared to observed fractures identified in optical imagery. We find that we can identify 4 main groups of ice shelf regions having similar characteristics. Our method of identifying fracture regions provides the initial step in the modelling of the propagation of crevasses and can form the basis for modelling ice shelf calving processes.

Description: Blowing snow detection from ground-based ceilometers: application to East Antarctica Alexandra Gossart, Niels Souverijns, Irina Valerievna Gorodetskaya, Stef Lhermitte, Jan Thérèse Maria Lenaerts, Jan Herbert Schween, Alexander Mangold, Quentin Laffineur, and Nicole Petra Marie van Lipzig The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-89,2017 Manuscript under review for TC (discussion: open, 0 comments) Blowing snow plays an important role on local surface mass balance of Antarctica. We present here the blowing snow detection algorithm, to retreive blowing snow occurence from the attenuated backscatter signal of ceilometers set up at two station. There is a good correspondance in detection of heavy blowing snow by the algorithm, and the visual observations performed at Neumayer station. Moreover, most of the blowing snow occurs duringevents bringing precipitation from the coast inland.

Description: Seafloor geomorphology of western Antarctic Peninsula bays: a signature of ice flow behaviour Yuribia P. Munoz and Julia S. Wellner The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-108,2017 Manuscript under review for TC (discussion: open, 0 comments) We mapped submarine landforms in western Antarctic Peninsula bays. These landforms were formed by flowing ice and can provide insight into the local controls on glacial ice advance and retreat. We used a sonar to collect bathymetric data and then created seafloor maps. We conclude that the length and width of bays exert a control on the number and type of landforms found in the bays, narrower bays tend to stabilize ice flow, and shallower water depths may accelerate ice retreat.

Description: Antarctic sub-shelf melt rates via PICO Ronja Reese, Torsten Albrecht, Matthias Mengel, Xylar Asay-Davis, and Ricarda Winkelmann The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-70,2017 Manuscript under review for TC (discussion: open, 0 comments) Floating ice shelves surround most of Antarctica and melting at their bases in contact with the ocean is a major reason for its current sea-level contribution. We developed a simple model that captures the vertical ocean circulation generally present in ice-shelf cavities and allows simulating melt rates in accordance with physical processes beneath the ice. We test the model for all Antarctic ice shelves and find that melt rates and melt patterns from our model agree well with observations.

Description: Submarine melt rates and mass balance for Greenland's remaining ice tongues Nat Wilson, Fiammetta Straneo, and Patrick Heimbach The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-99,2017 Manuscript under review for TC (discussion: open, 0 comments) We estimate submarine melt rates from ice tongues in northern Greenland using WorldView satellite imagery. At Ryder Glacier, melt is strongly concentrated around regions where subglacier channels likely enter the fjord. At 79 North Glacier, we find a large volume imbalance in which melting removes a greater quantity of ice than is replaced by inflow over the grounding line. This leads us to predict a reduction in the spatial extent of the ice tongue over the coming decade.

Description: Numerical homogenization of the viscoplastic behavior of snow based on X-ray tomography images Antoine Wautier, Christian Geindreau, and Frédéric Flin The Cryosphere, 11, 1465-1485, https://doi.org/10.5194/tc-11-1465-2017, 2017 The results shown in the present paper consist in predicting the overall viscous behavior of snow from the sole knowledge of the microstructure and the ice viscous behavior. This is done thanks to multi-scale modeling techniques and an original approach is used to handle the nonlinearity of the ice behavior. An application of the developed formulation can be found in the simulation of the densification of the snowpack in order to enhance avalanche risk forecasting.

Description: Transient modeling of the ground thermal conditions using satellite data in the Lena River delta, Siberia Sebastian Westermann, Maria Peter, Moritz Langer, Georg Schwamborn, Lutz Schirrmeister, Bernd Etzelmüller, and Julia Boike The Cryosphere, 11, 1441-1463, https://doi.org/10.5194/tc-11-1441-2017, 2017 We demonstrate a remote-sensing-based scheme estimating the evolution of ground temperature and active layer thickness by means of a ground thermal model. A comparison to in situ observations from the Lena River delta in Siberia indicates that the model is generally capable of reproducing the annual temperature regime and seasonal thawing of the ground. The approach could hence be a first step towards remote detection of ground thermal conditions in permafrost areas.

Description: A weekly Arctic sea-ice thickness data record from merged CryoSat-2 and SMOS satellite data Robert Ricker, Stefan Hendricks, Lars Kaleschke, Xiangshan Tian-Kunze, Jennifer King, and Christian Haas The Cryosphere, 11, 1607-1623, https://doi.org/10.5194/tc-11-1607-2017, 2017 We developed the first merging of CryoSat-2 and SMOS sea-ice thickness retrievals. ESA’s Earth Explorer SMOS satellite can detect thin sea ice, whereas its companion CryoSat-2, designed to observe thicker perennial sea ice, lacks sensitivity. Using these satellite missions together completes the picture of the changing Arctic sea ice and provides a more accurate and comprehensive view on the actual state of Arctic sea-ice thickness.

Description: Climate warming has led to the degradation of permafrost stability in the past half century over the Qinghai-Tibet Plateau Youhua Ran, Xin Li, and Guodong Cheng The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-120,2017 Manuscript under review for TC (discussion: open, 0 comments) Approximately 87.98 % of the permafrost area in the 1960s has been degraded to lower levels of stability in the past half century over the Tibetan Plateau. The mean elevations of the extremely stable, stable, sub-stable, transitional, unstable, and extremely unstable permafrost areas increased by 88 m, 97 m, 155 m, 185 m, 161 m and 250 m, respectively. This degradation has led to increases in risks to infrastructure, water resources, and positive climate feedback.

Description: No abstract is available for this article.

Description: Snow water equivalent in the Alps as seen by gridded data sets, CMIP5 and CORDEX climate models Silvia Terzago, Jost von Hardenberg, Elisa Palazzi, and Antonello Provenzale The Cryosphere, 11, 1625-1645, https://doi.org/10.5194/tc-11-1625-2017, 2017 The estimate of the current and future conditions of snow resources in mountain areas depends on the availability of reliable fine-resolution data sets and of climate models capable of properly representing snow processes and snow–climate interactions. This work considers the snow water equivalent data sets from remote sensing, reanalyses, regional and global climate models available for the Alps and explores their ability to provide a coherent view of the snowpack features and its changes.

Description: The significance of vertical moisture diffusion on drifting Snow sublimation near snow surface Ning Huang and Guanglei Shi The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-116,2017 Manuscript under review for TC (discussion: open, 0 comments) Snow is an important part of the cryosphere, and blowing snow sublimation is an important method to change the snow distribution. However, in the previous studies blowing snow sublimation near surface was ignored. Herein, we built a blowing snow sublimation model to study the sublimation in near-surface region. The results showed that the mass of snow sublimation near surface accounted for even more than half of the total. Therefore, blowing snow sublimation near surface can’t be neglected.

Description: Design and results of the ice sheet model initialisation experiments initMIP-Greenland: an ISMIP6 intercomparison Heiko Goelzer, Sophie Nowicki, Tamsin Edwards, Matthew Beckley, Ayako Abe-Ouchi, Andy Aschwanden, Reinhard Calov, Olivier Gagliardini, Fabien Gillet-Chaulet, Nicholas R. Golledge, Jonathan Gregory, Ralf Greve, Angelika Humbert, Philippe Huybrechts, Joseph H. Kennedy, Eric Larour, William H. Lipscomb, Sébastien Le clec´h, Victoria Lee, Mathieu Morlighem, Frank Pattyn, Antony J. Payne, Christian Rodehacke, Martin Rückamp, Fuyuki Saito, Nicole Schlegel, Helene Seroussi, Andrew Shepherd, Sainan Sun, Roderik van de Wal, and Florian A. Ziemen The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-129,2017 Manuscript under review for TC (discussion: open, 0 comments) We have compared a wide spectrum of different initialisation techniques used in the ice sheet modelling community to define the modelled present-day Greenland ice sheet state as starting point for physically-based future sea-level change projections. Compared to earlier community-wide comparisons we find better agreement across different models, which implies overall improvement of our understanding of what is needed to produce such initial states.

Description: Black carbon and mineral dust in snow cover on the Third Pole Yulan Zhang, Shichang Kang, Michael Sprenger, Zhiyuan Cong, Tanguang Gao, Chaoliu Li, Shu Tao, Xiaofei Li, Xinyue Zhong, Min Xu, Wenjun Meng, and Mika Sillanpää The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-111,2017 Manuscript under review for TC (discussion: open, 0 comments) Light-absorbing impurities deposited on snow can reduce surface albedo and contribute to the near-worldwide melting of snowpack and ice. This study focused on the black carbon and mineral dust in snow cover on the Third Pole region. We discussed their concentrations, distributions, possible sources, and albedo reduction and radiative forcing. Findings indicated that the impacts of black carbon and mineral dust needed to be properly accounted for in future regional climate projections.

Description: Mechanisms influencing seasonal-to-interannual prediction skill of sea ice extent in the Arctic Ocean in MIROC Jun Ono, Hiroaki Tatebe, Yoshiki Komuro, Masato I. Nodzu, and Masayoshi Ishii The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-122,2017 Manuscript under review for TC (discussion: open, 0 comments) Sea ice in the Arctic Ocean has experienced rapid decline since the satellite era. To assess the predictability of sea ice extent in the Arctic Ocean and to clarify the underlying physical processes, we conducted prediction experiments using an initialized climate model (MIROC5). The present study reveals that subsurface ocean heat content originating from the North Atlantic contributes to the predictability of winter sea ice extent up to 1 year ahead, through the advection and vertical mixing.

Description: Simulating ice thickness and velocity evolution of Upernavik Isstrøm 1849–2012 by forcing prescribed terminus positions in ISSM Konstanze Haubner, Jason E. Box, Nicole J. Schlegel, Eric Y. Larour, Mathieu Morlighem, Anne M. Solgaard, Kristian K. Kjellerup, Signe H. Larsen, and Kurt H. Kjær The Cryosphere Discuss., https//doi.org/10.5194/tc-2017-121,2017 Manuscript under review for TC (discussion: open, 0 comments) We investigate the effect of neglecting calving on Upernavik Isstrøm, West Greenland, between 1849 and 2012. Our simulation is forced with observed terminus positions in discrete time steps and is responsive to the prescribed ice front changes. Simulated frontal retreat is needed to obtain a realistic ice surface elevation and velocity evolution of Upernavik. Using the prescribed terminus position change we get insight to mass loss partitioning during different time periods.

Description: The shunt of photosynthetically derived particulate organic carbon (POC) from the euphotic zone and deep remineralization comprises the basic mechanism of the “biological carbon pump.” POC raining through the “twilight zone” (euphotic depth to 1 km) and “midnight zone” (1 km to 4 km) is remineralized back to inorganic form through respiration. Accurately modeling POC flux is critical for understanding the “biological pump” and its impacts on air-sea CO 2 exchange and, ultimately, long-term ocean carbon sequestration. Yet, commonly used parameterizations have not been tested quantitatively against global datasets using identical modeling frameworks. Here, we use a single one-dimensional physical-biogeochemical modeling framework to assess three common POC flux parameterizations in capturing POC flux observations from moored sediment traps and thorium-234 depletion. The exponential decay, Martin curve, and ballast model are compared to data from 11 biogeochemical provinces distributed across the globe. In each province, the model captures satellite-based estimates of surface primary production within uncertainties. Goodness-of-fit is measured by how well the simulation captures the observations, quantified by bias and the root-mean-squared-error and displayed using “target diagrams.” Comparisons are presented separately for the twilight zone and midnight zone. We find the ballast hypothesis shows no improvement over a globally or regionally parameterized Martin curve. For all provinces taken together, Martin's b that best fits the data is [0.70, 0.98]; this finding reduces by at least a factor of 3 previous estimates of potential impacts on atmospheric pCO 2 of uncertainty in POC export to a more modest range [-16 ppm, +12 ppm].

Description: The subpolar North Atlantic is a key region for the oceanic uptake of heat, oxygen and carbon dioxide. Centennial oxygen (O 2 ) changes are investigated in the upper 700 m of the North Atlantic Ocean using a subset of Earth System Models (ESMs) included in the Coupled Model Intercomparison Project Phase 5. The climatological distributions of dissolved O 2 averaged for the recent past period (1975-2005) are generally well captured although the convective activity differs among the models in space and strength, and most models show a cold bias south of Greenland. By the end of the 21st century, all models predict an increase in depth-integrated temperature of 2-3 o C, resultant solubility decrease, weakened vertical mass transport, decreased nutrient supply into the euphotic layer, and weakened export production. Despite an overall tendency of the North Atlantic to lose oxygen, patchy regions of O 2 increase are observed due to the weakening of the North Atlantic Current (NAC) causing a regional solubility increase (the warming hole effect) and a decrease in the advection of subtropical, low-O 2 waters into the subpolar regions (the nutrient stream effect). Additionally, a shift in the NAC position contributes to localized O 2 changes near the boundaries of water masses. The net O 2 change reflects the combination of multiple factors leading to highly heterogeneous and model-dependent patterns. Our results imply that changes in the strength and position of the NAC will likely play crucial roles in setting the pattern of O 2 changes in future projections.

Description: There is currently no consensus on how humans are affecting the marine nitrogen (N) cycle, which limits marine biological production and CO 2 uptake. Anthropogenic changes in ocean warming, deoxygenation, and atmospheric N deposition can all individually affect the marine N cycle and the oceanic production of the greenhouse gas nitrous oxide (N 2 O). However, the combined effect of these perturbations on marine N cycling, ocean productivity, and marine N 2 O production is poorly understood. Here we use an Earth system model of intermediate complexity to investigate the combined effects of estimated 21st century CO 2 atmospheric forcing and atmospheric N deposition. Our simulations suggest that anthropogenic perturbations cause only a small imbalance to the N cycle relative to preindustrial conditions (∼+5 Tg N y −1 in 2100). More N-loss from water-column denitrification in expanded oxygen minimum zones (OMZs) is counteracted by less benthic denitrification, due to the stratification-induced reduction in organic matter export. The larger atmospheric N load is offset by reduced N inputs by marine N 2 fixation. Our model predicts a decline in oceanic N 2 O emissions by 2100. This is induced by the decrease in organic matter export and associated N 2 O production, and by the anthropogenically-driven changes in ocean circulation and atmospheric N 2 O concentrations. After comprehensively accounting for a series of complex physical-biogeochemical interactions, this study suggests that N flux imbalances are limited by biogeochemical feedbacks that help stabilize the marine N inventory against anthropogenic changes. These findings support the hypothesis that strong negative feedbacks regulate the marine N inventory on centennial timescales.

Description: We present a theoretical model to investigate the potential of 13 CH 3 D and 12 CH 2 D 2 , the doubly substituted mass-18 isotopologues of methane, as tools for tracking atmospheric methane sources and sinks. We use electronic structure methods to estimate kinetic isotope fractionations associated with the major sink reactions of methane in air (reactions with OH and Cl radicals), and combine literature data with reconnaissance measurements of the relative abundances of 13 CH 3 D and 12 CH 2 D 2 to estimate the compositions of the largest atmospheric sources. This model atmospheric budget is investigated with a simplified box model in which we explore both steady state and dynamical (non-steady state) conditions triggered by changes in emission or sink fluxes. The steady-state model predicts that sink reactions will generate a marked (〉100‰) clumped isotope excess in atmospheric Δ 12 CH 2 D 2 relative to the net source composition. 12 CH 2 D 2 measurements may thus be useful for tracing both atmospheric source and sink fluxes. The effect of sinks on Δ 13 CH 3 D is much less pronounced, indicating that 13 CH 3 D in air will give a more focused picture of the source composition.

Description: Riverine ecosystems receive organic matter (OM) from terrestrial sources, internally produce new OM, and biogeochemically cycle and modify organic and inorganic carbon. Major gaps remain in the understanding of the relationships between carbon sources and processing in river systems. Here we synthesize isotopic, elemental, and molecular properties of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the Upper Mississippi River (UMR) system above Wabasha, MN, including the main stem Mississippi River and its four major tributaries (Minnesota, upper Mississippi, St. Croix, and Chippewa Rivers). Our goal was to elucidate how biological processing modifies the chemical and isotopic composition of aquatic carbon pools during transport downstream in a large river system with natural and man-made impoundments. Relationships between land cover and DOC carbon-isotope composition, absorbance, and hydrophobic acid content indicate that DOC retains terrestrial carbon source information, while the terrestrial POC signal is largely replaced by autochthonous organic matter, and DIC integrates the influence of in-stream photosynthesis and respiration of organic matter. The UMR is slightly heterotrophic throughout the year, but pools formed by low-head navigation dams and natural impoundments promote a shift towards autotrophic conditions, altering aquatic ecosystem dynamics and POC and DIC composition. Such changes likely occur in all major river systems affected by low-head dams and need to be incorporated into our understanding of inland water carbon dynamics and processes controlling CO 2 emissions from rivers, as new navigation and flood control systems are planned for future river and water resources management.

Description: Perfluorooctane sulfonate (PFOS) is an aliphatic fluorinated compound with eight carbon atoms that is extremely persistent in the environment and can adversely affect human and ecological health. The stability, low reactivity, and high water solubility of PFOS combined with the North American phase-out in production around the year 2000, make it a potentially useful new tracer for ocean circulation. Here we characterize processes affecting the lifetime and accumulation of PFOS in the North Atlantic Ocean and transport to sensitive Arctic regions by developing a 3-D simulation within the MITgcm. The model captures variability in measurements across biogeographical provinces (R 2 = 0.90, p =0.01). In 2015, the North Atlantic PFOS reservoir was equivalent to 60% of cumulative inputs from the North American and European continents (1400 Mg). Cumulative inputs to the Arctic accounted for 30% of continental discharges, while the remaining 10% was transported to the tropical Atlantic and other regions. PFOS concentrations declined rapidly after 2002 in the surface mixed-layer (half-life: 1-2 years) but are still increasing below 1000 m depth. During peak production years (1980-2000), plumes of PFOS enriched seawater were transported to the Subarctic in energetic surface ocean currents. However, Atlantic Meridional Overturning Circulation (AMOC) and deep ocean transport returned a substantial fraction of this northward transport (20%, 530 Mg) to southern latitudes and reduced cumulative inputs to the Arctic (730 Mg) by 70%. Weakened AMOC due to climate change is thus likely to increase the magnitude of persistent bioaccumulative pollutants entering the Arctic Ocean.

Description: Terrestrial ecosystem responses to temperature and precipitation have major implications for the global carbon cycle. Case studies demonstrate that complex terrain, which accounts for more than 50% of Earth's land surface, can affect ecological processes associated with land-atmosphere carbon fluxes. However, no studies have addressed the role of complex terrain in mediating ecophysiological responses of land-atmosphere carbon fluxes to climate variables. We synthesized data from AmeriFlux towers and found that for sites in complex terrain, responses of ecosystem CO 2 fluxes to temperature and precipitation are organized according to terrain slope and drainage area, variables associated with water and energy availability. Specifically, we found that for tower sites in complex terrain, mean topographic slope and drainage area surrounding the tower explained between 51% and 78% of site-to-site variation in the response of CO 2 fluxes to temperature and precipitation depending on the time scale. We found no such organization among sites in flat terrain, even though their flux responses exhibited similar ranges. These results challenge prevailing conceptual framework in terrestrial ecosystem modeling that assumes CO 2 fluxes derive from vertical soil-plant-climate interactions. We conclude that the terrain in which ecosystems are situated can also have important influences on CO 2 responses to temperature and precipitation. This work has implications for about 14% of the total land area of the conterminous US. This area is considered topographically complex and contributes to approximately 15% of gross ecosystem carbon production in the conterminous US.

Description: No abstract is available for this article.

Description: In this pilot study we link the yield of industrial fisheries to changes in the zooplankton mortality in an idealised way accounting for different target species (planktivorous fish - decreased zooplankton mortality; large predators - increased zooplankton mortality). This indirect approach is used in a global coupled biogeochemistry-circulation model to estimate the range of the potential impact of industrial fisheries on marine biogeochemistry. The simulated globally integrated response on phytoplankton and primary production is in line with expectations - a high (low) zooplankton mortality results in a decrease (increase) of zooplankton and an increase (decrease) of phytoplankton. In contrast, the local response of zooplankton and phytoplankton depends on the region under consideration: In nutrient-limited regions, an increase (decrease) in zooplankton mortality leads to a decrease (increase) in both zooplankton and phytoplankton biomass. In contrast, in nutrient-replete regions, such as upwelling regions, we find an opposing response: an increase (decrease) of the zooplankton mortality leads to an increase (decrease) in both zooplankton and phytoplankton biomass. The results are further evaluated by relating the potential fisheries-induced changes in zooplankton mortality to those driven by CO 2 emissions in a business-as-usual 21 st century emission scenario. In our idealised case, the potential fisheries-induced impact can be of similar size as warming-induced changes in marine biogeochemistry.

Description: The concentration of dissolved oxygen (O 2 ) plays fundamental roles in diverse chemical and biological processes throughout the oceans. The balance between the physical supply and the biological consumption controls the O 2 level of the interior ocean, and the O 2 supply to the deep waters can only occur through deep convection in the polar oceans. We develop a theoretical framework describing the oceanic O 2 uptake during open-ocean deep convection events and test it against a suite of numerical sensitivity experiments. Our framework allows for two predictions, confirmed by the numerical simulations. First, both the duration and the intensity of the winter-time cooling contribute to the total O 2 uptake for a given buoyancy loss. Stronger cooling leads to deeper convection and the oxygenation can reach down to deeper depths. Longer duration of the cooling period increases the total amount of O 2 uptake over the convective season. Second, the bubble-mediated influx of O 2 tends to weaken the diffusive influx by shifting the air-sea disequilibrium of O 2 towards supersaturation. The degree of compensation between the diffusive and bubble-mediated gas exchange depends on the dimensionless number measuring the relative strength of oceanic vertical mixing and the gas transfer velocity. Strong convective mixing, which may occur under strong cooling, reduces the degree of compensation so that the two components of gas exchange together drive exceptionally strong oceanic O 2 uptake.

Description: The relationship between dissolved cadmium (Cd) and phosphate (PO 4 -3 ) can elucidate a biological role for Cd in the ocean, and help to evaluate the usefulness of Cd as a tracer of past ocean circulation and nutrient distributions. Here we determine and analyze this relationship in the poorly-studied region of the tropical South Pacific. The dissolved Cd distribution is generally similar to PO 4 3- , but a plot of Cd vs. PO 4 3- shows a clear concavity resulting from distinct Cd:PO 4 3- ratios in waters local to our transect, and in waters formed distally in higher latitudes. To determine the factors affecting the subsurface Cd:PO 4 3- ratio along our transect, we used an ocean circulation model and a multi-linear regression model to determine the preformed and regenerated components of dissolved Cd and PO 4 3- . We found that both the preformed and regenerated Cd:PO 4 3- ratios are low in the shallow, locally-formed water masses along the transect, and significantly higher in the deeper and older water-masses. Overall, the regenerated:preformed Cd:PO 4 3- ratio in the deep waters (〉1000 m) along our transect is ~1.8:1, reflecting the basin-wide average Cd:PO 4 3- “fractionation factor” during biological uptake. However, we find a lower fractionation factor in local waters of 1.1 (± 0.6). We suggest that this locally lower biological fractionation factor is due to either the chemical speciation of Cd, or to a lower efficiency of Cd assimilation by the pico- and nano-plankton species found in our study region.

Description: Lithogenic material deposited as dust is one of the major sources of trace metals to the ocean, particularly in the tropical and subtropical Atlantic. On the other hand, it can also act as a scavenging surface for iron. Here we studied this double role of lithogenic material in the marine iron cycle by adding a new scheme for describing particle dynamics into a global biogeochemistry and ecosystem model including particle aggregation and disaggregation of two particle size classes as well as scavenging on both organic and lithogenic particles. Considering the additional scavenging of iron on lithogenic particles, the modelled dissolved iron concentration is reduced significantly in the tropical and subtropical Atlantic, bringing the model much closer to observations. This underlines the necessity to consider the double role of dust particles as iron source and sink in studies on the marine iron cycle in high dust regions and with changing dust fluxes.