Ice-covered Regions Research Articles

Measurements of wave damping by a grease ice slick in Svalbard using off-the-shelf sensors and open-source electronics

ABSTRACTVersatile instruments assembled from off-the-shelf sensors and open-source electronics are used to record wave propagation and damping measured by Inertial Motion Units (IMUs) in a grease ice slick near the shore in Adventfjorden, Svalbard. Viscous attenuation of waves due to the grease ice slick is clearly visible by comparing the IMU data recorded by the different instruments. The frequency dependent spatial damping of the waves is computed by comparing the power spectral density obtained from the different IMUs. We model wave attenuation using the one-layer model of Weber from 1987. The best-fit value for the effective viscosity is ν = (0.95 ± 0.05 × 10−2)m2 s−1, and the coefficient of determination is R2 = 0.89. The mean absolute error and RMSE of the damping coefficient are 0.037 and 0.044m−1, respectively. These results provide continued support for improving instrument design for recording wave propagation in ice-covered regions, which is necessary to this area of research as many authors have underlined the need for more field data.

Monitoring Deep Convection and Convective Overshooting From 60° S to 60° N Using MHS: A Cloudsat/CALIPSO-Based Assessment

Spaceborne passive microwave sounders allow to detect convective storms thanks to their sensitivity to ice cloud particles. However, a worldwide assessment of the conditions under which these instruments can detect storms as well as a characterization of the microphysics of the detected storms is still missing. In this letter, we used ten-year measurements from Cloudsat radar and CALIPSO lidar to assess and characterize two convection diagnostics, namely, deep convection (DC) and convective overshooting (COV), derived from microwave humidity sounder measurements. When snow and sea ice-covered regions, such as Siberia and elevated regions (>1800 m) are discarded, DC and COV are associated with convective clouds, as identified by Cloudsat, more than 90% of time. COV reaches the Tropopause in 51% of cases. Results also show that ice water content (IWC) profiles peak higher for COV (9 km) than DC (7 km), with a heavier average ice loading for DC (0.9 g/m 3 ) than for COV (0.8 g/m 3 ). Maximal altitude reached by ice clouds is higher in the tropics than in the midlatitudes (16.1 km against 12.7 km in average for COV events), while average IWC is slightly higher in the tropics (0.21 g/m 3 against 0.18 g/m 3 for DC events). This evaluation and characterization open the doors to the development of a unique 60° S/60° N, 1999-present database of DC and COV using spaceborne passive microwave sounders.

Numerical investigation of the Arctic ice–ocean boundary layer and implications for air–sea gas fluxes

Abstract. In ice-covered regions it is challenging to determine constituent budgets – for heat and momentum, but also for biologically and climatically active gases like carbon dioxide and methane. The harsh environment and relative data scarcity make it difficult to characterize even the physical properties of the ocean surface. Here, we sought to evaluate if numerical model output helps us to better estimate the physical forcing that drives the air–sea gas exchange rate (k) in sea ice zones. We used the budget of radioactive 222Rn in the mixed layer to illustrate the effect that sea ice forcing has on gas budgets and air–sea gas exchange. Appropriate constraint of the 222Rn budget requires estimates of sea ice velocity, concentration, mixed-layer depth, and water velocities, as well as their evolution in time and space along the Lagrangian drift track of a mixed-layer water parcel. We used 36, 9 and 2 km horizontal resolution of regional Massachusetts Institute of Technology general circulation model (MITgcm) configuration with fine vertical spacing to evaluate the capability of the model to reproduce these parameters. We then compared the model results to existing field data including satellite, moorings and ice-tethered profilers. We found that mode sea ice coverage agrees with satellite-derived observation 88 to 98 % of the time when averaged over the Beaufort Gyre, and model sea ice speeds have 82 % correlation with observations. The model demonstrated the capacity to capture the broad trends in the mixed layer, although with a significant bias. Model water velocities showed only 29 % correlation with point-wise in situ data. This correlation remained low in all three model resolution simulations and we argued that is largely due to the quality of the input atmospheric forcing. Overall, we found that even the coarse-resolution model can make a modest contribution to gas exchange parameterization, by resolving the time variation of parameters that drive the 222Rn budget, including rate of mixed-layer change and sea ice forcings.

Measuring ocean waves in sea ice using SAR imagery: A quasi-deterministic approach evaluated with Sentinel-1 and in situ data

Measurements of wave heights in marginal ice zones are limited to very few in situ data. Here we revisit the linear and quasilinear theories of Synthetic Aperture Radar imaging of waves in the particular case of waves in sea ice. Instead of only working with spectra, we have developed an iterative nonlinear algorithm to estimate phase-resolved deterministic maps of wave-induced orbital velocities, from which elevation spectra can be derived. Application of this algorithm to Sentinel 1A wave mode images in the Southern Ocean shows that it produces reasonable results for swells in all directions except when they propagate at a few degrees off the range direction. The estimate of wave parameters is expected to work best when the shortest wave components, those which cause a pixel displacement of the order of the dominant wavelength in azimuth, can be neglected. Otherwise short waves produce a blurring of the image, increasing exponentially with the azimuthal wavenumber and reducing the estimated wave amplitude. Given the expected spatial attenuation of waves in ice-covered regions, our deterministic method should apply beyond a few tens of kilometers in the ice, without any correction for short wave effects. In situ data collected around the ice edge as part of the 2015 SeaState DRI cruise in the Beaufort confirm the progressive image blurring caused by such short waves, and the apparent reduction in the wave modulation. When short waves propagate from the open ocean towards the ice, this blurring can produce an unrealistic apparent increase of wave height, from the open ocean up to a few tens of kilometers inside the ice.

Ocean acoustic tomography: Fortieth anniversary, 1976–2016

The year 1976 saw the publication of “Monitoring the ocean acoustically” by Munk and Worcester [Science, Technology, and The Modern Navy, Thirtieth Anniversary 1946-1976, 497-508] and arguably the first ocean acoustic tomography experiment. The field got off to a stormy start when a reviewer of an early proposal wrote that “travel times along ray paths are meaningless in a saturated environment,” but a 1978 experiment showed that ray arrivals could be “resolved, identified, and tracked” at ~900 km range, just in time to save the field from an early demise. The need to measure and predict travel times with millisecond accuracy pushed the field of underwater acoustics to develop the requisite instrumentation and mathematical tools to accurately characterize the impulse response of the ocean. Important contributions to our understanding of the oceans include (1) the demonstration that internal tides are coherently generated by surface tides over the Hawaiian submarine ridge (and other similar benthic features) and can be traced for thousands of km to distant regions of dissipation, (2) measurements of oceanic deep convection, and (3) the discovery using trans-Arctic transmissions that the heat content in the Arctic was increasing. In the future, ocean acoustic tomography will likely be one component of multipurpose acoustic systems for acoustic remote sensing, navigation, communications, and passive listening, with special applicability in ice-covered regions.

In-situ measured primary productivity of ice algae in Arctic sea ice floes using a new incubation method

Recent changes in climate and environmental conditions have had great negative effects such as decreasing sea ice thickness and the extent of Arctic sea ice floes that support ice-related organisms. However, limited field observations hinder the understanding of the impacts of the current changes in the previously ice-covered regions on sea ice algae and other ice-related ecosystems. Our main objective in this study was to measure recent primary production of ice algae and their relative contribution to total primary production (ice plus pelagic primary production). In-situ primary productivity experiments with a new incubation system for ice algae were conducted in 3 sea ice cores at 2 different ice camps in the northern Chukchi Sea, 2014, using a 13C and 15N isotope tracer technique. A new incubation system was tested for conducting primary productivity experiments on ice algae that has several advantages over previous incubation methods, enabling stable carbon and nitrogen uptake experiments on ice algae under more natural environmental conditions. The vertical C-shaped distributions of the ice algal chl-a, with elevated concentrations at the top and bottom of the sea ice were observed in all cores, which is unusual for Arctic sea ice. The mean chl-a concentration (0.05 ± 0.03 mg chl-a m−3) and the daily carbon uptake rates (ranging from 0.55 to 2.23 mg C m−2 d−1) for the ice algae were much lower in this study than in previous studies in the Arctic Ocean. This is likely because of the late sampling periods and thus the substantial melting occurring. Ice algae contributed 1.5–5.7% of the total particulate organic carbon (POC) contents of the combined euphotic water columns and sea ice floes. In comparison, ice algae contributed 4.8–8.6% to the total primary production which is greater than previously reported in the Arctic Ocean. If all of the ice-associated productions were included, the contributions of the sea ice floes to the total primary production would be greater in the Arctic Ocean and their importance would be greater in the arctic marine ecosystems.

Evaluation of satellite-based algorithms to estimate photosynthetically available radiation (PAR) reaching the ocean surface at high northern latitudes

Abstract Two satellite-based methods to estimate daily averaged photosynthetically available radiation (PAR) at the ocean surface are evaluated at high northern latitudes. The first method employs a precomputed Look-Up-Table (LUT) generated from radiative transfer simulations. The LUT associates spectral irradiance reaching the surface to a given set of input parameters, namely solar zenith angle, cloud optical thickness, cloud fraction, ozone concentration, and surface albedo. The second method, as implemented by NASA's Ocean Biology Processing Group (OBPG) in the standard Ocean Color data processing chain, expresses the energy budget of the atmosphere-surface-ocean system via a simple radiative transfer model. The performance of these methods is evaluated using an extensive in situ PAR dataset collected in the Arctic Ocean between 1998 and 2014, with daily values ranging from 0.08 to 61.07 Em − 2 d − 1 . A methodology is developed to compare in situ measurements and satellite products of different spatial and temporal resolution. Agreement is generally good between satellite-derived estimates and ship-based data and between methods. Specifically, both methods yield a small positive bias of 6% and 2% and a relative uncertainty larger than that observed at low latitude, with a root mean squared error (RMSE) of 33% and 20% for the LUT and OPBG methods, respectively. This is attributed to the peculiar environmental conditions encountered in the Arctic, namely low solar elevation, changing surface albedo due to sea ice, and persistent cloudiness. The RMSE difference among methods is due to the high temporal resolution (3 h) of the International Satellite Cloud Climatology Project (ISCCP) LUT input not fully compensating for its low spatial resolution (280 km grid cells). The LUT method has the major advantage of providing PAR estimates in all conditions, including ice-covered regions, while the OBPG method is currently limited to open waters and a solar zenith angle lower than 83°. Consequently, the OBPG method may not account for as much as 38% of PAR reaching the Arctic ocean surface annually. Both methods have the potential to provide useful PAR estimates just below the ice, by including information about ice transmittance.

Gas exchange of CO2 and O2 in partially ice-covered regions of the Arctic Ocean investigated using in situ sensors

Sea surface CO2 dynamics are not well characterized in the Arctic Ocean (AO). Most data are from ship-based studies during the low-ice period (May-September) and obtained from near shore areas because of accessibility. More CO2 data are needed to improve models for predicting the future of the carbon cycle in the region and its relationship to ocean acidification. Air-sea gas exchange rates are complicated by the presence of ice. Consequently gas exchange rates have a larger uncertainty in the AO compared to other ocean regions. To provide more information about CO2 dynamics and gas exchange in the AO, in situ time-series data have been collected from the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2 (pCO2), dissolved O2 (DO) concentration, temperature, and salinity were measured at ∼6-m depth under little ice and multi-year ice on two ice-tethered profilers (ITPs) for 40-50 days. The pCO2 levels were always below atmospheric saturation whereas DO was almost always slightly above saturation. Although the two ITPs were on an average only 222 km apart, one was further south and had 14 ± 12% ice cover; whereas the more northern ITP had 63 ± 16% ice cover. Consequently the two data sets differed significantly in external forcings. Modeled variability of CO2 and DO estimate that gas exchange would significantly alter sea surface pCO2 in the low ice cover record but minimally in the more extensively ice-covered region. If these conditions extended over the entire AO, the total uptake of atmospheric CO2 would be 28.6 Tg C yr-1 and 15.4 Tg C yr-1 under low and high ice-covered conditions, respectively.

Patterns and drivers of megabenthic secondary production on the Barents Sea shelf

Megabenthos plays a major role in the overall energy flow on Arctic shelves, but information on megabenthic secondary production on large spatial scales is scarce. Here, we estimated for the first time megabenthic secondary production for the entire Bar- ents Sea shelf by applying a species-based empirical model to an extensive dataset from the joint Norwe- gianRussian ecosystem survey. Spatial patterns and relationships were analyzed within a GIS. The environ- mental drivers behind the observed production pattern were identified by applying an ordinary least squares regression model. Geographically weighted regression (GWR) was used to examine the varying relationship of secondary production and the environment on a shelf- wide scale. Significantly higher megabenthic second- ary production was found in the northeastern, season- ally ice-covered regions of the Barents Sea than in the permanently ice-free southwest. The environmental parameters that significantly relate to the observed pat- tern are bottom temperature and salinity, sea ice cover, new primary production, trawling pressure, and bottom current speed. The GWR proved to be a versatile tool for analyzing the regionally varying relationships of benthic secondary production and its environmental drivers (R 2 = 0.73). The observed pattern indicates tight pelagicbenthic coupling in the realm of the productive marginal ice zone. Ongoing decrease of winter sea ice extent and the associated poleward movement of the seasonal ice edge point towards a distinct decline of benthic secondary production in the northeastern Barents Sea in the future.

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica fromS-wave receiver functions and implications for heat flow

Crustal and upper-mantle structure beneath ice-covered regions in Antarctica from<i>S</i>-wave receiver functions and implications for heat flow

Ice-sheet configuration in the CMIP5/PMIP3 Last Glacial Maximum experiments

Abstract. We describe the creation of a data set describing changes related to the presence of ice sheets, including ice-sheet extent and height, ice-shelf extent, and the distribution and elevation of ice-free land at the Last Glacial Maximum (LGM), which were used in LGM experiments conducted as part of the fifth phase of the Coupled Modelling Intercomparison Project (CMIP5) and the third phase of the Palaeoclimate Modelling Intercomparison Project (PMIP3). The CMIP5/PMIP3 data sets were created from reconstructions made by three different groups, which were all obtained using a model-inversion approach but differ in the assumptions used in the modelling and in the type of data used as constraints. The ice-sheet extent in the Northern Hemisphere (NH) does not vary substantially between the three individual data sources. The difference in the topography of the NH ice sheets is also moderate, and smaller than the differences between these reconstructions (and the resultant composite reconstruction) and ice-sheet reconstructions used in previous generations of PMIP. Only two of the individual reconstructions provide information for Antarctica. The discrepancy between these two reconstructions is larger than the difference for the NH ice sheets, although still less than the difference between the composite reconstruction and previous PMIP ice-sheet reconstructions. Although largely confined to the ice-covered regions, differences between the climate response to the individual LGM reconstructions extend over the North Atlantic Ocean and Northern Hemisphere continents, partly through atmospheric stationary waves. Differences between the climate response to the CMIP5/PMIP3 composite and any individual ice-sheet reconstruction are smaller than those between the CMIP5/PMIP3 composite and the ice sheet used in the last phase of PMIP (PMIP2).

RRNA and rDNA based assessment of sea ice protist biodiversity from the central Arctic Ocean

Sea ice is a large and diverse ecosystem contributing significantly to primary production in ice-covered regions. In the Arctic Ocean, sea ice consists of mixed multi-year ice (MYI), often several metres thick, and thinner first-year ice (FYI). Current global warming is most severe in Arctic regions; as a consequence, summer sea ice cover is decreasing and MYI is disappearing at an alarming rate. Despite its apparent hostility, sea ice is inhabited by a diverse microbial community of bacteria and protists, many of which are photosynthetic. Here we present an assessment of eukaryotic biodiversity in MYI and FYI from the central Arctic Ocean using high-throughput 454 sequencing of 18S rRNA and rDNA amplicons. We compared the rDNA-based ‘total’ biodiversity with the ‘active’ biodiversity from rRNA amplicons and found differences between them including an over-representation of Ciliophora, Bicosoecida and Bacillariophyceae operational taxonomic units (OTUs) in the active part of the community. Differences between the two libraries are more pronounced at the lower taxonomic level: certain genera, such as Melosira, are more abundant in the rRNA library, indicating activity of these genera. Furthermore, we found that one FYI station showed a higher activity of potential grazers which was probably due to the advanced stage of melt evident by higher ice temperatures and highly porous ice compared with the other stations.

High geothermal heat flux measured below the West Antarctic Ice Sheet.

The geothermal heat flux is a critical thermal boundary condition that influences the melting, flow, and mass balance of ice sheets, but measurements of this parameter are difficult to make in ice-covered regions. We report the first direct measurement of geothermal heat flux into the base of the West Antarctic Ice Sheet (WAIS), below Subglacial Lake Whillans, determined from the thermal gradient and the thermal conductivity of sediment under the lake. The heat flux at this site is 285 ± 80 mW/m(2), significantly higher than the continental and regional averages estimated for this site using regional geophysical and glaciological models. Independent temperature measurements in the ice indicate an upward heat flux through the WAIS of 105 ± 13 mW/m(2). The difference between these heat flux values could contribute to basal melting and/or be advected from Subglacial Lake Whillans by flowing water. The high geothermal heat flux may help to explain why ice streams and subglacial lakes are so abundant and dynamic in this region.

Modeling ocean wave propagation under sea ice covers

Operational ocean wave models need to work globally, yet current ocean wave models can only treat ice-covered regions crudely. The purpose of this paper is to provide a brief overview of ice effects on wave propagation and different research methodology used in studying these effects. Based on its proximity to land or sea, sea ice can be classified as: landfast ice zone, shear zone, and the marginal ice zone. All ice covers attenuate wave energy. Only long swells can penetrate deep into an ice cover. Being closest to open water, wave propagation in the marginal ice zone is the most complex to model. The physical appearance of sea ice in the marginal ice zone varies. Grease ice, pancake ice, brash ice, floe aggregates, and continuous ice sheet may be found in this zone at different times and locations. These types of ice are formed under different thermal-mechanical forcing. There are three classic models that describe wave propagation through an idealized ice cover: mass loading, thin elastic plate, and viscous layer models. From physical arguments we may conjecture that mass loading model is suitable for disjoint aggregates of ice floes much smaller than the wavelength, thin elastic plate model is suitable for a continuous ice sheet, and the viscous layer model is suitable for grease ice. For different sea ice types we may need different wave ice interaction models. A recently proposed viscoelastic model is able to synthesize all three classic models into one. Under suitable limiting conditions it converges to the three previous models. The complete theoretical framework for evaluating wave propagation through various ice covers need to be implemented in the operational ocean wave models. In this review, we introduce the sea ice types, previous wave ice interaction models, wave attenuation mechanisms, the methods to calculate wave reflection and transmission between different ice covers, and the effect of ice floe breaking on shaping the sea ice morphology. Laboratory experiments, field measurements and numerical simulations supporting the fundamental research in wave-ice interaction models are discussed. We conclude with some outlook of future research needs in this field.

Terrain-aided Navigation for an Underwater Glider

A terrain-aided navigation method for an underwater glider is proposed that is suitable for use in ice-covered regions or areas with heavy ship traffic where the glider may not be able to surface for GPS location updates. The algorithm is based on a jittered bootstrap algorithm, which is a type of particle filter that makes use of the vehicle's dead-reckoned navigation solution, onboard altimeter, and a local digital elevation model (DEM). An evaluation is performed through postprocessing offline location estimates from field trials that took place in Holyrood Arm, Newfoundland, overlapping a previously collected DEM. During the postprocessing of these trials, the number of particles, jittering variance, and DEM grid cell size were varied, showing that convergence is maintained for 1,000 particles, a jittering variance of 15 , and a range of DEM grid cell sizes from the base size of 2 m up to 100 m. Using nominal values, the algorithm is shown to maintain bounded error location estimates with root-mean-square (RMS) errors of 33 and 50 m in two sets of trials. These errors are contrasted with dead-reckoned errors of 900 m and 5.5 km in those same trials. Online open-loop field trials were performed for which RMS errors of 76 and 32 m- were obtained during 2-h-long trials. The dead-reckoned error for these same trials was 190 and 90 m, respectively. The online open-loop trials validate the filter despite the large dead-reckoned errors, single-beam altitude measurements, and short test duration.

An Improved 20-Year Arctic Ocean Altimetric Sea Level Data Record

For ocean and climate research, it is essential to get long-term altimetric sea level data that is as accurate as possible. However, the accuracy of the altimetric data is frequently degraded in the interior of the Arctic Ocean due to the presence of seasonal or permanent sea ice. We have reprocessed ERS-1/2/Envisat satellite altimetry to develop an improved 20-year sea level dataset for the Arctic Ocean. We have developed both an along-track dataset and three-day gridded sea level anomaly (SLA) maps from September 1992 to April 2012. A major improvement in data coverage was gained by tailoring the standard altimetric editing criteria to Arctic conditions. The new reprocessed data has significant increased data coverage with between 4 and 10 times the amount of data in regions such as the Beaufort Gyre region compared with AVISO and RADS datasets. This allows for a more accurate estimation of sea level changes from satellite altimetry in the Arctic Ocean. The reprocessed dataset exhibit a mean sea level trend of 2.1 ± 1.3 mm/year (without Glacial Isostatic Adjustment correction) covering the Arctic Ocean between 66°N and 82°N with significant higher spatial coherency in the ice-covered regions than the RADS and DUACS datasets.

Physical and biological controls on oxygen saturation variability in the upper Arctic Ocean

Employing continuous in situ measurements of dissolved O2/Ar and O2 in the Arctic Ocean, we investigate the mechanisms controlling the physical (abiotic) and biological oxygen saturation state variability in the surface ocean beneath sea ice. O2/Ar measurements were made underway using Equilibrator Inlet Mass Spectrometry (EIMS) during an icebreaker survey transiting the upper Arctic Ocean across the North Pole in late summer 2011. Using concurrently collected measurements of total oxygen, we devolve biological oxygen saturation and physical oxygen (Ar) saturation signals at unprecedented horizontal resolution in the surface ocean. In the Nansen Basin, Ar is undersaturated up to −7% while biological oxygen supersaturation peaks at 18.4%. We attribute this to ice melt, Atlantic Water influence and/or cooling. In the Canadian Basin, Ar is supersaturated up to 3%, likely because of Ar injection from freezing processes and long residence times of gas under ice cover. The overall Canadian Basin to Eurasian Basin gradient of Ar supersaturation to undersaturation may reflect net freezing in the Canadian Basin and net melting in the Eurasian Basin over several seasons, either by Pacific to Atlantic sector ice transport or local changes over time. Ar saturation could thereby provide large-scale high-resolution estimates of current and future changes in these processes. O2/Ar supersaturation averages 4.9% with peaks up to 9.8% where first year ice and abundant melt ponds likely allow sufficient light for blooms in ice-covered regions.

Paleozoic siliciclastic rocks from northern Victoria Land (Antarctica): Provenance, timing of deformation, and implications for the Antarctica-Australia connection

Paleozoic sequences exposed along the Transantarctic Mountains in Antarctica and in southeastern Australia are segments of a formerly contiguous accretionary orogen that developed along the eastern margin of Gondwana. The margin underwent amalgamation and eastward accretion in the early Cambrian to Early Ordovician Ross-Delamerian orogen and in the Ordovician to Carboniferous Lachlan orogen. Northern Victoria Land plays a key role in many geodynamic reconstructions because it has long been considered the along-strike continuation of Australia in Antarctica; however, the correlation between lithotectonic units in Antarctica (Wilson, Bowers, and Robertson Bay terranes) and those in southeastern Australia (Glenelg, Grampians-Stavely, and Stawell zones), as well as the presence of Lachlan-aged tectono-metamorphic events in northern Victoria Land, are still uncertain. 40 Ar- 39 Ar laser experiments on detrital and syndeformational white micas from low-grade siliciclastic rocks of northern Victoria Land, in conjunction with mineral-textural analysis and whole-rock geochemical and Nd isotope data, are used to constrain provenance and the timing of deformation, and to assess analogies with correlative structural zones in southeastern Australia. Detrital white micas of the western lithotectonic unit (Wilson terrane) yielded an age pattern dominated by late Cryogenian to Ediacaran ages (650–550 Ma), closely matching those of turbidites from the Australian Kanmantoo Group. Detrital white micas from the easternmost lithotectonic units (Bowers terrane and Robertson Bay terrane) yield indistinguishable age patterns, strikingly in agreement with those available for the western subprovince of the Lachlan orogen in Australia, which are dominated instead by younger ages with a dominant Ross orogen fingerprint (550–480 Ma). Deposition of siliciclastic detritus in the three lithotectonic units most likely occurred synchronously in the early–middle Cambrian, and the different signatures suggest that detritus was supplied from different source areas, with sediment supply from the west-southwest (East African orogen or ice-covered regions of East Antarctica) in the Wilson terrane and from the south (Ross orogen) in the Bowers and Robertson Bay terranes. Results also provide evidence for post-Ross orogen (ca. 462 Ma) contractional tectonics at the boundary between the Bowers and the Robertson Bay terranes, suggesting that available Ar data from the literature are variably affected by the presence of detrital micas. This finding establishes a cause-effect relationship between compressional tectonics at the plate margin and Middle–Late Ordovician intraplate reactivation processes in the western Wilson terrane. The accretion of northern Victoria Land was polyphase; it began with the amalgamation of the Bowers and Wilson terranes in the middle–late Cambrian and was followed by the docking of the Robertson Bay and Bowers terranes in the Middle–Late Ordovician. Results further support the link between northern Victoria Land and southeastern Australia. Together with a careful examination of data from the literature, they also suggest that the Bowers terrane and the across-strike contiguous Robertson Bay terrane are correlatives of the whole Stawell zone.

The influence of sea ice cover on air-sea gas exchange estimated with radon-222 profiles

Air-sea gas exchange plays a key role in the cycling of greenhouse and other biogeochemically important gases. Although air-sea gas transfer is expected to change as a consequence of the rapid decline in summer Arctic sea ice cover, little is known about the effect of sea ice cover on gas exchange fluxes, especially in the marginal ice zone. During the Polarstern expedition ARK-XXVI/3 (TransArc, August/September 2011) to the central Arctic Ocean, we compared 222Rn/226Ra ratios in the upper 50 m of 14 ice-covered and 4 ice-free stations. At three of the ice-free stations, we find 222Rn-based gas transfer coefficients in good agreement with expectation based on published relationships between gas transfer and wind speed over open water when accounting for wind history from wind reanalysis data. We hypothesize that the low gas transfer rate at the fourth station results from reduced fetch due to the proximity of the ice edge, or lateral exchange across the front at the ice edge by restratification. No significant radon deficit could be observed at the ice-covered stations. At these stations, the average gas transfer velocity was less than 0.1 m/d (97.5% confidence), compared to 0.5–2.2 m/d expected for open water. Our results show that air-sea gas exchange in an ice-covered ocean is reduced by at least an order of magnitude compared to open water. In contrast to previous studies, we show that in partially ice-covered regions, gas exchange is lower than expected based on a linear scaling to percent ice cover.

Role of environmental factors on phytoplankton bloom initiation under landfast sea ice in Resolute Passage, Canada

It has been common practice in scientific studies to assume negligible phytoplankton production when the ocean is ice-covered, due to the strong light attenuation properties of snow, sea ice, and ice algae. Recent observations of massive under-ice blooms in the Arctic challenge this concept and call for a re-evaluation of light conditions prevailing under ice during the melt period. Using hydrographic data collected under landfast ice cover in Resolute Passage, Nunavut, Canada between 9 May and 21 June 2010, we documented the exponential growth phase of a substantial under-ice phytoplankton bloom. Numerous factors appeared to influence bloom initi- ation: (1) transmitted light increased with the onset of snowmelt and termination of the ice algal bloom; (2) initial phytoplankton growth resulted in the accumulation of biomass below the devel- oping surface melt layer where nutrient concentrations were high and turbulent mixing was rela- tively low; and (3) melt pond formation rapidly increased light transmission, while spring-tidal energy helped form a surface mixed layer influenced by ice melt — both were believed to influ- ence the final rapid increase in phytoplankton growth. By the end of the study, nitrate+nitrite was depleted in the upper 10 m of the water column and the under-ice bloom had accumulated 508 mg chl a m �2 with a new production estimate of 17.5 g C m �2 over the upper 50 m of the water column. The timing of bloom initiation with melt onset suggests a strong link to climate change where sea ice is both thinning and melting earlier, the implication being an earlier and more ubiquitous phytoplankton bloom in Arctic ice-covered regions.