Field investigations of the properties of heavily melted “rotten” Arctic
sea ice were carried out on shorefast and drifting ice off the coast of
Utqiaġvik (formerly Barrow), Alaska, during the melt season. While no
formal criteria exist to qualify when ice becomes rotten, the objective
of this study was to sample melting ice at the point at which its structural and
optical properties are sufficiently advanced beyond the peak of the summer
season. Baseline data on the physical (temperature, salinity, density,
microstructure) and optical (light scattering) properties of shorefast ice
were recorded in May and June 2015. In July of both 2015 and 2017, small
boats were used to access drifting rotten ice within
The seasonal evolution of Arctic sea ice follows a fairly
predictable annual pattern: winter, snowmelt, pond formation, pond drainage,
rotten ice (De Abreu et al., 2001). Considerable attention has been given to
characterization of these various states and their transitions. In
situ observations during the summer melt season are typically
straightforward through the pond drainage stage, but, as ice conditions
deteriorate, it becomes increasingly difficult to work on or around the most
fragile state, rotten ice. During the summer of 1894, Nansen, in his seminal
work
The relatively high temperatures and abundant sunlight of summer cause sea ice to “rot”. While the microstructure of winter ice is characterized by small, isolated brine inclusions, with brine convection restricted to the lower reaches of the ice, and spring ice is characterized by increased permeability and brine convection through the full depth of the ice cover (Jardon et al., 2013; Zhou et al., 2013), the defining characteristics of rotten ice may be its high porosity and enhanced permeability. Warming causes changes in the ice structure including enlarged and merged brine and gas inclusions (see, e.g., Weeks and Ackley, 1986; Light et al., 2003a). Columnar ice permeability increases drastically for fluid transport when the brine volume fraction exceeds approximately 5 % (Golden et al., 2007; Pringle et al., 2009). In a previous study on shorefast ice, brine volume fractions were found to exceed this 5 % threshold for permeability through the entire depth of the ice from early May onwards (Zhou et al., 2013). While the term “rotten ice” is used in this paper to refer to heavily melted summer ice that has diminished structural integrity, has relatively large voids, and is highly permeable, it is also noted that this work is intended to provide a more refined and quantitative definition of this ice type.
Connectivity of the pore space in sea ice is known to contribute to ocean–atmosphere heat transfer (Eicken et al., 2002; Hudier et al., 1995; Lytle and Ackley, 1996; Weeks, 1998; Weeks and Ackley, 1986) and exchange of dissolved and particulate matter (Freitag, 1999; Krembs et al., 2000) including nutrients (Fritsen et al., 1994), salinity evolution of the ice cover (Untersteiner, 1968; Vancoppenolle et al., 2007; Wettlaufer et al., 2000), and surface melt pond distribution (Eicken et al., 2002). As a result of this notable connectivity, rotten ice also has reduced structural integrity, which can have implications for ice dynamics. Though it is known to have diminished tensile and flexural strength (Richter-Menge and Jones, 1993; Timco and Johnston, 2002; Timco and O.'Brien, 1994), such details have not been well-characterized. Measurements by Timco and Johnston (2002) demonstrated that in mid-May, the ice had about 70 % of its midwinter strength. By early June, the ice had about 50 % and by the end of June 15 %–20 % of its midwinter strength. The ice strength during July was only about 10 % of the midwinter strength. Such changes in strength may be relevant to the late summer behavior of Arctic ice-obligate megafauna. With increasing melt season length (Stroeve et al., 2014), the future could bring increasing areas of rotten ice. Because it represents the very end of summer melt, its presence matters for the longevity of the ice cover. If the ice melts completely, then the open ocean will form new ice in the autumn. Only ice remaining at the end of summer can become second-year and subsequently multiyear ice.
For rotten ice, permeability is typically large enough to render the ice cover in connection with the ocean throughout its depth. As a result, rotten ice may have a very different biogeochemical environment for sea ice microbial communities than ice with connectivity properties typical of winter, spring, or even early summer to midsummer. Increases in ice permeability result in an increase in the flow rate of surface meltwater that can penetrate through a melting ice cover, both from the top of the ice downwards (e.g., Untersteiner, 1968) and from the bottom of the ice upwards (e.g., Eicken et al., 2002; Jardon et al., 2013). The convective overturning of meltwater pooled beneath the ice can contribute significantly to enlargement of pores and internal melt. In fact, during the Surface Heat Budget of the Arctic Ocean (SHEBA) field campaign, Eicken et al. (2002) noted that high advective heat fluxes into the permeable ice found on melt pond bottoms and first-year ice likely contributed to the breakup and disintegration of the ice cover toward the end of the melt season.
To address questions about the physical characteristics of rotten sea ice, a
targeted field study was carried out at Utqiaġvik (formerly Barrow; lat
71.2906, long
Sea ice samples and field measurements were collected from locations near the northern coast of Alaska (Figs. 1–2, Table 1). Samples were collected at three different time points to help define the progression of melt: May to collect baseline data on the ice properties, June to observe its progression, and July to capture rotten ice (Fig. 3).
Locations sampled and cores collected are summarized in Table 1. All ice
cores were drilled using a 9 cm diameter Kovacs Mark II corer (Kovacs
Enterprise, Roseburg, Oregon, USA) through the full depth of the ice.
Extracted cores were photographed and either bagged whole or as 20 cm
subsections for subsequent laboratory analysis. At each sampling site, a
single core was used for temperature and density profiles. Bagged cores were
stored up to several hours in insulated coolers for transport back to the
Barrow Arctic Research Center (BARC) laboratory and immediately placed in
one of several walk-in freezers set to
Map of sea ice sample collection sites.
The first set of samples was collected on 6 May 2015 from landfast,
first-year, snow-covered congelation sea ice in a region of undeformed ice 2 km southwest of Point Barrow and
Sea ice in the vicinity of Utqiaġvik, Alaska, USA, during
summer melt.
The second set of samples was collected on 3 June 2015 from within 30 m of
the site sampled in May (Fig. 1b). The ice had begun to form melt ponds
(Fig. 2b), which we avoided during sampling. The June ice had a thickness
ranging from 149 to 159 cm, with
In 2015, the landfast ice broke away from the local coastline during the third week of June (Fig. 1f). July samples were drilled from isolated floes accessed by small boats within a radius of 32 km from Point Barrow (Fig. 1c). Floes in July varied greatly in size, thickness, and character.
On 3 July 2015, the sea was ice free within an
Cores collected on 10 July 2015 (JY10) came from a sediment-rich,
heavily ponded floe with an ice thickness measured in a non-ponded part of
the floe of 190 cm. Ice in non-ponded areas was solid and saline, similar to
what was observed in June and on 3 July. Cores from ponded areas of the floe
(collected from ponds
On 11 July 2015, additional cores (64–90 cm length) were sampled from a
ponded area of a clean (sediment-poor) floe of rotten ice (JY11). As with
the 10 July floe, ice in non-ponded areas was solid and saline, partially
drained but not heavily rotted. The upper portion of the ice was pitted and
drained. Ice beneath melt ponds (cores collected were submerged under 8–15 cm water) was heavily rotted and drained rapidly when cored. Ambient air
temperature during sampling was
The last cores sampled in 2015 were collected on 14 July from both ponded and non-ponded areas of two relatively thin, clean floes (JY14-L3 & JY14-L4). Ice collected in non-ponded areas ranged from 100 to 139 and 80–83 cm thick and was similar in character to the non-ponded ice of the other July floes. Ice collected in ponded areas (under 5 cm of water) ranged from 27 to 91 cm thick and was similar in character to the ice collected from beneath melt ponds in the other July floes.
In summer 2017, our team returned to the offshore waters near Utqiaġvik
in search of ice that had previously broken from the shore and was continuing to
melt (Fig. 3). Five distinct ice floes of varying degrees of melt were
sampled on 13 July 2017. Ice thicknesses ranged from 40 to 110 cm. Seawater in
open areas between floes had a salinity of 29.5 ppt and temperature of
Photographs of specific rotten ice sampling sites in July:
One core from each sampling site was used to measure vertical profiles of
temperature, density, salinity, and pH. Ice temperature was measured in the
field immediately following core removal. The core was placed on a PVC
cradle, and temperature was measured using a field temperature probe
(Traceable™ total-range thermometer, Fisher Scientific;
accuracy
Collected ice cores, sample locations, and in situ conditions.
Measured local conditions are ranges of hourly averages of meteorological data
measured between 10:00 and 18:00 local time from the NOAA Earth System Research
Laboratory Barrow Atmospheric Baseline Observatory (BRW) 8 km NE of
Utqiaġvik (lat 71.3230, long
Continued.
Representative horizontal and vertical sections were prepared from each
horizon of ice for each of the three time points sampled in 2015. Thin
sections (
To prepare samples for X-ray micro-computed tomography (micro-CT)
imaging, 10 cm subsections of ice cores returned from the field were stored
overnight in insulated coolers in a walk-in freezer set to working
temperatures. Subsections were then placed upright in Teflon centrifuge cups
(500 mL bottles with tops cut off) and spun out at
The masses of brine and spun-out ice were determined, and brines were saved for
later biological and chemical analysis. Spun-out ice horizons were returned
to the working-temperature walk-in freezer, where they were then placed
upright on top of corrugated cardboard circles placed inside the Teflon
centrifuge cups. Samples were casted with dimethyl phthalate (DMP) in an
attempt to minimize structural changes during transport, storage, and
processing and in order to use methods consistent with prior micro-CT work on
snow. Working temperature DMP was then carefully poured
down the sides of the container in order to flood the ice samples and form
casts of the brine networks in contact with the borders of the ice core as
described by Heggli et al. (2009) for casting snow. The DMP was
left to penetrate brine networks and slowly freeze at the working temperature
for at least 12 h before freezing fully at
Prepared samples were imaged at the U.S. Army Cold Regions Research and Engineering Laboratory using a micro-CT high-energy X-ray scanner
(SkyScan 1173, Bruker) housed in a
Shadow images generated by micro-CT were reconstructed into 2-D horizontal
slices using the software NRecon (Bruker). Thermal abnormalities were
corrected by performing
Reconstructed 8-bit 2-D images were analyzed using the software CTAn (Bruker).
Cylindrical subvolumes (height
CTAn was then used to calculate properties of the parsed phases, including 3-D volume, number of 3-D objects, closed and open porosity, and anisotropy. A description of the mathematical basis for these parameters as well as detailed best-practice methods for micro-CT imaging of sea ice can be found in Lieb-Lappen et al. (2017).
Further, 3-D prints of the reconstructed ice-only phase were made from the micro-CT reconstructions using polylactic acid fused deposition modeling (FlashForge Creator Pro, FDM print with MakerBot print program and layer height of 0.1 mm).
Field measurements of optical properties are generally limited to estimation of apparent optical properties (AOPs), e.g., albedo, transmittance, and extinction. Due to the tenuous working conditions on rotten sea ice floes and instrument reliability problems, we were not successful at obtaining estimates of in situ AOPs of rotten ice. Instead, we focused on assessing the optical properties of extracted ice samples in the laboratory. Inherent optical properties (IOPs), such as scattering and absorption coefficients and scattering phase functions, are intrinsically difficult to measure in multiple-scattering media, but estimates from laboratory measurements can build a picture of the evolution of sea ice optical properties. In fact, estimates of IOPs are particularly useful since they are independent of boundary conditions (e.g., ice thickness and floe size) and the magnitude, directionality, and spectral character of the incident light field (see, e.g., Katlein et al., 2014; Light et al., 2015).
The evolution of light-scattering coefficients for sea ice as it melts
determines the partitioning of solar radiation in the ice–ocean system.
Light et al. (2004) considered the evolution of the optical
properties of sea ice samples as they warmed in a laboratory setting, but
encountered practical limitations for handling small samples of ice with
large void space as the temperature approached 0
To track the evolution of how the ice in this study partitioned sunlight, a
laboratory optics study was carried out. Cores for optical property
assessment were sampled alongside cores for other characterizations,
returned to the lab, and stored intact at
To carry out optical property assessment, each core was cut into 10 cm long sections. Each section was placed in a chamber for the measurement of light transmittance using a technique developed to infer inherent scattering properties of a sea ice sample from a simple measurement and a corresponding model calculation (see Light et al., 2015). Figure 4 shows a schematic of this laboratory measurement, in which ice samples are placed in a dark housing and illuminated from above. Spectral light transmittance between the 400 and 1000 nm wavelengths of each subsample was recorded relative to the transmittance through pure liquid water. The relative transmittance was then compared with results from numerical radiative transport simulations using the model described by Light et al. (2003b) for a wide range of scattering coefficients. The scattering coefficient producing relative transmittance (at 550 nm) closest to the observed relative transmittance was then chosen. When subsamples from a full length of ice core are measured, this technique estimates the vertical profile of the light-scattering coefficient through the depth of the ice. By directly assessing scattering coefficient, an IOP, we avoid complications introduced by the interpretation of AOPs (e.g., albedo, total transmittance measured in situ), notably differences in total ice thickness and incident solar radiation conditions (e.g., diffuse or direct), as well as other physical boundary conditions. In each case, samples taken from ice sitting below freeboard were placed into the sample chamber and then gently flooded with a sodium chloride and water mixture in freezing equilibrium (temperature and salinity) with the sample. Light transmission was measured while the sample was flooded. Sample measurement was fast, with each sample in the chamber for less than 1 min. It is probable that the liquid did not completely fill the pore structure of the ice samples; however, the visible appearance of the samples indicated a dramatic reduction in backscatter during the flooding process, suggesting that flooding was effective.
Schematic depicting laboratory setup for measuring light transmittance through 10 cm tall ice core samples. Adapted from Light et al. (2015).
Samples were run in two modes. In the first mode, samples were analyzed
promptly after removal from the ice. These samples represent snapshots of
the rotting process as it occurs naturally. The second mode was run in an
attempt to use light-scattering measurements to inform our understanding of
ice rotting processes. To do this, an archived May core was cut into 10 cm
thick sections and placed in an insulated box in the freezer laboratory. The
sections were stored standing upright and were placed on a wire rack such
that the melt water drained away from the remaining sample material.
Initially, the freezer temperature was set to
Figure 5 shows photomosaics of the microstructure in representative cores collected at the different time points and from different rotten floes. The series shows the progression from recognizable congelation ice in May, to the development of a retextured snow layer in June, to the chaotic appearance of the ice structure in July.
In May, the interior of the ice was relatively translucent due to the small, isolated nature of brine and gas inclusions, a result of the still relatively low temperature of the ice. Obvious brighter white bands of concentrated bubbles were present within the ice. A weak layer was present in several cores between roughly 32 and 45 cm, which defined breaking points of the corresponding middle horizon samples. May cores also exhibited a brown discoloration in the ice proximal to the ice–ocean interface, which is indicative of algae; microscopy confirmed the presence of abundant pennate diatoms in ice bottom samples.
In June, the ice interior did not appear visibly distinct from May ice
except for the upper surface of the ice. Significant rains during the last
week of May fell on the snow-covered ice, saturated the snow, and refroze.
This produced a retextured snow layer that occupied the upper 20 cm of the
ice and was composed of grainy, bright ice with low structural integrity.
Ice below the retextured snow layer was soft and saline. Telltale
discoloration in the bottom
Photomosaics of representative cores collected and analyzed in
this study showing the sequence of rot. Core names correspond to samples
discussed elsewhere in this paper and are coded by sample site (as shown in
Fig. 1). The measured ice thickness at each core hole is indicated. For
the JY14 samples, measured core length is indicated instead of ice
thickness. Due to variability in the ice bottom, spreading or compression of
weak layers, and artifacts of image stitching, core images, which are shown
to scale, may not match the measured ice thickness. Asterisks (
Ice collected in July 2015 and July 2017 was highly variable. Cores
collected on 3 July 2015 were largely similar in character to samples
collected in June, including an apparent retextured snow layer in the upper
Ice sampled in mid-July in both 2015 and 2017 was found to be in various stages of rot. Ice sampled from thick floes was similar in character to the June 2015 and 3 July 2015 ice in non-ponded regions, but distinctly rotten below melt ponds. Uniformly thin floes were rotten throughout in both ponded and non-ponded regions. Visually, rotten ice was devoid of the microstructural inclusions that characterized the May and June ice interior, instead appearing to have large, isolated pores and a more chaotic structure. When cored, rotten ice crumbled or broke at many points along the length of cores, rendering it difficult to handle. Rotten ice drained copiously when cores were removed from drill holes, and the bottom portion of rotten cores consisted of optically clear fresh ice drained of brine and characterized by large (centimeter scale) voids. Figure 6 shows photomosaics of cores sampled on 14 July 2015 at Location 3. Images show variations in ice texture depending on whether the ice was ponded or non-ponded, although both types do appear to have at least some scattering layer with a bright white appearance. Many cores had holes exiting the bottom of the ice that were large enough to stick a finger into, although we did not have a means to quantitatively assess how vertically extensive these drainage tubes were.
The May temperature profile had values as low as
Ice core profiles of
Bulk salinity profiles (Fig. 7b) were also consistent with prior published observations. May ice showed the classical C-shaped salinity profile with enhanced salt content near the upper and lower boundaries (10 ppt) and lower salt content (< 5 ppt) in the interior of the ice. By June, significant fresh water flushing from rain and snowmelt reduced the salt content in the upper portions of the ice. The July profiles showed evidence of prolonged flushing, with salt content approaching zero in some cores.
Density values (Fig. 7c) measured in this study in May and June averaged
0.91 and 0.87 g cm
Thin sections show the evolution of the ice structure as it warmed (Fig. 8).
Each of the microphotographs in Fig. 8 is a stitched composite of 20
individual images taken at
Ice sample vertical thin section transmitted light photomicrograph
mosaics
Inclusions in the May sample had an average size of 80
Calculations performed on 3-D reconstructions generated from micro-CT show a
significant evolution in the internal structure of ice during the course of
melt and help define rotten ice. Figure 9 shows reconstructions of the
ice-only phase (top row) and reconstruction of the not-ice phase (air
Three-dimensional reconstructions from micro-CT scans of middle horizon cuts of
cores collected in May, June, and July (JY11 and JY14) 2015 showing the
evolution of pore space. Series
Note that micro-CT analyses only resolve structures with a short dimension
> 284
Porosity is defined as the percentage of total volume occupied by pores, as
measured from the ice-only phase perspective such that the porous space is
derived from air, brine, and DMP. Porosity in DMP-casted May and June
horizons (excluding June top horizons determined to represent a retextured
snow layer) ranged from 0.5 % to 7.5 % by volume (Fig. 10a). In contrast, the
DMP-casted rotten core (JY11-06) had a range in porosity of 37.5 %–47.9 %.
For non-casted rotten cores measured, the porosity ranged from 7.6 % to 23.1 %
(mean
Sea ice internal pore properties calculated from 3-D
reconstructions of micro-CT scans of cuts of cores collected in May, June,
and July.
In addition to becoming generally more porous, the nature of pores in the
ice changed as melt progressed (Fig. 10b). Open pores were those pores
connected to the exterior surface of the volume analyzed, while closed pores
were those fully interior within the 77.6 cm
Anisotropy roughly indicates deviation from spherical structures, with a
value of 0 indicating a perfectly isotropic sample (identical in all
directions) and 1 indicating a perfectly anisotropic sample (fully columnar).
This definition for degree of anisotropy (DA) follows from the equation
DA
In the rotten July cores, the C-shaped profile disappeared entirely. In the JY11 sample analyzed (from ponded ice), the middle portion of the core became more isotropic (0.38–0.57 in the DMP-casted sample, 0.24–34 in the non-casted sample), indicating a rounding of the core center brine channels. This trend was not apparent in the JY14 (thinner rotten floe of non-ponded ice) sample; however, in all July cores analyzed, the upper layer had a generally greater anisotropy value than core middle values, perhaps indicative of vertical channel formation in the upper portion of the ice due to melt and draining from the upper portion of the ice.
Vertically resolved scattering coefficients of sea ice measured during each phase of the field campaign. Coefficients are inferred from laboratory optical transmittance measurements (after Light et al., 2015) and interpretation of a radiative transport model in cylindrical domain (Light et al., 2003b). The April profile is included to show spring ice was measured on ice sampled in 2012 at a comparable geographic location. The May lab rot profile is for ice extracted in May during a field campaign and then warmed in the lab prior to subsample preparation. The shaded area shows the range of measurements on melting multiyear ice (Light et al., 2008) and melting first-year ice (Light et al., 2015).
As the sea ice cover progresses through the onset and duration of melt season, its optical properties respond to increased temperature and the absorption of increasing amounts of solar radiation. Typically, the albedo of the ice cover decreases (less light backscattered to the atmosphere) and its transmittance increases (more light propagating into the ocean). The bulk of this effect, however, is due to the loss of accumulated snow and the widespread formation of melt puddles on the ice surface (Perovich et al., 2002). While this net effect dominates the surface radiation balance, it overlooks effects due to changes in the properties of the ice itself. As the ice warms and becomes porous, permeable, and rotten, increases in void space increase the total amount of internal ice–liquid and ice–air boundaries and would thus be expected to increase total scattering. Increases in ice scattering should promote higher albedo and lower transmittance – exactly opposite the behavior of the aggregate ice cover.
The results of the laboratory optical measurements are shown in Fig. 11. Vertically resolved profiles of scattering coefficient are shown for ice obtained in April, May, June, and July. The April ice was extracted in the same vicinity as the May and June samples during an unrelated field campaign in 2012. In addition to the temporal trend in sampled ice, optical property assessment was also carried out for a May sample subjected to controlled melt in the laboratory (open circles). Scattering coefficients generally increased with time and individual profiles were typified by the characteristic C shape (higher scattering at top and bottom of the column, lower scattering in the middle) also seen in typical salinity profiles.
As sea ice warms, its microstructure changes as inclusions of brine and gas enlarge as required to maintain freezing equilibrium. This has been well established theoretically (Richter-Menge and Jones, 1993) as well as in laboratory experiments (Light et al., 2003a; Perovich and Gow, 1996) for ice with isolated inclusions of brine and gas. This study addresses the limits of sea ice microstructure when natural ice is in advanced stages of melt, in which these inclusions are typically no longer isolated, but rather are in connection with the ocean and/or the atmosphere.
The equations of Cox and Weeks (1983) describe the phase relations
of sea ice for temperatures less than or equal to
Photos of ice core samples shown in Fig. 5 illustrate the evolution of the
ice structure. Early in the season, the majority of the interior ice (areas
away from the top and bottom) appears mostly translucent and often milky
with the exception of isolated bright, bubble-rich weak layers. As the
season progresses, more of the ice appears opaque, losing its transparency
(Fig. 5). This highly scattering ice results from merging, connecting, and
draining inclusions. This effect is clearly seen in the cores that were
submerged when extracted (e.g., the cores indicated with
Submerged cores appear to have more porous ice structure. We hypothesize this is due to additional heating of submerged ice. This heating may come as a result of increased absorption of radiation as swamped or ponded ice will not maintain a substantial surface scattering layer, and as a result, its albedo is typically lower (Light et al., 2015) and more sunlight is absorbed within its interior. Or it may result simply from the contact between this ice and sunlight-warmed water. It is also possible this additional melting serves to enhance the connectivity of this ice to the ocean, promoting the invasion of seawater – and any associated heat – from beneath.
Rotten ice is isothermal, having warmed to approximately the freezing
temperature (0
Density profiles (Fig. 7c) reflect changes in temperature, bulk salinity, and
structure. We observed a marked decrease in density corresponding to summer
melt, a result of the dramatic increase in porosity that defines rotten ice.
May and June profiles had density measurements centered around 0.9 g cm
It is worth noting that sediment loading did not appear to influence the density and structure of rotten ice. Rotten cores collected on 10 July 2015 came from a floe with a visibly high sediment load, while rotten cores collected on 11 July 2015 and in July 2017 had much less sediment (Fig. 5). For all July cores, measured density values were similar within the large range of measurement error. Salinity in the core collected from a sediment-rich floe was, however, somewhat higher than the cores collected from clean floes.
The number and size of brine inclusions identified in this study through the
microscope imagery is commensurate with the number and size of inclusions
documented by Light et al. (2003a). That study reported a brine
inclusion number density range of 24 to 50 mm
Porosity (Fig. 10a) is low in May, with values of less than 10 % and
increases as the ice warms and melts. By July, the micro-CT-determined
porosity approached 50 %, commensurate with densities measured as low as
0.6 g cm
Permeability, and hence pore structure, is central to the hydrological evolution of summer sea ice (Eicken et al., 2002). This suggests that the documentation of highly permeable ice with large porosity may be central to understanding the mass balance of modern ice covers late in the summer melt season. In particular, Eicken et al. (2002) outlined a mechanism for significant ice melt whereby warmed surface waters run off the ice and accumulate beneath areas with shallow draft late in summer, and this pool of warmed fresh water experiences convective overturn and is entrained within the open structure of melting ice. It is expected that further melting from this additional heat could exacerbate the decay and structural frailty of the melting ice, literally melting it from the inside out.
The pore anisotropy results shown in Fig. 10d reinforce the overall trend that as the season progresses, the ice structure homogenizes, losing its characteristic C shape. Where strong vertical gradients in anisotropy existed in May and June, the July ice is more uniform. Our findings are consistent with those of Jones et al. (2012), who used cross-borehole direct current resistivity tomography to observe increasing anisotropy of brine structure as early spring (April) ice transitioned to early summer (June) ice. In that work, the brine phase was found to be connected both vertically and horizontally and the dimensions of vertically oriented brine channels gradually increased as the ice warmed.
There remain notable limitations associated with the characterization of sea ice using micro-CT techniques. Many small brine inclusions were not counted owing to the limited spatial resolution of the technique. Furthermore, the casting technique that was employed appears to have introduced artifacts, especially in connectivity. From all the derived properties (porosity, connectivity, and anisotropy), it appears that the introduction of the casting media may have forced channel connections where perhaps they did not exist naturally. However, the trend in casted samples and the values measured for non-casted samples reflect the substantial changes in ice character that are apparent in the field.
Increases in effective light-scattering coefficient over the course of
seasonal warming are shown to be approximately 5-fold for the interior ice
studied here (Fig. 11). The overall trend of increasing scattering with time
as the melt progresses is a result of the connecting and draining
microstructure, as assessed in the microstructure and tomography analyses.
Relative increases in the scattering would be expected to scale by the
inclusion number density multiplied by the square of the effective inclusion
radius (see Light et al., 2003a). Observed mean inclusion sizes increased from an average
May size of 80
Early in the season, the larger scattering near the ice bottom likely reflects the higher brine content (larger and/or more numerous brine inclusions) near the growth interface. The larger scattering near the top ice surface likely results from the less organized ice structure that forms prior to the onset of congelation growth during initial ice formation. As the melt season progresses, this uppermost portion of the ice has additional enhanced scattering due to the drainage of above-freeboard ice and the eventual development of a surface scattering layer. The enhanced scattering at the top and bottom of the ice results in a C-shaped profile, consistent with observed salinity profiles. This C shape appears to dominate the profiles for April, May, and June, but the July sample appears to have no memory of the characteristic C shape found earlier in the season. Given the significant structural retexturing that occurred by July, this should not be surprising.
Laboratory optical measurements made analogously to the ones in this study
were carried out for melting first-year sea ice in the open pack (see
Light et al., 2015). That data set included little information
about the temporal progression of the ice, as no one location was sampled
more than once. However, interior ice scattering coefficients between
0.1 and 0.3 cm
In an effort to use light-scattering measurements to inform our
understanding of ice rotting processes, we monitored the optical properties
of natural ice samples as they melted. Since most of the May core had an in situ
temperature >
As Arctic sea ice melts during the summer season, its microstructure, porosity, bulk density, salinity, and permeability undergo significant evolution. In situ measurements of sea ice documented off the northern coast of Alaska in May, June, and July indicate that sea ice transitioned from having 4–10 ppt salinity in May to near zero salt content in July. The ice became extremely porous, with porosity values exceeding 10 % through most of the depth of the ice compared to < 10 % for ice collected in May and June. Some July porosity values approached 50 % at places in the ice interior. Brine pockets in rotten ice are few; the ice is essentially fresh in composition and characterized by large visible voids and channels on the order of several millimeters in diameter. These changes result from increased air temperature, ocean heat, and prolonged exposure to sunlight and leave the ice with dramatically increased porosity, pore space with increased connectivity, and increased capacity to backscatter light. These changes have potential implications for the structural integrity, permeability to surface melt water as well as ocean water, light partitioning, habitability, and melting behavior of late summer ice. Specifically, increased connectivity with the ocean may affect how material (e.g., dissolved and particulate material, including biological organisms and their byproducts) is exchanged at the ice–ocean boundary. Subsequent surface meltwater flushing may in turn effectively rinse these constituents from the ice, making this enhanced connectivity central to the control of ice-associated constituents well into the summer season. Rotten ice is a very different physical and chemical habitat for microbial communities than earlier-season ice.
Reductions in bulk density were observed to occur from values of approximately
0.90–0.94 g cm
In addition to sampling naturally rotted sea ice, we have also attempted to simulate the rotting process in the laboratory. Our laboratory optics measurements suggest that natural samples extracted early in the season can be at least partially rotted in the laboratory. To achieve ice that is as rotted and structurally compromised as was observed to occur in nature, the absorption of solar radiation may be a necessary parameter. Sunlight is key to the formation of surface scattering layers at the air–ice interface. In the lab, ice was permitted to rot in air, so any melt that was produced would quickly drain away. In nature, the ice necessarily floats in its own melt, and this may be a critical difference in the way that heat is delivered to the ice. Increases in melt season length may bring increased occurrence of rotten ice, and the timing and character of the seasonal demise of sea ice may be related to the evolution of the ice microstructure.
Data are archived at the NSF Arctic Data Center
The research concept and general research plan were contributed by KJ, BL, and MO. BL, KJ, MO, CF, and SC designed the study and planned the fieldwork. BL, MO, KJ, SC, and CF conducted the fieldwork in 2015; BL, KJ, and SF conducted the fieldwork in 2017. CF compiled and analyzed all field data. BL collected and analyzed all optical data. CF performed the microscopy and SF analyzed the microstructure images. CF performed micro-CT measurements, and the micro-CT analyses were designed and conducted by CF, SF, RL, and ZC. CF and BL prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This work was supported by NSF Award PLR-1304228 to Karen Junge (lead PI), Bonnie Light, and Mónica V. Orellana. Samuel M. Farley had additional support from a Mary Gates Research Scholarship (UW). We thank Julianne Yip for help with sample collection and processing, Hannah DeLapp for data organization, and Michael Hernandez for GIS help. The field campaign was successful as a result of the enterprising support of the Ukpeaġvik Iñupiat Corporation Science staff and affiliates in Utqiaġvik. Logistical support was provided by CH2M Hill Polar Services. The authors also appreciate the constructive reviews of Sönke Maus and the anonymous reviewer, who helped to improve this paper. Edited by: Martin Schneebeli Reviewed by: Sönke Maus and one anonymous referee