you're reading...

Climate Change

Bergy Bits Are a Bit Burdensome

Paper: T. Moon, et al., 2017. Subsurface iceberg melt key to Greenland fjord freshwater budget. Nature Geoscience, v11: 49-54.

Icebergs off Ilulissat, Greenland. (Credit: Zoe Gentes)

It is widely known that icebergs come from breaking off of larger glaciers into the ocean. If you frequent oceanbites, you also know that the ocean has surface and deep-water currents that transport heat around the globe, and that freshwater from melting glaciers can affect the system. This is especially true in the North Atlantic off the coast of Greenland. These freshwater fluxes from the Greenland ice sheet affect ocean water properties and circulation not only on a global scale, but also on subtler, harder-to-define local scales (say, the size of a single fjord). The exact effect, however, depends on the volume and timing of these freshwater influxes, which, despite many studies and models, are poorly known.

One large influx of freshwater into these coastal systems comes from the “transformation,” or melting, of icebergs. Icebergs make up about 30–50% of the mass lost from the Greenland ice-sheet annually – quite a lot. By studying iceberg melt within the ice mélange – the slushy area in front of a glacier consisting of icebergs and “bergy” bits – in two Greenland fjords, earlier authors believe iceberg melt probably dominates the freshwater discharge in winter. However, previous models tend to overlook some perhaps major factors and components:

  • Models have treated iceberg melt as ocean-surface input only
  • Models have not reflected seasonal changes in water stratification and wind
  • Models have not incorporated the time it takes for an iceberg to fully transform

All the Bits

Greenland fjord freshwater flux consists of terrestrial runoff (such as rivers), subglacial discharge (primarily from ice-sheet surface melt), glacier terminus (the calving end) melt, and iceberg melt. These all feed a cycle of varying water properties (such as salinity and buoyancy) and circulation that further changes ocean waters and influences glacier stability. Thus, iceberg-melt-induced changes in the fjord could potentially trigger glacier retreat and greater ice loss.

Figure 1: Differing melt conditions in summer (a) and winter (b). (Credit: Moon, et al. 2017.)

Seasonal environmental conditions control most freshwater-flux processes (Figure 1). Summer snowmelt and rain/snow over land create terrestrial runoff, whereas ice-sheet surface snowmelt, ice melt, and precipitation create an active glacier hydrological system (water drainage system). This hydrological system creates seasonal subglacial discharge, which in turn increases the submarine glacier-end melt from fresher (and more buoyant) upwelling meltwater plumes. In winter, glacier hydrology shuts down as surface melt stops, which decreases the subglacial discharge and leaves primarily ambient melt at the glacier wall.

Conditions for each fjord can be different and can influence iceberg melt. For instance, if a fjord is not iced over in winter, strong katabatic (downward, cold-air driven) winds can produce large winter waves that erode and melt icebergs and bergy bits faster. All of this can in turn be affected by variable stratification (layering) in the water: a winter water column is strongly stratified with a cold fresh upper layer and a warm, salty layer below. Summer has warmer near-surface ocean temperatures and an upper mixed layer.

In short, there are many, many factors that were not taken into previous iceberg-melt models. One should never leave out the bergy bits. 

Delving Under the Surface Melt

Dr. Moon and colleagues wanted to develop a better approach to determine the mass and effect of iceberg melt. They focused on a particular area, the Helheim–Sermilik glacier–fjord system in southeast Greenland, one of the highest discharge systems of solid ice in Greenland.

They used a plethora of data. Their data included hydrographic data from six summer research cruises (2008–2013) and one winter survey in 2010; time series of hydrographic observations from mid-fjord CTD (conductivity/temperature/depth) casts and moorings that were installed in 2009; observations of water velocities from an upward-looking acoustic Doppler current profiler; vertical water velocities; wind speed; and sea-ice cover based on the analysis of satellite imagery.

Their first goal was to understand how icebergs were melting below the water surface. They estimated a model for individual icebergs that incorporates in situ ocean temperature, salinity, and water velocity profiles. The model includes several key components of the iceberg-melt process which are often modeled as one, including wave erosion and convection. With this, they assessed the ocean conditions and developed a full annual hydrographic profile (Fig 2), which was used to estimate yearly annual freshwater sources.

Figure 2: Differing components and magnitudes of fjord freshwater inputs over a year. (Credit: Moon, et al. 2017.)

They found that at the ocean surface, higher summer temperatures and low sea-ice concentrations increase surface melt three times faster than during winter, with wave-induced melt being the biggest factor for surface melt. Subsurface, however, the iceberg melting is controlled by water velocity, temperature, and stratification, all of which change seasonally. Summer subsurface iceberg-melt is generated by opposing water velocity directions (shear). In winter, the velocity shear increases, but the upper water temperatures are colder. The result is a reduced melt above the velocity shear zone (responding primarily to colder temperatures) but a greater melt below (responding primarily to higher velocities). They also found that the deeper the iceberg went below the upper surface layer, the upper-layer melt rates increase as deeper layer melt rates decrease.

The researchers could then take this model for an individual iceberg melt and calculate freshwater fluxes for each group of icebergs of a particular size. From there, they could estimate the total melt flux from all icebergs into the fjord using observed iceberg distributions from satellite imagery. They completed the fjord freshwater budget by calculating terrestrial runoff, subglacial discharge, and glacier-end melt.

From their completed profile, they reached a few conclusions. First, by estimating seasonal variations in all the freshwater sources, they show quantitatively that iceberg melt is the largest annual freshwater source in this system type. Additionally, 68–78% of that melt is released below a depth of 20 m and, seasonally, about 40–100% of that melt is likely to remain at depth, in contrast with previous model assumptions. Freshwater flux sources also have marked differences in timing and magnitude, which alter their importance throughout the year. Glacier terminus melt may be important for glacier stability, but it is the smallest contributor to freshwater flux. Ambient melt dominates in winter.  Summer terminus melt includes ambient melt and melt from subglacial discharge plumes. Terrestrial runoff is substantially larger than terminus melt. Iceberg melt also peaks two months after all the other freshwater sources peak.

Who Cares About Greenland?

Humpback whales off Greenland.(Credit: Zoe Gentes)

Me, for one. It’s a spectacular place with tons of marine life (Figure 3 (photo from my recent trip)), with a complex biological system. Also, as the top producers of ice-discharge, these fjord areas are important for understanding the future of ice loss in the Arctic and Antarctic. Given that icebergs account for ~30–50% of Greenland mass loss, there is no doubt that understanding their solid-to-liquid transition is important on an ice-sheet-wide scale. Moon et al.’s results show that the intricate complexities of iceberg melt must be a part of analyzing these climate systems. Additionally, the methods used in this study could be applied to other individual freshwater sources in tidewater systems, though the results are particularly applicable to coastal regions with a high ice discharge. So maybe don’t use these methods for your local swampland, and remember to account for the smaller bergy bits – they could be important.


No comments yet.

Post a Comment


  • by oceanbites 3 months ago
    Happy Earth Day! Take some time today to do something for the planet and appreciate the ocean, which covers 71% of the Earth’s surface.  #EarthDay   #OceanAppreciation   #Oceanbites   #CoastalVibes   #CoastalRI 
  • by oceanbites 4 months ago
    Not all outdoor science is fieldwork. Some of the best days in the lab can be setting up experiments, especially when you get to do it outdoors. It’s an exciting mix of problem solving, precision, preparation, and teamwork. Here is
  • by oceanbites 5 months ago
    Being on a research cruise is a unique experience with the open water, 12-hour working shifts, and close quarters, but there are some familiar practices too. Here Diana is filtering seawater to gather chlorophyll for analysis, the same process on
  • by oceanbites 6 months ago
    This week for  #WriterWednesday  on  #oceanbites  we are featuring Hannah Collins  @hannahh_irene  Hannah works with marine suspension feeding bivalves and microplastics, investigating whether ingesting microplastics causes changes to the gut microbial community or gut tissues. She hopes to keep working
  • by oceanbites 6 months ago
    Leveling up - did you know that crabs have a larval phase? These are both porcelain crabs, but the one on the right is the earlier stage. It’s massive spine makes it both difficult to eat and quite conspicuous in
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring Cierra Braga. Cierra works ultraviolet c (UVC) to discover how this light can be used to combat biofouling, or the growth of living things, on the hulls of ships. Here, you
  • by oceanbites 7 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Elena Gadoutsis  @haysailor  These photos feature her “favorite marine research so far: From surveying tropical coral reefs, photographing dolphins and whales, and growing my own algae to expose it to different
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on Oceanbites we are featuring Eliza Oldach. According to Ellie, “I study coastal communities, and try to understand the policies and decisions and interactions and adaptations that communities use to navigate an ever-changing world. Most of
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Jiwoon Park with a little photographic help from Ryan Tabata at the University of Hawaii. When asked about her research, Jiwoon wrote “Just like we need vitamins and minerals to stay
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  on  #Oceanbites  we are featuring  @riley_henning  According to Riley, ”I am interested in studying small things that make a big impact in the ocean. Right now for my master's research at the University of San Diego,
  • by oceanbites 8 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Gabby Stedman. Gabby is interested in interested in understanding how many species of small-bodied animals there are in the deep-sea and where they live so we can better protect them from
  • by oceanbites 9 months ago
    This week for  #WriterWednesday  at  #Oceanbites  we are featuring Shawn Wang! Shawn is “an oceanographer that studies ocean conditions of the past. I use everything from microfossils to complex computer models to understand how climate has changed in the past
  • by oceanbites 9 months ago
    Today we are highlighting some of our awesome new authors for  #WriterWednesday  Today we have Daniel Speer! He says, “I am driven to investigate the interface of biology, chemistry, and physics, asking questions about how organisms or biological systems respond
  • by oceanbites 9 months ago
    Here at Oceanbites we love long-term datasets. So much happens in the ocean that sometimes it can be hard to tell if a trend is a part of a natural cycle or actually an anomaly, but as we gather more
  • by oceanbites 10 months ago
    Have you ever seen a lobster molt? Because lobsters have exoskeletons, every time they grow they have to climb out of their old shell, leaving them soft and vulnerable for a few days until their new shell hardens. Young, small
  • by oceanbites 11 months ago
    A lot of zooplankton are translucent, making it much easier to hide from predators. This juvenile mantis shrimp was almost impossible to spot floating in the water, but under a dissecting scope it’s features really come into view. See the
  • by oceanbites 11 months ago
    This is a clump of Dead Man’s Fingers, scientific name Codium fragile. It’s native to the Pacific Ocean and is invasive where I found it on the east coast of the US. It’s a bit velvety, and the coolest thing
  • by oceanbites 12 months ago
    You’ve probably heard of jellyfish, but have you heard of salps? These gelatinous sea creatures band together to form long chains, but they can also fall apart and will wash up onshore like tiny gemstones that squish. Have you seen
  • by oceanbites 12 months ago
    Check out what’s happening on a cool summer research cruise! On the  #neslter  summer transect cruise, we deployed a tow sled called the In Situ Icthyoplankton Imaging System. This can take pictures of gelatinous zooplankton (like jellyfish) that would be
  • by oceanbites 1 year ago
    Did you know horseshoe crabs have more than just two eyes? In these juveniles you can see another set in the middle of the shell. Check out our website to learn about some awesome horseshoe crab research.  #oceanbites   #plankton   #horseshoecrabs 
WP2Social Auto Publish Powered By : XYZScripts.com