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Atmospheric Science

Not all freshwater is created equal

Meredith, M. P., S. E. Stammerjohn, H. J. Venables, H. Ducklow, D. G. Martinson, R. A. Iannuzzi, M. J. Leng, J. M. van Wessem, C. H. Reijmer, and N. E. Barrand (2016), Changing distributions of sea ice melt and meteoric water west of the Antarctic Peninsula, Deep Sea Research Part II: Topical Studies in Oceanography, doi:10.1016/j.dsr2.2016.04.019.

Figure 1. Glacier and sea ice on the West Antarctic Peninsula

Figure 1. Glacier and sea ice on the West Antarctic Peninsula

The role of freshwater in the Antarctic

Freshwater is an important part of the West Antarctic Peninsula (WAP) marine environment. Sea ice grows every winter and melts every summer leaving a relatively thin and fresh layer of water at the surface of the ocean. This becomes a stable and sunny environment for phytoplankton to bloom. Runoff from summertime glacial melt also feeds into this shallow surface layer and, as an added bonus for phytoplankton, brings important micronutrients like iron that it picks up from rocks and sediments along the way. When phytoplankton are happy, the entire Antarctic ecosystem can thrive, from the krill to the penguins, whales, and seals.

These shallow, fresh surface layers can also affect circulation of the water on the continental shelf. For example, a southward-flowing coastal current is usually strong in the summer when glacial runoff provides freshwater to the surface ocean close to the coast (Fig. 2).

Figure 2. The West Antarctic Peninsula (WAP) and schematic path of the Antarctic Peninsula Coastal Current, which tends to strengthen in the summer when a shallow layer of relatively fresh water exists in the coastal surface waters. (Figure 14 from Moffat 2008)

Figure 2. The West Antarctic Peninsula (WAP) and schematic path of the Antarctic Peninsula Coastal Current, which tends to strengthen in the summer when a shallow layer of relatively fresh water exists in the coastal surface waters. (Figure 14 from Moffat 2008)

Researchers from the British Antarctic Survey and the Palmer Long Term Ecological Research project were interested to map the distribution of different types of freshwater along the peninsula. They took water samples from dozens of surface stations on the continental shelf and measured the composition of oxygen isotopes to determine what fraction came from sea ice melt and what fraction came from “meteoric water,” which includes precipitation and glacial melt. (“Meteoric” here means the water relates to meteorology, i.e. it is water that has come from the atmosphere – it has nothing to do with meteors!)

How can oxygen isotopes tell us where freshwater comes from?

Every water molecule contains one atom of oxygen, which itself is made up of 8 protons and some number of neutrons. An atom of oxygen that contains 10 neutrons and 8 protons is written “18O” and is heavier than one that contains 8 neutrons and 8 protons, “16O”.

Water is constantly evaporating from the ocean at low latitudes, traveling through the atmosphere in clouds, and raining back down again (Fig. 3). Lighter 16O evaporates more easily than 18O, so clouds tend to be “isotopically lighter” than the water beneath them. When clouds lose water droplets in the form of precipitation, they tend to lose a higher fraction of 18O because it’s slightly heavier. This leaves the clouds even lighter, so that by the time they get all the way south to Antarctica, the rain and snow that falls out of them is very isotopically light. Meanwhile, sea ice forms from the water that surrounds it, so it tends to have a similar isotopic signature to seawater.

Figure 3. The isotopic fraction of oxygen in water changes as water moves from low to high latitudes in clouds. Clouds that formed in low latitudes and lost water along the way have a very low isotopic weight by the time they reach Antarctica. This makes precipitation that falls on Antarctica, and the glaciers that form over tens of thousands of years, very isotopically light.

Figure 3. The isotopic fraction of oxygen in water changes as water moves from low to high latitudes in clouds. Clouds that formed in low latitudes and lost water along the way have a very low isotopic weight by the time they reach Antarctica. This makes precipitation that falls on Antarctica, and the glaciers that form over tens of thousands of years, very isotopically light.

So, if the oxygen isotope composition of a water sample is very isotopically light, relatively more of the freshwater in it came from precipitation, either recent precipitation directly over the ocean, or millennia-old precipitation in the form of glacial runoff. If the sample has isotopically heavier freshwater, more of it it came from melting sea ice. At any given location, the seawater will actually be a mixture of freshwater form the two sources. The ratio of 18O to 16O allows us to calculate the relative amounts of each.

Cross-shelf patterns of meteoric melt and along-shelf patterns of sea ice melt

Meredith et al mapped the sources of freshwater on the shelf during each January from 2011 through 2014. In 2011, the majority of freshwater came from glacial water and precipitation and more of it was found closer to shore (Fig. 3). This makes sense for two reasons: first, the glaciers themselves are inshore and second, precipitation tends to fall more near land where mountains force warm air to rise and condense into cloud droplets. When meteoric water is the dominant source, there is an across-shelf gradient in freshwater. This kind of situation lends itself to the formation of that strong southward coastal current (Fig. 2).

Figure 4. Fraction of freshwater from glacial melt and precipitation in 2011-2014. The coastal regions tend to have more of this “meteoric” water than offshore regions. 2011 had the smallest fractions overall – in this year more freshwater came from glacial melt and precipitation than from sea ice melt. (Figure 5 in the paper.)

Figure 4. Fraction of freshwater from glacial melt and precipitation in 2011-2014. The coastal regions tend to have more of this “meteoric” water than offshore regions. (Figure 5 in the paper.)

2011 had the least amount of sea ice melt of all four years while 2014 had the greatest. These two years also saw differences in the regional wind patterns. In 2011, winds mainly came from the north and pushed sea ice generally south-eastward. When sea ice is pushed to the south, it is confined to a smaller area that tends to be colder than the more northern areas where most of it formed. As a result, sea ice doesn’t melt as much and when it does, the freshwater is confined to the south. In 2014, winds came from the south and pushed ice north-westward, opening up the ice pack and moving it into warmer areas. Here, it was more likely to melt and the distribution of freshwater was more evenly spread out over the continental shelf.

What can we expect for the future?

The freshwater system is complicated. This region of the world is one of the fastest warming on the planet and, as a result, glacial melt has been increasing and sea ice has not been lasting as long each winter. Precipitation has also been increasing. All these patterns should lead to a higher fraction of freshwater coming from meteoric sources. Unfortunately, four years isn’t quite enough time to establish a significant trend.

Changes in the wind also push the freshwater system more towards meteoric dominance (relatively more freshwater coming from glaciers and precipitation). Winds have been coming from the north more often and with greater strength. This could mean sea ice melts less and stays confined to the southern region of shelf instead of spreading freshwater all over the continental shelf.

Since conditions on the WAP are changing so quickly, we can use trends we see here to make better guesses about how conditions will change in other parts of Antarctica. Here, it looks like the changes are favoring the southward coastal current and this circulation pattern could affect the local ecology. The current is likely to be biologically productive, with phytoplankton thriving in the light-filled shallow surface layer, but it may move all that biological material swiftly out of the WAP continental shelf area. Luckily, the researchers plan to keep going back every January to see what happens.

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