Voyage to Iceberg Alley
If you’ve seen Titanic, you’re probably wondering why on Earth we would undertake such a voyage, especially now that the Southern Hemisphere summer is over and the days are getting colder, shorter, and stormier.
Yet, we will drill deep into the ocean floor to retrieve sediment that will help us reconstruct the melting history of icebergs breaking off of the Antarctic Ice Sheet, the thick layer of ice (on average 1.3 miles) covering the vast continent of Antarctica, which has the potential to raise global sea level by several meters and drastically affect coastal communities and the economy.
Rising seas from global warming and the melting of polar ice will impact the lives of hundreds of millions of people around the world who live near the coast. It is essential that we better understand the physics of polar ice sheets so we can better predict the future. ~Dr. Maureen Raymo, co-Chief Scientist
How Much, How Fast?
Today, scientists are concerned about melting of the ice sheet and Antarctica’s glaciers or “ice streams” as they are called. It’s projected that, at the current rate of warming, melt from Antarctica will raise global average sea levels by up to 1 meter by the end of the century, and possibly up to 10 meters by the year 2500. Children born today are likely to experience significant disruption to civilization in their lifetime if we continue on our current trajectory of greenhouse gas emissions.
If vulnerable glaciers, such as the massive Thwaites Glacier (popularly known as the “Doomsday Glacier”) and slightly smaller Pine Island Glacier melt, the West Antarctic Ice Sheet may collapse, raising global sea level even further.
Thwaites Glacier Velocity Map [Credit: Kevin Pluck, PixelMoversAndMakers.com]
But uncertainties remain as to how much ice will melt and how fast. Hence, we will endeavor to find out what happened to the Antarctic Ice Sheet in the past, which will inform our knowledge of how it may respond to warming now and in the future.
All About Icebergs
To understand how we’ll reconstruct the history of the Antarctic Ice Sheet, we must first talk about where icebergs come from.
Icebergs are masses of ice that break off—or “calve”—from ice that originated on land. (Compare sea ice, which forms when sea water freezes. When sea ice melts, it does not raise sea level, because it is floating, just like ice in your drink does not cause your glass to overflow when it melts.)
Icebergs calve from glaciers and ice shelves. The latter are floating sheets of ice attached to the land, and they’re important because of their “buttressing” effect—they act as a wall, holding back the ice behind them. Glaciers are great rivers of ice that flow across the land toward the sea.
Warming ocean currents are melting ice shelves and glaciers from below, thinning, weakening, and destabilizing them. Indeed, just recently, a hole the size of Manhattan was discovered under the Thwaites Glacier, and within the last three weeks, the ice tongue—the part of the glacier flowing out onto the sea—has undergone a partial collapse, perhaps retreating as far back as the grounding line (where the glacier is attached to the land).
As glaciers grind across the land, they pick up dirt and rocks. When an iceberg calves, it drifts with the Antarctic Coastal Current, carrying this “iceberg-rafted debris” until it melts. Many Antarctic icebergs reach the Weddell Sea and are deflected north by the Antarctic peninsula, reaching the warmer waters of the Antarctic Circumpolar Current.
There, they drop their debris, which accumulates on the seafloor over time, and this is where we will drill.
Iceberg Flux from Antarctica; yellow dots show our drilling sites [Credit: PixelMoversAndMakers.com, Kevin Pluck & Marlo Garnsworthy]
Reconstructing the History of the Antarctic Ice Sheet
The JOIDES Resolution will retrieve long cylinders of sediment called cores. Analyzing the properties of the sediment will allow us to discover when the icebergs that carried it calved and even where in Antarctica they came from.
To determine the age of the sediment, we’ll use a combination of “biostratigraphy” and “magnetostratigraphy”. Biostratigraphy looks at the fossils of microscopic algae, such as diatoms and dinoflagellates, and animals called radiolarians and foraminifera. Since we have known dates for the periods in which certain species existed, when we find them, we’ll know the surrounding sediment was likely deposited during those time periods.
Paleomagnetism compares changes in the alignment of minerals in sediments to known changes in the polarity of Earth’s magnetic field over time and helps confirm the time period in which the sediment was deposited.
The chemical and physical properties of the sediment can tell us which part of Antarctica the sediment came from.
Learning from the Past, Preparing for the Future
All of this information can tell us more about the stability of the Antarctic Ice Sheet in the past and its relation to changing climate and sea level rise, especially during times that were warmer than today. We hope to retrieve a few million years worth of sediment, perhaps even more, to provide a nearly continuous history of changes in melting of the Antarctic Ice Sheet.
At times when more debris was deposited, we know more icebergs were breaking away from the Antarctic Ice Sheet, which tells us the ice sheet was less stable. Previous research has shown that rapid discharge of icebergs from the Antarctic Ice Sheet has occurred multiple times in the past when the climate became warmer. This has huge implications for how the Antarctic Ice Sheet may behave in the future as our world warms.
My hope is that our expedition will unravel the mysteries of the Antarctic ice-sheet dynamics for the past, and this may tell us something about its course in the near future. ~Dr. Mike Weber, co-Chief Scientist
And so we sail forth into wild and frigid seas, ever watching for advancing sea ice—and icebergs, of course!—so that we may better understand the past in order to prepare for an uncertain future. Indeed, the winds are up this morning, the sea state has deteriorated, and a storm is on the way. It’s all part of polar research on the Southern Ocean!
The JOIDES Resolution is part of the International Ocean Discovery Program and is funded by the US National Science Foundation. Expedition 382 Iceberg Alley and Subantartic Ice and Ocean Dynamics will be at sea until May 20, 2019.
Marlo Garnsworthy is an author/illustrator, editor, science communicator, and Education and Outreach Officer for JOIDES Resolution Expedition 382 and previously NBP 17-02. She and Kevin Pluck are co-founders of science communication venture PixelMoversAndMakers.com, creators of the animations in this article.
Feature Image: Tabular iceberg, Ross Sea, Antarctica [Credit: Marlo Garnsworthy]