Peck, V. L., C. S. Allen, S. Kender, E. L. McClymont, and D. Hodgson (2015), Oceanographic variability on the West Antarctic Peninsula during the Holocene and the influence of upper circumpolar deep water, Quaternary Science Reviews, 119, 54–65, doi:10.1016/j.quascirev.2015.04.002.
Introduction – Wind, “warm water”, and melting ice
In the Southern Hemisphere, the westerly winds blow around Antarctica, uninhibited by land, and drive the strong and deep-flowing Antarctic Circumpolar Current (ACC) clockwise around the continent (Fig. 1). This current carries a nutrient-rich, and relatively warm water mass at depths between about 200 and 500 m below the surface. (In Antarctica, anything over 1°C counts as warm.) We’ll call this water mass “Warm Deep Water.”
In recent years, the ice shelves along the West Antarctic Peninsula have been shrinking rapidly. Warm Deep Water has been found underneath the ice shelves, melting them from below. It is hypothesized that as the westerly winds move closer to the continent (Fig. 1, left), they drive the warm water of the ACC onto the continental shelf more often or with greater force than if they were centered at a latitude farther north (Fig. 1, right).
Methods – How do you measure the wind that blew thousands of years ago?
We can’t directly measure where the Southern Westerly Winds were centered or how strongly they were blowing several thousand years ago, but we can infer where they were based on evidence they left behind in the sediments. Strong winds drive warm, nutrient-rich water onto the shelf, which creates conditions that favor plant and animal life. Remnants of tiny plants and animals that lived throughout the past 10,000 years have been preserved in the sediments. Researchers can read a sediment core like a timeline where the most recent history is preserved right at the seafloor and events that happened farther in the past are preserved in the deeper layers. In this paper, the authors measured three things in the sediments to infer the conditions of the past: diatoms, foraminifera, and the isotopic composition of foraminifera shells.
Diatoms are microscopic plants that live in the upper ocean where there is enough light to photosynthesize (Fig. 2, left). Foraminifera are tiny marine protists that can be found either in the upper ocean or at the seafloor (Fig. 2, right). There are many different species of diatoms and foraminifera, and each grows better in different environmental conditions. Some prefer warmer water, some live in sea ice, and some prefer the open ocean. When they die, their hard shells are often preserved in the sediments. Because we know what environmental factors the different species prefer, when researchers find a certain species in a core, they can infer what the conditions of the ocean were like at the time it lived.
Isotopes in the sediments are also important chroniclers of the past. They’re also a little complicated to understand… Let’s start by remembering that atoms come in different forms depending on how many neutrons they have: 12C is a carbon atom with 6 protons and 6 neutrons and is therefore lighter than 13C which has 6 protons and 7 neutrons. When diatoms take up carbon in the surface waters, they prefer the lighter 12C over 13C. That means that they bring a disproportionate amount of 12C with them when they sink to the bottom (Fig. 3). The shells dissolve back into the deep ocean and enrich those waters in 12C so that when foraminifera living in deep water build their shells, they end up using more 12C because more of it is available. Imagine this process happening over and over for hundreds of years – the result of all those sinking particles is that the deep water gets really enriched in 12C. Researchers can use a high 12C/13C ratio as a fingerprint that proves a shell either came from old deep water or from a time when tons of production was happening in the surface waters and sinking. And in the case of the west Antarctic Peninsula, it is likely that both would have happened at the same time: Warm Deep Water is one of those deep water masses that has been accumulating the dissolved carbon from sinking particles for hundreds of years, but it also contains a high concentration of nutrients required for primary productivity.
Results – What do the sediments tell us?
Some of the diatoms deposited between 10,000 and 7,000 years ago are a species known to come exclusively from the Antarctic Circumpolar Current, so they could only have reached the shelf waters through upwelling of the offshore Warm Deep Water. Over the next few thousand years, sea-ice-dwelling diatoms became more prevalent suggesting that less warm water was coming onto the shelf and ice was extending. By about 4,000 years ago, far fewer diatoms were growing in the surface waters and foraminifera that love really cold water were becoming more abundant.
The isotopes tell a similar story. Ten-thousand years ago, the 12C/13C isotope ratio was high, indicating a strong presence of Warm Deep Water on the shelf. This ratio increased over the millennia and then, about 3,000 years ago, it started to become quite variable. There have been short periods of upwelling influencing the shelf waters, but not a constant presence.
Conclusions – What has history taught us?
Between 10,000 and 7,000 years ago, the diatom, foraminifera, and isotope records all indicate there was a strong presence of Warm Deep Water and not a lot of ice (Fig. 4). Over the next 3,000 years, Warm Deep Water intruded less frequently and the coastal waters became less productive. Finally, over the last 4,000 years, Warm Deep Water became rare on the shelf, ice became more extensive, and production slowed down significantly.
In recent years, we are seeing a shift back toward the conditions of 10,000 years ago, but the pace of change is much quicker than it has been over past millennia. Concurrently, we have seen a shift of the westerly wind belt toward the south. Based on what we know from the past, we can make some predictions: the shifting wind will push Warm Deep Water closer to the shelf, which will flood the shelf with nutrient-rich, warm water. What does this mean? More heat means the Antarctic ice shelves may be in danger of disappearing, while more nutrients mean there will likely be more primary productivity. Now we are left with the bigger questions. How these changes will affect the overall ecosystem that depends on the planktonic life? And how will the melting of glacial ice alter the global sea level?