11 October 2016 (Lamont-Doherty Earth Observatory) – Twenty-three million years ago, the Antarctic Ice Sheet began to shrink, going from an expanse larger than today’s to one about half its modern size. Computer models suggested a spike in carbon dioxide levels as the cause, but the evidence was elusive – until now. Ancient fossilized leaves retrieved from a lake bed in New Zealand now show for the first time that carbon dioxide levels increased dramatically over a relatively short period of time as the ice sheet began to deteriorate.
The findings, appearing in the journal Earth and Planetary Science Letters, raise new questions about the stability of the Antarctic Ice Sheet today as atmospheric CO2 concentrations rise to levels never before experienced by humans.
“We see here that when the Antarctic ice sheet starts deteriorating, it is not that easy to get it back,” said study lead author Tammo Reichgelt, a postdoctoral research scientist at Columbia University’s Lamont-Doherty Earth Observatory and a Frontiers Teaching Fellow. “Some models have shown that, at the rate we’re going right now, the Antarctic Ice Sheet might reach a critical tipping point and start reducing the extent of ice very quickly. We see here that that has happened in the past.”
The scientists examined a 100,000-year period at the transition between the Oligocene and Miocene epochs. The Antarctic Ice Sheet was about 125 percent larger at the start of that period than it is today. By the end, it was about 50 percent smaller than today.
Just before the ice sheet began its decline, atmospheric CO2 levels began to spike, rising from about 500 parts per million (ppm) to between 750 and 1550 ppm over a span of just 20,000 years, the study found. The CO2 level returned to around 425 ppm after that, but positive feedback loops continued to drive melting, the scientists said.
“This is the first time we have found evidence that CO2 fluctuations of this magnitude can happen on relatively short time scales,” Reichgelt said. To compare the CO2 increase to modern times, atmospheric CO2 recently passed 400 ppm and is rising.
What caused the CO2 spike 23 million years ago is still unknown, Reichgelt said. Glacial periods have coincided with cyclical changes in Earth’s orbit, and one theory suggests that organic material that had built up in the Southern Ocean may have started oxygenating quickly and released large amounts of CO2.
Evidence captured in fossilized leaves
To dig into the past, the scientists analyzed sediment cores from the bottom of an ancient lake on New Zealand’s South Island where conditions at that time were ideal for preserving evidence of climates past. Foulden Maar was a humid, subtropical area surrounded by a diverse rainforest back then, and the lake was deep, creating low-oxygen conditions at the bottom that helped preserve forest leaves that fell in and sank.
In those now-fossilized leaves, the scientists can clearly see the size, shape and number of the leaves’ stomata, the opening through which leaves “breathe” CO2 in and release oxygen. The carbon isotope composition of the leaves together with the stomatal data provided the atmospheric CO2 estimates for the period 23 million years ago.
“The leaves living in the forest surrounding the lake were responding to a change in CO2 by reducing the number of stomata as CO2 increased,” said co-author William D’Andrea, a Lamont Associate Research Professor and climate scientist.
Destabilizing the Antarctic Ice Sheet
Other recent studies have found that Antarctica may be more sensitive to changes in CO2 and lower levels of CO2 than previously thought. One international group of scientists working with a sediment core from the Ross Sea off Antarctica and ice sheet modeling found that Antarctica was highly sensitive to changes in CO2 during the early and mid-Miocene, and that the ice had retreated far inland during times when atmospheric CO2 passed 500 ppm.
D’Andrea noted that even the lowest end of the CO2 estimates in the new study, around 750 ppm, appears to have been high enough to destabilize the Antarctic Ice Sheet.
“Prior to this study, it was unclear if atmospheric carbon dioxide increased during the termination of the Miocene Mi-1 glaciation, a factor critical to ending glaciations,” said Aaron Diefendorf, a paleoclimatologist at the University of Cincinnati who was not involved in the new study. ”This new study will provide Earth scientists new information to study this glaciation and provide a new framework to approach other glaciations and events in Earth’s past.”
The other co-author of the new study is Bethany Fox of the University of Waikato, New Zealand.
ABSTRACT: A rise in atmospheric CO2 is believed to be necessary for the termination of large-scale glaciations. Although the Antarctic Ice Sheet is estimated to have melted from ∼125% to ∼50% its modern size, there is thus far no evidence for an increase in atmospheric CO2 associated with the Mi-1 glacial termination in the earliest Miocene. Here, we present evidence from a high-resolution terrestrial record of leaf physiological change in southern New Zealand for an abrupt increase in atmospheric CO2 coincident with the termination of the Mi-1 glaciation and lasting approximately 20 kyr. Quantitative pCO2 estimates, made using a leaf gas exchange model, suggest that atmospheric CO2 levels may have doubled during this period, from 516±111ppm to 1144±410ppm, and subsequently returned back to 425±53ppm. The 20-kyr interval with high pCO2 estimates is also associated with a period of increased moisture supply to southern New Zealand, inferred from carbon and hydrogen isotopes of terrestrial leaf waxes. The results provide the first high-resolution record of terrestrial environmental change at the Oligocene/Miocene boundary, document a ∼20 kyr interval of elevated pCO2 and increased local moisture availability, and provide insight into ecosystem response to a major orbitally driven climatic transition.
By Marty Downs
13 October 2016
(Lamont-Doherty Earth Observatory) – A special section in the October issue of BioScience examines the effects of a single season of intense melting on two Antarctic ecosystems, tracking impacts all the way from microbial food webs to shifting penguin populations.
In the spring of 2001-2002, two climatic cycles (Southern Annular Mode and the El Niño Southern Oscillation) intersected to produce a particularly warm and windy spring season across Antarctica, with melting glaciers, thinning of lake ice, and changes in sea ice area. For the far-flung researchers who were prepared to capture it, this natural experiment offered a glimpse into the ecological future of this most remote continent.
Two Long-Term Ecological Research (LTER) sites operate in Antarctica. Palmer LTER, established on the West Antarctic Peninsula (WAP) with National Science Foundation funding in 1990, focuses on the ways that changing sea ice extent influences marine ecology and the multilayered food webs of the coastal, nearshore, and continental slope ecosystems. McMurdo LTER, established in 1992, explores the ecology of the terrestrial and freshwater ecosystems of the Antarctic dry valleys—a polar desert where glacial meltwater exerts a profound influence on connectivity, nutrient inputs, and the availability of sunlight.
“These two vastly different polar ecosystems offer insights into how diverse ecosystems around the world will respond to climate change,” said Hugh Ducklow, who leads the Palmer LTER and is an ecologist at Columbia University’s Lamont-Doherty Earth Observatory. “With long-term studies already in place, we were able to observe the effects on so many different levels.”
Sea Ice Changes Ricochet up the Food Web
In the West Antarctic Peninsula, 2001-2002 atmospheric conditions resulted in thicker sea ice compacted against the edge of the peninsula, with increased melting at the edge releasing large quantities of fresh water and ice algae directly into the upper mixed layer of the ocean. These nutrient inputs supported a large spring algal bloom and a population boom of Antarctic krill, a major food source for penguins, whales, seals, fish and flighted seabirds.
The same atmospheric patterns delivered greater snowfall and earlier spring snowmelt to the coastal region along the peninsula, flooding the nests of early-hatching Adélie penguins and lending an advantage to later-nesting Gentoo and Chinstrap species.
Wetting the Dry Valleys
Inland, in the dry valleys, the warm, windy spring accelerated melting of mountain glaciers, feeding a pulse of water across the parched landscape, scouring streambeds, raising lake levels, and revealing windblown dust deposits hidden in the glaciers. The characteristic thick layer of lake ice (typically 4-6 meters) also thinned rapidly, allowing more sunlight than usual to reach the upper layers of the plankton community that thrives in lake water under the ice.
At the same time, inputs of dissolved organic carbon from newly-active streams fed increased bacterial productivity deeper in the lakes. Hints from genomic data suggest that the competition for these two different carbon sources could drive long term changes in the structure of the lakes’ microbial communities.
Today’s Anomaly; Tomorrow’s Normal?
At Palmer LTER, the physical impacts of the 2001-2002 climate anomaly were transient. At McMurdo, that single season of warming marked the start of nearly a decade of rising lake levels and increased turbidity.
In so many ways, ice mediates the response of these two systems to environmental change. The relationships are rarely simple, but long term ecological research is allowing scientists to capture the insights from today’s brief climate excursions to understand how ecosystems may respond when the unusual becomes the typical.
The papers appearing in the BioScience special section leverage more than two decades of observations at Palmer and McMurdo. In one, Lamont post-doctoral research scientist Jeff Bowman, working with Ducklow and other colleagues, explores microbial ecosystem dynamics in the two polar extremes and how those ecosystems respond to changes in climate. Another paper, co-authored by Ducklow, looks at the sensitivity of Antarctic sea ice and lake ice to climate variations and the impact on ecosystems.
Marty Downs is the communications lead for the Long Term Ecological Research Network. The Long Term Ecological Research Network was created by the National Science Foundation to conduct research on ecological issues that can last decades and span huge geographical areas. It provides scientific expertise, research platforms, and long-term datasets necessary to document and analyze environmental change.