By Dr. David Harwood and Dr. Richard Levy
If it happened before, it can happen again.
A phrase often repeated to geology students curious about how past geological events preserved within layers of sedimentary rock can influence how we view and manage our modern world against natural hazards. Geologists, Earth’s scientific historians, identify and interpret past events by studying sedimentary layers back through time. The future is less uncertain when guided by knowledge of rates and magnitudes of change that are evident in sedimentary rock archives. For example, we know from sediments and landforms in northern Europe, Asia and North America that large, mile-thick ice sheets repeatedly covered broad regions of the northern continents. These ice masses vanished quickly, within several thousand years, melting and returning large volumes of water to the ocean.
The most recent of these interglacial sea-level rises occurred within the last 12,000 years, and is evident in the abundant drowned river valleys, or estuaries, well expressed by Chesapeake Bay and Thames Estuary on opposite sides of the Atlantic Ocean. Will the centers of U.S. and U.K. governments be further inundated by higher sea level as the Greenland and West Antarctic ice sheets melt in the future, forcing inland relocation of these and other cultural and industrial centers? Given current projections of global warming, and news of accelerating ice-sheet reduction and related sea-level rise, this relocation of human populations and infrastructure could be required within 100 or 200 years.
We look into the future, wondering how fast the Greenland and West Antarctic ice sheets will respond to the projected global-warming trends in the ocean and atmosphere. Scientists lack a clear understanding of how these ice sheets will behave, and can only estimate rates of change. Recent discoveries of a vast network of subglacial water beneath the Antarctic ice sheet raise questions of basal lubrication that can accelerate ice-sheet flow rates, leading to collapse. New scientific data and observations have increased estimated rates of change, and shortened ice-sheet response time. Where can we obtain information about the past behavior of these ice sheets to guide our predictions of future changes?
We need to look to Earth’s geologic past, recognizing that the Greenland and West Antarctic ice sheets melted repeatedly during many previous interglacial periods. By studying examples of past glacial to interglacial transitions, we will be better able to interpret at which point we currently sit within a "typical" climate cycle. We will be able to assess rate, frequency and magnitude of past climate change and the associated response of glaciers and ice sheets, and use these data to predict the rates of glacial processes in the future. We will be able to recognize how far off pace we are with changes evident in past "natural" climate cycles. This is a fundamental mission of an ambitious geological drilling program operating on the margin of Antarctica’s two large ice sheets.
The ANDRILL (Antarctic Geological Drilling) Program recently finished two highly successful drilling campaigns in the western Ross Sea region of Antarctica (see figure) with a goal of documenting the past behavior of the Antarctic ice sheets to help predict the timing and magnitude of future changes (see Prairie Fire, January 2008
). Nearly 2,500 meters of sedimentary core were extracted from two drill holes that recorded a detailed history of the past circa 20 million years of climate and ice-sheet history in this important region of Earth. The second season of drilling ended in late 2007 and the core is currently being shipped to the U.S. More than 100 scientists, students, educators and technicians from Germany, Italy, New Zealand and the United States will continue to study this core over the next two years.
The U.S. National Science Foundation and national funding agencies in the other participating nations pooled resources to provide support to build a new drilling system and drill ANDRILL’s two inaugural holes during the fourth International Polar Year.
Construction of the drilling system and on-ice drilling operations were coordinated through the ANDRILL Operations Management Office at Antarctica New Zealand in Christchurch, New Zealand. Research on these cores in Antarctica and in home laboratories in these four nations is coordinated through the ANDRILL Science Management Office at the University of Nebraska-Lincoln.
Results from initial study of the cores indicate a more complex and dynamic history for the South Polar Region than suspected previously. ANDRILL scientists identified more than 60 examples of glacial-interglacial cycles from preserved intervals of time over the past 14 million years. With these new data, we can now attempt to compare our current climate position with examples from the past. We now know that the West Antarctic Ice Sheet was a dynamic component in past glacial-interglacial cycles, and it will likely be a responsive element, shrinking as Earth continues to warm. We are also able to assess the likelihood of changes to the larger, colder, and assumed to be more stable East Antarctic Ice Sheet, by revealing its past response to times of even higher global temperatures back through the past 20 million years.
ANDRILL’s success depended on our ability to recover long and continuous sedimentary cores. Our "time machine" is the ANDRILL rig, which recovered more than 98 percent of the sediment and rock it cored through in 2006 and 2007 (1,284 and 1,138 meters) at two locations in the western Ross Sea. These cores enable us to travel into the past and examine the cycles of West Antarctic Ice Sheet advance and collapse, as open marine conditions returned to the Ross Sea. Selection of ANDRILL’s drilling sites targeted sedimentary rocks deposited during times in the geological past when Earth’s climate was warmer than present. Knowledge of conditions during the early Pliocene and middle Miocene warm Climatic Optima, five to three million years ago and 17.5 to 14.5 million years ago, respectively, will be instructive as a map of where our warmer future may lead us.
The age of events in the sedimentary cores are determined through the study of remains of fossil plankton, chiefly one-celled algae called diatoms, through paleomagnetic properties in the core that record times of past reversals of the Earth’s magnetic field, and through the geochemical dating of volcanic materials erupted from local Antarctic volcanoes. Combined, these dating tools provide a high-resolution timescale, to which we can compare Antarctic events to those known from other areas around the world, and estimate rates of glacial, climate and sea-level changes.
The drilling system
Design of the new ANDRILL drilling system utilized knowledge gained from previous Antarctic margin drilling efforts over the last 30 years. Improvements in drilling design and operations are evident in the above photographs. Modifications and new technology have allowed us to increase total drilling depth, operate in deeper water, and obtain core at faster drilling rates while maintaining excellent core recovery and condition. A major advance has been the development of an integrated hot-water-drilling system that allows ANDRILL to melt holes through thick ice and operate from ice-shelf platforms, in addition to sea-ice platforms. During the last two years, ANDRILL’s new tool for scientific exploration of the Antarctic region recovered the two deepest drill holes in Antarctica.
The drilling system is built around a mining industry rig, which is designed to recover cylindrical samples of rock and sediment (core) from great depth below the Earth’s surface. This is in contrast to most oil-and-gas industry drilling rigs that are typically designed to make a hole and drill to great depths quickly, but not recover core. ANDRILL’s drilling system utilizes sections of steel pipe that are screwed together and lowered below the drilling rig to enable us to "reach" deep into the earth. The steel pipe comes in four different diameters that fit inside each other like Russian nesting dolls. The largest diameter pipe, the sea-riser, is the first to be lowered from the rig; it is cemented into the seafloor, put under tension by floatation devices and attached to a moveable tide-compensation beam beneath the drill rig floor. The sea-riser acts as a conduit for drilling fluid and provides support for the other pipe that is lowered to the seafloor inside the sea-riser.
The other three sizes of pipe are the drill strings (photo). The largest diameter string is used first and is lowered to the seafloor inside the sea-riser. The drill bit (photo), which consists of a diamond-studded piece of drill pipe, is attached to the bottom end of this drill string. The drill rig spins the steel pipe rapidly and the bit on the end cuts through the sedimentary rocks. As the bit cuts and the string advances down, a cylinder of these rocks is preserved within an inner tube that sits inside the hollow drill pipe. Drillers can advance the drill bit and string into the layers of rock nine meters at a time, at which point the inner tube is filled up. Drillers then stop advancing the bit, and the inner tube containing the core is pulled to the surface on a wire that is deployed inside the drill string.
Drilling fluids of varying density are pumped through the center of the drill string down to the spinning drill bit, and are recycled back up to the drilling rig through the sea-riser in a closed system. The drilling fluids lubricate the drill bit, keep the cutting surface clean, and provide weight or pressure inside the borehole to keep the sediments from falling into the open hole. Average drilling rates ranged between 30 to 40 meters per day.
If problems develop in the borehole, such as encountering soft sediment layers or rock fragments that keep falling into the hole, or fractures in the rocks that cause the drilling fluid to disappear into the rock formation, the drilling team can cement the current drill string at the problem spot and begin drilling with a smaller drill string. The cemented drill string serves as a "casing" for the overlying borehole, and the new string is lowered inside this casing. At the first ANDRILL site, a total of nearly 6,000 meters of pipe were suspended from the drill rig when the sea-riser and all three drill strings were deployed.
Interpreting the rock layers
When each nine-meter section of core arrived at the surface, it was transferred from the drill rig platform to a series of converted shipping containers that served as the drill site laboratory. Drillers and scientists would gather beside the laboratory benches each time these cylinders of rock and sediment were opened up, exposing layers of Earth history that were hidden for millions of years. As each core section was recovered, more information regarding the glacial and climatic history of the region was revealed.
Collecting basic geological data was the focus for the more than 50 ANDRILL scientists who worked in Antarctica as the core was pulled from the deep. Integration and analysis of these environmental clues allow ANDRILL scientists to uncover the glacial history of the region. A diverse range of rock and sediment types were recovered and described during the two seasons of drilling.
Thick layers of massive sediment containing pieces of rock ranging in size from clay to boulders up to one meter in diameter (photo) were deposited when the ice sheets sat on the seafloor directly above the drill site. Thin layers of alternating mud and sand with dispersed pebbles (photo) were deposited when ice shelves and icebergs floated above the drill site. Finely layered rock composed almost entirely of fossilized remains of marine algae or diatoms (photo), formed when the ice retreated far from the drill site and only open marine conditions, with the occasional iceberg, prevailed. Repeated sequences of these different layers stacked on top of each other record cycles of glacial growth and retreat. These phases of ice-sheet advance and collapse are in response to changes in past climate.
Over 60 of these repetitive vertical successions of varying rock type were recorded in the 1,284 meters of core recovered during ANDRILL’s McMurdo Ice Shelf Project, completed in early 2007. The duration of each of the cycles is likely linked to known variation in astronomical phenomena that affect Earth’s movement in space (Milankovitch cycles). These cycles impact the amount of solar energy entering Earth’s atmosphere at any given time, and contribute to climate variation (radiative forcing) through changes in the shape of Earth’s orbit, the tilt of Earth’s rotational axis relative to the plane of its orbit and wobble on this rotational axis. These orbital cycles occur at regular and predictable frequencies of 100,000, 41,000 and ~22,000 years.
The vertical successions of sediment layers are not all exactly the same and can be grouped into three associations of rock type termed facies associations. These facies associations reflect climatically distinct periods during the last 14 million years. A cold polar climate similar to today prevailed between ~13 mya (million years ago) and 10 mya and returned at ~1 mya. During this climatic regime, the regional environment was dominated by persistent ice over the drill site (including grounded ice and sub-ice-shelf conditions). Between 9 and 6 mya, warmer than present climatic conditions occurred and are reflected in open water deposits interspersed with rock types that indicate sub-glacial conditions. Between 5 and 2 mya, extended periods of warmth are recorded by thick deposits of open marine sediments dominated by marine algae. During much of this time, the ice had retreated far from the drill site, and water temperatures were likely too warm to support growth of annual sea ice. This last period may represent an analogous condition for this region under future elevated global temperature.
ANDRILL’s Southern McMurdo Sound Project, completed in late 2007, recovered 1,138 meters of sediment core with a focus on extending the record obtained the previous season and documenting conditions and events during two periods of warm climatic optima between 17 and 15 mya (middle Miocene). Variations in the sediment and rock types recovered reflect changes in sea level, glacial proximity and climate. Sediments deposited close to or beneath grounded glaciers, likely flowing from East Antarctica, alternate with fine-grained sediments, which provide clear evidence for cycles of ice advance and substantial retreat during main climate transitions to warmer times.
These sediments and shell fossils suggest the persistence of another phase of warmer-than-present conditions over an extended period of the middle Miocene. At this time, the western Ross Sea and McMurdo Sound resembled the modern climate conditions of southernmost South America, southwestern New Zealand and southern Alaska. An unexpected absence of microfossil algae (diatoms) in many of the fine-grained lithologies suggests that the coastal marine environment was dominated by high sediment input, with substantial river runoff, high coastal turbidity, and meltwater input. The middle Miocene climate regime in Antarctica was quite different from the cold polar climate of today, much warmer and much wetter, producing a highly dynamic glacial environment.
Natural and unnatural cycles
Some climate cycles we understand well. The Milankovitch cycles described above have been termed the "pacemaker of ice ages," due to their regular and calculable pulse. The change in solar radiation that results from the above-mentioned variation of Earth’s orbit is not sufficiently large to drive climate changes. However, these minor variations in temperature are amplified by resultant changes in atmospheric and oceanic processes that change rates of biosynthesis of carbon by photosynthetic organisms. This change in rate of carbon extraction, and other factors that impact cycling and storage of carbon through various systems, leads to changes in atmospheric levels of CO2
The amount of carbon available on the Earth’s surface and in the atmosphere as CO2
contributes to greenhouse-effect regulation of Earth’s temperature. But the amount of available carbon and CO2
has not been constant through time. For example, if we start with conditions of small ice sheets and high sea level, this will lead to the burial of carbon-bound organic matter on continental shelves and in estuaries. This will lower CO2
in the atmosphere and eventually lead to global cooling and ice-sheet growth, due to inverse greenhouse effects. Continued cooling and ice-sheet growth will then lead to a lowering of sea level, exposure of continental shelves to erosion by rivers and waves, and the return of some of that buried carbon back into the ocean. This last step will lead to an increase in available carbon and higher CO2
in the atmosphere, which will promote progressive greenhouse warming, melting of ice sheets and sea-level rise. We are now back to the starting point. Repeat this cycle of carbon removal and return, and witness large-scale changes in Earth climate driven by the Milankovitch "pulse" and greenhouse-effect influences. This regular cycling of Antarctic climate and ice-sheet changes are clearly evident in ice cores and marine sediment cores through the past ~40 million years.
In the very recent geological past, however, humans have interrupted this cycle. Our ancestors cut forests across Europe and North America and now across vast areas of South America and Asia. We found, extracted and burned much of the organic carbon buried over many millions of years that is stored in the rocks as oil, gas and coal, and released it into the atmosphere and ocean. Hydrocarbons that accumulated through hundreds of million of years of geological time have been reintroduced into the atmosphere and oceans in less than 150 years!
It is unique in Earth history for a single species to disrupt Earth’s natural systems so rapidly. There is no analogous event for this rapid change in Earth’s atmospheric composition. We are indeed only starting to see the initial changes that this environmental perturbation will bring. The closest example of a similar rapid increase in atmospheric CO2
and resulting global temperature increase occurred 55 million years ago, as documented in numerous studies around the world. Then, global temperatures rose abruptly by up to 9 degrees Fahrenheit. The postulated cause for this temperature rise during the Paleocene-Eocene Thermal Maximum (PETM), as this event is called, was heat generated by tremendous volcanic activity associated with the tectonic birth of the North Atlantic region that cooked marine sediments rich in organic matter. It is thought that more than 1,500 billion tons of carbon (as CO2
and methane) were released into the atmosphere and oceans at this time. In contrast to the modern rapid rise in greenhouse gasses due to burning of fossil fuels, the PETM event 55 million years ago took a much longer time, ~20,000 years, to occur. It took more than 200,000 years (!) for Earth’s climate system to adjust to that ancient event, before global temperatures returned to normal. This well-documented event provides one example of the potential duration of the current global-warming event. Because the PETM event occurred during a time when large ice sheets were not players in Earth’s climate system, the future response is less certain; but the future of ice sheets on Earth is also uncertain.
How warm will ocean and atmospheric temperatures need to rise around Antarctica before the West and then East Antarctic ice sheets collapse and disappear? How warm before the larger and more stable East Antarctic Ice Sheet starts to change? Can records of past ice behavior help identify climate thresholds or "tipping points" when Earth’s interconnected cycles take a fundamental step into a new regime? Results from the ANDRILL Program will help to address these questions. The loss of seasonal sea ice may be a critical step. This is occurring now in the Arctic Ocean, and may begin to change in the Southern Ocean in the future.
Global warming — global warning
Climate has changed in the past through natural cycles, driven largely by interactions between complex systems in the atmosphere, oceans and biosphere. However, humans are changing the atmosphere, influencing climate change. We are driving fast, on a road we have never traveled before, and we don’t have a map. Many are coming to the realization that we should start slowing down. Each of us can take steps to slow down the rates of change, by reducing activities and items that add to the fossil-fuel problem. It is encouraging to see more discussion and attention given to this problem in this country and by other nations. It is a global problem that we need to address as individuals, as local groups, as nations and as a global community. Talk it up! Get involved. Make some changes in your behavior. Initiate changes in your workplace, home and community. We cannot stop what has started, but we can reduce the impact and hopefully delay what will be nature’s unkind response.
Related: ANDRILL: Antarctic geological drilling for climate history