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Investigating the Paleoclimate of an Arctic Gateway
Julie Brigham-Grette, Lloyd Keigwin and Neal Driscoll


Sidebars:

Studying a submerged land bridge
The original Healy
The drilling routine



In June of 2002, the icebreaker U.S. Coast Guard Cutter Healy set off from Nome, Alaska, for its first geoscience research mission in Alaskan waters. Carrying about 20 scientists from five institutions and a Coast Guard crew of 75, the Healy, under the command of Capt. Dave Visneski, was decked out with the best equipment for coring marine sediments and collecting sonar images of the seafloor and continental shelves.

The clues for this scientific investigation were beneath the treacherous waters of the Bering Sea and Chukchi Sea. Working within a project funded by the National Science Foundation (NSF), we were on a mission to collect sediments from these seafloors and the intervening shallow continental shelf to determine the recent sea history for the Bering Strait region. It was in this region that sea-level drop exposed a land bridge 20,000 years ago — the proverbial bridge across which early people and many other types of mammals presumably entered the New World.

Breaking the ice: The U.S. Coast Guard Cutter Healy breaks ice on the Arctic Ocean during a nine-week cruise in August 2001. With funding from the National Science Foundation, the cruise was the first scientific voyage for the Healy, and the scientists onboard set out to investigate a slow-spreading mid-ocean ridge, the Gakkel Ridge. They found that the ridge may be more volcanically active than previously thought.

Last summer, Healy set out again, this time to help a research team sample the seafloor beneath the Bering Sea and the Chukchi Sea to find clues about past climate and sea-level changes in the western Arctic. Photo by Henry Dick, Woods Hole Oceanographic Institution.


As we continue our research, we are interested in how this sea-level history influenced the past oceanography of the water masses on either side of the land bridge. Despite the importance of this marine gateway between the Atlantic and Pacific oceans, the recent geologic history of these sea floors remains poorly explored. Our goal was to collect sediment cores, seismic data and oceanographic measurements in order to unravel broad questions surrounding the paleoclimatic history of this western Arctic gateway.

By the end of our fieldwork last summer, we collected more than 100 different cores from the seafloor sediments.
The Bering Strait, today connecting the Bering and Chukchi seas, sits in the middle of a vast subcontinent known as Beringia. This entire region of land and sea stretching from the Kolyma River of northeastern Siberia to the Mackenzie River in the Yukon, is critically important to Earth’s climate system. Only 20,000 years ago during the last glaciation, when sea level was about 130 meters lower than it is today, the land bridge separated the deeper Bering Sea and North Pacific Ocean from the Arctic Ocean. As sea level goes up and down due to changes in the size of the world’s ice sheets and glaciers, the Bering Land Bridge serves at times as a continent and at times as an ocean gateway, producing a geographic bottleneck to the migration of terrestrial and marine biota.

The floors of the Bering and Chukchi seas are some of the most extensive continental shelves on Earth. Their low gradient makes them very sensitive to relative sea-level changes and they could play a key role in global climate.

The common research vessel for collecting cores of marine sediments is a Class-1 UNOLS ship (of the University-National Oceanographic Laboratory System) outfitted for long (at least 20 meters) piston coring. But none of these ships will go to the Chukchi Sea because of the threat of sea ice. Thus, we proposed using the USCGC Healy, which is designed to cut through sea ice, for its first geological science mission in Alaskan waters. We spent our first three-week research cruise in the Bering Sea in late June and early July and our second in the Chukchi Sea in late August to the middle of September.

Where ocean and climate interact

The history contained in the cores we collected can tell us a great deal about changes in sea level and climate, and add to our understanding of global change in Earth’s past. One of the major scientific goals in studies of past climate change is to quantify the range and dynamics of natural variability in Earth’s systems on a variety of time scales.

A key mechanism in Earth’s climate system is the coupling between ocean and climate. Studies that help us to understand this coupling — such as correlations between marine sedimentary records and terrestrial records — are of particularly high priority because they are tools for learning more about the climate system’s dynamics and feedbacks. The arctic regions are exceptionally sensitive to climate change and are thought to amplify changes in the global system.

Glacial-interglacial cycles have imposed on the Bering Strait region some of the most radical changes in paleogeography documented in the Northern Hemisphere. These changes, in turn, have helped to drive equally radical changes in arctic climate. With sea level rise, for example, the rapid migration of shorelines and encroachment of the sea would have locally cooled nearshore sites by changing summertime gradients in temperature and moisture. The greatest east-west heterogeneity across Beringia occurred during warm (flooded) or warming (partially flooded) periods of late Pleistocene summers, when the cool maritime influence bifurcated the relatively warm continental interior, as suggested by Patricia Anderson and Anatoly Lozhkin in the January 2001 Quaternary Science Reviews. When the Bering shelf is at least partially flooded, the sea seems to have considerable “down-wind” effects.

Oceanographic changes would also have been radically influenced by changes in sea level across the Bering Strait that regulated the northward flow of Pacific waters into the Arctic Ocean and North Atlantic. Today, the less saline Pacific waters supply nearly one-third of the total freshwater input to the Arctic Ocean (half that of riverine inputs), maintaining the modern halocline while influencing the strength of thermohaline circulation in the North Atlantic. Any change in this system would have impacted global ocean circulation in ways that remain unclear.

Despite their importance, the paleoceanography and sea-level history of the Bering Strait region, viewed by some as the Arctic Ocean’s leaky back door, remain poorly studied. In contrast, decades of detailed study and modeling of the paleoceanography of the North Atlantic, Norweigian Sea and Greenland Sea continue to yield important information about the cyclicity and dynamics of thermohaline circulation in these water bodies, and of this circulation’s impact on global climate. Studies of terrestrial and near-shore paleoclimate from across the Yukon, Alaska and northeast Russia are also extensive, as demonstrated by a compilation of this research in the January 2001 Quaternary Science Reviews.

Seeing this compilation of Beringian paleoenvironmental history, we recognized the need to study high-resolution records from the Bering Sea and Chukchi Sea for comparison. Sorely missing from existing literature are studies of how these seas participate in controlling Beringian climate. Understanding this relationship could give us a fuller appreciation for the role of the Beringian gateway in the global climate system.

Especially important is an understanding of how the flow of water through the Bering Strait influences thermohaline circulation in the North Atlantic by changing the flux of fresher water into the Arctic Ocean. While we know that sea-surface conditions such as ice extent, temperature and salinity likely influenced global climate through albedo effects, heat exchange and humidity, we previously had very little high-resolution proxy data for these properties from the Bering Sea and Chukchi Sea. Changes in the surface water conditions and the amount of water flowing toward the North Atlantic were also likely influenced by changes in sea level.

Stories of changing sea level

One of the major science goals of our project is to gain new insights into the history of sea-level change through the Bering Strait. We hypothesized it is likely to have differed from global sea-level change because of both global changes in glacial sea level and because of tectonic forces in the Bering Strait region.

For example, estimates have varied on how long and how late after the last glacial cycle the land bridge was exposed. In a 1997 issue of Palaeogeography, Palaeoclimatology, Palaeoecology, Scott Elias and his colleagues updated estimates of post-glacial sea level rise, saying that the Chukchi shelf was inundated and water began to flow through the Bering Strait when sea level was at negative 50 meters water depth by 11,000 years before the present. This new age estimate is significant because it suggests the land bridge was exposed 3,800 years longer than the time period postulated in 1984 by Dean McManus and Joe Creager in Quaternary Research.

However, the new estimate was based on only a few radiocarbon dates of terrestrial material from the Chukchi Sea. Nevertheless, submergence of the land bridge shortly after 11,000 years before the present is indirectly supported by evidence for the arrival of endemic Pacific mollusks and the reestablishment of a seasonal whale migration to the Beaufort Sea between about 10,000 and 10,500 years before the present, as first published in 1996 by Canadian geologist Art Dyke in the journal Arctic. These new dates, though significant, only provide data marking the position of sea level for isolated locations on the shelf at the same depth.

What was needed to help quantify how long the land bridge was exposed was a transect of cores from deep-water sites up onto the shallow continental shelf from a range of depths to more accurately define the migration of the shoreline through time. The cores we collected last summer cover such an area. Moreover, we needed to assess any new sea level history against the influence of subtle flexing of the lithosphere due to the subduction processes and tectonics across this region of Alaska and northeast Russia.

We chose the Chukchi for reconstructing relative sea level because it offered a greater likelihood for better coring locations on the shelf. Pleistocene marine and terrestrial deposits on the Bering shelf are more likely to be reworked and eroded than on the Chukchi shelf, because the former is less protected by sea ice and more exposed to powerful storms from the southwest, which have a long fetch. Previous work, especially that led by geologist Larry Phillips of the U.S. Geological Survey, documented the existence of numerous buried channels and valleys dissecting the Chukchi shelf that appear to record several changes in sea level. By sampling near the channel thalwegs (the steepest parts of the channels), we hoped to acquire thick estuarine sequences from which we could establish a chronostratigraphic framework for each estuarine section. Ideally we hoped to also find fossiliferous marine sediments overlying the sandy estuarine deposits. Such sediments would allow us to bracket the sea level rise with radiocarbon ages on the youngest terrestrial and oldest marine deposits.

Based on the seismic data from these cruises alone, we are encouraged that we have identified many of the complexities of the stratigraphy of the Chukchi shelf. Nested series of eroded and back-filled channels and channel systems across the shallow shelf indicate that a much longer record of sea level change is available in the sediments we cored. In deep-water sites, we located thick piles of flat-lying, undisturbed sediments that likely contain critical information for understanding the long history of the deeper portions of the Bering and Chukchi seas. In a few places, the coring equipment wasn’t long enough to sample all of the sediment. However, knowing where to find such “sweet spots” on the ocean floor is half of the scientific puzzle in this line of work.

Given this new knowledge and our future results from the cores we have collected, the data will allow us to ask new scientific questions of the regional paleoclimate history and the influence of the oceans on the terrestrial environment. By improving upon what was known from these oceans before, we will also be in a better position to help define the role and history of the Bering Strait in the global climate system.


Studying a submerged land bridge

The terrain and bathymetry map at near right shows the Bering Strait region as it might have appeared during the Last Glacial Maximum, about 20,000 years ago. Sea level might have been 125 meters lower than today (this comparison does not compensate for Holocene sedimentation or tectonic forces). This drop opened a land bridge that probably allowed humans and other mammals to migrate to the New World.

The map on the far right shows the region today. Sea level is higher and the land bridge is submerged. Stars indicate where the Healy team took cores from the seafloor. These stars are also indicated on the upper map for comparison.

Both maps were created by William F. Manley of the Institute of Arctic and Alpine Research at the University of Colorado. See online animations of the flooding of the land bridge over time.

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The original Healy

In the late 1800s, Capt. Michael Healy of the Cutter Bear was a celebrated officer in the Marine Revenue Cutter Service. He protected the interests of the U.S. government along the coast of Alaska. Known as one of the greatest arctic navigators of the time, “Hell Roaring Mike Healy” stood for justice in the early years of the new American Arctic.

He warned of the potential over-harvesting of marine mammals, and he protected the interests of native communities and new settlers alike, as both were vulnerable to the Alaskan frontier’s lawless times. At right is a photo of the oil painting of Capt. Healy that is onboard the ship named for him, the U.S. Coast Guard Cutter Healy.

True to its namesake, the USCGC Healy patrols and protects U.S. waters while carrying civilian scientists into the Arctic ice pack as far as the North Pole. Commissioned in 2000 as the largest icebreaker in the U.S. fleet, it is also a scientific platform for exploring the Arctic.

JBG

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The drilling routine

As with any major oceanographic cruise on a ship the size of Healy, time really does equal money. Our goal was to sample as many areas of the floors of the Bering Sea and Chukchi Sea as possible in each of our three-week cruises.

We employed a two-pronged approach to rapidly locate regions of continuous deposition with minimal unconformities caused by past continental slope failure. We used existing GLORIA side-scan sonar data (Geologic Long-Range Inclined Asdic) from the U.S. Geological Survey, along with accompanying single-channel seismic data, to identify broad sediment targets. We then used our Scripps Edgetech CHIRP system, a seismic system that creates images of the seabed at a resolution of less than a meter to penetration depths of more than 50 meters. By combining the CHIRP data with the shipboard swath bathymetry, we could easily locate the exact coring targets.

Core on deck: Deck operations included scientists from several institutions. From left to right are Ana Carmo of the University of Kentucky; graduate students Zach Lundeen of the University of Massachusetts, Amherst, and Mea Cooke of the Woods Hole Oceanographic Institution; and undergraduate student Alex Zaveruha of Western Washington University. Photo by Julie Brigham-Grette, University of Massachusetts, Amherst.


By day, we set up 40 coring stations to take four-inch-wide cores of the seafloor sediments. Depending on which of our coring systems we used, the cores would range between 80 centimeters long and 21 meters long.

By night the seismic crew monitored the charts, identified coring targets, and collaborated with the bridge to set the ship’s course on a grid of transects that mapped the seafloor in a pattern you might otherwise make mowing your lawn.

Coring sandy shelf sediments is nearly impossible using conventional piston coring equipment. To optimize our chances for recovery at shallow shelf depths, we used a submersible vibracore system — which literally vibrates the core barrel into the seafloor — designed by Neal Gielstra, Paul Gayes, Liz Johnstone and Jamie Phillips from Coastal Carolina University. It worked beautifully at first, but soon the summer’s unexpected lack of sea ice proved to be a mixed blessing.

On the one hand, the lack of sea ice meant rough waters, making it difficult to hold the ship on station without an anchor and take cores. More than once we exceeded the vibracore system’s design specifications, but we accomplished our goals. On the other hand, the open water and warm weather allowed us to use the CHIRP system freely and obtain very high-resolution images of the seafloor and the shelf stratigraphy in areas where even coarse resolution data had not been collected before.

By the end of our fieldwork, we had collected cores from between negative 2,800 meters to negative 50 meters water depth. We had an outstanding set of nearly 100 different marine cores together measuring more than 500 meters. Some are the longest cores ever taken in this part of the western Arctic.

JBG

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Brigham-Grette is a professor in the Department of Geosciences at the University of Massachusetts-Amherst. Email her at juliebg@geo.umass.edu.

Keigwin is a senior scientist and paleoceanographer at Woods Hole Oceanographic Institution. And Driscoll is an associate professor with the Scripps Institution of Oceanography, University of California-San Diego.

National Science Foundation (NSF) funding for this project is from the Arctic Natural Sciences program of the Office of Polar Programs (Jane Dionne, program manager) with contributions from the Paleoclimate Program (David Verardo, program director) and the Arctic System Science Program (Neil Swanberg, program manager).


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