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 Earths past. One of the major scientific goals in studies of past climate
change is to quantify the range and dynamics of natural variability in Earths
systems on a variety of time scales.
A key mechanism in Earths 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 systems 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 Oceans 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 circulations 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
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 wasnt 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.
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.
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.
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