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
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 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.
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
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. |
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. |
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