Remains of bearded seal (Erignathus barbatus) of all ages
have been recovered from the Amaknak Bridge site on
the Island of Unalaska in the eastern Aleutians. Shown
here are mandibles from a fully mature prehistoric
adult and a pup several weeks old (top). Bearded seals
give birth on sea ice in early April and nurse their young
for about three weeks.
Winter ice once clogged the Bering Strait until late summer and surrounded the rugged shores of the Pribilof Islands most of the year, according to a new study of ancient animal bones at an Aleutian archeaological site.
This frigid climate, occurring during a mysterious 2,000-year-long cold snap that ended about 2,500 years ago, was powerful enough to trigger decisive shifts in the migrations of whales and the breeding habits of Northern fur seals, ultimately creating a thriving ecology very different than the one observed during the past few hundred yeaers.
Two Canadian archaeozoologists — Susan Crockford and Gay Frederick of Pacific Identifications Inc — say the findings suggest a startling conclusion: that marine mammals like seals may be far more adaptable that people realize.
“Sea ice not only extended further south than it does today but persisted longer into the summer,” they added. “No other type of evidence has documented an expansion of sea ice in the Bering Sea during this time period, which appears to explain the prehistoric distribution of at least one North Pacific marine mammal.”
Among other things, the two scientists found remains of a newborn bearded seal pup, an ice seal, suggesting that floes existed near Unalaska Island as late as April.
The findings come from archeological material deposited during the Neoglacial period, a much colder era that lasted for 2,000 years in the Bering Sea, from about 4700 to about 2500 years ago.
No previous sources found evidence that this frigid epoch significantly expanded the sea ice cover in the Bering Sea. But the archeological dig on Unalaska Island — basically a study of the discarded bones and garbage by a community of Native marine hunters — found a sea change so cold and substantial that it altered the ranges and habits of marine mammals across the region.
Northern fur seal
Source: Doug O’Harra
The abstract, published in the journal Holocene, adds more:
Here we provide new evidence that Neoglacial sea ice expansion in the Bering Sea was substantial enough to have altered the distribution of North Pacific pinnipeds and cetaceans, using prehistoric skeletal remains recovered from an archaeological site on the island of Unalaska in the eastern Aleutians (Amaknak Bridge, occupied from c. 3500—2500 yr BP (radiocarbon years before present, uncalibrated).
Comprehensive archaeozoological analysis of the Amaknak Bridge fauna indicates that sea ice in the Bering Sea must have reached a more southerly position at the height of the Neoglacial and persisted longer than it does today.
We infer from this evidence that for most of the Neoglacial period, sea ice must have surrounded the Pribilof Islands until early summer and blocked the Bering Strait until late summer. Such an expansion and seasonal persistence of sea ice would have prevented fur seals from using the Pribilofs as a summer breeding rookery and whales from making summer migrations into arctic waters to feed, as they do today.
We suggest this expansion of sea ice in the Bering Sea during the Neoglacial may explain several unresolved phenomena of mammalian distributions, genetic partitioning and extinctions in the North Pacific.
The work, conducted funding from the Museum of the Aleutians, is another example of using archaeological middens as keys to unlock the ancient ecosystems. Over the past decade, scientists have been analyzing the remains of animals tossed aside by Aleut people outside communities that date back thousands of years.
When dated using carbon isotope techniques, the bones become a sort of an index to what animals lived nearby and what the marine world had to offer to eat.
Here’s more detail in a story posted by Pacific IDentifcations:
Although about 90 percent of all fur seals now give birth on the Pribilof Islands, from about 4,000 years ago until just before Europeans arrived, fur seals also maintained rookeries throughout the Aleutian Islands and along the west coast of North America from Alaska to southern California. While it seems likely that a combination of intense harvesting by native and early European hunters wiped out these southern rookeries, why they were established in the first place has been an unsolved ecological mystery.
Careful analysis of prehistoric marine mammal bones from the Amaknak Bridge site now provides a very plausible explanation.
Northern fur seal pup on St. Paul
Credit: Doug O’HarraCrockford and Frederick conclude that the expansion of sea ice had a huge impact on Northern fur seals and some whale species. They suggest that the Pribilof Islands were inaccessible from May through July of most years during the Neoglacial because of encroaching sea ice and that these condition prompted fur seals to establish breeding colonies along the Aleutian chain and down the west coast of North America.
They also conclude that the Bering Strait must have been blocked with ice virtually year round between about 4,700 BP and 2,500 yr BP, preventing migration of whales into Arctic waters. Whales that currently use the Beaufort and Chukchi Seas for summer feeding, including bowhead, gray whales and beluga, must either have established resident populations in the Bering Sea — or migrated elsewhere — for at least 2,000 years.
The inevitable repercussions of such a significant expansion of sea ice in the Bering Sea, over such a long period of time, suggests that life history patterns of many migratory marine mammals are much more flexible than they appear. In other words, species may be far more adaptable than we give them credit for.
In the face of a major climate-generated obstacle, whales accustomed to summer grazing in the Arctic simply shifted to alternative feeding grounds; fur seals found alternative birthing locations and changed their migration patterns.
Fur seals not only survived this challenging period, they thrived: evidence from archaeological sites along the west coast of North America indicate that prehistoric fur seal populations associated with southern rookeries must have been substantial.
When conditions improved after about 2,500 year ago, fur seals and whales not only re-established old migratory habits, they evidently continued to flourish because large populations of fur seals, gray whales and bowheads existed by the time European sealers and whalers hit the North Pacific in the late 18th century and began hunting with gleeful abandon.
