Ancient polar bear remains explained by sea ice expansion and open water polynyas

Lay summary authored by Susan Crockford. Read the full paper here: http://doi.org/10.5334/oq.107

No Arctic animal is more iconic than its apex predator, the polar bear (Ursus maritimus). Ice-edge habitats are essential for the survival of polar bears today because that’s where they consistently find seals, their primary prey. What we know about where and when polar bears lived in the past is limited to reports of their skeletal remains recovered from archaeological and geological sites. However, the distribution across time and space of these records has not previously been reported.

In her article in Open Quaternary, archaeozoologist Dr. Susan Crockford (Pacific Identifications Inc.) presents the first historical compilation of known records of ancient polar bear remains from fossil and archaeological contexts before AD 1910. The study also presents the locations of known polynyas (recurring areas of thin ice or open water surrounded by sea ice) across the Arctic.

Left lower jaw of polar bear found at Kjul Å near Asdal in northern Jylland (Denmark). Photo credit: Photo by Geert Brovad, courtesy Natural History Museum of Denmark.

It turns out that natural death skeletal specimens of this species (‘fossils’) are rare but archaeological remains are much more common. Most polar bear remains date to the Holocene (the last 11,700 years) and come from human habitation sites within the modern range of the species. Specimens found outside the modern range of polar bears have been documented in the north Atlantic during the late Pleistocene (ca. 115,000- 11,700) and the southern Bering Sea during the middle Holocene (ca. 8,300-4,200 years ago), and are associated with natural expansions of sea ice during known cold periods.

Surprisingly, the single largest find is also the oldest archaeological site with polar bear skeletal remains. Zhokhov Island, Russia is one of the northern-most land forms in the East Siberia Sea and sits at the edge of the Great Siberian Polynya, which polar bears still use today. The site was occupied primarily for about one hundred years (ca. 8,000-7,900 years ago) near the beginning of the Holocene Climatic Optimum (about 9,000-5,500 years ago), when the Arctic was warmer than today. Almost 6,000 polar bear bones were recovered, some of them chewed by dogs.

Crockford, author of the report, also pointed out a little-known fact. ‘The Zhokhov Island assemblage is our first evidence of the return of polar bears to the western Arctic after extraordinarily thick sea ice during the Last Ice Age drove seals and bears into the north Pacific.’

The study shows that ancient polar bear remains, whether archaeological or fossil, are most often found in proximity to areas where polynyas are known today and which likely also occurred in the past. This includes the oldest known fossil (dated to about 130-115 years ago) and the oldest known archaeological specimens from Zhokhov Island. This pattern indicates that as they do today, polar bears may have been most commonly found near polynyas throughout their known historical past because of their need for ice-edge habitats.

Full paper: Crockford, S.J., 2022. Polar Bear Fossil and Archaeological Records from the Pleistocene and Holocene in Relation to Sea Ice Extent and Open Water Polynyas. Open Quaternary, 8(1), p.7. DOI: http://doi.org/10.5334/oq.107

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