Rocks and plants<\/p>\n
The first method measures beryllium-10, an isotope that accumulates in the top few centimeters of rock that is exposed to the sky. By sampling a boulder\u2019s upper surface and counting the beryllium, researchers could learn how long it has been uncovered. That technique put a date on the ice sheet\u2019s retreat. It turned out the Laurentide had withdrawn to within today\u2019s ice margin by 3,600 years ago, probably shrinking further by around 3,000.<\/p>\n
The second dating method looked at the dead plants. \u201cThere were all these plants growing, just like today. And included in that are mosses, these little mosses that are rooted in the ground,\u201d Young says. \u201cSo they\u2019re there, they\u2019re living their life, and eventually the Laurentide Ice Sheet starts to re-expand. And when that ice margin covers the moss, it literally kills it.\u201d<\/p>\n
By radiocarbon dating the remnants of the moss, the team could pinpoint the moment it died, yielding what glaciologists call a kill date\u2014the moment the ice arrived at that spot. By stringing multiple kill dates of moss samples taken from different spots, scientists reconstructed a map of an advancing ice margin.<\/p>\n
About 70 percent of the team\u2019s radiocarbon dates cluster around 1800 CE, at the culmination of the Little Ice Age. \u201cIt\u2019s all sort of the same thing,\u201d he says. The Little Ice Age marks the point where Northern Hemisphere glaciers were at their most extensive in recent millennia. At Barnes, Young argues, it was just one late pulse within an advance that had been running for two thousand years.<\/p>\n
The most important plants for Young\u2019s team, however, are still hidden under ice. Barnes\u2019 margin has retreated 200 to 800 meters since 1960, exposing mosses killed from about 1000 CE onward. Anything older remains buried. To pinpoint the exact moment the ice started advancing, the team had to look at Barnes\u2019 neighbors.<\/p>\n
Pinning down the turn<\/p>\n
Scattered across the same plateau are other small ice caps, close enough to share the same climate. Earlier studies collected nearly 200 kill dates from beneath them\u2014not one is older than 2,000 years. \u201cWhat that means is that all of those ice caps up there didn\u2019t start advancing or re-expanding until about 2,000 years ago,\u201d Young says. \u201cSince the Barnes Ice Cap is right next door, it\u2019s probably a good bet that it also started expanding 2,000 years ago.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"Rocks and plants The first method measures beryllium-10, an isotope that accumulates in the top few centimeters of…\n","protected":false},"author":2,"featured_media":812839,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[7],"tags":[59,90,56,54,55],"class_list":["post-812838","post","type-post","status-publish","format-standard","has-post-thumbnail","category-science","tag-gb","tag-science","tag-uk","tag-united-kingdom","tag-unitedkingdom"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/812838","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/comments?post=812838"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/posts\/812838\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media\/812839"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/media?parent=812838"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/categories?post=812838"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/uk\/wp-json\/wp\/v2\/tags?post=812838"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}