Alan Wanamaker, working as a postdoctoral researcher from 2007 to 2009, was charged with beginning to compile a 1,000-year record of the marine climate for a spot in the North Atlantic just off the fjords and fishing villages of North Iceland.
He was at Bangor University in Wales, working with James Scourse and Chris Richardson, professors in the School of Ocean Sciences. Before Wanamaker were thousands of clams, each specimen of Arctica islandica taken from 80 meters of seawater on the North Icelandic Shelf.
Those clams – dead and alive, some able to live up to 500 years in the icy water – were the research group’s sensors under the sea. Just like tree rings say a lot about growing seasons over time, annual growth increments in the shells can tell researchers a lot about ocean conditions over time.
Wanamaker, now an Iowa State University associate professor of geological and atmospheric sciences, started building the marine archive by processing all those clams – painstaking lab work that involved sectioning shells, embedding shell slices in epoxy blocks, measuring down to a millionth of a meter, drilling samples, radiocarbon dating and determining oxygen isotopes.
Then he’d eye cross sections of shells, looking for patterns, matching growth rings, finding overlaps, all with the goal of eventually lining up enough shells to build a master chronology of growth increments covering a millennium. After Wanamaker left Wales, Paul Butler, now a research lecturer at Bangor University, continued with the shell chronology and David Reynolds, now a postdoctoral research associate at Cardiff University in Wales, completed the isotope analysis. Wanamaker called it a real team effort.
That open-access record, based on 10 years of work and analysis of nearly 1,500 isotope samples from dozens of clams, is now complete, Wanamaker and other researchers report in a paper just published online by the journal Nature Communications. Reynolds is the paper’s lead author. Wanamaker, Scourse, Richardson, Butler and six others are co-authors.
Grants from the United Kingdom’s Natural Environment Research Council and the European Union supported the researchers’ work.
“Here we report a new 1,048-year precisely dated, annually resolved marine oxygen isotope record that spans the entirety of the last 1,000 years (AD 953-2000),” the researchers wrote in their paper.
Isotopes are elements with varying numbers of neutrons. Heavier oxygen isotopes in the growth increments of shells are generally associated with colder, denser seawater; lighter isotopes are associated with warmer, less-dense seawater.