Sea-ice in the Arctic has declined about 40% over the past 40 years, as monitored by satellite-based techniques. Open questions still exist about how fast the Arctic will reach ice-free conditions, seasonally and annually, in the not-so-distant future as humans continue to warm our planet. This necessitates a deeper understanding of the processes that dictate changes in Arctic sea-ice coverage, and how this ice coverage responds to a shifting climate.

Along with collaborators at UMass Boston (Jesse Farmer), the USGS (Laura Gemery and Thomas Cronin), and Caltech (Jonathan Treffkorn and Kenneth Farley), we devised a new technique to study Arctic sea-ice coverage in the geologic past, using clues from cosmic dust accumulation in marine sediments. Our results were published today in Science. (See here for coverage in the New York Times)

We found increased Arctic sea-ice coverage during the last glacial maximum (30,000-20,000 years ago), a decrease in sea-ice during the transition from glacial to interglacial conditions (20,000-11,000 years ago) driven either by atmospheric heating or by Atlantic ocean heat incursions, and a minimum in ice during the early Holocene (~10,000 years ago) followed by an increase towards pre-industrial times. Sea-ice coverage was tightly coupled to nutrient use by surface-dwelling phytoplankton, with greater nutrient use during low sea-ice conditions — suggesting greater nutrient use by plankton in the Arctic as sea-ice continues to retreat.

We used measurements of helium-3, which tells us about how much cosmic dust was present in the past, and thorium-230, which tells us about how much helium-3 we expect to be present under ice-free conditions, in Arctic marine sediments to infer cosmic dust accumulation changes. When ice covered our sediment sites, there was a deficit in the amount of cosmic dust we would have otherwise found — because sea-ice intercepts cosmic dust falling towards Earth surface and prevents it from reaching the seafloor sediments below.

This paper is the first application of the helium-3 – thorium-230 sea-ice proxy. However, ongoing work in the lab led by current Ph.D. student Tara Kalia is now applying this proxy to additional locations in the Arctic, and time periods further back in the geologic record in order to deepen our understanding of the response of Arctic sea-ice to climate change. We are keen to continue our work devising new chemical methods to study feedbacks between the Earth and climate systems.