Around 250,000 years ago, most of North America and northern Europe was covered in glaciers. These massive ice sheets formed over thousands of years and waxed and waned in response to a long list of factors, including wobbles in Earth’s orbit, alterations in the atmosphere, and changing ocean currents. Now, scientists have added another mechanism to that list: distant monsoons.
Continental ice sheets are supposed to remain fairly stable during an ice age, but that is not always the case. During a frigid period of the late Quaternary, large chunks of the European Ice Sheet melted and refroze, temporarily creating large, warm pools of meltwater in the Bay of Biscay, the gulf between Spain and France.
Recent studies have shown that distant, low-latitude weather events such as the East Asian monsoon can impact glaciers far away, at much higher latitudes. To test whether monsoons could explain the European Ice Sheet’s ebb and flow, Kaboth-Bahr et al. examined cores of sediment extracted from the deep seafloor near the Strait of Gibraltar, as well as two other locations. Using the ratio of light to heavy minerals in the sediment as a proxy for the volume and force of water flow at the seafloor, they reconstructed ancient ocean currents dating back 250,000 years.
The sediment revealed that when monsoons periodically weakened in northeast Africa, thus drying up the Nile River, the change caused saltier water to rush out of the Mediterranean Sea through the Strait of Gibraltar and into the Atlantic. As this relatively warm, salty water hit the Atlantic, it produced what is known as the Azores Current, a branch of the Gulf Stream that drags warm Atlantic waters toward England and northern Europe. During glacial periods, the pooling of this warm Atlantic water generates the moisture that makes ice sheets grow.
This monsoon-driven current helps to explain the European Ice Sheet’s periodic growth, the team suggests. It could also help scientists understand how monsoons are affecting Earth’s glaciers today. (Geophysical Research Letters, https://doi.org/10.1029/2018GL078751, 2018)
—Emily Underwood, Freelance Writer