The last ice age was punctuated by many millennial scale abrupt warming (8–16 °C) and cooling events in the Northern Hemisphere. This study reports measurements from ocean floor sediments that provide the first direct evidence that not only do variations in the primary North/South Atlantic current correlate with these periods of rapid warming and slower cooling, but that the changes in the Atlantic overturning current occurred before and likely initiated these warming/cooling cycles.
During the last ice age, which began about 110,000 years ago and reached its maximum glacial extent 22,000 years ago, there were twenty five millennial scale warming and cooling events. Focussing on the more recent part of the ice age between 60,000 and 25,000 years ago there were 15 abrupt warming (8–16 °C) events, each followed by renewed cooling. These cycles on average lasted about about a millennium and half. The most severe of these cycles correlate with catastrophic iceberg discharges into the North Atlantic Ocean called Heinrich events. Most of the ice in these events has been shown to be sourced from the Hudson Strait (HS) region in modern northern Canada.
The Northern Hemisphere warming/cooling events are associated inversely with Antarctica cooling/warming events. During northern cooling events, ice cores have shown that Antarctica warmed, and each rapid northern hemisphere warming was followed shortly by cooling at high southern latitudes. Referred to as the north/south sea-saw it involves massive heat flows between the two hemispheres. Research has shown that the Northern Hemisphere events precede the Antarctica cooling/warming periods.
One of the explanations for these abrupt events in the Northern Hemisphere is changes in the ocean’s persistent circulation pattern, in particular the primary Atlantic north/south current. The Atlantic Meridional Overturning Current (AMOC) draws southern tropical surface waters to the North Atlantic where they become increasingly saline and sink to become deep water and flow south. It has been conjectured that the Atlantic overturning current is in a weak or shut-down state during Northern cold periods. The Northern warming events are caused by a strengthening or resumption of the Atlantic overturning current that greatly increases heat transport to the North Atlantic from the Southern Ocean. While there has been indirect support for this conjecture, this study is the first that provides solid evidence supporting the central role of the Atlantic overturning current in triggering these rapid Northern Hemisphere warming events.
In this study several radioisotopic ratios were measured in samples from a 35 meter long core taken from the Bermuda Rise in the northwestern Atlantic Ocean. The researchers measured the radioisotope protactinium and thorium ratio (Pa/Th) in the ocean sediments along with carbon-13 and oxygen-18 isotope ratios in the deposited microfossil shells containing calcium carbonate (CaCO3) of single cell organisms that lived on the ocean floor and in surface waters between 60,000 to 30,000 years ago.
The sedimentary Pa/Th ratios provide information about the strength of past ocean circulation. Low Pa/Th ratios are associated with vigorous overturning. Increased carbon-13 ratios indicate increased productivity in the ocean. Photosynthesizing organisms preferentially uptake carbon-12 during photosynthesis, which leaves more C13 in the water column with which marine organisms build their shells. When there is a lot of photosynthesis happening in the water column this usually means there is increased burial of carbon in the form of calcium carbonate on the seafloor. The oxygen-18 isotope ratio is a proxy for sea surface temperature. Together these records reveal information about the strength of current and residence times of deep water masses with a resolution of centuries.
The low Pa/Th ratios observed during the warming periods are evidence of a vigorous Atlantic overturning current. The increased values of carbon-13 data from the sea floor during each warming period are indicative of a deep, as opposed to a shallow, overturning current. The difference in measured values between deep, vigorous overturning during warming periods and shallower, weaker overturning during cooling periods is most pronounced during all the Hudson Strait cooling periods, when catastrophic discharge of melting icebergs from Canada covered the subpolar North Atlantic.
These new results reveal that variations in the Atlantic overturning current were associated with every cooling/warming event between 25,000 and 60,000 years ago. The largest difference in the measurements correspond to the Hudson Strait cooling periods. Together with previous investigations, these new results confirm that all Hudson Strait ice discharge events of the past 60,000 years were associated with a dramatic increase in Pa/Th and are evidence for major reduction in Atlantic overturning current in association with the largest ice rafting events.
The combined Pa/Th and carbon-13 ratio results therefore indicate a persistent pattern of AMOC weakening during cold periods and strengthening during warm periods, with the largest reductions in Atlantic overturning current associated with all Hudson Strait ice discharge events.
Relative timing of AMOC changes and warming/cooling events
To ascertain whether Northern Hemisphere climate events are triggered by or reinforced by changes in the Atlantic overturning current, the researchers investigated the phase relationship between surface and deep-sea properties. Cross-correlations were performed on each of the deep sea carbon-13 ratio, Pa/Th ratio, sea surface temperature (SST), and calcium carbonate composition observations with North Greenland Ice Core Project (NGRIP) oxygen-18 measurements. Both sea surface temperature and Greenland temperature proxies were found to lag the the changes in AMOC circulation.
The evidence for significant Atlantic circulation changes followed by northern hemisphere climate events, combined with the previously reported lag of Antarctic temperature variations, supports the conjecture that changes in heat transport by ocean currents are a trigger for abrupt northern hemisphere warming.