Introduction


A fundamental climate research question is the mechanism behind the Earth's warming (interstadials) and cooling (stadials) cycles. One conjecture is that variations in the Atlantic Meridional Overturning Circulation (AMOC), which is the major north/south current in the Atlantic, has played an important role in the abrupt warming/cooling cycles during the last ice age (110,000 to 12,000 years ago reaching a maximum glacial extent about 22,000 years ago). The Atlantic overturning current brings warm southern surface waters to the North Atlantic where they become saltier and drop to the ocean floor to form North Atlantic deep water which flows south. This study provides data from ocean floor sediments that strongly supports this conjecture.

Chronology of events during last ice age
Chronology of events during last ice age
Between 60,000 and 25,000 years ago, during what is called Marine Isotope Stage 3 (MIS 3) in Greenland, there were 15 abrupt warming (8–16 °C) events, each followed by renewed cooling. These cycles on average lasted about a millennium. The most severe of these cycles correlate with catastrophic iceberg discharges into the North Atlantic Ocean (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 with the Antarctica ice core record in what is called a north/south sea-saw. 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.

One of the explanations for these abrupt events and the sea-saw is changes in the ocean’s persistent circulation pattern, in particular the Atlantic overturning current. 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. 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 Northern warming events.

Observations and analysis


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 (δCarbon-13) and oxygen-18 (δOxygen-18) isotope ratios in the microfossil shells of single cell organisms living on the ocean floor and in surface waters. These shells are composed of calcium carbonate (CaCO3).

The sedimentary Pa/Th ratios provide information about the strength of past ocean circulation. Low Pa/Th ratios are associated with vigorous overturning. The Carbon-13 records of organisms living on the sea floor (BF for benthic foraminifera) are used as a proxy for changes in water mass nutrient concentrations. Low Carbon-13 values are indicative of high-nutrient concentrations and weakly ventilated water masses. The Oxygen-18 isotope ratios are a proxy for temperature. Together these records reveal information about the origin and residence times of deep water masses with a resolution of centuries.

Climate and circulation indicators
Climate and circulation indicators
Cool periods (stadials) are numbered with vertical bars. (A) NGRIP ice core δOxygen-18(ice) (B) SST (°C) (C) Calcium x-ray fluorescence (orange) mapped to %CaCO3 (D) Pa/Th from bulk sediment (green) (E) δCarbon-13(BF) from the sea floor

Low Pa/Th ratios observed during interstadials are evidence of a vigorous Atlantic overturning current during warming periods. The increased values of Carbon-13 data from the sea floor δCarbon-13(BF) during each interstadial are indicative of a deep, as opposed to a shallow, overturning current. The difference in measured values between deep, vigorous overturning during interstadials and shallower, weaker overturning during stadials is most pronounced during all the Hudson Strait cooling periods, when catastrophic discharge of melting icebergs from Canada covered the subpolar North Atlantic.

Results


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 δCarbon-13(BF) results therefore indicate a persistent pattern of 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. 

To ascertain whether Northern Hemisphere climate events are triggered by or reinforced by changes in Atlantic overturning current, the researchers investigated the phase relationship between surface and deep-sea properties. Cross-correlations were performed on each of δCarbon-13(BF), Pa/Th, sea surface temperature (SST), and CaCO3 observations with North Greenland Ice Core Project (NGRIP) δCarbon-18 measurements. Both sea surface temperature and Greenland temperature proxies were found to lag the ocean circulation data in a consistent fashion. In addition previous research has shown that the northern events lead Antarctic temperature changes. 

This evidence for significant 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.

North Atlantic ocean circulation and abrupt climate change during the last glaciation, L. G. Henry et al., Science 29 Jul 2016, Vol. 353, Issue 6298, pp. 470-474, DOI: 10.1126/science.aaf5529