A Heinrich event is a natural event where large groups of icebergs break away from the Laurentide ice sheet and move through the Hudson Strait into the North Atlantic Ocean. These events were first identified by scientist Hartmut Heinrich and happened during five of the last seven glacial periods over the past 640,000 years. They are most clearly recorded during the Wisconsin glaciation, part of the Last Glacial Period, but did not occur during the Penultimate Glacial Period. The icebergs carried rock and sediment that had been scraped by glaciers. When the icebergs melted, this material sank to the ocean floor, forming layers of rock called Heinrich layers.
The melting of these icebergs added large amounts of fresh water to the North Atlantic. This sudden addition of cold, fresh water might have changed the ocean’s thermohaline circulation, which is driven by differences in water density. These changes often happened at the same time as signs of global climate shifts.
Scientists have proposed several reasons for Heinrich events, most of which involve instability in the large Laurentide Ice Sheet, which covered much of northeastern North America during the Last Glacial Period. Other ice sheets in the Northern Hemisphere, such as the Fennoscandic and Iceland/Greenland ice sheets, may also have played a role. However, the exact cause of this instability is still not fully understood.
Description
A Heinrich event is a climate event that causes layers of ice rafted debris (IRD)—pieces of ice and rock carried by icebergs—to appear in marine sediment cores from the North Atlantic. This event happens when large ice shelves in the Northern Hemisphere collapse, releasing a huge number of icebergs into the ocean. The term can also describe similar climate changes observed in other parts of the world around the same time. These events occur quickly, lasting less than a thousand years, with each event lasting a different amount of time. Their sudden start can happen within just a few years. Heinrich events are clearly seen in many North Atlantic marine sediment cores from the Last Glacial Period. However, older sediment records are less detailed, making it harder to determine if these events happened during other glacial periods in Earth's history. Some scientists believe the Younger Dryas event is a Heinrich event, which would label it as event H0 (table, right). Heinrich events seem connected to some, but not all, cold periods that happened before the rapid warming events called Dansgaard–Oeschger events, which are best recorded in the North Greenland Ice Core Project. However, challenges in matching the timing of marine sediment cores and Greenland ice cores to the same time scale have raised questions about the accuracy of this connection.
Potential climatic fingerprint of Heinrich events
Heinrich first noticed six layers in ocean sediment samples that contained very high amounts of rock pieces, called "lithic fragments," measuring between 180 micrometers and 3 millimeters (about 1/8 inch). These larger rock pieces cannot be moved by ocean currents, so scientists believe they were carried by icebergs or sea ice that broke away from glaciers or ice shelves. As the ice melted, the icebergs dropped debris onto the ocean floor. Scientists use chemical tests on this debris, called IRD, to determine its origin. Most of the debris came from the large Laurentide Ice Sheet, which covered North America during Heinrich events 1, 2, 4, and 5. Smaller events, 3 and 6, likely involved European ice sheets instead. The effects of these events on sediment layers change depending on how far the layers are from the source of the ice.
For events linked to the Laurentide Ice Sheet, a band of IRD appears around 50° N, called the Ruddiman belt. This band stretches about 3,000 kilometers (1,900 miles) from North America toward Europe, becoming much thinner as it moves from the Labrador Sea to the European end of the modern iceberg route (Grousset et al., 1993). During Heinrich events, large amounts of fresh water entered the ocean. For Heinrich event 4, a model study estimated that about 0.29±0.05 units of water flow occurred over 250±150 years. This is equal to roughly 2.3 million cubic kilometers (0.55 million cubic miles) of fresh water, enough to raise sea levels by about 2±1 meters (6 feet 7 inches ± 3 feet 3 inches).
Several geological signs change around the same time as Heinrich events, but it is difficult to determine if these signs happened before, after, or are unrelated to the events due to challenges in precise dating. Heinrich events are often shown by the following changes:
The global spread of these records highlights the major effects of Heinrich events.
Unusual Heinrich events
H3 and H6 do not have the same clear set of features as H1, H2, H4, and H5, which has made some scientists think they may not be true Heinrich events. If that is true, then Gerard C. Bond's idea that Heinrich events follow a 7,000-year cycle ("Bond events") becomes questionable.
There is evidence showing that H3 and H6 differ in some ways from the other events.
Causes
Climate-related issues are often very complicated, making it difficult to link them to a single cause. Scientists have identified several possible reasons for these events, which can be grouped into two main categories.
One model explains that changes within ice sheets may cause large ice masses to break apart periodically, leading to events known as Heinrich events. This model describes two phases: the "binge phase," where ice accumulates slowly on the Laurentide Ice Sheet, and the "purge phase," where the ice sheet slides rapidly over soft, slippery sediment after reaching a critical mass. This process lasts about 750 years. The original model suggested that geothermal heat from Earth’s interior caused the sediment to melt when the ice sheet became large enough to trap heat.
The mathematics of this system support a 7,000-year cycle, similar to patterns observed in some Heinrich events (H3 and H6). However, if H3 and H6 are not actually Heinrich events, the binge-purge model becomes less reliable, as its predictions depend on this cycle.
Some scientists question why similar events are not seen in other ice ages, but this may be due to a lack of detailed sediment records. Additionally, the model predicts that smaller ice sheets during the Pleistocene should result in fewer and less intense Heinrich events, but evidence does not support this.
Other factors outside of ice sheets may also cause Heinrich events, but these would need to be very strong to overcome the massive ice volumes involved. Gerard C. Bond proposed that changes in solar energy over 1,500 years might be linked to climate cycles like the Dansgaard-Oeschger events and Heinrich events. However, the small changes in solar energy alone are unlikely to cause such large effects without significant feedback processes within Earth’s systems. Instead, rising sea levels caused by warming may destabilize ice shelves. When sea levels rise, they can erode the base of ice sheets, causing them to collapse. This collapse releases more ice, further raising sea levels and triggering more ice failures. Evidence supports this idea, as ice sheet breakups in some Heinrich events occurred before melting in Europe, up to 1,500 years earlier.
The Atlantic Heat Piracy model suggests that changes in ocean currents can shift heat between hemispheres. For example, the Gulf Stream currently carries warm water from the equator toward the northern Nordic Seas. Adding freshwater to northern oceans could weaken the Gulf Stream, allowing colder water to flow southward. This would cool the northern hemisphere and warm the southern hemisphere, altering ice accumulation and melting rates and possibly triggering Heinrich events.
Rohling’s 2004 Bipolar model suggests that rising sea levels may lift floating ice shelves, causing them to collapse. Without floating shelves to support them, continental ice sheets could flow into the ocean, breaking apart into icebergs and sea ice.
Climate models show that adding freshwater to the Nordic Seas can cause major changes in ocean and atmospheric systems, even with small increases or decreases in freshwater. These changes may lead to Heinrich events, but the cooling associated with these events occurs farther south, in the subtropical Atlantic, rather than near Greenland. This idea connects to Dansgaard-Oeschger events, as melting from one region can weaken ocean currents, causing warming in another region and triggering further melting. Eventually, enough melting could raise sea levels enough to destabilize the Laurentide Ice Sheet, leading to a Heinrich event and restarting the cycle.
Hunt & Malin (1998) proposed that Heinrich events might be caused by earthquakes near ice margins, triggered by rapid ice melting.