A Heinrich event is a natural event where large groups of icebergs break away from the Laurentide ice sheet and travel through the Hudson Strait into the North Atlantic. This event was first described by marine geologist Hartmut Heinrich. These events happened during five of the last seven glacial periods over the past 640,000 years. Heinrich events are most clearly recorded during the Wisconsin glaciation, which was part of the Last Glacial Period, but they did not occur during the Penultimate Glacial Period. The icebergs carried rock and sediment that had been eroded by glaciers. When the icebergs melted, this material sank to the ocean floor, forming layers of sediment 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 may have changed the ocean's thermohaline circulation, which is driven by differences in water temperature and salt content. These changes often coincide with signs of global climate shifts.

Scientists have proposed several explanations for Heinrich events. Most suggest that the massive Laurentide Ice Sheet, which covered much of northeastern North America during the Last Glacial Period, became unstable. 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) found in marine sediment cores from the North Atlantic. This event happens when large ice shelves in the Northern Hemisphere collapse suddenly, releasing a very large number of icebergs into the ocean. The term can also describe similar climate changes that happen at the same time in other parts of the world. These events happen quickly and last less than a thousand years, with the exact time varying between events. They can begin suddenly, even within just a few years. Heinrich events are clearly seen in many North Atlantic marine sediment cores from the Last Glacial Period. However, sediment records from earlier times are less detailed, making it harder to know if these events occurred during other ice ages in Earth's history. Some scientists believe the Younger Dryas event is a Heinrich event, which would make it event H0 (table, right).

Heinrich events seem to be connected to some, but not all, of the cold periods that come before the rapid warming events called Dansgaard–Oeschger events. These warming events are best recorded in the North Greenland Ice Core Project. However, challenges in matching the timing of marine sediment cores and Greenland ice cores have made it harder to confirm this connection.

Potential climatic fingerprint of Heinrich events

Heinrich first studied six layers found in ocean sediment cores. These layers contained a very high amount of rock pieces, called "lithic fragments," that were between 180 micrometers and 3 millimeters (about 1/8 inch) in size. Larger rock pieces cannot be moved by ocean currents, so scientists believe they were carried by icebergs or sea ice that broke off glaciers or ice shelves. As the ice melted, these rocks fell to the ocean floor. By studying the chemical makeup of these rock pieces, scientists can determine where they came from. Most of the rock pieces from Heinrich events 1, 2, 4, and 5 came from the large Laurentide Ice Sheet, which covered North America. Rock pieces from events 3 and 6 likely came from European ice sheets. The effects of these events on sediment layers vary depending on how far the layers are from the ice sheet’s source.

For events linked to the Laurentide Ice Sheet, a band of rock pieces, called the Ruddiman belt, is found around 50°N latitude. This band stretches about 3,000 kilometers (1,900 miles) from North America toward Europe. The rock pieces become much thinner as they move from the Labrador Sea to Europe. During Heinrich events, large amounts of fresh water entered the ocean. For Heinrich event 4, models suggest that about 0.29±0.05 Sverdrup of fresh water flowed into the ocean over 250±150 years. This is equivalent to about 2.3 million cubic kilometers (0.55 million cubic miles) of fresh water, which could raise sea levels by 2±1 meters (6 feet 7 inches ± 3 feet 3 inches).

Many geological signs change around the same time as Heinrich events, but it is difficult to determine if these changes happened before, after, or are unrelated to the events due to challenges in dating and matching data. Heinrich events are often marked by the following changes:

• Higher oxygen isotope ratios in northern seas and East Asian stalactites, suggesting lower global temperatures or more ice
• Lower ocean salinity due to fresh water entering the ocean
• Lower sea surface temperatures near the West African coast, shown by biochemical clues called alkenones
• Warmer temperatures in the deep ocean of the North Atlantic
• Changes in sediment disturbance caused by animals digging in the ocean floor
• Shifts in the chemical makeup of plankton, including lower oxygen isotope ratios
• Evidence of cold-loving pine trees replacing oak trees in North America
• Fewer foraminifera (tiny ocean organisms), which may be linked to lower salinity
• More sediment carried from continents, especially near the mouth of the Amazon River
• Larger grain sizes in wind-blown dust in China, suggesting stronger winds
• Changes in the amount of thorium-230, which may reflect changes in ocean current speed
• Increased sediment buildup in the northern Atlantic, with more rock pieces from land compared to usual levels
• Expansion of grasslands and shrublands across parts of Europe

These findings show how widely Heinrich events affected the Earth.

Unusual Heinrich events

H3 and H6 do not show the same clear signs of Heinrich events as H1, H2, H4, and H5. This has made some scientists think that H3 and H6 might not be real Heinrich events. If that is true, then Gerard C. Bond's idea that Heinrich events happen every 7,000 years ("Bond events") might be incorrect.

Several lines of evidence suggest that H3 and H6 were different from the other events.

• Lithic peaks: H3 and H6 have fewer lithic grains (3,000 vs. 6,000 grains per gram) compared to other events. This suggests that the continents contributed less sediment to the oceans during these times.
• Foram dissolution: Foraminifera tests are more eroded during H3 and H6 (Gwiazda et al., 1996). This may mean that nutrient-rich, corrosive Antarctic bottom water entered the area due to changes in ocean currents.
• Ice provenance: Icebergs from H1, H2, H4, and H5 have more Paleozoic "detrital carbonate" from the Hudson Strait region. However, icebergs from H3 and H6 have less of this material.
• Ice rafted debris distribution: Sediment carried by ice does not reach as far east during H3 and H6. Some scientists think some of the clasts from these events might have come from Europe. Since America and Europe were once next to each other, the rocks from both places look similar, making it hard to determine the exact source.

Causes

Climate-related events are often complex and influenced by many factors rather than a single cause. Scientists identify two main categories of possible causes for these events.

One model suggests that changes within ice sheets, such as the Laurentide Ice Sheet, lead to periodic ice disintegration known as Heinrich events. During the "binge phase," ice gradually builds up on the sheet. When the sheet reaches a critical size, soft sediment beneath the ice acts like a lubricant, causing the ice to slide rapidly during the "purge phase," which lasts about 750 years. An earlier idea proposed that heat from Earth’s interior melted the sediment under the ice once the ice volume became large enough to trap heat.

Mathematical analysis supports a repeating pattern every 7,000 years, similar to observations linked to Heinrich events H3 and H6. However, if H3 and H6 are not Heinrich events, the binge-purge model’s predictions about periodicity become less reliable.

Some scientists question why similar events are not seen in other ice ages, possibly due to a lack of detailed sediment records. Additionally, the model predicts that smaller ice sheets during the Pleistocene would result in fewer and less severe Heinrich events, but evidence does not support this.

External factors, such as changes in solar energy or ocean currents, may also trigger Heinrich events. However, these factors must be strong enough to overcome the massive ice volumes involved.

Gerard C. Bond proposed that changes in solar energy over 1,500 years might influence climate cycles called Dansgaard-Oeschger events, which are linked to Heinrich events. However, the small changes in solar energy alone are unlikely to cause major effects without strong feedbacks within Earth’s systems. Instead, rising sea levels during warming periods may destabilize ice shelves by eroding their bases. When one ice sheet collapses, it raises sea levels further, causing more ice sheets to destabilize. Evidence supports this idea, as ice sheet breakups in events like H1, H2, H4, and H5 occurred in different regions over time.

The Atlantic Heat Piracy model suggests that changes in ocean currents redistribute heat between hemispheres. The Gulf Stream currently carries warm water northward, but adding fresh water to northern oceans could weaken this current, allowing colder water to flow southward. This would cool the northern hemisphere and warm the southern, altering ice accumulation and potentially triggering Heinrich events.

Rohling’s 2004 Bipolar model suggests that rising sea levels lift floating ice shelves, weakening them and causing ice sheets on land to flow into the ocean and break apart.

Climate models show that adding freshwater to the Nordic Seas can cause major shifts in ocean and atmospheric circulation, even with small changes. Heinrich events are linked to cooling in the subtropical Atlantic, not Greenland, supported by paleoclimatic data. Maslin et al. (2001) connected this to Dansgaard-Oeschger events, explaining that melting ice reduces the strength of the North Atlantic Deep Water current, shifting heat to the southern hemisphere and melting Antarctic ice. This cycle repeats until sea level rise destabilizes the Laurentide Ice Sheet, causing a Heinrich event.

Hunt & Malin (1998) suggested that earthquakes near ice margins, caused by rapid ice melting, might trigger Heinrich events.