Heinrich events are one of the most discussed and debated phenomena related to global climate change. For each theory proposed related to the cause or effect of a Heinrich event, there is a theory contrary to the concept. Theories relating to the binging and purging of ice sheets, cyclic changes in atmospheric conditions, and the thermohaline circulation disruption of the North Atlantic Ocean all play a part in the discussion of Heinrich events. While the debate of the causes of Heinrich events is still ongoing, the effects of the events are well documented, and are clearly substantial in relation to changes in the global climate.
This paper discusses the history of Heinrich events, and will discuss current theories of their origin. Additionally, this paper will outline the scientific method for discovering more information of Heinrich events, and their relationship to the Bond Cycle, Milankovitch Cycles, and Dansgaard-Oeschger (D-O) oscillations. Finally, this paper will discuss possible effects on global climate as the result of Heinrich events, using the Younger Dryas as the basis for discussion.
Heinrich events were first discovered by Hartmut Heinrich of the German Hydrographic Institute of Hamburg, Germany in 1988. Heinrich extracted samples from northeastern Atlantic sediment cores, and analyzed the samples. He discovered that the number of lithic, or rock sediment, and planktonic foraminifera (zooplankton) shell formations fluctuated greatly in many of the core samples. Additionally, Heinrich noted that the components of the sediments seemed not to fluctuate gradually, but seemingly abruptly (Hackett, 1994).
In short, Heinrich found that, within the core samples, the planktonic foraminifera were dominant for long stretches of time. However, he noted that six separate layers of the sample distinctly showed the presence of lithic sediments, and nearly no foraminifera shell formations. Furthermore, the lithic sediment was made up of small pebbles and debris (Hackett, 1994). This type of sediment was not seen in other periods within the core.
These types of sediment deposits have been shown to be the result of rapidly changing ice sheets, and are known as Heinrich layers. As ice sheets and bergs move across the bedrock substrate during extreme cold periods, and as changes in the temperature of the ice sheet occurs, the sediment within the substrate becomes entrained to the basal layer of the ice sheet. These sediments, as a result of the ice sheet movement, are transported by the ice stream to the surface of the ice margin (Broecker, 2003). As portions of the ice sheet break off, the sediment is carried further into the ocean, either through the melting of the newly formed iceberg, or the simple movement of the current.
Additionally, meltwater at the base of the ice sheet rises at the ice margin. This meltwater contains high concentration of sediment. As the meltwater mixes with ocean water, and is absorbed by the ocean tide, this sediment is further carried into the ocean. The result is sediment high in lithic content and ice rafted detritus (Hesse, 2004).
The Heinrich layers were determined to be these types of ice rafted detritus (IRD). Bond and his colleagues (1992) examined the IRD sediment, and discovered sand grains, pebbles, and even stones, carried onto the shelf margin by ice bergs. Within the IRD, Bond (et al.) discovered high concentrations of light-colored detrital carbonate, which is usually consequential from continental weathering of carbonate rock (Bond, et al., 1992). In other words, the sediment found in the Heinrich layers originated within continental land, showing that the sediment had to have been deposited by ice sheet breakage. These breakages are known as Heinrich events.
Bond and his colleagues determined that the sediment found in the Heinrich layers of the core samples originated from limestone and dolomite, or calcium-magnesium carbonate, found in eastern Canada and northwestern Greenland. This, coupled with the map of sources used by Bond (et al.) suggested the cause of the Heinrich event responsible for the deposits was the Laurentide Ice Sheet, or North American Ice Sheet, and the Hudson Strait ice stream (1992).
To understand the causes of Heinrich events, it is imperative to first understand the conditions surrounding the events. One of the best records of a Heinrich event can be found during the last ice age, approximately 11,500 years ago. Researchers know that, at the time, the Laurentide Ice Sheet and many others like it covered North America, and have since retreated to areas including Greenland (Grousset, et al., 2000). By analyzing the ice sheets, it is possible to confirm atmospheric conditions involved in the creation of the layered sheets.
Water molecules within the ice of Greenland contain a chemical history of temperature at which the ice was created. Core samples taken from Greenland show that, over the last 10,000 years, known as "currently" to most environmental scientists, the climate in Greenland has been a stable warm period within a much colder period of 100,000 years. During this 100,000-year period, the temperatures in Greenland have been between 15 to 40 degrees Celsius cooler than current climate conditions. These cooler periods were unstable, in that in as little as a decade, the climate has altered between cold and relatively warm (Clark, et al., 1995). One such period, that of the Younger Dryas, shows such a switch to extremely cold climate conditions, and a just as abrupt switch to the current climate conditions.
These periods of slow cooling followed by abrupt warming are known as Dansgaard-Oeschger events. While these events do create a warming trend, this trend is not enough to return the climate to its previous temperatures. Thus, the next cooling cycle is even colder than the first. Following a number of these cycles, there is a terminal dramatic cooling, followed by a massive warming. This termination stage is known as the Bond Cycle.
Samples taken from Vostok, Antarctica, further clarify this wavering climate. In both samples, the climate appears to be warm for approximately 10,000 years, but is followed by drastic, erratic cooling period, lasting approximately 90,000 years. Additionally, there appears to be smaller variations occurring at 41,000 years and 20,000 years (Jouzel, 1999). Carbon dioxide bubbles trapped within the ice samples at both locations show similar time patterns.
A decade earlier, Milutin Milankovitch, a Serbian astronomer, had discovered changes in the amount of sunlight received by the earth, and these variations appeared to coincide with the temperature data from the core samples. Milankovitch had determined that the changes in sunlight were due to small changes in the orbit of the Earth. These changes altered the amounts of insolation received by the Northern hemisphere, and occurred in cycles of 100,000 years, 41,000 years and a 19,000 -- 23,000-year cycle (Hays, 1976). Hayes and his colleagues discovered that the Milankovitch cycles, or the changes from eccentric, obliquity, and precession orbital paths, corresponded to the major variations in temperature recorded in core samples (1976).
These orbital variations alter the amount of radiation which reaches the earth's surface. However, in order for the climate to be cooled dramatically, there must be some form of reflection of incoming radiation. This reflective surface, in the case of global climate, is ice. As small orbital variations cool the northern hemisphere, ice sheet formulation is initiated. This occurs during the 100,000 eccentric orbit. As an ice sheet continues to grow over the 100,000-year orbit cycle, it progressively cools the atmosphere of the northern hemisphere, which creates a faster growth rate (Bergeron, 1997).
So, in looking again at the Greenland core samples, researchers can begin to see a full picture. The Laurentide Ice Sheet and other ice sheets in North America probably came about as a result of the shifting orbit cycle of the Earth. The continued presence and growth of ice contributed to a further cooling of the climate, allowing for an even faster rate of ice formation. Since the Greenland and Heinrich Layer sediments found clearly showed sediment much like that found in the Hudson Bay area, there is a high likelihood that massive numbers of ice bergs broke from the ice sheets and entered the ocean, bringing with them large amounts of freshwater and debris, otherwise known as an episode of a Heinrich event (Bond, et al., 1992).
How these Heinrich events occur has been debated by the scientific community. One theory, proposed by MacAyeal (1993), is that of the binge-purge model. According to MacAyeal, as ice sheets begin to thicken, the insulation properties of the ice begin to allow the heating of the base. This heating is caused from the geothermal heat of the Earth itself. Once the ice sheet is thick enough, the base begins to melt, and the ice sheet slips along the bed, and eventually surges out into the ocean. This surge creates armadas of icebergs, which contain sediment such as those found in the ice cores of Greenland (MacAyeal, 1993).
Another theory of the cause of Heinrich events is the release of freshwater from within a jokulhlaup (glacier) or large lake. The result is a massive release of freshwater in a matter of minutes, or sometimes…