Annotated Bibliography

Heinrich Events and Climate Change in the Quaternary

Sara Gran

Andrews, J.T. and K. Tedesco (1992). "Detrital carbonate-rich sediments, northwestern Labrador Sea: Implications for ice-sheet dynamics and iceberg rafting (Heinrich) events in the North Atlantic." Geology 20: 1087-1090.

The presence of detrital carbonate sediments found in North Atlantic sediment cores (in layers known as Heinrich Events, after Heinrich, 1988) suggest that the Laurentide Ice Sheet (LIS) extended at least to the continental shelf out from the Labrador Sea (or the other Canadian outlets that contain carbonate rock outcrops). More sediment cores, taken from the far north-western Atlantic also show the carbonate layers (and corresponding drop in foraminifera) in a similar pattern to those studied from farther out in the Atlantic. Due to this core's proximity to the continent, it is likely that the detritus in these layers resulted from not icebergs, but rather the presence of the actual ice sheet (or ice stream) over the continental shelf. The material would have come off the bottom of the ice sheet, as well as from sediment-carrying streams flowing over the ice surface and turbidites from built-up sediment. The thickness of these sediments drops off sharply past the shelf, implying that the very thick detrital layers are due to the ice sheet being right there, whereas the thinner detrital layers found farther out came from icebergs. This situation (of IRD) does not occur in modern-day glacial environments (for example, Antarctica).

Andrews, John T. and Helmut Erlenkeuser, Katherine Tedesco, Ali E. Asku, A.J. Timothy Jull (1994). "Late Quaternary (Stage 2 and 3) Meltwater and Heinrich Events, Northwest Labrador Sea." Quaternary Research 41: 26-34.

There is evidence from within the Labrador Sea of Heinrich events 1, 2, and either 3 or 4. Similar to more distal to North America sections of the North Atlantic, Heinrich Events in the Labrador Sea are marked by a decrease in forams, increase in detrital carbonate, and a change in the oxygen isotopes (increase in d18O). The authors discuss Heinrich Events as meltwater events, as opposed to ice-rafting events (though there is evidence for ice rafting as well) mainly because their cores were taken so close to shore. Because of this proximity, the isotope change due to salt dilution should be more drastic than in more proximal cores. The sedimentation rate is also higher (in this study, determined to be higher than previously thought) so a better age-resolution was obtained. In addition to the higher sedimentation rate, the development of AMS 14C measuring techniques allows fairly detailed dating of the sediment in the cores. These are precise enough to be able to speculate that the "third" Heinrich event in the Labrador Sea may actually be event 4 (number 3 is absent). If this is the case, it is likely that event 3 was not North America-sourced.

Blanchon, Paul and John Shaw (1995). "Reef drowning during the last deglaciation: Evidence for catastrophic sea-level rise and ice-sheet collapse." Geology 23 no. 1: 4-8.

Drowned coral reefs in the Caribbean give evidence for three meter-scale increases in sea level during the past 30,000 years. These increases occur simultaneously with Heinrich events. The ocean is affected by the components involved with Heinrich events in that climate is different, ice sheets are collapsing, ocean circulation patterns change, and very large volumes of sub-ice sheet meltwater enters the ocean. What happens is that a certain species of coral (Acropora) has a very limited depth range for habitation. If sea level goes up a meter, corals near the lower limit of the range drown. By radiometrically-dating the corals and finding out when they drowned, one can tell when sea level rose (and get an idea how fast it rose). By determining that this massive meltwater influx into the ocean was pretty fast, Blanchon and Shaw suggest that this expulsion was itself the cause of the ice sheet collapse. Since mid-latitudes are most affected by the changes in summer insolation, and the northern hemisphere's mid-latitude region was large enough to sustain a huge ice cap, the summer insolation maxima was able to produce a huge volume of meltwater. The release of this water was (theoretically) the cause of ice sheet collapse and ocean circulation changes and sea level changes.

Bond, Gerard, and Wallace Broecker, Sigfus Johnson, Jerry McManus, Laurent Labeyrie, Jean Jouzel, Georges Bonani (1993). "Correlations between climate records from North Atlantic sediments and Greenland ice." Nature 365: 143-147.

Dansgaard-Oeschger temperature ocillations, seen in Greenland ice core data, seem to be bundled into slow-cooling cycles occurring every 10-15 kyr, culminating in a Heinrich event (massive iceberg discharge). These are seen in NA sediment core data. With a resolution of 300-500 years, Bond et al. match the sediment and ice core records (isotope and foraminifera proxies for temperature) by rubbersheeting (pinned to the abrupt terminations of the D-O cycles). They justify this matching by saying that the cores were taken near enough to each other (ice and sediment) that the climate should have been in phase at each location. Iceberg discharges (seen as detrital carbonates in the sediment cores) appear to occur at the ends of the cold cycles in the D-O pattern. It is still uncertain whether the ice sheet collapse caused the cooling in climate or vice versa. It is possible, however, that the sudden warming at the ends of these cooling cycles could have been caused by the iceberg discharge. Iceberg discharge cannot explain the other rapid temperature fluctuations occuring between Heinrich events.

Bond, Gerard and Hartmut Heinrich, Wallace Broecker, Laurent Labeyrie, Jerry McManus, John Andrews, Lylvain Huon, Ruediger Jantschik, Silke Clasen, Christine Simet, Kathy Tedesco, Mieczyslawa Klas, Georges Bonani, and Susan Ivy (1992). "Evidence for massive discharges of icebergs into the North Atlantic ocean during the last glacial period." Nature, 360: 245-249.

'Heinrich events,' defined as the phenomena that create layers poor in foraminifera within deep sea sediment cores from the North Atlantic, were likely created by the cooling of surface ocean water and the influx of large amounts of ice-rafted debris (IRD) into the ocean, between 20-70Ka. These layers appear to form a swath originating from the Labrador sea and extending almost all the way to Europe. In these layers, IRD consists of high amounts (25-30%) of limestone and dolomite fragments that are rare in the non-Heinrich layers. IRD does not occur in all Heinrich layers, but the decrease in planktonic d18O does. A possible scenario creating these layers is that first surface temperature of the ocean dropped, and whatever caused that caused the massive influx of icebergs out of the sea of Labrador, which followed a flow path (defined by ocean circulation patterns) across the Atlantic. These icebergs melt, lowering the salinity of the surface water (possibly shutting off the thermohaline circulation pattern) and drop carbonate material entrained in the ice. The timing and pattern of these events do NOT (according to the authors) correspond with any Milkankovich-type cycles, however they do not propose any other climate-forcing mechanism that could result in the cooling and discharge of ice.

Broecker, Wallace S. (1994). "Massive iceberg discharges as triggers for global climate change." Nature 372: 421-424.

There is likely some connection between the iceberg discharges recorded as Heinrich events and the heat transport mechanism of the North Atlantic. There is much evidence that these HE's are connected with rapid climate change. This evidence includes the correlation between the ice core record and the sediment record of the North Atlantic. It is still uncertain, however whether the iceberg discharges cause climate change or vice versa.

Heinrich layers consist of several components. First, there is a high percentage of carbonate fragments, which are not present in the "ambient" glacial material in the sediments. The K-Ar ages of the clay particles are higher in the Event layers than those not in the layers, and the clay is of a different composition. And there is a lack of foram shells in the layers. These characteristics point towards a period of colder, fresher water at the ocean surface and influx of icebergs of a Northeast Canadian source.

The events appear to occur at major climatic boundaries. The events are cyclic and fit in with D-O and Bond cycles (all of which appear to have global impact). The global aspect could be associated with the thermohaline circulation--the influx of fresh water caps the North Atlantic, shutting down the heat conveyor during a cold period.

Clark, Peter U. (1994). "Unstable Behavior of the Laurentide Ice Sheet over Deforming Sediment and Its Implications for Climate Change." Quaternary Research 41: 19-25.

Ice sheets behave differently over bedrock than over unconsolidated material. When a glacier flows over unconsolidated material, it is possible that the material can act as a deformable bed--a situation where the sub-glacial material has no cohesive or shear strength and can rapidly deform. Evidence for surging of the Laurentide Ice sheet is concetrated on areas overlain by unconsolidated sediment, implying (disclaimer--I think this is hooey) that surging only occurs over deforming beds. In this situation, ice sheet dynamics (and subsequently Heinrich Events and the concurrent ocean circulation and temperature changes) are controlled by the ice sheet and its substrate, not climate. By the time the ice retreated to the center of the Canadian Shield and was on solid bedrock again, it was about 10-14 Ky and since then the climate has been relatively stable.

Clark, Peter U. and Patrick J. Bartlein (1995). "Correlation of late Pleistocene glaciation in the western United States with North Atlantic Heinrich events." Geology 23 no. 6: 483-486.

There is evidence from alpine and Cordilleran ice sheet glaciers in the Western US of ice advance and retreat correlated to the North Atlantic Heinrich Events. Alpine glaciers respond rapidly to climate change, and if Heinrich events are related somehow to climate change, that should be recorded in places like the Rocky Mountains. It appears that, at least in some areas, glaciers advance to some stable terminus several thousand years prior to an HE, then rapidly retreat at the same time as the HE. This implies that the climate change associated with the collapse of the LIS affected western North America (the ice sheet dynamics in both areas are different so it would have to be climate as the forcing factor for the alpine glaciers). These glacial advances in the West have not been dated to a very high precision, but they occur at a similar frequency to Heinrich events and could therefore be related. (another disclaimer, these correlations are kind of shaky)

Gwiazda, R.H, and S.R. Hemming, W.S. Broecker (1996). "Provenance of icebergs during Heinrich event 3 and the contrast to their sources during other Heinrich episodes." Paleoceanography 11 no. 4: 371-378.

In order for Heinrich events to be caused by "binge and purge" model ice sheet behavior, it needs to be established that the model fits what all ice sheets tend to do over time. In the sequence of Heinrich Events, it appears that numbers 1, 2, 4 and 5 have very similar characteristics, whereas number 3 is quite different. The binge and purge model must therefore be able to explain the differences in number 3. Numbers 1, 2, 4, and 5 start abruptly whereas 3 is gradual, and the former contain IRD while number 3 does not. Number 3 also has a different istope signature, more consistent with the ambient (non-Event) material. Material from the former can be traced to the Canadian Shield, whereas from event 3 the Baltic shield of northern Europe is a more likely source. Since there still is a drop in forams for this event layer, it probably indicates a cooling of the surface water without an iceberg armada advance.

Heinrich, Hartmut (1988). "Origin and Consequences of Cyclic Ice Rafting in the Northeast Atlantic Ocean during the Past 130,000 Years." Quaternary Research 29: 142-152.

Deep sea sediment cores from underwater hills in the North Atlantic were studied in order to determine what information cores can reveal about climate change and glaciation, as well as better-defining the relationships between the ocean, atmosphere and landmasses. A major problem with studying sediments and their relationship to climate is that age resolution is often not sufficient to determine what was the causal event. For this study, a series of cores were taken, and sub-samples were sieved to obtain a fraction containing foraminifera and sand-sized particles. The foraminifera were dated and the rock fragments studied to determine whether they were ice-rafted debris (IRD) or not. Two graphs, one showing percentage of IRD and the other of foraminifera show that sharp peaks of IRD occur simultaneously with cold-water species of forams. Cores taken from near each other both show all these peaks (5). These peaks of IRD occur twice during the 11,000 year precessional cycle at fairly regular intervals, suggesting that iceberg flotillas occur during times of winter solar minima (really cold winters) and summer solar minima (really cold summers).

Keigwin, Lloyd D. and Scott J. Lehman (1994). "Deep circulation change linked to HEINRICH event 1 and Younger Dryas in a middepth North Atlantic core." Paleoceanography 9 no. 2: 185-194.

The focus of this research is the effects, if any, of Heinrich events on deep ocean circulation change. Cores were taken south of the central axis of ice rafted debris because the layers with most IRD do not have enough forams to get a good isotope record within those time periods. There is evidence of cold and fresh surface water conditions during Heinrich Event 1 (as well as the Younger Dryas), even though this core was taken out of that main iceberg-trajectory axis. This evidence is the following: "increased abundance of IRD, reduced abundance of total planktonic foraminifera, maximum percentage of the polar planktonic form species N.pachyderma (s) and decreased d18O of planktonic foraminifera.

Lehman, Scott (1993). "Ice Sheets, wayward winds and sea change." Nature 365: 108-110.

This paper is a review of the recent research on paleoclimatology related to Heinrich events. Greenland ice core data correspond with sediment cores taken from the North Atlantic in their recording of some sort of climatic/ice sheet shifting. Bond cycles (cycles in the ice-core and sediment records that show that temperature ocillations come in bundles, where the warm part of the cycle becomes progressively cooler and then rapidly rises again) contain the shorter-frequency temperature ocillations known as Dansgaard-Oeschger events. It is probable that all of these cycles are at least in part driven by ocean circulation patterns. Some researchers propose that the circulation cycles are driven solely by salt concentrations, others suggest that the influx of icebergs associated with the Heinrich events are the cause. A major question is how ice sheets are relted to the climate/ocean/ice core histories. MacAyeal proposes that an ice sheet (the LIS) starts out frozen to its bed and gets progressively taller (with steep marginal profiles). Eventually, geothermal heat (or whatever) warms the base of the ice sheet enough to "float" the base and cause massive collapse (and iceberg discharge). As the ice sheet grows, it changes air circulation (cooling and intensifying winds crossing the Atlantic). This postulated cooling could be the cause of the steady cooling seen in the Bond cycles. After collapse, the air circulation could have quickly returned to its warmer pattern. It does not appear that icebergs are the sole cause of ocean circulation patterns, because temperature ocillation occurs more frequently than do the layers of IRD in the Heinrich layers. It is still uncertain what was the cause of the circulation patterns.

Mayewski, P.A and 13 others (1994). "Changes in Atmospheric Circulation and Ocean Ice Cover over the North Atlantic During the Last 41,000 Years." Science 263: 1747-1751.

Greenland ice core records indicate that the massive iceberg discharges associated with Heinrich events appear to occur at times with increased ocean ice cover and increased air circulation (referred to as the Polar Circulation Index, or PCI). These two factors are determined by ice core isotopes and chemistry, based on temperature and dust and sea salt influx. For stadial/interstadial cycles not associated with Heinrich events, there appears to be an upper limit for ocean cover (which is different than ocean cover for the events).

Moores, Howard D. and J.D. Lehr (1997). "Terrestrial record of Laurentide Ice Sheet reorganization during Heinrich events." Geology 25 no. 11: 987-990.

If Heinrich events are caused by binging and purging of the Laurentide Ice sheet (proposed by MacAyeal, 1993), then it should be possible to see a terrestrial record of the ice sheet collapse. Late Wisconsinan deposits in Minnesota may indicate such a record of growth and quick retreat. There are three main glacial phases that each are marked by a prominent moraine system and different marker lithologies. With each Heinrich event/ice sheet collapse, the ice divide shifts, causing different lithologies to be "upstream" of Minnesota. One of the fundamental ideas about glacial surging is that the ice cap must undergo a shift in the location of the ice divide, because such a huge slug of ice is removed from the system. This appears to be the case in MN, though dating of the ice divide shifts is not discussed. A precautionary note: the MN geologic community didn't particularly agree with these findings.

Paillard, D. and L. Labeyrie (1994). "Role of the thermohaline circulation in the abrupt warming after Heinrich events." Nature 372: 162-164.

The process behind Heinrich events affect thermohaline circulation in two ways. As an ice sheet builds up and releases its huge iceberg armada and subglacial meltwater volumes, the thermohaline circulation slowly grinds to a halt because of all the fresh water. After the collapse, the freshwater source is effectively cut off (no more meltwater, no more icebergs) and the circulation pattern can start again rather abruptly. Also, after the ice sheet collapse, air circulation over the ice sheet is not cooled so much (the ice sheet elevation is lower and the sheet in general is smaller). Warmer water over the Atlantic increases evaporation and therefore salinity. During the time of no deep water formation and circulation, equatorial regions get warmer and polar regions get colder because the ocean circulation is not transferring heat. After the circulation starts up again, VERY warm water from the equator can rapidly warm the polar regions.