DANSGAARD-OESCHGER CYCLES AND HEINRICH LAYERS

The earlier part of this section of the course has emphasized the Milankovitch portion of the frequency spectrum because it is a region where there is well defined forcing that appears to generate responses in many component of the climate system. Likewise, although we have not emphasized high frequency climate change, El Nino and higher frequency signals are a current focus of paleoceanographic research. In between is a frequency band (corresponding roughly to periods of 10 to 20,000 years) where nothing is known about possible forcing mechanisms, yet where there is considerable evidence of climate variability.

The first time series to provide a continuous record of this variability were ¹⁸O profiles from Greenland ice cores. Figure DO-1 shows such an oxygen isotope record spanning about 200 ky in which the current interglacial displays little variability, whereas the preceding glacial record is very "spiky".

Figure DO-1. Variation of ¹⁸O with depth in a Greenland ice core. An approximate age scale is given in the middle of the figure. The record below about 2,750 m is suspect due to internal deformation of the ice.

Figure DO-2 shows the ¹⁸O record for the last 100 ky in a second, better dated, Greenland core, this time with the depth axis converted to time. The high frequency spikes (Dansgaard-Oeschger (D-O) cycles) between the current and prior interglacials have amplitudes of 4-5 per mil, which implies a warming of the air temperature of 6-7 degrees C, or roughly half the glacial-interglacial range. The onset of these brief warming periods was very rapid (decades or less) in each case, whereas cooling was more gradual. The pattern is reminiscent of glacial-interglacial ¹⁸O patterns in deep sea cores.

Figure DO-2. ¹⁸O versus age in a recent well-dated ice core from Greenland. Note the amplitude of the D-O cycles relative to the glacial-interglacial range.

More recent work (Figure DO-3) has shown that these D-O events coincide with lower dust concentrations in the ice, and with increases of about 50 microatm in the CO₂ content of bubbles of atmospheric gas trapped in the ice.

Figure DO-3. Variations in dust content and the CO₂ concentration in gas bubbles in Greenland ice relative to the D-O cycles.

Both the magnitude and rapidity of onset of the D-O cycles are significant on human time scales, but the forcing mechanism is unknown. Speculation centers on changes in either the global water vapor budget or the oceanic heat pumps of the North Atlantic, but neither hypothesis has yet been tested rigorously.

Shortly after the ice core records were recognized, sediment layers enriched in ice-rafted debris (called Heinrich layers after their discoverer) were identified in cores in a band across the North Atlantic (figure DO-4). The distribution pattern and types of rock fragments (which include very old rocks and carbonates) suggest that the primary source is the Hudson Bay area of Canada. Subsequent more detailed studies have also identified a component from Iceland and another from the St. Lawrence.

Figure DO-4. Locations of sediment cores containing Heinrich layers and inferred path of the icebergs that produced them.

The following table summarizes the distinctive characteristics of the Heinrich layers. They do not simply represent an increase in the background flux of glacial ice-rafted debris, but have characteristics that point to both a different source and contemporaneous changes in the oceanographic conditions in the area of deposition.

Property	Ambient glacial sediment	Heinrich layers	Reason for difference
FORAMINIFERA SHELLS (>150 microns)
Abundance	High	Low	Low productivity and/or high sedimentation rate
18O / 16O	High	Low	Melt water
Species	Variable	All N. Pachyderma (l)	Cold ocean
LITHIC FRAGMENTS (>150 microns)
Quartz	~80%	~80%	---
Detrital CaCO3	<2%	~20%	Icebergs from eastern Canada
FINE-GRAINED MATERIAL
Clays	Smectite present	Smectite absent	No debris from Iceland
K - Ar age of clays	0.44 billion	0.90 billion	Icebergs from eastern Canada
K - Ar age of individual amphibole grains	Large range	1.7 billion	Icebergs from Churchill province northeastern Canada
Pb isotopes of individual feldspar grains	Large range	2.7 billion crust, 1.7 billion metamorphics	Icebergs from Churchill province northeaastern Canada

Table DO-1. Comparison of Heinrich layers with the intervening ambient glacial sediment in North Atlantic cores.

Careful dating and correlation of the ice core and marine records (Figure DO-5) shows that the Heinrich events correlate with the cool intervals immediately before D-O warm spikes (note the reversed ¹⁸O scale relative to the normal convention).

Figure DO-5. Correlation of Heinrich and interspersed layers of ice rafted debris with D-O cycles in Greenland ice cores.

Figure DO-5 shows that smaller peaks in ice-rafted debris occur between the four major Heinrich layers, suggesting that the subarctic climate fluctuated with a period in the 2 ky range.

Similar fluctuations have been subsequently been identified in numerous other records as diverse as vegetation (pollen) indices in Florida Lakes and laminated sediments in the Santa Barbara Basin off southern California (Figure DO-6), the Cariaco Trench off Venezuela (Figure DO-7), and the upwelling region off southwest Africa (Figure DO-8).

Figure DO-6. Summer (red) and winter (blue) temperature fluctuations in Santa Barbara Basin (based on foram assemblage transfer functions) compared to D-O cycles (yellow numbered intervals). Horizontal lines at 10ºC and 15ºC are coolest and warmest modern temperatures observed at 25 m.

Figure DO-7. Correlation of fluctuations in the color of Cariaco Trench sediments with DO cycles in the GISP II ice core. Dark sediment bands are laminated and indicative of low bottom oxygen, high surface productivity, and high runoff from South America.

Figure DO-8. Correlation of fluctuations in the abundance of left coiling N. pachyderma beneath the Benguela current, west of South Africa (indicative of incursions of water from south of the subtropical convergence) with DO cycles in the GISP II ice core.

The limited geographic coverage of cores showing DO-type fluctuations (Figure DO-9) does not define the global pattern of such fluctuations, or seriously constrain theories as to their origin.

Figure DO-9, Sites with paleoclimatic records spanning the last 40-80 ky with resolution better than 200 years.

A recent paper by Shackleton and Hall (Paleoceanography v. 15, p. 565, 2000) does suggest that the effect is primarily Northern Hemisphere and upper water column; Antarctic air temperature (from D/H ratios) and deep-water ice volume/temperature time series do not show D-O fluctuations (Figure DO-10).

Figure DO-10. Fluctuations in the oxygen isotopic composition of planktonic and benthic forams from a core off the Iberian peninsula compared to the oxygen isotope record of the GRIP (greenland) and the D/H ratio in the Vostok (Antarctica) ice cores.

The frequency spectra of these records show barely significant peaks corresponding to periods of a few hundred to a few thousand years (e.g Figure DO-11). Wunch suggests (Paleoceanography v.15, p. 417, 2000), however, that the 1,460 peak that is prominent in many of the records results form aliasing of the seasonal cycle and, in any case, that the power in peaks is a small fraction of the total power under the spectral curves.

Figure DO-11. Examples of spectra of proxies that show D-O type fluctuations. Numbers at top show some of the peak periods in years (after van Kreveld et al., Paleoceanography v. 15, p. 425, 2000).

There is little question that these are global signals (both the temperature and CO₂ changes exceed reasonable local effects), but whether the cause is internal forcing (natural oscillations in the icecaps, hydrological cycle, or oceans, for example) or external forcing (by variations in the solar constant, for example) is still an open question.

Sub-Milankovitch cycles, particularly those associated with rapid temperature changes, are of interest to climatologists because they provide a lower limit on the rapidity with which climate can change, and because they can be combined with models of the impact of greenhouse gases to make more plausible empirical projections of future climate change (e.g. Figure DO-12).

Figure DO-12. Effect of including high frequency temperature variations inferred from Camp Century ice core record on projections of greenhouse gas warming of the earth.