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What’s below the Greenland Ice?

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By Andy May

An interesting PNAS article discusses the deepest portion of the Camp Century Greenland Ice core. It is not paywalled. The researchers, led by Andrew Christ (Dept. of Geology, University of Vermont) found evidence of an ice-free vegetated environment at the base of the Camp Century ice core roughly one million years ago. This means the glaciers, which are currently 1.4 km (0.9 miles) thick, at the Camp Century location completely melted and reformed sometime between 0.7 and 1.4 million years ago. The Camp Century location, along with other deep ice core locations, are shown in Figure 1.

The Greenland Ice Sheet was complete by 7.5 Ma (million years ago) and grew substantially between 3.3 and 2.7 Ma. So, generally we are discussing a period of time when Greenland was usually very much like today.

Figure 1. Camp Century location and Greenland Ice cross section.

The sediments below the ice, contain well-preserved fossil plants and other paleo-evidence of an ice-free environment at the Camp Century location. Macrofossils were also found at NGRIP, but in ice. Photographs of the macrofossils found in the Camp Century core are shown in Figure 2.

Figure 2. Macrofossil microphotographs and leaf wax concentrations from the basal cores.

The basal sediment portion of the Camp Century core is 3.4 meters thick. The researchers divided it into three units. The evidence suggests that the most recent sediment layer experienced melting and refreezing between 0.7 and 1.4 Ma. In Figure 3 we show Javier’s illustration of the past million years.

Figure 3. Javier’s figure of the last 25 “MIS” interglacials, with 0.7 to 1.4 Ma marked. The δ18O (change in the oxygen 18 ratio) pseudo-temperature anomaly is high, but not particularly unusual, for MIS 25, and the 65°N insolation anomaly is unusually high. Perhaps this is when the melting occurred.

Figure 4 shows the entire possible interval for the Camp Century melting event from Tzedakis, et al. (2017).

Figure 4. Orbital obliquity peaks are shaded in gray, the black line is the caloric summer half-year insolation at 65°N, the red circles are insolation maxima nearest the onset of interglacials, black diamonds are continued interglacials, light blue triangles are failed interglacials. The orange line is the δ18O stack representing temperature. The upper numbers are MIS numbers for interglacials and the lower are kyrs (thousands of years) before present or the number of a continued interglacial or a failed interglacial. The “Mid-Pleistocene Transition” toward lower-frequency higher-amplitude glacial cycles is apparent near MIS 38/37. Source Tzedakis, et al., Nature, 2017.

The earlier portion of the time of the melting is near the “Mid-Pleistocene Transition,” about 1.25 Ma. The Mid-Pleistocene transition (MPT) is a time defined by the total energy required for an interglacial to begin and be successful. Before 1.5 Ma, less total energy was required to melt the glacial ice and enter an interglacial period like today. The amount of energy required increases with time until about 0.6 Ma, when it leveled off, see Figure 5. The period of possible total melting seen in the Camp Century core, at 77°N and 61°W, is essentially the breadth of the Mid-Pleistocene transition.

Figure 5: Temperature peaks for the last 2.6 million years separated into successful interglacials (red dots), failed interglacials (blue triangles), continued interglacials (black diamonds) and uncertain assignments (open symbols). The dashed black line separates successful interglacials from unsuccessful interstadials with only two misclassifications (59 and 63). The ramp in the dashed line is the “Mid-Pleistocene transition.” The Y axis is the effective “melting” energy, that is the mean summer solstice insolation peak. Source: After Tzedakis, et al., 2017.

During the MPT transitional period, the stability of the northern ice sheet was in flux. As Figures 3 and 4 show, MIS 31 (1.7 Ma) and MIS 25 (9.6 Ma) were unusually warm and occurred during a period with few very cold glacials. In fact, the glacial periods prior to 600K years ago, just weren’t as cold as the glacials of the past 600K years. The Dye ice core suggests it was ice free between 424 and 374 ka (MIS-11), but Dye is much farther south than Camp Century (see Figure 1). MIS 11 is just after the end of the MPT.

The gold curve in figure 4 is δ18O, an oxygen isotope ratio that varies linearly with surface temperature. It peaks during MIS intervals. The other curve in Figure 4 is the 65°N insolation. These curves are also shown in Figure 3, where they are easier to read.

The current elevation of the underlying sediment layer at Camp Century is 500 meters above mean sea level, removing the 1.4 km. of overlying ice would result in a rebound of about 950 meters in roughly 10,000 years. So, if conditions were the same when the ice melted, the elevation would eventually be about 1,400 meters.

Conclusions

It is hard to draw any firm conclusions from the data shown in this article, but I find it very interesting, and it is a well written paper, it is recommend reading. It is amazing that the Greenland ice sheet completely melted to 77°N. However, that it occurred during the MPT is less surprising, since it was clearly a climatically unstable time. This geological event does not affect the current climate debate, but it does show that natural forces can cause extreme changes in climate.

Figures 3, 4 and 5 show how much colder glacial periods are in the past 600K years, than they were in the previous two million years. We currently live in a bitterly cold portion of Earth’s history. Something to think about, when governments are trying to force us back into the Little Ice Age (roughly 1300 to 1850) by limiting CO2 emissions.

via Watts Up With That?

May 11, 2022

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