Ice core studies of greenhouse gas concentrations prior to 800,000 years ago
One of the most important lines of evidence for the link between greenhouse gases and global temperature is the close covariation between global temperature and atmospheric CO2 concentration over the past 800 ka. The CO2 records come from trapped gases in ancient ice sampled by coring the 3-km thick ice sheet in East Antarctica. The timespan of the records is limited by the age of ice in the East Antarctic Ice Sheet.
There are no direct measurements of CO2 for earlier times, but there are proxy estimates based on certain physiological or geochemical properties of ancient plants or other fossils. Surprisingly, much of this evidence, while equivocal, is not supportive of a close link between CO2 and climate at earlier times. One example: at least 2 lines of evidence indicate that CO2 was lower than preindustrial (280 ppm) at around 7 Ma (7 million years ago). Such a change is unexpected, as at that time global temperature was about 2°C warmer, Greenland was unglaciated, and the Antarctic ice sheet was half its present size. The Bender Group has undertaken collaborative projects searching for much older ice, in two regions, Mullins Valley, and the Allan Hills (Figure 20). Both are near the coast of Antarctica, south of New Zealand. At both sites, there is some equivocal evidence that ice is present dating back to 2.5-9 Ma. Our earlier studies at one of these sites revealed an ice age of about 1 Ma, old but not as old as expected. It is unclear if this relatively young age was an artifact of contamination of shallow ice samples with modern air.
This past austral summer, the Bender team was involved in expeditions to drill deeper samples of ice at both sites. These expeditions were reasonably successful. Ice will be returned to the laboratory shortly, and the researchers will date it using an Ar isotope method recently developed with CMI support. If dating studies demonstrate the antiquity of this ice, they will carry out additional sampling, and undertake analyses of CO2 and methane in the trapped gases.
Studying glacial-interglacial changes in the polar ocean
As part of a long term collaboration with close colleague Gerald Haug, Professor in the Geological Institute at ETH Zurich, Daniel Sigman’s group continues to pursue evidence for reduced vertical exchange (i.e. “stratification”) in the halocline-bearing polar ocean regions under colder climates of the past 3 million years (Figure 21). A major motivation for this focus is that the reconstructed polar ocean changes have the capacity to affect atmospheric CO2 in the observed sense of its glacial/ interglacial oscillation, by reducing the natural leak of biologically sequestered CO2 out of the polar ocean and into the atmosphere during ice ages. Moreover, this work bears on the expected response of the different polar ocean regions to anthropogenic climate change, with implications for regional weather, fisheries, and the oceanic sink of anthropogenic carbon.
In 2009, ETH researcher Sam Jaccard authored a study on the uranium content and other properties of sediments from the deep North Pacific that together indicate a reduction in the dissolved oxygen content of the deep Pacific during the last two ice ages. Given the inverse relationship between the deep ocean storage of biologically sequestered CO2 storage and the dissolved oxygen content of deep water, this work identifies the CO2 that was kept away from the atmosphere during ice ages. Both the sense and amplitude of inferred deep ocean change are consistent with expectations of the polar stratification hypothesis for ice age CO2 drawdown.
Recent Princeton Ph.D. Brigitte Brunelle authored a manuscript reporting new microfossil nitrogen isotope records from three of the major domains in the subarctic North Pacific: the Okhotsk and Bering Seas as well as the open western basin. These records, in concert with records of other sediment properties, further strengthen the evidence for increased stratification and nutrient consumption across the entire region during the last and previous ice ages relative to the current and previous interglacial periods. Moreover, the records indicate that stratification in the North Pacific first weakens at the end of ice ages on the same early deglacial schedule as the Antarctic region of the Southern Ocean and the same time that atmospheric CO2 first starts to rise toward interglacial.
The North Pacific in itself was likely a minor player in the CO2 changes, which were most likely dominated by the Antarctic. However, the similarity in behavior of the North Pacific and Antarctic provides a critical constraint on the physical cause that can be imagined for polar stratification during ice ages as well as the processes that end a given ice age. Haug and Sigman elaborate on this in a short piece published in 2009, entitled “Polar Twins.”
Figure 21. Records of changing climate, atmospheric CO2, and Southern Ocean conditions over the last 800 thousand years. (a) A compilation of benthic foraminiferal δ18O records, which reflect changes in continental glaciation and deep ocean temperature. (b) Atmospheric CO2 concentration as reconstructed from Antarctic ice cores. (c) Antarctic air temperature reconstructed from the deuterium content an Antarctic ice core. (d) The sediment reflectance of an Antarctic deep sea sediment record. This varies with the concentration of biogenic opal, which is produced by diatoms in the surface ocean and provides a measure of the export of biogenic material (including organic carbon) out of the surface ocean. According to the “stratification” hypothesis for ice age CO2 reduction (b), the ice age reduction in Antarctic productivity (d) is due to reduced exposure of nutrient‐ and CO2‐rich (and slightly warmer) deep water around Antarctica, which cooled the region (c), reduced productivity there, and also reduced the leakage of deep ocean‐sequestered CO2 to the atmosphere.