Embedded within the Earth’s long-term cooling trend over the past 65 million years are several climate spikes—swift transitions to “hothouse” conditions—that had profound consequences for life. These spikes could serve as analogues for the future of our warming planet.
The cause of these spikes may in part be due to changes in the atmospheric concentration of carbon dioxide, an important greenhouse gas. But the complex feedbacks between the Earth’s climate and the carbon cycle have been hotly debated, and there is little scientific consensus on this issue.
To help unravel the relationship between the carbon cycle and climate during an extended warm period, Kochhann et al. present a data set of stable isotope and carbonate records. These records, indicators of changing temperature and the growth or contraction of ice sheets, are from an Integrated Ocean Drilling Program drill site in the eastern equatorial Pacific Ocean. The authors also correlate these results with data collected from other regional sites.
Their new record spans the Miocene Climatic Optimum (MCO), the hothouse interval between about 17 and 15 million years ago. During the MCO, the average global temperature was up to 4°C warmer than today, and carbon dioxide concentrations hovered at about modern levels (400 parts per million).
The results show that the stable isotope and carbonate records varied over regular cycles of both 100,000 and 400,000 years. Because these time frames correspond to the eccentricity of Earth’s orbit and its corresponding variations in incoming solar radiation, the authors argue that this finding suggests that this orbital parameter played an important role in controlling the climate as the planet warmed during the MCO. The study also provides strong evidence that the lysocline, the depth in the ocean below which carbonates are much more quickly dissolved, was very dynamic during the MCO, repeatedly fluctuating by up to 600 meters.
To see how the authors reached this conclusion, one has to envision how marine sediment cores store carbon as calcium carbonates. When organisms that hold carbon in their calcareous skeletons die, they sink through the water column to the seafloor. If a given drill site is located above the lysocline, these calcareous skeletons will be preserved in the sediments. If a site is located below the lysocline depth, some—or most—of these skeletons will be dissolved. By comparing carbonate records from drill sites located at different ocean paleodepths, it is possible to estimate vertical movements of the lysocline depth through time.
Notably, episodes of peak carbonate dissolution during the MCO coincided with warmer temperatures (as indicated by the oxygen isotope records), as well as a lightening of carbon isotopes. In contrast, during Pleistocene interglacial warming intervals, periods of higher temperatures (as indicated by the oxygen isotope records) corresponded to records of heavier carbon isotopes. This difference caused the team to conclude that the feedbacks between climate and the carbon cycle during the Miocene differed fundamentally from those at play during the more recent Pleistocene glacial-interglacial cycles.
The authors note that correlation between high temperature and light carbon isotopes, as seen during the MCO, was also observed during the Paleogene, an interval noted for its extremely rapid episodes of global warming and probable lack of ice caps. Therefore, they argue that climate and carbon cycle variability during the MCO, when global warming conditions probably limited ice cover at the poles, corresponded more closely to patterns observed during the Paleogene than those seen during more recent Pleistocene times, when Earth hosted two polar ice sheets. (Paleoceanography, doi:10.1002/2016PA002988, 2016)
—Terri Cook, Freelance Writer