During the late Pleistocene epoch, ice sheets advanced and retreated in tandem with changing atmospheric carbon dioxide levels. Researchers have long sought to understand the complex processes that modulate rising and falling carbon dioxide concentrations—a line of research with important implications today as levels reach highs not seen since roughly 3 million years ago in the Pliocene, when the Arctic was forested.

The world’s oceans are a major carbon sink today, collectively absorbing as much as a third of the carbon dioxide humans have pumped into the atmosphere since the Industrial Revolution. In a new study, Shao et al. further constrain the role of oceans in driving atmospheric carbon levels during the glacial cycles of the late Pleistocene.

The work builds on previous research that used boron isotope records as a proxy for ocean surface chemistry, which gives scientists insights into the exchange of carbon dioxide between the sea surface and the atmosphere. Researchers have measured boron isotopes extracted from planktic animals locked in marine sediment cores to reconstruct oceanic pH in the tropical and North Pacific, the Indian, and the Atlantic Oceans, but data from the southwestern Pacific, a major carbon sink today, were lacking.

To address this, the authors analyzed boron isotopes from specimens of the planktic foraminifera Globigerina bulloides found in two sediment core samples collected from Chatham Rise off the east coast of New Zealand to obtain a boron isotope–based pH reconstruction for the area. The cores provided a record of oceanic conditions dating back at least 25,000 years, during the late Pleistocene. The team found that pH was about 8.2 during the Last Glacial Maximum, when ice sheets covered most of North America, Europe, and Asia. The pH then fell between 16,500 and 14,000 years ago before rising again to 8.1 at the end of the Pleistocene and into the early Holocene. The results indicate that this region of the South Pacific was venting carbon dioxide as the ice sheets were retreating. The team noted that similar results have been obtained from the South Atlantic, suggesting that both the South Pacific and the South Atlantic Oceans were carbon sources, not sinks, during the last deglaciation.

The authors integrated their results with previously published boron isotope records from around the globe to create a more complete picture of carbon dioxide exchange over the past 25,000 years. They found widespread outgassing of carbon dioxide, particularly during the last deglaciation, which could be explained by an increase in upwelling of the gas from the deep ocean, according to the authors. The result is intriguing, they say, as none of the records in the data set are from the high-latitude Southern Ocean, where most carbon from the deep ocean first contacts the atmosphere. However, the researchers also note that the sites sampled may be biased toward upwelling regions.

The study fills an important gap in boron isotope–based reconstructions of the ocean-atmosphere carbon dioxide exchange throughout the last deglaciation. A better understanding of this exchange in the past could provide insights about impacts that rising atmospheric carbon dioxide levels will have on the climate today. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2018PA003498, 2019)

—Kate Wheeling, Freelance Writer