Signs of tipping point for oxygen minimum zone in the ocean
When ocean temperatures change, the natural variability of the oxygen supply and the associated biogeochemical cycles don’t respond in a lineal manner. Instead, circa 6,000 years ago a tipping point was reached relatively suddenly. This was the key finding of a study by group of researchers led by geologists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), which has now been published in the journal PNAS.
Like rising temperatures and acidification of the oceans, oxygen minimum zones are a phenomenon of global warming that threatens ecosystems. These zones could occur more frequently even with a temperature rise of less than two degrees compared to the preindustrial age. “Using a combined approach, we investigated for the first time how the ventilation in the North Pacific has changed during the current interglacial period (the Holocene), in other words, over the last 12,000 years,” explains Dr. Lester Lembke-Jene, first author of the study now published in the Proceedings of the National Academy of Sciences of the United States of America.
The team of authors collected high-resolution, paleooceanographic proxy data from sediment cores taken from the Sea of Okhotsk, a key region in which new water masses are formed, supplying large parts of the North Pacific with oxygen and nutrients. The Sea of Okhotsk is the most southerly region in the northern hemisphere where winter sea ice occurs.
Using a complex Earth system model, the geo- and climate scientists reconstructed the effects of temperature fluctuations on the physical conditions influencing ventilation and the spatial distribution of these new water masses. The proxy data and the models agree: circa 6,000 years ago the oxygen supply probably changed drastically. In addition, the character of the ventilation also changed, from a system that predominantly ventilated the intermediate water, supplying it with oxygen, to a system that in a warmer climate consumes more oxygen than it contributes. “The changes we observed were on a scale that we would otherwise expect with shifts between major glacial and interglacial periods,” reports Lembke-Jene.
In the recent geological past, it appears that in this region there were natural (not influenced by humans) changes in the ventilation dynamics, even when the upper water layers became “only” two degrees warmer. For comparison: in the last 50 years, the North Sea has become 1.7 degrees warmer. Even within the announced limits for anthropogenic global warming, in the future it’s possible that oxygen minimum zones will increase. “Until now, it has only been only possible to quantify oxygen depletion associated with ventilation changes to a limited extent. But our first estimations indicate that an oxygen loss of 25-50 % compared to today’s levels could be expected if it becomes warmer,” says Lembke-Jene. This would lead to episodic oxygen deficiency in large parts of the intermediate waters in the subpolar Pacific. This in turn could have a negative impact on the food web, including the economically important fishing grounds in the Bering Sea. To better understand the effects on the Pacific and to investigate the background and causes, this summer scientists from the AWI and the GEOMAR Helmholtz Centre for Marine Research are embarking on a joint expedition to the Northwest Pacific on board the research vessel Sonne.
Lester Lembke-Jene, Ralf Tiedemann, Dirk Nürnberg, Xun Gong, and Gerrit Lohmann: A Rapid Shift and Millennial-scale Variations in Holocene North Pacific Intermediate Water Ventilation. Proceedings of the National Academy of Sciences of the United States of America (PNAS). doi/10.1073/pnas.1714754115