As the world emerged from the last Ice Age, the North Pacific saw a sudden proliferation of tiny creatures such as phytoplankton and foraminifera. This brief, strong spike in biological productivity has been observed in marine sediment cores throughout the region, and represents up to six times the productivity levels at the Last Glacial Maximum and today. Because iron limits productivity in the modern day Subarctic Pacific, Equatorial Pacific, and Southern Ocean, it has long been thought that the spike was caused by an influx of iron from the continental shelves as they were flooded during the deglaciation.
Putting the hypothesis to the test
Camille Li, of the Geophysical Institute at University of Bergen and the Bjerknes Centre, has contributed to testing this hypothesis in a new international study published in the prestigious journal Nature Geoscience. According to Li and her co-authors, there is no evidence supporting a deglacial pulse of iron from any source during the time of the productivity spike. The results are based on a range of proxy reconstructions for productivity, iron flux and iron provenance along with climate modeling experiments. The study argues instead that the spike was caused by an increase in major nutrient supply from deep convection during the early deglaciation followed by melt-water induced stratification that relieved light limitation. In short, an interplay between nutrient and light limitation controlled productivity in the glacial and deglacial Subarctic North Pacific, with iron limitation only becoming important in the Holocene and modern times.
The satellite image shows a large phytoplankton bloom that occurred in the Northwest Pacific Ocean off the coast of Hokkaido, Japan in May 2009. Researchers have assumed that such blooms were driven by iron in the post-Ice Age ocean, but a new study by Lam et al. indicates otherwise (Norman Kuring, MODIS Ocean Color Team/NASA).
Implications for geo-engineering
The findings resolve conflicting ideas about the relationship between iron and biological productivity during this time period in the North Pacific. In addition, the study has potentially important implications for geo-engineering efforts to slow climate change by seeding the ocean with iron.
See also the news release from the WHOI Media Relations Office, including a complete summary of the study here .
Reference:
Phoebe J. Lam, Laura F. Robinson, Jerzy Blusztajn, Camille Li, Mea S. Cook, Jerry F. McManus, Lloyd D. Keigwin: Transient stratification as the cause of the North Pacific deglacial productivity spike. Nature Geoscience 6, 622–626 (2013)
Link: http://www.nature.com/ngeo/journal/v6/n8/full/ngeo1873.html