Collaboration with Laura Robinson, Andrea Burke, and Tina van der Flierdt
Project team includes Joe Stewart, Tianyu Chen, Tao Li, Jessica Crumpton-Banks and many other wonderful collaborators!
The Southern Ocean plays a central role in defining Earth's climate because it is a location where cold, deep waters rise to the surface and exchange gases and heat with the atmosphere. One of the most important gases for the climate system is carbon dioxide (CO2). Since the oceans contain about 60 times more carbon than the atmosphere, it only takes a small perturbation in the ocean to have a large climate impact. Atmospheric CO2 levels have shown systematic changes over the past 800,000 years as revealed by gasses trapped in ice cores, and recent evidence has come to light that shows that CO2 can increase rapidly over only hundreds of years. We still do not know how and why these changes in CO2 occur but their size and speed suggests that they must have been driven by changes in the deep ocean.
Mechanisms that have been put forward to explain lower atmospheric CO2 concentrations during past cold (glacial) periods focus on increased CO2 uptake in the Southern Ocean. This could have been achieved by a combination of increased sea ice cover and a more layered structure in the water column, which prevents CO2 from escaping to the atmosphere. The concept is supported by modeling evidence and predicts that we should find old, carbon-rich waters in the deep Southern Ocean during past cold times. If this layered water structure was removed, then these deep waters would release CO2 to the atmosphere as ice ages came to a close. Records from ice cores show us that the actual rise of CO2 during the end of the last ice age ('last deglaciation') happened in multiple steps. So far, however, it has been very difficult to obtain records from the Southern Ocean to test the hypothesis posed above, or the alternative hypothesis that carbon sequestration in the South was achieved due to more active CO2 uptake by planktonic marine plants. What we have been lacking is a suitable recorder ('archive') of past environmental conditions directly in the Southern Ocean that can resolve time increments of about 100 years or less, similar to in the record preserved in ice cores.
With our project we aim to transform understanding of the Southern Ocean's role in climate change by creating detailed records of the circulation, temperature, and CO2 chemistry of the Southern Ocean at the end of the last ice age and into the current warm period (past 25,000 years) at unprecedented temporal resolution. To achieve this we will make geochemical measurements on the skeletons of fossil deep-sea corals, a novel archive that allows us to create unique coupled records of past oceanographic change on a precise and accurate timescale. The skeletons of deep-sea corals are formed using the chemical ingredients of the seawater that they live in. This means that during the lifetime of a coral (~100 years) a record of water mass composition and temperature is captured as they grow. By performing a suite of geochemical measurements on each fossil coral, we can reconstruct environmental conditions at the time it grew. Repeating this exercise for hundreds of corals will allow us to construct the first directly dated record of the Southern Ocean's behavior since the last ice age. Our new record will allow comparison of the relative timing of environmental changes in the Southern Ocean with those of ice core records. It will therefore address one of the most hotly debated questions in global climate change research, the origin of changes in atmospheric CO2 and temperature on time scales of hundreds to thousands of years.
A simplified version of our key new data, showing deep Southern Ocean CO2 release over the last deglaciation. See the paper for the version that includes the centennial-scale excursions too!
Carbon storage in the deep Southern Ocean is widely thought to control atmospheric CO2on glacial-interglacial timescales, but few direct tests of this hypothesis exist. Here we present new deep-sea coral boron isotope data that track the pH – and thus CO2chemistry – of the deep Southern Ocean over the last 40,000 years. At sites closest to the Antarctic continental margin we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2ocean pH is low, reflecting enhanced ocean carbon storage; during intervals of rising CO2ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid pH decreases during abrupt CO2rise, reflecting the transfer of carbon from the deep to the upper ocean and atmosphere. These data thus confirm the importance of the deep Southern Ocean in ice age CO2change, and demonstrate that deep ocean CO2release can occur as a dynamic feedback to rapid climate change on centennial timescales.