Where did all the CO2 go? Insights from boron isotopes in deep-sea corals

点击翻译按钮获取中文摘要

Over the last 2.5 million years or so the Earth's climate has regularly oscillated between warm periods, like today called interglacials, and frigid cold periods called glacials when several kms of ice blanketed the Northern Hemisphere. Bubbles of ancient air trapped in ice cores tell us that, although the cycles are ultimately triggered by changes in the Earth's orbit around the Sun, they are largely driven by increases in the atmospheric concentration of the greenhouse gas carbon dioxide (CO2) - CO2 is low during glacial periods and high during interglacial periods. During each cycle, cooling into a glacial tends to be rather slow (taking between 90 to 30 thousand years) and the warming that terminates each glacial period tends to be very rapid (~10 thousand years in length). Since these warming events caused the dramatic and rapid retreat of the northern hemisphere ice sheets they are known as deglacials. The last deglacial began around 18 thousand years ago and was completed by around 10 thousand years ago. Despite these glacial-interglacial cycles being the most dramatic and significant recent examples of global climate change, their exact cause is not known. What we do know however is that during a deglacial CO2 is most likely being moved out of the deep oceans where it is stored during glacial periods, to the atmosphere, where it warms the Earth up and drives the retreat of the ice sheets, until the next cooling cycle begins. In order to tie down which mechanisms are responsible for moving the CO2 around like this we need to know exactly where in the ocean it is going. Some studies point to it being stored in the deep abyss in water that circulates around Antarctica, therefore suggesting it is mechanisms operating in this region that are responsible. Although this agrees with many of our observations, some other clues point to the North Pacific on the other side of the globe, as being important. And it has even been recently suggested that the deep ocean isn't involved at all. In this proposal we shed light on this debate by determining whether or not CO2 was stored around Antarctica.No actual measurements exist of the CO2 of seawater 18 thousand years ago, therefore we have to use indirect measurements known as proxies. The proxy we will use is based on boron in ancient deep-sea coral skeletons. Deep-sea corals, like their cousins found in warm tropical seas, make skeletons out of calcium carbonate. The isotopic composition of boron in their calcium carbonate skeleton is related to the pH in which the coral grew and the pH of seawater is proportional to the amount of CO2 it contains. Therefore, pH is a very useful and direct tracer ofthe CO2 stored in the glacial abyss. However, in order to get the best pH reconstructions we first need to calibrate the proxy better than it is currently. We will mainly do this by growing deep-sea corals at known pH in the laboratory and measuring their boron composition. Armed with this better understanding we will not only get an idea of how these animals will be affected by future ocean acidification, but, by making measurements of the boron isotopic composition of ancient deep-sea coral skeletons of different ages we can reconstruct how pH evolved in one location through the entire deglacial. We have a number of deep-sea coral samples from around 1500 m water depth in the SW Pacific that are from 30 to 8 thousand years old. We are interested in this region because it has been put forward as a key route for CO2 as it is mixed from the deep abyss into the upper levels of the ocean and then ultimately into the atmosphere. The pH record we will produce will be a thorough test of our current ideas of how CO2 moves between ocean and atmosphere during a deglacial; this study will therefore provide valuable insights into the mechanisms responsible for glacial-interglacial pCO2 change.