Characterising the Nd-isotopic composition of Cretaceous intermediate- and deep-water masses using fossil fish remains

The threat posed to our planet by increased greenhouse gas concentrations and global warming is being widely discussed by scientists and politicians alike. Our ability to deal with this threat is greatly aided by a thorough understanding our planet's climate history. Approximately 144 to 65 million years ago, during the Cretaceous period, the Earth's climate was considerable warmer than present, with atmospheric CO2 levels more than two to three times higher than present-day levels. There was little ice at the poles, and dinosaurs, crocodiles and tropical plants living in high latitudes regions like Antarctica and Alaska. Clearly, the Cretaceous represents a 'natural' example of global warming and provides valuable information concerning how our Planet might respond to future anthropogenic warming, driven by the burning of oil and coal. From ~120 to 90 million years ago large amounts of carbon produced by planktonic organisms and plants (that were washed-in from the land) were buried at the bottom of the North Atlantic in low-oxygen conditions. These sediments must have had a significant impact on the carbon-cycle and may have helped regulate the amount of CO2 in the atmosphere by storing carbon. However, this situation ended about 90 million years ago and waters containing much more oxygen filled the bottom of the North Atlantic, thereby preventing the burial of carbon. Prior to 90 million years ago, the North Atlantic was an isolated ocean, separated by land (that connected Africa and South America) from the South Atlantic. After 90 million years ago a connection may have existed between the two oceans allowing water to mix from south to north. It has been suggested that this deep-water connection caused the change in oxygen conditions in the North Atlantic. Additionally, this event may have allowed increased ocean circulation that would have transported heat away from the tropics to cooler high latitudes. In order to detect whether changes in deep-water circulation affected the deposition of sediments in the North Atlantic, it is necessary to have a method of 'fingerprinting' the deep-water from each ocean. Neodymium (a heavy metal with the symbol Nd) occurs in seven forms (or isotopes) and is transported to the oceans by rivers after it has been eroded from rocks. One of the isotopes, 143Nd, varies substantially between the Pacific, Atlantic and Indian Oceans because each ocean is surrounded by different rock-types that all have varying amounts of 143Nd in them. These differences are expressed as the ratio of 143Nd to 144Nd. At the present day, the Pacific Ocean has the highest ratio values, whereas the Atlantic has the lowest values. Thus, if at a single site there is a change from lower to higher values with time, this may indicate an increase in the proportion of Pacific Ocean water present at this site. In the geological past we can use fish teeth and bones to estimate bottom-water Nd-isotope values. After death, fish sink to the sea-floor and decay. Through a chemical reaction at the sea-floor fish teeth and bones gain Nd in the same ratio as the sea-water in which they lie. Deep-sea sediments recovered by ocean drilling often contain fish remains that can be used to construct records through time of seawater Nd-isotope ratios. I plan to construct such records from the Pacific, Atlantic and Indian Oceans for the Cretaceous that will allow us, firstly, to see if the the oceans had distinct, characteristic values at this time (in the same way as the modern oceans). I will then produce several records from the North Atlantic to see if the change in oxygen conditions approximately 90 million years ago was caused by increased ocean circulation due to the final separation of Africa from South America. This research will be important for understanding how ocean circulation regulates the global carbon cycle and global temperatures.