A Least-Squares Fit of an Ocean Model to Deglacial Radiocarbon Records
The deep ocean is renewed, or "ventilated", when surface waters present at high latitudes become sufficiently cold and very dense. These waters subsequently spread laterally, filling the various deep oceanic basins, and eventually return to the sea surface. Ventilation is therefore the very process by which the deep ocean communicates with our atmosphere and as such, is thought to play a particularly important role in the climate system. This project will estimate the changes in deep ocean ventilation which took place during the last deglaciation -- the largest manifestation of natural climate change that remains relatively well preserved in the geologic record. It will (i) promote the progress of science by elucidating the contribution of ocean ventilation changes to the deglacial rise in atmospheric concentration of carbon dioxide (CO2) observed in ice cores, (ii) be profitable to the broader scientific community by making freely available the computer codes developed for this project, (iii) support education by engaging three students and by drafting a little pocket book on the methods used for this project and aimed particularly at undergraduate and graduate students, and (iv) benefit society by furthering understanding of the ocean's role in the natural changes of atmospheric CO2 -- the most important greenhouse gas in our atmosphere after water vapor and the subject of major current environmental and societal concerns. In more technical terms, the history of the ventilation of deep oceanic basins over the past 20,000 yr will be estimated from the quantitative combination of ocean radiocarbon (14C) records with an ocean circulation model using recursive least-squares techniques (a Kalman filter and a related smoother). The challenge posed by the application of these powerful but computationally intensive techniques over geologic time scales will be addressed by using a coarse-resolution model with simplified dynamics in concert with efficient filtering and smoothing algorithms. The hypothesis that ocean ventilation changes contributed to the changes in atmospheric CO2 concentration of the last deglaciation will be tested with due regard for the uncertainties and sparse distribution of the radiocarbon records. The work plan of this research will comprise four tasks. (1) An ocean circulation model of intermediate complexity, based on coarse resolution and planetary geostrophy, will be applied. It will have a grid that accommodates the spatial distribution of 14C records and include a simplified sea-ice model and transport equation for 14C. (2) Modern and paleo-data, including more than 1000 14C age estimates from benthic foraminifera and deep-sea corals, will be assembled together with uncertainty estimates. (3) A Kalman filter will be used to combine the model with the data during a forward integration of the model from the Last Glacial Maximum (LGM) to today. (4) A smoother will be used to propagate backwards in time the information provided by the data (including modern data) and hence further constrain past ocean states. From (3-4), an estimate of basin-scale ocean ventilation rates from LGM to today will be derived, one that is consistent with the data and the model, given estimates of their respective error statistics.