Ocean circulation plays an essential role in Earth’s climate and the global carbon cycle. Indeed, due to its large volume the ocean is able to store or release large quantities of heat and carbon. The rate at which these quantities are exchanged with the atmosphere is set by the rate at which interior waters are replaced by surface water, or the ventilation rate. There is a critical need to strengthen our understanding of the mechanisms controlling this rate in order to more adequately represent them in climate models, and as a result, gain confidence in future climate projections.
Addressing past ocean circulation changes could provide the means for assessing the processes at stake and the ability of ocean general circulation models (OGCMs) to reproduce them. However, our understanding of such changes in terms of transport pathways and transit times is impeded by large uncertainties in data-based reconstructions which heavily rely on radiocarbon data from deep sea cores.
In addition to measurement errors and mixing processes in the sediment, there are two important reasons why the interpretation of field data is ambiguous. The first is intrinsic to the radiocarbon cycle. Radiocarbon, whose source is in the atmosphere, is characterized by a low air-sea exchange rate. Therefore, the interplay between slow sea surface adjustment and transit pathways in the ocean interior leads to significant differences between radiocarbon-based ventilation rates and true ventilation rates. Further, radiocarbon atmospheric levels changed dramatically over the last fifty thousand years. That this evolution is not well constrained has consequences on both the accuracy of sample dating and the assessment of ventilation changes. Second, the classical methods of interpreting the measured signal display significant shortcomings. Each of them calls for implicit assumptions in terms of water mass pathways, origin and composition of source water, which may be at odds with the actual properties, especially during dramatic climate transitions.
In the OCTANT project we examined assumptions underlying methods commonly used to assess past ocean ventilation. We also investigated how deep-sea radiocarbon ages scale to the actual ventilation timescales during the transition from the last glacial maximum (26 kyr ago) to the present-day. We further developed different tools based on age theory to help interpret the results.
This lead to develop an understanding of the mechanisms by which radiocarbon ages differ from the true ventilation ages. The project also succeed in providing insight on why studies of past ocean ventilation based on deep-sea core radiocarbon measurements reach contradictory conclusions.
The new results will help develop adequate methodology and strategies for the interpretation of deep-sea cores.
