Session: Benchmarking & cross-cutting Group 1 (Isotope modelling, COMPARE)
Author: Anne Mouchet / firstname.lastname@example.org / Max-Planck Institute for Meteorology, Hamburg, Germany
Co-author: Jean-Marie Beckers, Université de Liège, Astrophysics, Geophysics and Oceanography Department, Liège, Belgium;
Eric Deleersnijder, Université catholique de Louvain, Institute of Mechanics, Materials and Civil Engineering & Earth and Life Institute, Louvain-la-Neuve, Belgium;
Eric Delhez, Université de Liège, Aerospace and Mechanical Engineering Department, Liège, Belgium;
Uwe Mikolajewicz, Max-Planck Institute for Meteorology, Hamburg, Germany;
The assessment of past ocean ventilation changes most often relies on the difference between radiocarbon ages of co-existing benthic and planktonic species (BP ages). However, several factors limit the potential of the BP ages method in this purpose. The radiocarbon distribution in the ocean results from the complex interplay between air-sea exchange, and transit pathways and rates to the ocean interior. An additional difficulty stems from varying atmospheric 14C levels over the last 40 ka.
Here we examine the sensitivity of BP ages to these processes by means of experiments with 3-D OGCMs. Significant departures of the BP ages from the actual ventilation timescales (up to several hundred years) are observed., Most significantly, BP age biases, that is the difference between radiocarbon and ventilation BP ages, are far from uniform. They exhibit marked vertical and horizontal structures, even when homogeneous air-sea exchange rate is prescribed. The response of BP ages to evolving atmospheric radiocarbon levels also exhibit significant temporal and spatial variability.
With the help of idealized age tracers whose properties are established in the framework of The Constituent-oriented Age and Residence time Theory (CART, www.climate.be/cart) we investigate the reasons for such significant departures. As air-sea exchange rate decreases, contributions from distant ocean regions to the local tracer age increase. This behavior explains most of the departure between the actual ventilation timescale and the apparent ventilation age derived from BP radiocarbon ages. It also appears that heterogeneity in the air-sea exchange rate only plays a secondary role in setting BP age biases.