An improved method for estimating water-mass ventilation age from radiocarbon data

Tim Devries,François Primeau

doi:10.1016/j.epsl.2010.04.011

Tim Devries, François Primeau

Open Access

https://doi.org/10.1016/j.epsl.2010.04.011

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Abstract

Existing methods for inferring the ventilation age of water masses in the ocean using radiocarbon data neglect the effects of diffusive mixing. In the presence of varying atmospheric Δ 14C, this neglect produces spurious time dependence in the estimated ventilation ages. To correct this deficiency we propose a new method for estimating the ventilation age from sediment core radiocarbon data. The new method is formulated in terms of parameterized age distributions that account for the effects of advective and diffusive transport in the ocean. When applied to simulated radiocarbon data from an OGCM, the method is able to closely reproduce the modeled ventilation age, whereas other methods are not. We also applied the method to sediment-core radiocarbon data from the deep subarctic northeast Pacific. We estimated the model parameters using a Bayesian approach that allows for a careful quantification of the uncertainty in the inferred ventilation age. Results from the sediment-core analysis show larger ventilation ages during the last glacial maximum compared to the Holocene, although uncertainty on individual age estimates is high. Large excursions in ventilation ages for one of the cores may also indicate a change in the reservoir age of North Pacific surface waters during the last deglaciation.

Highlights

The ventilation age of a water mass is defined as the time that has elapsed since the water was last at the sea surface (e.g. Thiele and Sarmiento, 1990)
During the last glacial maximum (LGM) and early Heinrich I (H1), ventilation ages averaged about 1900 years, or about 400 years older than in the modern ocean
The results generally support the conclusion of (Galbraith et al, 2007) that ventilation ages at this location were older during the LGM than in the Holocene, and that there is a significant decrease in ventilation age at around 15 ka

Summary

Introduction

The ventilation age of a water mass is defined as the time that has elapsed since the water was last at the sea surface (e.g. Thiele and Sarmiento, 1990). The ventilation age of a water mass is defined as the time that has elapsed since the water was last at the sea surface Ventilation age records estimated from deep-sea sediment cores (Sikes et al, 2000; Broecker et al, 2004; Robinson et al, 2005; Galbraith et al, 2007; Broecker et al, 2008) provide key constraints on the ability of the deep-ocean reservoir to accumulate a sufficient amount of respired CO2 to account for the 100 ppm reduction in atmospheric CO2 during the last glacial maximum Ventilation ages are closely related to the 14C to 12C ratio recorded in foraminifera shells recovered from deep sea cores. Unlike current speeds or overturning rates, ventilation ages can be estimated without using an ocean circulation model and sophisticated data assimilation techniques.

Methods

Results

Conclusion