Ventilation of the deep ocean constrained with tracer observations and implications for radiocarbon estimates of ideal mean age

Abstract

Ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior. Quantifying the dominant source regions and time scales remains a major challenge in oceanography. A mathematically rigorous approach, that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy-diffusive flow such as the ocean, is to quantify ventilation in terms of a probability distribution that partitions fluid parcels according to the time and location of their last surface contact. Here, we use globally gridded radiocarbon data in combination with other transient (CFCs) and hydrographic (temperature, salinity, phosphate, and oxygen) tracer data to estimate the joint distribution of age and surface origin of deep ocean waters. Our results show that ~ 40% and 26% of the global ocean was last in contact with the Southern Ocean and North Atlantic, respectively. Some 80% of the global deep ocean below 1500 m is ventilated from these high latitude regions. However, contrary to the classical description of the deep ocean as a roughly equal mixture of “northern” and “southern” source waters, we find a significantly higher contribution from the Southern Ocean relative to the North Atlantic. We estimate the mean transit time from the surface to the deep North Pacific at 1360 ± 350 y, intermediate between two widely used radiocarbon-based estimates. To reconcile our estimate of the ideal mean age with ventilation age estimates based on radiocarbon, we apply the estimated distribution function to construct a 3-dimensional distribution of the water mass fraction-weighted surface “initial” radiocarbon concentration that can serve as an accurate reservoir age. Radiocarbon ages corrected for this initial reservoir age are found to be in good agreement (within 5%) with our ideal age estimate, demonstrating that it is essential to take into account the spatially variable surface radiocarbon field when computing ventilation ages using radiocarbon. A wide spectrum of ages contributes to the mean age, providing evidence for the fundamentally eddy-diffusive nature of the large-scale general circulation of the ocean.

Introduction

Ocean ventilation is the process that transports water from the surface mixed layer into the ocean interior. Understanding this fundamental aspect of the climate system, in particular quantifying where waters sink and how long on average they have remained isolated from the atmosphere, remains a major challenge in oceanography. It has implications in a variety of areas, ranging from the uptake of heat and anthropogenic CO₂, to interpretation of tracer observations, and reconstructing past variations in ocean circulation from paleoceanographic proxies recorded in marine sediments.

In this paper, we address the question of where waters sink and for how long they have been isolated from the atmosphere. While these two aspects are intricately linked, they have generally been treated as separate problems. Classical water mass analysis has long used hydrographic and other quasi-conservative tracers to decompose waters into constituent water masses (e.g., De Brauwere et al., 2007, Johnson, 2008, Tomczak, 1981, Tomczak and Large, 1989). Typically, only a small number of “end members” have been considered, although a more recent variant by Gebbie and Huybers (2010), allows for a much larger number of sources (in their approach any surface point is a potential source). On the other hand, the question of time scales has been addressed through the use of tracers such as chlorofluorocarbons (CFCs) and radiocarbon to yield some measure of the “age” of the water. However, such tracer ages are not an intrinsic property of the flow but weighted transit times that vary from tracer to tracer (Holzer and Hall, 2000, Khatiwala et al., 2001, Wunsch and Heimbach, 2008).

A fundamental difficulty with the traditional approaches described above is that in an advective–diffusive flow such as the ocean, there is no unique pathway or time scale by which a water parcel reaches any interior location. Instead, it is more appropriate to describe a water parcel in terms of a continuous distribution that partitions it according to the time and place of last surface contact. Mathematically, this joint distribution is a type of Green function, known in the present context as a “boundary propagator” (BP). While previous studies have simulated BPs and other age tracers in ocean models (England, 1995, Khatiwala et al., 2001, Peacock and Maltrud, 2006, Primeau, 2005), the solutions may be sensitive to model resolution (Peacock and Maltrud, 2006) and parameterization of sub-grid scale processes (England, 1995), thus limiting their applicability. The recent development of a new mathematical inverse technique (Holzer et al., 2010, Khatiwala et al., 2009) based on the maximum entropy approach (Tarantola, 2005) has, however, made it possible to estimate the ocean's boundary propagator directly from observations. This method was previously applied by Khatiwala et al. (2009) to reconstruct the history of anthropogenic CO₂ in the ocean over the industrial period, which required point-wise estimates of the boundary propagator throughout the global ocean. It has also been used by Holzer et al. (2010) to estimate the joint distribution of transit time and surface origin from cruise bottle data in the North Atlantic. Here, we apply data-constrained estimates of the ocean's boundary propagator made by Khatiwala et al. (2009) to provide the first quantitative description of the joint distribution of the age and surface origin of ocean waters for the global ocean. We will focus on the deep ocean, and the various source regions and time scales with which waters in the deep Pacific are ventilated. We also explore the implications of our results for estimates of age based on radiocarbon (¹⁴C), a widely used tracer in both modern and paleoceanography.