The Mean Age of Ocean Waters Inferred from Radiocarbon Observations: Sensitivity to Surface Sources and Accounting for Mixing Histories

Abstract

A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700-1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds given Global Ocean Data Analysis Project (GLODAP) radiocarbon observations, and they show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. The inversion requires decomposing the World Ocean into source waters, which is obtained here using the total matrix intercomparison (TMI) method at up to 28 3 28 horizontal resolution with 11 113 surface sources. The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. Accounting for the broadness of surface regions where waters originate leads to a reservoir-age correction of almost 100 yr smaller than would be estimated with a two or three water-mass decomposition and explains some of the discrepancy with previous observational studies. A best estimate of mean age is also presented using the mixing history along circulation pathways. Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200-1500 yr old, which is more in line with existing numerical model results.

The deep distributions of helium isotopes, radiocarbon, and noble gases along the U.S. GEOTRACES East Pacific Zonal Transect (GP16)

“…This feature is convincingly echoed in maps of the radiocarbon-based deep water age (see Figure 4 of Matsumoto, 2007), where around 110-120°W a tongue of younger water extends northward from 30-40°S to nearly the equator. The Matsumoto feature is further supported by a nuanced analysis deep radiocarbon ages by Gebbie and Huybers (2012, pg. 14 the basis of f/H (Coriolis parameter divided by ocean depth) control of barotropic flow (Pedlosky, 1979) and indeed it appears in global (see Plate 1 of Koblinksy, 1990) and regional (see figure 17 of Hautala and Riser, 1993) maps.…”

“…The in situ decay of any ancient natural (cosmogenic) tritium (Weiss and Roether, 1980) could contribute no more than about 0.05 fmol/kg to the excess 3 He, and would tend to appear as a constant offset to the data, thereby not affecting the slope of the 3 He vs. 4 He relationship. We claim the latter because the MOC transit time from water mass formation regions to the deep Pacific (at least several centuries and likely close to a millenium, e.g., see Gebbie and Huybers, 2012) greatly exceeds the half-life of tritium.…”

Distributions and sea-to-air fluxes of nitrous oxide in the South China Sea and the West Philippines Sea

“…Walter et al, (2006) showed that using "the contemporary atmospheric mole fraction" instead of "the initial atmospheric mole fraction" yielded a 7% smaller ΔN 2 O at depths of less than 2000 m, and a 17% smaller ΔN 2 O at depths of greater than 2000 m in the North Atlantic Ocean. This ΔN 2 O difference at a particular depth may be higher in the Pacific Ocean than in the Atlantic Ocean because the deep water masses in the Pacific Ocean are older (England, 1995;Gebbie and Huybers, 2012). Fig.…”

An inverse approach to understanding benthic oxygen isotope records from the last deglaciation

“…Gebbie and Huybers [2012] (hereafter GH12) sought upper and lower bounds as well as a mean estimate of the ages (the time since ventilation) of water parcels in the interior ocean using the GLODAP gridded dataset of pre-anthropogenic radiocarbon, which is based on hydrographic sections, and a correction for bomb radiocarbon using measured potential alkalinity [Rubin andKey, 2002, Key et al, 2004]. Similarly as in TMI procedure, GH12 first performed a local inversion for the residence time T of every grid box and then solved a system of tracer equations for the advection of a mean age tracer a,…”