Dissolved Organic Radiocarbon in the Central Pacific Ocean

Ellen R M Druffel,Ann P Mcnichol,Sheila Griffin,Robert M Key,Noreen G Garcia,Ning Wang,Brett D Walker

doi:10.1029/2019gl083149

Abstract

AbstractWe report marine dissolved organic carbon (DOC) concentrations, and DOC ∆14C and δ13C values in seawater collected from the central Pacific. Surface ∆14C values are low in equatorial and polar regions where upwelling occurs and high in subtropical regions dominated by downwelling. A core feature of these data is that 14C aging of DOC (682 ± 86 14C years) and dissolved inorganic carbon (643 ± 40 14C years) in Antarctic Bottom Water between 54.0°S and 53.5°N are similar. These estimates of aging are minimum values due to mixing with deep waters. We also observe minimum ∆14C values (−550‰ to −570‰) between the depths of 2,000 and 3,500 m in the North Pacific, though the source of the low values cannot be determined at this time.

Highlights

Dissolved organic carbon (DOC) is the largest pool of organic carbon in the ocean, equal in size to carbon in the atmosphere (Hansell et al, 2009)
We show that deep dissolved organic carbon (DOC) ∆14C values from the central Pacific are consistent with transport of Antarctic Bottom Water (AABW) northward, and that this aging is equivalent to that observed for dissolved inorganic carbon (DIC) in this water mass
The average at the slope station off Alaska was the highest (41.7±0.6μM). These values are within statistical uncertainty to averages of [DOC] measurements reported for nearby stations on this P16N cruise (Carlson, 2018). 3.2 DOC δ13C values The DOC δ13C values ranged from –22.7‰ to –20.3‰ in the upper 950m (Table S2 and Figure S2 in the Supporting Information and Figure 2b)

Summary

Introduction

Dissolved organic carbon (DOC) is the largest pool of organic carbon in the ocean, equal in size to carbon in the atmosphere (Hansell et al, 2009). The DOC in the deep North Pacific (>1500m) was found to be 6000 14C years old in 1985 and 1987 (Williams & Druffel 1987; Druffel et al, 1992). This very old age was surprising because DOC was thought to be produced in the surface ocean where 14C levels are near modern (∆14C = –50‰), and should be much younger. It was proposed that the surface DOC is composed of a 1:1 mixture of old DOC, similar to the radiocarbon (∆14C) value of deep water, and newly produced, post-bomb ∆14C values (Williams & Druffel 1987). We show that deep DOC ∆14C values from the central Pacific are consistent with transport of Antarctic Bottom Water (AABW) northward, and that this aging is equivalent to that observed for DIC in this water mass

Setting and Methods

Results

Discussion

DOC isotopic distributions in water masses of the upper ocean

Implications for the marine DOC cycle