Southern Ocean source of 14C-depleted carbon in the North Pacific Ocean during the last deglaciation

During the last deglaciation, atmospheric carbon dioxide concentrations rose at the same time that the 114C of that CO2 fell. This has been attributed to the release of 14C-depleted carbon dioxide from the deep ocean1, possibly vented through the Southern Ocean2–5. Recently, a sediment record from the eastern North Pacific Ocean spanning the last deglaciation was interpreted to reflect transport of such radiocarbon-depleted CO2 from the Southern Ocean through Antarctic Intermediate Water2. However, the suggestion that the record reflects intermediate water derived from the Southern Ocean remains controversial. Here we assess the source of the deglacial intermediate water by measuring the neodymium isotopes of fossil fish teeth/debris from the same eastern North Pacific core used in the earlier study2. The isotopic signature of a water mass, which is captured in the fossil fish teeth, reflects the location in which it formed. Our data exhibit a clear shift in the neodymium isotope values towards Southern Ocean values about 18,000 years ago, coinciding with the negative 114C excursion. We conclude that these data support a Southern Ocean source for the deglacial radiocarbon-depleted CO2 detected in the eastern North Pacific. Negative radiocarbon excursions2,6 observed in the eastern tropical Pacific Ocean are contemporaneous with increased upwelling in the Southern Ocean7 and onset of warming and rising CO2 detected in Antarctic ice cores8, highlighting a possible cause and effect relationship between circulation in the Southern Hemisphere and radiocarbon in the eastern tropical Pacific Ocean. Despite these observations, there is no direct evidence that this older water was sourced from the Southern Ocean. An alternative possible source is the high-latitude North Pacific. There is no clear way to distinguish between northern and southern routing on the basis of 114C, δ13C or δ18O because similar oceanic and atmospheric processes could theoretically act tomodify the isotopic characteristics of either endmember water mass. In contrast, Nd isotopes in fossil fish teeth/debris are considered quasiconservative tracers of water mass (Supplementary Information), meaning that major water masses carry distinct Nd isotopic ratios (reported here as εNd) that are generally altered only by mixing, although modification by local weathering inputs is possible. Our core location and depth is at present situated at the boundary between Equatorial Intermediate Water (EqIW) (a mixture of Antarctic Intermediate Water (AAIW) and upwelled Pacific Deep Water (PDW))9, and North Pacific Intermediate Water (NPIW) within the east Pacific shadow zone (Fig. 1; Supplementary Information). Limited data for PDW from the deep tropical waters indicate very little variation in εNd since the Last Glacial Maximum10 (LGM). Thus, the εNd value at Baja is