This study extends the 1991-1995 records of marine dissolved organic carbon (DOC) concentrations and Δ¹⁴C values at hydrographic Station M (34°50′N, 123°00′W) with new measurements from a frozen (-20 °C) archive of samples collected between April 1998 and October 2004. The magnitudes and synchronicity of major Δ¹⁴C anomalies throughout the time-series imply transport of DOC from the surface ocean to depths of at least 450 m on the timescale of months. Keeling plots of all measurements at Station M predict a continuum of possible background DOC compositions containing at least 21 μM of -1000‰ (i.e., ≥57,000 ¹⁴C years) DOC, but are more consistent with mean deep DOC (38 μM, -549‰; i.e., ∼6,400 ¹⁴C years). These results and coral records of surface dissolved inorganic carbon (DIC) Δ¹⁴C were used to estimate pre-bomb DOC Δ¹⁴C depth profiles. The combined results indicate that bomb-¹⁴C has penetrated the DOC pool to depths of ≥450 m, though the signal at that depth is obscured by short-term variability.

We analyzed radiocarbon in annual coral bands from the Galápagos Islands in the tropical east Pacific to understand natural variability of past ocean circulation associated with climate change. Variability was observed on El Niño timescales (3–7 years) as well as at decadal and multi-decadal periods. Low radiocarbon levels persisted for ten years during the early 1600s and for sixteen years during the early 1800s, both coincident with periods of high volcanic aerosol loading in the atmosphere from massive eruptions. Intensification of tropical circulation at this time may be linked to climatically-controlled changes in the structure of the tropical thermocline that is responsible for delivering cool, upwelled waters to the tropical east Pacific.