New constraints on deglacial marine radiocarbon anomalies from a depth transect near Baja California

Previous studies have shown that radiocarbon activities (Δ 14 C) in the low‐latitude, middepth Pacific and Indian Oceans were anomalously low during Heinrich Stadial 1 (HS1, ~17.8–14.6 ka) and the Younger Dryas (YD, ~12.8–11.5 ka), coincident with intervals of rising atmospheric CO 2 concentration and declining atmospheric Δ 14 C. However, a full explanation of these events remains elusive due to sparse and sometimes conflicting data. Here we present new 14 C measurements on benthic and planktic foraminifera that, in combination with previously published measurements, enable us to reconstruct the Δ 14 C depth gradient near Baja California. Vertical profiles were similar to present during the Last Glacial Maximum and Bølling/Allerod (14.6–12.8 ka) but display a pronounced middepth (~700 m) Δ 14 C minimum during HS1 and the YD. The latter observation, along with a comparison to other regional reconstructions, appears to rule out intermediate waters from the north or from directly below as proximate sources of aged 14 C‐depleted ocean carbon during deglaciation and point instead to changes in the composition of Equatorial Pacific intermediate waters. Simple mixing constraints require Equatorial Pacific intermediate waters to be only slightly lower in Δ 14 C than at Baja California, in contrast with previous observations of extremely low Δ 14 C at Galapagos Rise. While the latter may have been influenced by localized releases of geologic ( 14 C‐dead) CO 2 , the smaller and more widespread deglacial Δ 14 C anomalies in the Arabian Sea and North Pacific seem to require a source of aged carbon in the glacial deep Southern and Pacific Oceans for which there is growing evidence.