UC Santa Cruz

Abstract Proxy records of seawater radiocarbon ( 14 C/C) provide strong constraints on how changes in ocean ventilation contributed to the increase in atmospheric CO 2 during the termination of the last ice age (≈18,000‐to‐12,000 years ago). One outstanding problem, however, is the existence of anomalously low deglacial benthic foraminiferal 14 C/C in the intermediate‐depth Eastern Tropical North Pacific (ETNP) near the Gulf of California (GoC). This deglacial ETNP 14 C/C anomaly is hypothesized to reflect either (a) an artifact of the proxy record, (b) the advection of low 14 C/C seawater, or (c) the input of 14 C/C‐depleted geologic carbon related to local seafloor volcanism. To test these hypotheses, we first use new sediment‐trap and seaweed 14 C/C to establish a new baseline understanding of ETNP seawater 14 C/C, which suggest that anomalously low 14 C/C is upwelled in the modern GoC. We then apply new geochemical experiments to test and ultimately validate the utility of the benthic foraminiferal 14 C/C as a proxy for seawater 14 C/C. Finally, we present a compilation of published and new glacial‐interglacial benthic foraminiferal 14 C/C records, specifically developed to map the spatial and temporal variability of the intermediate‐depth water mass containing the deglacial ETNP 14 C/C anomaly. These results clearly show that the ETNP deglacial 14 C/C anomaly develops near the GoC mouth, concomitant with local hydrothermal systems. Considering these results and those of our companion paper (Green et al., 2026, https://doi.org/10.1029/2025pa005217 ), we argue that the input of pH‐neutral geologic carbon from hydrothermal vents near and within the GoC could explain the anomalous intermediate‐depth 14 C/C values both during the deglaciation and today. Plain Language Summary Climate scientists and oceanographers reconstruct past ocean and climate conditions using a variety of indirect observations or “proxies,” and a key tool in this work is the geochemistry of marine microfossils. Here, we examine a well‐known mystery in ocean carbon chemistry in the Eastern Tropical North Pacific (ETNP), involving extremely low seawater radiocarbon content ( 14 C/C) after the last ice age. We first present new proxy measurements of modern ETNP seawater 14 C/C using seaweed and sediment trap‐collected planktic foraminifera, to provide a baseline for interpreting the much older proxy measurements. We then develop and apply several new tests of the benthic foraminiferal 14 C/C proxy, which show that the proxy is robust and truly recording the deglacial seawater 14 C/C anomalies. We then produce a regional map of eastern North Pacific subsurface seawater since the last ice age with 5 new and 7 published benthic foraminiferal radiocarbon records. These new sites were chosen to exploit known subsurface currents, thereby enabling us to observe the spatial and temporal variability of 14 C/C in this water mass. We find that the deglacial ETNP seawater 14 C/C anomaly develops as subsurface waters move over areas of known seafloor volcanism. We propose that this variability may be related to increased carbon flux that does not affect seawater acidity. Key Points We created a glacial‐deglacial map tracing the subsurface changes in Eastern Tropical North Pacific (ETNP) radiocarbon content Very low radiocarbon anomalies develop at ETNP hydrothermal vents, with no radiocarbon anomalies upstream and diluted anomalies downstream The proxy records appear robust and therefore may record an increased flux of pH‐neutral geologic carbon during the deglaciation