Equatorial Pacific bulk 𝛿15N supports a secular increase in subantarctic zone nitrate utilization after the mid-Pleistocene Transition

Pacific-wide measurements of nitrate and its isotopic composition have furthered our understanding of modern subsurface circulation and have revealed basin-scale connections between oceanographic and nitrogen cycle processes. From the Eastern Tropical Pacific (ETP), the isotopic signature of denitrification is spread zonally and meridionally via subsurface currents. From the Pacific sector of the Southern Ocean, Subantarctic Mode Water (SAMW) penetrates to the low latitudes, delivering nitrate (and likely its isotopic signature) to equatorial surface waters via upwelling. These two regional processes combine to inform much of the thermocline nitrogen dynamics of the Pacific. Here, we compare a new 1.4-Myr bulk sediment 𝛿15N record from the New Guinea margin (IODP Site U1486) to other Pacific 𝛿15N records to track Pleistocene changes in denitrification and SAMW properties. Our results highlight a dramatic increasing 𝛿15N trend after the mid-Pleistocene Transition (MPT) at equatorial sites that is not observed at the New Guinea and California margin sites. Strong 41-ky forcing at equatorial sites and little detectable influence from denitrification (counter to larger denitrification signals at margin sites) suggests increasing 𝛿15N within upwelled SAMW. Because the New Guinea and California margin sites are not below equatorial upwelling, thermocline nitrate is less influenced by SAMW, but rather tracks denitrification in the ETP.

As equatorial Pacific nitrate utilization has not dramatically increased in the late Pleistocene, an increase in subantarctic zone nitrate utilization is proposed. Initiation of increased nitrate utilization appears to commence near the end of the MPT and accelerate near the Mid-Brunhes Event (~430 ka). The observed southward shift of the polar front at this time (associated with increased sea surface temperature), combined with elevated dust/iron flux, may have contributed to greater nitrate utilization and a more efficient biological pump in the subantarctic zone. Through the production (via denitrification) and sequestration (via nitrate utilization) of greenhouse gases, these biogeochemical processes potentially participated in feedbacks associated with both the MPT and the Mid-Brunhes Event. Until reconstructions of subantarctic zone nitrate are extended beyond the last two glacial cycles, this reconstruction of SAMW properties via equatorial Pacific bulk 𝛿15N may provide the best record of long-term changes in nitrogen dynamics in the subantarctic zone.