Thermocline-level connectivity between the tropical equatorial Pacific and tropical northeast Pacific during deglaciation

Abstract

The connection of the Southern Ocean and Eastern Equatorial Pacific (EEP) has been used to explain the presence of an old, respired water mass into the Eastern Tropical North Pacific (ETNP) and Eastern North Pacific (ENP) during deglaciation. This water mass and its geochemical properties are transported along the thermocline via subsurface and intermediate circulation. Some questions remain regarding an intermediate water mass with old and less ventilated characteristics in the eastern Pacific because certain sedimentary records of δ13C, δ18O, and Δ14C in benthic foraminifera have failed to prove its presence. The top-down hypothesis has been proposed to explain the absence of this carbon isotopic signal in benthic and its presence in planktonic foraminifera in the EEP. To prove this hypothesis in the ETNP, oxygen and carbon isotopic compositions were determined for Neogloboquadrina dutertrei in a core collected at 700 m depth over the Magdalena margin, Mexico. From these new data, a reconstruction of the water column structure and the change in ocean circulation at the thermocline level during deglaciation was inferred. N. dutertrei δ18O values in the ETNP and those from Globigerina bulloides in the ENP, similar to those of N. dutertrei in the EEP, suggest that the tropical water mass extended as far as 32 °N. Specifically, N. dutertrei δ18O values were more positive in the Heinrich Stadial-1 (HS-1) and Younger Dryas (YD) than the Bolling Allerod-Antarctic Cold Reversal (BA-ACR) and Early Holocene (EH) suggesting the presence of a saline water mass with origin in the EEP. N. dutertrei δ13C depleted values during the HS-1 and YD suggested the presence of a water mass with old and respired carbon. This is consistent with the salinification or “spicy” inferred from δ18O for the ETNP. These isotopic findings in N. dutertrei corroborate the existence of a tropical water mass that probably originated in the Southern Ocean. Additionally, the present data do not support the top-down hypothesis.