Alternating Influence of Northern Versus Southern‐Sourced Water Masses on the Equatorial Pacific Subthermocline During the Past 240 ka

Abstract

The eastern equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low‐latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient‐rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial subthermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (δ13C) record of subthermocline waters derived from deep‐dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published δ13C records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern‐sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial subthermocline waters is recognized during glacial terminations and marks changes to modern‐like conditions in nutrient concentrations and biological productivity in the EEP.

Deep Hydrography of the South China Sea and Deep Water Circulation in the Pacific Since the Last Glacial Maximum

“…More recently, vertical profiles of benthic δ 18 O and δ 13 C in the southwest Pacific also suggested a ∼500 m shoaling of the interface between AAIW and UCDW during the LGM than at present (Sikes et al, ). Furthermore, the increasing difference of intermediate water δ 13 C was reported between the South Pacific and eastern equatorial Pacific during glacial maxima relative to the modern conditions, which was interpreted to indicate reduced contribution of AAIW into equatorial intermediate waters during peak glacials (Rippert et al, ). Therefore, it seems unlikely that AAIW expanded and exerted a greater impact on the low‐latitude northwest Pacific during the LGM than nowadays, mainly due to an enhanced bathyal ocean stratification (Allen et al, ; Skinner et al, ) and a possible upward displacement of AAIW (Ronge et al, ; Sikes et al, ).…”

Ventilation of Northern and Southern Sources of Aged Carbon in the Eastern Equatorial Pacific During the Younger Dryas Rise in Atmospheric CO2

“…At Ocean Drilling Program site 1240, located east of the Galapagos on the equator, carbon isotopes measured on thermocline dwelling planktonic foraminifera appear to track carbon isotope variations within NPIW at the end of the last glacial period and early deglaciation. These data are used to argue for greater penetration of NPIW to the EEP during the late glacial (Max et al, ; Rippert et al, ). Lastly, though sparsely sampled, neodymium isotopes at the same site indicate broad swings in water mass contributions to the EEP thermocline across this interval (Pena et al, ).…”

Plio‐Pleistocene Variability of the East Pacific Thermocline and Atmospheric Systems

“…Replacing the influence of SOIW with slightly warmer NPIW would thus result in a TT G. tumida increase at Site 1236 similar to the pattern observed by us (Figure ). The rivaling influence of NPIW and SOIW in the equatorial East Pacific was previously demonstrated for the last two glacial‐interglacial cycles (Rippert et al, ). Reconstructed δ 13 C records on the subthermocline species Globorotaloides hexagonus indicate the dominant presence of NPIW during peak glacial, and SOIW during interglacial periods (Rippert et al, ).…”

“…The rivaling influence of NPIW and SOIW in the equatorial East Pacific was previously demonstrated for the last two glacial‐interglacial cycles (Rippert et al, ). Reconstructed δ 13 C records on the subthermocline species Globorotaloides hexagonus indicate the dominant presence of NPIW during peak glacial, and SOIW during interglacial periods (Rippert et al, ). The ACD of G. hexagonus (340–430 m; Rippert et al, ) is close to the ACD of G. inflata (Groeneveld & Chiessi, ).…”