Evidence for enhanced convection of North Pacific Intermediate Water to the low‐latitude Pacific under glacial conditions

Abstract

We provide high‐resolution foraminiferal stable carbon isotope (δ13C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extratropical and tropical North Pacific during the past 60 kyr. We measured the δ13C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201‐2‐101KL; 58°52.52′N, 170°41.45′E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the δ13C of subthermocline water masses in the EEP is derived from the deep‐dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31′N, 82°27.76′W; 2921 m water depth). Apparent similarities in the long‐term evolution of δ13C between GNPIW, intermediate waters in the eastern tropical North Pacific and subthermocline water masses in the EEP suggest the expansion of relatively 13C‐depleted, nutrient‐enriched, and northern sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface waters of the glacial EEP. Our findings highlight potential links between North Pacific mid‐depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.

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

“…To validate the depth habitat of G. hexagonus , we determined its apparent calcification depth (ACD) using the uppermost core top sample (at 10 cm; details described in Max et al, ). The ACD assessment indicates an ACD of 340–430 m water depth that agrees with studies from the central equatorial Pacific (Rippert et al, ) and North Pacific (Ortiz et al, ).…”

“…(North Pacific, NP; dark green; Knudson & Ravelo, ), and EW9504‐04 from the California Margin (CM; Stott et al, ). Please note that the first 60 ka of the δ 13 C G.hexagonus values were previously published in Max et al (). Benthic δ 18 O stack (gray; Lisiecki & Raymo, ) is shown for stratigraphic orientation.…”

“…Given that GNPIW shows slightly higher δ 13 CEQ G.hex values, the Δδ 13 CNP‐EQ G.hex is slightly positive during glacial maxima. This is in line with a recent high‐resolution δ 13 C comparison between western Bering Sea (Shirshov Ridge) core SO201‐2‐101KL and ODP Site 1240 G. hexagonus over the past 60 ka that clearly demonstrates the similar absolute δ 13 C values and the long‐term trend between subthermocline waters and North Pacific waters during early MIS 2 (Figure b; Max et al, ).…”

“…Hence, the enhanced ventilated GNPIW seems to have been progressively expanding southward, reaching the California margin and ETNP under glacial conditions, and by the glacial maxima, the volume of GNPIW further expanded as far as the equatorial subthermocline waters (Figure ). The expansion of relatively nutrient‐rich northern‐sourced intermediate water should have important implications for the nutrient concentration of the equatorial subthermocline waters, consistent with the findings of Loubere et al (, ) and Max et al (). Given the persistent stratification and nearly complete surface nutrient utilization with low organic carbon accumulation that prevailed under glacial conditions in the subarctic Pacific (Riethdorf et al, ), the “North Pacific Nutrient Leakage” might have been further enhanced and possibly explains the larger Δδ 13 CNP‐EQ G.hex during MIS 6 compared to MIS 2.…”

“…In the equatorial Pacific, benthic carbon isotope signatures show apparent similarities between Bering Sea records and EEP subthermocline waters during glacial boundary conditions, highlighting that the enhanced North Pacific middepth circulation reaches further south during glacial maxima than today (Knudson & Ravelo, ). These similarities and trends in δ 13 C signatures further point to an increased nutrient injection from GNPIW into EEP subthermocline waters during Marine Isotope Stage (MIS) 2, which was termed the “North Pacific Nutrient Leakage” (Max et al, ).…”