Coupled nutricline and productivity variations during the Pliocene in the western Pacific warm pool and their paleoceanographic implications

Highlights • Accurate age model during Pliocene for site U1490 established • Co-variant nutricline depth and productivity in WPWP throughout Pliocene • Deeper nutricline and lower productivity during 4.8–3.5 Ma linked to CAS closure • Nutricline shoaling during 3.5–3.0 Ma due to restriction of Indonesian Seaway Abstract The tropical Pacific played an important role in modulating global climate change during the Pliocene. Studies of tropical Pacific sea surface temperatures covering the period from the Pliocene onwards indicate that changes in the thermal mean state over the tropical Pacific can significantly influence global climate feedbacks and connect the high- and low-latitude climates. Tropical productivity fluctuations are a significant mechanism with respect to the operation of the global carbon cycle. Yet, temporal changes in primary productivity are not well constrained in the western Pacific warm pool (WPWP), where the ocean–climate system is not dominated by upwelling systems. Furthermore, the role of nutricline dynamics in forcing productivity over tectonic timescales remains uncertain. Here we use relatively high-resolution foraminiferal carbon isotope records combined with Ba/Ti ratios obtained from International Ocean Discovery Program (IODP) Site U1490 in the WPWP to reconstruct nutricline depth and paleoproductivity over the period 5.1–2.6 Ma. Our records imply that nutricline and productivity variations were closely coupled over tectonic timescales, implying that the dynamics of the nutricline play a significant role in regulating productivity in the WPWP. The deeper nutricline and lower productivity during 4.8–3.5 Ma might have been fostered by the closure of the Central American Seaway through the thickening of the mixed layer in the WPWP. We relate the overall shallower nutricline and increased productivity during 3.5–3.0 Ma to the restriction of the Indonesian Seaway via the enhanced influence and upwelling of high-latitude southern-source waters

Influences of Atlantic Ocean thermohaline circulation and Antarctic ice-sheet expansion on Pliocene deep Pacific carbonate chemistry

Highlights • High-resolution deep-water record for Pliocene western tropical Pacific. • Changes in NADW production at ∼4.6 Ma and ∼2.7 Ma influenced Pacific • These changes controlled a seesaw fluctuation between deep Pacific and Atlantic oceans. • Antarctic ice-sheet/sea-ice expansions influenced deep Pacific • These ice-mass changes resulted in long-term decline during late Pliocene. Abstract Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific . We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene

Precession-driven changes in air-sea CO2 exchange by East Asian summer monsoon in the Western Tropical Pacific since MIS 6

Surface waters of the modern Western Tropical Pacific (WTP) are in equilibrium with atmospheric CO2. However, air-sea exchange of CO2 in this region may have been modulated in the past by oceanic-atmospheric fluctuations in the tropical Pacific such as the East Asian monsoon and the El Niño-Southern Oscillation (ENSO) and extratropical mode waters such as Antarctic Intermediate Water. Thus, understanding controls on the sea-surface carbonate system in the WTP is important for forecasting future carbon-cycle changes in this region. Here, we reconstruct sea-surface pH and pCO2 since Marine Isotope Stage 6 (MIS 6; 155 ka) based on B/Ca ratios of the planktic foraminifer Globigerinoides ruber (white) in sediment Core MD06–3052 from the western Philippine Sea, and we then calculate the difference between oceanic and atmospheric pCO2 (ΔpCO2(sw-atm)) in order to evaluate the history of air-sea CO2 exchange. ΔpCO2(sw-atm) changes were strongly modulated by the ~20-kyr precession cycle. The results of cross-spectral analysis demonstrate a close connection between the East Asian summer monsoon (EASM) and air-sea CO2 exchange since MIS 6, demonstrating that precession-driven EASM can affect air-sea CO2 exchange through regulation of surface productivity and thermocline depth. In contrast, the East Asian winter monsoon (EAWM) and ENSO-like conditions are not major influences on air-sea CO2 exchange in the study area at precession-band frequencies. In addition, enhanced upwelling of Southern Ocean-sourced deepwater rich in dissolved inorganic carbon (DIC) affected the upper water column during transitions from cold to warm stages (i.e., deglaciations). In conclusion, these findings suggest that orbital precession influences can affect oceanic conditions not only through climate change and biological processes but also through sea-surface carbonate chemistry