Plio‐Pleistocene glacial‐interglacial productivity changes in the eastern equatorial Pacific upwelling system

Abstract

The eastern equatorial Pacific Ocean (EEP) upwelling system supports >10% of the present-day global ocean primary production, making it an important component in Earth's atmospheric and marine carbon budget. Traditionally, it has been argued that since intensification of Northern Hemisphere glaciation (iNHG,~2.7 Ma), changes in EEP productivity have predominantly depended on trade wind strength-controlled upwelling intensity. An alternative hypothesis suggests that EEP productivity is primarily controlled by nutrient supply from the high southern latitudes via mode waters. Here we present new high-resolution data for the latest Pliocene/early Pleistocene from Ocean Drilling Program Site 849, located within the equatorial divergence system in the heart of the EEP upwelling regime. We use carbon isotopes in benthic and planktic foraminiferal calcite and sand accumulation rates to investigate glacial-interglacial (G-IG) productivity fluctuations between 2.65 and 2.4 Ma (marine isotope stages (MIS) G1 to 94). This interval includes MIS 100, 98, and 96, three large-amplitude glacials (~1‰ in benthic δ 18 O) representing the culmination of iNHG. Our results suggest that latest Pliocene/early Pleistocene G-IG productivity changes in the EEP were strongly controlled by nutrient supply from Southern Ocean-sourced mode waters. Our records show a clear G-IG cyclicity from MIS 100 onward with productivity levels increasing from full glacial conditions and peaking at glacial terminations. We conclude that enhanced nutrient delivery from high southern latitudes during full glacial conditions together with superimposed intensified regional upwelling toward glacial terminations strongly regulated primary productivity rates in the EEP from MIS 100 onward.

Thermocline state change in the Eastern Equatorial Pacific during the late Pliocene/early Pleistocene intensification of Northern Hemisphere Glaciation

“…Measurements of geochemical parameters (δ 13 C, δ 18 O, Mg/Ca) separately on sinistral-and dextral-coiling specimens from 25 the same samples have demonstrated that the reconstructed values are independent of the coiling directions (see also Jakob et al [2016]). …”

“…These intervals are from 2.68 to 2.65 .65 mcd corresponding to MIS G3) and from 2.73 to 2.69 .79 mcd corresponding to MIS G7-G5). We decided not to 30 fill these gaps by using another size fraction because (i) this might have biased our Mg/Ca and stable-isotope records due to ontogenetic effects (Elderfield et al, 2002;Friedrich et al, 2012), and (ii) the abundance of G. crassaformis specimens <315 For G. ruber, the previously published Mg/Ca-based SST and δ 18 O datasets of Jakob et al (2017) were augmented by δ 13 C data, except for the interval from ~2.65 to 2.4 Ma for which δ 13 C values have already been published by Jakob et al (2016). For this purpose, an average of twelve specimens was picked from the 250-315 µm size fraction.…”

“…1). This site has been selected because of (i) its position within the equatorial "cold tongue" west of the East Pacific Rise in the open ocean that makes it less prone to continental influence compared to sites east of the East Pacific Rise (Mix et al, 1995); (ii) the good preservation of 20 foraminifera (Jakob et al, 2016(Jakob et al, , 2017 despite the fact that the present-day water depth at Site 849 is close to the lysocline (Adelseck and Anderson, 1978;Berger et al, 1982); and (iii) high sedimentation rates (2.7 cm kyr -1 for our study interval [Jakob et al, 2017]) with continuous sedimentation (Mayer et al, 1992).…”

“…The >250 µm size fraction was split down to at least 400 individual planktic foraminifera using a microsplitter. Abundance counts were converted to mass-accumulation rates using linear sedimentation rates and dry bulk density data (calculated from high-resolution GRAPE density shipboard measurements [IODP JANUS database; Mayer et al, 1992]) following the methodology described in Jakob et al (2016). 20…”

“…This so-called "intensification of Northern Hemisphere Glaciation" (iNHG) represents the most recent major tipping point in Earth's climate history. It strongly influenced oceanographic and climatic patterns worldwide, affecting, for example, the amount of 5 biological production in the Eastern Equatorial Pacific Ocean (EEP) (Etourneau et al, 2010;Jakob et al, 2016) -a region that exerts a strong influence on the Earth's climate system through its effects on the global carbon and nutrient cycles (e.g., Schlitzer, 2004;Takahashi et al, 2009). …”