Pliocene Changes in Tropical East Pacific Upper Ocean Stratification: Response to Tropical Gateways?

Abstract

A combination of stable isotope records and Mg/Ca temperature estimates of four different planktonic foraminiferal species from Ocean Drilling Program Site 1241 allows differentiation between temperature and salinity changes in the tropical east Pacific (TEP) upper water column during the Pliocene (~5.7-2.1 Ma). The deviation of δ 18 O records and Mg/Ca temperature estimates from thermocline-dwelling planktonic foraminifers suggests that local changes in salinity exerted a much stronger control on Pliocene TEP upper ocean water mass signatures than previously assumed. The most pronounced Pliocene change in TEP upper ocean stratification was the shoaling of the thermocline from ~4.8 to 4.0 Ma that was possibly triggered by changes in the configuration of low-latitude ocean gateways. During this time interval, mixed-layer temperatures and salinities remained relatively constant in contrast to a pronounced temperature (~6°C) and salinity decrease at the bottom of the photic zone. This change led to a new state in the thermal structure of the TEP, as the thermocline remained relatively shallow until ~2.1 Ma.

Thermocline state change in the eastern equatorial Pacific during the late Pliocene/early Pleistocene intensification of Northern Hemisphere glaciation

“…As a consequence, small changes in its depth through east-west tilting regulated by short-term climate phenomena such as the El Niño-Southern Oscillation can affect surface-water properties and ocean-atmosphere exchange processes (Fedorov et al, 2004;Ma et al, 2013). In general, proxy records and modeling results consistently document a long-term shoaling of the thermocline in the EEP and other (sub-)tropical upwelling regions throughout the Plio-Pleistocene (Wara et al, 2005;Fedorov et al, 2006;Steph et al, 2006aSteph et al, , 2010Dekens et al, 2007;Ford et al, 2012). However, the dynamics of the thermocline in the EEP and its potential links to the iNHG currently remained enigmatic.…”

“…The timing led to the hypothesis that thermocline shoaling and the development of the EEP "cold tongue" (Wyrtki, 1981) were necessary preconditions for the iNHG, as they reduced poleward atmospheric heat transport (Cane and Molnar, 2001). However, other studies only identified fundamental shifts in the thermocline depth prior to ∼ 3.5 Ma (Wara et al, 2005;Steph et al, 2006aSteph et al, , 2010Ford et al, 2012), which would imply that the thermocline depth in the EEP did not play an important role in the development of large-scale glaciation in the Northern Hemisphere. In light of these contradictory views, and to ultimately shed new light on potential links between low-latitude thermocline dynamics and high-latitude ice-sheet buildup, we investigated thermocline state changes for Ocean Drilling Program (ODP) Site 849 in the EEP during the final phase of the late Pliocene/early Pleistocene iNHG (∼ 2.75 to 2.…”

“…1a, b and 6). In comparison to thermocline temperature records from Site 1241 (Steph et al, 2006a;Groeneveld et al, 2014) our new G. crassaformis-based temperatures appear to be relatively low. However, this discrepancy can likely be explained by different calcification depths of the investigated species (which means that different temperatures are reflected; Cléroux et al, 2013) and differences in the oceanographic setting of the investigated sites (i.e., Site 849 in the "cold tongue" versus Site 1241 in the "warm pool" with, for example, a modern-day SST difference of ∼ 3 • C; Locarnini et al, 2013; see also Sect.…”

Data report: Mg/Ca values of Globorotalia tumida from early Pliocene to present, Site U1338

“…In the eastern equatorial Pacific, subsurface temperatures steeply cooled by ~3°C between 4.0 and 4.8 Ma and gradually cooled an additional ~2°C toward the present day (Ford et al, 2012;Steph et al, 2006Steph et al, , 2010. In comparison, subsurface temperatures gradually cooled ~3°C from the Pliocene to the present day (Ford et al, 2015).…”

“…In contrast, the Western Pacific Warm Pool is characterized by warm sea-surface temperatures and a deep thermocline. During the Pliocene warm period (~3 to 5 Ma), subsurface temperatures were warm across the equatorial Pacific, suggesting the tropical thermocline was warm and/or deep (Ford et al, 2012(Ford et al, , 2015Steph et al, 2006Steph et al, , 2010.…”