A Deep Eastern Equatorial Pacific Thermocline During the Last Glacial Maximum

Abstract

The mean state and variability of the tropical Pacific is influenced by the depth of the thermocline. During the Last Glacial Maximum (~21,000 years ago), the zonal sea surface temperature gradient across the equatorial Pacific was reduced and productivity was generally lower than modern. To understand the thermocline depth's role in determining the Last Glacial Maximum tropical mean state, we reconstruct the upper ocean δ18O profile from multiple species of planktic foraminifera. We synthesize existing records of surface and subsurface dwelling foraminifera to reconstruct the vertical δ18O gradient throughout the eastern equatorial Pacific. We find the thermocline was deeper during the Last Glacial Maximum than the Holocene throughout the eastern equatorial Pacific region. The thermocline depth's role in the dynamic forcing of the cold tongue contributed to the reduced zonal SST gradient across the equatorial Pacific, decreased productivity, and presumably impacted El Niño‐Southern Oscillation variability relative to the Holocene.

Field Testing the Fidelity of δ18O and δ13C in Reconstructing Upper Ocean Hydrography

“…The sum of analytical and replication error yields +/−0.15‰ for δ 13 C and +/−0.06‰ for δ 18 O. These analytical and replication error estimates are comparable to published errors reported for G. menardii (e.g., Farmer et al, ; Regenberg et al, ), G. truncatulinoides (e.g., Anand et al, ; Birch et al, ; Reynolds et al, ), G. inflata (e.g., Anand et al, ; Ford et al, ), and G. conglobatus (e.g., Anand et al, ).…”

“…Shallow habitat depths reported for this species include 0–100 m in the Sargasso Sea (Deusser & Ross, ; Fairbanks et al, ), which may be explained by its preference for the seasonal thermocline there (Cleroux et al, ) in late winter to early spring (Deusser & Ross, ). G. inflata has been observed at shallower habitat depths in the equatorial regions of the Atlantic and Pacific Oceans (e.g., Berger, ; Ford et al, ; Patrick & Thunell, ) and the SW African continental margin (Wilke et al, ), which can all be explained by the shoaling of the thermocline due to upwelling. At higher latitudes, the habitat depth of G. inflata is reported to be widely variable (50–350 m; Mortyn & Charles, ; Loncaric et al, ) probably due to a weak thermocline in those regions.…”

“…We preformed 10,000 Monte Carlo simulations using depth ranges reported in the literature, temperatures calculated from δ 18 O, their pooled standard deviations, and δ 18 O‐based depths and their pooled standard deviations (Table ). These methods were based on those of Ford et al ().…”

“…We preformed 10,000 Monte Carlo simulations using depth ranges reported in the literature, temperatures calculated from δ 18 O, their pooled standard deviations, and δ 18 Obased depths and their pooled standard deviations (Table 2). These methods were based on those of Ford et al (2018). (Figures 2a and 2b) which corresponds to a large variation of calculated [PO 4 ] and temperature of habitat waters (Figures 2c and 2d; Table 1).…”

“…Lastly, many researchers interpret a species-specific, narrow, stratified, and often basin-wide habitat depth range when using stable isotopes from tests of planktonic foraminifers to reconstruct surface ocean hydrography. This assumption is critical to reconstructing oceanographic events in the past that rely on understanding changes in the hydrography and stratification of the upper ocean, (Kroon & Ganssen, 1989;Watkins et al, 1996;Mulitza et al, 1997;Rohling & Cooke, 1999;Peeters et al, 2002;Spero et al, 2003;Kuroyanagi & Kawahata, 2004;Mohtadi et al, 2011;Friedrich et al, 2012;Ford et al, 2018). Similarly, LeGrande et al (2004) concluded that δ 18 O from G. truncatulinoides tests reflects seawater density at intermediate depths (300-800 m) and can be used to reconstruct upper ocean geostrophic flows.…”