Progressive shoaling of the equatorial Pacific thermocline over the last eight glacial periods

Abstract

International audienceThe depth of equatorial Pacific thermocline is diagnostic of the main modes of tropical climates. Past estimates of Pacific thermocline dynamics have been reconstructed either for the Last Glacial Maximum or on longer timescales at low resolution. Here we document a new high-resolution set of reconstructed past sea surface and subsurface waters temperatures from the southwestern subequatorial Pacific, core MD05-2930, in the Gulf of Papua, over the last 800ka. We used two morphotypes of Globigerinoides ruber known to live at different water depths to reconstruct past stratification. We estimated calcification temperature of each morphotypes by Mg/Ca paleothermometry. Our subequatorial Pacific thermocline paleotemperature record indicates a response of the thermocline to both direct orbital forcing and glacial-interglacial changes. Our stratification record shows a systematic shallower glacial thermocline, whereas sea surface temperatures are characterized by precessional forcing. The record is indicative of a progressive long-term shoaling of the thermocline during the glacial stages during the late Pleistocene. The shoaling of the subequatorial Pacific thermocline is consistent with regional estimates. An enhanced South Pacific shallow overturning wind-driven circulation could have driven this progressive shoaling. We speculate that this late Pleistocene glacial shoaling of the thermocline could be related to an increase in the amplitude of the obliquity

Stable Oxygen Isotopes and Mg/Ca in Planktic Foraminifera From Modern Surface Sediments of the Western Pacific Warm Pool: Implications for Thermocline Reconstructions

“…There is an ongoing debate how the thermocline depth varied throughout the past. For example, some records indicate a thermocline deepening during the Last Glacial Maximum (e.g., Bolliet et al, ), others indicate a thermocline shoaling during the same period (Andreasen & Ravelo, ; Beaufort et al, ; de Garidel‐Thoron et al, ; Regoli et al, ; Sagawa et al, ), and yet others indicate no change compared to the modern ocean (Patrick & Thunell, ). Many of these reconstructions are based on the calculation of differences between shell Mg/Ca‐derived temperature and/or δ 18 O of planktic foraminifera calcifying at different depth levels to estimate vertical temperature gradients within the upper water column (e.g., Bolliet et al, ; de Garidel‐Thoron et al, ; Regoli et al, ).…”

“…For example, some records indicate a thermocline deepening during the Last Glacial Maximum (e.g., Bolliet et al, ), others indicate a thermocline shoaling during the same period (Andreasen & Ravelo, ; Beaufort et al, ; de Garidel‐Thoron et al, ; Regoli et al, ; Sagawa et al, ), and yet others indicate no change compared to the modern ocean (Patrick & Thunell, ). Many of these reconstructions are based on the calculation of differences between shell Mg/Ca‐derived temperature and/or δ 18 O of planktic foraminifera calcifying at different depth levels to estimate vertical temperature gradients within the upper water column (e.g., Bolliet et al, ; de Garidel‐Thoron et al, ; Regoli et al, ). Previous studies used, for example, the difference between shell Mg/Ca in G. ruber as surface indicator and P. obliquiloculata or N. dutertrei as thermocline depth indicators (e.g., Bolliet et al, ) or the difference between G. ruber sensu strictu and G. ruber sensu lato (here referred to as G. ruber and G. elongatus following Aurahs et al, ) (Regoli et al, ).…”

“…Many of these reconstructions are based on the calculation of differences between shell Mg/Ca‐derived temperature and/or δ 18 O of planktic foraminifera calcifying at different depth levels to estimate vertical temperature gradients within the upper water column (e.g., Bolliet et al, ; de Garidel‐Thoron et al, ; Regoli et al, ). Previous studies used, for example, the difference between shell Mg/Ca in G. ruber as surface indicator and P. obliquiloculata or N. dutertrei as thermocline depth indicators (e.g., Bolliet et al, ) or the difference between G. ruber sensu strictu and G. ruber sensu lato (here referred to as G. ruber and G. elongatus following Aurahs et al, ) (Regoli et al, ). However, to choose species and interpret such proxy records correctly, it is essential to understand how modern hydrographic conditions are reflected in foraminiferal calcite.…”

A general theory on frequency and time–frequency analysis of irregularly sampled time series based on projection methods – Part 2: Extension to time–frequency analysis

“…-Studies in palaeoclimate: analysis of the astronomical forcing in Berger et al (1998), analysis of the midPleistocene transition in Elderfield et al (2012), or the analysis of the equatorial Pacific thermocline over the last eight glacial periods in Regoli et al (2015).…”

Assessing the reliability of foraminiferal Mg/Ca thermometry by comparing field-samples and culture experiments: a review

“…Mg/Ca palaeothermometry has the po ten tial of re cord ing rel a tively small ocean T changes, which is par tic u larly use ful when study ing high-fre quency cli mate changes dur ing the Pleis to cene (Lea et al, 2000;Me dina-Elizalde and Lea, 2005;Rodríguez-Sanz et al, 2012;Regoli et al, 2015;Lynch-Stieglitz et al, 2015;Lear et al, 2015), and is based on the prem ise that sea wa ter tem per a ture is the pri mary con trol on Mg 2+ in cor po ration into foraminiferal tests dur ing cal ci fi ca tion. The un der ly ing ba sis for Mg/Ca ther mom e try, as cor rob o rated by both ther mody nam ics and phys i o log i cal pro cesses, is that the sub sti tu tion of Mg in cal cite is en do ther mic and there fore is fa voured at higher tem per a tures Erez, 2003;Bentov and Erez, 2006;Lea, 2014).…”