Water Mass Versus Sea Level Effects on Benthic Foraminiferal Oxygen Isotope Ratios in the Atlantic Ocean During the LGM

Abstract

Depth transects of benthic foraminiferal oxygen isotopes from the Atlantic Ocean show that glacial‐interglacial changes are larger at deep (> ~2,000 m) than at intermediate water levels. Our model results suggest that the smaller changes in the upper 1,000 m of the water column are a result of the glacial sea level lowering of about 120 m, leading to warmer temperatures of around 1 °C and hence a smaller glacial‐interglacial stable oxygen isotope difference. In contrast, a shoaling of the water mass boundary to ~2,000‐m water depth between the northern and southern source waters is caused by the expansion of a cold (close to the freezing point) southern source water in the abyssal ocean, increasing the oxygen isotope values of benthic foraminifera from the Last Glacial Maximum in the deep Atlantic. These two effects explain the different amplitudes of glacial‐interglacial stable oxygen isotope differences in the upper and deeper water column of the Atlantic Ocean.

Cancellation of the Precessional Cycle in δ18O Records During the Early Pleistocene

“…This is due to the hemispheric antiphasing of the precessional signal in insolation for a given season (Milankovic, 1920), and because of the inflow of meltwater from the ice sheets into the Southern Ocean and North Atlantic deep‐water formation regions (Figure 1). Indeed, sediment records of δ 18 O archive the influence from northern and southern deep‐water sources on glacial‐interglacial timescales (e.g., Völpel et al., 2019). Our approach builds on the study by Raymo et al.…”

A dynamical reconstruction of the Last Glacial Maximum ocean state constrained by global oxygen isotope data

“…Their optimized control variables were also used with a similar model configuration to obtain an equilibrium LGM simulation that showed a shallower and weaker AMOC, again highlighting the influence of the simulation length or of slight changes in the model configuration on the result (Völpel et al, 2018).…”

“…In this study the model was driven by monthly air temperature, meridional and zonal wind velocities, wind speed, specific humidity, precipitation, downward shortwave radiation, downward longwave flux, and river run-off, based on a fully coupled LGM simulation of the Community Climate System Model Version 3 (Merkel et al, 2010). Following Völpel et al (2018) and Breitkreuz et al (2019, in review), the Mediterranean Sea was excluded from the model domain, because a shallow passage trough the Strait of Gibraltar was not possible due to the coarse vertical resolution. The isotopic composition of seawater…”

“…We use a general circulation model enhanced with a water isotope module (Völpel et al, 2017(Völpel et al, , 2018 that enables it to simulate the oxygen isotopic composition of seawater (δ 18 O sw ) in the complete water column. This study investigates the robustness of previous ocean state estimates and the influence of the additional data constraint placed by global surface data and deep-ocean data from outside of the Atlantic Ocean on the estimate, in particular, on the AMOC strength and the vertical extent of the NADW.…”

A reduced-order Kalman smoother for (paleo-)ocean state estimation: assessment and application to the LGM

“…Subgrid-scale mixing was parameterized with a GM/Redi scheme (Redi, 1982;Gent and McWilliams, 1990). Following Völpel et al (2018), we excluded the Mediterranean Sea from our model domain because a shallow passage through the Strait of Gibraltar was not possible due to the coarse vertical resolution. The MITgcm is extended with a water isotope module that comprises the fractionation processes of water isotopes during evaporation over the ocean and allows the model to simulate stable water isotopes in the entire water column (Völpel et al, 2017).…”