Modeling water isotopologues during the last glacial: Implications for quantitative paleosalinity reconstruction

Abstract

Quantitative paleosalinity reconstructions with reasonable uncertainties remain a challenge in paleoceanography. In this study, we focus on stable isotope-based methods (δ 18 O and δ 2 H) to derive paleosalinity. We use the water isotopes-enabled fully coupled atmosphere/ocean/vegetation/land surface three-dimensional model of intermediate complexity iLOVECLIM to simulate the climate and water isotopes during the Last Glacial Maximum (LGM) and Heinrich event 1. We investigate how the isotopes in water can be used as reliable proxies to quantitatively reconstruct past changes in seawater salinity. Our results demonstrate that a quantitative salinity reconstruction during glacial conditions based on present-day regional δ 18 O-salinity relationships can lead to considerable errors (up to 25 g/kg in certain regions). However, we show that these eventual uncertainties on paleosalinity reconstruction can be reduced by allowing for model-derived regional δ 18 O-salinity relationships to vary through time. Our results indicate a rather stable dependence between δ 2 H and δ 18 O in surface seawater during the LGM. This suggest that quantitative reconstruction of salinity based on the δ 2 H measurement of alkenones (δ 2 H a ) might be possible if the slope and the intercept of the regression between the fractionation factor δ 2 H a -δ 2 H sw and salinity can be sufficiently tightly constrained in open ocean conditions. We confirm that pairing water isotopologues has a strong potential to reduce uncertainties on quantitative paleosalinity reconstructions, also under glacial boundary conditions.

Pronounced centennial-scale Atlantic Ocean climate variability correlated with Western Hemisphere hydroclimate

“…However, this assumption should be valid considering the lack of abrupt and significantly large changes in the late Holocene. This assumption is further validated by an isotopic modeling study that shows little-to-no salinity bias in the salinity-δ 18 O sw relationship in the Gulf of Mexico during the Last Glacial Maximum and Heinrich Event 1, two time periods with significant changes compared to the late Holocene 88 . Furthermore, the structure of the resultant δ 18 O sw and SST reconstruction at the Garrison Basin is unchanged even if we use the conventional methodology of a straightforward Mg/Ca-SST relationship 89 , though the absolute magnitudes differ.…”

Constraining past seawater δ 18 O and temperature records developed from foraminiferal geochemistry

“…PSU Solver can be suitably modified to address problems on timescales or foraminiferal species outside our example data sets, including important paleoclimate targets where seawater Mg/Ca was different from the modern ocean [ O ' Brien et al ., ; Ravelo et al ., ; Zhang et al ., ; Lear et al ., ]. The underlying code can also be suitably modified to address the nonstationarity of past δ 18 O sw ‐salinity relationships where modeling studies can provide important constraints on this problem [ Caley and Roche , ; Tindall and Haywood , ; Holloway et al ., ]. Such a temporal bias may be explicitly incorporated into PSU Solver by applying appropriate δ 18 O sw ‐salinity equations, furnished by paleoclimate simulations, in a piecewise manner.…”

“…However, there are multiple sources of uncertainty inherent in transferring the paired Mg/Ca‐δ 18 O measurements into deconvolved time series of temperature, δ 18 O sw , and salinity. Propagating analytical, sampling [ Thirumalai et al ., , ], and calibration errors [ Khider et al ., ] along with age uncertainty [ Trauth et al ., ; Blaauw and Christen , ] into a climatic time series is complex [ Rohling and Bigg , ; Schmidt , ; Rohling , , ] due to the nonlinear nature of the associated Mg/Ca and δ 18 O relationships [ Dekens et al ., ; Anand et al ., ], the potential effect of salinity and postdepositional effects on Mg/Ca [ Ferguson et al ., ; Kısakürek et al ., ; Mathien‐Blard and Bassinot , ; Arbuszewski et al ., ; Dueñas‐Bohórquez et al ., ; Hertzberg and Schmidt , ; Hönisch et al ., ; Khider et al ., ], the effect of ice volume on δ 18 O sw [ Shackleton , ; Mix and Ruddiman , ; Shackleton , ; Elderfield et al ., ], and the potential nonstationarity of the spatially dependent δ 18 O sw ‐salinity relationship [ Rohling and Bigg , ; Bigg and Rohling , ; Leduc et al ., ; Caley and Roche , ; Holloway et al ., ]. Each of these factors can complicate the interpretation of downcore foraminiferal geochemistry, depending on the signal‐to‐noise ratio being reconstructed.…”

Precession-band variance missing from East Asian monsoon runoff

“…Quantitative reconstruction of salinity is not attempted, given the unconstrained nature of the δ 18 O sw salinity relationship over time 23 , 54 , 77 . No reliable information is available on the potentially variable slope of the δ 18 O sw -salinity relationship in the ECS over the past 400,000 years.…”