Reconstructing paleosalinity from δ18O: Coupled model simulations of the Last Glacial Maximum, Last Interglacial and Late Holocene

Abstract

Reconstructions of salinity are used to diagnose changes in the hydrological cycle and ocean circulation.A widely used method of determining past salinity uses oxygen isotope (δ Ow ) residuals after the extraction of the global ice volume and temperature components. This method relies on a constant relationship between δ Ow and salinity throughout time. Here we use the isotope-enabled fully coupled General Circulation Model (GCM) HadCM3 to test the application of spatially and time-independent relationships in the reconstruction of past ocean salinity. Simulations of the Late Holocene (LH), Last Glacial Maximum (LGM), and Last Interglacial (LIG) climates are performed and benchmarked against existing compilations of stable oxygen isotopes in carbonates (δ Oc ), which primarily reflect δ Ow and temperature. We find that HadCM3 produces an accurate representation of the surface ocean δ Oc distribution for the LH and LGM. Our simulations show considerable variability in spatial and temporal δ Ow -salinity relationships. Spatial gradients are generally shallower but within ⇠50 % of the actual simulated LH to LGM and LH to LIG temporal gradients and temporal gradients calculated from multi-decadal variability are generally shallower than both spatial and actual simulated gradients. The largest sources of uncertainty in salinity reconstructions are found to be caused by changes in regional freshwater budgets, ocean circulation, and sea ice regimes. These can cause errors in salinity estimates exceeding 4 psu. Our results suggest that paleosalinity reconstructions in the South Atlantic, Indian and Tropical Pacific Oceans should be most robust, since these regions exhibit relatively constant δ Ow -salinity relationships across spatial and temporal scales. Largest uncertainties will affect North Atlantic and high latitude paleosalinity reconstructions. Finally, the results show that it is difficult to generate reliable salinity estimates for regions of dynamic oceanography, such as the North Atlantic, without additional constraints.

Evolution of South Atlantic density and chemical stratification across the last deglaciation

“…Salinity is calculated from δ w-ice assuming that the modern relationship between salinity and δ w for the Southern Ocean (21) holds over the deglacial period. Recent isotope-enabled fully coupled general circulation model (GCM) experiments have suggested that the salinity-δ w relationship in the South Atlantic is more temporally constant than in other ocean basins (22), ruling out significant biases in our inferred salinity estimates due to potential variations in the salinity-δ w relationship. However, brine rejection during the formation of sea ice increases salinity without fractionating oxygen isotopes; therefore, we suggest that salinity values derived from δ w-ice are minimum estimates.…”

What drives interannual variation in tree ring oxygen isotopes in the Amazon?

“…It is therefore feasible that variation in Amazon hydrology could account for most of the change in δ 18 O ICE since the LGM (i.e., a decrease in rainout since the LGM causing an increase in δ 18 O ICE ), without needing to invoke large shifts in temperature [ Pierrehumbert , ]. However, it should be noted that during the LGM δ 18 O SW would have been ~1‰ higher due to the difference in global ice volume, although spatial gradients in tropical δ 18 O SW were similar to the present day [ Holloway et al , ].…”

Simulating the 128‐ka Antarctic Climate Response to Northern Hemisphere Ice Sheet Melting Using the Isotope‐Enabled HadCM3

“…We carry out a series of isotope‐enabled (Holloway, Sime, Singarayer, Tindall, & Valdes, ; Tindall et al, ) coupled atmosphere‐ocean general circulation model experiments using the U.K. MetOffice HadCM3 model. The simulated distribution of isotopes in Antarctic precipitation compare well to present‐day observations (e.g., Sime et al, ).…”