Paleosalinity and δ¹⁸O: A critical assessment

Abstract

Abstract. In paleoceanography, a traditional application of oxygen isotope ratios in fossil carbonate from deep-sea cores concerns the reconstruction of palcotemperatures. Recently, isotopic data have been increasingly employed in the reconstruction of paleosalinities as well. This application involves a number of basic assumptions, a critical review of which is presented here. As the calculation of paleosalinity from •80 residuals assumes constant and linear surface water salinity (S):•80 relationships, we investigate this basic assumption for areas with important and variable freshwater budget and/or sea-ice influences. We show that it is particularly unwarranted to assume such temporally invariant linearity in temperate and subpolar seas because of the effects of freezing and melting. More critically, we argue that the determinant of many regions' S:•80 is advection, not the local water balance, so that the basic •80 of seawater in many regions of the world ocean would likely have changed in a manner somewhat different from that expected from the ice volume record. Although the differences may not be significant for the gross reconstruction of such features as past sea level, we believe they do potentially bias paleosalinity reconstructions, mostly toward high salinities. To fully exploit the potential of oxygen isotope records as quantitative paleosalinity indicators, both the spatial and temporal variabilities need to be constrained. Hence we recommend that (1) for any time slice of specific interest to paleoceanographic and paleoclimatological studies, a coupled ocean-atmosphere-isotope model, or at least an ocean-isotope model driven by a paleoclimate-isotope model, should be used to define the distribution of the base, mixing end-member, b•80 of sea water; (2) much further regional-scale research is needed on flux rates and isotopic compositions of the various terms in the freshwater budget, as well as on the variability in the various advective terms. The latter variability may be assessed from reconstructions of past seaice distributions and of volume fluxes/isotopic compositions of outflow from marginal seas and through sea straits/passages.

Recent unprecedented warming and oligotrophy of the eastern Mediterranean Sea within the last millennium

“…The average δ 18 O Ver of living vermetids from reef tops, 0.5‰ ± 0.13‰ (±1σ SD; n = 300 individual vermetids), translates to an average deposition temperature [Bohm et al, 2000] [Bohm et al, 2000]. EM δ 18 O sw was assumed to be constant [Grauel et al, 2013] for the SST reconstruction as it is dominated by the advection and mixing of the Atlantic input [Rohling and Bigg, 1998], which was stable over the last millennium. The decrease of river discharge, especially due to the Nile damming during the second half of the twentieth century, has resulted with a salinity increase of about 0.3 psu [Skliris and Lascaratos, 2004].…”

Stable water isotopes in the MITgcm

“…In OGCMs, the focus was mainly on the relationship between δ 18 O w and salinity, which are affected by similar physical processes. This topic is of significant interest in paleoceanography, because it is likely that changes in advection and freshwater budgets as well as the source of precipitation may have altered this relationship (Rohling and Bigg, 1998). Using real freshwater flux boundary conditions in conjunction with the nonlinear free surface (Huang, 1993) is essential to simulate it properly.…”

Pliocene climate and seasonality in North Atlantic shelf seas

“…This increased the r 2 value to 0.9 but did not significantly change the diagnostic predictions of d 18 O sw generated using PKE. Nevertheless, it is important to recognize that the use of a salinity : d 18 O or PKE : d 18 O co-variation from present-day observations as a diagnostic for the d 18 O composition of seawater during the Early or Mid-Pliocene is complicated by the fact that latitudinal temperature gradients are steeper today than they were during the Pliocene, a reflection of cooler temperatures in polar regions, which result in different patterns of Rayleigh distillation and hence different d 18 O values in the hydrological cycle (Rohling & Bigg 1998;Rohling 2000).…”