Sea surface salinity reconstruction as seen with foraminifera shells: Methods and cases studies

Eur. Phys. J. Conferences 1, 177–188 (2009) c EDP Sciences, 2009
DOI: 10.1140/epjconf/e2009-00919-6 THE EUROPEAN PHYSICAL JOURNAL CONFERENCES Sea surface salinity reconstruction as seen with foraminifera shells: Methods and cases studies B. Malaizé and T. Caley Université Bordeaux 1, UMR 5805 EPOC, Avenue des Facultés, 33405 Talence, France Abstract. Reconstruction of past salinities in surface oceans (SSS) can be done by measuring the isotopic composition of foraminifera shells found in the deep sea sediments. The proportion of heavy oxygen isotopes (18 O) in the calcite of these shells depend on the temperature and the isotopic oxygen composition of the surrounded waters (δ 18 Osw), this latter parameter depending on the water salinity. Mainly two equations allows to reconstructed past SSS, one estimating past temperature variations and the other one changes in the δ 18 Osw through time. Uncertainties linked with these calculation can be important, and therefore quantitative reconstructions need to be taken with cautions. For some specific cases, uncertainties on temperature and δ 18 Osw estimations can be reduced. For such cases, salinity reconstructions showing amplitude changes higher than 1 per mil can be considered as significative. 1 Introduction The role of salinity in oceanography is crucial: It plays an important part in the hydrography, i.e. intensities and directions of major current. As a main parameter in the oceanographic engine, it also contributes to heat transfert throughout ocean circulation, and therefore to regional and global climate changes. To reconstruct past oceanographic conditions, scientists need to gather a huge compilation of data set (oceanic temperatures and salinities) over a long period of time and covering major parts of worldwide oceans. Furthermore, quantitative reconstructions of oceanic parameters are needed as inputs for oceanographic or climatic models. Some sea sediments displays interesting geologic archives, as long as its sedimentation process has not been disturbed, i.e. without any time gap or physical disruption (such as turbiditic flows on continental shelf or bioturbation). Microfossils of past oceanic life can be found in these archives, and their assemblages oftenly give indications on the conditions in which they developed, as for example mean sea surface temperatures (hereafter SST). Only few geological archives are known to be directly related to sea surface salinities (hereafter SSS), and therefore, some indirect method, based on foraminifera microfossils, have been developed to reconstruct past SSS. As long as some foraminifera species have specific living requirements, studies have investigated past SSS reconstructions based on foraminiferal abundance data. The use of Artificial Neural Network, or Modern Analog Technique, have led to the conclusion that such estimations were unrealistic [1], and can’t be considered as quantitative. Therefore, scientists turned their attention to geochemical analysis of foraminifera species, which could lead to quantitative estimations of SSS. The aim of this paper is to present an overview of past sea surface salinities reconstructions through the chemistry of foraminifera microfossils, to estimate uncertainties of such paleorecords, and to discuss wether it can be used as quantitative parameters or not. Article published by EDP Sciences and available at http://www.epj-conferences.org or http://dx.doi.org/10.1140/epjconf/e2009-00919-6 178 The European Physical Journal Conferences P/E and discharge Ice Volume SSS SST δ18Oc δ18Osw Fig. 1. Main parameters influencing the isotopic composition of planktonic foraminifera shells (δ 18 Oc): Sea surface temperature (SST) and the isotopic composition of the surrounded waters (δ 18 Osw). This last parameter is also dependant on sea surface salinity (SSS), linked with the evaporation-precipitation balance (P/E) and with the volume of continental ice sheets (depleted in heavy isotopes). 2 Stable isotopes fractionation in foraminifera shells 2.1 Today Marine invertebrates such as foraminifera are building exoskeletons made of calcium carbonate to protect themselves from predators. Isotopic fractionations are taking place during this process, depending on both the proportion of stable isotopes available in the waters surrounding the foraminifera and the temperature of these waters (Figure 1). Many experimental and theoretical studies have been held since the middle of last century to understand the incorporation of oxygen isotopes in foraminifera shells. Epstein et al., in 1953, established a paleotemperature equation, linking the temperature with the isotopic composition of the calcite and of the surrounded waters [2]. Shackleton and Opdyke have adapted this equation in 1973 [2] into the following one: T = 16.9 − 4.38(δ 18 Oc − δ 18 Osw) + 0.13(δ 18 Oc − δ 18 Osw)2 (1) (δ 18 Oc is the isotopic value of the calcite, and δ 18 Osw the isotopic composition of the sea water). This relationship holds only if foraminifera deposit their shells in isotopic equilibrium with their growth medium. An overview of several oceanic sites revealed that only few species fits these requirements. Meanwhile, some quantitative temperature reconstructions seems to be possible. This equation underlines the double dependance of the δ 18 Oc of the foraminiferal shell with temperature changes together with the marine isotopic composition changes δ 18 Osw. We have one equation with two unknowned. To complicate the solving, δ 18 Osw is also dependent on salinity changes. A first overview of experimental measurements, made in 1965, following a series of observations made during oceanic cruises over different oceans (top cores), allows Craig and Gordon to established a first salinity–water isotope relationship (Figure 2) [4]: δ 18 Osw = 0.66 SSS − 23.5. (2) Since this pioneer work, many other observations have shown a wide range of slopes, depending on oceans, and on the nature of these waters (deep or shalow). These descrepancies will be discussed in a following section. ERCA 8 179 2.0 1.0 18 δ O(‰) 1.5 0.5 0.0 -0.5 Craig and Gordon, 1965 -1.0 GEOSECS (Östlund et al., 1987) Arabian Sea (Delaygue et al., 2001) -1.5 31 32 33 34 35 SSS ( ‰ ) 36 37 38 39 Fig. 2. Different δ 18 Osw-salinity relationships deduced from two different data sets of the worldwide oceans [4] from GEOSECS cruises [24], and from modeled output gathered with observed data from the Arabian Sea [36]. Data compiled by Gavin Schmidt [25], http://data.giss.nasa.gov/o18data/. To conclude, the δ 18 Oc of a monospecific species of foraminifera is directly dependent on the temperature, but also undirectly dependent on the salinity which follows the δ 18 Osw. Equations (1) and (2) were build using specific foraminifera species, within a certain range of temperature and salinity. They could be applied only for some specific situation. Within these requirements and according to equa (...truncated)