Elsevier

Chemical Geology

Volume 232, Issues 1–2, 30 August 2006, Pages 54-66
Chemical Geology

Strontium isotopic composition of modern and Holocene mollusc shells as a palaeosalinity indicator for the Baltic Sea

https://doi.org/10.1016/j.chemgeo.2006.02.010Get rights and content

Abstract

The 87Sr/86Sr isotope ratio in subfossil carbonate mollusc shells from raised-beach sediments is used as a palaeosalinity indicator for the Baltic Sea. The main purpose is to demonstrate the applicability of the method in a formerly glaciated area, using 87Sr/86Sr ratios found in modern shells (Mytilus edulis, Macoma baltica, and Radix balthica) to elucidate the parameters and conditions to be used for palaeosalinity determinations based on subfossil shells. Proxy salinity data are calculated assuming that the Sr concentration in Baltic Sea water is controlled by a two-component, conservative mixing between seawater and river water. Based on replicate determinations of the 87Sr/86Sr ratio in modern shells, proxy salinity data can be quantified with a relative precision of better than ± 5% for salinities up to ∼ 10‰. Comparison with measured, present-day salinities indicates that the accuracy is generally better than ± 5% for the proxy data. With this precision and accuracy, the present-day salinity differences between the major Baltic sub-basins (Bothnian Bay, Bothnian Sea, and Baltic Proper) can be detected.

Palaeosalinities were determined using subfossil shells (M. edulis and M. baltica) with calibrated 14C ages of 6770–3070 cal BP. The shell deposits occur from 65 m a.s.l. down to the present-day sea level. The palaeosalinities determined for the Bothnian Bay (4.8–10.3‰) and the Bothnian Sea (7.3–10.3‰) are in good agreement with earlier estimates of the maximum Littorina Sea stage palaeosalinity in these sub-basins (8–10‰). At one locality, two different shell species from the same shell deposit were dated (M. edulis and M. baltica). The shells differed in age by 460 ± 70 14C years, a possible indication that the ecological conditions at a locality may have been favourable to different faunas at different times.

Introduction

The glacio-eustatic sea-level rise of approximately 130 m that occurred from ∼ 18 000 to 6000 BP was one of the most profound environmental changes of the Late Quaternary. In formerly glaciated areas that have experienced regional postglacial isostatic rebound, the resulting transgressions/regressions can be expected to have influenced the palaeoenvironment of coastal bays and epicontinental seas (cf. Fredén, 1988). This is clearly demonstrated by the dynamic Holocene history of the Baltic Sea, which resulted in regional changes in salinity and redox conditions (e.g., Sohlenius et al., 1996, Sohlenius et al., 2001) as well as in palaeoproductivity and palaeoecological conditions (e.g., Westman and Sohlenius, 1999). The absolute salinity changes experienced during the Holocene development of the Baltic Sea have been discussed extensively since the late nineteenth century (summarised by Westman et al., 1999). Earlier studies of the palaeosalinity of the Baltic Sea have been based on fossil assemblages (Munthe, 1894, Witkowski, 1994); on δ18O in mollusc shells (Punning et al., 1988), foraminifera (Winn et al., 1986) and rhodochrosite (Huckriede et al., 1996); and on the occurrence of cyanobacterial blooms (Bianchi et al., 1998). However, the palaeosalinity of the Baltic Sea is still incompletely understood, and estimates of the maximum surface salinity of the Baltic Proper over the last 8500 years range from 10‰ to 20‰, compared to the present-day salinity of 6–8‰.

Precise knowledge of the palaeosalinity of the Baltic Sea is required when reconstructing the environmental history of the Holocene Baltic (e.g., Winterhalter, 1992, Sohlenius et al., 1996, Westman and Sohlenius, 1999, Sternbeck et al., 2000, Sohlenius et al., 2001). Combined with the sedimentary record of palaeoproductivity and palaeoredox conditions (Westman and Sohlenius, 1999, Sohlenius et al., 2001), palaeosalinity data could also be used to predict the future, long-term effects of an expected climate-induced reduction of the salinity of the Baltic Sea (cf. Stigebrandt and Gustafsson, 2003).

It is well known that the Sr isotopic composition of fossil carbonate mollusc shells can be used as a proxy indicator of palaeosalinity (e.g., Ingram and Sloan, 1992, Ingram and DePaolo, 1993, Klingberg and Andersson, 1997, Israelson and Buchardt, 1999). The determination of proxy salinity data is based on the fact that the 87Sr/86Sr ratio in biogenic carbonate closely reflects that of dissolved Sr in the ambient water (Veizer, 1989). Due to the generally conservative nature of Sr in estuarine environments, proxy salinity data can then be obtained from the relationship between the 87Sr/86Sr ratio and the Sr concentration in the water (Ingram and Sloan, 1992, Faure and Mensing, 2005).

This study for the first time uses the Sr isotopic composition of 14C-dated carbonate mollusc shells as a palaeosalinity indicator for the Baltic Sea. Our aim is primarily to demonstrate the applicability of the method in a formerly glaciated area, using 87Sr/86Sr ratios in modern shells to elucidate the parameters and conditions to be used for palaeosalinity determinations based on subfossil shells.

Section snippets

Study area

The brackish Baltic Sea is a shallow epicontinental sea with a surface area of 370 000 km2 and a volume of 21 000 km3 (Fig. 1). Within the Baltic Sea, three major sub-basins–the Baltic Proper, Bothnian Sea, and Bothnian Bay–are separated by sills and shallow-water areas. Water exchange with the Atlantic Ocean occurs through the Belt Sea and Öresund (with approximate sill depths of 18 m and 8 m, respectively), via a transitional area in the Skagerrak-Kattegat (Ehlin, 1981).

The outflow of low-saline

Mixing model for Sr in the Baltic Sea and palaeosalinity calculation

In this study, the Sr isotopic composition is expressed as 87Sr/86Sr ratios or using the ε notation, i.e., as fractional deviations in parts in 104 from that of modern seawater, as follows:εSr=[( Sr87/ Sr sample86 Sr87/Srseawater86)1]×104

The general equations for a mixture of two components having different Sr concentrations and 87Sr/86Sr ratios can be found in Faure and Mensing (2005, Chap. 16). In Baltic Sea water (BW) the concentration of Sr (CSrBW) is controlled by a two-component,

Strontium isotope data and present-day salinity

To validate the proxy salinity data obtained from mollusc shells, salinities calculated using Sr isotope data for modern shells from seven localities were compared with measured, present-day salinities (Fig. 1, Table 2, Table 3). In most cases, M. edulis was used for the salinity determination. Today, M. edulis colonises hard bottoms to water depths of 35–40 m (Kautsky, 1981), so this can be considered as the maximum depth to which modern shells of this species reflect the salinity. The salinity

Conclusions

This study shows that reliable palaeosalinity data for the Baltic Sea can be obtained from the Sr isotopic composition of Holocene carbonate mollusc shells occurring in raised-beach sediments. However, this presupposes that the post-depositional alteration of shells is of little or no significance. Based on replicate determinations of the 87Sr/86Sr ratio in modern shells, proxy salinity data can be quantified with a relative precision of better than ± 5% for salinities up to ∼ 10‰. The comparison

Acknowledgements

This study was funded by grants from Magnus Bergvall's Foundation. We greatly appreciate the assistance of Marina Fischerström and Hans Schöberg with sample preparation and TIMS instrument operation. We are grateful for the comments and assistance with mollusc species identification provided by Tommy Sörlin. We also thank Analytica AB for their support with chemical analyses and Milan Vnuk for skilfully drafting the figures. Salinity data for the Baltic Sea (SHARK database) were kindly provided

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