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Evidence for a Rapid Sea-Level Rise 7600 Yr Ago

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Evidence for a Rapid Sea-Level Rise 7600 Yr Ago Evidence for a rapid sea-level rise 7600 yr ago Shi-Yong Yu* Björn E. Berglund Per Sandgren Department of Geology/Quaternary Sciences, Lund University, Sölvegatan 12, SE–223 62 Lund, Sweden Kurt Lambeck Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ABSTRACT Dating the transgression and subsequent regression in marginal basins of the southeastern Swedish Baltic Sea provides a new perspective of global ice-volume changes and the isostatic adjustment of the mantle after the retreat of the Scandinavian Ice Sheet from this area. Super- imposed upon a smooth pattern of local sea-level rise, acceleration occurred ca. 7600 cali- brated (cal) yr B.P., evidenced as a nearly synchronous fl ooding in six elevated basins ranging from 3.0 to 7.2 m above present sea level. We ascribe this rapid local sea-level rise of ~4.5 m to a sudden increase in ocean mass, most likely caused by the fi nal decay of the Labrador sector of the Laurentide Ice Sheet. The subsequent monotonic fall of local sea level from ca. 6500 cal yr B.P. to the present is mainly an expression of the slow isostatic adjustment of the mantle. Keywords: Holocene, Baltic Sea, rapid sea-level rise, lake isolation, crustal rebound, Laurentide Ice Sheet decay. INTRODUCTION ties to mean sea level and the temporal variabil- level. Another signifi cant advantage is that these Dating the accretion of coral reefs reveals ity of marine radiocarbon reservoir age (Clark, sites are distributed more or less along the 10 m a smooth pattern of postglacial sea-level rise 1995). Sea-level reconstructions through dating isobase, a contour of postglacial land uplift (Fairbanks, 1989; Chappell and Polach, 1991; salt-marsh basal peat in open marine settings are (Fig. 1A, inset), thereby reducing the possible Bard et al., 1996) as major continental ice sheets subject to the uncertainties of tidal amplitude. infl uences of differential isostatic uplift. These in the Northern Hemisphere disappeared. How- Therefore, these data sets should be tested with preconditions ensure excellent constraints on ever, in the Caribbean-Atlantic region, a reef other independent evidence, ideally from areas past sea-level changes for the period after the back stepping ca. 7500 calibrated (cal) yr B.P. of tectonic stability and small tidal range. retreat of the Scandinavian Ice Sheet, when the has been interpreted as a rapid sea-level rise of Baltic was in open contact with the Atlantic. 6.5 m (Blanchon and Shaw, 1995). The validity STUDY AREA AND METHODS The sill of most basins, composed of thin of this event has been subject to debate, because In previously glaciated areas, local sea-level boulder-rich till or sandy sediments resting no rapid sea-level rise corresponding to this changes are primarily governed by the relative on Precambrian granite, may have been partly event has been observed from coral records in rates of crustal rebound and global sea-level eroded during the transgression or aggraded the neighboring areas (e.g., Bard et al., 1996; rise. We reconstruct Holocene sea-level history after the regression. We estimate that this altera- Montaggioni et al., 1997; Toscano and Lund- on the southeastern Swedish Baltic coast by tion may introduce a maximum error of ~40 cm, berg, 1998). Recent work on dating salt-marsh systematically dating the lacustrine to marine according to the thickness of the undisturbed till drowning on the eastern U.S. coast (Bratton and marine to lacustrine transitions in sediment in this area. Age control for the cores was pro- et al., 2003), together with data from north- sequences from eight closed basins with a sill vided by radiocarbon dating of terrigenous plant west Europe (Christensen, 1995, 1997; Lemke, of known elevation (Fig. 1). In addition to these macrofossils except for Inlängan, Siretorp, and 2004; Behre, 2007), has renewed interest in this sites, a beach ridge at Olsäng (Fig. 1A), situ- Hallarum, where bulk organic matter samples question. All of these records indicate the same ated 8.0 m above sea level (asl) and representing were dated. A regional radiocarbon reservoir amplitude of sea-level rise, and thus imply a the highest Holocene sea level in this area, was age anomaly of 108 ± 24 yr (Berglund, 1971) global expression of this event. If it is global, dated. A sample from peat immediately below was used to correct the bulk organic matter and because it occurred under climate bound- the beach ridge yields a radiocarbon date of samples for the marine stage of the Baltic Sea. ary conditions similar to the present, determin- 5690 ± 70 yr B.P., a maximum age for the beach All radiocarbon dates were calibrated using ing its magnitude and rate may provide a better ridge. Due to the narrow connection with the the CALIB 5.0 program (Reimer et al., 2004). understanding on how future sea level responds Atlantic, the Baltic Sea is a brackish-water body Identifi cations of sedimentary facies are based to global warming and the potential melting of with an extremely low amplitude tide of <5 cm on analyses of diatoms, dinofl agellate cysts, and polar ice sheets (Alley et al., 2005), particularly (Ekman and Stigebrandt, 1990). The meteoro- aquatic macrofossils. We use a fully coupled and the land-based Greenland Ice Sheet. logical tide caused by storm surges can result in gravitationally consistent ice-water-earth model The accuracy of coral records depends on the a perturbation of ±25 cm in the sea level (Stige- to calculate the isostatic uplift and its contribu- knowledge of growth position of coral communi- brandt and Gustafsson, 2003), and this transient tion to sea level at each site using the model sea-level change can be refl ected in sedimentary parameters described in Lambeck and Purcell *Current address: Large Lakes Observatory, Uni- records of the basin, when the mean sea level is (2003). The time-dependent eustatic sea-level versity of Minnesota–Duluth, Duluth, Minnesota close to the sill. However, such an event has no function used here was described in Lambeck 55812, USA; E-mail: [email protected]. signifi cance for the reconstructions of mean sea and Chappell (2001). © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, October October 2007; 2007 v. 35; no. 10; p. 891–894; doi: 10.1130/G23859A.1; 3 fi gures. 891 10°E 15°E Ryssjön Olsäng published elsewhere, e.g., Hallarums mosse SWEDEN A 40 m Smygen SWEDEN Färsksjön Hallarums mosse and Inlängans mosse (Berglund, 1964), Spjälkö 58°N Spjälkö 20 m Hunnemara (Liljegren, 1982), Ryssjön (Yu et al., 2003), 10 m 56°N BALTIC Färsksjön and Siretorp (Yu et al., 2004), and 0 m ° ′ SEA Kalvöviken Bay 56 06 N Smygen and Hunnemara (Yu et al., 2005). We obtained 65 radiocarbon dates (Fig. 1B) that provide a fi rm age control on the timing Siretorp Inlängans mosse BALTIC SEA 56°N of the sedimentary facies, particularly the OB facies. Note that the levels exactly represent- 14°30′E 15°E15°30′E16°E ing the lacustrine-marine and marine-lacustrine transitions at most sites were well dated, except B 0.5 Smygen Ryssjön Spjälkö Färsksjön –1.0 m Facies 4.5 m Facies6.0 m Facies 7.2 m Facies for Smygen (Fig. 1B). For this site we have 1.5 established an age-depth model to refi ne the 4110±65 2.5 2820±110 2100±100 onset of transgression (Yu et al., 2005). 5260±65 L L L 3.5 3200±90 5520±70 6080±70 4830±100 2920±95 L L L 4610±105 6250±75 5250±90 ± Local Holocene Mean Sea-Level Record 4.5 5575±105 L L L 6480 75 5705±80 6670±75 ± ± 6150 95 Measuring the elevation of the sills and dat- 3090±105 6850 80 6695±105 6705±95 5.5 L L L OF SF SB OB 3950±85 L L L 7245±90 ing transgressions into and regressions out of LL L L L 6.5 L L L 4555±105 L L 9005±95 these basins (i.e., the timing of the OB sedi- L L L LL L OF SF SB OB 7.5 9565±120 mentary facies) enabled us to reconstruct local 5645±85 OF SF SB OB mean sea-level changes when the Baltic Sea 8.5 6020±95 was widely connected to the Atlantic (Fig. 2). Depth below water level (m) 9.5 The SB facies occur very briefl y and are miss- ± L L L 7310 100 10.5 L L ing in the stratigraphic context of some basins. L L L ± L L L 8475 95 Where present, these facies indicate an inter- 11.5 L L L L L LL L L L 9650±95 mittent connection of the basin to the Baltic 12.5 Sea due to extreme meteorological tides, and OF SF SB OB Hunnemara Inlängans mosse Siretorp 3820±65 Hallarums mosse thus have not been considered in our recon- ± 3.0 m Facies 4.0 m Facies 5.0 m Facies 3950 60 5.2 m Facies 0.0 4270 ±60 struction of mean sea level. 4470±80 5040±65 ± Our data show that local sea level rose L L L L L 3145 65 LL L 5290±65 2935±80 LL L ± 1.0 3545±65 LL L 5430 65 steadily from 8600 to 6500 cal yr B.P. (Fig. 2), 4170±75 L L L 5700±70 5930±70 OF SF SB OB ± coherent with the ice-volume–equivalent global 6090 70 4585±70 2.0 6220±70 OF SF SB OB ± 6640±70 L L L 5040 85 sea-level rise (Fairbanks, 1989; Chappell and L L 5665±85 L L L 5945±75 L L ± Polach, 1991; Bard et al., 1996).
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