FLORIDA AND BERMUDA RECORDS OF SEA LEVEL DURING THE LATEST PART OF THE LAST INTERGLACIAL PERIOD AT ~80,000 YR BP

INTRODUCTION

Figure 1: Comparison of insolation values for June at latitude 60&#deg;N (data from Berger and Loutre, 1991) and sea level record from coral reefs on tectonically rising New Guinea (solid circles from Chappell and Shackleton, 1986; open circles from Bloom and Yonekura, 1985).

The last interglacial period in its broadest sense has been correlated with all of deep-sea oxygen isotope stage 5 (~130-80 ka). Three d18O minima during stage 5 are generally inferred as reflecting high sea stands at ~125 ka, ~105 ka and ~80 ka and are commonly correlated with emergent coral reefs of these ages found on tectonically rising coastlines. Though considerable attention has been given to the oldest, and apparently highest, sea-level stand (~125 ka), much less study has been made of the younger two. Assuming constant uplift rates, studies on tropical island coasts such as New Guinea calculated that sea level at ~80 ka could have been as high as -6.6 m (Bloom and Yonekura, 1985; 1990) or as low as -19 ± 5 m (Chappell and Shackleton, 1986) relative to present (Fig. 1). Based on data from tectonically rising Barbados, sea-level estimates at ~80 ka vary from -10 m to -18 m (Bender et al., 1979; Gallup et al., 1994). In the tectonically stable Bahamas, U-series dating of submerged speleothems suggest that at ~80 ka, sea level was below -15 m (Lundberg and Ford, 1994) or below -18 m (Richards et al., 1994). A recent study of deep-sea sediments, using Mg/Ca in ostracodes combined with oxygen isotopes in foraminifera, suggests that sea level at ~80 ka could even have been as low as -60 m to -70 m (Dwyer et al., 1995). However, recent data from the tectonically active Pacific coast of North America suggest that sea level at ~80 ka could have been closer to the present, perhaps around -1 m relative to present (Muhs et al., 1994). In addition, in tectonically stable Virginia and North Carolina, several corals from the Norfolk and Kempsville Formations (or their equivalents), at elevations of 4 to 10 m above sea level, give ages ranging from 66 ± 8 to 78 ± 10 ka (2 sigma) and average about 72 ka (Szabo, 1985). These formations may therefore correlate with the ~80 ka high stand of sea, and if so, imply sea level above the present at that time. Thus, there is not a consensus on the position of sea level at 80 ka, and the various coastal records imply vastly different ice volumes. Summer insolation values at high latitudes are considered to be critical for the growth and decay of ice sheets according to the orbital forcing theory of climate change (Milankovitch, 1941), and at ~84 ka, June insolation at 60&#deg;N was actually higher than at 11 ka (Berger and Loutre, 1991). The rise toward the 11 ka insolation high is considered to have been the trigger for the decay of the late Wisconsin ice sheets at the close of the last glaciation (Mix, 1987; Ruddiman, 1987). In order to investigate further the magnitude of sea level at ~80 ka, we studied two localities on the tectonically stable Florida Keys and Bermuda, where any deposits laid down at 80 ka, whether onshore or offshore, should be unaffected by uplift or subsidence.

STUDY AREAS

Figure 2: Location of submerged offshore reef south of Florida Keys (black), and sample locality. Reef location data generalized from Lidz et al. (1991).

High-resolution seismic-reflection profiles by Lidz et al. (1991) have demonstrated the presence of a submerged outlier-reef tract system extending discontinuously for ~57 km, seaward of the emergent, last-interglacial Florida Keys (Fig. 2). We determined U-series ages of samples of Montastrea annularis and Acropora palmata from two newly acquired cores, using thermal-ionization mass-spectrometric (TIMS) analyses.

Bermuda

Figure 3: Study localities on island of Bermuda, and distribution of outer reef and lagoon on Bermuda platform (from Vacher et al., 1989). FSC = Fort St. Catherine, SB = Shelly Bay.

The island of Bermuda (Fig. 3) has a core of pre-Quaternary volcanic rocks derived from the Mid-Atlantic Ridge, but its surficial deposits are Quaternary carbonates, mostly eolianite, but also consisting isolated sublittoral marine and beach deposits and some emergent, fossil patch reefs (Vacher et al., 1989). Most of the marine deposits have been mapped as belonging to the Belmont Formation (penultimate interglacial age) or the Devonshire marine member of the Rocky Bay Formation (last-interglacial age). Both formations have eolianite facies which overlie the marine facies. An eolianite unit that is slightly younger than the Rocky Bay Formation, called the Southhampton Formation, can be differentiated from the Rocky Bay Formation on the basis of stratigraphic relations (Vacher et al., 1989) and amino acid racemization data (Hearty et al., 1992). It has a marine facies that is 1-2 m above sea level. We examined a locality at Fort St. Catherine on Bermuda and collected fossil Oculina corals from the Southhampton Formation there for dating.

Results from the submerged reef off the Florida Keys confirms the earlier >38 ka radiocarbon result of Lidz et al. (1991) and indicates that the Montastrea corals in this reef grew during the ~80 ka high sea stand. A sample of this coral from a core depth of 1.2 m gives an age of 80.9 ± 1.7 ka, and a sample from a depth of 5.5 m in the same core gives a stratigraphically consistent age of 83.2 ± 0.9 ka (all errors at 2 sigma).

Our analyses of Oculina corals from the +1 m to +2 m marine deposit at Fort St. Catherine on Bermuda give U-series ages of 82.3 ± 3.6 ka, 82.1 ± 0.9 ka, 77.7 ± 0.4 ka, and 77.2 ± 2.5 ka (Fig. 4 & 5). These ages agree with the single analysis from this deposit reported by Harmon et al. (1983), and also show that the Bermuda corals grew during the same sea-level high stand at ~80 ka as those from the Florida Keys.

Figure 4: Stratigraphy of marine and eolian deposits of the Southampton Formation at Fort St. Catherine, Bermuda, and U-series ages of Oculina corals from this study.

Figure 5: Calculated initial 234U/238U (activity) for ~80 ka corals from Florida and Bermuda, plotted against their 230Th/U ages (shown by their 2s error ellipses). Diagonal-rule band shows initial 234U/238U of modern sea water, taken as mean and 95% confidence-limit for 27 Holocene corals analyzed in our laboratory. Curved line shows relative insolation and for 60-100 ka (Berger and Loutre, 1991).

IMPLICATIONS FOR SEA LEVEL AT ~80,000 YR BP

The new U-series data and geological setting of our samples allow estimates of the position of sea level at ~80 ka from tectonically stable platforms. Off the Florida Keys, the reported depth of the topographic crest of the main reef body (Lidz et al., 1991) is 10-12 m. Montastrea occupies a considerable depth range down to 80 m, but has optimum growth depths of 3-45 m (Shinn et al., 1989). If we assume conservatively that the Montastrea dated in the present study grew in water depths as shallow as 3 m, the 10-12 m depth of the submerged reef implies a sea level no lower than -7 to -9 m, relative to present, at ~80 ka. Sea level could have been much higher if the corals grew in water deeper than 3 m, which is likely.

The paleo-sea level significance of the marine deposits at Fort St. Catherine have been controversial. Harmon et al. (1983) interpreted the deposits to be storm-generated and concluded that they provided no constraints on sea level at 80 ka. However, Vacher and Hearty (1989) pointed out that storm waves (Fig. 1) would have had to traverse an outer reef, followed by a 15-20 km passage through an 11-15 m-deep lagoon on the Bermuda Platform before reaching the vicinity of Fort St. Catherine. In addition, Vacher and Hearty (1989) reported amino acid ratios for mollusks in marine facies of the Southampton Formation at two other localities on Bermuda that correlate these deposits with those at Fort St. Catherine. Finally, they pointed out that the marine deposits at Fort St. Catherine do not appear in any way different from those of older marine deposits such as the Rocky Bay Formation and the Belmont Formation. We conclude, as did Vacher and Hearty (1989), that the marine deposits at Fort St. Catherine and correlative deposits elsewhere on Bermuda imply a sea level close to, or above, the present at ~80 ka.

CONCLUSIONS

Our results have important implications for late Quaternary sea level history. An 80 ka sea level close to the present, derived from tectonically stable platforms, conflicts with previous estimates of -15 to -20 m, derived from tectonically rising New Guinea and Barbados. The difference in ice volume between previous sea level estimates and the present study is greater than the volume of the Greenland ice sheet (~7 m sea-level equivalent) or West Antarctic ice sheet (5-10 m sea-level equivalent). However, our hypothesis of a sea level close to the present at 80 ka agrees with records from the tectonically stable Atlantic Coastal Plain of the U.S. and the slowly rising Pacific coast of North America. Recent evidence from New Guinea (Ota et al., 1993; Pandolfi et al., 1994) suggests that emergent Holocene terraces there may be the result of coseismic uplift, which need not occur regularly through time, although Ota et al. (1993) thought that mean Holocene uplift rates and mean late Quaternary rates of uplift were similar. Nevertheless, platforms distant from plate boundaries, such as the Florida Keys and Bermuda, require fewer assumptions about tectonic history, and may be a more confident record of sea level history.

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