Chertsey #112, P.O. Box N-337, Nassau, Bahamas. We reply to Kindler and Strasser's comments as follows: Beach Deposits. It is true that the sub- and intertidal ...
Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology: Comment and Reply COMMENT
Cay notch (Neumann and Moore, 1975) by a later (possibly Sangamonian) sea-level event provides support for this assumption. Eolianite Structures. According to Neumann and Hearty (1996), vertiPascal Kindler cally buried tree trunks and palm fronds in eolianites indicate strong winds Department of Geology and Paleontology, University of Geneva, and rapid sedimentation of mobile oolitic sands at the close of substage 5e. Maraîchers 13, 1211 Geneva 4, Switzerland However, reexamination of the Two Pines site on Eleuthera showed that the André Strasser “trunk” represented in their Figure 6 consists of bifurcating rhizoliths that Institute of Geology, University of Fribourg, Pérolles, 1700 Fribourg, originate at about 1.5 m above the paleosol separating the lower and upper 5e Switzerland eolianites, barely penetrate it, but do not spread out horizontally or affect the Neumann and Hearty (1996) proposed a new interpretation of sea-level underlying dune. In addition, fine-scale eolian structures can be followed changes during the last interglacial period (isotopic substage 5e) based on the across the root complex, clearly indicating that the latter postdates dune emstudy of coastal notches, fossil coral reefs, and eolianites from various Ba- placement. We did not observe the buried palm fronds shown in Figure 7, but hamian islands. According to these authors, sea level stayed near +2 m from we wonder how palm fronds can be fossilized in a vertical position below a 132 to 118 ka, except for a brief negative excursion at around 125 ka, and massive sediment load, even if they are rooted. Contrary to Neumann and experienced a rapid rise, short crest at +6 m, and rapid fall at the close of 5e. Hearty’s (1996) claim, the upper 5e eolianite at Two Pines and at other numerThis interpretation revives Wilson’s (1964) ice surge theory, which suggests ous locations in the Bahamas (e.g., Collins Ave, New Providence) includes that the end of interglacial periods is marked by catastrophic sea-level and cli- many rhizocretions, protosols, and reactivation surfaces that suggest multiple mate changes linked to Antarctic ice-sheet instability. Neumann and Hearty’s sedimentation phases rather than one unique and catastrophic event. Conclusions. The geologic evidence presented by Neumann and Hearty (1996) report is extremely attractive, but partly relies on questionable and incomplete data. We would like to comment on the record from beach deposits, (1996) does not support the occurrence of rapid sea-level changes and stormy climates at the close of the last interglacial. The Sangamonian age of the the age of coastal notches, and the internal structures of eolianites. Beach Deposits. Neumann and Hearty (1996) infer from the +2 m observed notches is questionable, and the internal structure of the upper 5e elevation of many substage 5e reefs throughout the Bahamas that sea level eolianites suggests multiple phases of sedimentation rather than one catastayed at or near this datum during most of the last interglacial. However, sea strophic event during this time period. In addition, the sea-level record from level could have been higher because reefs do not necessarily grow up to sea coastal deposits, in particular that from beach facies, has been overlooked by level. In addition, the record from perched beach deposits of 5e age clearly these authors. Nonetheless, we agree with Neumann and Hearty (1996) that shows that sea level went over the +2 m datum at several occasions during sea-level fluctuations during isotopic substage 5e display a complex pattern, this time interval. Following are a few published cases. At Clifton Pier, New and that reconstructing sea-level changes during the last interglacial is of Providence Island, a well-studied succession (Ball, 1967; Garrett and Gould, prime importance to better understand the mechanisms of climate change. 1984) dated at 146 ± 9 ka by Neumann and Moore (1975) includes beach facies at +6 m. At Lyford Cay, New Providence Island, two shoaling-upward REFERENCES CITED Ball, M. M., 1967, Carbonate sand bodies of Florida and the Bahamas: Journal of sequences (Garrett and Gould, 1984) dated at 128 ± 4 and 117 ± 3 ka by Sedimentary Petrology, v. 37, p. 556–591. Muhs et al. (1990) show a transition from beach to eolian facies at 8.7 and Chen, J. H., Curran, H. A., White, B., and Wasserburg, G. J., 1991, Precise chronology of the last interglacial period: 234U-230Th data from fossil coral reefs in the 10.0 m, respectively. At Cockburn Town, San Salvador Island, a ca. 123 ka Bahamas: Geological Society of America Bulletin, v. 103, p. 82–97. coral is entombed by coastal deposits showing beach facies at +4 m (Chen Garrett, P., and Gould, S. J., 1984, Geology of New Providence Island, Bahamas: et al., 1991). Clearly, sea-level history during isotopic substage 5e is complex Geological Society of America Bulletin, v. 95, p. 209–220. and cannot be correctly deciphered without using all available data. Hearty, P. J., and Kindler, P., 1995, Sea-level highstand chronology from stable carNotches. Neumann and Hearty’s (1996) hypothesis of a rapid rise and bonate platforms (Bermuda and the Bahamas): Journal of Coastal Research, v. 11, p. 675–689. fall of sea level at the end of the last interglacial relies on the observation of notches of 5e age cut into middle Pleistocene material and preserved at or Kindler, P., and Hearty, P. J., 1995, Pre-Sangamonian eolianites in the Bahamas? New evidence from Eleuthera Island: Marine Geology, v. 127, p. 73–86. near +6 m at three locations in the Bahamas. We question the Sangamonian Kindler, P., and Hearty, P. J., 1996, Carbonate petrography as an indicator of climate age of these notches. The age of coastal notches cannot be directly measand sea-level changes: New data from Bahamian Quaternary units: Sedimentology, v. 43, p. 381–399. ured, but only bracketed from the age of the older host rock and that of younger infilling sediments. In a highly schematic geological cross section Kindler, P., and Hearty, P. J., 1997, Geology of the Bahamas: Architecture of Bahamian Islands, in Vacher, H. L., and Quinn, T. M., eds., Geology and hydrogeology of of Eleuthera Island, Neumann and Hearty (1996, Fig. 2) depicted a notch carbonate islands: Developments in Sedimentology, v. 54, p. 141–160. cut into stage 9 rocks at +6 m, but did not provide any geochemical or petro- Neumann, A. C., and Hearty, P. J., 1996, Rapid sea-level changes at the close of the logical data to support this age. High-resolution stratigraphic studies in last interglacial (substage 5e) recorded in Bahamian island geology: Geology, v. 24, p. 775–778. northern and central Eleuthera (Kindler and Hearty, 1995; Kindler and Hearty, 1997) now extend the Bahamian Pleistocene record as far back as Neumann, A. C., and Moore, W. S., 1975, Sea level events and Pleistocene coral ages in the Northern Bahamas: Quaternary Research, v. 5, p. 215–224. isotopic stage 13 and provide geological evidence of several higher-than- Muhs, D. R., Bush, C. A., Stewart, K. C., Rowland, T. R., and Crittenden, R. C., 1990, present sea-level events associated with stages 9 and 11 (Hearty and Geochemical evidence of Saharan dust parent material for soils developed on Quaternary limestones of Caribbean and western Atlantic islands: Quaternary Kindler, 1995; Kindler and Hearty, 1996). Refined dating of host rock is Research, v. 33, p. 157–177. thus necessary before correlating the studied notches with isotopic substage Wilson, A. T., 1964, Origin of ice ages: An ice shelf theory for Pleistocene glaciation: 5e. These erosional features could also correspond to the carving of stage 13 Nature, v. 201, p. 147–149. rocks during a stage 9 or 11 highstand. Evidence of cliffing of the Little Sale GEOLOGY, December 1997
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REPLY A. Conrad Neumann Curriculum in Marine Sciences, University of North Carolina, Chapel Hill, North Carolina 27599-3300 Paul J. Hearty Chertsey #112, P.O. Box N-337, Nassau, Bahamas
press), it is clear that the limited number of stage 13/11 outcrops that could be notched by stage 11/9 sea levels would vastly reduce the probability of occurrence and/or preservation potential of middle Pleistocene notches. Eolianites. Kindler and Strasser propose that the “tree trunk” in our Figure 6 is a root mass and postdates the enclosing eolianite. Bahamian trees grow in sandy sediments and typically maintain a broad surface network of roots in order to increase the stability of the plant in the loose substrate. Rarely do they have deeply penetrating tap root systems in this environment. The tree cast in our Figure 6 clearly shows a narrower top, or trunk, thickening downward toward the contact with the protosol. In the protosol are seen the rhizomorphs of the tree above. The entire tree was cast with fine eolian sand. The rhizoliths to which Kindler and Strasser refer in the vertical cast of the tree were initially missed by us and were probably infiltrated later from plants growing on the surface of the enclosing dune. Rootlets would be expected to grow down the natural conduit created by the tree cast. Our later observation indeed revealed even roots of modern plants within the same vertical structure of the tree cast. Fine eolian structures extending around (or across in two-dimensional outcrop) the stump would be expected as sand accumulates and buries the existing vegetation. Delicate fronds entombed on a steep angle relative to the bedding must be evidence of rapid burial while attached to the tree. A detached frond would lie on the bedding plane. From this and similar evidence, we conclude that the vertical casts are indeed those of tree trunks, and that the palmetto fronds were buried alive. Kindler and Strasser have exaggerated our position on the speed of eolian mobilization at the termination of substage 5e. The massive eolian buildup may encompass hundreds of years between the +6 m peak and mobilization accompanying the fall of sea level below the shelf margin. This fall must have been fast when it did occur or the peritidal lagoon sands would be expected to undergo cementation, rather than mobilization. We would expect some eolian reactivation surfaces during storms and some plant growth to occur during calmer intervals. Conclusions. Kindler and Strasser attack several minor points related to interpretation of isolated features at a few sites, but in no way do they impugn the extent and convergence of observations provided in Neumann and Hearty (1996). Despite a considerable number of reef outcrops in the Bahamas, none of the in situ coral heads rise above +2.25 m. Had sea level remained at the +6 m level for any duration during the 14 ka substage 5e cycle, at least some of the corals, particularly the fast-growing Acropora palmata, would have grown to levels higher than +2 m. Records from reefs, notching, and sedimentary environments indicate sea level rose quickly to a higher level late in the substage. Although some of the individual site-specific features observed in eolianites are subject to interpretation, the fact remains that large, well-bedded dune complexes revealing relatively scant rhizomorph development characterize the stratigraphic termination of 5e in the Bahamas. Obvious tree trunks and upright fronds further attest to rapid burial of a vegetated landscape by remobilization of large volumes of sand. Taken together, the evidence from stranded reefs, perched beaches, suspended notches, fossil and buried sea cliffs, and rapidly thrown up eolianites presents a persistent picture of catastrophic climate change and complex sea-level events at the close of the last interglacial. We seem to agree on the complexity of the events within 5e. The exact sequence and dimension of events may change as more work is done. Because Kindler and Strasser provide no alternative scenario, we find no cogent reason to alter our view of the 5e events as originally presented.
We reply to Kindler and Strasser’s comments as follows: Beach Deposits. It is true that the sub- and intertidal deposits at Clifton Pier record a sea level higher than +2 m; however, they occupy a terminal stratigraphic position and thus probably represent the rise to, and fall from, the +6 m terminal 5e sea level described in Neumann and Hearty (1996). It is not certain whether the coral from which the 146 ka U-series date (Neumann and Moore, 1975) was obtained was in situ or was rubble oriented in living position (due to its hemispheric shape) within the beach deposits. The provenance of this sample was questioned in their Table 2. Regardless, the coral provides a maximum age of the deposits, and if in situ, indicates submergence at +2 m. Kindler and Strasser propose that sea level “could have been higher on several occasions,” yet provide no unambiguous data (elevation of subtidal to intertidal facies transition) that are contrary to, or refute, our description of sea level during substage 5e. Garrett and Gould (1984) factored a subsidence rate of 3 m/125 k.y. into their field measurements of substage 5e sea level on New Providence Island, and thus the actual elevation of the last interglacial shorelines is 3 m lower than indicated in their publication. Kindler and Strasser argue that the transition from beach to dune at 8.7 and 10.0 m (actually at 5.7 and 7 m) at Lyford Cay during two successive 5e highstands is an indication of much higher sea levels during substage 5e than proposed in Neumann and Hearty (1996) while, in fact, these levels only record the upper limit of the swash zone. Fundamentally, Garrett and Gould’s (1984) assumption of 3 m subsidence over the past 125 k.y. is unprovable (Hearty and Neumann, 1997) and thus improperly incorporated into both their paper and this discussion. Thus, we do not consider that these findings invalidate our hypothesis in any way. It is recognized that ancient high energy beach environments such as that at Lyford Cay may have had an elevated swash zone of a few meters or more, depending upon exposure and climate extremes. Emplacement of beach deposits can be rapid and ephemeral as compared to reefs, which respond more slowly and better represent the broad interglacial cycle. In reference to Figure 12 (Chen et al., 1991) vs. Figure 4 (Neumann and Hearty, 1996), we point out that at both Cockburn Town, San Salvador, and Devil’s Point, Inagua, all in situ corals (including the best sea-level indicator, Acropora palmata) were sampled at elevations lower than +2.25 m. All dated corals higher than +2.25 m are from rubble deposits that do not pertain to actual sea level. The youngest corals (ca. 120 ka) are the highest and may indicate the initial, but unsuccessful attempt of the reef to grow upward during the brief +6 m level. Notches. Our interpretation of the +6 m notches of 5e age depends on (1) the observed morphology being identical to that of actively forming bioerosional notches, (2) the middle Pleistocene age of the preexisting host rocks, and (3) the occurrence of a higher-elevation sea level that cliffed older, more interior 5e eolianites at and above +6 m. One excellent example of (3) is at a small quarry near St. Augustine’s Monastery in eastern New Providence Island (Hearty and Kindler, 1997). This late 5e cliffing of interior 5e eolianites indicates a parallel sedimentary record of sea level rising to a +6 m level late in the period, after the bulk of 5e beach/dune ridges had been em- REFERENCES CITED Chen, J. H., Curran, H. A., White, B., and Wasserburg, G. J., 1991, Precise chronolplaced. The Geology cover photo (1996, v. 24) and our Figure 3 show the ogy of the last interglacial period: 234U-230Th data from fossil coral reefs in the primary, unaltered, original bioerosional morphology of the 5e notch. WeathBahamas: Geological Society of America Bulletin, v. 103, p. 82–97. ering, corrosion, and dissolution would be expected to reduce or destroy Garrett, P., and Gould, S. J., 1984, Geology of New Providence Island, Bahamas: Geological Society of America Bulletin, v. 95, p. 209–220. most older middle Pleistocene notches. Furthermore, although recent studies Hearty, P. J., in press, The geology of Eleuthera Island, Bahamas: A rosetta stone of have determined that the middle Pleistocene record is more widespread in Quaternary stratigraphy and sea-level history: Quaternary Science Reviews. the Bahamas than previously thought (Kindler and Hearty, 1996; Hearty, in Hearty, P. J., and Kindler, P., 1997, The stratigraphy and surficial geology of New 1148
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Providence and surrounding islands, Bahamas: Journal of Coastal Research, v. 13, p. 798–812. Hearty, P. J., and Neumann, A. C., 1997, Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology: Reply: Geology, v. 25, p. 574–575. Kindler, P., and Hearty, P. J., 1996, Carbonate petrology as an indicator of climate and sea level changes: New data from Bahamian Quaternary units: Sedimentology, v. 43, p. 381–399.
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Neumann, A. C., and Hearty, P. J., 1996, Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian Island geology: Geology, v. 24, p. 775–778. Neumann, A. C., and Moore, W. S., 1975, Sea level events and Pleistocene coral ages in the northern Bahamas: Quaternary Research, v. 5, p. 215–224.
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