The geomorphology and morphostratigraphy of numerous worldwide sites reveal the relative movements of sea level during the peak of the Last Interglaciation (Marine Isotope Stage (MIS) 5e, assumed average duration between 13072 and 11972 ka). Because sea level was higher than present, deposits are emergent, exposed, and widespread on many stable coastlines. Correlation with MIS 5e is facilitated by similar morphostratigraphic relationships, a low degree of diagenesis, uranium-thorium (U/Th) ages, and a global set of amino-acid racemization (AAR) data. This study integrates information from a large number of sites from tectonically stable areas including Bermuda, Bahamas, and Western Australia, and some that have experienced minor uplift ($2.5 m/100 ka), including selected sites from the Mediterranean and Hawaii. Significant fluctuations during the highstand are evident at many MIS 5e sites, revealed from morphological, stratigraphic, and sedimentological evidence. Rounded and flat-topped curves derived only from reef tracts are incomplete and not representative of the entire interglacial story. Despite predictions of much different sea-level histories in Bermuda, the Bahamas, and Western Australia due to glacio-and hydro-isostatic effects, the rocks from these sites reveal a nearly identical record during the Last Interglaciation.The Last Interglacial highstand is characterized by several defined sea-level intervals (SLIs) that include: (SLI#1) post-glacial (MIS 6/5e Termination II) rise to above present before 130 ka; (SLI#2) stability at +2 to +3 m for the initial several thousand years ($130 to $125 ka) during which fringing reefs were established and terrace morphology was imprinted along the coastlines; (SLI#3) a brief fall to near or below present around 125 ka; (SLI#4) a secondary rise to and through $+3-4 m ($124 to $122 ka); followed by (SLI#5) a brief period of instability ($120 ka) characterized by a rapid rise to between +6 to +9 m during which multiple notches and benches were developed; and (SLI#6) an apparently rapid descent of sea level into MIS 5d after 119 ka. U/Th ages are used to confirm the Last Interglacial age of the deposits, but unfortunately, in only two cases was it possible to corroborate the highstand subdivisions using radiometric ages.Sea levels above or at present were relatively stable during much of early MIS 5e and the last 6-7 ka of MIS 1, encouraging a comparison between them. The geological evidence suggests that significant oceanographic and climatic changes occurred thereafter, midway through, and continuing through the end of MIS 5e. Fluctuating sea levels and a catastrophic termination of MIS 5e are linked to the instability of grounded and marine-based ice sheets, with the Greenland (GIS) and West Antarctic (WAIS) ice sheets being the most likely contributors. Late MIS 5e ice volume changes were accompanied by oceanographic reorganization and global ecological shifts, and provide one ominous scenario for a greenhouse world.
Measured profiles of the submerged portion of the steep cliffs that ring Harrington Sound Bermuda, show that they are undercut as much as 4 to 5 m by a notch whose flat rooi coincides closely with the level of extreme low tides. Nips, intertidal indentations with sloping roofs, are not present.Because of the largely subtidal position of the notch, the morphology of the undercut region, and the restricted wave energy, wave cutting is ruled out and hioerosion is proposed as the mechanism of undercutting,Biocrosion is suggested as a term for the removal of consolidated mineral or lithic substrate by the direct action of organisms.Observations of the emerged and submerged carbonate coastline of Bermuda reveal that organisms of many types alter the exposed rock surface from the upper reaches of the spray zone to depths well below sea level by processes that vary with degree of exposure, tidal range, rock type, organic community, and probably other ecological variables.A series of expcrimcnts with common rock-destroying organisms has been initiated. Experiments in the laboratory and under natural conditions show that Cliona Zumpa, a boring sponge, is capable of removing as much as 6 to 7 kg of material from 1 m2 of carbonate substrate in 100 clays. This corresponds to an erosion rate of calcarenite of more than 1 cm/year, If previous estimates are correct (that 90% of the boring by C. Zampa is mechanical and not chemical), then 5 to 6 kg of fine-grained carbonate detritus can be supplied from 1 m2 of sponge-infected substrate in I.00 days.The present investigation of rock destruction and coastal erosion by biological agencies is an outgrowth of earlier studies on the origin of fine-grained carbonate sediments in Harrington Sound, Bermuda (Neumann 1963(Neumann , 1966. As much as 30% of this sedimcnt is composed of calcite containing very little magnesium in solid solution. The only common local source of this mineral phase is the eolian limestone that is cxposcd at the short steep cliffs ringing the small enclosed lagoon. It is unlikely that the fine calcitic detritus is produced by mechanical grinding ' bccausc there is littlc wave energy, and there are hardly any beach sites for abrasion, Furthermore, sand would be the common product of the disaggrcgation of the calcareous eolianite and, except close to shore, this size class is not volumetrically important in the largely silt and clay-size sediments of the sound. A likely origin was found when it was realized that many rockboring organisms bore wholly or in part by mechanical means. Rock-browsing organisms also contribute fine detritus by rasping rock surfaces. Rock-boring and browsing organisms of several types arc abundant in the sound, and the observations indicate the significance of their activity.Rock-destroying organisms of many descriptions play an important role in the erosion of reefs and coastlines as well as being producers of both fine detritus and coarse rubble. As a general term, the destruction and removal of consolidated mincral or Ethi...
Landward-pointing V-shaped sand ridges several kilometers long are common along the windward margin of the Bahama Islands. Their axes share a northeast–southwest trend. Internally, the ridges contain low-angle oolitic beds with few erosional truncations. Commonly interbedded are tabular, fenestrae-rich beds such as those formed by the sheet flow of water over dry sand. Defined here as “chevron ridges,” these landforms appear to have originated in the rapid remobilization of bank margin ooid bodies by the action of long-period waves from a northeasterly source. Deposits along adjacent coastlines also preserve evidence of the impact of large waves. Reworked eolian sand bodies preserve beach fenestrae and hydraulic scour traces up to +40 m on older ridges. On cliffed coasts, 1000-ton boulders have been thrown well inland, recording the impact of large waves. Amino acid ratios confirm a correlation of the ridges across the archipelago, while stratigraphy, spacing, and cross-cutting relationships indicate emplacement as sea level fell rapidly from the substage 5e maximum at or above +6 m.
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