Paul J. Hearty

Rapid sea-level changes at the close of the last interglacial (substage 5e) recorded in Bahamian island geology

Rapid and abrupt relative sea-level changes within the last interglacial (substage 5e) are recorded in the island geology of the tectonically stable Bahamas. From 132 to 118 ka, reef growth reached a maximum elevation near +2 m, as indicated by fossil reef elevation across the platform, whereas bioeroded notches are incised in coastal cliffs as high as +6 m. The end of the interval is characterized by voluminous eolianites exhibiting palm tree and frond impressions. It is inferred that sea level for most of the interval remained near +2 m, restraining reef growth, and that the notch at +6 m represents a rapid and brief excursion just before the close of the substage. The subsequent fall must have been rapid in order to leave the notch profile intact and mobilize windward lagoon ooids into dunes before cementation could anchor them. In order to explain the rapid rise to +6 m, glacial surging is invoked. The subsequent fall, also rapid, may be a consequence of the surge flooding high latitudes and providing enough moisture to initiate reglaciation and drawdown.

Amino acid geochronology of individual foraminifer (Pulleniatina obliquiloculata) tests, north Queensland margin, Australia: A new approach to correlating and dating Quaternary tropical marine sediment cores

[1] In this study, we demonstrate the utility of amino acid geochronology based on single-foraminiferal tests in Quaternary sediment cores from the Queensland margin, Australia. The large planktonic foraminifer Pulleniatina obliquiloculata is ubiquitous in shelf, slope, and basin sediments of north Queensland as well as pantropical oceans. Fossil tests are resistant to dissolution, and retain substantial concentrations of amino acids (2–4 nmol mg � 1 of shell) over hundreds of thousands of years. Amino acid D and L isomers of aspartic acid (Asp) and glutamic acid (Glu) were separated using reverse phase chromatography, which is sensitive enough to analyze individual foraminifera tests. In all, 462 Pulleniatina tests from 80 horizons in 11 cores exhibit a systematic increase in D/L ratios down core. D/L ratios were determined in 32 samples whose ages are known from AMS 14 C analyses. In all cases, the Asp and Glu D/L ratios are concordant with 14 C age. D/L ratios of equal-age samples are slightly lower for cores taken from deeper water sites, reflecting the sensitivity of the rate of racemization to bottom water temperature. Beyond the range of 14 C dating, previously identified marine oxygen-isotope stage boundaries provide approximate ages of the sediments up to about 500,000 years. For this longer time frame, D/L ratios also vary systematically with isotope-correlated ages. The rate of racemization for Glu and Asp was modeled using power functions. These equations can be used to estimate ages of samples from the Queensland margin extending back at least 500,000 years. This analytical approach provides new opportunities for geochronological control necessary to understand fundamental sedimentary processes affecting a wide range of marine environments. INDEX TERMS: 4267 Oceanography: General: Paleoceanography; 4850 Oceanography: Biological and Chemical: Organic marine chemistry; 3022 Marine Geology and Geophysics: Marine sediments— processes and transport; KEYWORDS: Queensland margin, marine sediment, amino acid racemization, geochronology, foraminifera, Quaternary stratigraphy Citation: Hearty, P. J., M. J. O’Leary, D. S. Kaufman, M. C. Page, and J. Bright (2004), Amino acid geochronology of individual foraminifer (Pulleniatina obliquiloculata) tests, north Queensland margin, Australia: A new approach to correlating and dating Quaternary tropical marine sediment cores, Paleoceanography, 19, PA4022, doi:10.1029/2004PA001059.

THE GEOLOGY OF ELEUTHERA ISLAND, BAHAMAS: A ROSETTA STONE OF QUATERNARY STRATIGRAPHY AND SEA-LEVEL HISTORY

Abstract A 5-km stretch of coastline in north Eleuthera reveals a long and detailed stratigraphy that includes all known surficial limestone units in the Bahamas, and supplements the record with several previously unrecognized ones. Eight paleosol-bounded limestone parasequences comprise at least six interglacial periods. The lithostratigraphy demonstrates cyclicity at several frequencies (10 5 , 10 4 (20–40xa0ka), and 10 3 xa0years) and displays a variety of distal to proximal shoreline facies indicative of shifting depocenters associated with changing sea-levels. Stratigraphy, petrology, pedology and whole-rock aminostratigraphy are used to correlate units and subunits among the 12 described sections. Amino acid ratios are also converted to absolute age estimates which support the lithostratigraphy. The parasequences are correlated with marine Oxygen Isotope Stages 1 to 13 or older. Evidence of middle Pleistocene highstands are abundant in the Eleutheran stratigraphy, including paleo-sea-levels of decreasing age at +2xa0m, +7xa0m, +20xa0m associated with Stages 11 and/or 9, and two near-present highstands during Stage 7. A complex sea-level history is associated with Substage 5e, while Substage 5a is represented by near shore aggradation of coastal dune complexes in Eleuthera and throughout the Bahamas. Concordance of sea-level deposits between Bermuda and the Bahamas reinforce their tectonic stability, while the abundance of highstand evidence during the middle Pleistocene contradicts suggestions that platform subsidence has obscured all evidence of these events below present sea-level. The high-resolution late Quaternary stratigraphy of Eleuthera is unrivaled among geologic records from stable carbonate coastlines, and thereby offers a ‘Rosetta Stone’ for interpretation of the Quaternary evolution of the Bahamas and sea-level history over the past 500xa0ka.

Probable extirpation of a breeding colony of Short-tailed Albatross (Phoebastria albatrus) on Bermuda by Pleistocene sea-level rise

Albatrosses (Diomedeidae) do not occur in the North Atlantic Ocean today except as vagrants, although five species were present in the early Pliocene. No fossil breeding sites of albatrosses were known previously. The timing of extinction of albatrosses in the North Atlantic was likewise unknown. Deposits that formed near present-day sea level along the southeastern shore of Bermuda contain remains of a former breeding colony and include intact eggshells and bones of embryos, juveniles, and adults of Short-tailed Albatross (Phoebastria albatrus), a critically endangered species now confined to a few islets in the northwestern Pacific Ocean. These deposits are correlated with the middle Pleistocene Lower Town Hill Formation, which at other sites have a radiometric age of ≈405,000 years ago. This equates with the marine isotope stage 11 interglacial, which culminated in a rise in sea-level to >+20 m. Bones of a juvenile Short-tailed Albatross were also found in beach deposits at +21.3 m from this same interglacial. We interpret the extirpation of albatrosses on Bermuda as probably resulting from lack of nesting sites protected from storm surges over the little emergent land that remained at the height of the marine isotope stage 11 sea level rise.

QUATERNARY STRATIGRAPHY OF BERMUDA: A HIGH-RESOLUTION PRE-SANGAMONIAN ROCK RECORD

Abstract Carbonate islands such as Bermuda are created by climatic change. Warm climates and high sea levels stimulate carbonate sediment production that may ultimately result in island growth, while cold glacials expose the platforms to weathering, dissolution and soil formation. Of great importance in Quaternary studies is the ability to decipher this climatic history. Mapping and geochronologic studies have established that Bermuda may have one of the most continuous and detailed Quaternary interglacial depositional records on a carbonate platform. Advances in racemization dating (AAR) have offered a means of deciphering this climatic history and generating a high-resolution stratigraphic and age framework for the Quaternary. Bermudian interglacial units consist predominantly of eolianites, with less voluminous occurrences of beach deposits and calcarenite protosols (Entisols). Glacial or stadial-age terra rossa (aluminous laterite) paleosols, whose degree of development is a function of time of exposure, form boundaries between interglacial units. d -alloiso-leucine/ l -isoleucine ( A I ) ratios have been determined on marine pelecypods, land snails and whole-rock samples from mapped sections; aminozones have been defined for two Sangamonian and at least five pre-Sangamonian depositional intervals. From kinetic models based on calibration with previously published U-series coral dates, estimated ages of middle Pleistocene and older aminozones are: F = 190,000–265,000 years; G = 300,000–400,000 years; H = 400,000–500,000 years; J = >700,000 years; and K = > 900,000 years. Aminozone G, which is correlated with the upper Town Hill Formation and Isotope Stage 9, is volumetrically the most important depositional event of the middle Pleistocene. The great mass of sediment deposited during this period suggests an interglacial of significant duration and prolonged shelf submergence, during which the island grew to over half its present size. Only the Sangamonian ( sensu lato ) rivals Stage 9 in volume of eolianite deposited on the island. Sea-level amplitude, as determined from dated outcrops, appears to correlate well with amplitudinal variations in the oxygen isotope record.

Geological Constraints on Evolution and Survival in Endemic Reptiles on Bermuda

Abstract Paleontological and geological evidence suggest that the distinctive endemic skink Eumeces longirostris could potentially be as old as continuously emergent land on the Bermuda seamount (approximately > 1–2 million yr). The species has experienced sustained evolutionary stasis for at least the past 400,000 yr, during which time there has been no perceptible change in skeletal morphology. The tortoise Hesperotestudo bermudae is known from a single fossil from interglacial period marine isotope stage (MIS) 9—approximately 300,000 yr ago. A viable population of tortoise on Bermuda could be reconciled with the geological record and the lack of any other fossils of the species to date if tortoises colonized the island at or after the end of the maximal sea-level rise of interglacial MIS 11, evolved during the single glacial episode of MIS 10, and become extinct as a result of the interglacial sea-level rise of MIS 9, a period of about 100,000 yr. Such rapid evolution and extinction has a close parallel in the giant tortoises of Aldabra Island in the Indian Ocean.

Predation as the primary selective force in recurrent evolution of gigantism in Poecilozonites land snails in Quaternary Bermuda.

During the last half million years, pulses of gigantism in the anagenetic lineage of land snails of the subgenus Poecilozonites on Bermuda were correlated with glacial periods when lower sea level resulted in an island nearly an order of magnitude larger than at present. During those periods, the island was colonized by large vertebrate predators that created selection pressure for large size and rapid growth in the snails. Extreme reduction in land area from rising seas, along with changes in ecological conditions at the onset of interglacial episodes, marked extinction events for large predators, after which snails reverted to much smaller size. The giant snails were identical in morphology during the last two glacials when the predators included a large flightless rail Rallus recessus (marine isotope stages (MIS) 4-2) and a crane Grus latipes and a duck Anas pachysceles (MIS 6). In a preceding glacial period (MIS 10), when the fauna also included the tortoise Hesperotestudo bermudae, the snails were not only large, but the shells were much thicker, presumably to prevent crushing by tortoises. Evolution of Poecilozonites provides an outstanding example of dramatic morphological change in response to environmental pressures in the absence of cladogenesis.

40Ar/39Ar age of a young rejuvenation basalt flow: implications for the duration of volcanism and the timing of carbonate platform development during the Quaternary on Kaua'i, Hawaiian Islands

Abstract Remnants of an extensive carbonate platform crop out along the southeast coast of Kauai, Hawaii. A basalt flow within this succession has a whole‐rock 40Ar/39Ar step‐heating plateau age of 375 ± 4 ka. The plateau age, which we interpret as the eruption age, indicates that rejuvenation volcanism persisted on Kauai for considerably longer (c. 200 000 yr) than previously thought, and also that published whole‐rock K‐Ar determinations may not accurately reflect eruption ages. The succession of younger sedimentary deposits and age of the basalt imply that the eruption occurred near the end of marine isotope stage (MIS) 11. Preservation of limestone dune assemblages and extensive paleosols above present‐day sea level indicates that Kauai underwent a period of emergence during the early and middle Pleistocene, probably due to passage over the lithospheric arch or forebulge created by crustal loading of Maui Nui. The presence of at least eight major limestone‐soil “couplets”, together with extrapolated ages from the 40Ar/39Ar dating, make this the oldest surficial record of limestone formation in the Hawaiian Islands. This work provides a framework for further interpretation of the stratigraphy and paleoecology of Kauai and the tropical Hawaiian Islands.

Fossilized egg indicates probable breeding of Brown Pelican (Pelecanus occidentalis) on Bermuda in the Middle Pleistocene

Abstract A large, relatively intact fossil birds egg collected on Bermuda over a century ago is identified here as that of a Brown Pelican Pelecanus occidentalis, a species known historically only as an occasional vagrant in Bermuda. Although the exact provenance and age were originally unknown, contemporary descriptions of the stratigraphy and subsequent amino acid ratios of the attached limestone matrix establish the age as a Middle Pleistocene interglacial, probably Marine Isotope Stage (MIS) 11 or perhaps slightly older. The presence of an egg is a very likely indication of at least a small former breeding colony of Brown Pelicans on Bermuda. Ecological conditions during interglacial periods and presence of potential fish prey would have made such a colonization feasible.