Pascal Kindler

A +20 m middle Pleistocene sea-level highstand (Bermuda and the Bahamas) due to partial collapse of Antarctic ice

Marine deposits at +20 ± 3 m on the tectonically stable coastlines of Bermuda and the Bahamas support the hypothesis of a partial collapse of the Antarctic ice sheet during the middle Pleistocene. Beach sediments fill a sea cave at +22 m in Bermuda, and horizontal, fenestrae-filled beds crop out on platforms at two sites as high as +21 m in Eleuthera, Bahamas. Carbonate beach sands are bound by an early generation of isopachous fibrous cement that is characteristic of a phreatic marine environment. Amino acid racemization and TIMS (thermal-ionization mass spectrometry) dates constrain the age of the deposits to between 390 and 550 ka, while proxy evidence supports a correlation with oxygen isotope stage 11. This direct geologic evidence of a 20% decrease in polar ice during the middle Pleistocene has important implications for the stability of ice sheets during warm interglaciations.

Tyrrhenian coastal deposits from Sardinia (Italy): a petrographic record of high sea levels and shifting climate belts during the last interglacial (isotopic substage 5e)

Abstract Detailed petrographic and sedimentological analysis of Tyrrhenian coastal deposits from four sites in Sardinia provides new data about climate and sea-level changes during the early part of the last interglacial (isotopic substage 5e) and shows that facies analysis is not only useful for the identification of ancient depositional environments, but also represents a powerful tool for evaluating past climatic conditions and sea-level forcing mechanisms. Two shallowing-upward sequences (sensu Ward, W.C., Brady, M.J., 1979. Strandline sedimentation of carbonate grainstones, Upper Pleistocene, Yucatan Peninsula, Mexico. Am. Assoc. Pet. Geol. Bull. 63, 362–369), commonly separated by an erosion surface and displaying complex geometric relationships, occur above modern sea level at several locations along the shorelines of Sardinia. These sequences include upper shoreface to backshore deposits and were formed during separate sea-level highstands attributed to isotopic substage 5e. Our recent revision of these deposits indicates that each sequence presents distinctive petrographic characteristics. The older succession essentially contains mixed calcarenites, rich in quartz grains and lithic fragments, whereas the younger sediments are dominated by carbonate bioclasts such as red algae, coral and mollusk fragments. Both units apparently accumulated in a similar depositional setting, but during different climatic regimes. The petrography of the lower unit reflects a temperate-humid period characterized by increased weathering and continental erosion, whereas the composition of the upper one suggests a dryer, and possibly warmer, climate favoring active carbonate production. The disparity in petrographic composition between sequences probably records the migration of atmospheric circulation cells, i.e. a shift of climate belts, before and after the ∼128-ka insolation maximum. Our climatic data are coherent with the calculated insolation curves for isotopic substage 5e and thus support the Milankovitch hypothesis for the origin of Pleistocene climates. In contrast, our sedimentological data clearly show the occurrence of two distinctive highstands of sea level during this time period, which does not agree with the hypothesis. It follows that the relation between orbitally induced insolation variations, climate and sea level is not necessarily straightforward.

Elevated marine terraces from Eleuthera (Bahamas) and Bermuda: sedimentological, petrographic and geochronological evidence for important deglaciation events during the middle Pleistocene

Sedimentological, petrographic and geochronological (uranium series and amino acid racemization dating) study of middle Pleistocene deposits from the archipelagos of Bermuda and The Bahamas revealed the occurrence of marine terraces of possible stage 11 age at +2, +7 and over 20 m above mean sea level. Considering the tectonic stability of the investigated regions, these elevated deposits likely correspond to three discrete, higher than present sea levels during this time period, which is regarded by many as the warmest interglacial of the late Quaternary. It follows that warmer than present climatic conditions might profoundly modify water distribution between the cryosphere and the oceans. The punctuated nature of our stratigraphy further suggests that future deglaciation might not be a smooth process, but could be marked by rapid ice-sheet breakdown leading to abrupt, meter-scale sea-level rises. Given the long period of warm climate and stable sea level of the past few thousands of years and CO2 loading of the atmosphere, the probability of a rapid eustatic rise must be seriously considered.

Pre-Sangamonian eolianites in the Bahamas? New evidence from Eleuthera Island

Abstract The present study provides new insight into the Quaternary stratigraphy of the Bahamas. Refining the stratigraphic record of tectonically stable carbonate islands, such as the Bahamas, is important for improving our perception of eustatic and climatic variations during this time interval. Two key-sections located 15 km apart in the northern portion of Eleuthera display three vertically stacked rock-units: (1) a basal unit consisting of up to four eolianites separated by paleosols, (2) intermediate oolitic deposits showing beach facies at +6 m, and (3) an upper bioclastic eolianite overlain by a thick calcrete. The occurrence of beach facies at an elevation of 6 m above MSL suggests an Early Sangamonian age (isotopic substage 5e) for the intermediate oolitic deposits. Because of its position below a thick calcrete, the upper unit was more likely formed during a later highstand of the Sangamonian (substage 5c or 5a), rather than during the Holocene. It follows that the basal unit clearly predates the last interglacial. This new evidence from Eleuthera shows that pre-Sangamonian rocks may be more extensive in the Bahamas Archipelago than previously thought. It also emphasizes the widespread occurrence of bioclastic limestones in an area often considered as being entirely composed of oolites.

Submerged upper Holocene beachrock on San Salvador Island, Bahamas: implications for recent sea-level history

Sedimentological, petrographic and radiometric data from a submerged beachrock on San Salvador Island, Bahamas, provide new information about the Late Holocene sea-level history in this area.At French Bay, on the southern shore of the island, samples of beachrock collected at a depth of 1 m below low tide level yielded an average 14C age of 965 ± 60 years before present. These samples further display a well developed fenestral porosity and present an early generation of low Mg calcite meniscus cement. These features characterize intertidal and supratidal settings; they are not consistent with the present beachrock position and the reported Late Holocene sea-level history in the Bahamas. A 1.5–2m low stand of the sea about 1000 years ago would best explain the observed particularities of the French Bay beachrock.This example from San Salvador shows that the smooth trend of Late Holocene sea-level rise proposed by previous workers might be overprinted by high frequency, low amplitude fluctuations. Recognition of these fluctuations is fundamental when calculating rates of sea-level rise and evaluating the coastal response to a marine transgression.

Canyon morphology on a modern carbonate slope of the Bahamas: Evidence of regional tectonic tilting

New high-quality multibeam data presented here depict the northern slope of the Little Bahama Bank (Bahamas). The survey reveals the details of large- and small-scale morphologies that look like siliciclastic systems at a smaller scale, including large-scale slope failure scars and canyon morphologies, previously interpreted as gullies and creep lobes. The slope exhibits mature turbidite systems built by mass-fl ow events and turbidity currents. The sediment transport processes are probably more complex than expected. Slope failures show sinuous head scarps with various sizes, and most of the scars are fi lled with recent sediment. Canyons have amphitheater-shaped heads resulting from coalescing slump scars, and are fl oored by terraces that are interpreted as slump deposits. Canyons rapidly open on a short channel and a depositional fan-shaped lobe. The entire system extends for ~40 km. The development of these small turbidite systems, similar to siliciclastic systems, is due to the lack of cementation related to alongshore current energy forcing the transport of fi ne particles and fl ow differentiation. Detailed analyses of bathymetric data show that the canyon and failurescar morphology and geometry vary following a west-east trend along the bank slope. The changing parameters are canyon length and width, depth of incision, and canyon and channel sinuosity. Accordingly, failure scars are larger and deeper eastward. These observations are consistent with a westward tectonic tilt of the bank during the Cenozoic.

Chapter 3B - Geology of the Bahamas: Architecture of Bahamian Islands

This chapter focuses on the three-dimensional mosaic making up the islands. The chapter uses the stratigraphic scheme, which differs from the formal lithostratigraphic column. It has become clear that there is variability among Bahamian islands with respect to their stratigraphic architecture, and that there are patterns to this variability. Such observations present in the chapter provide tentative classification of islands where one can find or expect to find different types of stratigraphic architecture. Although the classification is still a hypothesis, which one can test by studying more Bahamian islands, the chapter includes it as an organizer and provides a potential framework for (1) understanding Bahamian islands fully, and (2) comparing Bahamian islands to other kinds of carbonate islands. The architecture of Bahamian islands shows that they grow by lateral accretion or vertical stacking depending on their exposure to the open ocean and the width of the outer platform.

Sedimentology and petrography of dredged carbonate sands from Stocking Island (Bahamas). Implications for meteoric diagenesis and aeolianite formation

Abstract Piles of carbonate sand dredged onto Stocking Island (Exuma Chain, Bahamas) during World War II are now relatively well-lithified down to a depth of about 80 cm and may contain up to 8% of interstitial calcite cement, a volume comparable to that measured from 1000-yr-old beach ridges elsewhere in the Bahamas. Rates of meteoric diagenetic changes are thus more rapid than previously assessed in this area (∼600 cm3 of calcite cement produced by m3 of rock per year). Preservation of hydrodynamic sedimentary structures, footprints, and the lack of aeolian bedforms (e.g. dunes, ripples) on the surface of these sediments further show they have not been reworked by the wind at all. Therefore, the carbonate aeolianites of Holocene and Pleistocene ages forming the bulk of the islands of Bermuda and the Bahamas were likely deposited during relatively dry periods when the effects of meteoric diagenesis were limited. Reduced moisture, along with sediment availability and wind energy, appears as a key player in carbonate aeolianite genesis.

Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American margin

We report here the discovery of Miocene, Pliocene, and early Pleistocene shallow-marine carbonates on Mayaguana Island (southeastern Bahamas) that have so far not been observed on any other Bahamian island. Spanning more than 17 m.y., but <12 m thick, this stratigraphic succession only occurs along the northern coast of the island, indicating that the Mayaguana Bank underwent minor subsidence throughout the late Cenozoic and was tilted toward the south during the Quaternary. In addition to considerably extending the stratigraphic record of the Bahamas Islands, our findings demonstrate that these carbonate banks were at different elevations and subsided at different rates during the Neogene. The young age of the tilting event detected on Mayaguana further shows that parts of the southeastern margin of North America have recently undergone tectonic activity a long way from its actual boundary with the Caribbean plate.

Palaeoclimatic significance of co-occurring wind- and water-induced sedimentary structures in last-interglacial coastal deposits from Bermuda and the Bahamas: response to Hearty et al.'s comment

Abstract We respond to Hearty et al.s objections to our interpretation of upper Pleistocene coastal landforms from Bermuda and the Bahamas as aeolian dunes rather than wave-induced deposits. Their remarks concern the origin of fine-scale laminae and fenestral pores observed in these landforms, the apparent lack of steeply dipping foresets, the inference that carbonate coastal dunes can migrate, the vicinity of wave-deposited boulders that we did not mention, and finally, the palaeoclimatic reconstruction we derived from our interpretation. Although, we do not exclude—and never did—that occasional storm-generated waves washed up and over these dunes and created some scours, we maintain that these controversial deposits have an aeolian origin.