Abstract
Modern and palaeo-shores from Pleistocene Marine Isotope Substage 5e (MIS 5e) featuring prominent cobble/boulder deposits from three locations, on the southern and eastern coast of Santa Maria Island in the Azores Archipelago, were compared, in order to test the idea of higher storminess during the Last Interglacial. A total of 175 basalt clasts from seven transects were measured manually in three dimensions perpendicular to one another. Boulders that exceeded the minimum definitional diameter of 25 cm contributed to 45% of the clasts, with the remainder falling into the category of large cobbles. These were sorted for variations in shape, size, and weight pertinent to the application of two mathematical formulas to estimate wave heights necessary for traction. Both equations were based on the “Nott-Approach”, one of them being sensitive to the longest axis, the other to the shortest axis. The preponderance of data derived from the Pleistocene deposits, which included an intertidal invertebrate fauna for accurate dating. The island’s east coast at Ponta do Cedro lacked a modern boulder beach due to steep rocky shores, whereas raised Pleistocene palaeo-shores along the same coast reflect surged from an average wave height of 5.6 m and 6.5 m. Direct comparison between modern and Pleistocene deposits at Ponta do Castelo to the southeast and Prainha on the island’s south shore produced contrasting results, with higher wave heights during MIS 5e at Ponta do Castelo and higher wave heights for the modern boulder beach at Prainha. Thus, our results did not yield a clear conclusion about higher storminess during the Last Interglacial compared to the present day. Historical meteorological records pit the seasonal activity of winter storms arriving from the WNW-NW against the scant record of hurricanes arriving from the ESE-SE. The disparity in the width of the marine shelf around Santa Maria Island with broad shelves to the north and narrow shelves to the south and east suggested that periodic winter storms had a more regular role in coastal erosion, whereas the rare episodic recurrence of hurricanes had a greater impact on southern and southeastern rocky shores, where the studied coastal boulder deposits were located.
Highlights
Survey models regarding the level of storm intensity during the Last Interglacial stage of the Pleistocene, the Marine Isotope Substage 5e (MIS 5e), have been conducted on a global scale [1], but studies organized on a more regional scale provide the potential for higher resolution
coastal boulder deposits (CBDs) were present all-year-round at Prainha, the location of this site combined with the fact that Prainha was found within a bay, and taking into consideration the wave regime, it resulted that, in some cases, the modern CBD was covered by sand
Table 8), where it was possible to compare the modern CBD with the MIS 5e CBD, the results showed contrasting site-dependency
Summary
Survey models regarding the level of storm intensity during the Last Interglacial stage of the Pleistocene, the Marine Isotope Substage 5e (MIS 5e), have been conducted on a global scale [1], but studies organized on a more regional scale provide the potential for higher resolution. Localized studies on the propensity for storms in the Bahamas and Bermuda (Western Atlantic) [2,3] reveal patterns in agreement with global results. For higher latitudes in the North Atlantic Ocean (e.g., the Azores Archipelago), such analyses have been scarce [4]. With ongoing conditions of global warming, there is even more urgency for increased knowledge about the deep history of storm patterns associated with oceanic circulation
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