A review of the MIS 5e highstand deposits from Santa Maria Island (Azores, NE Atlantic): palaeobiodiversity, palaeoecology and palaeobiogeography

Towards a ‘Sea‐Level Sensitive’ dynamic model: impact of island ontogeny and glacio‐eustasy on global patterns of marine island biogeography

“…These studies indicate that the Pleistocene glacial–interglacial episodes and associated eustatic sea‐level oscillations had different effects on continental as well as insular shallow marine communities, mainly due to two factors. Firstly, insular shelves are narrower and steeper than continental shelves, such that the accommodation space and the residence time of sedimentary bodies is smaller on islands compared to continents (Ávila et al ., , ; Quartau et al ., ; Ávila, ). Secondly, in continents, the shelf‐break depth varies considerably, but is generally at depths of around −200 m (Paris, Walsh & Corbett, ), whereas in volcanic oceanic islands the shelf break is often situated at shallower depths (Ávila et al ., ; Quartau et al ., , ).…”

“…Ávila et al . (, b , , ) showed that the type of substrate in which marine organisms live (rocky shores, gravel, or sand/mud) plays an important role in the survival of volcanic oceanic island populations/species during glacial episodes. These studies indicate that the Pleistocene glacial–interglacial episodes and associated eustatic sea‐level oscillations had different effects on continental as well as insular shallow marine communities, mainly due to two factors.…”

“…In order to understand the repeated local disappearances of shallow‐water species associated with sandy substrates from islands, it is crucial to understand the differences in shelf‐break depths of continents versus volcanic oceanic islands (see Ávila et al ., ). When the sea falls below the level of the present highstand in reefless volcanic oceanic islands, sediment supply becomes sparse due to lack of major river systems, reduced precipitation in glacial periods (at least for presently subtropical to temperate islands), and reduced subaerial erosion of the palaeomarine cliffs left behind (Ávila et al ., , , , ; Quartau et al ., ). Therefore, during sea‐level falls, and notwithstanding a higher sediment production as a result of an increase in erosion caused by the fall of the riverine base levels, the sediments that are deposited on the shelf accompany the sea‐level drop because they are often transported offshore during storms (Quartau et al ., ).…”

“…A well‐studied fossil outcrop at Prainha, Santa Maria Island (Azores) illustrates the biogeographic significance of this process that affected reefless volcanic oceanic islands globally (Ávila et al ., , , b , ). Off the present‐day shore of Prainha, the shelf has a maximum gradient of ∼5% in the shallow areas between the intertidal and − 90 m, and an average gradient that ranges 30% from −90 m down to −250 m (Fig.…”

“…At around 69 ka, sea level reached the shelf break below 60–80 m depth and it became physically impossible to keep the sand in the littoral zone with such high gradients. While winter storms continued to remove sand, fair‐weather conditions could no longer restore it because it was permanently lost to the steeper island slopes (Ávila et al ., , ). As a result, sand beaches became progressively poorer and depleted of sand.…”