Ecology influenced by island growth, subsidence, and isolation
The Hawaii hot spot is beneath the southern end of the island chain. The Emperor Seamounts, Northwest Hawaiian Islands (Hawaiian Ridge), and the main Hawaiian Islands were built in succession over the hot spot. The northwest motion of the Pacific Plate slowly draws them away from the hot spot. Removed from the source of lava, they cease to erupt and erosion whittles them away until they disappear beneath the sea. The growth and subsidence of the islands as they pass over the hot spot influences the distances between islands, the climate zones and ecosystems available on the islands, and the evolution of the animal and plant species.
Our research on biogeography of the Hawaiian Islands
Subsidence of Koko Seamount
KOKO SEAMOUNT, EMPEROR SEAMOUNT CHAIN -
Aim To determine if Koko Seamount submerged below sea level before Kure Island and Pearl and Hermes Reef formed, resulting in a period in which there were no extant islands. A period with no islands would eliminate prior terrestrial and shallow marine biotas that could migrate from island to island and require a restart of colonization from distant shores to populate the younger islands of the Hawaiian volcanic chain.
Methods We estimate subsidence rates for Koko Seamount using ages determined from fossil large foraminifera and Sr-isotopes, and maximum depths using palaeodepth estimates based on coralline algae. These data are combined with palaeolatitude changes as the Pacific Plate moved northwards, sea level variations, and sea surface temperature variations at the seamount through time to reconstruct the time and causes of submergence.
Results Rounded carbonate clasts include three facies: zooxanthelate corals, bioclastic packstones to rudstones, and rhodolith floatstones. Two rudstones contain relatively deep-water, coralline algal rhodoliths and large foraminifera indicative of Aquitanian (20.4–20 Ma) and Burdigalian (20–16 Ma) stages of the Early Miocene, consistent with Sr-isotope ages of algae and one sample of large foraminifera. Corals grew on Koko Seamount from c. 50 to 27.1 ± 0.4 Ma, the youngest Sr-isotope age of a coral sample. These shallow, warm-water coral reefs came under increasing stress as the volcano subsided at 0.012 ± 0.003 mm yr)1, and migrated northwards, and as global climate cooled. The summit submerged and shallow coral reef growth ceased before 29 Ma, probably around 33 Ma. The volcano continued its slow subsidence, and deep-water carbonates accumulated until they too were unable to keep pace, dying out at c. 16 Ma.
Main conclusions The final submergence of the summit of Koko Seamount by about 33 Ma confirms that biota on older Hawaiian–Emperor Islands could not have migrated from island to island along the entire chain to eventually colonize the present Hawaiian Islands. There was a period between at least 33 and 29 Ma in which no islands existed, and distant colonization had to repopulate the younger portion of the Hawaiian chain, which began to emerge between about 29 and 23 Ma.
Reference: Clague, D.A., Braga, J.C., Bassi, D., Fullagar, P.D., Renema, W., Webster, J.M. (2010) The maximum age of Hawaiian terrestrial lineages: geological constraints from Koko Seamount. Journal of Biogeography, 37: 1022-1033, doi:10.1111/j.1365-2699.2009.02235.x. [Article]
How old is the island biota?
HAWAIIAN CHAIN - The long-term landscape changes in the Hawaiian archipelago impact dispersal, speciation and extinction of species. To quantify this, models were developed of elevations of and spacing between the islands for the last 32 million years, accounting for volcano growth, subsidence and erosion. The size, spacing, and total number of volcanic islands have varied greatly over time. The current landscape of large, closely spaced islands was preceded by a period with smaller, more distantly spaced islands. Considering that rates of dispersal and speciation must also have changed, much of the present species pool is probably the result of recent colonization from outside the archipelago and divergence within the islands now present, with limited dispersal from older islands. This view is consistent with abundant phylogenetic studies of Hawaiian organisms. Twelve out of fifteen multi-species lineages have diverged within the lifetime of the current high islands (last 5 million years). Three of these, and an additional seven (mostly single-species) lineages, have colonized the archipelago within this period. The timing of colonization of other lineages remains uncertain.
Reference: J.P. Price and D.A. Clague (2002) How old is the Hawaiian biota? Geology and phylogeny suggest recent divergence, Proceedings of the Royal Society of London, 269: 2429-2435.
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