[Taxacom] Pacific biogeography

Michael Heads m.j.heads at gmail.com
Mon Dec 17 02:07:32 CST 2012

 Dear Taxacomers,

A few months ago we were debating Hawaiian biogeography. In a discussion of
this topic in the book cited below, I mapped the 2000, 4000 and 5000 m
isobaths of the central Pacific. O’Grady et al. (Taxon 61: 702. 2012) have
now suggested that this was ‘disingenuous’, because sea-level has not
dropped by more than 100 m or so. But the authors overlooked the *1000s* of
meters of subsidence that the Pacific seafloor itself has undergone. This
is well-known to geologists and I discussed it in the book.

As the seafloor has drifted away from the East Pacific Rise - the spreading
ridge that produced it - it has cooled (increasing its density) over tens
of millions of years and has subsided by these large amounts. This has
led to the submergence of most of the islands that were perched on it.
The current high islands are new ones. Evidence for the subsidence is seen
in the numerous atolls of the region, formed by coral reefs which have
grown as the seafloor subsided. The many flat-topped seamounts (guyots)
located north, south, east and west of Hawaii are former high islands that
were eroded to sea-level before being submerged with the tectonic

Michael Heads
Wellington, New Zealand.

My new book: *Molecular panbiogeography of the tropics. *
University of California Press, Berkeley.

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