0198566123.pdf

archipelago is primarily a result of isostatic or of
eustatic changes. However, it is important to recog-
nize these complexities because of the need to under-
stand past land–sea configurations (Keast and Miller
1996). That isostatic effects associated with
Quaternary glaciations have not been confined to
high-latitude continents and their margins, but also
extended to low latitudes and ocean basins, was
something that became clear only in the 1970s, neces-
sitating a cautious approach to the construction of
regional eustatic curves (Nunn 1994). Stratigraphic
data from isolated oceanic islands have been of
particular value in such analyses (Fig. 2.7).
Given the general emphasis on Quaternary events,
it is noteworthy that certain of the sea-level oscilla-
tions in the Tertiary appear to have been of greater
amplitude, principally as a consequence of high
stands at 29, 15, and 4.2 Ma (Nunn 1994)—the wider
biogeographical consequence of which are only
beginning to be explored (Nores 2004). Within the
Quaternary the pattern has been one of glacial

episodes in the northern latitudes corresponding with

lowered sea levels, and interglacials associated with

levels not dissimilar to those of the present day, yet

superimposed on a falling trend. At a more detailed

and precise level of analysis it has, however, proved

difficult to construct global or even regional models of

sea-level change for the Quaternary (Stoddart and

Walsh 1992). Data for the previous and current inter-

glacial indicate that sea-level maxima have varied in

magnitude and timing across the Earth’s surface

(Nunn 2000). One intriguing, if controversial, expla-

nation for some of the irregularities in sea-level

changes relates to the configuration of the oceanic

geoid surface (i.e. the sea surface itself), which, rather

than being perfectly ellipsoid, is actually rather irreg-

ular, with a vertical amplitude of about 180 m relative

to the Earth’s centre. Relatively minor shifts in the

configuration of the geoid surface, which might be

produced by underlying tectonic movements, would

be sufficient to cause large amounts of noise in the

glacio-eustatic picture (Nunn 1994; Benton and

ENVIRONMENTAL CHANGES OVER LONG TIMESCALES 25

Stratigraphic age (Ma)

25 20 15 10 5 0

Late Early Middle Late

Oligocene Miocene

Atoll at sea

surface

Material removed

by erosion

Solution

unconformity

formed but

eroded sub-

sequently

Reef

hole

A

B

C

limestoneReef D

Volcanic

basement

Subsidence path

Solution

unconformity

preserved

Sea-level

curve

Elevation (m)

+200

+100

0

–100

–200

–300

–400

0

Time since island formation (million years)

510152025

–169

–381

0

Depth

(m)

Solution

unconformity

Pliocene-

Pleistocene

Figure 2.7The complex sequence of sea-level changes proposed for Midway Island for the last 25 million years. Solution unconformities
formed during low stages A–C were all removed during lowering of the atoll surface by subaerial processes during stage D (low sea level). The
solution unconformity formed at this stage has remained preserved because subsidence carried it below the reach of subsequent periods of atoll
surface lowering. (Redrawn from Nunn 1994: original sources given therein.)