135.8 to 115.8 ka which shows little change even when
corrected for such open-system effects (Gallup et al.,
2002; Thompson and Goldstein, 2005). Yet a continuous
20 m-thick unit of reef-crest corals could develop in
a mere 7,000 years using conservative rates of reef accretion, and in as little as 3,000 years at maximum rates.
Given the lack of breaks in the reef-crest sequences on
Barbados and the large true-age variation between closely
spaced samples, it is highly unlikely that this timing or
duration is accurate. Indeed, higher precision dating of
speleothems has recently shown that Termination II
started at 129 ka (Cheng et al., 2009) and implies therefore
that the coral ages are inaccurate.
Although the timing and duration of reef development
on the Huon Peninsula is similarly afflicted by opensystem effects, the greater rates of tectonic uplift have
exhumed almost the entire deglacial and interglacial reef
sequence. Sections through the upper 30 m of the fossil
fringing barrier-reef complex show that the barrier crest
and slope facies are over-thickened implying a keep-up
response to SL rise. Although supported by an exposure
of shallow-water corals 90 m below the barrier crest, the
interpretation of a keep-up architecture for the entire
sequence is premature because catch-up responses may
be obscured by the more complex reef zonation scheme
found in the Pacific region. In addition, this complexity
has made it difficult to reconstruct the internal architecture
of the terrace and delineate separate stages of reef development. In turn, this has led to an unhealthy reliance on
radiometric age inversions in establishing reef stratigraphy, and has resulted in ill-conceived claims of double
highstands and extreme SL excursions.
There are also two main stable regions around the world
where LIG reefs have now been well studied: the southern
passive margin of the North-American Plate (Florida,
Bahamas, Caymans, and the Yucatan), and the western
Australian passive margin of the Indo-Australian Plate
(from Cape Range to Cape Leeuwin). Remarkably, all
sites in these two regions show almost identical reef development at identical elevations and therefore have likely
had identical relative sea-level histories.
LIG sites in these regions show a ubiquitous and extensive stage of reef development that reached þ3 m and
consisted of reef-crest units with intertidal encrusters,
and back-reef and lagoonal patch reefs that in several areas
coalesced to form extensive flat-topped units (Yucatan and
Florida). The character and elevation of these units is
therefore consistent with reef development during a SL
stand of þ3 m. In Western Australia, these þ3 m reefs
have a significantly expanded latitudinal range and extend
550 km beyond the modern limit of reef development.
This widespread development and expanded range clearly
represents the acme of reef development during the LIG.
Yet there is compelling evidence to indicate that this
þ3 m reef stage was abruptly terminated before the next
stage commenced and SL rose to its þ6 m highstand. This
is based on the discovery in all areas (including two of the
uplifted sites) of a discontinuous marine-erosion surface
and capping of crustose coralline algae. Most interpretations of this erosion surface, termed the Devils-Point
Event, have related it to a transient SL fall of several
meters which exposed the reef unit to subaerial conditions
for up to 2,000 years. Yet the discontinuous nature of this
surface and an absence of any convincing evidence of subaerial exposure or intertidal sculpturing are inconsistent
with such an interpretation. An alternative scenario is that
it represents a marine-erosion event resulting from a SL
jump that drowned the þ3 m reefs (Blanchon et al.,
2009). This scenario is also supported by reef-surface
cap of corallines which implies that reefs died off suddenly while still submerged in the marine environment.
In all areas where the þ3 m reefs have been
documented, overlying deposits of LIG age clearly indicate that SL subsequently rose from þ3 m to its final
highstand at þ6 m. In most of the areas investigated, however, the overlying sequence is non-reefal and composed
of either shallow subtidal sands and beach-derived sand
and gravel, or there is no deposition and the þ3 m reefal
units onlap a bedrock coast with a distinct intertidal notch
at þ6 m. An important exception to this general sequence,
recently reported from the Yucatan, is the discovery of reef
back-stepping and reef-crest development up to the final
highstand elevation of þ6 m (Blanchon et al., 2009). This
clearly supports the idea that the SL rise to þ6 m was rapid
and drowned the þ3 m reefs and caused reef backstepping. But it does not explain why reefs survived and
back-stepped in the Yucatan and yet perished in most other
areas. A possible explanation might be that the jump to
þ6 m inundated large coastal tracts in these low-lying terrains and helped establish new sediment-production areas
and transport pathways which prevented reef recovery and
further development. Such a scenario is supported by sediment-tolerant reef assemblages in the back-reef and
lagoonal zones of the þ6 m reef in the Yucatan and indicates that even here reef development was not completely
normal.
Although confirmation of rapid SL change during the
highstand requires duplication of the Yucatan results, there
are sufficient data to draw some final conclusions about
the timing and duration of the highstand and how it compares with the full interglacial interval. As shown in
Table 2, the duration of the LIG as measured from the
range of strictly reliable ages in uplifted areas is
136–115 ka. However, very few of these ages are
stratigraphically consistent and it is therefore not possible
to determine a reliable estimate from these data. The duration of the highstand, as measured in stable areas from the
range of strictly reliable ages, is slightly less, between 128
and 116 ka. However, even this reduced range is excessive
when the amount of reef accretion is considered. For
example, data from the Huon Peninsula indicate that the
total amount of accretion during the LIG must have
exceed 90 m, yet reef accretion during the highstand was
a mere 6 m. It seems very unlikely that this 6 m of accretion took 12 ka to develop. However, if the stratigraphic
context of these samples is considered, and inconsistent
LAST INTERGLACIAL AND REEF DEVELOPMENT
635
corrected for such open-system effects (Gallup et al.,
2002; Thompson and Goldstein, 2005). Yet a continuous
20 m-thick unit of reef-crest corals could develop in
a mere 7,000 years using conservative rates of reef accretion, and in as little as 3,000 years at maximum rates.
Given the lack of breaks in the reef-crest sequences on
Barbados and the large true-age variation between closely
spaced samples, it is highly unlikely that this timing or
duration is accurate. Indeed, higher precision dating of
speleothems has recently shown that Termination II
started at 129 ka (Cheng et al., 2009) and implies therefore
that the coral ages are inaccurate.
Although the timing and duration of reef development
on the Huon Peninsula is similarly afflicted by opensystem effects, the greater rates of tectonic uplift have
exhumed almost the entire deglacial and interglacial reef
sequence. Sections through the upper 30 m of the fossil
fringing barrier-reef complex show that the barrier crest
and slope facies are over-thickened implying a keep-up
response to SL rise. Although supported by an exposure
of shallow-water corals 90 m below the barrier crest, the
interpretation of a keep-up architecture for the entire
sequence is premature because catch-up responses may
be obscured by the more complex reef zonation scheme
found in the Pacific region. In addition, this complexity
has made it difficult to reconstruct the internal architecture
of the terrace and delineate separate stages of reef development. In turn, this has led to an unhealthy reliance on
radiometric age inversions in establishing reef stratigraphy, and has resulted in ill-conceived claims of double
highstands and extreme SL excursions.
There are also two main stable regions around the world
where LIG reefs have now been well studied: the southern
passive margin of the North-American Plate (Florida,
Bahamas, Caymans, and the Yucatan), and the western
Australian passive margin of the Indo-Australian Plate
(from Cape Range to Cape Leeuwin). Remarkably, all
sites in these two regions show almost identical reef development at identical elevations and therefore have likely
had identical relative sea-level histories.
LIG sites in these regions show a ubiquitous and extensive stage of reef development that reached þ3 m and
consisted of reef-crest units with intertidal encrusters,
and back-reef and lagoonal patch reefs that in several areas
coalesced to form extensive flat-topped units (Yucatan and
Florida). The character and elevation of these units is
therefore consistent with reef development during a SL
stand of þ3 m. In Western Australia, these þ3 m reefs
have a significantly expanded latitudinal range and extend
550 km beyond the modern limit of reef development.
This widespread development and expanded range clearly
represents the acme of reef development during the LIG.
Yet there is compelling evidence to indicate that this
þ3 m reef stage was abruptly terminated before the next
stage commenced and SL rose to its þ6 m highstand. This
is based on the discovery in all areas (including two of the
uplifted sites) of a discontinuous marine-erosion surface
and capping of crustose coralline algae. Most interpretations of this erosion surface, termed the Devils-Point
Event, have related it to a transient SL fall of several
meters which exposed the reef unit to subaerial conditions
for up to 2,000 years. Yet the discontinuous nature of this
surface and an absence of any convincing evidence of subaerial exposure or intertidal sculpturing are inconsistent
with such an interpretation. An alternative scenario is that
it represents a marine-erosion event resulting from a SL
jump that drowned the þ3 m reefs (Blanchon et al.,
2009). This scenario is also supported by reef-surface
cap of corallines which implies that reefs died off suddenly while still submerged in the marine environment.
In all areas where the þ3 m reefs have been
documented, overlying deposits of LIG age clearly indicate that SL subsequently rose from þ3 m to its final
highstand at þ6 m. In most of the areas investigated, however, the overlying sequence is non-reefal and composed
of either shallow subtidal sands and beach-derived sand
and gravel, or there is no deposition and the þ3 m reefal
units onlap a bedrock coast with a distinct intertidal notch
at þ6 m. An important exception to this general sequence,
recently reported from the Yucatan, is the discovery of reef
back-stepping and reef-crest development up to the final
highstand elevation of þ6 m (Blanchon et al., 2009). This
clearly supports the idea that the SL rise to þ6 m was rapid
and drowned the þ3 m reefs and caused reef backstepping. But it does not explain why reefs survived and
back-stepped in the Yucatan and yet perished in most other
areas. A possible explanation might be that the jump to
þ6 m inundated large coastal tracts in these low-lying terrains and helped establish new sediment-production areas
and transport pathways which prevented reef recovery and
further development. Such a scenario is supported by sediment-tolerant reef assemblages in the back-reef and
lagoonal zones of the þ6 m reef in the Yucatan and indicates that even here reef development was not completely
normal.
Although confirmation of rapid SL change during the
highstand requires duplication of the Yucatan results, there
are sufficient data to draw some final conclusions about
the timing and duration of the highstand and how it compares with the full interglacial interval. As shown in
Table 2, the duration of the LIG as measured from the
range of strictly reliable ages in uplifted areas is
136–115 ka. However, very few of these ages are
stratigraphically consistent and it is therefore not possible
to determine a reliable estimate from these data. The duration of the highstand, as measured in stable areas from the
range of strictly reliable ages, is slightly less, between 128
and 116 ka. However, even this reduced range is excessive
when the amount of reef accretion is considered. For
example, data from the Huon Peninsula indicate that the
total amount of accretion during the LIG must have
exceed 90 m, yet reef accretion during the highstand was
a mere 6 m. It seems very unlikely that this 6 m of accretion took 12 ka to develop. However, if the stratigraphic
context of these samples is considered, and inconsistent
LAST INTERGLACIAL AND REEF DEVELOPMENT
635