www.elsevier.com/locate/gloplacha

Global and Planetary Chan

Episodes of reef growth at Lord Howe Island, the southernmost

reef in the southwest Pacific

C.D. Woodroffe a,*, M.E. Dickson b, B.P. Brooke c, D.M. Kennedy d

a School of Earth and Environmental Sciences and GeoQuEST Research Centre, University of Wollongong, NSW 2522, Australiab National Institute of Water and Atmospheric Research, P.O. Box 8602, Christchurch, New Zealandc Petroleum and Marine Division, Geoscience Australia, GPO Box 378, Canberra ACT, Australia

d School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand

Received 27 June 2005; received in revised form 5 September 2005; accepted 6 September 2005

Abstract

Lord Howe Island lies at the present latitudinal limit to reef growth in the Pacific and preserves evidence of episodes of reef

development over the Late Quaternary. A modern fringing reef flanks the western shore of Lord Howe Island, enclosing a

Holocene lagoon, and Late Quaternary eolianites veneer the island. Coral-bearing beach and shallow-water calcarenites record a

sea level around 2–3 m above present during the Last Interglacial. No reefs or subaerial carbonate deposits occur on, or around,

Balls Pyramid, 25 km to the south. The results of chronostratigraphic studies of the modern Lord Howe Island reef and lagoon

indicate prolific coral production during the mid-Holocene, but less extensive coral cover during the late Holocene. Whereas the

prolific mid-Holocene reefs might appear to reflect warmer sea-surface temperatures, the pattern of dates and reef growth history

are similar to those throughout the Great Barrier Reef and across much of the Indo-Pacific and are more likely correlated with

availability of suitable substrate. Little direct evidence of a Last Interglacial reef is now preserved, and the only evidence for older

periods of reef establishment comes from clasts of coral in a well-cemented limestone unit below a coral that has been dated to the

Last Interglacial age in a core at the jetty. However, a massive reef structure occurs near the centre of the wide shelf around Lord

Howe Island, veneered with Holocene coralline algae. Its base is 40–50 m deep and it rises to water depths of less than 30 m. This

fossil reef is several times more extensive than either Holocene or Last Interglacial reefs appear to have been. Holocene give-up

reef growth is inferred during the postglacial transgression, but an alternative interpretation is that this is a much older landform,

indicating reefs that were much more extensive than modern reefs at this marginal site.

D 2005 Elsevier B.V. All rights reserved.

Keywords: reef growth; submerged reef; sea level; Late Quaternary; geomorphology; Lord Howe Island

1. Introduction

Extensive coral reefs occur within the tropics and

subtropics but reefs are marginal where sea-surface

temperatures fall below 18 8C. The latitudinal limit to

0921-8181/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.gloplacha.2005.09.003

* Corresponding author. Tel.: +61 2 4221 3359; fax: +61 2 4221

4250.

E-mail address: colin@uow.edu.au (C.D. Woodroffe).

coral-reef formation is a sensitive threshold in the

world’s oceans, known as the Darwin Point (Grigg,

1982). Not only does the composition of carbonate

sediments change, with the coralgal assemblage of trop-

ical waters (dominated by coral and coralline algae)

being replaced by a foramol assemblage (foraminifera

and molluscs), but there are significant physical differ-

ences as well. Those volcanic islands that lie outside reef

seas are planated by marine abrasion and are flanked by

ge 49 (2005) 222–237

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 223

broad near-horizontal shelves (Menard, 1986). Lord

Howe Island (31830VS) sits at this threshold in the

southwest Pacific (Fig. 1); a broad shelf around it indi-

cates a long history of planation of the volcano, but it

now has a fringing reef developed on the windward side

of the island. Balls Pyramid, 25 km to the south, rises as

a striking monolith from the middle of a truncated shelf

representing the penultimate stage of marine planation.

Reginald Daly, one of the first geoscientists to ap-

preciate the significance of Quaternary glaciations, be-

lieved that climate and sea-level changes had been

accompanied by substantial shifts in the latitudinal

limit to coral-reef development. He postulated that

there were dmarginal seasT from which reefs had been

Fig. 1. The Tasman Sea showing the location o

eradicated during glacial periods, with gradual re-estab-

lishment during the postglacial (Daly, 1915, 1934).

However, sea-surface temperature reconstructions un-

dertaken as part of CLIMAP (1976) demonstrated that

at the glacial maximum, there had not been extensive

shifts in the 18 8C isotherm. It appears that the pole-

ward limits to reef development are less sensitive to

Quaternary climate and sea-level changes than Daly

envisaged.

By comparison with their modern distribution, great-

er poleward extent has been interpreted for reefs during

the Last Interglacial from several sites. For example,

limestones of Last Interglacial age are prominent within

the Miami Limestone in Florida and show many of the

f Lord Howe Island and Balls Pyramid.

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237224

characteristics of the more tropical Bahamian reefs

(Hoffmeister et al., 1967). On the west coast of Aus-

tralia, the well-developed reef at Fairbridge Bluff on

Rottnest Island (328S) and reef limestones on the adja-

cent mainland at the mouth of the Swan River estuary

in Fremantle suggest more prolific coral than occurs at

this latitudinal limit at present (Szabo, 1979). Similarly

in eastern Australia, corals in the Last Interglacial

deposits at Evans Head, New South Wales (298S),occur further south than coral reefs presently reach on

that coast (Marshall and Thom, 1976). These reefs

imply conditions warmer than present during the Last

Interglacial in that they grew in more poleward loca-

tions than their Holocene equivalents.

There is also evidence that reefs may have extended

further, or have been more prolific, at the latitudinal

limit during mid-Holocene than at present. Veron de-

scribed a reef at Tateyama, near Tokyo in Japan, that is

now fossil but which contained 72 species 6000–5000

years BP, about twice the present diversity of nearby

reefs. On the basis of this diversity, temperature appears

to have been 1.2–1.7 8C warmer than at present (Veron,

1992, 1995). Recognition of recent expansion of reefs

in Florida (Precht and Aronson, 2004) has also focused

attention on an early Holocene reef that flourished

along 85 km of the southeastern Florida coast, 9000–

7000 years BP (Lighty, 1977; Lighty et al., 1978;

Toscano and Macintyre, 2003).

Concern about future climate change has again fo-

cused attention on the latitudinal limits to reef growth;

these are of special significance if it can also be shown

that reefs have flourished further poleward than now at

times in the past. Discovery in 1998 of the recent

establishment of the branching coral Acropora cervi-

cornis off the coast of Fort Lauderdale in Florida

(Vargas-Angel et al., 2003), contrary to an almost Ca-

ribbean-wide trend for coral deterioration, has led to an

interpretation that global warming may result in exten-

sion of coral range (Precht and Aronson, 2004). The

relatively limited favourable habitat available at the

present poleward limits to reef growth provides a con-

straint and imparts a particular significance to those

places where reef expansion is possible and where

evidence of former, more extensive reefs can be

found (Guinotte et al., 2003; Buddemeier et al., 2004).

In this paper, we describe the evidence for previous

phases of reef growth on, and around, Lord Howe Island,

the southernmost coral reef in the southwestern Pacific

Ocean. As the island is at, or close to, the latitudinal limit

to coral reef growth, studies of former reef development

in the region provide insights into past changes that have

significant implications for future climate change.

2. Area of study

Lord Howe Island (31830VS, 159800VE) and Balls

Pyramid (31846VS, 159815VE) are located 600 km east

of the Australian mainland, on the Australian plate,

which is migrating northward at a rate of 5–6 cm/year

(Fig. 1). These volcanic edifices sit on the margin of the

Lord Howe Rise, a rifted continental crustal block. Lord

Howe Island presently occurs at the southernmost limit

of coral-reef formation, on a transition from tropical to

subtropical open-ocean carbonate environments, and is

gradually moving into reef-forming seas. Both Lord

Howe Island and Balls Pyramid have undergone exten-

sive marine planation and sit in the centre of broad

shelves (see Fig. 6); planation would appear to have

occurred largely before these islands migrated into seas

in which coral-reef growth is possible.

2.1. Geological characteristics

The basalts of Lord Howe Island have been dated to

Miocene, having erupted 6–7 Ma (McDougall et al.,

1981). Several stages of eruption can be detected; the

oldest rocks are the Roach Island Tuff to the north,

forming the Admiralty Islands. North Ridge is com-

posed of basalts erupted during a second phase, termed

the North Ridge basalt and potassium–argon dated to

6.9 Ma (Fig. 2). Intermediate Hill consists of Boat

Harbour Breccia deposited after the caldera of the

original volcano collapsed, and the youngest basalts

are formed of Mount Lidgbird basalt, which consists

of nearly horizontally bedded lava flows, and has

formed the peaks of Mount Gower and Mount Lidg-

bird, potassium–argon dated to 6.4 Ma. After their

eruption as a shield volcano, the volcanic pedestal

has been truncated to form the broad shelves that

circumscribe both Lord Howe Island and adjacent

Balls Pyramid.

A sequence of calcarenites, primarily deposited as

eolianite but containing shallow-water marine and

beach units, veneer the central section of Lord Howe

Island below 100 m elevation. The chronostratigraphy

of these has been examined in detail and dating indi-

cates that they have been deposited within the past

350,000 years (Brooke et al., 2003a,b). No subaerial

carbonate deposits occur on Balls Pyramid, which rises

to a height of 550 m, nor is there any indication of

former reefs on the shelf around this monolith. Lord

Howe Island appears to have undergone little subsi-

dence, based on the elevation of Last Interglacial shore-

line at around 2–3 m above present sea level

(Woodroffe et al., 1995).

Fig. 2. The geology of Lord Howe Island showing the location of transects across the lagoon.

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 225

The coral reef along the western side of Lord Howe

Island is the southernmost in the Pacific (Slater and

Phipps, 1977). Drilling and dating of the reef have

provided only an incomplete mid–late Holocene history

of reef growth, but the history of lagoonal sediment

infill behind the reef implies that a reef structure was in

place by at least 5000 years BP at which time rapid

sedimentation was occurring in its lee (Kennedy, 1999;

Kennedy and Woodroffe, 2000).

2.2. Oceanographic characteristics

The coral communities at Lord Howe Island, togeth-

er with those on Elizabeth and Middleton Reefs to the

north, are biogeographically related to, but separate

from, those of the Great Barrier Reef (Veron, 1995).

Reefs extend for over 2500 km along the Queensland

coast, south to Lady Elliot Island (248S) between Glad-

stone and Bundaberg (Fig. 1). There are extensive

fringing reefs in Moreton Bay (Brisbane) and sub-

merged banks and shoals off the southern Great Barrier

Reef to 288S. Coral reefs are also found on the Solitary

Islands (308S) offshore from Coffs Harbour (Hopley,

1982). Coral reefs occur further south on Lord Howe

Island than on the mainland because of the influence of

the warm-water East Australian Current. The boundary

between this current and the southern temperate Tas-

man Current, termed the Tasman Front, sweeps past the

island annually, its location varying from 308S in winter

to 348S in summer (Martinez, 1994). Mean annual sea-

surface temperature varies between 18 8C and 23 8C(Veron and Done, 1979). Low sea-surface temperatures

in winter are stressful to corals, and it is probable that

the endemic corals have become adapted to this.

The tidal range at Lord Howe Island is 1.5 m at

springs and 0.8 m at neaps, and the predominant winds

generate easterly swells in summer whereas southern

swells dominate in winter. The island has a maritime

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237226

climate and storms can approach from the north during

summer and autumn and from the west during autumn

and spring. GEOSAT altimeter data collected between

1995 and 1996 indicate mean significant wave heights

of 2.25–2.50 m, but the wave climate is not well

recorded and storms are frequent, occurring throughout

the year.

3. Methods

The stratigraphy of the calcareous sediments on, and

around, Lord Howe Island has been examined through

detailed observations of the calcarenites at a series of

exposures. Their age relationships are based on com-

parative U-series dating of coral and speleothems, ther-

moluminesence (TL) dating of dune and palaeosols,

amino-acid racemisation (AAR) dating of whole-rock

and pulmonate gastropods, and AMS radiocarbon dat-

ing of fossil shells (Brooke et al., 2003a,b). The stra-

tigraphy of lagoon deposits is based on 40 km of

continuous seismic profiling, using a Uniboom sound

source triggered every 0.5 s at an energy of 200 J and a

single channel 8-element hydrophone filtered at 500

Hz, towed at 4 knots with GPS fixes (Fig. 3). The

seismic interpretation was linked with observation of

reef sediments in 22 vibrocores (with variable penetra-

tion, compaction and recovery to about 5 m depth), 3

triple barrel diamond drill cores (to a maximum depth

of 18 m) and several hand-held diamond drill cores (to

a maximum depth of 10 m). These stratigraphic obser-

vations, together with 71 radiocarbon dates, have been

reported by Kennedy and Woodroffe (2000).

Fig. 3. Seismic reflection profiling and the stratigraphy of cores LH12 and LH

east–west, see Fig. 2 for location). Reflector A marks the contact between

underlying basalt.

Further evidence for offshore reefs near Lord Howe

Island has been based on reconnaissance during cruise

12/98 aboard the CSIRO research oceanographic vessel

R.V. Franklin in October 1998 and on the chartered

motor yacht, Advance II in 2001. The bathymetry of the

shelves surrounding Lord Howe Island and Balls Pyr-

amid, based on more than 40,000 soundings, has been

synthesised using ArcGIS from acoustic data supplied

by the Hydrographic Office of the Royal Australian

Navy, together with several swaths of coverage from

a Laser Airborne Depth Sounder (LADS). The broad

pattern of depths across the shelves has been interpreted

using an interpolation across a triangulated irregular

network (TIN) resampled into a raster grid (25 m cell

size) based on an inverse distance-weighted (IDW)

algorithm.

The surface sediments on the shelves around Lord

Howe Island and Balls Pyramid have been described

from 73 grab samples collected with a Smith–McIntyre

grab sampler with locations recorded using GPS (Ken-

nedy et al., 2002). Limited continuous acoustic sub-

bottom profiling across the shelf was undertaken using

an E.G.&G. Sparker 3-element array system at 500 J

and a GeoAcoustics Boomer system with amplifier and

filter at 200 J and a 10-element streamer of hydro-

phones. Dives were undertaken at two sites, but

attempts to drill the offshore reef limestones were not

successful.

Radiocarbon dates upon which this interpretation is

based were determined at Beta Analytic, Australian

National University (ANU), Australian Nuclear Science

and Technology Organisation (ANSTO) ANTARES

13, showing the relation between the different reflectors (transect runs

Holocene and Pleistocene calcarenites, and reflector B records the

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 227

facility, and Waikato University Radiocarbon Labora-

tories. Ages are in radiocarbon years BP and have been

corrected for the marine reservoir effect by subtracting

450F35 years from the conventional age (Gillespie

and Polach, 1979); details are reported in Kennedy

and Woodroffe (2000) and Kennedy et al. (2002).

Calibrated ages are reported to 2 standard deviations

where specific ages are reported.

4. Results

In this section, the modern reef is described, and the

evidence for its development in the Holocene is out-

lined. Evidence for earlier phases of reef growth in

previous interglacials is examined, and the nature and

significance of fossil reefs are addressed.

4.1. Holocene reef

The fringing reef along the western margin of Lord

Howe Island consists of a 6-km-long fringing reef

enclosing a shallow lagoon that is up to 2 km wide

(Guilcher, 1973). The morphology and Holocene his-

tory of the reef and lagoon have been described by

Kennedy and Woodroffe (2000). The lagoon has an

average depth of about 1.5 m at high tide, with a few

isolated holes up to 10 m deep. Coral is presently found

in luxuriant communities at several places in the la-

goon, but particularly within the backreef communities

where Acropora and Pocillopora dominate (Veron and

Done, 1979; Harriott et al., 1995). The reef front con-

Fig. 4. Cross section of northern lagoon (see Fig. 2 for location), Lord Howe

deposition. LAT, lowest astronomical tide.

sists of spur and groove to 5 m depth beyond which

there is a narrow terrace and then a drop off into deeper

water of about 20 m depth. The reef crest is typically an

algal pavement but the backreef contains an imbricated

gravel sheet (Fig. 2). The lagoon itself is floored by

sand. The composition of this sand has been described

in detail by Kennedy (2003); it is medium–coarse, and

although with scattered live coral and macroalgae,

much is bare and rippled implying intermittent sand

movement.

The stratigraphy underlying the lagoon has been

examined by seismic profiling which revealed three

prominent reflectors throughout the lagoon (Fig. 3).

These are interpreted on the basis of exposures (e.g.,

at Windy Point) and the stratigraphy of drill cores

(and in a few vibrocores, although these generally

only penetrated the top unit of the lagoon). The first

reflector marks the lagoon floor; reflector A represents

the unconformity between the underlying Pleistocene

and the overlying Holocene lagoonal sediments, and

the deeper reflector B marks the upper surface of the

underlying basalt. Drilling in mid-lagoon encountered

the underlying Pleistocene calcarenite, from 7 to 18 m

depth in LH12, although with poor recovery. The

thickness of Holocene sediments is up to 23 m but

the most detailed stratigraphy of this unit is available

for the northern part of the lagoon where it is only 7–

8 m thick. Vibrocores revealed a Holocene stratigra-

phy comprising an upper gravelly sand, a mid-gravelly

mud, and a lower clast-supported branching coral

gravel (Fig. 4). The upper unit is up to 2 m thick,

Island (based on Kennedy and Woodroffe, 2000) showing isochrons of

Fig. 5. Histogram of radiocarbon dates on sedimentation within Lord

Howe Island lagoon (dates in radiocarbon years reported in Kennedy

and Woodroffe, 2000).

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237228

but vibrocores typically penetrated about 0.5 m of this

unit and then entered the gravelly mud (oldest age

4480 years BP; gravel typically algal rhodoliths). Ra-

diocarbon ages on the mid-unit span the period 5190–

4600 radiocarbon years BP.

Despite drilling on the seaward margin of the gravel

field, recovery of material from beneath the Holocene

reef crest itself was not successful and the chronology

of reef growth on the margin is consequently only

poorly constrained. Nevertheless, the sequence of sedi-

ments from vibrocores in the lagoon yielded a consis-

tent sedimentology and chronology across the northern

part of the lagoon (Kennedy and Woodroffe, 2000)

based on clast-supported gravel in which branches of

Acropora are frequent. Radiocarbon ages of 6200 ra-

diocarbon years BP mark the initial phases of coral

establishment over the underlying eolianite as the rising

sea flooded the truncated fossil dunes. Rapid accretion,

at average rates of 5 mm/year and up to 11 mm/year,

then occurred infilling the available accommodation

space by 4000 years BP. Comparative radiocarbon

ages on coral, algae and the sediment in which they

occur further suggest rapid deposition (Kennedy and

Woodroffe, 2004). The cores indicate prolific coral

growth and rapid sedimentation in mid-Holocene. The

reef crest has not been investigated because it was

inaccessible and must be inferred to have caught up

with sea level, partly on the basis of mangrove remains

at the jetty (see Fig. 4) which have been dated at around

6000 years BP (Woodroffe et al., 1995).

The age structure of the reef is summarised in Fig. 5

which shows a histogram of radiocarbon dates (n =71,

details in Kennedy and Woodroffe, 2000) serving to

emphasise that the major part of the sediment was

deposited in the period 6500–4000 years BP, as can

be seen from the isochrons in Fig. 4 which shows a

cross section from the northern lagoon. The southern

lagoon is less accessible and the calcarenite is found at

greater depth based on the seismic results; sedimenta-

tion within this deeper southern lagoon lagged the

northern lagoon by up to 500 years. However, the

majority of the lagoon appears to have infilled by

4000 years BP (Woodroffe et al., in press).

Once the lagoon had infilled to near present levels,

waves and wind-driven currents were able to transport

sediment directly across the lagoon floor and onto the

foreshore. Mud-sized material was winnowed offshore

and sandy beaches began to prograde seaward. Coastal

plain development is most likely to have commenced at

around 3000 radiocarbon years BP after the lagoon had

infilled. Dating within the coastal plain suggests an age

younger than the gravelly mud units in the lagoon;

however, the allochthonous nature of the sediments

means that an exact chronology of beach progradation

cannot be established. Foredune height on the present

coastal plain increases towards the lagoon suggesting

that rates of progradation may have slowed in the late

Holocene.

Sea level was higher than present during the mid-

Holocene in this part of the Pacific. This is recorded by

platforms cut across the calcarenite within the lagoon

and now above the level of the highest tides and 1 m or

more above modern platforms that occur close to the

level of low tide (Woodroffe et al., 1995). Radiocarbon

dating of coral boulders from a small pedestal of

cemented coral conglomerate and a cemented boulder

ridge in the northern lagoon implies that sea level was

higher than at present around 3000 years BP. A further

date on an in situ bivalve, Fragum unedo, and com-

parison with the intertidal environments in which it now

lives, indicates that sea level was still above present 900

years ago (Woodroffe et al., 1995). This pattern of

gradually falling sea level, in conjunction with lagoonal

infill, may have further reduced the extent of habitat

suitable for coral on the western side of the island.

4.2. Pleistocene reefs

The calcarenites that veneer the central part of the

island and which extend below sea level in several

places comprise cross-bedded units 3–25 m thick.

Based on AAR and TL age estimates (Brooke et al.,

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 229

2003a,b), these dunes were emplaced during the Last

Interglacial with phases of deposition continuing into

oxygen isotopes stages 5a or 4. Shallow-marine units

have also been identified within the calcarenite. At

the type site at the southern end of Neds Beach, there

is a beach unit, with low-angle beds at the base of

which coral clasts (boulders to large pebbles) have

been found. U-series ages for these corals of around

125,000 years BP are supported by TL ages on traces of

quartz separated from the predominantly calcareous

sands. Further beach or shallow subtidal units of equiv-

alent age have been described from the west coast of

the island at the boat ramp and in cores beneath the jetty

and at Lovers Bay. Although these low-angle beds

contain coral clasts, they represent beach environments

and not an in situ reef. The location of the Last Inter-

glacial reef remains unresolved; reefs may have oc-

curred in locations similar to those of modern reefs

and may have been largely destroyed during emergence

when the sea was lower.

Evidence for the existence of a reef older than Last

Interglacial is sparse. An apparently older, more recrys-

tallised, better lithified limestone, included within the

Searles Point Formation, occurs in places beneath, or

adjacent to, the Neds Beach Formation (Brooke et al.,

2003a). Coral grains are not a conspicuous component

of the eolian units of the calcarenite. Drill core LH11

taken at the jetty penetrated through a thin calcarenite

veneer and then into a coral-rich lithified carbonate,

eventually hitting the basalt at a depth of 10.8 m. In this

core, a U-series age of Last Interglacial, 110,000–

115,000 years BP, on a coral at 5 m depth indicates

that the marine unit was deposited during oxygen iso-

tope stage 5e. Corals below 6.5 m in the core are

calcitic and towards the base at least two phases of

calcitic cementation are observed, infilling many of the

void spaces. The complete recrystallisation below the

Last Interglacial limestone suggests that this lower unit

may represent an older interglacial phase. The core

therefore indicates that coral growth was occurring

around the island possibly as early as Stage 7, with

glacial phases being recorded by diagenesis and infill

with calcitic cements.

4.3. Submerged mid-shelf reef

By contrast with the highly porous modern reef, of

limited extent along the western side of the island, and

the detrital evidence for former interglacial corals

(which it is presumed formed a reef), there is a very

large structure, interpreted as a fossil reef, on mid-shelf.

Lord Howe Island sits near the centre of a rhomboidal

shelf with a width (W–E) of 24 km and a length (N–S)

of 36 km. Balls Pyramid lies at the centre of a smaller

shelf with a width (W–E) of 15 km and a length (N–S)

of 22 km (Fig. 6). The two shelves are separated by a

trough that is on average 600 m deep.

Both shelves are relatively flat with slopes of less

than 18, in contrast to the steep slopes of the subaerial

volcanic edifices. There is a distinct shelf break at

around 70–100 m depth. Slopes are steepest near the

shelf break, typically 15–208 (maximum 308). The

surficial carbonate sediments on the shelf around

Lord Howe can be divided into three major zones: the

inner shelf, the outer shelf, and the shelf edge (Kennedy

et al., 2002). On the Lord Howe shelf, the fossil reef

separates the inner and outer shelves, whereas on the

Balls Pyramid shelf, the absence of any reef feature

means that the inner shelf extends with little differen-

tiation to 50 m depth. The carbonate assemblages on

the shelves around Lord Howe Island and Balls Pyra-

mid are predominantly temperate in composition with

coralline algae (rhodalgal) the dominant sediment com-

ponent (Kennedy et al., 2002). Coral and algal rhodo-

lith growth appears to have been more abundant during

the early and mid-Holocene (algal rhodoliths from the

outer margin of the shelf returned radiocarbon dates of

6700F95 and 3510F105 radiocarbon years BP, 7768–

7415 and 4190–3606 calibrated years, respectively).

The presence of these sediments suggests that there is

little active deposition on the shelf and that modern

carbonate productivity is low.

Prominent in the centre of the Lord Howe shelf is a

ridge reaching up from a water depth 40–50 m to

shallowest at depths of 25–30 m, but with an average

upper surface depth of approximately 30 m. This

feature appears to be a fossil reef, and it lies between

1.5 and 8 km from shore, on the western, southern and

eastern sides of Lord Howe Island (Fig. 6). This

submerged reef is widest (N3 km) on the western

side of the island. On the eastern side of the shelf,

the ridge is less continuous, located farther offshore

and characterised by a series of elongate high patches.

It appears largely absent along the northern edge of

the shelf, which gradually slopes towards the shelf

break. Interpretation of this feature as a fossil reef is

based on its morphology and surface lithology (i.e., it

is composed of limestone where it has been observed

directly). Fig. 7 shows a histogram based on the

gridded topography of the water depths across the

shelf. The predominance of depths in the range 25–

45 m can be seen.

Continuous acoustic sub-bottom profiling undertak-

en across the feature reinforces its protruding morphol-

Fig. 7. Histogram of elevations of gridded shelf bathymetry. Dots indicate cumulative area of 10-m depth increments.

Fig. 6. Bathymetry on Lord Howe and Balls Pyramid shelves (in metres), showing the mid-shelf reef, location of seismic traces, and selected grab

samples.

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237230

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 231

ogy, but neither Sparker nor Boomer seismic traces

enabled differentiation of lithological differences or

sediment thickness across the shelves (Fig. 8). The

lack of seismic differentiation between the fossil reef

limestone and the underlying basalt is attributed to the

highly lithified nature and relatively small thickness of

the limestone in contrast with Holocene reef limestones

which have been clearly discriminated in similar Uni-

boom seismic profiling in the lagoon (Fig. 3). The

selected seismic profiles shown in Fig. 8 emphasise

the irregular upper surface topography of the reef in

contrast to the relatively uniform sand-covered shelves

from which these reefs protrude. Whereas the lithology

is unclear, the seismic traverse does indicate the prom-

inent steep margins to the submerged reef and, in

particular, the inner margin. Sand within the trough

on the inner shelf could also not be discriminated in

seismic profiles, presumably because it is not thick

enough to be distinguished from the sea floor. Sand

ripples characterise the floor of the trough at around 30

m depth, as indicated by divers. The sand is presumably

periodically mobilised and is still accumulating; a ra-

diocarbon date of 4060F95 years BP (4854–4389

calibrated years) was determined on bulk sand at a

depth of 2.06 m in a piston core on the inner shelf

(Fig. 6) indicating accumulation at a minimum rate of

0.5 mm/year (Kennedy et al., 2002).

Grab samples from the surface of the fossil reef

contained very coarse and angular material that

appeared to have been cemented to the substrate (sam-

ples G15, G66, G84 from the fossil reef and G52, G78

from water depths of 40–50 m on the front of the reef).

Fig. 8. Acoustic sub-bottom profiles over m

Crustose sheets from the surface of the reef appeared to

be composed of multiple layers of coralline algae grow-

ing over a coral/algal substrate. The coralline algae

were generally alive and growing in association with

encrusting bryozoans and foraminifers, macroalgae,

and less often Halimeda. Significant concentrations of

branching coral gravel were recovered from the lee of

the fossil reef. A radiocarbon date on Acropora indi-

cated that this was Holocene in age (a radiocarbon age

of 8370F125 radiocarbon years BP, 9746–9095 cali-

brated years BP, on branching coral from G19, contrasts

with a modern date from the in situ clast in G18). This

implies a thin veneer of early Holocene surficial growth

on an older feature and give-up as the structure was

drowned during the postglacial marine transgression. A

high proportion of miscellaneous, or indeterminate,

grains was recorded on the inner Lord Howe shelf

and the fossil reef (Kennedy et al., 2002).

There is a relatively uniform shelf around Balls

Pyramid. The shelf reaches 50 m depth close to the

pyramid and gradually increases in depth towards the

shelf break. A few isolated undulations of around 10 m

relief occur in the central part of the shelf, although

numerous rocky outcrops to the south mean that depth

observations on that part of the shelf are sparse.

5. Discussion

Global climate change poses particular challenges

for coral reefs (Kleypas et al., 1999). Whereas the threat

of coral bleaching has received particular attention

because reefs in tropical waters are subject to thermal

id-shelf reef (see Fig. 6 for location).

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237232

stress (Douglas, 2003), a further possibility in response

to greenhouse warming is a poleward shift of the

latitudinal limit to reef growth (Buddemeier et al.,

2004). Episodes during which reefs were more exten-

sive than present at marginal locations like Lord Howe

Island need to be examined to see whether they provide

evidence of warmer conditions or whether there are

other explanations for past expansion or contraction.

Although the modern fringing reef on the western

side of Lord Howe Island supports luxuriant coral

communities (Veron and Done, 1979; Harriott et al.,

1995), coring and dating of lagoonal sediments imply a

phase of more prolific coral growth and sediment pro-

duction in the mid-Holocene (Kennedy and Woodroffe,

2000). This coincides with a time at which higher

temperatures have been indicated by isotopic proxies

in corals on the Great Barrier Reef (Gagan et al., 1998),

and in worm tubes in eastern Australia (Baker et al.,

2001). It remains unproven, however, whether sea-sur-

face temperatures were higher at Lord Howe Island. An

alternative explanation is that there was a broad ex-

panse of suitable substrate available as the sea rose

across the bench bevelled into calcarenites that had

been deposited during a previous interstadial. Dates

on mid-Holocene robust branching corals, of around

6200 years BP (including some clearly in their position

of growth at the base of a vibrocore [LV8] in North

Bay where upright branches were recovered in the core

catcher) indicate that the early phase of lagoonal sedi-

mentation was characterised by widespread coral growth

across the eroded pre-Holocene calcarenite surface. We

interpret that the seaward reef grew up to the modern

reef crest rapidly, although we have no direct evidence

for this from Lord Howe Island. Radiocarbon ages on

the reef crest on Middleton and Elizabeth Reefs to the

north (29830VS and 308S, respectively) indicate that

these reefs caught up to sea level shortly after it stabi-

lised around 6000 years ago (Woodroffe et al., 2004).

The newly formed reef crest inferred on Lord Howe

Island impounded the lagoon which then rapidly filled

with low-energy gravelly mud sediments.

The much sparser coral growth during the past 4000

years may be a response to less favourable climatic

conditions since the mid-Holocene optimum, but it is

more likely that the depositional environments have

undergone intrinsic changes with a decrease in the

area of habitat suitable for coral growth. Such a de-

crease is expected, first because vertical reef growth

and sediment infill will have occupied the available

accommodation space, and second as a result of slight

sea-level fall. The sedimentological response was a

switch in the locus of deposition from the lagoon to

the foreshore, although overall sedimentation rates also

significantly reduced. It has been recognised elsewhere

that as reef environments mature under decelerating or

stable sea level, they often get dshot in the backT by

their own lagoons (Neumann and Macintyre, 1985).

The contrast between reef growth before 4000 years

BP, and the lesser extent of coral production thereafter

is indicated by the histogram of dates (Fig. 5).

It is necessary to interpret Fig. 5 with some caution

because the material has not been randomly selected.

The frequency of ages around 5000 years BP is accen-

tuated through targeted dating (described in Kennedy

and Woodroffe, 2000), such as the clast-matrix com-

parison involving dating of coral, encrusting algae and

sand grains comprising the sediment within which the

clasts are deposited (Kennedy and Woodroffe, 2004).

Nevertheless, the frequency of ages reinforces the vol-

umetric impression that can be gained from the iso-

chrons in Fig. 4. The pattern of dates, and the growth

history of the reef that can be inferred from it, is directly

comparable with radiocarbon dating results and reef

formation across the Great Barrier Reef throughout

which climatic conditions are favourable for coral

growth (Hopley, 1982). Indeed, a similar reef-growth

history has been demonstrated for the majority of reefs

in the Indo-Pacific, and the greater volume of reef

material associated with mid-Holocene compared with

late Holocene reflects prolific reef growth as reefs track

(keep up with) or catch up with sea level, and the less

extensive suitable shallow-water habitat since sea level

has stabilised and fallen slightly (Montaggioni, 2005).

In comparison with the Holocene, the extent of Last

Interglacial reef remains unclear; any reef is likely to

have undergone dissolution and physical erosion, but

the apparent absence of a structure comparable in size

with the modern reef is surprising. Still more unexpect-

ed is the massive submerged reef that occurs on mid-

shelf. Submerged reefs are common around the world,

and water depths of 30–40 m represent a mode at which

there are many reefs worldwide (Vecsei, 2003). There

are extensive areas of submerged reefs at similar depths

in the Indian Ocean; for example, much of the Chagos

Archipelago has a mean depth of around 30 m (Stod-

dart, 1969). Submerged reefs at similar depths are

found between Cairns and Townsville and occur

south as far as the Pompey Reefs in the southern part

of the Great Barrier Reef (Hopley, in press). Recently,

hitherto uncharted submerged coral reefs at this water

depth have been reported from the Gulf of Carpentaria

(Harris et al., 2004). However, the occurrence of the

Lord Howe shelf reefs at the latitudinal limit to reef

growth is remarkable; reefs would be expected to be

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 233

very poorly developed or absent as a result of the cooler

water temperatures anticipated when sea level was

lower, a central argument of Daly’s marginal seas.

Despite the large reef feature on the shelf, coral is

not a major component of the modern shelf sediments.

It is most abundant on the inner parts of both shelves,

comprising generally less than 5% of the sediment. On

the Lord Howe shelf, the highest proportions of coral

grains occurred around the fossil reef. Coral gravel

tends to be highly abraded and sub- to well-rounded,

often with a few millimetres thick coating of coralline

algae (Kennedy et al., 2002). The pattern of eustatic

sea-level fluctuations over the most recent glacial cycle

is shown in Fig. 9, on the basis of which it can be seen

that the postglacial sea transgressed the shelf 12,000

years ago. The reef occurs at a depth that would have

been favourable for reef growth during marine oxygen-

isotope stages 1, 3 and 5, but such an extensive reef

seems out of keeping with the size of reefs of these ages

elsewhere in the world.

Some early Holocene coral growth on the Lord

Howe mid-shelf feature is indicated by deposits of

branching corals on the shelf close to the fossil reef

and in particular a radiocarbon age of 8000–9000 years

BP on branching coral from G19 (Kennedy et al.,

2002). It is possible that a phase of luxuriant coral

growth occurred on the shelf, terminated either by

temperature change or by the drowning of these tropical

carbonate environments and their replacement by more

temperate communities as water depths increased and

the reef dgave upT. The submerged fossil reef on the

Lord Howe shelf indicates much more prolific coral

growth was possible than during the Holocene. It has

recently been realised that there is a complex aggrega-

tion of reefs recording a lower sea level in oxygen

isotope stage 5a and further reefs associated with an

early Holocene phase at a similar high latitude in

Florida (Toscano and Lundberg, 1998, 1999).

Fig. 9. Sea-level curve for Late Quaternary (based on Chappell and Shackl

known to have occurred close to present sea level (heavy stipple) and periods

follow Brooke et al. (2003b). Marine oxygen isotope stages are indicated. T

One possible scenario for the development of this

submerged fossil reef, linking it with the broader pat-

tern of carbonate production over a sea-level cycle, is

shown in Fig. 10. In the first stage under conditions of

high sea level (particularly the Last Interglacial, Fig.

10a), carbonate began to accumulate on the shelf and

some was blown into dunes that develop against the

steep margins of the island. At the peak of the intergla-

cial, sea-surface temperatures were warm enough for

reef formation (Fig. 10b). During the subsequent inter-

stadials, the carbonate sediments continued to form

eolianite deposits across the island and shelf (Fig.

10c). During the ensuing glacial lowstand, there was

extensive erosion and dissolution (especially of the

older and best lithified limestones, Fig. 10d). As the

subsequent postglacial sea level rose, it enabled reef

growth over the eroded remnants of calcarenite on the

shelf with a reef developing that was then drowned in

the final stages of sea-level rise (Fig. 10e). This inter-

pretation would imply that the mid-shelf reef is an early

Holocene veneer over older deposits (probably includ-

ing oxygen isotope stage 5 dunes), but that it gave up in

those water depths at around the time that the founda-

tions of the modern reef were forming. However, this

interpretation remains largely speculative and the origin

and age of the fossil reef will remain problematic until

it is possible to recover samples from within it.

It is also possible that the core of the mid-shelf reef

may not be Late Quaternary in age. There are several

alternative explanations by which the foundation of the

fossil reef might be older than marine oxygen-isotope

stage 5e. First, the island and shelf morphology super-

ficially resembles that of dmakateaT islands in the Pacificwhere a similar rim of reef limestone surrounds a vol-

canic interior (although on makatea islands the reef

limestone is above sea level, not below it). The makatea

islands have a central core that comprises rounded and

weathered volcanic slopes (being older volcanics and

eton, 1986; Chappell et al., 1996), showing times at which reefs are

when calcarenite units were deposited (light stipple). Formation names

he grey band shows the depth range of the mid-shelf reef.

Fig. 10. A possible scenario of carbonate deposition on the shelf around Lord Howe Island (see text for explanation).

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237234

contrasting with the precipitous slopes of much of Lord

Howe Island), with a rim of Tertiary limestone around

their margin (Stoddart et al., 1990). The steep inner

margin to the Lord Howe mid-shelf reef is one morpho-

logical feature that lends support to this idea. A similar

inner cliff was initially interpreted as indicating that the

reef limestones on makatea islands preserved a barrier-

reef morphology (Chubb, 1927). It has since been

shown that the steep inner cliff actually results from

karst erosion and parallel retreat of the inner margin of a

former fringing reef as a result of dissolution by acidic

waters from the volcanic slopes (Hoffmeister and Ladd,

1935; Stoddart et al., 1990). However, for the mid-shelf

reef to be formed in this manner, it would have been

necessary for prior planation of the Lord Howe volcanic

edifice to have occurred soon after the volcano erupted,

6–7 Ma, presumably with marine abrasion occurring at a

decelerating rate as the shelf widened. It seems highly

unlikely that the reef could be Late Tertiary in age, as the

island would have been considerably south of its present

location at that time (100–300 km), and this would

require that reefal seas were also much more extensive

then than they are now.

Alternatively, the reef might have formed during a

Middle Pleistocene sea-level highstand prior to the Last

Interglacial. Evidence for higher sea level, or more pro-

lific reef growth during earlier interglaciations has re-

cently been described from both Atlantic and Pacific reef

provinces (Hearty et al., 1999; Stirling et al., 2001), and

it is possible that this reef formed around Lord Howe

Island during one of these periods. The island would

have been south of its present location at that time, so this

explanation also implies considerable extension of reef-

forming seas poleward of the present limit.

The absence of reef or fossil reef on Balls Pyramid is a

further complication requiring explanation. If such a

large reef could develop on the Lord Howe platform,

then it is difficult to see why conditions less than 25 km

further south should inhibit reef development. It could be

that any reef formed on the smaller platform around Balls

Pyramid has been eroded away, either through dissolu-

tion or as a result of the effective rates of cliff retreat

C.D. Woodroffe et al. / Global and Planetary Change 49 (2005) 222–237 235

observed around modern shorelines (Dickson et al.,

2004). Rapid dissolution of limestone can be demon-

strated. For example, at North Bay on Lord Howe Island,

a bedrock valley was infilled with Middle Pleistocene

eolianite but most has been removed by dissolution, and

that which remains contains caves with stalactites

(Brooke, 1999). Rapid degradation of the landscape

can be anticipated during stages of low sea level.

Our studies suggest that the extent of coral reef has

varied substantially around Lord Howe Island during the

Late Quaternary. Although inferences about climate

change are possible, the extent of reef development is

not dependent solely on water temperature. If sea-sur-

face temperature was to increase in the future, we would

not anticipate that coral cover would extend across the

lagoon at Lord Howe Island as it did prior to lagoonal

infill, because suitable substrate and water depth has

altered as a result of sediment production and the filling

of accommodation space. The development of pre-Ho-

locene reefs appears to have been considerably more

complex than previously anticipated. The chronology of

reef development is similar to that across the Great

Barrier Reef and much of the Indo-Pacific reef province

and is likely to reflect reef response to patterns and rates

of sea-level change rather than temperature alone. Last

Interglacial reefs can be inferred, although their extent is

not well understood. However, the history of the im-

pressive mid-shelf reef remains enigmatic, and the tim-

ing of its formation is uncertain and can only be

discovered with further subsurface study.

6. Conclusions

In the face of concern about the impact of global

warming, particularly in relation to coral bleaching and

the threat of thermal stress on tropical reefs, there has

been a reawakening of interest in marginal reefs. Lord

Howe Island is a key location at, or close to, the

southernmost limit of reef growth in the southwest

Pacific; it has a modern fringing reef that contains

flourishing coral communities. Detailed coring and dat-

ing of the reef on the western side of the island shows

that the extent of coral has changed considerably. There

was prolific coral 6000–5000 years ago, but subsequent

reduction in the extent of coral seems likely based on

the lagoonal stratigraphy and the lesser frequency of

coral of late Holocene age in cores. This is a trend that

is seen throughout the region, and it is interpreted as an

intrinsic change as a result of progressive infill of the

lagoon and consequent changing habitat conditions and

need not have been a function of any decline in sea-

surface temperature or other regional climatic control.

U-series ages on coral clasts indicate a Last Inter-

glacial beach at Neds Beach and other locations

around the island. Older calcarenite deposits, associ-

ated with the Searles Point Formation, contain evi-

dence of at least one previous phase of high sea level

during which corals occurred at this marginal location.

However, neither the modern reef nor the minor Last

Interglacial reef limestones compare with the enor-

mous submerged structure, in places 2–3 km wide,

that occurs in 30–40 m water depth. Whereas there is

increasing evidence for submerged reefs in tropical

areas, this submerged reef at the limits to reef growth

implies that there have been periods of more extensive

reef development in the past 6 million years than

occur now. At least some of that reef growth seems

to have occurred in the Late Quaternary, but the entire

history of its formation will require further subsurface

investigation. With the wider recognition of sub-

merged reefs in other parts of the world, this extensive

submerged reef associated with the latitudinal limit to

reef growth poses a series of challenging and unan-

swered questions.

Acknowledgments

This study was funded by the Australian Research

Council. Research was conducted with permission and

support from the Lord Howe Island Board and the Lord

Howe Island Marine Park Authority. The authors thank

the captain and crew of R.V. Franklin for their assis-

tance during cruise 12/98 and the crew of Advance II

for their skill during fieldwork in 2001. Further bathy-

metric data was provided under license from the Hy-

drographic Office of the Royal Australian Navy. Vicki

Harriott (James Cook University), David Mitchell (Uni-

versity of Sydney), Stewart Fallon, John Marshall,

Damien Kelleher and Eugene Wallensky (Australian

National University), Brian Jones, Colin Murray-Wal-

lace, Ted Bryant and John de Carli (University of

Wollongong) and Dean Hiscox (Lord Howe Island

Board) provided valuable field assistance. David Hop-

ley and Adam Vecsei are thanked for insightful review

comments on the regional and global significance of

submerged reefs.

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