Milankovitch orbital forcing theory has been used to assign time scales to many paleoclimate records. However, the validity of this theory remains uncertain, and independent sea-level chronologies used to test its applicability have been restricted largely to the past approximately 135,000 years. Here, we report U-series ages for coral reefs formed on Henderson Island during sea-level high-stands occurring at approximately 630,000 and approximately 330,000 years ago. These data are consistent with the hypothesis that interglacial climates are forced by Northern Hemisphere summer solar insolation centered at 65 degrees N latitude, as predicted by Milankovitch theory.

Benthic communities were assessed and 22 environmental variables were monitored at seven leeward localities (L1ŒL7) in the Whitsunday Islands, Queensland, Australia. L1 was near the Proserpine and O’Connell river mouths and L7 ~80 km north of the river mouths. Distinct physico Œchemical and biological gradients were evident. Sparse scleractinian coral communities, dominated by faviids, Montipora spp. and encrusting Porites colonies, were present at L1, L2 and L3, whereas diverse reef-building communities, dominated by Acropora spp., were more common at and beyond L4. The number of coral recruits (age <6 months) did not differ significantly among localities, suggesting that coral recruitment was near random and that the environment shapes the adult community from those recruits. The study demonstrates strong negative relationships between chlorophyll a and the following: percentage coral cover, coral species richness and coral abundance. The reef-building capacities of the coral communities and the extent of Holocene reef development were inconsistent at L2 and L3, which is interpreted as a sign of anthropogenic effects.

No abstract

Henderson Island is an emergent limestone island. It rises to 33.5 m above modern sea-level (MSL) from a sea-floor depth of about 3,500 m and conforms to the pattern of an elevated atoll, although no field evidence was found pertaining to the pre-atoll volcanic history of the island. The emergence of this coral atoll can be explained by lithospheric flexure processes subsequent to the emplacement of the Pitcairn Island volcano, built by two phases of volcanism (estimated at 855 and 540 kyr by K-Ar dating). U-Th dates obtained from Henderson Island indicate that the majority of the presently visible fossil corals have an age between 404-225 kyr. Henderson Island first became emergent above MSL when sea level dropped subsequent to 380 kyr, as the period 440-380 is thought to have been characterized by sea level at least several metres above MSL in the Central Pacific. As a result Henderson Island would have become subaerially exposed from 380 kyr onwards. The uplift rate of Henderson Island is approximately 0.093-0.10 mm/yr indicating that flexure-induced uplift component above MSL commenced between 360-335 kyr, 500 kyr after the initial, and 190 kyr after the last phase of construction of the Pitcairn volcano. The time-lag following the loading of the Pitcairn volcano and the present continuation of uplift on Henderson support the theory that the lithosphere behaves in a visco-elastic manner. Field relations and U-Th dates indicate three main periods of reef development: (1) A prolific reef-building period (440-380 kyr wnit 71 and 330-300 kyr [Units 2 6' 31) dominated by large, stout branching coral colonies; (2) A shorter period of reef growth j285-275 kyr [Unit 41) dominated by well-formed large in situ coral colonies and Tridacna maxima; (3) A less prolific period of reef growth (230-215 kyr [Unit 51) dominated by platy corals enveloping the previous lithologies below 19.6 m. The absence of dateable material from the last interglacial, oxygen isotope substage ii.5 (=i;e) at Henderson Island is in marked contrast to the Late Pleistocene reefs in the southern Cook Islands where reefs of substage 5.5 age skirt older carbonate complexes. C l!)!J,i The Linnean Society of London ADDITIONAL KEY WORDS:-carbonate stratigraphylithospheric flexurefossil atollcoral reef -Pleistocenesea-level -U-series dating.