Alexander W. Tudhope

Patterns and possible environmental controls of skeletogenesis of Porites lutea, South Thailand

Eleven fringing reef sites were investigated over a distance of about 50 km in the Phuket Region. There is a wide range in exposure to wave energy, and also water turbidity across the area. Annual increments of growth of shallow-water reef-front colonies of Porites lutea were calculated for the period November 1984 November 1986 using seasonal fluorescent banding (revealed with ultraviolet light) and Alizarin staining. Measurement of linear extension rate, skeletal bulk density, calcification rate, polyp numbers per unit area and colony surface morphology were made and compared. Linear extension rate and skeletal bulk density are inversely related within and between reef sites. Linear extension rate decreases and bulk density increases along a gradient of increasing hydraulic energy of the setting. Calcification (the product of linear extension rate and bulk density), although varying slightly from site to site, does not appear to relate to any obvious environmental inshore-offshore gradient. Skeletal bulk density is the most sensitive discriminator between reef sites, and we suggest that hydraulic energy of the setting is the main control on these spatial variations in skeletogenesis.

Recent changes in climate in the far western equatorial Pacific and their relationship to the Southern Oscillation; oxygen isotope records from massive corals, Papua New Guinea

The western equatorial Pacific warm pool is believed to be the source region of a substantial proportion of the worlds inter-annual climate variability, including the globally significant El Nin˜o-Southern Oscillation. Here new data are presented on changes in the climate of this region over the past 70 yr, based on analysis of the stable oxygen isotopic composition of annually banded, massive corals living on the north coast of Papua New Guinea. In this area, the coincidence of abundant and isotopically light rainfall with small seasonal and inter-annual changes in temperature means that the coral skeletal δ18O record may be reliably interpreted in terms of rainfall variation. Since the abundance of rainfall is governed by the intensity of deep atmospheric convection, the coral records provide regionally significant, and regionally reproducible, climatic indices. In particular, the coral records indicate larger changes in the degree of coupling of the climate of the region with the Southern Oscillation than have hitherto been described. These changes in coupling, which accompany shifts in the dominant periodicities of inter-annual climate variation, are indicative of rapid and large-scale reorganisation of ocean-atmosphere interactions. The data indicate that from the 1920s to 1950s the western equatorial Pacific was less important in modulating Pacific and global inter-annual climatic variability than it has appeared to be subsequently. The reproducibility of the coral climate records, combined with the presence on the north coast of Papua New Guinea of extensive and well preserved raised reef terraces, leads to the potential for extending understanding of climatic variability in this globally important region back into the late Quaternary.

Excitation-emission-matrix fluorescence spectroscopy applied to humic acid bands in coral reefs

An investigation of the three-dimensional excitation-emission-matrix (EEM) fluorescence spectra of humic acids and other organic matter from corals and other sources is reported, the results of which have implications for the use of corals as paleoenvironmental indicators. Four characteristic excitation/emission peaks were identified in the samples studied, at 310430, 340450, 390490 and 280(320–350) nm, the last of these attributed to protein fluorescence. The positions of these peaks were essentially invariant between different samples, including coral extracts, solid corals, seawater, and commercially-available humic acids, although their relative intensities showed considerable variation. The use of the EEM technique in the present work has demonstrated that changes in sample concentration can considerably change the intensity distribution of the fluorescence spectra, particularly if strongly absorbant, non-fluorescent species are present. Comparison of the EEM spectra of organic matter extracted from bright and dull coral bands showed the spectroscopic structure of the fluorescence emitted to be essentially identical for both bands; the emissions appear to differ only in their absolute intensities.

Factors influencing the stable carbon and oxygen isotopic composition of Porites lutea coral skeletons from Phuket, South Thailand

We determined the δ18O and δ13C composition of the same fixed growth increment in severalPorites lutea coral skeletons from Phuket, South Thailand. Skeletal growth rate and δ18O are inversely related. We explain this in terms of McConnaugheys kinetic isotopic disequilibria model. Annual trends in δ18O cannot be solely explained by observed variations in seawater temperature or salinity and may also reflect seasonal variations in calcification rate. Coral tissue chlorophylla content and δ13C of the underlying 1 mm of skeleton are positively related, suggesting that algal modification of the dissolved inorganic carbonate pool is the main control on skeletal δ13C. However, in corals that bleached during a period of exceptionally high seawater temperatures in the summer of 1991, δ13C of the outer 1 mm of skeleton and skeletal growth rate (over 9 months up to and including the bleaching event) are inversely related. Seasonal variations in °13C may reflect variations in calcification rate, zooxanthellae photosynthesis or in seawater δ13C composition. Bleached corals had reduced calcification over the 9-month period up to and including the bleaching event and over the event they deposited carbonate enriched in13C and18O compared with unaffected corals. However, calcification during the event was limited and insufficient material was deposited to influence significantly the isotopic signature of the larger seasonal profile samples. In profile, overall decreases in δ18O and δ13C were observed, supporting evidence that positive temperature anomalies caused the bleaching event and reflecting the loss of zooxanthellae photosynthesis.

Monsoon climate and Arabian Sea coastal upwelling recorded in massive corals from southern Oman

Corals living in the coastal waters of southern Oman experience the influence of the seasonally reversing Asian monsoon system. The objective of the research reported here is to assess the potential for using the skeletal chemistry of these corals to investigate past variability in the monsoon climate. To this end, 20-year long, monthly resolution geochemical records are presented for cores from two massive Porites corals, located 20 km apart near Marbat on the Arabian Sea coast of southern Oman. We consider four aspects of skeletal chemistry: oxygen and carbon isotopic composition, barium content and the nature and occurrence of annual fluorescent bands within the coral skeletons. Coral skeletal 180 documents variations in sea surface temperature which have regional and basin-wide significance. In particular, the 80 of coral skeleton precipitated during the period of the NE monsoon is strongly correlated with annual rainfall anomalies in India, whilst that precipitated during the period of the SW monsoon appears to provide information on variability in the strength of coastal upwelling. The stable carbon isotope composition and barium content of these particular corals display strong annual cycles,

Tropical-North Pacific Climate Linkages over the Past Four Centuries*

Analyses of instrumental data demonstrate robust linkages between decadal-scale North Pacific and tropical Indo-Pacific climatic variability. These linkages encompass common regime shifts, including the noteworthy 1976 transition in Pacific climate. However, information on Pacific decadal variability and the tropical high-latitude climate connection is limited prior to the twentieth century. Herein tree-ring analysis is employed to extend the understanding of North Pacific climatic variability and related tropical linkages over the past four centuries. To this end, a tree-ring reconstruction of the December–May North Pacific index (NPI)—an index of the atmospheric circulation related to the Aleutian low pressure cell—is presented (1600–1983). The NPI reconstruction shows evidence for the three regime shifts seen in the instrumental NPI data, and for seven events in prior centuries. It correlates significantly with both instrumental tropical climate indices and a coral-based reconstruction of an optimal tropical Indo-Pacific climate index, supporting evidence for a tropical–North Pacific link extending as far west as the western Indian Ocean. The coral-based reconstruction (1781–1993) shows the twentieth-century regime shifts evident in the instrumental NPI and instrumental tropical Indo-Pacific climate index, and three previous shifts. Changes in the strength of correlation between the reconstructions over time, and the different identified shifts in both series prior to the twentieth century, suggest a varying tropical influence on North Pacific climate, with greater influence in the twentieth century. One likely mechanism is the low-frequency variability of the El Nino–Southern Oscillation (ENSO) and its varying impact on Indo-Pacific climate.

The Effects of Callianassa Bioturbation on the Preservation of Carbonate Grains in Davies Reef Lagoon, Great Barrier Reef, Australia

ABSTRACT Coring, air-life excavation, and tracer-sediment experiments in the lagoon-floor sediments of Davies Reef revealed that callianassid shrimp had produced, during their feeding, a 5-60-cm-thick surface layer of moderately to poorly sorted gravel-free sediment, above very poorly sorted gravelrich sediment. The subsurface gravel is epilithic in origin close to reef framework but consists predominantly of infaunal molluscs through most of the lagoon. These infaunal skeletons tend to be buried rapidly in a well-preserved state escaping the microboring that attacks all grains on the surface. Two geologically significant points emerge; first, the surface layer will not get preserved in its present form and any analyses of surface samples can give a false impression of the accumulating sedimen pile and the prevailing hydraulic regime, and second, the combination of continuous fine-sediment recycling and surface microboring can lead to a bias in the fossil-reefal limestone record, with epilithic organisms seemingly subordinate to infaunal molluscs.

SEDIMENTARY ENVIRONMENTS OF THE CENTRAL REGION OF THE GREAT BARRIER-REEF OF AUSTRALIA

The sediments and calcareous organisms on the outer reefal shelf of the Central Region of the Great Barrier Reef were collected and observed by SCUBA diving and research vessel techniques (including underwater television) to understand the production and processes of deposition of the sediment. The carbonate grains are mainly sand and gravel size and solely of skeletal origin. Over the whole area the major CaCO3 producers, in order of decreasing importance are: benthic foraminiferans (chiefly Operculina, Amphistegina, Marginopora, Alveolinella and Cycloclypeus), the calcareous green alga Halimeda, molluscs and corals. Coral abundance is high only close to reefs and submerged rocky substrates. Benthic foraminiferal sands dominate the inter-reef areas i.e. the bulk of the shelf, and Halimeda gravels form an outer shelf band between 60 and 100 m depths. Seven distinct facies are recognised after quantitative analyses of the sediments. These are: A. Shelf edge slope (>120 m depth); B. Shelf edge (with rocky outcrops); C. Outer shelf with high Halimeda (>40%); D. Inter-reef I; E. Inter-reef II ( 100 m depth but >2% pelagics); F. Lee-ward reef talus wedge (<2 km from sea level reefs); G. Lagoonal.

Mid-Holocene ENSO: Issues in quantitative model-proxy data comparisons

(1) Evaluation of climate model simulations using observed data contributes to the assessment of confidence in model predictions of future climate change. The mid-Holocene represents an opportunity to evaluate model simulations of El Nino-Southern Oscillation (ENSO) in comparison with coral proxy evidence of reduced ENSO amplitude. Quantitative comparisons between coral records and model output have been limited by (1) the use of different measures of ENSO amplitude, (2) possible sampling of natural variability in short records, and (3) uncertainty about the stationarity of the relationship between central Pacific sea surface temperature (SST) variability and ENSO signals at the coral site. We examine these issues using modern and fossil coral records from the western Pacific and model simulations of preindustrial and mid-Holocene climate. As a measure of ENSO amplitude, the standard deviation is found to be preferable to event frequency or size as event-based measures are highly dependent on the choice of threshold and may be unreliable for a small number of events. Model ENSO amplitude is found to be strongly dependent on the choice of averaging period, with calendar year averages smoothing the seasonal ENSO signal. A relatively robust relationship between SST variability in the NINO3.4 region and the ENSO SST and precipitation anomalies archived in corals is demonstrated for the instrumental period and for a set of model simulations. Remaining uncertainty about changes in ENSO teleconnections under paleoclimate conditions implies the need for additional proxy records from ENSO- sensitive regions before quantitative reconstructions of ENSO amplitude can be used to evaluate model sensitivity.

RHODOLITHS AND CORALLITHS OF MURI LAGOON, RAROTONGA, COOK-ISLANDS

Free-living massive and branching spheroidal growths (about 5 cm diameter) of calcareous red algae (rhodoliths) and corals (coralliths) occur in abundance on the sea bed of shallow Muri Lagoon on Rarotongas reef flat. The rhodoliths are composed of one or more species of Neogoniolithon, Lithophyllum, Tenarea, and Porolithon; the coralliths are Pavona varians (Verrill) and Porites lutea (Milne-Edwards and Haime). Muri Lagoon is the only area on Rarotongas reef flat that is sheltered by reef islands from ocean waves. The tidal currents, which are predominantly unidirectional in Muri Lagoon, are concentrated by the reef islands into channels through which sand and gravel sediment is regularly transported. However, these prevailing currents do not normally roll the rhodoliths and coralliths. The results of field experiments on the pick-up velocity of the various types of spheroidal structure, combined with observations on growth histories of massive coralliths as revealed by the non-concentric nature of skeletal density banding, indicate that the rhodoliths and coralliths may remain static for periods up to several months yet maintain a complete envelope of living tissue. This downward survival may depend on the strong currents. Not only is the water flushing through the upper millimetre or so of the sediment substrate, but it is also capable of moving the sand and gravel grains which laterally support the rhodoliths and coralliths so that no one point of a spheroidal structure is in direct contact with the substrate for a fatal length of time. Massive rhodoliths have a high preservation potential as discrete spheroidal structures; in contrast, branching rhodoliths and coralliths are prone to fragmentation, and massive coralliths grow into stable microatolls. We conclude that a similar assemblage of rhodoliths, coralliths and microatolls in the fossil record may be indicative of the former existence of contemporary reef flat islands.