Sea-surface temperature from coral skeletal strontium/calcium ratios

Interdecadal variation in the extent of South Pacific tropical waters during the Younger Dryas event

During the Younger Dryas event, about 12,000 years ago, the Northern Hemisphere cooled by between 2 and 10 degrees C (refs 1, 2) whereas East Antarctica experienced warming. But the spatial signature of the event in the southern mid-latitudes and tropics is less well known, as records are sparse and inconclusive. Here we present high-resolution analyses of skeletal Sr/Ca and 18O/16O ratios for a giant fossil Diploastrea heliopora coral that was preserved in growth position on the raised reef terraces of Espiritu Santo Island, Vanuatu, in the southwestern tropical Pacific Ocean. Our data indicate that sea surface temperatures in Vanuatu were on average 4.5 +/- 1.3 degrees C cooler during the Younger Dryas event than today, with a significant interdecadal modulation. The amplified annual cycle of sea surface temperatures, relative to today, indicates that cooling was caused by the compression of tropical waters towards the Equator. The positive correlation in our record between the oxygen isotope ratios of sea water and sea surface temperatures suggests that the South Pacific convergence zone, which brings 18O-depleted precipitation to the area today, was not active during the Younger Dryas period.

Inductively coupled plasma atomic emission spectrometry: an efficient tool for precise determination of Mg/Ca and Sr/Ca ratios in biocarbonates

A method is developed to precisely determine Mg/Ca and Sr/Ca ratios in biocarbonates by inductively coupled plasma atomic emission spectrometry (ICP-AES). This method precision (RSD%) is 0.52% for Mg/Ca and 0.28% for Sr/Ca, respectively. The precision suggests that ICP-AES is satisfactory for supplying good quality Mg/Ca and Sr/Ca data of biocarbonates for paleo-reconstruction. This ICP-AES technique was applied to 51 continuous coral subsamples, and the results show annually periodical variations in coral Mg/Ca and Sr/Ca ratios, which are consistent with previous findings.

The effect of algal symbionts on the accuracy of Sr/Ca paleotemperatures from coral

The strontium-to-calcium ratio (Sr/Ca) of reef coral skeleton is commonly used as a paleothermometer to estimate sea surface temperatures (SSTs) at crucial times in Earth's climate history. However, these estimates are disputed, because uptake of Sr into coral skeleton is thought to be affected by algal symbionts (zooxanthellae) living in the host tissue. Here, we show that significant distortion of the Sr/Ca temperature record in coral skeleton occurs in the presence of algal symbionts. Seasonally resolved Sr/Ca in coral without symbionts reflects local SSTs with a temperature sensitivity equivalent to that of laboratory aragonite precipitated at equilibrium and the nighttime skeletal deposits of symbiotic reef corals. However, up to 65% of the Sr/Ca variability in symbiotic skeleton is related to symbiont activity and does not reflect water temperature.

The ecology of extinction: molluscan feeding and faunal turnover in the Caribbean Neogene

Molluscan faunal turnover in the Plio-Pleistocene of the tropical western Atlantic has been attributed to drops in temperature or primary productivity, but these competing hypotheses have not been assessed ecologically. To test these alternatives, we compiled data on changing molluscan life habits and trophic composition over 12 million years derived from 463 newly made collections from the southwestern Caribbean. Shelf ecosystems have altered markedly in trophic structure since the Late Pliocene. Predatory gastropods and suspension-feeding bivalves declined significantly in abundance, but not in diversity, and reef-dwellers became common. By contrast, all other ecological life habits remained remarkably stable. Food-web changes strongly support the hypothesis that declining regional nutrient supply had an increasing impact on regional macroecology, culminating in a faunal turnover.

Occurrence, distribution, and localisation of metals in cnidarians

The Cnidaria are simple organisms that have remarkable physiological features susceptible to microscopic investigation. As a group they produce cnidae, the most complex intracellular organelles known, form symbioses with a range of unicellular algae, contain mucocytes that account for a very substantial fraction of their body mass, and form complex skeletal structures of calcium carbonate. This review summarises contributions dealing with the distribution and localisation of metals of physiological and pathological importance within soft tissues and skeletons. Whilst there have been detailed studies of microscale metal distribution, using X-ray microanalysis, in the stinging organelles or cnidocysts and in mucocytes, other cells such as symbiotic algae and the epithelial cells have received little attention. In the skeleton-producing scleractinian corals X-ray microanalysis has provided tenuous, but persistent, evidence of Ca associated with intracellular vesicles or granules in the skeletogenic epithelium, even though the investigations were technically limited. These observations may be germane to the intriguing and intransigent problem of the mechanism of coral calcification. Metal localisation in coral skeleton at the resolution of annual growth rings has been concerned with the validity of Sr/Ca and Mg/Ca ratios as thermometers for paleoclimatic studies. It is not clear whether these ratios are influenced primarily by environmental or biological parameters. Microscale analyses by X-ray microanalysis and ion microprobe indicate a much greater variability of metal ratios which suggests biological control of metal deposition. New data are provided on the elemental composition, measured by X-ray microanalysis, of cells and cell compartments in the coral Galaxea fascicularis and zooxanthellae in the anemone Aiptasia sp. New information is also presented on changing Ca/Sr ratios at the skeletal interface in Galaxea fascicularis.

Decadal sea surface temperature variability in the subtropical South Pacific from 1726 to 1997 A.D

We present a 271-year record of Sr/Ca variability in a coral from Rarotonga in the South Pacific gyre. Calibration with monthly sea surface temperature (SST) from satellite and ship measurements made in a grid measuring 1 degrees by 1 degrees over the period from 1981 to 1997 indicates that this Sr/Ca record is an excellent proxy for SST. Comparison with SST from ship measurements made since 1950 in a grid measuring 5 degrees by 5 degrees also shows that the Sr/Ca data accurately record decadal changes in SST. The entire Sr/Ca record back to 1726 shows a distinct pattern of decadal variability, with repeated decadal and interdecadal SST regime shifts greater than 0. 75 degrees C. Comparison with decadal climate variability in the North Pacific, as represented by the Pacific Decadal Oscillation index (1900-1997), indicates that several of the largest decadal-scale SST variations at Rarotonga are coherent with SST regime shifts in the North Pacific. This hemispheric symmetry suggests that tropical forcing may be an important factor in at least some of the decadal variability observed in the Pacific Ocean.

Cool glacial temperatures and changes in moisture source recorded in oman groundwaters

Concentrations of atmospheric noble gases (neon, argon, krypton, and xenon) dissolved in groundwaters from northern Oman indicate that the average ground temperature during the Late Pleistocene (15,000 to 24,000 years before present) was 6.5 degrees +/- 0.6 degrees C lower than that of today. Stable oxygen and hydrogen isotopic groundwater data show that the origin of atmospheric water vapor changed from a primarily southern, Indian Ocean source during the Late Pleistocene to a dominantly northern, Mediterranean source today. The reduced northern water vapor source is consistent with a drier Last Glacial Maximum through much of northern Africa and Arabia.

El Niño-Southern Oscillation mass mortalities of reef corals: a model of high temperature marine extinctions?

Protracted high sea temperature anomalies accompanying El Niño-Southern Oscillation (ENSO) events have caused reef-building coral bleaching (loss of zooxanthellae) and mortality in all major coral reef biogeographic regions during the past two decades. Coral reef degradation in the eastern tropical Pacific has resulted from reductions in live coral cover, declines in coral species population abundances, local to regional scale extinctions, disruption of predator/prey spatial relations and relative abundances, bioerosion of reef frameworks, and low coral recruitment. None of the coral species that have suffered regional extinctions has reappeared after 15 years. Intense external and internal bioerosion by fishes, echinoids, lithophagine bivalves and clionid sponges has occurred on reefs affected by the 1982/83 El Niño coral bleaching event, and 1000–5000 year old reef framework accumulations in the Galápagos Islands have been completely eroded and reduced to gravel and sand. Because tropical zooxanthellate reef species are more vulnerable to rising (2–3°C) than falling (8–10°C) temperatures, greenhouse conditions may be more critical in limiting reef growth than icehouse conditions. ENSO warming episodes elicit physiological stress responses resulting in widespread mass coral mortality, leaving scant traces relating to causation. Signals that may help to identify past ENSO disturbances are: (a) temperature-related oxygen isotopic signatures, (b) skeletal stress bands and growth discontinuities, (c) coral debris in beach storm deposits, (d) increases in coral clastics resulting from intensified bioerosion and (e) the skeletal elements of bioeroders. Because this disturbance is the most pronounced and widespread of any known natural perturbation, and may increase markedly in scope with projected global warming predictions, it is considered a likely agent of future and possibly some ancient bioevents.

Oceanic minerals: their origin, nature of their environment, and significance

The chemical and isotopic compositions of oceanic biogenic and authigenic minerals contain invaluable information on the evolution of seawater, hence on the history of interaction between tectonics, climate, ocean circulation, and the evolution of life. Important advances and greater understanding of (a) key minor and trace element cycles with various residence times, (b) isotopic sources and sinks and fractionation behaviors, and (c) potential diagenetic problems, as well as developments in high-precision instrumentation, recently have been achieved. These advances provided new compelling evidence that neither gradualism nor uniformitarianism can explain many of the new important discoveries obtained from the chemistry and isotopic compositions of oceanic minerals. Presently, the best-developed geochemical proxies in biogenic carbonates are 18O/16O and Sr/Ca ratios (possibly Mg/Ca) for temperature; 87Sr/86Sr for input sources, Cd/Ca and Ba/Ca ratios for phosphate and alkalinity concentrations, respectively, thus also for ocean circulation; 13C/12C for ocean productivity; B isotopes for seawater pH;, U, Th isotopes, and 14C for dating; and Sr and Mn concentrations for diagenesis. The oceanic authigenic minerals most widely used for chemical paleoceanography are barite, evaporite sulfates, and hydrogenous ferromanganese nodules. Marine barite is an effective alternative monitor of seawater 87Sr/86Sr, especially where carbonates are diagenetically altered or absent. It also provides a high-resolution record of seawater sulfate S isotopes, (evaporite sulfates only carry an episodic record), with new insights on factors affecting the S and C cycles and atmospheric oxygen. High-resolution studies of Sr, Nd, and Pb isotopes of well-dated ferromanganese nodules contain invaluable records on climate driven changes in oceanic circulation.

Coral record of equatorial sea-surface temperatures during the penultimate deglaciation at huon peninsula

Uplifted coral terraces at Huon Peninsula, Papua New Guinea, preserve a record of sea level, sea-surface temperature, and salinity from the penultimate deglaciation. Remnants have been found of a shallow-water reef that formed during a pause, similar to the Younger Dryas, in the penultimate deglaciation at 130,000 +/- 2000 years ago, when sea level was 60 to 80 meters lower than it is today. Porites coral, which grew during this period, has oxygen isotopic values and strontium/calcium ratios that indicate that sea-surface temperatures were much cooler (22 degrees +/- 2 degreesC) than either Last Interglacial or present-day tropical temperatures (29 degrees +/- 1 degreesC). These observations provide further evidence for a major cooling of the equatorial western Pacific followed by an extremely rapid rise in sea level during the latter stages of Termination II.

Temperature and surface-ocean water balance of the mid-holocene tropical western pacific

Skeletal Sr/Ca and 18O/16O ratios in corals from the Great Barrier Reef, Australia, indicate that the tropical ocean surface approximately 5350 years ago was 1 degrees C warmer and enriched in 18O by 0.5 per mil relative to modern seawater. The results suggest that the temperature increase enhanced the evaporative enrichment of 18O in seawater. Transport of part of the additional atmospheric water vapor to extratropical latitudes may have sustained the 18O/16O anomaly. The reduced glacial-Holocene shift in seawater 18O/16O ratio produced by the mid-Holocene 18O enrichment may help to reconcile the different temperature histories for the last deglaciation given by coral Sr/Ca thermometry and foraminiferal oxygen-isotope records.

Geochemistry of corals: proxies of past ocean chemistry, ocean circulation, and climate

This paper presents a discussion of the status of the field of coral geochemistry as it relates to the recovery of past records of ocean chemistry, ocean circulation, and climate. The first part is a brief review of coral biology, density banding, and other important factors involved in understanding corals as proxies of environmental variables. The second part is a synthesis of the information available to date on extracting records of the carbon cycle and climate change. It is clear from these proxy records that decade time-scale variability of mixing processes in the oceans is a dominant signal. That Western and Eastern tropical Pacific El Niño-Southern Oscillation (ENSO) records differ is an important piece of the puzzle for understanding regional and global climate change. Input of anthropogenic CO2 to the oceans as observed by 13C and 14C isotopes in corals is partially obscured by natural variability. Nonetheless, the general trend over time toward lower delta18O values at numerous sites in the world's tropical oceans suggests a gradual warming and/or freshening of the surface ocean over the past century.

Constancy and change of life in the sea

Most marine species appear abruptly in the fossil record and persist unchanged for millions of years. Speciation and extinction commonly occur in pulses so that groups of species come and go as ecological units that dominate the seascape for millions of years. Dramatic turnover of mollusc, coral and planktonic foraminifera species occurred throughout tropical America about two million years ago in apparent response to the onset of northern hemisphere glaciation. In contrast, subsequent glacial cycles, temperature fluctuations and sea-level change had little lasting biological effect. There is no necessary correlation between the magnitude of environmental change and the subsequent ecological and evolutionary response.

Recent Variability in the Southern Oscillation: Isotopic Results from a Tarawa Atoll Coral

In the western tropical Pacific, the interannual migration of the Indonesian Low convective system causes changes in rainfall that dominate the regional signature of the El Niño-Southern Oscillation (ENSO) system. A 96-year oxygen isotope record from a Tarawa Atoll coral (1 degrees N, 172 degrees E) reflects regional convective activity through rainfall-induced salinity changes. This monthly resolution record spans twice the length of the local climatological record and provides a history of ENSO variability comparable in quality with those derived from instrumental climate data. Comparison of this coral record with a historical chronology of EI Niño events indicates that climate anomalies in coastal South America are occasionally decoupled from Pacific-wide ENSO extremes. Spectral analysis suggests that the distribution of variance in this record has shifted among annual to interannual periods during the present century, concurrent with observed changes in the strength of the Southern Oscillation.