Michael K. Gagan

A high-resolution Sr/Ca and δ18O coral record from the Great Barrier Reef, Australia, and the 1982–1983 El Niño

Abstract A high-resolution (near weekly) Sr/Ca and oxygen isotopic record is presented for a coral from the Pandora Reef in the Great Barrier Reef (GBR) of Australia during the period 1978 to 1984. The records are well correlated except for periods of high rainfall when river runoff has significantly modified the δ18O value of seawater. Using the Sr/Ca temperature calibration of De Villiers et al. (1994), the Sr/Ca records exhibit seasonally controlled cyclical SST (sea surface temperature) variations of from ~21 to ~28δC. During the very strong El Nino of 1982–1983, the Sr/Ca systematics indicate a sharp drop in the winter SST to ~ 18.5δC. This represents a temperature anomaly of −3δC which is approximately twice that given by the δ18O variations, suggesting an ~×2 amplification of the anomaly by the Sr/Ca system, possibly due to the increasing dominance of inorganically controlled aragoniteseawater fractionation. The oxygen isotopic systematics show the combined effects of both temperature and changing seawater δ18O values, the latter reflecting the influx of 18O-depleted runoff during periods of high rainfall. Due to the extremely low (~10−3) Sr and Ca contents of river run off relative to seawater, it is possible to use the Sr/Ca thermometer to calculate temperatures independent of major floods and hence deconvolve the combined effects in the oxygen isotopic record of variable temperature and the δ18O value of seawater. Using this approach it is possible to quantitatively reproduce the volume of runoff from the Burdekin River during the periods of major flooding that occurred in early 1979 and 1981. The results of this study demonstrate that the combined use of high-resolution Sr/Ca and δ18O systematics in scleractinian corals is a powerful tool for providing quantitative constraints on past climate.

Post-glacial evolution of the Indo-Pacific Warm Pool and El Nino-Southern Oscillation

Recent research has revealed new insights into the temperature, size, and variability of the Indo-Pacific Warm Pool (IPWP) and the nature of the El Nino-Southern Oscillation (ENSO) since the Last Glacial Maximum (LGM). Sea surface temperature (SST) reconstructions from foraminiferal Mg/Ca, alkenone, and revised coral Sr/Ca palaeothermometry agree that SSTs in the IPWP during the LGM were similar to 3degreesC cooler than at present. In the central portion of the IPWF, the rapid post-glacial rise in SST led the deglaciation by similar to 3000 years to produce near-modern SSTs by the early Holocene. In contrast, further west and north, post-glacial shifts in SSTs in the South China and Sulu Seas are synchronous with abrupt climate changes in the North Atlantic. New evidence for the nature of the Little Ice Age in the tropics has been obtained from a 420-year record of coral Sr/Ca and 6180 from the Great Barrier Reef, Australia. This indicates that SSTs and salinity were higher in the 18th century than in the 20th century. The results suggest that the tropical Pacific played a role as a source region of water vapour during the global expansion of Little Ice Age glaciers. The onset of modern ENSO periodicities is identified by palaeo-ENSO records throughout the tropical Pacific region similar to 5000 years ago, with an abrupt increase in ENSO magnitude similar to 3000 years ago. Individual ENSO events recorded by corals reveal that the precipitation response to El Nino temperature anomalies was subdued in the mid-Holocene. The apparent non-linear onset of ENSO in the late Holocene appears to reflect abruptly enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. Comparisons of precipitation variability recorded by Great Barrier Reef corals with ENSO indices for the last 350 years confirms that non-stationarity of ENSO teleconnections is a natural characteristic of modern climate

High-resolution isotopic records from corals using ocean temperature and mass-spawning chronometers

Abstract We present a 6-year-long (1978–1984) δ 18 O and δ 13 C record from a Great Barrier Reef (Pandora Reef) Porites lutea coral based on near-weekly sample intervals. A sampling technique was designed to minimise any smoothing or distortion of the isotopic record due to complex coral growth, calyx architecture, and calcification at depth within the tissue layer. The arrival-time of the mid-winter minimum sea-surface temperature is very consistent (±2 weeks) near Pandora Reef and provides an annual time-marker offering more precision than the traditional density band chronometer. The improved chronology and high-resolution record demonstrate that signal distortion in Porites , due to calcification within the tissue layer and variable intra-annual coral extension, is generally negligible. Also confirmed is that the arrival-time of monsoonal floods is precisely preserved (±1 week) within the skeleton of Porites . The sensitivity of weekly sampling allows the detection of a subtle warm-to-cool sea-surface temperature anomaly which preceded, by more than one year, a similar temperature anomaly associated with the 1982–83 El Nin˜o in the east Pacific. Sharply higher δ 13 C values coincide with the time of the annual coral mass-spawning event in the Great Barrier Reef. Recognition of this mass-spawning signal should simplify the interpretation of coral δ 13 C records and provides an additional, precise time-marker with which to adjust chronologies when intra-annual coral extension is not constant.

Increasing Australian–Indonesian monsoon rainfall linked to early Holocene sea-level rise

The Australian–Indonesian summer monsoon affects rainfall variability across the Indo–Pacific region. Reconstructions of monsoon strength from stalagmites show that precipitation increased from 11,000 to 7,000 years ago, as rising global sea level caused the flooding of the Indonesian continental shelf. The Australian–Indonesian summer monsoon affects rainfall variability and hence terrestrial productivity in the densely populated tropical Indo–Pacific region. It has been proposed that the main control of summer monsoon precipitation on millennial timescales is local insolation1,2,3, but unravelling the mechanisms that have influenced monsoon variability and teleconnections has proven difficult, owing to the lack of high-resolution records of past monsoon behaviour. Here we present a precisely dated reconstruction of monsoon rainfall over the past 12,000 years, based on oxygen isotope measurements from two stalagmites collected in southeast Indonesia. We show that the summer monsoon precipitation increased during the Younger Dryas cooling event, when Atlantic meridional overturning circulation was relatively weak4. Monsoon precipitation intensified even more rapidly from 11,000 to 7,000 years ago, when the Indonesian continental shelf was flooded by global sea-level rise5,6,7. We suggest that the intensification during the Younger Dryas cooling was caused by enhanced winter monsoon outflow from Asia and a related southward migration of the intertropical convergence zone8. However, the early Holocene intensification of monsoon precipitation was driven by sea-level rise, which increased the supply of moisture to the Indonesian archipelago.

New evidence for episodic post-glacial sea-level rise, central Great Barrier Reef, Australia

Abstract We present an extensive database of 364 radiocarbon dates from coastal and marine sediments of the central Great Barrier Reef (GBR) shelf, of which 110 are previously unpublished. The elevation data have been reduced to a common datum (Australian Height Datum, AHD) and the various sources of error have been assessed. Using modern lithological and biological relationships with sea level, the elements of the radiocarbon database have been converted into sea-level indicators. The upper bound of the assembled dataset corresponds to a best-estimate sea-level curve, and the full dataset provides a narrow envelope for sea-level rise on the GBR shelf for the last 11–12 kyr (not including hydro-isostatic crustal flexing). The envelope is consistent with episodic rise in post-glacial sea levels. The rising post-glacial sea level probably included Stillstands (or minor falls), at ca. −45 m AHD (at ca. 10.5 kyr B.P.), −5 m (7.8 kyr B.P.), −2 m (ca. 6 kyr B.P.) and +1.7 m (5.5 kyr B.P.). There is evidence for a significant fall in sea level between stillstands at −11 m (8.5 kyr B.P.) and −17 m (8.2 kyr B.P.). Stillstand durations apparently ranged between The vertical spread in the derived sea-level data is very wide. The use of shell material for dating seems unreliable and prone to large and unpredictable errors. Data from bulk mangrove muds appear reliable for determination of ancient sea level, but may at times result in sea level being placed up to 4 m below the true level. In-situ biogenic carbonates such as preserved oyster beds and coral micro-atolls are the most reliable indicators of sea-level position, while deposits of mangrove mud give a useful first-order approximation of ancient sea levels. Caution should be used in drawing ‘sea-level curves’ from few data points. We conclude that the post-11–12 kyr B.P. relative rise in sea level was episodic on the central GBR continental margin. More data are required to define clearly sea-level change up to ca. −20 m at 9 kyr B.P.

Diagenesis and geochemistry of porites corals from Papua New Guinea: Implications for paleoclimate reconstruction

Coral proxy records of sea surface temperature (SST) and hydrological balance have become important tools in the field of tropical paleoclimatology. However, coral aragonite is subject to post- depositional diagenetic alteration in both the marine and vadose environments. To understand the impact of diagenesis on coral climate proxies, two mid-Holocene Porites corals from raised reefs on Muschu Island, Papua New Guinea, were analysed for Sr/Ca, 18 O, and 13 C along transects from 100% aragonite to 100% calcite. Thin-section analysis showed a characteristic vadose zone diagenetic sequence, beginning with leaching of primary aragonite and fine calcite overgrowths, transitional to calcite void filling and neomorphic, fabric selective replacement of the coral skeleton. Average calcite Sr/Ca and 18 O values were lower than those for coral aragonite, decreasing from 0.0088 to 0.0021 and 5.2 to 8.1‰, respectively. The relatively low Sr/Ca of the secondary calcite reflects the Sr/Ca of dissolving phases and the large difference between aragonite and calcite Sr/Ca partition coefficients. The decrease in 18 O of calcite relative to coral aragonite is a function of the 18 O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though generally lower relative to aragonite, ranging from 2.5 to 10.4%. The variability of 13 C in secondary calcite reflects the amount of soil CO 2 contributing 13 C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on 18 O; the calcite compositions reported here convert to SST anomalies of 115°C and 14°C, respectively. Based on calcite Sr/Ca compositions in this study and in the literature, the sensitivity of coral Sr/Ca-SST to vadose-zone calcite diagenesis is 1.1 to 1.5°C per percent calcite. In contrast, the rate of change in coral 18 O-SST is relatively small (0.2 to 0.2°C per percent calcite). We show that large shifts in 18 O, reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca-SSTs, cannot be caused by calcite diagenesis. Low-level calcite diagenesis can be detected through X-ray diffraction techniques, thin section analysis, and high spatial resolution sampling of the coral skeleton and thus should not impede the production of accurate coral paleoclimate reconstructions. Copyright

Seasonal Characteristics of the Indian Ocean Dipole during the Holocene epoch

The Indian Ocean Dipole (IOD)—an oscillatory mode of coupled ocean–atmosphere variability—causes climatic extremes and socio-economic hardship throughout the tropical Indian Ocean region. There is much debate about how the IOD interacts with the El Niño/Southern Oscillation (ENSO) and the Asian monsoon, and recent changes in the historic ENSO–monsoon relationship raise the possibility that the properties of the IOD may also be evolving. Improving our understanding of IOD events and their climatic impacts thus requires the development of records defining IOD activity in different climatic settings, including prehistoric times when ENSO and the Asian monsoon behaved differently from the present day. Here we use coral geochemical records from the equatorial eastern Indian Ocean to reconstruct surface-ocean cooling and drought during individual IOD events over the past ∼6,500 years. We find that IOD events during the middle Holocene were characterized by a longer duration of strong surface ocean cooling, together with droughts that peaked later than those expected by El Niño forcing alone. Climate model simulations suggest that this enhanced cooling and drying was the result of strong cross-equatorial winds driven by the strengthened Asian monsoon of the middle Holocene. These IOD–monsoon connections imply that the socioeconomic impacts of projected future changes in Asian monsoon strength may extend throughout Australasia.

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 °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 °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.

Impact of skeletal dissolution and secondary aragonite on trace element and isotopic climate proxies in Porites corals

Restricted zones of recent dissolution and secondary aragonite infilling were identified in a coral core collected in 1986 from a living massive Porites colony from the central Great Barrier Reef, Australia. Secondary aragonite needles, ≥20 μm long, cover skeletal surfaces deposited from 1972 to late 1974 and increase bulk density by 10%. Dissolution is observed above this zone, whereas older skeleton is pristine. We investigate the impact of both types of early marine diagenesis on skeletal geochemistry and coral paleoclimate reconstructions by comparison with similar records from eight contemporary Porites colonies collected at nearby reefs. Secondary aragonite overgrowth causes anomalies in skeletal density, Mg/Ca, Sr/Ca, U/Ca, δ O, and δC. The secondary aragonite is consistently associated with a cool temperature anomaly for each of the sea surface temperature (SST) proxies (δO-SST -1.6°C; Sr/Ca-SST -1.7°C; Mg/Ca-SST -1.9°C; U/Ca-SST -2.8°C). Dissolution, through incongruent leaching, also causes cool SST artifacts but only for trace element ratios (Mg/Ca-SST -1.2°C; Sr/Ca-SST -1.2°C; U/Ca-SST -2.1°C). The sequence of preference with respect to dissolution of coral skeleton in seawater is Mg > Ca > Sr > U. Rigorous screening of coral material for paleoclimate reconstructions is therefore necessary to detect both dissolution and the presence of secondary minerals. The excellent agreement between apparent SST anomalies generated by different modes of diagenesis means that replication of tracers within a single coral cannot be used to validate climate-proxy interpretations. Poor replication of records between different coral colonies, however, provides a strong indication of nonclimatic artifacts such as dissolution and secondary aragonite. Copyright 2007 by the American Geophysical Union.

Climatic significance of seasonal trace element and stable isotope variations in a modern freshwater tufa

Abstract We present a continuous ∼14-yr-long (1985 to 1999) high-resolution record of trace element (Mg, Sr, Ba, U) and stable isotope (δ13C, δ18O) variations in a modern freshwater tufa from northwestern Queensland, Australia. By utilizing the temperature dependence of the δ18O signal, an accurate chronology was developed for the sampled profile, which allowed a comparison of the chemical records with hydrological and meteorological observations. As a consequence, it was possible to constrain the relevant geochemical processes relating climate variables, such as temperature and precipitation, to their chemical proxies in the tufa record. Temperatures calculated from the Mg concentrations of the tufa samples provide close approximations of average annual water temperature variations. Furthermore, we demonstrate that temporal changes in (Mg/Ca)water can be estimated using an empirically derived equation relating (Mg/Ca)water to the (Sr/Ba) ratio measured in the tufa samples. By means of this relationship, it is theoretically possible to determine the (Mg/Ca) ratio of paleowaters, and hence to derive reliable estimates of former water temperatures from the Mg concentrations of fossil tufas from the study area. Sympathetic variations in Sr, Ba, and δ13C along the sampled profile record changes in water chemistry, which are most probably caused by variable amounts of calcite precipitation within the vadose zone of the karst aquifer. This process is thought to be markedly subdued whenever the amount of wet-season precipitation exceeds a given threshold. Accordingly, distinct minima in Sr, Ba, and δ13C are interpreted to reflect years with above-average rainfall. The pronounced seasonal and annual variability of the U concentration along the profile is thought to primarily record changes in the U flux from the soil to the water table. We suggest that during intensive rain events U is transported to the phreatic zone by complexing organic colloids, giving rise to conspicuous U maxima in the tufa after above-average wet seasons. This study demonstrates the potential of freshwater tufas to provide valuable information on seasonal temperature and rainfall variations. If tufa deposits turn out to be reasonably resistant to secondary processes, combined investigation of speleothems and tufas from the same area could become a promising approach in future research. While speleothems offer continuous records of long-term paleoenvironmental changes, tufas could provide high-resolution time windows into selected periods of the past.