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Untitled Document
Abstracts
Temperature and surface-ocean water balance of the mid-Holocene
tropical western Pacific
Gagan MK, Ayliffe LK, Hopley D, Cali JA, Mortimer GE, Chappell J,
McCulloch MT, Head MJ (1998). Science 279: 1014-1018.
Abstract:
Skeletal Sr/Ca and O-18/O-16 ratios in corals from the Great Barrier
Reef, Australia, indicate that the tropical ocean surface ~5350 years
ago was 1 degree C warmer and enriched in O-18 by 0.5 per mil
relative to modern seawater. The results suggest that the temperature
increase enhanced the evaporative enrichment of O-18 in seawater.
Transport of part of the additional atmospheric water vapor to
extratropical latitudes may have sustained the O-18/O-16 anomaly. The
reduced glacial-Holocene shift in seawater O-18/O-16 ratio produced
by the mid-Holocene O-18 enrichment may help to reconcile the
different temperature histories for the last deglaciation given by
coral Sr/Ca thermometry and foraminiferal oxygen-isotope-records.
65,000 years of vegetation change in central Australia and the
Australian summer monsoon
Johnson BJ, Miller GH, Fogel ML, Magee JW, Gagan MK, Chivas AR (1999). Science 284: 1150-1152.
Abstract:
Carbon isotopes in fossil emu (Dromaius novaehollandiae) eggshell
from Lake Eyre, South Australia, demonstrate that the relative
abundance of C-4 grasses varied substantially during the past 65,000
years. Currently, C-4 grasses are more abundant in regions that are
increasingly affected by warm-season precipitation. Thus, an
expansion of C-4 grasses likely reflects an increase in the relative
effectiveness of the Australian summer monsoon, which controls summer
precipitation over Lake Eyre. The data imply that the Australian
monsoon was most effective between 45,000 and 65,000 years ago, least
effective during the Last Glacial Maximum, and moderately effective
during the Holocene.
Coral microatolls from the central Pacific record late Holocene El Nino
Woodroffe CD, Gagan MK (2000). Geophysical Research Letters 27: 1511-1514.
Abstract:
Microatolls are discoid corals that have grown laterally because
vertical growth is constrained by exposure at lowest tides. We
demonstrate that a modern reef-flat Porites microatoll from Christmas
(Kiritimati) Island preserves an oxygen isotope record of substantial
sea surface temperature variations related to El Nino-Southern
Oscillation (ENSO). We also show that a late Holocene fossil
microatoll from the centre of the island contains interannual oxygen
isotope variations over an approximate 20-year period. Three
pronounced negative isotope anomalies attributed to warm EI Nino
events are superimposed on an annual cycle. El Nino events similar to
those seen in recent decades appear to have been a feature of
mid-Pacific climate in the late Holocene. Analysis of further
microatolls offers a source of pre-instrumental El Nino tropical
climate data. It may be possible to extend the NINO-C proxy to
indicate intensity and frequency of ENSO over the past three
millennia.
New views of tropical paleoclimates from corals
Gagan MK, Ayliffe LK, Beck JW, Cole JE, Druffel ERM, Dunbar RB,
Schrag DP (2000). Quaternary Science Reviews 19: 45-64.
Abstract:
Corals offer a rich archive of past climate variability in tropical
ocean regions where instrumental data are limited and where our
knowledge of multi-decadal climate sensitivity is incomplete. In the
eastern equatorial Pacific, coral isotopic records track variations
in ENSO-related changes in sea-surface temperature; further west,
corals record variability in sea-surface temperature and rainfall
that accompanies zonal displacement of the Indonesian Low during ENSO
events. These multi-century records reveal previously unrecognised
ENSO variability on time scales of decades to centuries. Outside the
ENSO-sensitive equatorial Pacific, long-term trends towards recent
warmer/wetter conditions suggest the tropics respond to global
forcings. New coral paleothermometers indicate that surface-ocean
temperatures in the tropical southwestern Pacific were depressed by
4-6 degrees C during the Younger Dryas climatic event and rose
episodically during the next 4000 yr. High temporal-resolution
measurements of Sr/Ca and delta(18)O in corals provide information
about the surface-ocean hydrologic balance and can resolve the
seasonal balance between precipitation and evaporation. Radiocarbon
measurements in corals, coupled with ocean circulation models, may be
used to reconstruct near-surface ocean circulation, past mixing
rates, and the distribution of fossil fuel CO2 in the upper ocean.
Most recently, seasonal to interannual variations in the radiocarbon
of corals from the equatorial Pacific have been linked to the
redistribution of surface waters associated with the ENSO.
New views of tropical paleoclimates from corals
Gagan MK, Ayliffe LK, Beck JW, Cole JE, Druffel ERM, Dunbar RB, Schrag DP (2000). Quaternary Science Reviews 19: 45-64.
Abstract:
Corals offer a rich archive of past climate variability in tropical ocean regions where instrumental data are limited and where our knowledge of multi-decadal climate sensitivity is incomplete. In the eastern equatorial Pacific, coral isotopic records track variations in ENSO-related changes in sea-surface temperature; further west, corals record variability in sea-surface temperature and rainfall that accompanies zonal displacement of the Indonesian Low during ENSO events. These multi-century records reveal previously unrecognised ENSO variability on time scales of decades to centuries. Outside the ENSO-sensitive equatorial Pacific, long-term trends towards recent warmer/wetter conditions suggest the tropics respond to global forcings. New coral paleothermometers indicate that surface-ocean temperatures in the tropical southwestern Pacific were depressed by 4-6 degrees C during the Younger Dryas climatic event and rose episodically during the next 4000 yr. High temporal-resolution measurements of Sr/Ca and delta(18)O in corals provide information about the surface-ocean hydrologic balance and can resolve the seasonal balance between precipitation and evaporation. Radiocarbon measurements in corals, coupled with ocean circulation models, may be used to reconstruct near-surface ocean circulation, past mixing rates, and the distribution of fossil fuel CO2 in the upper ocean. Most recently, seasonal to interannual variations in the radiocarbon of corals from the equatorial Pacific have been linked to the redistribution of surface waters associated with the ENSO.
Last Interglacial coral record of enhanced insolation seasonality and seawater O-18 enrichment in the Ryukyu Islands, northwest Pacific
Suzuki A, Gagan MK, De Deckker P, Omura A, Yukino I, Kawahata H (2001). Geophysical Research Letters 28: 3685-3688.
Abstract:
We present a calibrated, high-resolution O-18/O-16 and C-13/C-12 record for a well-preserved Last Interglacial Porites sp. coral (U-Th age of 127 +/- 6 ka) from the sea-level high-stand terrace of Yonaguni Island, Japan. Seasonal variations in the delta O-18 and delta C-13 values for the fossil coral are greater than those observed in modem coral records from the same reef setting and appear to be driven by the enhanced insolation seasonality in the northern hemisphere during the Last Interglacial maximum. The O-18 enrichment of 1.1 parts per thousand in the fossil coral compared to the modem analogue cannot be due entirely to a reduction in sea-surface temperature because corals in this region are already growing at their lower thermal limit. Instead, most of the O-18 enrichment must be due to a change in the delta O-18 of the surface seawater, probably in response to enhanced evaporation of the ocean or a higher volume flux of the Kuroshio Current.
Early marine diagenesis in corals and geochemical consequences for paleoceanographic reconstructions
Muller A, Gagan MK, McCulloch MT (2001). Geophysical Research Letters 28: 4471-4474.
Abstract:
Detecting the potential geochemical consequences of early marine diagenesis is essential for establishing the validity of past climate reconstructions from coral. We present coral skeletal delta O-18 and Sr/Ca data for two long coral cores spanning 1839-1994 AD at Ningaloo Reef, Western Australia, one of which includes significant secondary precipitation of marine inorganic aragonite. Long-term trends in reconstructed sea surface temperatures (SSTs) for the well preserved coral correlate strongly with instrumental SST records spanning the 20th century. In contrast, the delta O-18 and Sr/Ca for the diagenetically altered coral give identical cool SST anomalies of 4-5 degrees C, as a consequence of the addition of secondary aragonite enriched in O-18 and Sr. Our results indicate that cross-checking of paleoclimate reconstructions with two supposedly independent paleothermometers may not be valid, and that coral records showing cooler SSTs in the past need to be interpreted with caution. Furthermore, modern coral records with long-term trends in delta O-18 indicating recent warming and freshening of the ocean can be potentially explained by early marine diagenesis.
Abrupt decrease in tropical Pacific sea surface salinity at end of Little Ice Age
Hendy EJ, Gagan MK, Alibert CA, McCulloch MT, Lough JM, Isdale PJ (2002). Science 295: 1511-1514.
Abstract:
A 420-year history of strontium/calcium, uranium/calcium, and oxygen isotope ratios in eight coral cores from the Great Barrier Reef, Australia, indicates that sea surface temperature and salinity were higher in the 18th century than in the 20th century. An abrupt freshening after 1870 occurred simultaneously throughout the southwestern Pacific, coinciding with cooling tropical temperatures. Higher salinities between 1565 and 1870 are best explained by a combination of advection and wind-induced evaporation resulting from a strong latitudinal temperature gradient and intensified circulation. The global Little Ice Age glacial expansion may have been driven, in part, by greater poleward transport of water vapor from the tropical Pacific.
Diagenesis and geochemistry of Porites corals from Papua New Guinea: implications for paleoclimate reconstruction.
McGregor, H.V. and M.K. Gagan (in press). Geochimica et Cosmochimica Acta.
Abstract:
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, (18O, and (13C 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 (18O values were lower than coral aragonite, decreasing from 0.0088 to 0.0021 and -5.2 to -8.1î, 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 (18O of calcite relative to coral aragonite is a function of the (18O of precipitation. Carbon-isotope ratios in secondary calcite are variable, though lower relative to aragonite, changing from -1.3 to -1.4î and -0.1 to -10.5î for each coral. The variability of (13C in secondary calcite reflects the amount of soil CO2 contributing 13C-depleted carbon to the precipitating fluids. Diagenesis has a greater impact on Sr/Ca than on (18O; the calcite compositions reported here convert to SST anomalies of 115(C and 14(C, respectively. Thus, 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 (18O SST is relatively small (-0.2 to 0.2(C per percent calcite). We show that large shifts in (18O, `reported for mid-Holocene and Last Interglacial corals with warmer than present Sr/Ca SSTs, cannot be caused by diagenesis. Low-level 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.
Historical die-offs in massive Porites from the central Great Barrier Reef, Australia: evidence of past environmental stress?
Hendy, E.J., Lough, J.M. and M.K. Gagan (in press). Coral Reefs.
Abstract: Not yet available
Coral oxygen isotope evidence for recent groundwater CO2 fluxes to the Australian Great Barrier Reef.
Gagan, M.K., Ayliffe, L.K., Opdyke, B.N., Hopley, D., Scott-Gagan, H. and J. Cowley (in press). Geophysical Research Letters.
Abstract:
High-resolution measurements of Sr/Ca and 18O/16O in mid-Holocene and modern Porites corals were used to derive a history of freshwater fluxes to the Australian Great Barrier Reef (GBR). Interannual variations in dry-season seawater 18O/16O evident in the coral record for the late 20th century, but not 6200 years ago, correlate with variations in groundwater recharge of coastal aquifers. The results suggest that a delay in groundwater recharge following the post-glacial sea-level rise, and recent deforestation for agriculture, may have lead to higher land-sea hydraulic gradients and greater groundwater discharge to the GBR today. Oxygen-isotope mass balance calculations show that ~3% in-mixing of CO2-enriched groundwater could raise the partial pressure of CO2 in coastal seawater by ~90 _atm following strong summer monsoons. Consequently, modern nearshore corals in the GBR, and elsewhere, may now be subjected to lower aragonite saturation states, which could reduce coral calcification and contribute to recent reef degradation.
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