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A concerted effort has begun to gather and preserve archives of marine samples and descriptive data, giving scientists ready access to insights on ancient environments.

A concerted effort has begun to gather and preserve archives of marine samples and descriptive data, giving scientists ready access to insights on ancient environments.

In our paper describing a new coral record from Butaritari, we hypothesized that comparing the temporal trends in our records to coral records from farther east in the equatorial Pacific may support the evidence for a weakening of a Walker circulation, documented elsewhere in the literature [Power and Smith, 2007; Tokinaga et al., 2012]. Weakening of the Walker circulation is expected under global warming due to an imbalance in the rate of change in different aspects of the hydrological cycle [Vecchi and Soden, 2007]. Here we provide additional analyses, which confirm that our reconstruction [Carilli et al., 2014] shows a long-term weakening of the Walker circulation over 1972-1998. We also document that significant decadal variations in Walker circulation strength, and for particular choices of start and end years over which trends are calculated, are able to show slight Walker strengthening. Overall, we conclude that Walker circulation variations are more nuanced than either our original publication [Carilli et al., 2014] or the subsequent Karnauskas et al. [2015] comment would suggest. Karnauskas et al. [2015] also provide a detailed analysis of Equatorial Undercurrent (EUC) activity near the Gilbert Islands and argue that the EUC does not strongly affect Butaritari. Our original publication did not claim to find significant EUC/Butaritari linkages, and we appreciate the diligence of Karnauskas et al. [2015] for ruling this out as a possibility. more...

In our paper describing a new coral record from Butaritari, we hypothesized that comparing the temporal trends in our records to coral records from farther east in the equatorial Pacific may support the evidence for a weakening of a Walker circulation, documented elsewhere in the literature [Power and Smith, 2007; Tokinaga et al., 2012]. Weakening of the Walker circulation is expected under global warming due to an imbalance in the rate of change in different aspects of the hydrological cycle [Vecchi and Soden, 2007]. Here we provide additional analyses, which confirm that our reconstruction [Carilli et al., 2014] shows a long-term weakening of the Walker circulation over 1972-1998. We also document that significant decadal variations in Walker circulation strength, and for particular choices of start and end years over which trends are calculated, are able to show slight Walker strengthening. Overall, we conclude that Walker circulation variations are more nuanced than either our original publication [Carilli et al., 2014] or the subsequent Karnauskas et al. [2015] comment would suggest. Karnauskas et al. [2015] also provide a detailed analysis of Equatorial Undercurrent (EUC) activity near the Gilbert Islands and argue that the EUC does not strongly affect Butaritari. Our original publication did not claim to find significant EUC/Butaritari linkages, and we appreciate the diligence of Karnauskas et al. [2015] for ruling this out as a possibility. more...

The tropics are the major source of heat and moisture for the Australasian region. Determining the tropics' response over time to changes in climate forcing mechanisms, such as summer insolation, and the effects of relative sea level on exposed continental shelves during the Last Glacial period, is an ongoing process of re-evaluation. We present a synthesis of climate proxy data from tropical Australasia spanning the last 30,000 years that incorporates deep sea core, coral, speleothem, pollen, charcoal and terrestrial sedimentary records. Today, seasonal variability is governed largely by the annual migration of the inter-tropical convergence zone (ITCZ), influencing this region most strongly during the austral summer. However, the position of the ITCZ has varied through time. Towards the end of Marine Isotope Stage (MIS) 3, conditions were far wetter throughout the region, becoming drier first in the south. Universally cooler land and sea-surface temperature (SST) were characteristic of the Last Glacial Maximum, with drier conditions than previously, although episodic wet periods are noted in the fluvial records of northern Australia. The deglacial period saw warming first in the Coral Sea and then the Indonesian seas, with a pause in this trend around the time of the Antarctic Cold Reversal (c. 14.5 ka), coincident with the flooding of the Sunda Shelf. Wetter conditions occurred first in Indonesia around 17 ka and northern Australia after 14 ka. The early Holocene saw a peak in marine SST to the northwest and northeast of Australia. Modern vegetation was first established on Indonesia, then progressively south and eastward to NE Australia. Flores and the Atherton Tablelands show a dry period around 11.6 ka, steadily becoming wetter through the early Holocene. The mid-late Holocene was punctuated by millennial-scale variability, associated with the El Niño-Southern Oscillation; this is evident in the marine, coral, speleothem and pollen records of the region. more...

The tropics are the major source of heat and moisture for the Australasian region. Determining the tropics' response over time to changes in climate forcing mechanisms, such as summer insolation, and the effects of relative sea level on exposed continental shelves during the Last Glacial period, is an ongoing process of re-evaluation. We present a synthesis of climate proxy data from tropical Australasia spanning the last 30,000 years that incorporates deep sea core, coral, speleothem, pollen, charcoal and terrestrial sedimentary records. Today, seasonal variability is governed largely by the annual migration of the inter-tropical convergence zone (ITCZ), influencing this region most strongly during the austral summer. However, the position of the ITCZ has varied through time. Towards the end of Marine Isotope Stage (MIS) 3, conditions were far wetter throughout the region, becoming drier first in the south. Universally cooler land and sea-surface temperature (SST) were characteristic of the Last Glacial Maximum, with drier conditions than previously, although episodic wet periods are noted in the fluvial records of northern Australia. The deglacial period saw warming first in the Coral Sea and then the Indonesian seas, with a pause in this trend around the time of the Antarctic Cold Reversal (c. 14.5 ka), coincident with the flooding of the Sunda Shelf. Wetter conditions occurred first in Indonesia around 17 ka and northern Australia after 14 ka. The early Holocene saw a peak in marine SST to the northwest and northeast of Australia. Modern vegetation was first established on Indonesia, then progressively south and eastward to NE Australia. Flores and the Atherton Tablelands show a dry period around 11.6 ka, steadily becoming wetter through the early Holocene. The mid-late Holocene was punctuated by millennial-scale variability, associated with the El Niño-Southern Oscillation; this is evident in the marine, coral, speleothem and pollen records of the region. more...

The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between ∼5500 and 4300 years BP (∼1.2 °C ± 0.3 °C) and were similarly cool before ∼6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between ∼6600 and 6300 years BP (∼1.3 °C ± 0.3 °C), while similarly warm conditions may have also existed after ∼4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesti more...

The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between ∼5500 and 4300 years BP (∼1.2 °C ± 0.3 °C) and were similarly cool before ∼6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between ∼6600 and 6300 years BP (∼1.3 °C ± 0.3 °C), while similarly warm conditions may have also existed after ∼4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesti more...

The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between ∼5500 and 4300 years BP (∼1.2 °C ± 0.3 °C) and were similarly cool before ∼6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between ∼6600 and 6300 years BP (∼1.3 °C ± 0.3 °C), while similarly warm conditions may have also existed after ∼4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesti more...

The Indo-Pacific Warm Pool (IPWP) is thought to play a key role in the propagation and amplification of climate changes through its influence on the global distribution of heat and water vapour. However, little is known about past changes in the size and position of the IPWP. In this study, we use a total of 48 modern and fossil coral records from the Mentawai Islands (Sumatra, Indonesia) and Muschu/Koil Islands (Papua New Guinea) to reconstruct oscillations in the extent of the IPWP since the mid-Holocene. We show that reliable estimates of mean sea surface temperature (SST) can be obtained from fossil corals by using low-resolution Sr/Ca analysis of a suite of corals to overcome the large uncertainties associated with mean Sr/Ca-SST estimates from individual coral colonies. The coral records indicate that the southeastern and southwestern margins of the IPWP were cooler than at present between ∼5500 and 4300 years BP (∼1.2 °C ± 0.3 °C) and were similarly cool before ∼6800 years BP. This mid-Holocene cooling was punctuated by an abrupt, short-lived shift to mean SSTs that were warmer than at present between ∼6600 and 6300 years BP (∼1.3 °C ± 0.3 °C), while similarly warm conditions may have also existed after ∼4300 years BP. We suggest that mid-Holocene cooling at our study sites was related to contractions of the southeastern and southwestern margins of the IPWP, associated with the more northerly position of the Inter-tropical Convergence Zone (ITCZ) that accompanied mid-Holocene strengthening of the Asian summer monsoon. Conversely, intervals of abrupt warming appear to correspond with widespread episodes of monsoon weakening and accompanying southward migrations of the ITCZ that caused the IPWP to expand beyond our coral sites. Intervals of a strengthened Asian monsoon and cooling in the southwestern IPWP during the mid-Holocene appear to correspond with a more positive Indian Ocean Dipole (IOD)-like mean configuration across the tropical Indian Ocean, suggesti more...

The problem of the worldwide extinction of a diverse assemblage of Late Pleistocene and Holocene large-bodied animals continues to cause debate (Brook and Bowman, 2002; Barnosky et al., 2004; Burney and Flannery, 2005; Koch and Barnosky, 2006). The most recent contribution on the Australian megafaunal extinction (Wroe and Field, 2006), argues for a staggered series of extinctions over multiple glacial cycles, with most megafaunal extinctions predating the arrival of humans and driven primarily by climate change eventually causing a ‘hydrological threshold’ to be breached. At this point, accessible water became too scarce for megafauna to forage successfully. Although Wroe and Field (2006) highlight a range of ideas under consideration, they provide a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence. more...

The problem of the worldwide extinction of a diverse assemblage of Late Pleistocene and Holocene large-bodied animals continues to cause debate (Brook and Bowman, 2002; Barnosky et al., 2004; Burney and Flannery, 2005; Koch and Barnosky, 2006). The most recent contribution on the Australian megafaunal extinction (Wroe and Field, 2006), argues for a staggered series of extinctions over multiple glacial cycles, with most megafaunal extinctions predating the arrival of humans and driven primarily by climate change eventually causing a ‘hydrological threshold’ to be breached. At this point, accessible water became too scarce for megafauna to forage successfully. Although Wroe and Field (2006) highlight a range of ideas under consideration, they provide a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence. more...

The problem of the worldwide extinction of a diverse assemblage of Late Pleistocene and Holocene large-bodied animals continues to cause debate (Brook and Bowman, 2002; Barnosky et al., 2004; Burney and Flannery, 2005; Koch and Barnosky, 2006). The most recent contribution on the Australian megafaunal extinction (Wroe and Field, 2006), argues for a staggered series of extinctions over multiple glacial cycles, with most megafaunal extinctions predating the arrival of humans and driven primarily by climate change eventually causing a ‘hydrological threshold’ to be breached. At this point, accessible water became too scarce for megafauna to forage successfully. Although Wroe and Field (2006) highlight a range of ideas under consideration, they provide a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence. more...

The problem of the worldwide extinction of a diverse assemblage of Late Pleistocene and Holocene large-bodied animals continues to cause debate (Brook and Bowman, 2002; Barnosky et al., 2004; Burney and Flannery, 2005; Koch and Barnosky, 2006). The most recent contribution on the Australian megafaunal extinction (Wroe and Field, 2006), argues for a staggered series of extinctions over multiple glacial cycles, with most megafaunal extinctions predating the arrival of humans and driven primarily by climate change eventually causing a ‘hydrological threshold’ to be breached. At this point, accessible water became too scarce for megafauna to forage successfully. Although Wroe and Field (2006) highlight a range of ideas under consideration, they provide a selective interpretation which does not come to terms with biology and ignores or misinterprets current evidence. more...

Oxygen isotope ratios in Porites microatolls from Christmas Island in the central Pacific provide high-resolution proxy records of El Niño-Southern Oscillation (ENSO) variability since 3.8 thousand years ago (ka). Compared with modern microatolls, reconstructions from fossil microatolls imply that interannual variations in ENSO sea-surface temperature and precipitation were less intense 3.8–2.8 ka, but more pronounced at 1.7 ka. Amplification of ENSO at ∼2 ka is consistent with precessional changes in insolation seasonality, but exceeds model predictions and may reflect stronger rainfall teleconnections through enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. more...

Oxygen isotope ratios in Porites microatolls from Christmas Island in the central Pacific provide high-resolution proxy records of El Niño-Southern Oscillation (ENSO) variability since 3.8 thousand years ago (ka). Compared with modern microatolls, reconstructions from fossil microatolls imply that interannual variations in ENSO sea-surface temperature and precipitation were less intense 3.8–2.8 ka, but more pronounced at 1.7 ka. Amplification of ENSO at ∼2 ka is consistent with precessional changes in insolation seasonality, but exceeds model predictions and may reflect stronger rainfall teleconnections through enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. more...

Oxygen isotope ratios in Porites microatolls from Christmas Island in the central Pacific provide high-resolution proxy records of El Niño-Southern Oscillation (ENSO) variability since 3.8 thousand years ago (ka). Compared with modern microatolls, reconstructions from fossil microatolls imply that interannual variations in ENSO sea-surface temperature and precipitation were less intense 3.8–2.8 ka, but more pronounced at 1.7 ka. Amplification of ENSO at ∼2 ka is consistent with precessional changes in insolation seasonality, but exceeds model predictions and may reflect stronger rainfall teleconnections through enhanced interaction between the Southern Oscillation and the Pacific Intertropical Convergence Zone. more...

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 Niño-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 El Niño events are superimposed on an annual cycle. El Niño 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 Niño 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. more...

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 Niño-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 El Niño events are superimposed on an annual cycle. El Niño 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 Niño 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. more...

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 Niño-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 El Niño events are superimposed on an annual cycle. El Niño 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 Niño 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. more...