Tezer M. Esat

Reconciliation of late Quaternary sea levels derived from coral terraces at Huon Peninsula with deep sea oxygen isotope records

A major discrepancy between the Late Quaternary sea level changes derived from raised coral reef terraces at the Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores is resolved. The two methods agree closely from 120 ka to 80 ka and from 20 ka to 0 ka (ka = 1000 yr before present), but between 70 and 30 ka the isotopic sea levels are 20–40 m lower than the Huon Peninsula sea levels derived in earlier studies. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces aged between 30 and 70 ka now give similar sea levels to those based on deep sea oxygen isotope data planktonic and benthic δ18O data. Using the sea level and deep sea isotopic data, oxygen isotope ratios are calculated for the northern continental ice sheets through the last glacial cycle and are consistent with results from Greenland ice cores. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.

Links between climate and sea levels for the past three million years

The oscillations between glacial and interglacial climate conditions over the past three million years have been characterized by a transfer of immense amounts of water between two of its largest reservoirs on Earth — the ice sheets and the oceans. Since the latest of these oscillations, the Last Glacial Maximum (between about 30,000 and 19,000 years ago), ∼50 million cubic kilometres of ice has melted from the land-based ice sheets, raising global sea level by ∼130 metres. Such rapid changes in sea level are part of a complex pattern of interactions between the atmosphere, oceans, ice sheets and solid earth, all of which have different response timescales. The trigger for the sea-level fluctuations most probably lies with changes in insolation, caused by astronomical forcing, but internal feedback cycles complicate the simple model of causes and effects.

Timing and duration of the Last Interglacial: evidence for a restricted interval of widespread coral reef growth

Abstract We report new mass spectrometric U-series ages for eight Last Interglacial fossil reefs along the continental margin of Western Australia. Corals were selected in growth position from localities that are characterized by apparently low levels of diagenesis and relative tectonic stability so that the fossil reefs provide critical information on Last Interglacial sea-levels without requiring corrections for tectonic movements. In addition, we have improved the constraint on the timing of onset of reef growth by recovering drill core coral from the base of the reefs. Uranium and thorium isotopes were measured with high levels of precision, leading to improvements in age resolution and allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified and discarded. These data supplement our previous results for Rottnest Island and Leander Point, leading to more than seventy mass spectrometric U-series ages from which constraints can be placed on the timing, duration and character of the Last Interglacial sea-level highstand. Reliable ages show that reef growth started contemporaneously at 128 ± 1 ka along the entire Western Australian coastline, while relative sea-levels were at least 3 m above the present level. Because Western Australia is located far from the former Penultimate Glacial Maximum ice sheets and are not significantly effected by glacial unloading, these data constrain the timing of onset of the Last Interglacial period to 128 ± 1 ka, assuming reef growth started soon after sea-level approached interglacial levels. A unique regressive reef sequence at Mangrove Bay constrains the timing of termination of the Last Interglacial period to 116 ± 1 ka. The major episode of reef building, however, both globally and locally along the Western Australian coast, is restricted to a very narrow interval occurring from ∼128 ka and ∼121 ka, suggesting that global ocean surface temperatures were warm and/or sea-levels were stable enough to allow prolific reef growth only during the earlier part of the Last Interglacial.

High-precision U-series dating of corals from Western Australia and implications for the timing and duration of the Last Interglacial

Abstract U-series ages using methods of thermal ionisation mass spectrometry (TIMS) are reported for Last Interglacial fossil reefs along the stable coastal margin of Western Australia. Thorium isotope ratios were measured with superior precision using methods of charge collection. High levels of precision in the measurement of both uranium and thorium isotopes has reduced the age uncertainty due to analytical errors, excluding the uncertainty in the decay constants, by a factor of four over the precisions reported by many earlier TIMS workers. Uncertainties in δ234U(T), determined from both 230Th/238U and 234U/238U, are also significantly smaller than previously reported, allowing samples which have undergone diagenetic exchange of uranium and thorium to be more easily identified. Strict criteria were adopted to screen the new Western Australian data. Reliable ages range from 127 to 122 ka. Published TIMS observations from other localities have been assessed using the same strict criteria. When these are combined with glacio-hydro isostatic sea-level models they indicate that the Last Interglacial period occurred from at least 130 to 117 ka. However, these age constraints are largely determined from single data points and need to be verified with additional ages before considering them to be robust estimates for the timing of onset and termination of the Last Interglacial. Globally, the main episode of reef growth appears to be confined to a narrow interval occurring from 127 to 122 ka, in direct agreement with the narrow range in ages obtained from the Western Australian sites. This may indicate that the Last Interglacial was of short duration, extending from 127 to 122 ka only. Alternatively, this interval may reflect a major reef-building event in the middle of a longer duration (130-117 ka) interglacial interval.

The persistence of off-cratonic lithospheric mantle : Os isotopic systematics of variably metasomatised southeast Australian xenoliths

The ReOs systematics of a suite of spinel peridotite xenoliths from southeast Australia provide insight into the effects of melt extraction and metasomatism on Re and Os and place strong constraints on the evolution and long-term stability of post-Archean lithospheric mantle in a tectonically complex region. Data from variably melt-depleted and non-modally metasomatised xenoliths demonstrate that Re abundances are largely controlled by melt extraction, with Re similarly distributed to Os. Ratios of ReOs correlate strongly with indices of melt extraction (e.g. Al2O3, Ni and Yb), and with the calculated bulk partition coefficient of Re, comparable to that of Yb over a large range of melt extraction (∼ 4–20%). Hence, if Re is controlled by sulfide phases, sulfur:clinopyroxene ratios should remain essentially constant over large degrees of melt extraction. Eight of the 24 samples analysed were wehrlites or apatite-bearing peridoties, representing residual peridotite which has interacted with a carbonatitic melt. In comparison with the non-modally metasomatised xenoliths, these samples show no evidence for disturbance of Os isotopic composition, or addition of Re or Os during metasomatism. The entire suite provides a 220 km long, WNW-ESE lithospheric mantle transect, east of, and perpendicular to, the presumed Australian Precambrian shield margin. The Os model ages indicate at least three episodes of mantle depletion: ca. 1960 Ma, 800–1000 Ma and < 500 Ma. The older age is found only in the two westernmost localities where a subset of four samples define a ReOs age of 1959 ± 100 Ma, with an initial γOs = +0.2. Although the oldest exposed rocks in the region are of Cambrian age, and the presence of early Proterozoic basement is highly contentious, the Os isotopic data require that early Proterozoic basement extends some 400 km further east than the easternmost exposed early Proterozoic crust. Model ages of 800–1000 Ma are common to all but one locality, indicating at least two melt extraction events in the western localities. Paleozoic ages are only identified in the eastern localities, suggesting the lithospheric mantle becomes younger to the east. Importantly, this and other ReOs isotopic studies provide increasing evidence for the long-term stability and persistence of lithospheric mantle of Proterozoic as well as of Archean age.

Coupled climate and sea-level changes deduced from Huon Peninsula coral terraces of the last ice age

Abstract Huon Peninsula, Papua New Guinea, is a tectonically unstable, uplifting shoreline ringed by emergent coral terraces. The terraces were formed during episodes of rapid sea-level rise when corals constructed large, discrete coral platforms that were subsequently uplifted. Uranium series ages of four prominent Huon Peninsula last glacial (OIS 3) coral terraces coincide with the timing of major North Atlantic climate reversals at intervals of 6000–7000 yr between 30 000 yr and 60 000 yr ago. Terrace elevations, when combined with uplift, indicate 10–15-m high sea-level excursions at these times. We attribute the growth of the terraces directly to sea-level rises arising from ice-calving episodes from major North Atlantic ice-sheets and the Antarctic ice-sheet that precipitated extremes of cold climate called Heinrich events. These periods are associated with major discharges of land-based ice and enhanced concentrations of ice-rafted debris in deep-sea cores. Sea-levels at this time were 60–90 m lower than present.

The coral record of last interglacial sea levels and sea surface temperatures

Abstract The rise and fall of the Last Interglacial (LI) sea levels and sea surface temperatures (SSTs) are evaluated using U-series dating combined with Sr/Ca ratios in corals from both stable and tectonically uplifted sites. Along the stable coastal margin of Western Australia, an extensive series of LI coral reefs occur at heights of 2–3 m above present-day sea level. These corals have a very tight cluster of 234U–230Th ages ranging from 129±1 to 119±1 ka, as well as a narrow range of initial δ234U values of 150±5, similar to modern seawater. Bahamas, which is also a stable site, has an essentially identical pattern of U-series ages from 130±1 to 120±1 ka. Barbados and Huon Peninsula are tectonically active sites where the LI terraces are found at elevations of >50 and >200 m, respectively. U-series ages from corals exposed in the lower footwall of these uplifted reefs, allow better constraints to be placed on the rate of sea level rise which initiated the LI. Corals from the Huon Peninsula constrain sea level at −80±10 m at 131±2 ka, and from Barbados, at −30±5 m at 129±1 ka. Combined with constraints from stable sites, these observations require an exceedingly rapid rise in sea level of 30–50 m per 1000 years at 130±1 ka. This indicates that large-scale catastrophic melting of the once massive continental ice sheets occurred in phase with the rapidly increasing northern hemisphere (NH) summer insolation, consistent with the orbital forcing being the main driver of glacial–interglacial climate change. There is also some evidence from Huon Peninsula, although still not conclusive, for a precursor oscillation in sea level during the penultimate deglaciation, that may have been within ∼−20 m of present-day levels at ∼135 ka. SSTs for the LI Porites corals from the Huon Peninsula and Western Australia have mean annual temperatures and seasonal ranges that are remarkably similar to present-day patterns. The tropical site of Huon Peninsula has SSTs of 29±1°C, which is indistinguishable from the SSTs given by modern corals. At Ningaloo Reef in Western Australia, similar mean annual (∼24°C) and summer maximum SSTs of 27–29°C are found in both LI and modern corals. The only significant difference is the ∼1°C cooler winter minimum SSTs of ∼21°C for the LI compared to present-day minimums of ∼22°C. LI SSTs from these southern hemisphere (SH) sites were thus very similar, or at most, only slightly cooler than today, despite sea levels being up to 4 m higher. This maybe indicative of asymmetric warming of the Earth, with the increased NH insolation during the LI period being responsible for the extensive melting of the mainly NH-based ice sheets, and hence, higher global sea levels. The observation of relatively high sea levels in the LI, together with the rapid pulses of sea level rise, indicates that the potential now exists for greenhouse warming to initiate increases in sea level of at least several metres on relatively short time-scales (102 years).

High resolution windows into early Holocene climate: SrCa coral records from the Huon Peninsula

High-precision measurements of SrCa ratios are reported for Porites corals from the uplifted Holocene coral terraces at Huon Peninsula, Papua New Guinea. The early Holocene Porites have UTh mass spectrometric ages of 8920 ± 60 yr and 7370 ± 50 yr, and δ234U(t) values of 145 ± 2, similar to modern seawater. The SrCa coral records provide 5–6 year high resolution (near weekly) time windows into early Holocene sea surface temperatures. Seasonal temperature fluctuations are generally in the range of ± 1°C, with occasional excursions of ± 2°C, which may indicate the more frequent recurrence of very strong ENSO (El Nino-Southern Oscillation) events. Mean annual SrCa temperatures of 24.2 ± 1.1°C and 22.9 ± 0.8°C have been obtained, which are ∼ 2–3°C cooler than that exhibited by a modern Porites. These results indicate that, during the early Holocene, the equatorial western Pacific ocean was at least several degrees cooler than present-day temperatures. This is consistent with late glacial coral records from the Caribbean that indicate lower (∼ 6°C) sea surface temperatures for the equatorial oceans. The Huon Peninsula corals also indicate that SSTs were several degrees cooler than those in the Caribbean during the early Holocene. Thus, although the northern hemisphere summer radiation maximum occurred at ∼ 10 ka, there appears to have been a significant lag in the response of the equatorial western Pacific ocean to this warming. Cooler early Holocene sea surface temperatures in the western Pacific may have been due to changing patterns of ocean-atmosphere circulation, resulting from the exposure of large areas of continental shelf in the southeast Asia region, a consequence of lower glacial sea levels. It is likely that ocean temperatures in the Huon Peninsula were influenced by the opening at ∼ 7 ka of the Torres Strait, that now separates New Guinea from the Australian mainland.

Last ice age millennial scale climate changes recorded in Huon Peninsula corals

Uranium series and radiocarbon ages were measured in corals from the uplifted coral terraces of Huon Peninsula (HP), Papua New Guinea, to provide a calibration for the (super 14) C time scale beyond 30 ka (kilo annum). Improved analytical procedures, and quantitative criteria for sample selection, helped discriminate diagenetically altered samples. The base-line of the calibration curve follows the trend of increasing divergence from calendar ages, as established by previous studies. Superimposed on this trend, four well-defined peaks of excess atmospheric radiocarbon were found ranging in magnitude from 100% to 700%, relative to current levels. They are related to episodes of sea-level rise and reef growth at HP. These peaks appear to be synchronous with Heinrich Events and concentrations of ice-rafted debris found in North Atlantic deep-sea cores. Relative timing of sea-level rise and atmospheric (super 14) C excess imply the following sequence of events: An initial sea-level high is followed by a large increase in atmospheric (super 14) C as the sea-level subsides. Over about 1800 years, the atmospheric radiocarbon drops to below present ambient levels. This cycle bears a close resemblance to ice-calving episodes of Dansgaard-Oeschger and Bond cycles and the slow-down or complete interruption of the North Atlantic thermohaline circulation. The increases in the atmospheric (super 14) C levels are attributed to the cessation of the North Atlantic circulation.

Phasing and amplitude of sea-level and climate change during the penultimate interglacial

The penultimate interglacial period was punctuated by three sea-level highstands. Uranium–thorium ages obtained from speleothems in Italian caves show that the relationship between the timing of the peaks in sea level and Northern Hemisphere insolation is dependent on the previous extent of continental ice sheets.