Kurt Lambeck

Sea-level and deep water temperature changes derived from benthic foraminifera isotopic records.

We show that robust regressions can be established between relative sea-level (RSL) data and benthic foraminifera oxygen isotopic ratios from the North Atlantic and Equatorial Pacific Ocean over the last climatic cycle. We then apply these regressions to long benthic isotopic records retrieved at one North Atlantic and one Equatorial Pacific site to build a composite RSL curve, as well as the associated confidence interval, over the last four climatic cycles. Our proposed reconstruction of RSL is in good agreement with the sparse RSL data available prior to the last climatic cycle. We compute bottom water temperature changes at the two sites and at one Southern Indian Ocean site, taking into account potential variations in North Atlantic local deep water δ18O. Our results indicate that a Last Glacial Maximum (LGM) enrichment of the ocean mean oxygen isotopic ratio of 0.95‰ is the lowest value compatible with unfrozen deep waters in the Southern Indian Ocean if local deep water δ18O did not increase during glacials with respect to present. Such a value of the LGM mean ocean isotopic enrichment would impose a maximum decrease in local bottom water δ18O at the North Atlantic site of 0.30‰ during glacials.

Timing of the Last Glacial Maximum from observed sea-level minima

During the Last Glacial Maximum, ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today. But few direct estimates exist of the volume of the ice sheets, or the timing and rates of change during their advance and retreat. Here we analyse four distinct sediment facies in the shallow, tectonically stable Bonaparte Gulf, Australia—each of which is characteristic of a distinct range in sea level—to estimate the maximum volume of land-based ice during the last glaciation and the timing of the initial melting phase. We use faunal assemblages and preservation status of the sediments to distinguish open marine, shallow marine, marginal marine and brackish conditions, and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio-hydro-isostatic modelling. Our results indicate that from at least 22,000 to 19,000 (calendar) years before present, land-based ice volume was at its maximum, exceeding todays grounded ice sheets by 52.5 × 106 km3. A rapid decrease in ice volume by about 10% within a few hundred years terminated the Last Glacial Maximum at 19,000 ± 250 years.

Refining the eustatic sea-level curve since the Last Glacial Maximum using far- and intermediate-field sites

The eustatic component of relative sea-level change provides a measure of the amount of ice transferred between the continents and oceans during glacial cycles. This has been quantified for the period since the Last Glacial Maximum by correcting observed sea-level change for the glacio-hydro-isostatic contributions using realistic ice distribution and earth models. During the Last Glacial Maximum (LGM) the eustatic sea level was 125±5 m lower than the present day, equivalent to a land-based ice volume of (4.6–4.9)×107 km3. Evidence for a non-uniform rise in eustatic sea level from the LGM to the end of the deglaciation is examined. The initial rate of rise from ca. 21 to 17 ka was relatively slow with an average rate of ca. 6 m ka−1, followed by an average rate of ca. 10 m ka−1 for the next 10 ka. Significant departures from these average rates may have occurred at the time of the Younger Dryas and possibly also around 14 ka. Most of the decay of the large ice sheets was completed by 7 ka, but 3–5 m of water has been added to the oceans since that time.

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.

Estimates of the Regional Distribution of Sea Level Rise over the 1950–2000 Period

Abstract TOPEX/Poseidon satellite altimeter data are used to estimate global empirical orthogonal functions that are then combined with historical tide gauge data to estimate monthly distributions of large-scale sea level variability and change over the period 1950–2000. The reconstruction is an attempt to narrow the current broad range of sea level rise estimates, to identify any pattern of regional sea level rise, and to determine any variation in the rate of sea level rise over the 51-yr period. The computed rate of global-averaged sea level rise from the reconstructed monthly time series is 1.8 ± 0.3 mm yr−1. With the decadal variability in the computed global mean sea level, it is not possible to detect a significant increase in the rate of sea level rise over the period 1950–2000. A regional pattern of sea level rise is identified. The maximum sea level rise is in the eastern off-equatorial Pacific and there is a minimum along the equator, in the western Pacific, and in the eastern Indian Ocean. A g...

Sea level and global ice volumes from the Last Glacial Maximum to the Holocene

Significance Several areas of earth science require knowledge of the fluctuations in sea level and ice volume through glacial cycles. These include understanding past ice sheets and providing boundary conditions for paleoclimate models, calibrating marine-sediment isotopic records, and providing the background signal for evaluating anthropogenic contributions to sea level. From ∼1,000 observations of sea level, allowing for isostatic and tectonic contributions, we have quantified the rise and fall in global ocean and ice volumes for the past 35,000 years. Of particular note is that during the ∼6,000 y up to the start of the recent rise ∼100−150 y ago, there is no evidence for global oscillations in sea level on time scales exceeding ∼200 y duration or 15−20 cm amplitude. The major cause of sea-level change during ice ages is the exchange of water between ice and ocean and the planet’s dynamic response to the changing surface load. Inversion of ∼1,000 observations for the past 35,000 y from localities far from former ice margins has provided new constraints on the fluctuation of ice volume in this interval. Key results are: (i) a rapid final fall in global sea level of ∼40 m in <2,000 y at the onset of the glacial maximum ∼30,000 y before present (30 ka BP); (ii) a slow fall to −134 m from 29 to 21 ka BP with a maximum grounded ice volume of ∼52 × 106 km3 greater than today; (iii) after an initial short duration rapid rise and a short interval of near-constant sea level, the main phase of deglaciation occurred from ∼16.5 ka BP to ∼8.2 ka BP at an average rate of rise of 12 m⋅ka−1 punctuated by periods of greater, particularly at 14.5–14.0 ka BP at ≥40 mm⋅y−1 (MWP-1A), and lesser, from 12.5 to 11.5 ka BP (Younger Dryas), rates; (iv) no evidence for a global MWP-1B event at ∼11.3 ka BP; and (v) a progressive decrease in the rate of rise from 8.2 ka to ∼2.5 ka BP, after which ocean volumes remained nearly constant until the renewed sea-level rise at 100–150 y ago, with no evidence of oscillations exceeding ∼15–20 cm in time intervals ≥200 y from 6 to 0.15 ka BP.

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.

Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound

Sea-level change around the British Isles since the time of the last glacial maximum is largely due to of the crustal rebound from the glacial unloading of northern Britain and the concomitant melt-water loading of the adjacent seas and Atlantic Ocean. Minor, but not insignificant, contributions also result from the rebound caused by the unloading of the distant ice sheets, including Fennoscandia and North America. Observations of sea-level change for this period constrain the glacio-hydro-isostatic rebound model parameters describing the effective lithospheric thickness or rigidity and the effective mantle viscosity, as well as certain ice sheet characteristics such as the ice thickness at the time of the last glacial maximum. The models permit palaeobathymetry and palaeoshorelines to be predicted for the British Isles region, including the North Sea. The resulting evolution of the coastlines exhibits a complex behaviour through time, one that is quite different from the usual models in which sea-level change is assumed to be a function of time only. In part this is because of the delayed response of the mantle to the spatially variable and time-dependent ice and water loads, and in part because the unloading history of the British ice sheet is different from those of the major global ice sheets. Thus, maximum emergence of the North Sea occurred after deglaciation had started and lasted for an extended period from about 15 000 to 12 000 (radiocarbon) years BP. During this relative sea-level still-stand shoreline features could have formed, for example, along the western edge of the Norwegian Trough when access to the firths of eastern Scotland would have been via a long and shallow marine inlet. Shoreline retreat across the North Sea became relatively rapid after about 10000 years. The model predictions for the Irish and Celtic Seas also suggest a complex behaviour, with the formation of a wide land bridge between about 20000 and 13 000 years ago. The model also suggests that as long as the Scottish ice extended across the northern Irish Sea, until about 14 000 years ago, there would have been a large freshwater periglacial lake located further south. Both the predicted sea-level height-age relations and the shoreline positions are consistent with a large body of observational evidence but some discrepancies occur, particularly in northern Scotland and Ireland where the ice heights may have been somewhat greater than assumed in the model.

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.

On a tectonic mechanism for regional sealevel variations

Abstract No satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm/1000 years and a magnitude of up to a few hundred meters that have been proposed by Vail et al. [1]. We propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales. The proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading. Apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction. The mechanism is most effective for young margins that are subject to rapid sediment loading. By its nature, the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments. In principle, it implies the possibility of regional correlations in different basinal settings.