Effect of sedimentation on ice-sheet grounding-line stability

Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level of at least several meters. Recent Antarctic changes thus cannot be attributed to sea-level rise, strengthening earlier interpretations that warming has driven ice-sheet mass loss. Large sea-level rise, such as the approximately 100-meter rise at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past.     

Orbital forcing of the marine isotope stage 9 interglacial

Milankovitch orbital forcing theory has been used to assign time scales to many paleoclimate records. However, the validity of this theory remains uncertain, and independent sea-level chronologies used to test its applicability have been restricted largely to the past approximately 135,000 years. Here, we report U-series ages for coral reefs formed on Henderson Island during sea-level high-stands occurring at approximately 630,000 and approximately 330,000 years ago. These data are consistent with the hypothesis that interglacial climates are forced by Northern Hemisphere summer solar insolation centered at 65 degrees N latitude, as predicted by Milankovitch theory.     

Long-term sea-level fluctuations driven by ocean basin dynamics

Earth's long-term sea-level history is characterized by widespread continental flooding in the Cretaceous period (approximately 145 to 65 million years ago), followed by gradual regression of inland seas. However, published estimates of the Late Cretaceous sea-level high differ by half an order of magnitude, from approximately 40 to approximately 250 meters above the present level. The low estimate is based on the stratigraphy of the New Jersey margin. By assimilating marine geophysical data into reconstructions of ancient ocean basins, we model a Late Cretaceous sea level that is 170 (85 to 270) meters higher than it is today. We use a mantle convection model to suggest that New Jersey subsided by 105 to 180 meters in the past 70 million years because of North America's westward passage over the subducted Farallon plate. This mechanism reconciles New Jersey margin-based sea-level estimates with ocean basin reconstructions.     

Ice-sheet and sea-level changes

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level approximately 70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered.     

Thorium-230 ages of corals and duration of the last interglacial sea-level high stand on oahu, hawaii

Thorium-230 ages of emergent marine deposits on Oahu, Hawaii, have a uniform distribution of ages from approximately 114,000 to approximately 131,000 years, indicating a duration for the last interglacial sea-level high stand of approximately 17,000 years, in contrast to a duration of approximately 8000 years inferred from the orbitally tuned marine oxygen isotope record. Sea level on Oahu rose to >/=1 to 2 meters higher than present by 131,000 years ago or approximately 6000 years earlier than inferred from the marine record. Although the latter record suggests a shift back to glacial conditions beginning at approximately 119,000 years ago, the Oahu coral ages indicate a near present sea level until approximately 114,000 years ago.     

Ice sheet and solid Earth influences on far-field sea-level histories

Previous predictions of sea-level change subsequent to the last glacial maximum show significant, systematic discrepancies between observations at Tahiti, Huon Peninsula, and Sunda Shelf during Lateglacial time (approximately 14,000 to 9000 calibrated years before the present). We demonstrate that a model of glacial isostatic adjustment characterized by both a high-viscosity lower mantle (4 x 10(22) Pa s) and a large contribution from the Antarctic ice sheet to meltwater pulse IA (approximately 15-meters eustatic equivalent) resolves these discrepancies. This result supports arguments that an early and rapid Antarctic deglaciation contributed to a sequence of climatic events that ended the most recent glacial period of the current ice age.     

Open-system coral ages reveal persistent suborbital sea-level cycles

Sea level is a sensitive index of global climate that has been linked to Earth's orbital variations, with a minimum periodicity of about 21,000 years. Although there is ample evidence for climate oscillations that are too frequent to be explained by orbital forcing, suborbital-frequency sea-level change has been difficult to resolve, primarily because of problems with uranium/thorium coral dating. Here we use a new approach that corrects coral ages for the frequently observed open-system behavior of uranium-series nuclides, substantially improving the resolution of sea-level reconstruction. This curve reveals persistent sea-level oscillations that are too frequent to be explained exclusively by orbital forcing.     

A chronology of Paleozoic sea-level changes

Sea levels have been determined for most of the Paleozoic Era (542 to 251 million years ago), but an integrated history of sea levels has remained unrealized. We reconstructed a history of sea-level fluctuations for the entire Paleozoic by using stratigraphic sections from pericratonic and cratonic basins. Evaluation of the timing and amplitude of individual sea-level events reveals that the magnitude of change is the most problematic to estimate accurately. The long-term sea level shows a gradual rise through the Cambrian, reaching a zenith in the Late Ordovician, then a short-lived but prominent withdrawal in response to Hirnantian glaciation. Subsequent but decreasingly substantial eustatic highs occurred in the mid-Silurian, near the Middle/Late Devonian boundary, and in the latest Carboniferous. Eustatic lows are recorded in the early Devonian, near the Mississippian/Pennsylvanian boundary, and in the Late Permian. One hundred and seventy-two eustatic events are documented for the Paleozoic, varying in magnitude from a few tens of meters to approximately 125 meters.     

Meltwater pulse 1A from Antarctica as a trigger of the Bølling-Allerød warm interval

Meltwater pulse 1A (mwp-1A) was a prominent feature of the last deglaciation, which led to a sea-level rise of approximately 20 meters in less than 500 years. Concurrent with mwp-1A was the onset of the B?lling-Aller?d interstadial event (14,600 years before the present), which marked the termination of the last glacial period. Previous studies have been unable to reconcile a warm Northern Hemisphere with mwp-1A originating from the Laurentide or Fennoscandian ice sheets. With the use of a climate model of intermediate complexity, we demonstrate that with mwp-1A originating from the Antarctic Ice Sheet, consistent with recent sea-level fingerprinting inferences, the strength of North Atlantic Deep Water (NADW) formation increases, thereby warming the North Atlantic region and providing an explanation for the onset of the B?lling-Aller?d warm interval. The established mode of active NADW formation is then able to respond to subsequent freshwater forcing from the Laurentide and Fennoscandian ice sheets, setting the stage for the Younger Dryas cold period.     

Worldwide initiation of holocene marine deltas by deceleration of sea-level rise

Radiocarbon-dated deltaic sequences of Holocene age from different parts of the world began to accumulate within a restricted time range, from about 8500 to 6500 years ago. Evaluation of major delta processes indicates that deceleration in sea-level rise was the key factor in Holocene delta formation. Within many deltas, there is as much as a 2000-year age range between basal deposits in seaward and landward cores. This age difference records the progressive landward migration of near mean sea-level depositional environments during the lower to mid-Holocene. Establishment of a chronostratigraphic framework for Holocene delta development provides a fundamental global baseline for distinguishing sea-level change from vertical land motion by tectonism and isostasy, and for evaluating rates of future marine incursion into low-lying deltas.     

A semi-empirical approach to projecting future sea-level rise

A semi-empirical relation is presented that connects global sea-level rise to global mean surface temperature. It is proposed that, for time scales relevant to anthropogenic warming, the rate of sea-level rise is roughly proportional to the magnitude of warming above the temperatures of the pre-Industrial Age. This holds to good approximation for temperature and sea-level changes during the 20th century, with a proportionality constant of 3.4 millimeters/year per degrees C. When applied to future warming scenarios of the Intergovernmental Panel on Climate Change, this relationship results in a projected sea-level rise in 2100 of 0.5 to 1.4 meters above the 1990 level.     

Comment on "A semi-empirical approach to projecting future sea-level rise"

Rahmstorf (Reports, 19 January 2007, p. 368) presented an approach for predicting sea-level rise based on a proposed linear relationship between global mean surface temperature and the rate of global mean sea-level change. We find no such linear relationship. Although we agree that there is considerable uncertainty in the prediction of future sea-level rise, this approach does not meaningfully contribute to quantifying that uncertainty.     

Isotopic evidence for glaciation during the Cretaceous supergreenhouse

The Turonian (93.5 to 89.3 million years ago) was one of the warmest periods of the Phanerozoic eon, with tropical sea surface temperatures over 35 degrees C. High-amplitude sea-level changes and positive delta18O excursions in marine limestones suggest that glaciation events may have punctuated this episode of extreme warmth. New delta18O data from the tropical Atlantic show synchronous shifts approximately 91.2 million years ago for both the surface and deep ocean that are consistent with an approximately 200,000-year period of glaciation, with ice sheets of about half the size of the modern Antarctic ice cap. Even the prevailing supergreenhouse climate was not a barrier to the formation of large ice sheets, calling into question the common assumption that the poles were always ice-free during past periods of intense global warming.     

Kinematic constraints on glacier contributions to 21st-century sea-level rise

On the basis of climate modeling and analogies with past conditions, the potential for multimeter increases in sea level by the end of the 21st century has been proposed. We consider glaciological conditions required for large sea-level rise to occur by 2100 and conclude that increases in excess of 2 meters are physically untenable. We find that a total sea-level rise of about 2 meters by 2100 could occur under physically possible glaciological conditions but only if all variables are quickly accelerated to extremely high limits. More plausible but still accelerated conditions lead to total sea-level rise by 2100 of about 0.8 meter. These roughly constrained scenarios provide a "most likely" starting point for refinements in sea-level forecasts that include ice flow dynamics.     

Sea-level fingerprinting as a direct test for the source of global meltwater pulse IA

The ice reservoir that served as the source for the meltwater pulse IA remains enigmatic and controversial. We show that each of the melting scenarios that have been proposed for the event produces a distinct variation, or fingerprint, in the global distribution of meltwater. We compare sea-level fingerprints associated with various melting scenarios to existing sea-level records from Barbados and the Sunda Shelf and conclude that the southern Laurentide Ice Sheet could not have been the sole source of the meltwater pulse, whereas a substantial contribution from the Antarctic Ice Sheet is consistent with these records.     

钢框架正常使用极限状态 考虑二阶效应的侧移性能研究

针对我国钢框架高等分析研究主要针对承载力极限状态的现状,分别采用传统一阶侧移计算方法、二阶简化侧移计算方法和二阶精确侧移计算方法对1榀2跨15层钢框架在正常使用极限状态时的侧移变形进行了计算,获得了钢框架在3种计算方法下的顶点侧移及层间侧移,并将获得的侧移计算结果进行了对比研究了钢框架考虑二阶效应后对结构侧移变形的影响,探讨了钢框架考虑二阶效应影响后在正常使用极限状态下的侧移验算准则结果表明：钢框架考虑二阶效应后,结构的侧移变形明显增加,其柱顶及层间侧移均比传统一阶分析结果增加很多,结构考虑二阶效应后,如仍采用我国现行钢结构设计规范中的侧移验算准则,将会显著增加结构用钢量.二阶简化侧移计算方法与二阶精确侧移计算方法相比,精度较高,可用于钢框架的二阶侧移计算.     

The Phanerozoic record of global sea-level change

We review Phanerozoic sea-level changes [543 million years ago (Ma) to the present] on various time scales and present a new sea-level record for the past 100 million years (My). Long-term sea level peaked at 100 +/- 50 meters during the Cretaceous, implying that ocean-crust production rates were much lower than previously inferred. Sea level mirrors oxygen isotope variations, reflecting ice-volume change on the 10(4)- to 10(6)-year scale, but a link between oxygen isotope and sea level on the 10(7)-year scale must be due to temperature changes that we attribute to tectonically controlled carbon dioxide variations. Sea-level change has influenced phytoplankton evolution, ocean chemistry, and the loci of carbonate, organic carbon, and siliciclastic sediment burial. Over the past 100 My, sea-level changes reflect global climate evolution from a time of ephemeral Antarctic ice sheets (100 to 33 Ma), through a time of large ice sheets primarily in Antarctica (33 to 2.5 Ma), to a world with large Antarctic and large, variable Northern Hemisphere ice sheets (2.5 Ma to the present).     

The structure of ferrihydrite, a nanocrystalline material

Despite the ubiquity of ferrihydrite in natural sediments and its importance as an industrial sorbent, the nanocrystallinity of this iron oxyhydroxide has hampered accurate structure determination by traditional methods that rely on long-range order. We uncovered the atomic arrangement by real-space modeling of the pair distribution function (PDF) derived from direct Fourier transformation of the total x-ray scattering. The PDF for ferrihydrite synthesized with the use of different routes is consistent with a single phase (hexagonal space group P6(3)mc; a = approximately 5.95 angstroms, c = approximately 9.06 angstroms). In its ideal form, this structure contains 20% tetrahedrally and 80% octahedrally coordinated iron and has a basic structural motif closely related to the Baker-Figgis delta-Keggin cluster. Real-space fitting indicates structural relaxation with decreasing particle size and also suggests that second-order effects such as internal strain, stacking faults, and particle shape contribute to the PDFs.     

Sea Levels during the Past 35,000 Years

A sea-level curve of the past 35,000 years for the Atlantic continental shelf of the United States is based on more than 80 radiocarbon dates, 15 of which are older than 15,000 years. Materials include shallow-water mollusks, oolites, coralline algae, beachrock, and salt-marsh peat. Sea level 30,000 to 35,000 years ago was near the present one. Subsequent glacier growth lowered sea level to about -130 meters 16,000 years ago. Holocene transgression probably began about 14,000 years ago, and continued rapidly to about 7000 years ago. Dates from most shelves of the world agree with this curve, suggesting that it is approximately the eustatic curve for the period.     

Ocean science. Coral clues to rapid sea-level change

Earth's climate can change substantially on time scales of 1000 years or so, but given the time it takes for an ice sheet to grow or melt, it has been unclear whether continental ice sheets-and hence global sea levels-mirror these rapid changes. In his Perspective, Henderson discusses the report by Thompson and Goldstein, who have used a new correction method to date coral samples that are up to 250,000 years old. The corals can be used to deduce past sea levels. The resulting sea-level record shows that sea levels have varied on millennial time scales even during times of high sea level and relative climate stability.