Milankovitch forcing of the last interglacial sea level

During the last interglacial, sea level was as high as present, 4000 to 6000 years before peak Northern Hemisphere insolation receipt 126,000 years ago. The sea-level results are shown to be consistent with climate models, which simulate a 3 degrees to 4 degrees C July temperature increase from 140,000 to 130,000 years ago in high latitudes, with all Northern Hemisphere land areas being warmer than present by 130,000 years ago. The early warming occurs because obliquity peaked earlier than precession and because precession values were greater than present before peak precessional forcing occurred. These results indicate that a fuller understanding of the Milankovitch-climate connection requires consideration of fields other than just insolation forcing at 65 degrees N.     

The deep ocean during the last interglacial period

Oxygen isotope analysis of benthic foraminifera in deep sea cores from the Atlantic and Southern Oceans shows that during the last interglacial period, North Atlantic Deep Water (NADW) was 0.4 degrees +/- 0.2 degrees C warmer than today, whereas Antarctic Bottom Water temperatures were unchanged. Model simulations show that this distribution of deep water temperatures can be explained as a response of the ocean to forcing by high-latitude insolation. The warming of NADW was transferred to the Circumpolar Deep Water, providing additional heat around Antarctica, which may have been responsible for partial melting of the West Antarctic Ice Sheet.     

Ice core evidence for extensive melting of the greenland ice sheet in the last interglacial

Evidence from ice at the bottom of ice cores from the Canadian Arctic Islands and Camp Century and Dye-3 in Greenland suggests that the Greenland ice sheet melted extensively or completely during the last interglacial period more than 100 ka (thousand years ago), in contrast to earlier interpretations. The presence of dirt particles in the basal ice has previously been thought to indicate that the base of the ice sheets had melted and that the evidence for the time of original growth of these ice masses had been destroyed. However, the particles most likely blew onto the ice when the dimensions of the ice caps and ice sheets were much smaller. Ice texture, gas content, and other evidence also suggest that the basal ice at each drill site is superimposed ice, a type of ice typical of the early growth stages of an ice cap or ice sheet. If the present-day ice masses began their growth during the last interglacial, the ice sheet from the earlier (Illinoian) glacial period must have competely or largely melted during the early part of the same interglacial period. If such melting did occur, the 6-meter higher-than-present sea level during the Sangamon cannot be attributed to disintegration of the West Antarctic ice sheet, as has been suggested.     

Local orbital forcing of antarctic climate change during the last interglacial

During the last interglacial, Antarctic climate changed before that of the Northern Hemisphere. Large local changes in precession forcing could have produced this pattern if there were a rectified response in sea ice cover. Results from a coupled sea ice-ocean general circulation model supported this hypothesis when it was tested for three intervals around the last interglacial. Such a mechanism may play an important role in contributing to phase offsets between Northern and Southern Hemisphere climate change for other time intervals.     

The last interglacial: paleotemperatures and chronology

The O(18)/O(16) analysis and Th(230)/Pa(231) dating of deep-sea cores showed that the last interglacial age, with an early major temperature maximum followed by two smaller ones, extended from 100,000 to 70,000 years ago and was preceded by a glacial age extending from 120,000 to 100,000 years ago. The O(18)/O(16) analysis and Th(230)/U(234) dating of speleothems confirm and refine these ages.     

Timing, duration, and transitions of the last interglacial Asian monsoon

Thorium-230 ages and oxygen isotope ratios of stalagmites from Dongge Cave, China, characterize the Asian Monsoon and low-latitude precipitation over the past 160,000 years. Numerous abrupt changes in 18O/16O values result from changes in tropical and subtropical precipitation driven by insolation and millennial-scale circulation shifts. The Last Interglacial Monsoon lasted 9.7 +/- 1.1 thousand years, beginning with an abrupt (less than 200 years) drop in 18O/16O values 129.3 +/- 0.9 thousand years ago and ending with an abrupt (less than 300 years) rise in 18O/16O values 119.6 +/- 0.6 thousand years ago. The start coincides with insolation rise and measures of full interglacial conditions, indicating that insolation triggered the final rise to full interglacial conditions.     

Precise timing of the last interglacial period from mass spectrometric determination of thorium-230 in corals

The development of mass spectrometric techniques for determination of (230)Th abundance has made it possible to reduce analytical errors in (238)U-(234)U-(230)Th dating of corals even with very small samples. Samples of 6 x 10(8) atoms of (230)Th can be measured to an accuracy of +/-3 percent (2sigma) and 3 x 10(10) atoms of (230)Th can be measured to an accuracy of +/-0.2 percent. The time range over which useful age data on corals can be obtained now ranges from about 50 to about 500,000 years. For young corals, this approach may be preferable to (14)C dating. The precision with which the age of a coral can now be determined should make it possible to critically test the Milankovitch hypothesis concerning Pleistocene climate fluctuations. Analyses of a number of corals that grew during the last interglacial period yield ages of 122,000 to 130,000 years. The ages coincide with, or slightly postdate, the summer solar insolation high at 65 degrees N latitude which occurred 128,000 years ago. This supports the idea that changes in Pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth's orbit and rotation axis.     

The percolation phase transition in sea Ice

Sea ice exhibits a marked transition in its fluid transport properties at a critical brine volume fraction pc of about 5 percent, or temperature Tc of about -5 degreesC for salinity of 5 parts per thousand. For temperatures warmer than Tc, brine carrying heat and nutrients can move through the ice, whereas for colder temperatures the ice is impermeable. This transition plays a key role in the geophysics, biology, and remote sensing of sea ice. Percolation theory can be used to understand this critical behavior of transport in sea ice. The similarity of sea ice microstructure to compressed powders is used to theoretically predict pc of about 5 percent.     

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.     

Rapid fluctuations in sea level recorded at huon peninsula during the penultimate deglaciation

About 140,000 years ago, the breakup of large continental ice sheets initiated the Last Interglacial period. Sea level rose and peaked around 135,000 years ago about 14 meters below present levels. A record of Last Interglacial sea levels between 116,000 years to 136, 000 years ago is preserved at reef VII of the uplifted coral terraces of Huon Peninsula in Papua New Guinea. However, corals from a cave situated about 90 meters below the crest of reef VII are 130, 000 +/- 2000 years old and appear to have grown in conditions that were 6 degreesC cooler than those at present. These observations imply a drop in sea level of 60 to 80 meters. After 130,000 years, sea level began rising again in response to the major insolation maximum at 126,000 to 128,000 years ago. The early (about 140,000 years ago) start of the penultimate deglaciation, well before the peak in insolation, is consistent with the Devils Hole chronology.     

Sea level change through the last glacial cycle

Sea level change during the Quaternary is primarily a consequence of the cyclic growth and decay of ice sheets, resulting in a complex spatial and temporal pattern. Observations of this variability provide constraints on the timing, rates, and magnitudes of the changes in ice mass during a glacial cycle, as well as more limited information on the distribution of ice between the major ice sheets at any time. Observations of glacially induced sea level changes also provide information on the response of the mantle to surface loading on time scales of 10(3) to 10(5) years. Regional analyses indicate that the earth-response function is depth dependent as well as spatially variable. Comprehensive models of sea level change enable the migration of coastlines to be predicted during glacial cycles, including the anthropologically important period from about 60,000 to 20,000 years ago.     

Global sea level and Earth rotation

Recent analyses of long time scale secular variations of sea level, based on tide gauge observations, have established that sea level is apparently rising at a globally averaged rate somewhat in excess of 1 millimeter per year. It has been suggested that the nonsteric component of this secular rate might be explicable in terms of ongoing mass loss from the small ice sheets and glaciers of the world. Satellite laser ranging and very long baseline interferometry data may be used to deliver strong constraints on this important scenario because of the information that these systems provide on variations of the length of day and of the position of the rotation pole with respect to the earth's surface geography. These data demonstrate that the hypothesis of mass loss is plausible if the Barents Sea was covered by a substantial ice sheet at the last maximum of the current ice age 18,000 years ago.     

Ice core records of atmospheric N2O covering the last 106,000 years

Paleoatmospheric records of trace-gas concentrations recovered from ice cores provide important sources of information on many biogeochemical cycles involving carbon, nitrogen, and oxygen. Here, we present a 106,000-year record of atmospheric nitrous oxide (N2O) along with corresponding isotopic records spanning the last 30,000 years, which together suggest minimal changes in the ratio of marine to terrestrial N2O production. During the last glacial termination, both marine and oceanic N2O emissions increased by 40 +/- 8%. We speculate that our records do not support those hypotheses that invoke enhanced export production to explain low carbon dioxide values during glacial periods.     

Ice core records of atmospheric CO2 around the last three glacial terminations

Air trapped in bubbles in polar ice cores constitutes an archive for the reconstruction of the global carbon cycle and the relation between greenhouse gases and climate in the past. High-resolution records from Antarctic ice cores show that carbon dioxide concentrations increased by 80 to 100 parts per million by volume 600 +/- 400 years after the warming of the last three deglaciations. Despite strongly decreasing temperatures, high carbon dioxide concentrations can be sustained for thousands of years during glaciations; the size of this phase lag is probably connected to the duration of the preceding warm period, which controls the change in land ice coverage and the buildup of the terrestrial biosphere.     

Ice core evidence for Antarctic sea ice decline since the 1950s

The instrumental record of Antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal Antarctica. Here we report a significant correlation (P < 0.002) between methanesulphonic acid (MSA) concentrations from a Law Dome ice core and 22 years of satellite-derived sea ice extent (SIE) for the 80 degrees E to 140 degrees E sector. Applying this instrumental calibration to longer term MSA data (1841 to 1995 A.D.) suggests that there has been a 20% decline in SIE since about 1950. The decline is not uniform, showing large cyclical variations, with periods of about 11 years, that confuse trend detection over the relatively short satellite era.