Variations In Summer Insolation Research Articles

Thermocline variability in the subtropical northwestern Pacific since the last deglaciation

Thermocline conditions in the tropical western Pacific are closely linked to regional- and basin-scale hydrological variation and may have played a critical role in global climate evolution on various timescales. However, long-term changes in thermocline hydrology in the subtropical northwestern Pacific and their connection to the tropical Pacific remain unclear. Based on planktonic foraminiferal Mg/Ca and δ18O, we reconstructed the temperature and salinity of thermocline water since the last deglaciation, using core OKI02 from the Okinawa Trough (OKT) in the subtropical northwestern Pacific. The results indicated a discrepancy in evolution of both temperature and δ18O between the thermocline and surface water over the past 19 ka. Unlike for surface water, changes in thermocline hydrology correlated with changing conditions in the tropical western Pacific, indicating a close relationship between these regions since the last deglaciation. Thermocline temperature and salinity gradients between the OKT and the tropical western Pacific increased from ∼16.5 to 10 ka BP, reached their maximum values at approximately 10 ka BP, and then decreased from the Early Holocene to present. Variations in temperature and salinity gradients correlated with changes in the El Niño–Southern Oscillation (ENSO) over the past 19 ka, which were coupled with equatorial summer insolation variations. We suggest that the ENSO-like process, forced by equatorial summer insolation, regulated thermocline dynamics in the subtropical northwestern Pacific Ocean since the last deglaciation, by influencing the strength of the Kuroshio Current.

Holocene environmental evolution in the Qinghai Lake area recorded by aeolian deposits

Aeolian deposits, widely distributed in the Qinghai Lake area, provide independently palaeoclimatic information. Herein we report the detailed investigation results of Gangcha (GC) and Reshui (RS) sections, located at the northern Qinghai Lake area. The chemical index of alteration (CIA) and Al2O3–(CaO*+Na2O)–K2O (A−CN–K) ternary diagram suggest that most of the sediments experienced the degree of weak and moderate chemical weathering. The study area had undergone the great alternation between wet and dry climate, reflected by the variation of CIA values. The accumulation hiatus in the palaeosol layer was identified in the RS section. Our comprehensive results indicate that the relatively dry climate conditions and strong aeolian activity occurred from 11.0 to 7.5 ka in the Qinghai Lake area, in agreement with widespread aeolian sand deposits and loess deposits in the investigated aeolian sections. The middle Holocene (from 7.5 to 3.0 ka) was characterized by humid climate conditions and weak aeolian activity, supported by commonly developed palaeosol deposits in the Qinghai Lake area. After 3.0 ka, the aeolian activity was again strengthened due to drier climate conditions. The variations of summer insolation and the Asian monsoon may cause the increase or decrease of effective moisture, thereby result in the climate changes between dry and wet in the Qinghai Lake area.

Revised chronology of the ICDP Dead Sea deep drill core relates drier-wetter-drier climate cycles to insolation over the past 220 kyr

The Dead Sea Deep Drilling Project drilled 456 meters into the deepest floor of the Dead Sea and recovered a record of the past ∼220 kyr of the Levant hydroclimate history, that is, Marine Isotope Stages 1–7, including the last three interglacials and the last two glacials. We present an updated chronology of the core from DSDDP Hole 5017-1-A, from the Dead Sea’s deepest basin, that refines our previous chronology (Torfstein et al. 2015) based on new information. The updated chronology uses the following approaches: (1) radiocarbon ages of Kitagawa et al. (2017); (2) correlation of specific layers in the core with U–Th-dated sediments on the Dead Sea margin, particularly during the interval of glacial Lake Lisan (MIS 2,3,4); (3) tuning of the δ18O data of DSDDP core aragonite to the U–Th dated oxygen isotopes of regional cave speleothems; and (4) tuning of the DSDDP aragonite δ18O data to summer insolation curves when the cave δ18O chronology is less clear. The updated chronology reveals a strong relationship between the sedimentary facies comprising the core and Northern Hemisphere summer insolation variations. It shows that sequences of sediments marking drier/wetter/drier climate conditions in the lake’s watershed (e.g., salt/muds/salt, respectively) appear across the flank/peak/flank segments of several summer insolation peaks. In particular, the transition from drier to wetter sedimentary facies during mid-latitude insolation peaks coincides with the intervals of sapropel conditions in the Mediterranean, reflecting enhanced Nile flow due to intense African monsoonal conditions, and marking the impact of the tropical precession cycles on Eastern Mediterranean hydroclimate. This pattern was lost during MIS 2,3,4, when mostly sequences of primary aragonite are punctuated by gypsum precipitation during Heinrich events, marking the strong impact of the North Atlantic on the last glacial Levant hydroclimate.

Indian Summer Monsoon variations and competing influences between hemispheres since ~35 ka recorded in Tengchongqinghai Lake, southwestern China

The southwestern Yunnan Province of China, which is located at the southeastern margin of the Tibetan Plateau and close to Bay of Bengal, is significantly influenced by the Indian Summer Monsoon (ISM). In this study, we reconstruct proxies for the ISM from 35 to 1 ka through detailed analysis of grain-size distribution, geochemical composition and environmental magnetism from a 7.96 m sediment core from Tengchongqinghai Lake, Yunnan Province, China. Globally recognized, abrupt climatic events, including Heinrich Events 0–3 (H0−H3) and the Bølling-Allerød (B/A) warm period are identified in most of our proxies, and the long-term trend is consistent with other published records such as stalagmite oxygen isotopes (δ18O) from Sangxing Cave. Northern Hemisphere (NH) temperature, which is influenced by NH solar insolation, is commonly suggested to play a dominant role in controlling the ISM. A comparison of our record with the δ18O variations of ice cores from Greenland and Antarctica, a sea surface temperature (SST) record from the Bay of Bengal, and summer solar insolation at 25°N latitude demonstrates that the general pattern of ISM change does follow variations in summer insolation; however, the ISM lags summer insolation by thousands of years. While the ISM fluctuations are highly correlated with NH temperature on shorter timescales (centennial-millennial), the gradually weakened ISM from 22.5 ka until the Last Glacial Maximum (LGM) indicates a close relationship with the rise of Southern Hemisphere (SH) temperature and the relatively cold background of the SH. Our record expands on the findings of ISM records from Heqing paleolake basin in southwestern China and the Arabian Sea sediments, suggesting that the NH and SH have a competitive influence on ISM by controlling the cross-equatorial pressure gradient. This relationship means that when NH temperatures are relatively high, it has a stronger influence on the ISM than SH influences. In contrast, when the SH temperature is relatively low, it has a dominant influence on ISM. In addition, we speculate that the change of SH temperature not only influences the cross-equatorial pressure gradient directly, but also likely modulates the circulation system of ocean energy by influencing the Atlantic Meridional Overturning Circulation (AMOC).

Precessional control on ocean productivity in the Western Pacific Warm Pool for the last 400 kyr: Insight from biogenic magnetite

AbstractsThe Western Pacific Warm Pool plays a significant role in large‐scale atmospheric circulation and global hydrology. We conducted an environmental magnetic study of two late Pleistocene sediment cores from the western equatorial Pacific Ocean offshore of New Guinea in order to better constrain climatic and oceanographic variability, particularly spatiotemporal ocean productivity variations. Magnetic property measurements and transmission electron microscopy reveal that the magnetic mineral assemblages in the studied sediments are a mixture of biogenic and terrigenous magnetite. Variations in the acid soluble sediment component, interpreted as carbonate content, and the proportion of biogenic to terrigenous magnetite estimated from the ratio of anhysteretic to saturation remanent magnetizations are in‐phase with northern hemisphere summer insolation variations. We interpret that ocean productivity increased during insolation maxima, which induced higher populations of magnetotactic bacteria through a larger nutrient supply to the seafloor. This interpretation assumes that magnetotactic bacterial populations are greatest in sediments just below the seafloor. Precessional frequencies in magnetic mineral concentration variations are suppressed after correction for carbonate dilution, whereas cyclic changes with a ∼100 kyr periodicity remain in carbonate‐free magnetic concentration variations. Glacial/interglacial changes in bottom water currents may have influenced transportation and deposition of magnetic minerals. We demonstrate the usefulness of magnetic proxies for paleoceanographic studies, particularly of biogenic magnetite proxies for estimating paleoproductivity variations.

Environmental control on the occurrence of high-coercivity magnetic minerals and formation of iron sulfides in a 640 ka sediment sequence from Lake Ohrid (Balkans)

Abstract. The bulk magnetic mineral record from Lake Ohrid, spanning the past 637 kyr, reflects large-scale shifts in hydrological conditions, and, superimposed, a strong signal of environmental conditions on glacial–interglacial and millennial timescales. A shift in the formation of early diagenetic ferrimagnetic iron sulfides to siderites is observed around 320 ka. This change is probably associated with variable availability of sulfide in the pore water. We propose that sulfate concentrations were significantly higher before ∼ 320 ka, due to either a higher sulfate flux or lower dilution of lake sulfate due to a smaller water volume. Diagenetic iron minerals appear more abundant during glacials, which are generally characterized by higher Fe / Ca ratios in the sediments. While in the lower part of the core the ferrimagnetic sulfide signal overprints the primary detrital magnetic signal, the upper part of the core is dominated by variable proportions of high- to low-coercivity iron oxides. Glacial sediments are characterized by high concentration of high-coercivity magnetic minerals (hematite, goethite), which relate to enhanced erosion of soils that had formed during preceding interglacials. Superimposed on the glacial–interglacial behavior are millennial-scale oscillations in the magnetic mineral composition that parallel variations in summer insolation. Like the processes on glacial–interglacial timescales, low summer insolation and a retreat in vegetation resulted in enhanced erosion of soil material. Our study highlights that rock-magnetic studies, in concert with geochemical and sedimentological investigations, provide a multi-level contribution to environmental reconstructions, since the magnetic properties can mirror both environmental conditions on land and intra-lake processes.

Synchronicity of Antarctic temperatures and local solar insolation on orbital timescales

The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes. The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation. Antarctic climate is therefore thought to be driven by northern summer insolation. A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the 'recorder' system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.

Late Pleistocene–Holocene rise and collapse of Lake Suguta, northern Kenya Rift

The Late Pleistocene to Middle Holocene African Humid Period (AHP) was characterized by dramatic hydrologic fluctuations in the tropics. A better knowledge of the timing, spatial extent, and magnitude of these hydrological fluctuations is essential to decipher the climate-forcing mechanisms that controlled them. The Suguta Valley (2°N, northern Kenya Rift) has recorded extreme environmental changes during the AHP. Extensive outcrops of lacustrine sediments, ubiquitous wave-cut notches, shorelines, and broad terrace treads along the valley margins are the vestiges of Lake Suguta, which once filled an 80 km long and 20 km wide volcano–tectonic depression. Lake Suguta was deep between 16.5 and 8.5 cal ka BP. During its maximum highstand, it attained a water depth of ca 300 m, a surface area of ca 2150 km 2, and a volume of ca 390 km 3. The spatial distribution of lake sediments, the elevation of palaeo-shorelines, and other geomorphic evidences suggest that palaeo-Lake Suguta had an overflow towards the Turkana basin to the north. After 8.5 cal ka BP, Lake Suguta abruptly disappeared. A comparison of the Lake Suguta water-level curve with other reconstructed water levels from the northern part of the East African Rift System shows that local insolation, which is dominated by precessional cycles, may have controlled the timing of lake highstands in this region. Our data show that changes of lake levels close to the Equator seem to be driven by fluctuations of spring insolation, while fluctuations north of the Equator are apparently related to variations in summer insolation. However, since these inferred timings of lake-level changes are mostly based on the radiocarbon dating of carbonate shells, which may have been affected by a local age reservoir, alternative dating methods are needed to support this regional synthesis. Between 12.7 and 11.8 cal ka BP, approximately during the Northern Hemisphere high-latitude Younger Dryas, the water level of Lake Suguta fell by ca 50 m, suggesting that remote influences also affected local hydrology.

Vegetation and climate dynamics during the early Middle Miocene from Lake Sinj (Dinaride Lake System, SE Croatia)

Pollen data from sediments from an outcrop section in the ancient Lake Sinj (Dinaride Lake System, SE Croatia) record variations in vegetation during the early Middle Miocene. Good correspondence between the palaeo-vegetation and the sedimentological profile suggests that both records are mainly controlled by variations in the past climatic conditions. Periods of frequent coal deposition generally coincide with high abundance of thermophilous and xeric indicators suggesting warm and dry climate. Pollen and sediment records show two large-scale warming and shallowing-upward cycles and smaller-scale cyclicity. The high relative percentage of warm pollen taxa in the entire record, with Engelhardia as the main component, generally indicates a subtropical humid climate for this area during the early Middle Miocene, probably related to the Miocene Climatic Optimum. This study shows a response of the vegetation to climate change at short-term, probably related to orbital cyclic variations in summer insolation during the Middle Miocene.

Millennial- and centennial-scale vegetation and climate changes during the late Pleistocene and Holocene from northern New Mexico (USA)

High-resolution pollen and magnetic susceptibility (MS) data from a sediment core from an alpine bog (3100 m) in the Sangre de Cristo Mountains (New Mexico) record variations in treeline elevation and in sedimentation for the last 14 ka (cal yr BP). Strong correspondence between the paleovegetation and the MS profile suggests that both records can be used to reconstruct past climatic conditions. The coldest conditions in this area occurred at the end of the late Pleistocene during the Pinedale glaciation and the Younger Dryas chron. A general warming trend took place during the early Holocene, lasting until ∼5.6 ka, when the warmest conditions occurred. A progressive climate cooling is then observed until today. Millennial- and centennial-scale changes are also observed throughout the vegetation and MS records. The higher amplitude millennial-scale cold events appear to correlate with the timing and duration of episodes of enhanced sea-ice drift in the North Atlantic and the lower amplitude centennial-scale cold events may correlate with periods of lower solar activity. A solar-climate connection is suggested from these records by a prominent ca 200-yr cycle in tree pollen abundance, which may correlate with the 208 yr Suess solar cycle. This study shows an immediate response of the vegetation (treeline) to climate change at millennial- and centennial-scales, probably related to variations in summer insolation and solar activity during the late Pleistocene and Holocene. Such high-resolution studies are very important in order to predict future climate change and particularly in very arid areas, where human activity and economies are strongly influenced by climate.

Timing and significance of maximum and minimum equatorial insolation

Variations in summer insolation at high northern latitudes on a timescale of 100 ka are very small. Thus a common belief is that the pronounced ∼100 ka glacial cycles are not directly linked to the very weak 100 ka insolation periodicity. Here we show, analytically and numerically, that the annual maximum (and minimum) of daily equatorial insolation has pronounced eccentricity periodicities, with timescales of ∼400 ka and ∼100 ka, as well as a pronounced half‐precession periodicity with timescale of ∼11 ka. The timing of the maximum (and minimum) annual equatorial insolation may change around the equinoxes (solstices), alternating between the vernal and autumnal equinoxes (summer and winter solstices) where the time of the maximum (minimum) equatorial insolation may occur up to more than 1 month from the equinoxes (solstices). We also show that when considering the mean insolation of periods larger than 1 d, the ∼11 ka periodicity becomes less dominant, and it vanishes when the averaging period is half a year; for the later case the maximum (minimum) may occur for any day in the annual cycle. The maximum equatorial insolation may alter the timing and amplitude of the maximum surface temperature of the summer hemisphere and in this way may drastically affect the Hadley circulation. Changes in Hadley circulation affect the heat and moisture transport from low to high latitudes, affecting the buildup of the high‐latitude Northern Hemisphere ice sheets.

Northern Hemisphere forcing of climatic cycles in Antarctica over the past 360,000 years

The Milankovitch theory of climate change proposes that glacial-interglacial cycles are driven by changes in summer insolation at high northern latitudes. The timing of climate change in the Southern Hemisphere at glacial-interglacial transitions (which are known as terminations) relative to variations in summer insolation in the Northern Hemisphere is an important test of this hypothesis. So far, it has only been possible to apply this test to the most recent termination, because the dating uncertainty associated with older terminations is too large to allow phase relationships to be determined. Here we present a new chronology of Antarctic climate change over the past 360,000 years that is based on the ratio of oxygen to nitrogen molecules in air trapped in the Dome Fuji and Vostok ice cores. This ratio is a proxy for local summer insolation, and thus allows the chronology to be constructed by orbital tuning without the need to assume a lag between a climate record and an orbital parameter. The accuracy of the chronology allows us to examine the phase relationships between climate records from the ice cores and changes in insolation. Our results indicate that orbital-scale Antarctic climate change lags Northern Hemisphere insolation by a few millennia, and that the increases in Antarctic temperature and atmospheric carbon dioxide concentration during the last four terminations occurred within the rising phase of Northern Hemisphere summer insolation. These results support the Milankovitch theory that Northern Hemisphere summer insolation triggered the last four deglaciations.

Orbital- and millennial-scale vegetation and climate changes of the past 225 ka from Bear Lake, Utah–Idaho (USA)

Continuous high-resolution pollen data for the past 225 ka from sediments in Bear Lake, Utah–Idaho reflect changes in vegetation and climate that correlate well with variations in summer insolation and global ice-volume during MIS 1 through 7. Spectral analysis of the pollen data identified peaks at 21–22 and 100 ka corresponding to periodicities in Earth's precession and eccentricity orbital cycles. Suborbital climatic fluctuations recorded in the pollen data, denoted by 6 and 5 ka cyclicities, are similar to Greenland atmospheric temperatures and North Atlantic ice-rafting Heinrich events. Our results show that millennial-scale climate variability is also evident during MIS 5, 6 and 7, including the occurrence of Heinrich-like events in MIS 6, showing the long-term feature of such climate variability. This study provides clear evidence of a highly interconnected ocean–atmosphere system during the last two glacial/interglacial cycles that extended its influence as far as continental western North America. Our study also contributes to a greater understanding of the impact of long-term climate change on vegetation of western North America. Such high-resolution studies are particularly important in efforts of the scientific community to predict the consequences of future climate change.

The pollen record from El’gygytgyn Lake: implications for vegetation and climate histories of northern Chukotka since the late middle Pleistocene

Three types of pollen assemblages (shrub-dominated, mixed herb- and shrub-dominated, and herb-dominated) characterize the ~ 300,000 year palynological record from El’gygytgyn Lake. Despite major changes in global climatic forcings, all pollen spectra, with a few isolated exceptions, have strong to possible analogs in the modern plant communities of Northeast Siberia and Alaska. Paleoclimatic reconstructions based on squared chord-distance analog analyses indicate two periods (~8600–10,700 14C year B.P. and OIS 5e) when summers were perhaps ~2 to 4°C warmer than modern. January temperatures were also warmer than present, and both July and January were wetter than today. Palynological data remain inconclusive as to the establishment of forests near El’gygytgyn Lake at these times. The wettest Julys occurred during OIS 5 d. July temperatures were near modern, and Januarys were colder and drier than now. January temperatures, even into the Middle Pleistocene, generally show little variability, suggesting that the suppression of arboreal taxa during glaciations was likely caused by cool summers with low effective moisture and not by frigid winters. Because age schemes that correlate magnetic susceptibility to variations in summer insolation or ∂18O have cool plant taxa persisting in warm times (and vice versa), we propose an alternative age model based on the palynological data.

Reconstruction of regional atmospheric circulation features during the late Pleistocene in subtropical Brazil from oxygen isotope composition of speleothems

Two high-resolution oxygen isotope records of speleothems from caves located in subtropical Brazil provide a broad view of regional climate variations and related forcing during the late Pleistocene and Holocene. Here, we present a new record precisely dated by U-series extending back to 131 kyr BP from a speleothem collected in Santana Cave. Comparison with the 116 kyr BP record from Botuverá Cave confirms that the seasonal change in regional rainfall distribution is primarily controlled by variations in summer insolation on Milankovitch timescales. However, significant negative anomalies of δ 18O from 60 kyr BP to 10 kyr BP in the records suggest that the Northern Hemisphere glacial boundary conditions, in particular Heinrich events, impacted the mean location and/or convective activity of the South American summer monsoon (SASM), resulting in enhanced transport of depleted moisture from the Amazon Basin into the region. In addition, comparison of the two records indicates a steep north–south gradient in δ 18O of rainfall throughout, which is interpreted as reflecting different relative contribution of extratropical and monsoonal precipitation to the two locations. For some time periods, more pronounced negative δ 18O anomalies occur in the northern part and more positive anomalies in the southern part of the region, which appear to be associated with more abundant monsoonal and extratropical rainfall, respectively. The last interglacial period is characterized by the highest δ 18O values in the entire St8 record, indicating a dominance of extratropical winter rainfall and thus, the most northward position of the summer SASM for the last 131 kyr BP.

Climatic controls on central African hydrology during the past 20,000 years

Past hydrological changes in Africa have been linked to various climatic processes, depending on region and timescale. Long-term precipitation changes in the regions of northern and southern Africa influenced by the monsoons are thought to have been governed by precessional variations in summer insolation. Conversely, short-term precipitation changes in the northern African tropics have been linked to North Atlantic sea surface temperature anomalies, affecting the northward extension of the Intertropical Convergence Zone and its associated rainbelt. Our knowledge of large-scale hydrological changes in equatorial Africa and their forcing factors is, however, limited. Here we analyse the isotopic composition of terrigenous plant lipids, extracted from a marine sediment core close to the Congo River mouth, in order to reconstruct past central African rainfall variations and compare this record to sea surface temperature changes in the South Atlantic Ocean. We find that central African precipitation during the past 20,000 years was mainly controlled by the difference in sea surface temperatures between the tropics and subtropics of the South Atlantic Ocean, whereas we find no evidence that changes in the position of the Intertropical Convergence Zone had a significant influence on the overall moisture availability in central Africa. We conclude that changes in ocean circulation, and hence sea surface temperature patterns, were important in modulating atmospheric moisture transport onto the central African continent.

Vegetation and climate change in northwest America during the past 125 kyr

Vegetation records spanning the past 21 kyr in western North America display spatial patterns of change that reflect the influence of variations in the large-scale controls of climate1. Among these controls are millennial-scale variations in the seasonal cycle of insolation and the size of the ice sheet, which affect regional climates directly through changes in temperature and net radiation, and indirectly by shifting atmospheric circulation. Longer vegetation records provide an opportunity to examine the regional response to different combinations of these large-scale controls, and whether non-climatic controls are important. But most of the longer North American records2,3 are of insufficient quality to allow a robust test, and the long European records4,5,6,7,8,9 are in regions where the vegetation response to climate is often difficult to separate from the response to ecological and anthropogenic controls. Here we present a 125-kyr record of vegetation and climate change for the forest/steppe border of the eastern Cascade Range, northwest America. Pollen data disclose alternations of forest and steppe that are consistent with variations in summer insolation and global ice-volume, and vegetational transitions correlate well with the marine isotope-stage boundaries. The close relationship between vegetation and climate beyond the Last Glacial Maximum provides evidence that climate variations are the primary cause of regional vegetation change on millennial timescales, and that non-climatic controls are secondary.

A Model for Northern Hemisphere Continental Ice Sheet Variation

The marine record shows that over the last 350 ka Northern Hemisphere ice sheet volumes have fluctuated widely and only on rare short occasions have they been reduced to the present interglacial state. The fluctuations are well synchronized with hemispheric average summer insolation variations of 20 ka periodicity caused by changing orbital parameters. The development of a model which explains the varied amplitudes of the fluctuations and is consistent with the geological record embodies the following arguments: The transition from an interglacial state like today's to a glacial state is initiated when a summer insolation deficit causes a southerly extension of the North Atlantic-Arctic pack ice to 60°N latitude. The extension alters the subpolar low pressure patterns and thus causes a southward diversion of the European Gulf Stream flow. It also produces an enhanced warm West Greenland current. This current causes open seas as far north as Baffin Bay which provides moisture for rapid northern Laurentide ice sheet growth. After several glacial fluctuations driven by insolation variations, the southern Laurentide ice front may reach an extreme extension. This diverts the westerlies and the Gulf Stream thus weakening a dominant subpolar North Atlantic gyre and consequently producing a prolonged cutoff of the West Greenland current and a reduction of high latitude glacial precipitation. The subsequent high insolation can then melt back the eastern pack ice and restore the northern European Gulf Stream. This warms the high latitudes for a time sufficient to melt the continental ice, thus causing the transition back to the interglacial state.An analysis of the record in the context of model suggests that the threshold deficit in average summer insolation that is required to initiate major glacial growth is influenced by the cooling effect of the Greenland ice cap on the seas to the east. The threshold level under conditions like today's is found to lie between −7 and −17 ly/day relative to the present. This threshold will not be crossed for at least 54 millenia due to an interval of smaller orbital eccentricity. Probable melting of the Greenland ice cap about 30 ka AP would ensure the extension of the present interglacial beyond 120 ka AP.

Evidence for Pleistocene Ice Lags

FOR more than one hundred years many different authors have claimed a causal relationship between the astronomical motions of the Earth and the alternating glacial and interglacial ages of the Pleistocene epoch. Koppen and Wegener1, using the insolation curves calculated by Milankovitch, concluded that glaciation was controlled by summer insolation variations at the high northern latitudes. This theory was brought up to date and amplified by Emiliani and Geiss2. Wilson3 suggested that summer insolation variations at the high southern latitudes controlled glaciation, making Antarctic ice surges and retreats the principal controlling agents for world climate. There is an average lag of 10,000 yr between the northern and the southern curves, with the latter leading. The purpose of this work is to determine the lag between the summer insolation curve at lat. 65° N and the temperature maxima and minima of core P-6304–9, as measured by the oxygen isotopic method. The method of visual correlation is difficult because noise coming from various mechanisms is superimposed on the core data. In an article by Kemp et al.4 core P-6304–9 was compared with the summer insolation at 65° N lat. by the method of power spectral analysis. The estimators from the auto-correlation functions in this case were those of Evans et al.5. Now these estimators are valid only for auto-correlation functions and not for cross-correlation, so the classical method is used in this communication. The auto and cross-correlation functions are expanded in the orthonormal set of Laguerre functions. The advantage is that all have rigorous Fourier transforms and thus the resulting spectra will be continuous functions of the frequency. This is not the case when the above integrals are evaluated numerically (the usual procedure) because the wavelengths chosen must be commensurate with the total record length. This method was checked by simulation such that the frequencies and relative phases involved were known in advance. One hundred and fifty correlation coefficients were calculated and the data were interpolated 2 for 1 which resulted in 289 data points. Only that part of the spectra is used which leaves the coherency matrix hermitian. The expansion in Laguerre functions and their transforms was carried to the thirtieth order.