Early onset and tropical forcing of 100,000-year Pleistocene glacial cycles

Between 1.5 and 0.6 Myr ago, the period of the Earth's glacial cycles changed from 41 kyr, the period of the Earth's obliquity cycles, to 100 kyr, the period of the Earth's orbital eccentricity, which has a much smaller effect on global insolation. The timing of this transition and its causes pose one of the most perplexing problems in palaeoclimate research. Here we use complex demodulation to examine the phase evolution of precession and semiprecession cycles--the latter of which are phase-coupled to both precession and eccentricity--in the tropical and extratropical Atlantic Ocean. We find that about 1.5 Myr ago, tropical semiprecession cycles (with periods of about 11.5 kyr) started to propagate to higher latitudes, coincident with a growing amplitude envelope of the 100-kyr cycles. Evidence from numerical models suggests that cycles of about 10 kyr in length may be required to explain the high amplitude of the 100-kyr cycles. Combining our results with consideration of a modern analogue, we conclude that increased heat flow across the equator or from the tropics to higher latitudes around 1.5 Myr ago strengthened the semiprecession cycle in the Northern Hemisphere, and triggered the transition to sustained 100-kyr glacial cycles.     

Stable sea surface temperatures in the western Pacific warm pool over the past 1.75 million years

About 850,000 years ago, the period of the glacial cycles changed from 41,000 to 100,000 years. This mid-Pleistocene climate transition has been attributed to global cooling, possibly caused by a decrease in atmospheric carbon dioxide concentrations. However, evidence for such cooling is currently restricted to the cool upwelling regions in the eastern equatorial oceans, although the tropical warm pools on the western side of the ocean basins are particularly sensitive to changes in radiative forcing. Here we present high-resolution records of sea surface temperatures spanning the past 1.75 million years, obtained from oxygen isotopes and Mg/Ca ratios in planktonic foraminifera from the western Pacific warm pool. In contrast with the eastern equatorial regions, sea surface temperatures in the western Pacific warm pool are relatively stable throughout the Pleistocene epoch, implying little long-term change in the tropical net radiation budget. Our results challenge the hypothesis of a gradual decrease in atmospheric carbon dioxide concentrations as a dominant trigger of the longer glacial cycles since 850,000 years ago. Instead, we infer that the temperature contrast across the equatorial Pacific Ocean increased, which might have had a significant influence on the mid-Pleistocene climate transition.     

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3-4 degrees C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1-23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).     

Closing of the Indonesian seaway as a precursor to east African aridification around 3-4 million years ago

Global climate change around 3-4 Myr ago is thought to have influenced the evolution of hominids, via the aridification of Africa, and may have been the precursor to Pleistocene glaciation about 2.75 Myr ago. Most explanations of these climatic events involve changes in circulation of the North Atlantic Ocean due to the closing of the Isthmus of Panama. Here we suggest, instead, that closure of the Indonesian seaway 3-4 Myr ago could be responsible for these climate changes, in particular the aridification of Africa. We use simple theory and results from an ocean circulation model to show that the northward displacement of New Guinea, about 5 Myr ago, may have switched the source of flow through Indonesia-from warm South Pacific to relatively cold North Pacific waters. This would have decreased sea surface temperatures in the Indian Ocean, leading to reduced rainfall over eastern Africa. We further suggest that the changes in the equatorial Pacific may have reduced atmospheric heat transport from the tropics to higher latitudes, stimulating global cooling and the eventual growth of ice sheets.     

Atlantic overturning responses to Late Pleistocene climate forcings

The factors driving glacial changes in ocean overturning circulation are not well understood. On the basis of a comparison of 20 climate variables over the past four glacial cycles, the SPECMAP project proposed that summer insolation at high northern latitudes (that is, Milankovitch forcing) drives the same sequence of ocean circulation and other climate responses over 100-kyr eccentricity cycles, 41-kyr obliquity cycles and 23-kyr precession cycles. SPECMAP analysed the circulation response at only a few sites in the Atlantic Ocean, however, and the phase of circulation response has been shown to vary by site and orbital band. Here we test the SPECMAP hypothesis by measuring the phase of orbital responses in benthic delta(13)C (a proxy indicator of ocean nutrient content) at 24 sites throughout the Atlantic over the past 425 kyr. On the basis of delta(13)C responses at 3,000-4,010 m water depth, we find that maxima in Milankovitch forcing are associated with greater mid-depth overturning in the obliquity band but less overturning in the precession band. This suggests that Atlantic overturning is strongly sensitive to factors beyond ice volume and summer insolation at high northern latitudes. A better understanding of these processes could lead to improvements in model estimates of overturning rates, which range from a 40 per cent increase to a 40 per cent decrease at the Last Glacial Maximum and a 10-50 per cent decrease over the next 140 yr in response to projected increases in atmospheric CO(2) (ref. 4).     

Late Miocene decoupling of oceanic warmth and atmospheric carbon dioxide forcing

Deep-time palaeoclimate studies are vitally important for developing a complete understanding of climate responses to changes in the atmospheric carbon dioxide concentration (that is, the atmospheric partial pressure of CO(2), p(co(2))). Although past studies have explored these responses during portions of the Cenozoic era (the most recent 65.5 million years (Myr) of Earth history), comparatively little is known about the climate of the late Miocene (～12-5 Myr ago), an interval with p(co(2)) values of only 200-350?parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents. Here we present quantitative geochemical sea surface temperature estimates from the Miocene mid-latitude North Pacific Ocean, and show that oceanic warmth persisted throughout the interval of low p(co(2)) ～12-5 Myr ago. We also present new stable isotope measurements from the western equatorial Pacific that, in conjunction with previously published data, reveal a long-term trend of thermocline shoaling in the equatorial Pacific since ～13?Myr ago. We propose that a relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene. Additional shoaling of the thermocline after 5?Myr ago probably explains the stronger coupling between p(co(2)), sea surface temperatures and climate that is characteristic of the more recent Pliocene and Pleistocene epochs.     

Obliquity pacing of the late Pleistocene glacial terminations

The 100,000-year timescale in the glacial/interglacial cycles of the late Pleistocene epoch (the past approximately 700,000 years) is commonly attributed to control by variations in the Earth's orbit. This hypothesis has inspired models that depend on the Earth's obliquity (approximately 40,000 yr; approximately 40 kyr), orbital eccentricity (approximately 100 kyr) and precessional (approximately 20 kyr) fluctuations, with the emphasis usually on eccentricity and precessional forcing. According to a contrasting hypothesis, the glacial cycles arise primarily because of random internal climate variability. Taking these two perspectives together, there are currently more than thirty different models of the seven late-Pleistocene glacial cycles. Here we present a statistical test of the orbital forcing hypothesis, focusing on the rapid deglaciation events known as terminations. According to our analysis, the null hypothesis that glacial terminations are independent of obliquity can be rejected at the 5% significance level, whereas the corresponding null hypotheses for eccentricity and precession cannot be rejected. The simplest inference consistent with the test results is that the ice sheets terminated every second or third obliquity cycle at times of high obliquity, similar to the original proposal by Milankovitch. We also present simple stochastic and deterministic models that describe the timing of the late-Pleistocene glacial terminations purely in terms of obliquity forcing.     

Eastern Pacific cooling and Atlantic overturning circulation during the last deglaciation

Surface ocean conditions in the equatorial Pacific Ocean could hold the clue to whether millennial-scale global climate change during glacial times was initiated through tropical ocean-atmosphere feedbacks or by changes in the Atlantic thermohaline circulation. North Atlantic cold periods during Heinrich events and millennial-scale cold events (stadials) have been linked with climatic changes in the tropical Atlantic Ocean and South America, as well as the Indian and East Asian monsoon systems, but not with tropical Pacific sea surface temperatures. Here we present a high-resolution record of sea surface temperatures in the eastern tropical Pacific derived from alkenone unsaturation measurements. Our data show a temperature drop of approximately 1 degrees C, synchronous (within dating uncertainties) with the shutdown of the Atlantic meridional overturning circulation during Heinrich event 1, and a smaller temperature drop of approximately 0.5 degrees C synchronous with the smaller reduction in the overturning circulation during the Younger Dryas event. Both cold events coincide with maxima in surface ocean productivity as inferred from 230Th-normalized carbon burial fluxes, suggesting increased upwelling at the time. From the concurrence of equatorial Pacific cooling with the two North Atlantic cold periods during deglaciation, we conclude that these millennial-scale climate changes were probably driven by a reorganization of the oceans' thermohaline circulation, although possibly amplified by tropical ocean-atmosphere interaction as suggested before.     

Impacts of orbital forcing and atmospheric carbon dioxide on Miocene ice-sheet expansion

The processes causing the middle Miocene global cooling, which marked the Earth's final transition into an 'icehouse' climate about 13.9 million years ago (Myr ago), remain enigmatic. Tectonically driven circulation changes and variations in atmospheric carbon dioxide levels have been suggested as driving mechanisms, but the lack of adequately preserved sedimentary successions has made rigorous testing of these hypotheses difficult. Here we present high-resolution climate proxy records, covering the period from 14.7 to 12.7 million years ago, from two complete sediment cores from the northwest and southeast subtropical Pacific Ocean. Using new chronologies through the correlation to the latest orbital model, we find relatively constant, low summer insolation over Antarctica coincident with declining atmospheric carbon dioxide levels at the time of Antarctic ice-sheet expansion and global cooling, suggesting a causal link. We surmise that the thermal isolation of Antarctica played a role in providing sustained long-term climatic boundary conditions propitious for ice-sheet formation. Our data document that Antarctic glaciation was rapid, taking place within two obliquity cycles, and coincided with a striking transition from obliquity to eccentricity as the drivers of climatic change.     

Evidence for decoupling of atmospheric CO2 and global climate during the Phanerozoic eon

Atmospheric carbon dioxide concentrations are believed to drive climate changes from glacial to interglacial modes, although geological and astronomical mechanisms have been invoked as ultimate causes. Additionally, it is unclear whether the changes between cold and warm modes should be regarded as a global phenomenon, affecting tropical and high-latitude temperatures alike, or if they are better described as an expansion and contraction of the latitudinal climate zones, keeping equatorial temperatures approximately constant. Here we present a reconstruction of tropical sea surface temperatures throughout the Phanerozoic eon (the past approximately 550 Myr) from our database of oxygen isotopes in calcite and aragonite shells. The data indicate large oscillations of tropical sea surface temperatures in phase with the cold-warm cycles, thus favouring the idea of climate variability as a global phenomenon. But our data conflict with a temperature reconstruction using an energy balance model that is forced by reconstructed atmospheric carbon dioxide concentrations. The results can be reconciled if atmospheric carbon dioxide concentrations were not the principal driver of climate variability on geological timescales for at least one-third of the Phanerozoic eon, or if the reconstructed carbon dioxide concentrations are not reliable.     

临夏盆地1.95MaBP的气候转型事件与青藏高原隆升作用于大气的临界高度

湖相地层孢粉分析的结果表明，临夏盆地在1．95MaB．P．前后发生过一次重要的气候转型事件。在此之前，南海热带季风信号在地层记录中存在较强的表现，在此之后热带季风信号减弱而高纬冰盖信号加强。CaCO3指标的周期分析同样表明，在1．95MaBP前后确实出现了气候的主导周期转型，在此之前，气候变化周期以类岁差的25kyr周期为主，而在此之后，以类倾角的38kyr周期为主。发生在临夏盆地1．95MaBP的这次气候转型事件表明青藏高原地面在当时隆升到一个可以改变周围地区大气环流的临界高度，如果以青藏高原在2．6MaB．P．- 0．6MaB．P．的平均抬升速率推算，这一临界高度可能在2500m左右。     

Early Pleistocene formation of the asymmetric east-west pattern of upper water structure in the equatorial Pacific Ocean

Surface-and subsurface-dwelling planktonic foraminifera from the upper 43 m of Hole A at the Ocean Drilling Program (ODP) Site 807,which was recovered from the western Pacific warm pool during ODP Leg 130,were analyzed for stable oxygen and carbon isotopes.By comparing these results with data from ODP Site 851 in the eastern equatorial Pacific,this study has reconstructed the paleoceanographic changes in upper ocean waters in the equatorial Pacific since 2.5 Ma.During the period from 1.6-1.4 Ma,the oxygen isotopes of surface and subsurface waters were found to markedly change in the western and eastern equatorial Pacific,further confirming the final formation of the well-defined asymmetric east-west (E-W) pattern at that time.This feature was similar to the zonal temperature gradient (sea surface temperature is higher in the west and lower in the east) and the asymmetric upper water structure (thermocline depth is deeper in the west and shallower in the east) in the modern equatorial Pacific.The zonal gradient change of subsurface water δ18O was greater than that of surface water δ18O,indicating that the formation of the asymmetric E-W pattern in the equatorial Pacific should be much more related to the shoaled thermocline and markedly decreased subsurface water temperature in the eastern equatorial Pacific.Moreover,since ～1.6 Ma,the carbon isotopic differences between surface and subsurface waters clearly decreased in the equatorial Pacific,and their long-term eccentricity periods changed from 400 ka to ～500 ka,reflecting the reorganization of the ocean carbon reservoir.This probably resulted from the deep water reorganization in the Southern Ocean at that time and its enhanced influence on the tropical Pacific (especially subsurface water).Our study demonstrates that the tropical ocean plays an important role in global climate change.     

Decline of surface temperature and salinity in the western tropical Pacific Ocean in the Holocene epoch

In the present-day climate, surface water salinities are low in the western tropical Pacific Ocean and increase towards the eastern part of the basin. The salinity of surface waters in the tropical Pacific Ocean is thought to be controlled by a combination of atmospheric convection, precipitation, evaporation and ocean dynamics, and on interannual timescales significant variability is associated with the El Ni?o/Southern Oscillation cycles. However, little is known about the variability of the coupled ocean-atmosphere system on timescales of centuries to millennia. Here we combine oxygen isotope and Mg/Ca data from foraminifers retrieved from three sediment cores in the western tropical Pacific Ocean to reconstruct Holocene sea surface temperatures and salinities in the region. We find a decrease in sea surface temperatures of approximately 0.5 degrees C over the past 10,000 yr, whereas sea surface salinities decreased by approximately 1.5 practical salinity units. Our data imply either that the Pacific basin as a whole has become progressively less salty or that the present salinity gradient along the Equator has developed relatively recently.     

Signal propagations and linkages of subsurface temperature anomalies in the tropical Pacific and Indian Ocean

The propagation characteristics of signals along different zonal-time profiles are analyzed using surface and subsurface temperature anomalies over the tropical Pacific and Indian oceans. Analyses show that there are intrinsic relationships between El Nio events in the eastern equatorial Pacific and dipole events in the equatorial Indian Ocean. In the region of tropical North Pacific between the equator and 16°N, there is a circle of propagation of subsurface temperature anomalies. El Nio events only happen when the warm subsurface signals reach the eastern equatorial Pacific. Dipole events are characterized when a warm subsurface signal travels along off-equatorial Indian Ocean to the western boundary. From these analyses, we believe that subsurface temperature anomalies can be considered to be the oceanographic early signal to forecast El Nio events in Pacific Ocean and dipole events in Indian Ocean, respectively.     

Warm tropical ocean surface and global anoxia during the mid-Cretaceous period

The middle of the Cretaceous period (about 120 to 80 Myr ago) was a time of unusually warm polar temperatures, repeated reef-drowning in the tropics and a series of oceanic anoxic events (OAEs) that promoted both the widespread deposition of organic-carbon-rich marine sediments and high biological turnover. The cause of the warm temperatures is unproven but widely attributed to high levels of atmospheric greenhouse gases such as carbon dioxide. In contrast, there is no consensus on the climatic causes and effects of the OAEs, with both high biological productivity and ocean 'stagnation' being invoked as the cause of ocean anoxia. Here we show, using stable isotope records from multiple species of well-preserved foraminifera, that the thermal structure of surface waters in the western tropical Atlantic Ocean underwent pronounced variability about 100 Myr ago, with maximum sea surface temperatures 3-5 degrees C warmer than today. This variability culminated in a collapse of upper-ocean stratification during OAE-1d (the 'Breistroffer' event), a globally significant period of organic-carbon burial that we show to have fundamental, stratigraphically valuable, geochemical similarities to the main OAEs of the Mesozoic era. Our records are consistent with greenhouse forcing being responsible for the warm temperatures, but are inconsistent both with explanations for OAEs based on ocean stagnation, and with the traditional view (reviewed in ref. 12) that past warm periods were more stable than today's climate.     

Slowdown of the meridional overturning circulation in the upper Pacific Ocean

Decadal temperature fluctuations in the Pacific Ocean have a significant effect on marine ecosystems and the climate of North America. The physical mechanisms responsible for these fluctuations are poorly understood. Some theories ascribe a central role to the wind-driven meridional overturning circulation between the tropical and subtropical oceans. Here we show, from observations over the past 50 years, that this overturning circulation has been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9 degrees N and 9 degrees S. This reduction in equatorial upwelling of relatively cool water, from 47 x 10(6) to 35 x 10(6) m3 s(-1), is associated with a rise in equatorial sea surface temperatures of about 0.8 degrees C. Another effect of the slowing circulation is a reduction in the outgassing of CO2 from the equatorial Pacific Ocean-at present the largest oceanic source of carbon dioxide to the atmosphere.     

Evidence for deep-water production in the North Pacific Ocean during the early Cenozoic warm interval

The deep-ocean circulation is responsible for a significant component of global heat transport. In the present mode of circulation, deep waters form in the North Atlantic and Southern oceans where surface water becomes sufficiently cold and dense to sink. Polar temperatures during the warmest climatic interval of the Cenozoic era (approximately 65 to 40 million years (Myr) ago) were significantly warmer than today, and this may have been a consequence of enhanced oceanic heat transport. However, understanding the relationship between deep-ocean circulation and ancient climate is complicated by differences in oceanic gateways, which affect where deep waters form and how they circulate. Here I report records of neodymium isotopes from two cores in the Pacific Ocean that indicate a shift in deep-water production from the Southern Ocean to the North Pacific approximately 65 Myr ago. The source of deep waters reverted back to the Southern Ocean 40 Myr ago. The relative timing of changes in the neodymium and oxygen isotope records indicates that changes in Cenozoic deep-water circulation patterns were the consequence, not the cause, of extreme Cenozoic warmth.     

Moisture transport across Central America as a positive feedback on abrupt climatic changes

Moisture transport from the Atlantic to the Pacific ocean across Central America leads to relatively high salinities in the North Atlantic Ocean and contributes to the formation of North Atlantic Deep Water. This deep water formation varied strongly between Dansgaard/Oeschger interstadials and Heinrich events-millennial-scale abrupt warm and cold events, respectively, during the last glacial period. Increases in the moisture transport across Central America have been proposed to coincide with northerly shifts of the Intertropical Convergence Zone and with Dansgaard/Oeschger interstadials, with opposite changes for Heinrich events. Here we reconstruct sea surface salinities in the eastern equatorial Pacific Ocean over the past 90,000 years by comparing palaeotemperature estimates from alkenones and Mg/Ca ratios with foraminiferal oxygen isotope ratios that vary with both temperature and salinity. We detect millennial-scale fluctuations of sea surface salinities in the eastern equatorial Pacific Ocean of up to two to four practical salinity units. High salinities are associated with the southward migration of the tropical Atlantic Intertropical Convergence Zone, coinciding with Heinrich events and with Greenland stadials. The amplitudes of these salinity variations are significantly larger on the Pacific side of the Panama isthmus, as inferred from a comparison of our data with a palaeoclimate record from the Caribbean basin. We conclude that millennial-scale fluctuations of moisture transport constitute an important feedback mechanism for abrupt climate changes, modulating the North Atlantic freshwater budget and hence North Atlantic Deep Water formation.     

Interannual atmospheric variability forced by the deep equatorial Atlantic Ocean

Climate variability in the tropical Atlantic Ocean is determined by large-scale ocean-atmosphere interactions, which particularly affect deep atmospheric convection over the ocean and surrounding continents. Apart from influences from the Pacific El Ni?o/Southern Oscillation and the North Atlantic Oscillation, the tropical Atlantic variability is thought to be dominated by two distinct ocean-atmosphere coupled modes of variability that are characterized by meridional and zonal sea-surface-temperature gradients and are mainly active on decadal and interannual timescales, respectively. Here we report evidence that the intrinsic ocean dynamics of the deep equatorial Atlantic can also affect sea surface temperature, wind and rainfall in the tropical Atlantic region and constitutes a 4.5-yr climate cycle. Specifically, vertically alternating deep zonal jets of short vertical wavelength with a period of about 4.5?yr and amplitudes of more than 10?cm?s(-1) are observed, in the deep Atlantic, to propagate their energy upwards, towards the surface. They are linked, at the sea surface, to equatorial zonal current anomalies and eastern Atlantic temperature anomalies that have amplitudes of about 6?cm?s(-1) and 0.4?°C, respectively, and are associated with distinct wind and rainfall patterns. Although deep jets are also observed in the Pacific and Indian oceans, only the Atlantic deep jets seem to oscillate on interannual timescales. Our knowledge of the persistence and regularity of these jets is limited by the availability of high-quality data. Despite this caveat, the oscillatory behaviour can still be used to improve predictions of sea surface temperature in the tropical Atlantic. Deep-jet generation and upward energy transmission through the Equatorial Undercurrent warrant further theoretical study.     

Interconnections between the Asian monsoon, ENSO, and high northern latitude climate during the Holocene

The International Geosphere Biosphere Program (IGBP), which promotes better understanding of the living environment, was initiated in the early 1990s. IGBP and other programs have uncovered much evi-dence that the Earth system is complex and nonlinear, ex…