Late Pleistocene Ice Ages Research Articles

GLOBAL CHOKE POINTS MAY LINK SEA LEVEL AND HUMAN SETTLEMENT AT THE LAST GLACIAL MAXIMUM

ABSTRACT Global choke points are preeminent nodes in geographic networks and geopolitical touchpoints subject to control by nations. They appear today as recurring theaters of conflict worldwide and also in archaeological investigations delving thousands of years back in time. How different were today’s global choke points at the Last Glacial Maximum (LGM) ~ 20,000 years ago? For the first time, we map nine of them to visualize their conditions at LGM. The global feature aquaterra—all lands inundated and exposed repeatedly during the Late Pleistocene ice ages—initially was mapped as first approximations of sea level. Here we refine its boundaries using Glacial Isostatic Adjustment (GIA) models to account for the Earth’s deformation and horizontal migrations of shorelines in response to glacial melting. We found three choke points sufficiently open to navigation, but six others presented substantially greater barriers than today. Implications include strategic insights on where to search for submerged evidence of human settlement.

Historical isolation and contemporary gene flow drive population diversity of the brown alga Sargassum thunbergii along the coast of China

BackgroundLong-term survival in isolated marginal seas of the China coast during the late Pleistocene ice ages is widely believed to be an important historical factor contributing to population genetic structure in coastal marine species. Whether or not contemporary factors (e.g. long-distance dispersal via coastal currents) continue to shape diversity gradients in marine organisms with high dispersal capability remains poorly understood. Our aim was to explore how historical and contemporary factors influenced the genetic diversity and distribution of the brown alga Sargassum thunbergii, which can drift on surface water, leading to long-distance dispersal.ResultsWe used 11 microsatellites and the plastid RuBisCo spacer to evaluate the genetic diversity of 22 Sargassum thunbergii populations sampled along the China coast. Population structure and differentiation was inferred based on genotype clustering and pairwise FST and allele-frequency analyses. Integrated genetic analyses revealed two genetic clusters in S. thunbergii that dominated in the Yellow-Bohai Sea (YBS) and East China Sea (ECS) respectively. Higher levels of genetic diversity and variation were detected among populations in the YBS than in the ECS. Bayesian coalescent theory was used to estimate contemporary and historical gene flow. High levels of contemporary gene flow were detected from the YBS (north) to the ECS (south), whereas low levels of historical gene flow occurred between the two regions.ConclusionsOur results suggest that the deep genetic divergence in S. thunbergii along the China coast may result from long-term geographic isolation during glacial periods. The dispersal of S. thunbergii driven by coastal currents may facilitate the admixture between southern and northern regimes. Our findings exemplify how both historical and contemporary forces are needed to understand phylogeographical patterns in coastal marine species with long-distance dispersal.

Breathing more deeply: Deep ocean carbon storage during the mid-Pleistocene climate transition

Abstract The ∼100 k.y. cyclicity of the late Pleistocene ice ages started during the mid-Pleistocene transition (MPT), as ice sheets became larger and persisted for longer. The climate system feedbacks responsible for introducing this nonlinear ice sheet response to orbital variations in insolation remain uncertain. Here we present benthic foraminiferal stable isotope (δ18O, δ13C) and trace metal records (Cd/Ca, B/Ca, U/Ca) from Deep Sea Drilling Project Site 607 in the North Atlantic. During the onset of the MPT, glacial-interglacial changes in δ13C values are associated with changes in nutrient content and carbonate saturation state, consistent with a change in water mass at our site from a nutrient-poor northern source during interglacial intervals to a nutrient-rich, corrosive southern source during glacial intervals. The respired carbon content of glacial Atlantic deep water increased across the MPT. Increased dominance of corrosive bottom waters during glacial intervals would have raised mean ocean alkalinity and lowered atmospheric pCO2. The amplitude of glacial-interglacial changes in δ13C increased across the MPT, but this was not mirrored by changes in nutrient content. We interpret this in terms of air-sea CO2 exchange effects, which changed the δ13C signature of dissolved inorganic carbon in the deep water mass source regions. Increased sea ice cover or ocean stratification during glacial times may have reduced CO2 outgassing in the Southern Ocean, providing an additional mechanism for reducing glacial atmospheric pCO2. Conversely, following the establishment of the ∼100 k.y. glacial cycles, δ13C of interglacial northern-sourced waters increased, perhaps reflecting reduced invasion of CO2 into the North Atlantic following the MPT.

ExploringGloborotalia truncatulinoidescoiling ratios as a proxy for subtropical gyre dynamics in the northwestern Atlantic Ocean during late Pleistocene Ice Ages

We explore the use of the coiling direction of planktic foraminifer Globorotalia truncatulinoides in sediment cores from the northwestern subtropical Atlantic Ocean as a proxy for variations in the intensity of the western boundary of the subtropical gyre over the past 280 kyr. Core-top sediments from the study region are dominated by the left coiling variety consistent with the deep permanent thermocline at the study sites (KNR140-37PC and Ocean Drilling Program Site 1059). Downcore G. truncatulinoides (sinistral) maxima occur in conjunction with 14 out of the 25 (Northern and Southern Hemisphere) precession maxima contained in the study interval. The agreement between the dominance of left coiling tests and the precession index of the Southern Hemisphere, in particular, supports a link between a deep thermocline in the northwestern subtropical Atlantic and northward flow of equatorially sourced warm surface currents, a situation analogous to the Late Holocene. Interglacial marine oxygen isotope stage (MIS) 5 lacks G. truncatulinoides (s) minima attesting to the relative stability of the western boundary during an interval of prolonged global warmth. G. truncatulinoides (s) disappear during the glacial extremes of MIS 2, 6, and 8 implying a weaker western boundary current at these times. Our results support that the coiling direction of this species is sensitive to variations in hydrography of the western boundary of the subtropical gyre. Because of the association between G. truncatulinoides (s) and precession maxima in both hemispheres, results support the importance of oceanic heat transport in half-precession climate variability in the North Atlantic.

State dependence of climate sensitivity: attractor constraints and palaeoclimate regimes

Equilibrium climate sensitivity (ECS) is a key predictor of climate change. However, it is not very well constrained, either by climate models or by observational data. The reasons for this include strong internal variability and forcing on many time scales. In practise this means that the 'equilibrium' will only be relative to fixing the slow feedback processes before comparing palaeoclimate sensitivity estimates with estimates from model simulations. In addition, information from the late Pleistocene ice age cycles indicates that the climate cycles between cold and warm regimes, and the climate sensitivity varies considerably between regime because of fast feedback processes changing relative strength and time scales over one cycle. In this paper we consider climate sensitivity for quite general climate dynamics. Using a conceptual Earth system model of Gildor and Tziperman (2001) (with Milankovich forcing and dynamical ocean biogeochemistry) we explore various ways of quantifying the state-dependence of climate sensitivity from unperturbed and perturbed model time series. Even without considering any perturbations, we suggest that climate sensitivity can be usefully thought of as a distribution that quantifies variability within the 'climate attractor' and where there is a strong dependence on climate state and more specificially on the 'climate regime' where fast processes are approximately in equilibrium. We also consider perturbations by instantaneous doubling of CO$_2$ and similarly find a strong dependence on the climate state using our approach.

‘Neanderthal bone flutes’: simply products of Ice Age spotted hyena scavenging activities on cave bear cubs in European cave bear dens

Punctured extinct cave bear femora were misidentified in southeastern Europe (Hungary/Slovenia) as ‘Palaeolithic bone flutes’ and the ‘oldest Neanderthal instruments’. These are not instruments, nor human made, but products of the most important cave bear scavengers of Europe, hyenas. Late Middle to Late Pleistocene (Mousterian to Gravettian) Ice Age spotted hyenas of Europe occupied mainly cave entrances as dens (communal/cub raising den types), but went deeper for scavenging into cave bear dens, or used in a few cases branches/diagonal shafts (i.e. prey storage den type). In most of those dens, about 20% of adult to 80% of bear cub remains have large carnivore damage. Hyenas left bones in repeating similar tooth mark and crush damage stages, demonstrating a butchering/bone cracking strategy. The femora of subadult cave bears are intermediate in damage patterns, compared to the adult ones, which were fully crushed to pieces. Hyenas produced round–oval puncture marks in cub femora only by the bone-crushing premolar teeth of both upper and lower jaw. The punctures/tooth impact marks are often present on both sides of the shaft of cave bear cub femora and are simply a result of non-breakage of the slightly calcified shaft compacta. All stages of femur puncturing to crushing are demonstrated herein, especially on a large cave bear population from a German cave bear den.

Recycling of Badger/Fox Burrows in Late Pleistocene Loess by Hyenas at the Den Site Bad Wildungen-Biedensteg (NW, Germany): Woolly Rhinoceros Killers and Scavengers in a Mammoth Steppe Environment of Europe

The Late Pleistocene (MIS 5c-d) Ice Age spotted hyena open air den and bone accumulation site Bad Wildungen-Biedensteg (Hesse, NW, Germany) represents the first open air loess fox/badger den site in Europe, which must have been recycled by Crocuta crocuta spelaea (Goldfuss, 1823) as a birthing den. Badger and fox remains, plus remains of their prey (mainly hare), have been found within the loess. Hyena remains from that site include parts of cub skeletons which represent 10% of the megafauna bones. Also a commuting den area existed, which was well marked by hyena faecal pellets. Most of the hyena prey bones expose crack, bite, and nibbling marks, especially the most common bones, the woolly rhinoceros Coelodonta antiquitatis (NISP = 32%). The large amount of woolly rhinoceros bones indicate hunting/scavenging specializing on this large prey by hyenas. Other important mammoth steppe hyena prey remains are from Mammuthus primigenius, Equus caballus przewalskii, Bison/Bos, Megaloceros giganteus, Cervus elaphus, and Rangifer tarandus. The few damaged bone remains of a scavenged cave bear Ursus spelaeus subsp. are unique for an open air situation. Abundant micromammal, frog, and some fish remains were concentrated in “pellets” that contain mainly mammoth steppe micromammals and also frog and fish remains that seem to originate from the nearby river/lake.

Enhanced stratification and seasonality in the Subarctic Pacific upon Northern Hemisphere Glaciation–New evidence from diatom-bound nitrogen isotopes, alkenones and archaeal tetraethers

Coincident with the intensification of Northern Hemisphere Glaciation (NHG) around 2.73 million years (Ma) ago, sediment cores from both the open subarctic North Pacific and the Antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed, representing one of the most abrupt and dramatic changes in the marine sediment record associated with the development of Pleistocene glacial cycles. In the North Pacific, bulk sediment nitrogen isotope data and alkenone-derived sea surface temperature (SST) estimates suggest that the productivity decline was driven by reduced exchange between surface and deep water, due to weaker wind-driven upwelling and/or a strengthening of the halocline (i.e. “stratification”). In this study of the 2.73Ma transition at Ocean Drilling Program (ODP) Site 882 in the western subarctic North Pacific, diatom-bound nitrogen isotopes (δ15Ndb), alkenone mass accumulation rate, and alkenone- and archaeal tetraether-based SST reconstructions support the stratification hypothesis, indicating perennially lower export production, generally higher nitrate consumption, and greater inter-seasonal variation in SST after the 2.73Ma transition. In addition, the δ15Ndb of large and small size fractions of Coscinodiscus spp. suggest that these diatoms grew mostly during the spring bloom during the late Pliocene, switching to their current fall-to-winter growth period at the 2.73Ma transition; this view is consistent with their decline in dominance and provides further evidence for increased stratification (reduced vertical exchange) in the North Pacific after 2.73Ma. The δ15Ndb data indicate that, over the ∼100kyr period after the 2.73Ma transition studied here, nitrate consumption did not reach late Pleistocene ice age levels and that nitrate consumption in post-2.73Ma warm stages was similar to that before the transition, even though productivity was greatly reduced. We tentatively attribute this to relatively weak dust-borne iron inputs in the early post-2.73Ma period.

Late Pleistocene Crocuta crocuta spelaea (Goldfuss, 1823) clans as prezewalski horse hunters and woolly rhinoceros scavengers at the open air commuting den and contemporary Neanderthal camp site Westeregeln (central Germany)

Late Pleistocene Ice Age Crocuta crocuta spelaea (Goldfuss, 1823) hyenas from the open-air gypsum karst site Westeregeln (Saxony-Anhalt, central Germany) is dated into the early to middle Late Pleistocene. Hyena clans apparently used the karst for food storage and as “commuting den”, where typical high amounts (15% of the NISP) of hyena remains appear, also faecal pellets in concentrations for den marking purposes. Additionally small carnivores Meles, Vulpes and Mustela appear to have used some cavities as dens. Several hundreds of lowland “mammoth steppe fauna” bones (NISP = 572) must have been accumulated primarily by hyenas, and not by Neanderthals at the contemporary hyena/human camp site. Abundant caballoid horse remains of “E. germanicusNehring, (1884)” are revised by the holotype and original material to the small E. c. przewalskii horse. Woolly rhinoceros Coelodonta antiquitatis remains are also abundant, and were left in several cases with typical hyena scavenging damages. Rangifer tarandus (11%) is mainly represented by numerous fragments of shed female antlers that were apparently gathered by humans, and antler bases from male animals that were collected and chewed in few cases (only large male antlers) by hyenas. The large quantities of small reindeer antlers must have been the result of collection by humans; their stratigraphic context is unclear but such large quantities most probably resulted from schamanic activities. The hyena site overlaps with a Middle Palaeolithic Neanderthal camp, as well as possibly with a later human Magdalénian site.

Phylogeographic heterogeneity of the brown macroalga Sargassum horneri (Fucaceae) in the northwestern Pacific in relation to late Pleistocene glaciation and tectonic configurations.

Pleistocene glacial oscillations and associated tectonic processes are believed to have influenced the historical abundances and distribution of organisms in the Asia Northwest Pacific (ANP). Accumulating evidence indicates that factors shaping tempospatial population dynamics and distribution patterns of marine taxa vary with biogeographical latitude, pelagic behaviour and oceanographic regimes. To detect what kinds of historical and contemporary factors affected genetic connectivity, phylogeographic profiles of littoral macroalga Sargassum horneri in the ANP were analysed based on mitochondrial (Cox3) and chloroplast (rbcL) data sets. Five distinct clades were recovered. A strong signature of biogeographical structure was revealed (Φ(CT) = 0.487, P < 0.0001) derived from remarkable differentiation in clade distribution, as clade I is restricted to Chinese marginal seas (Yellow-Bohai Sea, East China Sea and South China Sea), whereas clades II-V are discontinuously scattered around the main Islands of Japan. Furthermore, two secondary contact regions were identified along the south Japan-Pacific coastline. This significant differentiation between the two basins may reflect historical glacial isolation in the northwestern Pacific, which is congruent with the estimates of clade divergence and demographic expansion during the late Quaternary low sea levels. Analysis of molecular variance and the population-pair statistic F(ST) also revealed significant genetic structural differences between Chinese marginal seas and the Japanese basin. This exceptional phylogeographic architecture in S. horneri, initially shaped by historical geographic isolation during the late Pleistocene ice age and physical biogeographical barriers, can be complicated by oceanographic regimes (ocean surface currents) and relocating behaviour such as oceanic drifting.

The Crocuta crocuta spelaea (Goldfuss 1823) population and its prey from the Late Pleistocene Teufelskammer Cave hyena den besides the famous Paleolithic Neandertal Cave (NRW, NW Germany)

Remains from at least seven individuals of the Late Pleistocene Ice Age spotted hyena Crocuta crocuta spelaea (Goldfuss, 1823) from the Teufelskammer Cave in the Neandertal valley (North Rhine-Westphalia, northwest Germany) are described. The small cave was a well-frequented hyena den of the Early to Middle Late Pleistocene which was only 100 m from the famous small Feldhofer Cave, where the first Neandertal human skeleton was found. The high amount of hyena bone material (37%) and its strongly chewed and incomplete prey remains of the mixed mammoth steppe and boreal forest megafauna prove one more of 11 recently known hyena den caves in the Rhenish Massif. Hyenas and cave bears have used the cave, but Neandertal humans lived possibly not at the same time in the same valley. Although hyenas occupied mainly the smaller caves such as the Teufelskammer Cave, humans preferred large portal cave entrances such as in the Neandertal valley with the Small Feldhofer Cave.

A clan of Late Pleistocene hyenas, Crocuta crocuta spelaea (Goldfuss 1823), from the Rösenbeck Cave (Germany) and a contribution to cranial shape variability

The remains of a large population of Late Pleistocene Ice Age spotted hyenas (Crocuta crocuta spelaea Goldfuss 1823) are described from the Rosenbeck Cave in the Sauerland Karst of Germany. They include four skulls and 79 other skeletal parts, mainly from adult to senile animals, making this an important Late Pleistocene hyena cave-den site in Europe. The skulls have been compared with 30 other hyena skull specimens from open air and cave-den sites in central Europe (Germany, Austria, the Czech Republic and Romania) in order to achieve an understanding of sexual dimorphism in the crania of Ice Age spotted hyenas from the Upper Pleistocene cold period (Weichselian/Wuermian), and the types of injuries that they acquired during their lifetimes. Three different types of cranial shape have been distinguished, one of which appears to have been a consequence of pathologies that developed in response to injuries caused by bites received during the animal's lifetime, as a result of either intraspecies fights or fights with lions. Although cave bears penetrated to great depths within the Rosenbeck Cave for hibernation purposes, hyenas appear to have utilized only a short section of the cave that branched off directly from the entrance area. Hyena cub material is scarce, suggesting that this area was used as a communal den rather than for cub rearing. Bones exhibiting gnaw marks, representing prey imported by hyenas, are also rare but include horse (Equus caballus przewalskii) and reindeer (Rangifer tarandus) remains. The scarcity of bones from hyena prey suggests that this cave was not used as a food storage site. Some Ursus spelaeus cave bear remains, including skulls, show evidence of having been gnawed, chewed and cracked by hyenas, indicating that the hyenas periodically fed on cave bear carcasses in a specialization response to the mammoth steppe megafauna absence of the boreal mountain forest regions. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 191–220.

Eccentricity cycles shown by early Pleistocene planktonic foraminifers of the Omma Formation, Sea of Japan

There is a continuous record of planktonic foraminifers for oxygen isotope stages 50 to 26 (ca. 1.5–1.0 Ma) in the early Pleistocene Omma Formation near Kanazawa City, Central Japan, on the Sea of Japan coast. The warm-water species Globigerinoides ruber entered the Sea of Japan with the Tsushima Current during all interglacial periods and went locally extinct in the succeeding glacial periods. This implies that the marine climate of the Sea of Japan varied predominantly with the 41,000-year period of Earth's orbital obliquity. However, the relative abundances of G. ruber in marine isotope stages 47, 43 and 31 are significantly higher than those in other interglacial stages. These stages correspond to periods when eccentricity-modulated precession extremes were aligned with obliquity maxima. The Tsushima Current is a branch of the warm Kuroshio Current which is the strong northwestern component of the subtropical North Pacific Ocean gyre. Our data imply that the early Pleistocene climate in the northwestern Pacific was influenced not only by obliquity cycles but also by eccentricity cycles. This study also supports the climate model regarding eccentricity's role in the origin of low-frequency climate changes before the late Pleistocene ice ages.

EU SET TO APPROVE GERMAN AID FOR STENDAL MILL

Late Pleistocene Ice Age Crocuta crocuta spelaea (Goldfuss, 1823) hyenas from the open-air gypsum karst site Westeregeln (Saxony-Anhalt, central Germany) is dated into the early to middle Late Pleistocene. Hyena clans apparently used the karst for food storage and as “commuting den”, where typical high amounts (15% of the NISP) of hyena remains appear, also faecal pellets in concentrations for den marking purposes. Additionally small carnivores Meles, Vulpes and Mustela appear to have used some cavities as dens. Several hundreds of lowland “mammoth steppe fauna” bones (NISP = 572) must have been accumulated primarily by hyenas, and not by Neanderthals at the contemporary hyena/human camp site. Abundant caballoid horse remains of “E. germanicus Nehring, (1884)” are revised by the holotype and original material to the small E. c. przewalskii horse. Woolly rhinoceros Coelodonta antiquitatis remains are also abundant, and were left in several cases with typical hyena scavenging damages. Rangifer tarandus (11%) is mainly represented by numerous fragments of shed female antlers that were apparently gathered by humans, and antler bases from male animals that were collected and chewed in few cases (only large male antlers) by hyenas. The large quantities of small reindeer antlers must have been the result of collection by humans; their stratigraphic context is unclear but such large quantities most probably resulted from schamanic activities. The hyena site overlaps with a Middle Palaeolithic Neanderthal camp, as well as possibly with a later human Magdalénian site.

Impact of thermomechanical ice sheet coupling on a model of the 100 kyr ice age cycle

Simulations of the most recent 100 kyr glaciation‐deglaciation cycle of the late Pleistocene Ice Age are presented which feature a newly constructed thermomechanically coupled three‐dimensional ice sheet model which is itself coupled to a previously employed global energy balance climate model with realistic geography. The model incorporates both orbital insolation forcing due to the slow time evolution of orbital geometry (arising from many body effects in the solar system) as well as the forcing due to varying atmospheric concentrations of greenhouse gases. Simulations of the Greenland ice sheet are presented with which we are able to investigate the extent to which the ice flow law employed in the ice dynamics component of the model is constrained. The good agreement that we are able to achieve between the model‐generated ice sheet topography and the observed Greenland topography provides a clear demonstration of the quality of the ice dynamical model we have developed. The incorporation of full thermomechanical coupling using the standard Glen flow law in the ice dynamics component of the model is shown to increase the difficulty of achieving complete termination of the 100 kyr cycle that the model delivers. Furthermore, a 20 fold flow parameter enhancement relative to that used for Greenland is required to match the aspect ratio of the ICE‐4G reconstruction [Peltier, 1994]. Analyses presented herein suggest that basal processes are unlikely to account for this need for flow parameter retuning. However, the new thermomechanically coupled model now provides a clear separation of the Cordilleran and Hudson Bay domes at Last Glacial Maximum in contradistinction to analyses previously performed with isothermal ice sheet models. This innovation therefore leads to a major improvement of the generated ice sheet topography in relation to geological inferences. Aside from this important difference, the overall results regarding the ability of the model to fully account for the most recent 100 kyr cycle of glaciation and deglaciation without the necessity of introducing additional ad hoc feedbacks confirms the validity of this conclusion, reached previously on the basis of isothermal model integrations.

Terminating the 100 kyr ice age cycle

We report a simulation of the most recent 100,000‐year glaciationdéglaciation cycle of the late Pleistocene ice age, a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record. Our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one‐level energy balance model (EBM) and vertically integrated ice sheet model (ISM). The theory fully incorporates the influences of orbital insolation forcing, glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases. It correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21,000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently. The latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model, a limitation of all previous attempts to understand this global scale climate cycle. Our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of CO2 Is critical to the ability of the model to deliver a synthetic history of glaciation and déglaciation that is in accord with inferences based upon surface geological and geomorphological evidence. With the incorporation of this influence, model‐predicted ice thickness distributions at last glacial maximum (LGM) are very similar to those of the recently described ICE‐4G reconstruction that was based upon the inversion of postglacial relative sea level histories.

The Mid-Pleistocene climate transition: onset of 100 ka cycle lags ice volume build-up by 280 ka

The Mid-Pleistocene Climate Transition (MPT) is the complex climatic change which brought the Late Pleistocene ice ages. We explore the MPT in the time and frequency domains by new methods of time series analysis. High-resolution oxygen isotope records reveal that the ice volume-related increase in δ 18O mean (amplitude: 0.29 ± 0.05 (1 − σ μ) ‰, transition midpoint: 922 ± 12 ka, duration: 40 ± 9 ka) significantly preceded the abrupt increase in the amplitude of the ∼ 100 ka cycle at 641 ± 9 ka. This finding can be quantitatively simulated using a simple ice-bedrock model in which, due to the additional ice, the calving threshold is exceeded. The simulated calving events prior to ∼ 650 ka are separated by ∼ 77 ka, whereas after ∼ 650 ka they occur pseudo-periodically with a mean period of nearly 100 ka. The cause for the delay of the ∼ 100 ka calving cycle was not a slow bedrock relaxation; rather, the coincidental combination of insolation, existing ice mass, and bedrock depression.

Exploring the structure of the mid-Pleistocene revolution with advanced methods of time-series analysis

The mid-Pleistocene climate transition is a complex global change leading to Late Pleistocene ice ages with increased mean ice volume and dominant 100-ka cycle. A thorough understanding of this transition demands quantitative investigations in the time and frequency domains, and in the ''stochastic domain''. Three methods of time-series analysis are presented which have been adapted for this purpose. They are tested by means of predefined, artificial time series and applied to benthic oxygen isotope (d18O) records which serve as ice volume indicator. Results are as follows: (a) The time-dependent mean shows an increase of 0.35& vs PDB from 942 to 892 ka. (b) Evolutionary spectral analysis reveals an abrupt in- crease of 100-ka cycle amplitude at approximately 650 ka. (c) Probability density function exhibits a bifur- cation behavior at approximately 725 ka. These find- ings point to a multiple transition from a more linear climate system to a strong nonlinear system. The signif- icant lead of the transition in mean, in relation to the 100-ka amplitude change and bifurcation is left open for explanation.

Eccentricity forcing of Pliocene–Early Pleistocene climate revealed in a marine oxygen-isotope record

Milankovitch theory—that climate is controlled by variations in the Earth's orbital parameters—has gained wide acceptance for its ability to account for two climate cycles: a 23-kyr cycle that is phase-locked to the precession-driven insolation cycle, and a 41-kyr cycle that is phase-locked to the obliquity-driven insolation cycle1–6. But, explaining the observed ∼100-kyr climate cycle in terms of Milankovitch theory—especially for the Late Pleistocene ice-age cycle—remains controversial in spite of a strong correlation with the ∼100-kyr cycle in the Earth's orbital eccentricity5. One problem is that eccentricity affects insolation mainly by modulating the precession cycle; its direct contribution to radiation change is too small (<0.1%) to effect the observed climate change directly5,7. Another is the absence of a Late Pleistocene ice-volume cycle in oxygen-isotope records to match the ∼404-kyr component of the eccentricity cycle5,8. Here we examine an oxygen-isotope record spanning the interval 1.2 to 5.2 million years ago, before the Late Pleistocene ice-age regime. We find 404-kyr and ∼100-kyr climate cycles which are coherent with eccentricity and which have amplitudes that are similar to the coexisting 23-kyr cycle. Analysis of these low-frequency cycles suggests that they originate through an asymmetrical response mechanism that preferentially introduces variance into the climate system from the warmer portions of the eccentricity-modulated precession cycle. Our data thus support eccentricity's role in the origin of low-frequency oxygen-isotope cycles before the Late Pleistocene ice age.

Mantle viscosity from the simultaneous inversion of multiple data sets pertaining to postglacial rebound

We employ the formalism of Bayesian inference to invert three distinct data sets related to the phenomenon of glacial isostatic adjustment. These data consist, respectively, of the wavenumber spectrum of relaxation times for Fennoscandian rebound inferred by McConnell (1968), the characteristic relaxation times for a large collection of distinct 14C dated relative sea level curves from both Canada and Fennoscandia, and two data related to the rotational response of the Earth to the glaciation‐deglaciation cycle of the late Pleistocene ice age. Our analyses show that the spherically symmetric viscosity structure objectively inferred on the basis of these data from the once glaciated regions of the Northern Hemisphere allows a fully satisfactory reconciliation of both US East Coast relative sea level observations as well as providing a close fit to the requirements of aspherical geoid anomalies.