Impacts of post-depositional processes on rapid monsoon signals recorded by the last glacial loess deposits of northern China

Rapid East Asian Monsoon oscillations recorded by Chinese loess are thought to be dynamically linked to north Atlantic climate. However, few efforts have been made to assess the effects of post-depositional processes (e.g., surface mixing and pedogenesis) on loess paleoclimatic records. Here a detailed optically stimulated luminescence dating of a thick loess sequence from the western Loess Plateau is presented, offering a reliable chronology for last glacial deposits. Magnetic susceptibility and mean grain size records from three loess–paleosol sequences along a northwest–southeast transect are investigated to evaluate impacts of post-depositional processes on these loess-based proxy records. Our results indicate that: (1) loess sequences developed within the flat tableland of the central and western Loess Plateau are nearly continuous during the last glaciation; and (2) post-depositional processes have distinct impacts on rapid monsoon signals recorded in loess sequences from different regions. In the central Loess Plateau, rapid monsoon signals have been attenuated to various degrees depending on the sedimentation rate and pedogenic intensity. In the northwestern Loess Plateau, however, due to high sedimentation rate and relatively weak pedogenesis, high-resolution grain size oscillations reliably record rapid monsoon changes and can be well correlated to rapid climate changes recorded in the Greenland ice core and Hulu cave stalagmite.     

Chemical weathering history of the southern Tajikistan loess and paloclimate implications

Major chemical elements and Rb, Sr, Ba abundances were measured for loess samples from Chashmanigar loess-soil sequence in southern Tajikistan. And the degree of element loss and intensity of weathering of the samples were calculated. From these calculations we found that the paleosol horizons of the Chashmanigar section were chemically weathered to some extent and the weathering intensity of the soils is stronger than that of loess horizons, indicating that paleosols in southern Tajikistan were deposited in relatively warm and humid interglacial ages. In addition, the southern Tajikistan loess deposited before 0.9Ma BP was more intensely weathered than that deposited after 0.9Ma BP, which may imply the intensification of aridity in this area since 0.9Ma BP.     

Chemical weathering history of the southern Tajikistan loess and paleoclimate implications

In recent years, the loess deposits in Central Asia have attracted increasing attention of Qua-ternary paleoclimate researchers[1—8]. The centre of the loess deposits in this area is located in southern Tajikistan, where thick and continuous loess sectio…     

Molecular fossil and paleovegetation records of paleosol S4 and adjacent loess layers in the Luochuan loess section, NW China

Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identi- fied, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the pa- leoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predomi- nance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data dem- onstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.     

First high-resolution luminescence dating of loess in Western Siberia

The south of Western Siberia is an important part of the Eurasian loess belt, containing an extensive record of Quaternary landscape and climate evolution in up to 100 m thick loess deposits with as many as 10 pedocomplexes. However, this important Quaternary archive lacks a reliable absolute chronology, and this has prevented the linking of the widely accepted regional chronostratigraphic correlations with those of other parts of the Eurasian loess belt. Here we present the first results of detailed luminescence dating of the Late Pleistocene loess-palaeosol sequence at the Western Siberian stratotype section of Lozhok. According to the classical regional chronostratigraphic scheme, this sequence records the main stages of the environmental evolution of the region, including three palaeosols correlated with the warming stages of MIS 5e, MIS 5c and MIS 3. Our absolute chronology is based on 38 new luminescence ages (OSL, IR₅₀, pIRIR₂₉₀). Good agreement between the OSL and pIRIR₂₉₀ ages suggests sufficient bleaching before deposition. The resulting chronology reveals that, rather than being only Upper Pleistocene in age, the loess-palaeosol sequence at Lozhok actually formed in the Middle and Upper Pleistocene. The ages of individual horizons do not correspond to the previously accepted stratigraphic units and morphological features of pedocomplexes. Our Bayesian chronological model reveals remarkable variation in dust accumulation and preservation at the site. The new results unambiguously identify the presence of an erosional boundary with a hiatus lasting ∼90 ka. The upper pedocomplex, immediately below this discontinuity, formed in sediment deposited between 131 ± 9 ka and 122 ± 11 ka and clearly corresponds to MIS 5. The lower pedocomplex is found in sediment deposited between 240 ± 12 and 199 ± 9 ka, and correlates closely with MIS 7. These new findings demonstrate the urgent need for a wider programme to date the main stratotypes of loess-palaeosol sections in Western Siberia. Only then can the global implications of the regional climate record in this important continental-scale archive be correctly interpreted.     

Evaluating OSL-SAR protocols for dating quartz grains from the loess in Ili Basin,Central Asia

Late Pleistocene records of loess deposition are a critical archive for understanding terrestrial paleoenvironment changes in Central Asia. The age of loess is not well known for the deserts regions and surrounding high plateaus in Central Asia. Previous studies have shown that there remains a disparity between ages for loess deposition by luminescence and ¹⁴C dating. This study evaluates the potential of optically stimulated luminescence (OSL) to date a loess sequence resting on fluvial sands in the east Ili Basin, Central Asia. The single-aliquot regenerative-dose (SAR) protocol on coarse grain quartz was employed for equivalent dose determinations. The basal fluvial sand returned a secure OSL age, with low overdispersion value in equivalent doses (19 ± 2%) of ca. 36 ka and provides a close, but maximum age estimate (within 5 ka) on the initiation of loess deposition. However, the loess yielded high overdispersion values for equivalent doses and age reversals, coincident with diffuse paleosols; indicating that pedoturbation with loess deposition may be a dominant process. OSL ages between ca. 45 and 14 ka calculated using a maximum age model and OSL ages from other sites in the Basin suggests that the latest major period of loess deposition was between 70 and 10 ka ago. A future hypothesis to test based on these analyses is that there may be three periods of heightened loess deposition at ca. 45, 35 to 19 and 14 ka, when desert source areas to the west were particularly dry.     

Magnetic indicator of global paleoclimate cycles in Siberian loess–paleosol sequences

The loess–paleosol sequences of China, Siberia, Alaska and many other regions, along with lake sediments and glaciers, provide the only accurate paleoclimatic terrestrial records for intervals of thousands to hundreds of thousand years. The frequency dependence (FD) of magnetic susceptibility (MS) in such sequences has become the leading parameter for analyzing climatic change and Milankovitch (astronomical) periodicity in Siberian sequences; it is always higher in soil horizons than in loess. The enhanced FD parameter in soils is associated with ferromagnetic minerals, mostly magnetite, produced during pedogenesis. The MS and FD parameters of 670 samples from five sections in Siberia are reported here. Inter-section correlation is used to produce a combined FD time series for the studied sections. Chronological control is established by absolute dating and stratigraphic correlation. Spectral analysis of the FD time series reveals the presence of Milankovitch signals at ～100 kyr (eccentricity), ～40 kyr (obliquity) and ～23 kyr (precession) and demonstrates that Siberian loess–paleosol sequences are excellent continental recorders of long-term paleoclimatic changes. This suggests that the FD parameter can potentially be used more widely for evaluation of climate periodicity in loess/paleosol sequences in other parts of the world.     

巴基斯坦Bahawalpur黄土岩石磁学特征及磁化率变化机制研究

风成黄土是陆地上分布最广泛的沉积物之一,记载了各种古气候演化信息.目前巴基斯坦的黄土研究甚少,磁化率与气候对应的变化机制研究尚未开展.本文对位于巴基斯坦印度河平原Bahaw alpur地区新发现的黄土-古土壤剖面进行系统的岩石磁学研究,结合粒度和漫反射光谱(DRS)数据,讨论巴基斯坦黄土的磁化率变化机制.实验结果显示:...     

Rock-magnetic proxies of climate change from loess -paleosol sediments of the Czech Republic

Summary Rock-magnetic characteristics of late Pleistocene loess-paleosol sequences in the Czech Republic show patterns of variation that reflect climate-related depositional and diagenetic processes which acted on the sedimentary profiles. Mass-normalized magnetic susceptibility is high in interglacial and interstadial paleosols, while uniformly low values are measured in unweathered loess horizons. Normalized ferrimagnetic susceptibility and anhysteretic remanent magnetization show an enhancement of ultrafine (superparamagnetic, SP) and fine (single-domain, SD, and pseudo-single-domain, PSD) grains in chernozem paleosols correlated with δ¹⁸O substages 5c and 5a as well as in the Holocene soil. The parabraunerde paleosol associated with peak interglacial conditions, correlated with δ¹⁸O substage 5e, shows evidence of diagenetic loss of fine grained magnetic minerals, although coarse (multidomain, MD) grains appear to be preserved. Low temperature remanence behavior plus high temperature susceptibility measurements of representative samples from each lithologic unit indicate that magnetite and maghemite are the dominant magnetic minerals within the sediments. Variations in concentration-independent rock-magnetic parameters are therefore primarily a function of grain size variations through the profile. It is anticipated that with additional magnetic and non-magnetic sedimentological and geochemical tests, a quantitative rock-magnetic — paleoclimate model can be developed for the central European loess region.     

significance of high-resolution loess stratification based on grain size and magnetic susceptibility analysis to paleo-earthquake study: a case study of dongyugou loess section,at hongtong,shanxi province

As an important technology to paleoseismologic research, trenching has been used to identify paleo-earthquakes recorded in strata, combined with dating technology. However, there have been some bigger uncertainties and limitations. For instance, subtle strata in loess sediment cannot be interpreted only by naked-eye, which seriously affects identifying paleo-earthquake horizon and time. Therefore, how to improve the accuracy and reduce the uncertainty of paleo-earthquake identification is the important problem we are currently facing. Dongyugou loess section, located in the northeastern corner of Linfen Basin, Shanxi Province, cuts across the Huoshan piedmont fault. The section exposes not only the well-developed loess sequence, but also several obvious faulting events. Thus, this loess section is a better site to make a high resolution study to improve the accuracy and reduce the uncertainty of paleo-earthquake identification. Based on the high-resolution grain size and magnetic susceptibility analysis, and associated with visual interpretation by naked-eye, we made a high-resolution stratification of Dongyugou loess section, including high-resolution thickness of each stratum and its upper and bottom boundaries. Based on the high-resolution stratification and their comparison between two fault walls, we identified three earthquake events, which occurred after formation of u5-7, u4 and u2, corresponding to their stratification depth of 7.1m, 4.7m and 2.9m in hanging wall. Based on results of OSL dating and average sedimentation rate of hanging wall, we estimated that the three events occurred around 45.8ka(between (48.1±1.5)～(43.2±2.5)ka), 32.8ka(between (35.0±2.4)～(30.6±1.3)ka) and 23.3ka(between (26.4±0.8)～(20.9±0.7)ka). According to the thickness difference of three loess-paleosol sedimentary cycles between two fault walls, we calculated the coseismic vertical displacements of the three events as 0.5m, 0.4 and 1.3m, respectively. Compared with other segments of the Huoshan piedmont fault zone, we found the southernmost segment is the weakest, with longer recurrence interval of about 11ka and lower vertical slip rate of 0.048mm/a. The high-accuracy grain size and magnetic susceptibility analysis offers an effective method for reducing the uncertainties of the paleo-earthquake research in loess area.     

Climate change response to astronomical forcing during the Oligocene-Miocene transition in the equatorial Atlantic (ODP Site 926)

The Oligocene-Miocene transition period was characterized by a decrease in global CO₂ levels, expansion of polar ice sheet, fall in global sea-level, etc. However, the reasons for, and mechanisms of, this global, extreme-cold climate change event (Mi-1) still remain controversial. Our samples from the core of the Ocean Drilling Program (ODP) Leg 154, Site 926, located in the equatorial Atlantic, mainly consist of light-gray, nannofossil chalk with foraminifers interbedded with green-ish-gray, clayey, nannofossil chalk sediments. Color variation from light-gray layers (up to 80% carbonate content) to dark layers (～60% carbonate content) was observed to occur cyclically at the meter scale. Therefore, we chose color reflectance lightness (L*) data as the paleoclimate proxy on which to perform cyclostratigraphic analysis because it could reflect carbonate content changes. Based on the recognition of the 405 kyr long eccentricity and ～40 kyr obliquity cycles of the L* series, we tuned the series to establish an absolute astronomical time scale using the published age of the Oligocene-Miocene boundary (OMB) as the anchor for an absolute age control point. The power spectra of the tuned L* series showed that the long eccentricity signals became significantly weak, while the obliquity signals became strong, from the Late Oligocene to the Early Miocene. The 405 kyr long eccentricity minimum coincided with the 1.2 Myr obliquity node at the OMB, and similar convergences might be closely related to other extreme-cold events in Earth’s history. In addition, the sedimentation accumulation rate, oxygen isotopes of benthonic foraminifers, and rodents’ per-taxon turnover rate from Central Spain showed the same ～2 Myr cyclicity, which indicates the significant influence of Earth-orbital forcing on the Earth system and ecological evolution on the million-year time scale.     

Astronomical calibration of the Matuyama-Brunhes boundary: Consequences for magnetic remanence acquisition in marine carbonates and the Asian loess sequences

We have compiled 19 records from marine carbonate cores in which the Matuyama-Brunhes boundary (MBB) has been reasonably well constrained within the astronomically forced stratigraphic framework using oxygen isotopes. By correlation of the δ¹⁸O data to a timescale based on astronomical forcing, we estimate astronomical ages for each of the MBB horizons. In all but one record the MBB occurs within Stage 19.

Most magnetostratigraphic sections in Asian Loess place the MBB within a loess interval. Since loess deposition is presumed to be associated with glacial intervals, loess horizons should correspond to even-numbered oxygen isotope stages. A glacial age for the MBB is at odds with the results presented here, which firmly place the MBB within interglacial Stage 19. Inconsistency among the many loess sections and between the loess and the marine records suggests that the magnetic interpretation of loess sections may be more complicated than hitherto supposed.

The mean of the Stage 19 age estimates for the MBB is 777.9 ± 1.8 (N = 18). Inclusion of the single Stage 20 age results in a mean of 778.8 ± 2.5 (N = 19). The astronomical age estimate of the MBB compares favorably with an (unweighted) mean of 778.2 ± 3.5 (N = 10) from a compilation of ⁴⁰Ar/39^Ar results of transitional lava flows. Combining the two independent data sets yields a grand mean of 778.0 ± 1.7 (N = 28).

The new compilation shows virtually no trend in placement of the MBB within isotope Stage 19 as a function of sediment accumulation rate. We interpret this to mean that the average depth of remanence acquisition is within a few centimeters of the sediment-water interface.

Separating the cores into two geographic regions (an Indo-Pacific-Caribbean [IPC] Group and an Atlantic Group) results in a significant difference in the position of the mid-point of the reversal with respect to the astronomical time scale. The data presented here suggest a difference of several thousand years between the two regions. This observation could be caused by systematic differences between the two regions in sedimentation rate within the interval of interest, systematic differences in remanence acquisition, or by genuine differences in the timing of the directional changes between the two regions.     

A high resolution palaeoclimatic record for the last 27,500 years in tropical West Africa from the carbon and nitrogen isotopic composition of lacustrine organic matter

Variations in the carbon and nitrogen isotopic composition of bulk organic matter in the sediments of Lake Bosumtwi, Ghana reflect climatically induced changes to the lake and the catchment flora. Cores spanning the last 27.5 kyr of sedimentation in the lake show large oscillations inδ¹³C_org PDB andδ¹⁵N_org Air. The late Pleistocene record is particularly detailed, showing changes of ca. 20‰ in carbon and ca. 10‰ in nitrogen isotopic compositions. These variations are of complex origin. Although different in magnitude, major isotopic excursions in the two records are generally in phase and reveal the occurrence of two major dry intervals at and immediately following the Last Glacial Maximum. The Allerød-Younger Dryas period also seems to have been marked by generally dry conditions in this part of tropical West Africa. Nitrogen isotopic evidence suggests that during the period 9.2–3.2 kyr the lake had an extremely stable water column, probably due to the absence of a windy or cool, dry season, or both. Regular circulation of the water column recommenced during the late Holocene and has persisted until the present day.     

Nonlinear coupling between 100 ka periodicity of the paleoclimate records in loess and periodicities of precession and semiprecession

It is apparent that there are three periods in various Quaternary paleoclimate records, the pe-riod of the Earth抯 orbital eccentricity cycles, ~100 ka, the period of the Earth抯 obliquity cycles, 41 ka, and the periods of the precession cycles, 23 ka and…     

塔里木盆地南缘8.5 ka以来的黄土岩石磁学特征及其磁化率变化机制

黄土岩石磁学参数是古气候研究中的重要指标,其中磁化率应用最为广泛,并在黄土高原地区取得重大进展,其受控于成壤作用的变化机制也被普遍接受.然而在黄土高原外缘的新疆地区,磁化率的变化机制仍不明确,导致磁化率的古气候意义在该区存在较大争议.本文选取塔里木盆地南缘具有精确年代控制的典型黄土剖面（羊场剖面）开展岩石磁学和高分辨率磁化率研究,利用交叉小波分析方法并结合剖面粒度、矿物及元素特征对该地区磁化率变化机制进行初步探讨.结果显示,羊场剖面的岩石磁学性质主要由粗颗粒软磁性矿物所控制,同时也表现出一定的顺磁性特征.根据载磁矿物和磁化率变化特征可将剖面进一步划分为两个阶段：阶段Ⅰ（8.5~2.5 ka）,载磁矿物以亚铁磁性的磁铁矿为主,磁化率值整体较高;阶段Ⅱ（2.5~0.2 ka）,亚铁磁性矿物依然占据主导地位,但硬磁性矿物和以黄铁矿为代表的顺磁性矿物相对增多,磁化率值显著降低.相关性研究和交叉小波分析表明：阶段Ⅰ磁化率与粗颗粒组分的变化具有一致性,符合"风速论"模式;阶段Ⅱ磁化率不仅与粗颗粒组分具有明显的正相关关系,而且与指示成壤作用强度的频率磁化率百分含量呈现出显著的负相关关系,暗示了阶段Ⅱ的磁化率变化可能受到"风速论"和"还原性成壤"模式的共同影响.本文拓宽了对新疆地区黄土岩石磁学特征及其磁化率变化机制的深入理解,也为利用磁化率恢复新疆及中亚地区全新世以来的古气候变化历史提供了新的线索.     

Assessment of long‐term Holocene soil erosion rates in Polish loess areas using sedimentary archives from closed depressions

Soil erosion is a particularly important problem in the loess areas of Central Europe. Numerous studies of past and present soil erosion based on colluvial sediments have so far been conducted. The main problem is the fact that colluvia usually do not represent the complete sedimentation record. Closed depressions (CDs) collect all colluvial sediments from their catchment, therefore, constitute sediment stores enabling the calculation of soil erosion rates. Colluvial sediments and fossil soils, infilling four CDs in the Polish loess belt, were OSL and C‐14 dated. Human settlements near the studied CDs were analyzed. Phases of soil erosion and colluviation from the Neolithic (5400–2900 bc ), from the Middle Bronze Age to the Early Iron Age (1600–0 bc ), and from the Early Middle Ages to Modern Times (500 AD until today) were documented within the CDs studied. Phases of low soil erosion rate and pedogenesis occurred from the Late Vistulian to the Early Neolithic and from the Iron Age to Early Middle Ages. This study reveals that these phases are not synchronous with the soil erosion phases in Central Europe, as the latter mainly occurred in the Bronze Age, Roman Period and Middle Ages. The obtained soil erosion rates were compared with erosion rates in different areas of Central Europe. This study indicates that in loess regions with long‐term agricultural land use, mean erosion rates (i.e. 3.7–5.9 t ha^‐1 yr^‐1) from the Middle Ages to Modern Times were ten times higher than during the entire prehistoric period (0.39–0.67 t ha^‐1 yr^‐1). The mean soil erosion rates for forested CDs was 0.24–0.74 t ha^‐1 yr^‐1. Soil erosion phases are most probably caused by human activities (i.e. land use change) but the early Holocene erosion phase (7.96 +/‐ 0.67 kyr) could have been induced by a climatic fluctuation (e.g. a 8.2 kyr Bond event). Copyright © 2018 John Wiley & Sons, Ltd.     

黄土高原西北缘末次冰期晚期以来黄土沉积物的岩石磁学性质

以黄土高原西北缘的靖远和古浪剖面(包含黄土层L1上部和占土壤层SO)作为研究对象,选取代表性样品进行磁化率、频率磁化率、热磁曲线、等温剩磁获得曲线和磁滞回线等测定.结果表明,靖远和古浪L1黄土和SO古土壤具有相似的岩石磁学特征.磁性矿物含量相对较低,载磁矿物均以磁铁矿为主,同时含有磁赤铁矿和赤铁矿,且SO占土壤中的磁赤...     

Early onset and origin of 100-kyr cycles in Pleistocene tropical SST records

The large 100-kyr cycles evident in most late-Pleistocene (0–0.6 Ma) paleoclimatic records still lack a satisfactory explanation. Previous studies of the nature of the transition from the early Pleistocene (1.2–1.8 Ma) 41-kyr-dominated climate regime to the 100-kyr world have been based almost exclusively on benthic foraminiferal oxygen isotopic (δ¹⁸O) data. It is generally accepted that the late Pleistocene 100-kyr cycles represent a newly evolved sensitivity to eccentricity/precession, superimposed on an earlier, and largely constant, response to obliquity and precession forcing. However, orbitally-resolved Pleistocene sea surface temperature (SST) records from a variety of oceanic regions paint a rather different picture of the global climate transition across the mid-Pleistocene transition (MPT, 0.6–1.2 Ma). Reanalysis of these SST records shows that: (1) an early onset of strong 100-kyr-like cycles in two low-frequency bands (～ 120–145 kyr and ～ 60–80 kyr), derived from the bundling of two/three obliquity cycles into grand cycles (obliquity subharmonics), occurred in tropical SST records during the early Pleistocene, (2) these two early Pleistocene periods converge into the late-Pleistocene 100-kyr period in tropical SST records, (3) the dominance of 100-kyr SST power in the late Pleistocene coincides with a dramatic decline in the 41-kyr SST power, and (4) the correlation of timing of glacial terminations with eccentricity/precession variation could well extend back into the early Pleistocene. We demonstrate that most of these features also occur in δ¹⁸O records, but in a much more subtle manner. These features could be explained in two plausible ways: a shift in climate sensitivity from obliquity to eccentricity/precession (a modified version of the conventional view) or an increasingly nonlinear response to orbital obliquity across the MPT. However, our examination of the development of ～100-kyr cycles favors an obliquity bundling mechanism to form late Pleistocene 100-kyr cycles. We therefore suggest that the late Pleistocene 100-kyr climatic cycles are likely a nonlinear response to orbital obliquity, although the timing of late Pleistocene 100-kyr climatic cycles and their early forms appears to be paced by eccentricity/precession.     

Role of glacial Arctic Ocean ice sheets in Pleistocene oxygen isotope and sea level records

A comparison of the oxygen isotope signal in deep-sea benthic foraminifera with the record of glacio-eustatic sea level for the last 160,000 years reveals that the amplitude of the benthic δ¹⁸O records predicts more continental ice volume than appears to be reflected in lowered sea level stands. These differences between the benthic δ¹⁸O ice volume estimates and radiometrically-dated records of eustatic sea level are consistent with the presence of a large floating Arctic Ocean ice mass during glacial intervals. The presence of an Arctic Ocean ice sheet during glacial intervals may account for the two climatic modes observed in oxygen isotope records which span the entire Pleistocene. The early Pleistocene (1.8 to 0.9 Myr B.P.) interval is characterized by low-amplitude, high-frequency δ¹⁸O fluctuations between glacial and interglacial periods, while the late Pleistocene (0.9 Myr B.P. to present) is characterized by large-amplitude, low-frequency δ¹⁸O changes. These two climatic modes can be explained by the initiation of earth orbital conditions favoring the co-occurrence of glacial period Arctic Ocean ice sheets and large continental ice sheets approximately 900,000 years before present.     

The evolution of Pleistocene climate: A time series approach

In the later part of the Pleistocene, variations in global ice volume have been dominated by an approximate 100,000-year cycle. Analysis of 2-Myr-long oxygen isotope record from an equatorial Pacific core indicates that this is true only for the last 900,000 years. Prior to this time the amplitude of the 100,000-year cycle is much reduced, as is the variance of all oscillations with periods greater than 60,000 years. Based on results of time series analysis of this 2-Myr-long record, the Pleistocene glacial cycles can be divided into three sections: (1) the late Pleistocene (0–900 kyr B.P.) where the variations in the isotope record are dominated by the 100,000-year cycle; (2) the middle Pleistocene (900–1450 kyr B.P.) in which low-frequency components are not as important as in the later period of the Pleistocene, and (3) the early Pleistocene/late Pliocene (1450–2000 kyr B.P.) where general reductions of importance at all frequencies is seen as compared to the later intervals. Recent modeling efforts which describe variations in global ice volume show that the dominant low-frequency component observed in the late Pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the Earth's orbital parameters. It is hypothesized that during the early Pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice-cap formation to below sea level. When the ice caps became marine-based, more rapid decay of the ice became possible.