Productivity cycles in Lake Hovsgol, NW Mongolia, during the last 1 Ma and the age model of the HDP-04 drill core record

Repetitive patterns in the records of total organic carbon (TOC), total nitrogen (TN) and δ¹³C_org observed in the Lake Hovsgol sediment section from HDP-04 drill core reflect past changes in productivity of Lake Hovsgol and in the isotopic composition of the lake's carbon pool. Lake Hovsgol productivity proxy signals are interpreted to represent the response of the Hovsgol lacustrine system to glacial–interglacial cycles of the Pleistocene. This interpretation is supported by the apparent orbitally-forced pattern in the TOC, TN and δ¹³C_org records of the past 250 ka in the BDP-96-2 drill core from neighboring Lake Baikal.The intervals with independent age control, such as the radiocarbon-dated last glacial–interglacial transition and the paleomagnetic reversals, make it evident that productivity proxy signals are reliable indicators of past cold-to-warm and warm-to-cold climate transitions, as seen from the agreement with the pattern of global climate change in marine δ¹⁸O records. The Brunhes/Matuyama reversal during the MIS 19 interglacial coincides with a distinct peak of TOC and TN in the Hovsgol record, similar to the signal during the Holocene interglacial. By contrast, the upper Jaramillo reversal in the Lake Hovsgol record occurs in a diatom-free calcareous interval characterized by minima in TOC, TN and by a ‘glacial’-type range of δ¹³C_org values. In both Lake Baikal and Lake Hovsgol records, peaks in TOC and TN contents help distinguishing past interglacials and interstadials, and isotopically-heaviest δ¹³C_org values help identify past glacial intervals.An age model for the HDP-04 drill core section is proposed based on recognizing the repetitive patterns in Lake Hovsgol productivity and lithologic records as regional paleoclimate cycles of middle to late Pleistocene. Absolute dates and diatom biostratigraphic correlation ties to the Lake Baikal record are used as key controls. In the proposed age model, the interval 81–24 m in the HDP-04 sediment section below the major unconformity is correlated to MIS 27 through late MIS 13, whereas the upper 24 m of the HDP-04 section is suggested to have recovered the sedimentary record of late MIS 7 to MIS 1.     

Paleoenvironmental proxy records from Lake Hovsgol, Mongolia, and a synthesis of Holocene climate change in the Lake Baikal watershed

Here we discuss paleoenvironmental evolution in the Baikal region during the Holocene using new records of aquatic (diatom) and terrestrial vegetation changes from Hovsgol, Mongolia's largest and deepest lake. We reconcile previous contradictory Baikal timescales by constraining reservoir corrections of AMS dates on bulk sedimentary organic carbon. Synthesis of the Holocene records in the Baikal watershed reveals a northward progression in landscape/vegetation changes and an anti-phase behavior of diatom and biogenic silica proxies in neighboring rift lakes. In Lake Baikal, these proxies appear to be responsive to annual temperature increases after 6 ka, whereas in Lake Hovsgol they respond to higher precipitation/runoff from 11 to 7 ka. Unlike around Lake Baikal, warmer summers between 6 and 3.5 ka resulted in the decline, not expansion, of forest vegetation around Lake Hovsgol, apparently as a result of higher soil temperatures and lower moisture availability. The regional climatic proxy data are consistent with a series of 500-yr time slice Holocene GCM simulations for continental Eurasia. Our results allow reevaluation of the concepts of ‘the Holocene optimum’ and a ‘maximum of the Asian summer monsoon’, as applied to paleoclimate records from continental Asia.     

Obliquity and precessional forcing of continental snow fall and melt: implications for orbital forcing of Pleistocene ice ages

Milankovitch theory posits that Earth's orbital cycles were the primary forcing of Pleistocene ice-age cycles through their strong influence on summer insolation at high latitudes. Accordingly, Milankovitch theory predicts ice volume should vary at both obliquity and precessional periods. However, early Pleistocene global ice volume varied mainly at the obliquity period with weak variability at the precessional period suggesting that Milankovitch theory is not sufficient to explain the ice-age cycles. Here we describe the results from a series of coupled ocean-atmosphere general circulation model experiments, using the Fast Ocean Atmosphere Model, that systematically investigate the influence of precession and obliquity on continental snowfall and potential ablation.Our model results identify three factors that magnify the influence of obliquity forcing on the global ice volume: First, high-latitude snowfall variability is dominated by changes in Earth's axial tilt. Second, hemispheric changes in net snowfall due to Earth's precession are out-of-phase, and largely cancel to produce a very small global snowfall change. Third, snowmelt variability over Antarctica responds greatly to changes in obliquity that intensify accumulation over obliquity cycle. We discuss the implications of these factors for existing hypotheses that account for the variability in the ice volume record.     

A HIGH-RESOLUTION DIATOM RECORD OF THE PALAEOCLIMATES OF EAST SIBERIA FOR THE LAST 2.5 My FROM LAKE BAIKAL

Two parallel drilling cores, BDP96-1 (200 m), and BDP96-2 (100 m), have been taken from top of the underwater Akademichesky Ridge in Lake Baikal and dated by paleomagnetic techniques. These cores are part of the Baikal Drilling Project (Kuzmin et al., 1997a, b). Diatom analysis of the first 100 m of these sediments reveals that they store a 2.5 Myr high-resolution continuous record of the palaeoclimates of East Siberia. Sediments belonging to warmer climates have a high content of diatom algae frustules, whereas those belonging to global glaciations are diatom-barren. The record of Upper Pleistocene (500 kyr BP) strongly correlates with the oceanic SPECMAP curve and contains distinct 100, 41, 23, and 19 kyr periods, as revealed by Fourier analysis. Diversity of diatom species was small in Pliocene. Abrupt and frequent changes of the climate in Pleistocene resulted in frequent dramatic changes in the diatom communities. The extant dominating endemics, Cyclotella minuta and Aulacoseira baicalensis, became important in Lake Baikal 760 kyr and less than 120 kyr BP, respectively.     

The impact of varying ice sheets and greenhouse gases on the intensity and timing of boreal summer monsoons

Climate models, forced only with insolation, indicate that boreal summer monsoons respond to orbital forcing with a zero phase both at the precession and obliquity bands. Discrepancies exist among data with respect to the timing of the response. Some late Pleistocene monsoon records show small lags of 2–3 kyr, close to model results, while many others show considerably longer lags of 5–8 kyr. It has been hypothesized that such lags arise from factors that were, up till now, not included in the modelling experiments, namely variations in glacial-age boundary conditions.Here we address this issue using long, time-dependent climate simulations that do include varying ice sheets and greenhouse gas concentrations. Inclusion of these additional forcings introduces a small peak in the monsoon spectra at the 100 kyr period, while monsoon variance remains dominated by precession with a smaller contribution from obliquity. At the precession band orbital forcing remains the dominant control, with lags close to zero. At the obliquity band varying ice sheet and greenhouse gases explain most of the simulated African and Indian monsoon variance, with orbital forcing playing a minor role. For the East Asian monsoon orbital forcing remains dominant. As a result the simulated obliquity phase of different monsoon systems lies between summer insolation maxima and ice minima/greenhouse gas maxima, with a lag that varies with distance to the Eurasian ice sheet.     

新生代的气候节律:赤道太平洋IODP320、321航次

新生代以来,地表气候在大趋势上逐渐变冷,并呈现出冷暖交替的变化节律.影响气候变化的因素主要包括太阳辐射量和地内气候系统的反馈效应.揭示气候变化的最终规律需要更准确的天文计算和更长、更可靠的气候替代性指标的重建.综合大洋钻探计划IODP 320、321航次在东赤道太平洋钻取了一系列保存完好、连续的深海沉积钻孔,为揭示新生代气候演变的规律提供了良好的研究材料.从全球视野的角度研究古海洋学和古气候学问题是揭示气候演变规律的良好方法,值得中国研究者借鉴.     

Orbital forcing of climate over South Africa: A 200,000-year rainfall record from the pretoria saltpan

Late Pleistocene variations in rainfall in subtropical southern African are estimated from sediments preserved in the Pretoria Saltpan, a 200000 year-old closed-basin crater lake on the interior plateau of South Africa. We show that South African summer rainfall covaried with changes in southern hemisphere summer insolation resulting from orbital precession. As predicted by orbital precession geometry (Berger, 1978), this South African record is out of phase with North African palaeomonsoon indices (Street and Grove, 1979; Rossignol-Strick, 1983; McIntyre et al., 1989); the amplitude of the rainfall response to insolation forcing agrees with climate model estimates (Prell and Kutzbach, 1987). These results document the importance of direct orbital insolation forcing on both subtropical North and South African climate as well as the predicted antiphase sensitivity to precessional insolation forcing.     

Phytoplankton assemblage in the Plio-Pleistocene record of Lake Baikal as indicated by sedimentary steryl chlorin esters

Past changes in phytoplankton assemblages in Lake Baikal over the last 4.5 Ma, both in population and composition, are inferred from the downcore profiles of the relatively stable chlorophyll derivatives steryl esters of pyropheophorbides a and b (steryl chlorine esters; SCEs) in the 0–200 m section of the BDP-98 drill core, supplemented by the data on biogenic silica (BSi) and total organic carbon (TOC) contents. SCEs-a and -b dominate among sedimentary chlorophyll derivatives in the BDP-98 sediments except for the upper few meters, indicating their high stability during diagenetic alteration of sediments. The depth (age) profiles of SCEs-a are consistent with BSi and TOC profiles and are interpreted as reflecting primary productivity of the lake in the past. Baikal proxies reveal close correlation with marine oxygen isotope records (MIS stratigraphy). These observations confirm that climate change in the northern hemisphere has been a primary factor controlling the total phytoplankton productivity in Lake Baikal during the last several million years.Among SCEs-a, C₃₀ (dinostanol)-SCE-a, a marker of dinoflagellates was identified by GC–MS analysis. SCE-b, a marker of green algae, was identified by its UV–vis spectrum. The ratio of C₃₀-SCE-a to total SCEs-a (TSCEs-a) was higher during 4.5–4.2 and 1.7–1.3 Ma, suggesting that dinoflagellates proliferated preferentially in those periods. The early Pleistocene maximum of this ratio corresponds to the broad minimum of diatom abundance previously suggested to have recorded a prolonged regional cooling. An abrupt increase in the SCE-b/TSCEs-a ratio was observed at 2.5–2.6 Ma, indicating that green algae containing chlorophyll b have proliferated in Lake Baikal during this period. This interval has also been suggested to contain evidence for a significant regional cooling based on minima of diatom abundance and BSi in sediments. The depth profile of C₂₇Δ⁵ (cholesterol)-SCE-a relative to TSCEs-a showed a trend similar to that of BSi, suggesting that C₂₇Δ⁵-SCE-a/TSCEs-a ratio is a potential marker of diatoms in Lake Baikal.Certain mismatches between the Lake Baikal profiles of biological indicators and the marine oxygen isotope records, as well as the slight temporal offsets between different Lake Baikal biological marker signals suggest that the regional component of climatic and/or lacustrine environmental changes also have played a role in determining the composition of the Lake Baikal Plio-Pleistocene phytoplankton assemblage.     

气候环境变化的湖泊沉积学响应

湖泊沉积可作为气候和环境变化的信息库,它能提供时间分辨率高达百年至十年的气候变化信息。湖泊沉积能灵敏地反映与地球轨道力变化相关的气候效应,以及在此背景上叠加的非线性气候事件。湖泊沉积与其中的古生物组合、元素同位素组成是古气候环境的重要替代指标,为检验大气动力学气候模型和进一步改进模型,预测未来气候提供了依据。     

A radiometric calibration of the SPECMAP timescale

The astronomical theory of climate change asserts that Earth's climate is affected by changes in its orbit, which vary the seasonal and latitudinal distribution of solar radiation. This theory is the basis of the orbitally tuned SPECMAP timescale. A key constraint for this important chronology was the mid-point of the Penultimate Deglaciation, initially dated to 127,000 years ago. Recent work suggests this event may be considerably older, casting doubt on the astronomical theory, the SPECMAP timescale, and the accuracy of orbitally tuned chronologies. Difficulties with U/Th coral dating of sea-level events have impeded progress on this problem, because most corals are not closed systems. Here, we use a new approach to U/Th dating that corrects for open-system behavior and produces a sea-level curve of sufficient resolution to confidently correlate with SPECMAP over the last 240,000 years, permitting a reassessment of both this critical chronology and a central tenet of climate change theory. High-precision ages for 24 oxygen isotope events provide a 240,000-year chronology for marine δ¹⁸O records that is independent of orbital tuning assumptions. Although there appear to be significant differences between the radiometric and orbitally tuned timescales near the lastglacial maximum and at the Marine Isotope Stage 7/6 boundary, a comparison of radiometric and SPECMAP ages for identical isotope events suggest that the SPECMAP timescale is quite accurate and that its errors were, in general, overestimated. Despite suborbital complexity, orbital cyclicity is clearly evident in our record. High-amplitude sea-level oscillations at periods greater than 20,000 years are very close in phase to summer insolation in the Northern Hemisphere. Although sea-level changes cannot be uniquely tied to a specific season or latitude of insolation forcing, the simplest explanation is that long-period, high-amplitude sea-level change is linked to Northern Hemisphere insolation forcing. These results validate the principles of orbital tuning and suggest such timescales are generally robust.     

Cenozoic deposits of the underwater Akademicheskii Ridge in Lake Baikal

This paper presents data on the lithological composition of Cenozoic deposits penetrated for the first time by boreholes BDP-96-1, BDP-96-2, and BDP-98 down to a depth of 600 m on the underwater Akademicheskii Ridge in Lake Baikal. The deposits are subdivided into the upper (Angara) and lower (Barguzin) sequences, which span the Middle Miocene-Holocene period. They formed under different climatic conditions and tectonic settings. Sources of the terrigenous material were also different. Outbursts of diatom-and mineral formation in Lake Baikal can be related to not only climatic fluctuations in the Miocene-Holocene, but also the endogenous activity. By the analogy with the World Ocean, underwater gas-hydrothermal fluid discharge detected at the water-bottom interface in this lake may be accompanied by the formation of diatomaceous oozes and ferromanganese nodule fields and the concentration of rare elements.     

Multi-Wavelength Synchrotron Radiation XRF Determination of U and Th in Sedimentary Cores from Lake Baikal

A new technique for the determination of uranium and thorium in lacustrine sediments is based on non-destructive synchrotron radiation X-ray fluorescence analysis (SR-XRF) of sediment samples using monochromatic beams of different energies as excitation sources, with the recording and subsequent iterative processing of the X-ray fluorescence spectra. The technique has a multielement capability and enables fast, simultaneous analysis of a few tens of elements. The proposed SR-XRF technique was tested against INAA and ICP-MS methods and showed a number of advantages with a generally good correlation of results by the three methods. Uranium and Th profiles have been measured at a time resolution of 2 kyr in a drill core (BDP-96) from Lake Baikal bottom sediments deposited between 780 and 40 kyr B P. During this time span, peaks in the U and U/Th concentration marked warm climates. Oscillations of U and U/Th in Baikal bottom sediments during the Brunhes chron reflect climate-induced global change in the volume of polar ice, controlled by long-term cyclicity of the Earths orbital parameters. The response of these warm periods is similar to that observed earlier in shorter cores that span the two last interglacials (220-0 kyr BP).     

早上新世非洲季风与地中海表层生产力变化的岁差节律

地质历史上留下了许多反映非洲季风的记录, 意大利南方晚新生代的海相地层便是其中著名的一例.Cape Sper-tivento剖面位于意大利卡拉布里亚半岛, 属于早上新世(5.3~4.8Ma) 地中海泥灰岩-灰岩沉积物.在借鉴前人工作的基础上, 重新为该剖面建立一个精度更高的天文年代标尺.反映古生产力的指标显示, 早上新世在北半球夏季辐射量增大期间, 地中海有机碳含量及其堆积速率增大, C/N出现高值, 碳酸盐含量及其堆积速率减小, G. obliquus稳定氧碳同位素出现负偏移, 这是非洲夏季风带来的降雨增强的结果.尼罗河泛滥让更多陆源营养物和淡水输入地中海.各个古环境指标的频谱分析结果存在强烈的岁差周期和丰富的半岁差周期, 说明早上新世非洲季风主要受低纬过程控制.      

A unified approach to orbital,solar, and lunar forcing based on the Earth’s latitudinal insolation/temperature gradient

Widespread empirical evidence suggests that extraterrestrial forcing influences the Earth’s climate, but how this could occur remains unclear. Here we describe a new approach to this problem that unifies orbital, solar and lunar forcing based on their common control of the Earth’s latitudinal insolation gradient (LIG). The LIG influences the climate system through differential solar heating between the tropics and the poles that gives rise to the latitudinal temperature gradient (LTG), which drives the Earth’s atmospheric and (wind driven) ocean circulation. We use spectral analysis of recent changes in the Earth’s LTG to support earlier work on orbital timescales (Davis and Brewer, 2009) that suggests the climate system may be unusually sensitive to changes in the LIG. Identification of LIG forcing of the LTG is possible because the LIG varies according to seasonally specific periodicities based on obliquity in summer (41 kyr orbital and 18.6 yr lunar cycle), and precession (21 kyr orbital cycle) and total solar irradiance (11 yr solar cycle) in winter. We analyse changes in the Northern Hemisphere LTG over the last 120 years and find significant (99%) peaks in spectral frequencies corresponding to 11 years in winter and 18.6 years in summer, consistent with LIG forcing. The cross-seasonal and multi-frequency nature of the LIG signal, and the diffuse effect of the LTG driver on the climate system may account for the complexity of the response to extraterrestrial forcing as seen throughout the climatic record. This hypersensitivity of the LTG to the LIG appears poorly reproduced in climate models, but would be consistent with the controversial theory that the LTG is finely balanced to maximise entropy.     

Sedimentary record from Lake Hovsgol, NW Mongolia: Results from the HDP-04 and HDP-06 drill cores

The Hovsgol Drilling Project retrieved Pleistocene sediment section with the basal age of ca. 1 Ma from the Hovsgol rift basin, NW Mongolia. Detailed lithologic data on drill cores is presented and compared with analogous sediment facies in the radiocarbon-dated records of the last glacial–interglacial transition. Drill cores from two sites, presently in 239 m and 235 m water depth, represent somewhat different depositional settings. The shorter HDP-06 drill core (26 m) at the base of the gentler SE slope of the rift basin contains lithologic evidence for several recent lake lowstands on the order of −200 m. The longer HDP-04 drill core (81 m) some 8 km away at the base of the steep NW underwater slope is composed of finer sediments and contains at least 10 characteristic transitions from calcareous to carbonate-free (diatomaceous) layers. These lithologic transitions are interpreted here as signals of repeated Pleistocene lake transgressions in the Hovsgol basin. Transgressions appear to have been associated with lower sedimentation rates and with the deposition of thin turbidite beds at the drill site. Comparison of drill core lithology with the available seismic data shows reasonable agreement in terms of the number of lowstand events and the general trends of changing lake level. HDP-04 drill core retrieved shallow-water facies containing sand and carbonate oolites deposited at the time of the most dramatic mid-Pleistocene regression of the lake. At ca. 24 m core depth, this interval corresponds to a major basinwide angular unconformity apparent in the seismic pattern. Lake Hovsgol, a smaller sister rift lake of the grand Lake Baikal, has a confined local catchment, which makes it very sensitive to regional variations in the effective moisture. Consisting primarily of calcareous mud, the sedimentary record of Lake Hovsgol provides a unique regional sedimentary archive. Future multi-proxy studies of the Hovsgol sedimentary records will allow constraint of the mid-late Pleistocene history of the hydrologic budget in the Baikal region of continental interior Asia.     

轨道调谐建立西宁厚层黄土堆积时间标尺的探索

青藏高原东北部西宁-互助地区堆积的厚层黄土是认识高原第四纪环境演化和气候变化的重要信息载体。对西宁泮子山厚层黄土钻孔（181.7m）进行了间隔5cm的采样,在实验室测量了全部样品的粒度,以粒度和北半球高纬太阳辐射数据为材料,选择古地磁年龄作为独立年代控制点,采用改进的自动轨道调谐方法,建立了西宁黄土早更新世以来天文轨道调谐的时间标尺。调谐后的粒度时间序列与ETP曲线在轨道周期上高度相关,在一定程度上证明了轨道调谐方法建立西宁厚层黄土堆积年代标尺的可行性。泮子山厚层黄土的沉积速率在1.0Ma前后发生了明显变化,1.0Ma以来沉积速率的相对高值可能反映了粉尘源区的干旱化程度增强,也指示了亚洲中部和青藏高原干旱环境的急剧发展。西宁厚层黄土年代标尺的建立,对深入认识晚新生代亚洲干旱气候发展过程和青藏高原的环境演化具有积极意义。     

Native iron in the sediments of Lake Baikal (borehole BDP-98): results of thermomagnetic analysis

We performed a thermomagnetic analysis of 91 samples and a probe microanalysis of five samples of sedimentary rocks from the lower zone of the borehole BDP-98 drilled at the bottom of Lake Baikal. The results show the scarcity of native iron: It was found only in five samples. Its concentration varies from ～10^–5 to 7 × 10^–4%. The distribution of native iron by content is bimodal, with a distinct “zero” mode. This scarcity of native iron in the Baikal sediments distinguishes them from continental (Eurasia) and oceanic (Atlantic) sediments of different ages. It is due to the high rate of sedimentation in the studied interval of BDP-98.     

Sedimentation processes and new age constraints on rifting stages in Lake Baikal: results of deep-water drilling

 With this paper we present a first attempt to combine the direct results on lithology, composition and age dating in the boreholes BDP-93, BDP-96 and BDP-97 with geological and seismic data from the areas where those sections were drilled. The sedimentary environments represented by the BDP boreholes are markedly different and possess characteristic lithological features. The results of the deep drilling provide the essential means for testing numerous age models used in geological reconstructions of the Lake Baikal rifting dynamics. Neither the basin-wide unconformity interpreted from seismic data, nor the interpreted change from shallow-water to deep-water facies at the boundary of the seismic stratigraphic complexes were found in the BDP-96 boreholes on Academician Ridge. Also, lithology does not support the proposed reconstructions of intense lake level fluctuations and transgressions during the Pliocene at Academician Ridge. The continuous deep-water hemipelagic sedimentation at Academician Ridge has existed for the past 5 Ma. The beginning of an intense rifting phase of the Neobaikalian sub-stage and related drastic changes in sedimentation processes were interpreted on seismic sections as the basin-wide unconformity B10. Different age estimates for this boundary ranged from Late Pliocene (3.5 Ma) to Plio-Pleistocene boundary. As shown by BDP-96 borehole, B10 is associated with a lithological change from diatomaceous ooze to dense silty clay and not with an erosional contact. The new age for this boundary in BDP-96 is approximately 2.5 Ma. This new age constraint suggests that the upper sedimentary strata of Northern Baikal (1.5–1.7 km thick) have formed during the past 2.5 Ma with average sedimentation rates of 60–70 cm/ka. The BDP-93 boreholes at Buguldeika suggest that uplift in Primorsky Range took place prior to 1.07–1.31 Ma, a date which exceeds the age of previous geological models. Received: 12 March 1999 / Accepted: 10 February 2000     

2.5Ma以来地球轨道参数变化对黄土粒度变化的线性驱动

本文对2.5Ma以来宝鸡黄土剖面粒度曲线与ETP曲线做了互功率谱和凝聚函数分析,以期反映风力强度与地球轨道参数变化的关系。结果表明,2.5Ma以来粒度记录中始终存在着与地轴倾斜度、岁差呈线性响应的41000a、23000a和19000a周期;同时,在0.6—0.0MaB.P.时段存在与偏心率呈线性响应的0.1Ma周期,在2.5—1.6MaB.P.时段存在与偏心率0.4Ma呈线性响应的周期。上述结果佐证了地球轨道变化对内陆古气候变化的线性驱动作用,但该理论不能解释在约0.9—0.6MaB.P.和1.6MaB.P.前后出现的二次较大的主导气候周期转型事件。     

Cyclostratigraphic reasoning and orbital time calibration

Cyclostratigraphy postulates that the stratigraphic record cyclically encodes the periodic orbital forcing of terrestrial insolation, thus providing a time calibration. Regular cycles are sought using spectral analysis of lithological data series, but there are inherent ambiguities in this method. It may detect more cyclicities than conventional orbital forcing allows, but only those with the closest correspondence to estimated orbital frequencies are used for time calibration. Irregular cycles are subjectively defined in terms of non-rhythmic repetitions of facies. The calibrations assume that they record the spatially distorted sedimentary effects of orbitally forced periodicity in insolation. The null hypothesis that such non-rhythmic repetitions are autogenic, rather than orbitally forced, cannot, however, be rejected. Both types of cyclicity conform to an 'expected universe' where orbital forcing is reliably and recognisably encoded in the stratigraphic record. Neither form of cyclicity rules out the presence of hiatuses; thus, even if orbital in origin, neither can provide dependably refined, orbitally scaled, time calibration.