Kyr Period Research Articles

Astronomical cycle origin of bedded chert: A middle Triassic bedded chert sequence, Inuyama, Japan

Astronomical forcing is one of the main drivers of climate change, and astronomical cyclicity recorded in sediments provides a clue to understand the dynamics of the global climate system. Bedded cherts consist of rhythmic alternations of chert and shale beds. Although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles (e.g. Fischer, 1976; Hori et al., 1993), conclusive proof remains elusive. To explore this possibility, we established a continuous, high-resolution lithostratigraphy of middle Triassic bedded chert in Central Japan. The average duration of each chert–shale couplet is 20 kyr, similar to that of the precession cycle. Spectral analysis of a bed number series of thickness variations in chert beds was performed assuming that each chert–shale couplet represents a 20-kyr precession cycle. The results reveal cycles involving approximately 200, 20, 5, and 2–3 beds, corresponding to periodicities of approximately 4000, 400, 100, and 40–60 kyr, respectively. By further assuming that the 20-bed cycle represents a 405-kyr eccentricity cycle of constant and stable periodicity, we converted the bed number series to a time series. Spectral analysis of the time series revealed distinct periodicities of 3600, 117, 97, and 38 kyr, in addition to 405 kyr. Besides 3600 kyr, these periodicities agree well with the 120, 95, and 37 kyr periodicities for eccentricity cycles and the obliquity cycle during the Triassic. Moreover, we detected amplitude modulation of the approximately 100-kyr cycle of thickness variations in chert beds with a 405-kyr periodicity, which may correspond to amplitude modulation of 100-kyr climatic precession cycle with the 405-kyr periodicity. The approximately 3600-kyr periodicity described above and 1800-kyr periodicity manifested as the amplitude modulation of the 405-kyr cycle are correlated to present-day long-term eccentricity cycles of 2400 and 4800 kyr evolved by chaotic behavior of solar planets. Collectively, these similarities in the periodicities of dominant cycles, their hierarchy, and the nature of amplitude modulation of cycles in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles, thereby indicating the potential of bedded chert as a template for a Mesozoic cyclostratigraphy.

Late Pliocene equatorial Pacific

[1] Late Pliocene foraminiferal Mg/Ca and δ18O records from Ocean Drilling Program Hole 806B in the western equatorial Pacific (WEP) reveal warm pool climate evolution during the onset of Northern Hemisphere glaciation, 3.1–2.3 Myr B.P. Mg/Ca data indicate an average late Pliocene sea surface temperature (SST) of 27.8°C, a small long-term cooling of 0.3°C between 3.1 and 2.3 Ma, and a glacial-interglacial (G-I) SST range of 2°C throughout this time interval. For comparison, Pleistocene SSTs at this site over the last 0.9 Myr average 27.7°C with a G-I range of 3°C. Orbital-scale variability in Hole 806B SSTs during the late Pliocene occurs predominantly at ∼100 ka, in contrast to foraminiferal δ18O records, which show a dominant 41 kyr period. Variability at a 41 kyr period, out of phase with local annual insolation changes driven by obliquity, is also observed in the new WEP SST record. The WEP SST record suggests that an ∼3°C equatorial Pacific SST zonal gradient prevailed during the late Pliocene, compatible with a weaker Walker circulation. Adjustment of Hole 806B SSTs for past changes in seawater Mg/Ca suggests that SSTs higher than 30°C prevailed at 3 Myr B.P., followed by a progressive cooling of the warm pool through the late Pliocene. The characteristics of late Pliocene tropical climate evolution suggest that atmospheric greenhouse gas forcing played a major role in driving the observed G-I SST changes.

Potential origins of 400–500 kyr periodicities in the ocean carbon cycle: A box model approach

Cenozoic proxy records of both the isotopic composition of dissolved inorganic carbon in the oceans (δ13CDIC) and deep ocean carbonate preservation show significant periodicities in the range 400–500 kyr. Sensitivity analysis of the ocean carbon cycle to potential variability on this timescale in patterns of global oceanic primary productivity and/or continental weathering fluxes is performed using a 7 box ocean‐atmosphere model. The data constraints imposed by Plio‐Pleistocene proxy records of δ13CDIC, carbonate preservation, and atmospheric pCO2 variability allow different scenarios to be evaluated. Forcing with the global oceanic primary production ratio of inorganic and organic carbon leads to the relative phases of response most consistent with the proxy data. However, only when changes also occur in total marine primary production can the observed relative amplitudes of δ13CDIC and pCO2 variability also be reproduced. This scenario is consistent with oscillations between a highly productive, coccolithophore‐rich global ocean and a less productive, diatom‐rich one. Such oscillations have been proposed to originate from the orbital eccentricity cycle leading to changes in seasonality and thus silica utilization in the Southern Ocean. However, the period of response in model δ13CDIC is always close to that of the forcing function used, and thus a significant discrepancy remains between the orbital eccentricity cycle period of 413 kyr and the observed ∼500 kyr δ13CDIC periodicity seen in the Pleistocene.

Palaeo methane-seepage history traced by biomarker patterns in a carbonate crust, Nile deep-sea fan (Eastern Mediterranean Sea)

We investigated lipid biomarkers in three depth-intervals of a methane-derived carbonate crust collected from a pockmark in the Nile Deep-Sea fan sampled during the Nautinil cruise of the R/V l'Atalante. A diverse set of 13C-depleted archaeal and bacterial lipids were detected along this crust with δ 13C values as low as − 118‰. This provides evidence that anaerobic oxidation of methane was the main process that induced the formation of this carbonate crust. Archaeal lipid biomarker distributions revealed the presence of archaea belonging both to the ANME-1 and ANME-2 phylogenetic clusters, with the former predominating. The distribution and compound specific δ 13C values of AOM-derived lipid biomarkers showed no significant variations with depth. These results infer that the archaeal community structure and thus the local seepage environment remained relatively stable during the 5000 kyr period of the carbonate crust precipitation.

Geomagnetic field variations during the last 400 kyr in the western equatorial Pacific: Paleointensity‐inclination correlation revisited

A paleomagnetic study was conducted on four piston cores newly obtained from the West Caroline Basin in the western equatorial Pacific in order to investigate variations in paleointensity and inclination during the last 400 kyr. An inclination‐intensity correlation was previously reported in this region using giant piston cores, but the quality of the paleomagnetic data of the younger end, the last ca. 300 kyr, was needed to be checked because the upper part of the giant piston cores could suffer from perturbation by oversampling. Age control is based on the oxygen‐isotope ratios for one core and inter‐core correlation using relative paleointensity for other cores. Stacked curves of paleointensity and inclination were constructed from the four cores. It was confirmed that variations on the order of 104 to 105 years occur in inclination as well as paleointensity. A cross‐correlation analysis showed that significant in‐phase correlation occurs between intensity and inclination for periods longer than about 25 kyr, and power spectra of both paleointensity and inclination variations have peaks at ∼100 kyr periods. The regional paleointensity stack with higher resolution than the Sint‐800 stack (Guyodo and Valet, 1999) should be useful for paleointensity‐assisted chronostratigraphy.

Coexisting stochastic and coherence resonance in a mean-field dynamo model for Earth’s magnetic field reversals

Using a spherically symmetric mean-field α2-dynamomodel for Earth’s magnetic field reversals, we show thecoexistence of the noise-induced phenomena coherence resonance and stochastic resonance. Stochastic resonance was recentlyinvoked to explain the 100 kyr periodicity in the distribution ofresidence times between reversals.The comparison of the resulting residence time distribution withthe paleomagnetic one allows for some estimate ofthe effective diffusion time of the Earth’s core which may be100 kyr or slightly below rather than200 kyr as it would result from the molecular resistivity.

Origin of orbital periods in the sedimentary relative paleointensity records

Orbital cycles with 100 kyr and/or 41 kyr periods, detected in some sedimentary normalized remanence (relative paleointensity) records by power spectral analysis or wavelet analysis, have been attributed either to orbital forcing of the geodynamo, or to lithologic contamination. In this study, local wavelet power spectra (LWPS) with significance tests have been calculated for seven relative paleointensity (RPI) records from different regions of the world. The results indicate that orbital periods (100 kyr and/or 41 kyr) are significant in some RPI records during certain time intervals, and are not significant in others. Time intervals where orbital periods are significant are not consistent among the RPI records, implying that orbital periods in these RPI records may not have a common origin such as orbital forcing on the geodynamo. Cross-wavelet power spectra (|XWT|) and squared wavelet coherence (WTC) between RPI records and orbital parameters further indicate that common power exists at orbital periods but is not significantly coherent, and exhibits variable phase relationships, implying that orbital periods in RPI records are not caused directly by orbital forcing. Similar analyses for RPI records and benthic oxygen isotope records from the same sites show significant coherence and constant in-phase relationships during time intervals where orbital periods were significant in the RPI records, indicating that orbital periods in the RPI records are most likely due to climatic ‘contamination’. Although common power exists at orbital periods for RPI records and their normalizers with significant coherence during certain time intervals, phase relationships imply that ‘contamination’ (at orbital periods) is not directly due to the normalizers. Orbital periods are also significant in the NRM intensity records, and ‘contamination’ in RPI records can be attributed to incomplete normalization of the NRM records. Further tests indicate that ‘contamination’ is apparently not directly related to physical properties such as density or carbonate content, or to the grain size proxy κ ARM/ κ. However, WTC between RPI records and the grain size proxy ARM/IRM implies that ARM/IRM does reflect the ‘contamination’ in some RPI records. It appears that orbital periods were introduced into the NRM records (and have not been normalized when calculating RPI records) through magnetite grain size variations reflected in the ARM/IRM grain size proxy. The orbital power in ARM/IRM for some North Atlantic sites is probably derived from bottom-current velocity variations that are orbitally modulated and are related to the vigor of thermohaline circulation and the production of North Atlantic Deep Water (NADW). In the case of ODP Site 983, the orbital power in RPI appears to exhibit a shift from 41-kyr to 100-kyr period at the mid-Pleistocene climate transition (∼750 ka), reinforcing the climatic origin of these orbital periods. RPI records from the Atlantic and Pacific oceans, and RPI records with orbital periods eliminated by band-pass filters, are highly comparable with each other in the time domain, and are coherent and in-phase in time-frequency space, especially at non-orbital periods, indicating that ‘contamination’, although present (at orbital periods) is not debilitating to these RPI records as a global signal that is primarily of geomagnetic origin.

Multiple Brunhes Chron excursions recorded in the West Eifel (Germany) volcanics: Support for long-held mantle control over the non-axial dipole field

Volcanic records of excursional geomagnetic field behavior, in particular paleointensity estimates, are fragmentary for the Pleistocene. The West Eifel volcanic field is unique in that 12 of 66 measured lava flow sites record Virtual Geomagnetic Poles (VGPs) clustered between 34–45°N latitude and 30–50°E longitude over the Middle East and Southeast Asia. Paleointensity determinations reveal that 13 of 37 lavas erupted during times when the ambient field was weak. Specifically, results on nine transitionally magnetized and four normally magnetized lavas indicate paleofield intensities less than 30 μT associated with virtual dipole moments (VDMs) <4 × 10 22 Am 2 [Schnepp, E., Hradetzky, H., 1994. Combined paleointensity and 40Ar/ 39Ar age spectrum data from volcanic rocks of the West Eifel field (Germany): evidence for an early Brunhes geomagnetic excursion. J. Geophys. Res 99, 9061–9076]. Until now, the age of these lava flows has been known only from imprecise 40Ar/ 39Ar dating that implied acquisition of magnetic remanence between about 570 and 450 ka. Thus, they have been interpreted to record a single, but poorly defined, excursion in the lower Brunhes Chron. To incorporate these paleofield data into the global high-resolution Geomagnetic Instability Time Scale (GITS), we have determined precise ages of groundmass from 12 excursional and 7 normally magnetized lava flows using the 40Ar/ 39Ar incremental heating method. Unspiked K–Ar age determinations from two excursional samples are indistinguishable from their 40Ar/ 39Ar results. Ages of the excursional lava flows fall into five groups at 722 ± 38, 626 ± 24, 578 ± 8, 555 ± 4, and 528 ± 16 ka (±2 σ analytical uncertainties, relative to 1.194 Ma Alder Creek sanidine). The group of three excursional flows with an age of 578 ± 8 ka correspond temporally with lava flow sequences on Tahiti and La Palma that record the Big Lost excursion at 579 ± 6 ka. Eight other excursional lavas erupted from 722 to 528 ka correlate with less well-defined excursions or paleointensity minima differentially expressed in marine sediment records. Our findings that at least four, and perhaps five, temporally distinct excursions are recorded between 722 and 528 ka further suggest that each weakening of the dynamo during this period was associated with demise of the axial dipole. The VGPs produced during these excursions are found to be dispersed longitudinally across Eurasia and are remarkably similar to those associated with the calculated south VGPs at West Eifel for the non-axial dipole (NAD) field during the 20th century. A simple model in which the axial dipole repeatedly collapses leaving behind a residual NAD field that is largely held by stationary sources, probably in the lowermost mantle, provides an explanation for the recurrent excursional behavior during this ∼200 kyr period.

A relative paleointensity record of the geomagnetic field since 1.6 Ma from the North Pacific

Abstract A paleomagnetic study was conducted on a sediment core KR0310-PC1 taken from the central North Pacific in order to obtain a relative paleointensity record in the Matuyama chron from this region. The core reached to about 1.6 Ma. The age control is based on the correlation of the S ratio (S −0.1T) variations with a global oxygen-isotope stack. Isothermal remanent magnetization (IRM) was used as the normalizer of the relative paleointensity estimation; anhysteretic remanent magnetization (ARM) was not adopted because ARM is sensitive to magnetostatic interaction among magnetic particles, which is evidenced in these sediments by an inverse correlation between the ratio of ARM to saturation IRM (SIRM) and SIRM without significant magnetic grain-size changes. For the last 350 kyrs, the record of core NGC65, which was obtained at practically the same site as KR0310-PC1 and covers the Brunhes chron (Yamazaki, 1999), was incorporated because the upper part of KR0310-PC1 was physically disturbed. In the record of NGC65/KR0310-PC1, the average paleointensity in the late Matuyama chron is not lower than that during the Brunhes chron, which does not support the conclusion of Valet et al. (2005) based on their Sint-2000 stack. A spectral analysis on the NGC65/KR0310-PC1 paleointensity record shows a power at the ∼100 kyr eccentricity period. The relative paleointensity and magnetic properties of NGC65/KR0310-PC1 were compared with those of MD982185 from the western equatorial Pacific (Yamazaki and Oda, 2002, 2005). The two sites belong to different oceanographic regimes. Coherent variations in the relative paleointensity despite incoherent changes in the magnetic properties suggest that rock-magnetic contamination to the relative paleointensity is small, if any, and the ∼100 kyr period in the relative paleointensity records would reflect the geomagnetic field behavior.

Coherent obliquity band and heterogeneous precession band responses in early Pleistocene tropical sea surface temperatures

The nature of the connection between high‐ and low‐latitude climates during the early Pleistocene “41 kyr world” has important implications for our understanding of the feedbacks involved in translating insolation changes into global climate states. Here we focus on the tropical marine record, presenting alkenone‐derived sea surface temperature (SST) and productivity records from the eastern equatorial Atlantic, eastern equatorial Pacific, the Arabian Sea, and the South China Sea for a time interval covering the heart of the 41 kyr world (1.2–2.0 Ma). All four SST records are dominated by variance in the obliquity band, suggesting that high‐latitude dynamics and low‐latitude climate were tightly coupled in the 41 kyr world, despite smaller ice volume variability during this interval as compared to the late Pleistocene. At the 41 kyr period, SST varied coherently and nearly synchronously between the four study regions, suggesting a tropic‐wide feedback to high‐latitude processes. Productivity variations at our equatorial Atlantic and Pacific sites were also coherent in the obliquity band, implying tropical trade wind variability at this frequency during the early Pleistocene. In contrast, we observe heterogeneous SST and productivity responses in the precession band between each of the tropical locations. Local atmospheric circulation patterns, rather than a globally coordinated response to precessional insolation forcing, apparently determined SSTs and productivity in the tropics at precessional frequencies during the early Pleistocene.

Equatorial insolation: from precession harmonics to eccentricity frequencies

Abstract. Since the paper by Hays et al. (1976), spectral analyses of climate proxy records provide substantial evidence that a fraction of the climatic variance is driven by insolation changes in the frequency ranges of obliquity and precession variations. However, it is the variance components centered near 100 kyr which dominate most Upper Pleistocene climatic records, although the amount of insolation perturbation at the eccentricity driven periods close to 100-kyr (mainly the 95 kyr- and 123 kyr-periods) is much too small to cause directly a climate change of ice-age amplitude. Many attempts to find an explanation to this 100-kyr cycle in climatic records have been made over the last decades. Here we show that the double maximum which characterizes the daily irradiation received in tropical latitudes over the course of the year is at the origin in equatorial insolation of not only strong 95 kyr and 123 kyr periods related to eccentricity, but also of a 11-kyr and a 5.5-kyr periods related to precession.

Climatic oscillations over the past 120 kyr recorded in the Guliya ice core, China

Recently, several ice cores were drilled on the Tibetan Plateau to examine climate and environmental changes. Based on the temperature proxy records in the Guliya ice core, climate oscillations over the past 120 kyr were examined using wavelet analysis and singular spectrum analysis methods. The results showed two main Milankovitch-type oscillatory modes, 22 and 44 kyr. The 22 kyr period corresponds to the precession period and 44 kyr period corresponds to the Earth axis's obliquity inclination period. The two periods combined explain ∼43% of the variance. In addition, there are millennial and 10 thousand years time scales periods, which may be related to the Dansgaard–Oeschger (D–O) cycles and the Heinrich (H) events.

Western Caribbean sea surface temperatures during the late Quaternary

Mg/Ca ratios in the planktonic foraminifera Globigerinoides ruber from Colombian Basin core ODP 999A suggest that Caribbean sea surface temperatures (SSTs) were from 2.1 to 2.7°C colder than the present during the last three glacial maximums. In comparison, faunal derived SSTs (SIMMAX method) show that August SSTs in the Caribbean varied &lt;2°C over the past 360 kyr, whereas February SSTs varied between 21.0°C and 26.5°C. Changes in the Mg/Ca‐SST record contain a strong 23 kyr periodicity, suggesting the Mg/Ca‐SST record reflects a warm season weighted SST average rather than an annual mean SST. Combining several dissolution indices, we identify brief periods of decreased carbonate preservation in our record and show that MIS 11 stands out as the most intensive dissolution cycle in the Caribbean over the last 460 kyr. Comparison of Caribbean SST change with a similar estimate of tropical SST variability in the western Pacific over the past 360 kyr reveals shifts in the east‐west tropical SST gradient that are coeval with glacial‐interglacial climate change and consistent both with a southward migration of the glacial ITCZ and with a glacial El Niño‐like mode of tropical circulation.

Drowned coralline algal dominated deposits off Lanai, Hawaii; carbonate accretion and vertical tectonics over the last 30 ka

We present detailed bathymetry, remotely operated vehicle (ROV) and submersible observations, and sedimentary and radiocarbon age data from carbonate deposits recovered from two submerged terraces at − 150 m (T1) and − 230 m (T2) off Lanai, Hawaii. The tops of the terraces are veneered by relatively thin (< 5 m) in situ accumulations of coralline algal nodule, coralgal nodule, Halimeda and a derived oolitic facies deposited in intermediate (30–60 m) to deep fore-reef slope settings (60–120 m). The data are used to develop a sedimentary facies model that is consistent with eustatic sea-level variations over the last 30 ka. Both nodule facies on T1 and T2 initiated growth 30–29 ka following a fall in sea level of ∼50 m and increase in bottom currents during the transition from Marine Isotope Stage 3 to 2. The nodules accreted slowly throughout the Last Glacial Maximum when sea-level was relatively stable. Drowning occurred during the early deglaciation (17–16 ka) and was marked by the complete drowning of coralline algal nodules facies on T2 and incipient drowning of coralgal facies on T1. Abrupt sea-level rise during the middle deglaciation, perhaps associated with global meltwater pulse 1A (14–15 ka), finally drowned the coralgal facies on T1, which in turn was overlain by a deep-water Halimeda facies or an oolitic facies derived from upslope. Our data indicates that Lanai has experienced relatively little vertical tectonic movement over the last 30 ka. Using paleobathymetric data derived from the sedimentary facies, age vs. depth relationships, and published sea-level curves, we estimate that Lanai could be either slowly uplifting or subsiding, but at rates < 0.1 m/kyr (uplift) or < 0.4 m/kyr (subsidence) over this 30 kyr period.

Variations in atmospheric trace elements in Dome C (East Antarctica) ice over the last two climatic cycles

Concentrations of Li, Mg, Cr, Mn, Co, Cu, As, Rb, Cd, Ba and Bi have been determined by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) in various sections of the new Dome C EPICA Antarctic ice core, down to the depth of 2193 m, covering a time period of two climatic cycles. The time resolution of these records is at least twice as good as previously published ultra trace elements profiles obtained from the Vostok ice core. During the ∼217 kyr period spanned by this record, a high variability in concentrations is observed for most elements, with low values during warm periods and high values during cold periods. The highest concentrations are recorded at the times of the last two glacial maxima (∼20 and ∼140 kyr BP). The timing and the amplitude of the main concentration peaks match remarkably well the insoluble dust concentration profile. It confirms that dust was the main carrier of atmospheric trace elements to East Antarctica during the cold periods. For Ba, Co, Cu and Rb the crustal contribution was also dominant during warm periods. For other elements the situation is more complex during interglacial periods, when other sources such as volcanic quiescent emissions, became possibly significant for several trace elements such as Cd and Bi. Peculiarly high concentration values are observed for Cd and Bi for a short depth interval dated at ∼18 kyr BP. It is the same depth interval in which elevated F − values were previously observed. These very high concentrations are attributed to fallout from major local volcanic emissions at that time.

Orbitally paced paleoproductivity variations in the Timor Sea and Indonesian Throughflow variability during the last 460 kyr

A high‐resolution (∼1–2 kyr) multiproxy record from the Timor Sea in the easternmost Indian Ocean (International Marine Global Change (IMAGES) Program Core MD01‐2378, latitude 13°04.95′S, longitude 121°47.27′E, 1783 m water depth) closely tracks changes in intermediate water ventilation and paleoproductivity over the last 460 kyr within one of the main outflow passages of the Indonesian Throughflow. Spectral analysis of five different flux‐based productivity proxies indicates spectral power concentrated in the 100 kyr (glacial‐interglacial) and the 23 kyr and 19 kyr (precessional) periods. Paleoproductivity maxima lead ice volume (benthic δ18O) maxima by 20° to 40° (∼1300 to 2600 years) at the precession band. The spectral differences in tropical paleoproductivity records from the Pacific and Indian oceans suggest that local processes (wind and circulation patterns driven by insolation) are dominant in driving productivity rather than large‐scale tropical features. In the Timor Sea, productivity fluctuations over the last 460 kyr were strongly influenced by monsoonal wind patterns offshore NW Australia (23 and 19 kyr) and were also modulated by sea level–related variations in the intensity of the Indonesian Throughflow (100 kyr).

Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc

Geologic, chronologic, and U–Th isotope data from Pleistocene–Recent basaltic to rhyolitic lavas and their phenocrysts constrain the long-term evolution of magmatic processes at Seguam Island, Aleutian Island arc, and suggest a model in which erupted magmas were derived from a single, deep seated reservoir for ∼130 kyrs of its eruptive history. The monotonic evolution in ( 230Th / 232Th) 0 ratios is consistent with radiogenic ingrowth of 230Th in a long-lived magma reservoir between 142 and 9 ka. Internal U–Th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40Ar / 39Ar dating, which implies a short period (i.e., 10 3 yrs or less) of crystal residence in the magma prior to eruption. These results can be reconciled if small batches of magma are repeatedly extracted from a deep, thermally buffered, basaltic reservoir, followed by decompression-driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption. Stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering ∼12 km 2. The ignimbrite, as well as subsequent rhyolitic and basaltic lava flows, signal a decrease in ( 230Th / 232Th) 0 and 87Sr / 86Sr ratios. The abrupt change in magma composition at 9 ka most likely reflects disruption of the long-lived reservoir by rapid ascent of basaltic magma with low ( 230Th / 232Th) and 87Sr / 86Sr ratios and excess 226Ra. While the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored, the shift in isotopic composition and evidence for a protracted > 100 kyr period of magma storage, would have been missed had the focus been on only the most recent historical eruptions. Our results suggest that: not all arc basalt produced in the mantle wedge separates and ascends rapidly, magma ascent rates and storage times may vary significantly over periods of 10 4 to 10 5 yrs at a single arc volcano, and U–Th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs, provided a sufficiently long period of volcanic activity is examined.

Variations of the Last Glacial Warm Pool: Sea surface temperature contrasts between the open western Pacific and South China Sea

High‐resolution paleoceanographic records from the southern South China Sea (SCS) and the tropical open western Pacific reveal multicentennial‐ to millennial‐scale variations of the Western Pacific Warm Pool (WPWP) over the last 40 kyr. During the Last Glacial Maximum (LGM), in marine isotope stage (MIS) 2, planktonic Globigerinoides sacculifer δ18O in core NS93‐5 from the southern SCS and in core WP92‐5 from the central WPWP registered a similar average value of about −1.5‰. However, the average δ18O value was about 0.5‰ more negative in the SCS than in the central WPWP, as was their difference before and after the LGM in the later part of MIS 3 and early portion of MIS 1. These results indicate weaker monsoonal and fluvial effects on the southern SCS during the last glacial than during pleniglacial and postglacial periods owing to different oceanic circulation modes and regional topography. The calculated seasonal temperature differences of summer and winter sea surface temperatures (SSTs) between the open WPWP and the southern SCS were up to 3–4.5°C during the LGM but only slightly over 3°C in the southern SCS and less than 2°C in the open WPWP during other periods. From about 29 to 25 ka the MIS 2/3 transition was marked by a sudden SST increase in the southern SCS but a decrease in the open WPWP. Over this 4 kyr period the average annual SST appeared to be only 28°C in the central WPWP but more than 28°C in the southern SCS, indicating a shift of the central WPWP toward the far west during that time. Furthermore, our results imply a period of nearly 6 kyr from 22.5 to 16.5 ka without an apparent warm pool of 28°C in the western Pacific.

Oligocene climate dynamics

A planktonic and benthic foraminiferal stable isotope stratigraphy of the Oligocene equatorial Pacific (Ocean Drilling Program, Site 1218) was generated at 6 kyr resolution between magnetochrons C9n and C11n.2n (∼26.4–30 Ma on a newly developed astronomically calibrated timescale). Our data allow a detailed examination of Oligocene paleoceanography, the evolution of the early cryosphere, and the influence of orbital forcing on glacioeustatic sea level variations. Spectral analysis reveals power and coherency for obliquity (40 kyr period) and eccentricity (∼110, 405 kyr) orbital bands, with an additional strong imprint of the eccentricity and 1.2 Myr obliquity amplitude cycle, driving ice sheet oscillations in the Southern Hemisphere. Planktonic and benthic foraminifera δ18O are used to constrain the magnitude and timing of major fluctuations in ice volume and global sea level change. Glacial episodes, related to obliquity and eccentricity variations, occurred at 29.16, 27.91, and 26.76 Ma, corresponding to glacioeustatic sea level fluctuations of 50–65 m. Alteration of high‐latitude temperatures and Antarctic ice volume had a significant impact on the global carbon burial and equatorial productivity, as cyclic variations are also recorded in the carbon isotope signal of planktonic and benthic foraminifera, the water column carbon isotope gradient, and estimated percent carbonate of bulk sediment. We also investigate the implications of a close correspondence between oxygen and carbon isotope events and long‐term amplitude envelope extrema in astronomical calculations during the Oligocene, and develop a new naming scheme for stable isotope events, on the basis of the 405 kyr eccentricity cycle count.

Black body temperature, orbital elements, the Milankovitch precession index, and the Seversmith psychroterms

The temperature T of a black or gray body orbiting the Sun can be expressed in terms of spherical harmonics in latitude and longitude, its Keplerian orbital elements, and a variable describing rotation about its axis. Assuming that the Earth is a black or gray body without thermal inertia, the resulting equation for T exhibits previously unrecognized odd-degree zonal terms dubbed “Seversmith psychroterms.” They cause a hemispheric temperature difference which depends upon e sin ω S , where e is the orbital eccentricity and ω S is the Sun’s argument of perigee measured in an Earth-centered frame. The hemisphere containing perihelion is the cooler. For a gray body with the Earth’s average albedo of 0.3, an emissivity of unity, and an obliquity of 23.5°, the pole-to-pole temperature difference for the combined first and third degree spherical harmonic psychroterms can reach 3.4 K for the present eccentricity of 0.016, and 12.9 K for the maximum eccentricity of 0.06. While a thermally inertia-less black or gray body with boiling hot subsolar points and nights at absolute zero are poor models for the Earth, the Seversmith psychroterms will survive in more realistic models (although with smaller amplitudes) because the Earth radiates nonlinearly in T. The psychroterms acts in the direction opposite to the Milankovitch precession index, which also depends on e sin ω S : by warming the cool northern summers, the psychroterms make it harder for the traditional Milankovitch mechanism to operate. The Seversmith psychroterms could in fact be responsible for the ice sheets that cycle with e sin ω S , instead of the Milankovitch mechanism. By cooling the southern hemisphere for thousands of years when perihelion is in the south, the psychroterms may somehow cause the southern hemisphere to control the northern ice sheets associated with the 23 kyr and 19 kyr periods (kyr = 103 years), possibly through ice-albedo feedback in the sea ice surrounding Antarctica. Two other unexpected features besides the psychroterms are: while the average insolation increases with increasing e, the average temperature of the Earth paradoxically decreases; and the equator-to-pole temperature difference of 21 K on a gray body with an albedo equal to 0.3 and an emissivity of unity is actually smaller than the observed difference of 28 K on the real Earth.