Earth's climate underwent a fundamental change between 1250 and 700 thousand years ago, the mid-Pleistocene transition (MPT), when the dominant periodicity of climate cycles changed from 41 thousand ...to 100 thousand years in the absence of substantial change in orbital forcing. Over this time, an increase occurred in the amplitude of change of deep-ocean foraminiferal oxygen isotopic ratios, traditionally interpreted as defining the main rhythm of ice ages although containing large effects of changes in deep-ocean temperature. We have separated the effects of decreasing temperature and increasing global ice volume on oxygen isotope ratios. Our results suggest that the MPT was initiated by an abrupt increase in Antarctic ice volume 900 thousand years ago. We see no evidence of a pattern of gradual cooling, but near-freezing temperatures occur at every glacial maximum.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Coupled relative geomagnetic paleointensity (RPI) and oxygen isotope records are used to construct RPI and oxygen isotope stacks for the last 1.5 Myr. The coupled correlations are accomplished using ...the Match algorithm (Lisiecki, L.E., and Lisiecki, P.A., 2002. Application of dynamic programming to the correlation of paleoclimate records. Paleoceanography, 17: 1049, doi:10.1029/2001PA000733) to simultaneously correlate isotope and RPI records. The simultaneous match reduces the degree of freedom associated with correlations using RPI or oxygen isotope records alone. The overall compatibility of RPI and oxygen isotopes indicates a dominant global (but independent) component in both signals. The local wavelet power spectrum (LWPS) for the RPI stack indicates little significant orbital power, although the accompanying oxygen isotope stack has all the LWPS characteristics expected of a high-resolution oxygen isotope record containing orbital frequencies. The PISO-1500 stack represents a new stratigraphic template that can be used to correlate among marine sediment records and link them to polar ice cores via variations in cosmogenic nuclide production. Scaling the stack to values for virtual axial dipole moment (VADM) indicate maxima at ~
15
×
10
22 Am
2 and minima that imply a threshold of ~
2.5
×
10
22 Am
2 below which values are associated with either polarity reversals or magnetic excursions.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Export of organic carbon from surface waters of the Antarctic Zone of the Southern Ocean decreased during the last ice age, coinciding with declining atmospheric carbon dioxide (CO₂) concentrations, ...signaling reduced exchange of CO₂ between the ocean interior and the atmosphere. In contrast, in the Subantarctic Zone, export production increased into ice ages coinciding with rising dust fluxes, thus suggesting iron fertilization of subantarctic phytoplankton. Here, a new high-resolution productivity record from the Antarctic Zone is compiled with parallel subantarctic data over the past million years. Together, they fit the view that the combination of these two modes of Southern Ocean change determines the temporal structure of the glacial-interglacial atmospheric CO₂ record, including during the interval of "lukewarm" interglacials between 450 and 800 thousand years ago.
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The clumped isotopic composition of carbonate‐derived CO2 (denoted Δ47) is a function of carbonate formation temperature and in natural samples can act as a recorder of paleoclimate, burial, or ...diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R13VPDB, R17VSMOW, R18VSMOW, and λ) impact calculated Δ47 values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed interlaboratory differences in the measured T‐Δ47 relationship. Using the updated parameters, we reprocess 14 published calibration data sets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite data set (n = 1,253 sample replicates), we investigate the possibility for a “universal” clumped isotope calibration. We find that applying updated parameters improves the T‐Δ47 relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all interlaboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect interlaboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ47 (Δ*25‐X), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of interlaboratory disagreement in the T‐Δ47 relationship, but many more remain to be investigated.
Plain Language Summary
Measured stable and clumped isotope values are fundamentally tied to established compositions of international standard materials. When these standard compositions are updated, it impacts previously published isotope measurements such as those used to define the clumped isotope calibration relationship (the foundation for use of this isotopic proxy as a paleothermometer, recorder of burial history or past diagenetic conditions). Here we reprocess 14 published clumped isotope calibration studies using updated international standard compositions and identical data processing procedures to see if these changes would eliminate previously observed interlaboratory discrepancies in clumped isotope calibration relationships. We find that this update tightens the clumped isotope calibration relationship within most laboratories and improves overall agreement between laboratories but does not eliminate all interlaboratory differences. We also propose “best practices” for data processing and dissemination going forward. This study makes progress toward resolving discrepancies in clumped isotope calibration relationships between laboratories by eliminating a number of possible causes and moves the clumped isotope community closer toward our ultimate goal of applying this powerful new proxy routinely to exciting science questions.
Key Points
Updates to 17O correction parameters and international standard compositions affect clumped isotopic compositions
Reprocessing published calibration data using new parameters and consistent methodology slightly improves interlaboratory agreement
There is no evidence (within error) for mineral‐specific offsets in calibration equation or temperature dependence of acid digestion fractionation
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Cooling threshold
The end of each interglacial period over the past 800,000 years was characterized by large, abrupt cooling episodes that were distinct from the gradual decrease in insolation that ...was occurring. Why did these sudden coolings occur? Yin
et al
. used a suite of climate models to show that an insolation threshold exists, beneath which the slow decrease of energy from the sun leads to rapid climate cooling. This effect results from weakening of the Atlantic meridional overturning circulation involving sea ice feedbacks in the Nordic and Labrador Seas. —HJS
The slow decrease in insolation triggers the abrupt climate cooling events at the end of interglacials.
Abrupt cooling is observed at the end of interglacials in many paleoclimate records, but the mechanism responsible remains unclear. Using model simulations, we demonstrate that there exists a threshold in the level of astronomically induced insolation below which abrupt changes at the end of interglacials of the past 800,000 years occur. When decreasing insolation reaches the critical value, it triggers a strong, abrupt weakening of the Atlantic meridional overturning circulation and a cooler mean climate state accompanied by high-amplitude variations lasting for several thousand years. The mechanism involves sea ice feedbacks in the Nordic and Labrador Seas. The ubiquity of this threshold suggests its fundamental role in terminating the warm climate conditions at the end of interglacials.
Considerable ambiguity remains over the extent and nature of millennial/centennial-scale climate instability during the Last Interglacial (LIG). Here we analyse marine and terrestrial proxies from a ...deep-sea sediment sequence on the Portuguese Margin and combine results with an intensively dated Italian speleothem record and climate-model experiments. The strongest expression of climate variability occurred during the transitions into and out of the LIG. Our records also document a series of multi-centennial intra-interglacial arid events in southern Europe, coherent with cold water-mass expansions in the North Atlantic. The spatial and temporal fingerprints of these changes indicate a reorganization of ocean surface circulation, consistent with low-intensity disruptions of the Atlantic meridional overturning circulation (AMOC). The amplitude of this LIG variability is greater than that observed in Holocene records. Episodic Greenland ice melt and runoff as a result of excess warmth may have contributed to AMOC weakening and increased climate instability throughout the LIG.
The influence of ocean circulation changes on atmospheric CO2 hinges primarily on the ability to alter the ocean interior's respired nutrient inventory. Here we investigate the Atlantic overturning ...circulation at the Last Glacial Maximum and its impact on respired carbon storage using radiocarbon and stable carbon isotope data from the Brazil and Iberian Margins. The data demonstrate the existence of a shallow well-ventilated northern-sourced cell overlying a poorly ventilated, predominantly southern-sourced cell at the Last Glacial Maximum. We also find that organic carbon remineralization rates in the deep Atlantic remained broadly similar to modern, but that ventilation ages in the southern-sourced overturning cell were significantly increased. Respired carbon storage in the deep Atlantic was therefore enhanced during the last glacial period, primarily due to an increase in the residence time of carbon in the deep ocean, rather than an increase in biological carbon export.
We have developed a new method for measuring the isotopic composition (δ18O and δD) of different types of bonded water (e.g., molecular water, hydroxyl) contained in hydrated minerals by coupling a ...thermal gravimeter (TG) and a cavity ringdown laser spectrometer (CRDS). The method involves precisely step-heating a mineral sample, allowing the separation of the different types of waters that are released at different temperatures. Simultaneously, the water vapor evolved from the mineral sample is analyzed for oxygen and hydrogen isotopes by CRDS. Isotopic values for the separate peaks are calculated by integrating the product of the water amounts and its isotopic values, after correcting for background. We provide examples of the application of the differential thermal isotope analysis (DTIA) method to a variety of hydrous minerals and mineraloids including gypsum, clays, and amorphous silica (opal). The isotopic compositions of the total water evolved from a set of natural gypsum samples by DTIA are compared with the results of a conventional offline water extraction method followed by CRDS analysis. The results from both methods are in excellent agreement, and precisions (1σ) for δ18O (±0.12‰) and δD (±0.8‰) of the total gypsum hydration water from the DTIA method are comparable to that obtained by the offline method. A range of analytical challenges and solutions (e.g., spectroscopic interferences produced by VOCs in natural samples, isotopic exchange with structural oxygen, etc.) are discussed. The DTIA method has wide ranging applications for addressing fundamental problems across many disciplines in earth and planetary sciences, including paleoclimatology, sedimentology, volcanology, water exchange between the solid earth and hydrosphere, and water on Mars and other planetary bodies.
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IJS, KILJ, NUK, PNG, UL, UM
Pulses of the pastBursts of carbon dioxide, released into the atmosphere and occurring on centennial time scales, were seen during the cold periods of the last glacial cycle but not in older or ...warmer conditions. Nehrbass-Ahles et al. present a record of atmospheric carbon dioxide concentrations retrieved from the European Project for Ice Coring in Antarctica Dome C ice core showing that these carbon dioxide jumps occurred during both cold and warm periods between 330,000 and 450,000 years ago. They relate these pulses to disruptions of the Atlantic meridional overturning circulation caused by freshwater discharge from ice sheets. Such rapid carbon dioxide increases could occur in the future if global warming also disrupts this ocean circulation pattern.Science, this issue p. 1000Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.
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