Publications by authors named "Aradhna Tripati"

16 Publications

  • Page 1 of 1

Factors controlling the oxygen isotopic composition of lacustrine authigenic carbonates in Western China: implications for paleoclimate reconstructions.

Sci Rep 2020 10 1;10(1):16370. Epub 2020 Oct 1.

Department of Earth, Planetary, and Space Sciences, Department of Atmospheric and Oceanic Sciences, Institute of the Environment and Sustainability, Center for Diverse Leadership in Science, University of California, Los Angeles, CA, 90095, USA.

In the carbonate-water system, at equilibrium, the oxygen isotopic composition of carbonate is dependent not only on the temperature but also on the isotopic composition of host water in which the carbonate is formed. In this study, lake surface sediment and water samples were collected from 33 terminal lakes in Western China to evaluate controls on the oxygen isotopic composition of lacustrine authigenic carbonates (δO) and its spatial distribution. Our results show that water oxygen isotopic composition (δO) rather than lake summer water temperature (T), is the main determinant of δO, irrespective of whether oxygen isotope equilibrium is achieved. There are significant linear correlations between δO and elevation, as well as that between δO and latitude for lakes located on the Tibetan Plateau. In Western China, the spatial distribution of δO is consistent with that of δO, and is ultimately controlled by the isotopic composition of local precipitation (δO) that depends on the source of water vapor. Therefore, changes in δO can be predominantly interpreted as variations of δO, which in turn represent changes in δO for paleoclimate reconstructions in this region, and may be relevant to studies of other areas.
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http://dx.doi.org/10.1038/s41598-020-73422-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7529879PMC
October 2020

Massive formation of early diagenetic dolomite in the Ediacaran ocean: Constraints on the "dolomite problem".

Proc Natl Acad Sci U S A 2020 06 8;117(25):14005-14014. Epub 2020 Jun 8.

Institute for Geology, Mineralogy, and Geophysics, Ruhr University Bochum, D-44801 Bochum, Germany.

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO)] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the "dolomite problem." Accordingly, the genesis of these dolostones is usually attributed to burial-hydrothermal dolomitization of primary limestones (CaCO) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (T) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our T record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our T record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δO values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.
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http://dx.doi.org/10.1073/pnas.1916673117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7321997PMC
June 2020

Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ ) measurements: Insights from a long-term dataset.

Rapid Commun Mass Spectrom 2020 Apr;34(8):e8678

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, 430074, China.

Rationale: Carbonate clumped isotope (Δ ) thermometry examines the temperature-dependent excess abundance of the C- O bond in the carbonate lattice. Inconsistent temperature calibrations and standard values have been reported among laboratories, which has led to the use of equilibrated gases and carbonate standards for standardization. Furthermore, different acid fractionation factors and isotopic parameter sets have been proposed for improving inter-laboratory data comparability. However, few long-term datasets have been generated to explore the effects of these factors on the long-term reproducibility of Δ data within a laboratory.

Methods: Four standards (ISTB-1, NBS-19, GBWO4416, and GB04417) were analyzed as unknowns by isotope ratio mass spectrometry from 2015 to 2019. The values of Δ were calibrated using the ETH standards. We investigated the Assonov, Brand, and Gonfiantini isotope parameter sets for carbon and oxygen isotopes, as well as two correction schemes of equilibrated gas and carbonate standardization, using the same sample measurements to determine which procedures enhanced reproducibility. ISTB-1 (calcite) and ZK312-346W (dolomite) were measured to determine the 90°C acid fractionation factor.

Results: The corrected 90°C acid fractionation factors are 0.076 ± 0.008‰ for ISTB-1 and 0.077 ± 0.009‰ for ZK312-346W. The choice of isotope parameter set had no significant influence on final Δ values in this study. However, using the Assonov parameters to calculate Δ values improved the reproducibility of the results. The use of carbonate standards improved reproducibility through time compared with the use of equilibrated gases for standardization.

Conclusions: At 90°C, the acid fractionation factors of calcite and dolomite are statistically indistinguishable. We find an insignificant effect from changing the isotope parameter set, suggesting that the choice of isotope parameter set among laboratories is not a major factor affecting inter-laboratory reproducibility. We find that using carbonate standards improved the reproducibility of results, suggesting that the use of carbonate standards may help to achieve inter-laboratory comparability of results in future studies.
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http://dx.doi.org/10.1002/rcm.8678DOI Listing
April 2020

Analytical effects on clumped isotope thermometry: Comparison of a common sample set analyzed using multiple instruments, types of standards, and standardization windows.

Rapid Commun Mass Spectrom 2020 Apr;34(8):e8666

Department of Earth, Planetary, and Space Sciences, Department of Atmospheric and Oceanic Sciences, Institute for the Environment and Sustainability, University of California, Los Angeles, CA, USA.

Rationale: Carbonate clumped isotope geothermometry is being increasingly used in multiple disciplines in the geosciences. However, potential interlaboratory issues are arising from different standardization procedures that may contribute to the multiple Δ -temperature calibrations reported in the literature. We investigate this issue by comparing a common temperature calibration sample set across three different mass spectrometers, using multiple standardization methods.

Methods: The same temperature calibration sample set was analyzed on three different mass spectrometers. Several standardization methods were utilized, including the use of carbonate versus gas standards, and different types of background correction were applied to the raw data.

Results: All standardization types applied resulted in statistically indistinguishable Δ -temperature slopes, with the exception of standardization calculations that did not correct for background effects. Some instruments and standardizations showed different intercepts relative to each other. The use of carbonate standards improved comparability between different instruments relative to gas standards.

Conclusions: Our results show that background effects are the largest factor potentially affecting Δ results, and there may be an improvement in interlaboratory precision using carbonate standards. Critically, all techniques used for standardizing Δ results converge on a common slope as long as background effects are properly corrected. The use of carbonate standards is recommended as a component of standardization procedures.
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http://dx.doi.org/10.1002/rcm.8666DOI Listing
April 2020

Influence of surface ocean density on planktonic foraminifera calcification.

Sci Rep 2019 01 24;9(1):533. Epub 2019 Jan 24.

Department of Earth Sciences, University of Cambridge, Cambridge, UK.

This study provides evidence that ambient seawater density influences calcification and may account for the observed planktonic foraminifera shell mass increase during glacial times. Volumes of weighed fossil Globigerina bulloides shells were accurately determined using X-ray Computer Tomography and were combined with water density reconstructions from Mg/Ca and δO measurements to estimate the buoyancy force exerted on each shell. After assessment of dissolution effects, the resulting relationship between shell mass and buoyancy suggests that heavier shells would need to be precipitated in glacial climates in order for these organisms to remain at their optimum living depth, and counterbalance the increased buoyant force of a denser, glacial ocean. Furthermore, the reanalysis of bibliographic data allowed the determination of a relationship between G. bulloides shell mass and ocean density, which introduces implications of a negative feedback mechanism for the uptake of atmospheric CO by the oceans.
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http://dx.doi.org/10.1038/s41598-018-36935-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6346091PMC
January 2019

Evidence for ephemeral middle Eocene to early Oligocene Greenland glacial ice and pan-Arctic sea ice.

Nat Commun 2018 03 12;9(1):1038. Epub 2018 Mar 12.

Department of Ocean, Earth, and Atmospheric Sciences, Old Dominion University, Norfolk, VA, 23529, USA.

Earth's modern climate is defined by the presence of ice at both poles, but that ice is now disappearing. Therefore understanding the origin and causes of polar ice stability is more critical than ever. Here we provide novel geochemical data that constrain past dynamics of glacial ice on Greenland and Arctic sea ice. Based on accurate source determinations of individual ice-rafted Fe-oxide grains, we find evidence for episodic glaciation of distinct source regions on Greenland as far-ranging as ~68°N and ~80°N synchronous with ice-rafting from circum-Arctic sources, beginning in the middle Eocene. Glacial intervals broadly coincide with reduced CO, with a potential threshold for glacial ice stability near ~500 p.p.m.v. The middle Eocene represents the Cenozoic onset of a dynamic cryosphere, with ice in both hemispheres during transient glacials and substantial regional climate heterogeneity. A more stable cryosphere developed at the Eocene-Oligocene transition, and is now threatened by anthropogenic emissions.
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http://dx.doi.org/10.1038/s41467-018-03180-5DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5847593PMC
March 2018

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene.

Proc Natl Acad Sci U S A 2016 Mar 22;113(13):3453-8. Epub 2016 Feb 22.

Institute of Marine Sciences, National Research Council, 40129 Bologna, Italy.

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.
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http://dx.doi.org/10.1073/pnas.1516030113DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4822588PMC
March 2016

Isotopic ordering in eggshells reflects body temperatures and suggests differing thermophysiology in two Cretaceous dinosaurs.

Nat Commun 2015 Oct 13;6:8296. Epub 2015 Oct 13.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.

Our understanding of the evolutionary transitions leading to the modern endothermic state of birds and mammals is incomplete, partly because tools available to study the thermophysiology of extinct vertebrates are limited. Here we show that clumped isotope analysis of eggshells can be used to determine body temperatures of females during periods of ovulation. Late Cretaceous titanosaurid eggshells yield temperatures similar to large modern endotherms. In contrast, oviraptorid eggshells yield temperatures lower than most modern endotherms but ∼ 6 °C higher than co-occurring abiogenic carbonates, implying that this taxon did not have thermoregulation comparable to modern birds, but was able to elevate its body temperature above environmental temperatures. Therefore, we observe no strong evidence for end-member ectothermy or endothermy in the species examined. Body temperatures for these two species indicate that variable thermoregulation likely existed among the non-avian dinosaurs and that not all dinosaurs had body temperatures in the range of that seen in modern birds.
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http://dx.doi.org/10.1038/ncomms9296DOI Listing
October 2015

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle.

Proc Natl Acad Sci U S A 2013 May 13;110(22):8813-8. Epub 2013 May 13.

Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA.

The East Asian monsoon is one of Earth's most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6-7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.
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http://dx.doi.org/10.1073/pnas.1213366110DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3670372PMC
May 2013

Dinosaur body temperatures determined from isotopic (¹³C-¹⁸O) ordering in fossil biominerals.

Science 2011 Jul 23;333(6041):443-5. Epub 2011 Jun 23.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

The nature of the physiology and thermal regulation of the nonavian dinosaurs is the subject of debate. Previously, arguments have been made for both endothermic and ectothermic metabolisms on the basis of differing methodologies. We used clumped isotope thermometry to determine body temperatures from the fossilized teeth of large Jurassic sauropods. Our data indicate body temperatures of 36° to 38°C, which are similar to those of most modern mammals. This temperature range is 4° to 7°C lower than predicted by a model that showed scaling of dinosaur body temperature with mass, which could indicate that sauropods had mechanisms to prevent excessively high body temperatures being reached because of their gigantic size.
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http://dx.doi.org/10.1126/science.1206196DOI Listing
July 2011

The magnitude and duration of Late Ordovician-Early Silurian glaciation.

Science 2011 Feb 27;331(6019):903-6. Epub 2011 Jan 27.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate "clumped" isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician-Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.
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http://dx.doi.org/10.1126/science.1200803DOI Listing
February 2011

Body temperatures of modern and extinct vertebrates from (13)C-(18)O bond abundances in bioapatite.

Proc Natl Acad Sci U S A 2010 Jun 24;107(23):10377-82. Epub 2010 May 24.

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA.

The stable isotope compositions of biologically precipitated apatite in bone, teeth, and scales are widely used to obtain information on the diet, behavior, and physiology of extinct organisms and to reconstruct past climate. Here we report the application of a new type of geochemical measurement to bioapatite, a "clumped-isotope" paleothermometer, based on the thermodynamically driven preference for (13)C and (18)O to bond with each other within carbonate ions in the bioapatite crystal lattice. This effect is dependent on temperature but, unlike conventional stable isotope paleothermometers, is independent from the isotopic composition of water from which the mineral formed. We show that the abundance of (13)C-(18)O bonds in the carbonate component of tooth bioapatite from modern specimens decreases with increasing body temperature of the animal, following a relationship between isotope "clumping" and temperature that is statistically indistinguishable from inorganic calcite. This result is in agreement with a theoretical model of isotopic ordering in carbonate ion groups in apatite and calcite. This thermometer constrains body temperatures of bioapatite-producing organisms with an accuracy of 1-2 degrees C. Analyses of fossilized tooth enamel of both Pleistocene and Miocene age yielded temperatures within error of those derived from similar modern taxa. Clumped-isotope analysis of bioapatite represents a new approach in the study of the thermophysiology of extinct species, allowing the first direct measurement of their body temperatures. It will also open new avenues in the study of paleoclimate, as the measurement of clumped isotopes in phosphorites and fossils has the potential to reconstruct environmental temperatures.
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http://dx.doi.org/10.1073/pnas.0911115107DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2890843PMC
June 2010

Coupling of CO2 and ice sheet stability over major climate transitions of the last 20 million years.

Science 2009 Dec 8;326(5958):1394-7. Epub 2009 Oct 8.

Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles (UCLA), Los Angeles, CA 90095, USA.

The carbon dioxide (CO2) content of the atmosphere has varied cyclically between approximately 180 and approximately 280 parts per million by volume over the past 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth's history, the partial pressure of CO2 (pCO2) is much less certain, and the relation between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the past 20 million years. During the Middle Miocene, when temperatures were approximately 3 degrees to 6 degrees C warmer and sea level was 25 to 40 meters higher than at present, pCO2 appears to have been similar to modern levels. Decreases in pCO(2) were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (approximately 14 to 10 million years ago) and Late Pliocene (approximately 3.3 to 2.4 million years ago).
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http://dx.doi.org/10.1126/science.1178296DOI Listing
December 2009

The heartbeat of the Oligocene climate system.

Science 2006 Dec;314(5807):1894-8

National Oceanography Centre, Southampton, School of Ocean and Earth Science, European Way, Southampton SO14 3ZH, UK.

A 13-million-year continuous record of Oligocene climate from the equatorial Pacific reveals a pronounced "heartbeat" in the global carbon cycle and periodicity of glaciations. This heartbeat consists of 405,000-, 127,000-, and 96,000-year eccentricity cycles and 1.2-million-year obliquity cycles in periodically recurring glacial and carbon cycle events. That climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle, solar forcing, and glacial events. Box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass. The pronounced 405,000-year eccentricity cycle is amplified by the long residence time of carbon in the oceans.
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http://dx.doi.org/10.1126/science.1133822DOI Listing
December 2006

Eocene bipolar glaciation associated with global carbon cycle changes.

Nature 2005 Jul;436(7049):341-6

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.

The transition from the extreme global warmth of the early Eocene 'greenhouse' climate approximately 55 million years ago to the present glaciated state is one of the most prominent changes in Earth's climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica approximately 34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world's oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from approximately 42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to approximately 1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.
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http://dx.doi.org/10.1038/nature03874DOI Listing
July 2005

Deep-sea temperature and circulation changes at the Paleocene-Eocene Thermal Maximum.

Science 2005 Jun;308(5730):1894-8

Department of Earth Sciences, University of Cambridge, Downing Street, CB2 3EQ, UK.

A rapid increase in greenhouse gas levels is thought to have fueled global warming at the Paleocene-Eocene Thermal Maximum (PETM). Foraminiferal magnesium/calcium ratios indicate that bottom waters warmed by 4 degrees to 5 degrees C, similar to tropical and subtropical surface ocean waters, implying no amplification of warming in high-latitude regions of deep-water formation under ice-free conditions. Intermediate waters warmed before the carbon isotope excursion, in association with downwelling in the North Pacific and reduced Southern Ocean convection, supporting changing circulation as the trigger for methane hydrate release. A switch to deep convection in the North Pacific at the PETM onset could have amplified and sustained warming.
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http://dx.doi.org/10.1126/science.1109202DOI Listing
June 2005