Aradhna K. Tripati

The Heartbeat of the Oligocene Climate System

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.

Eocene bipolar glaciation associated with global carbon cycle changes

The transition from the extreme global warmth of the early Eocene ‘greenhouse’ climate ∼55 million years ago to the present glaciated state is one of the most prominent changes in Earths climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica ∼34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the worlds 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 ∼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 ∼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.

Coupling of CO2 and Ice Sheet Stability Over Major Climate Transitions of the Last 20 Million Years

CO2 and Miocene Climate Change Atmospheric carbon dioxide is a powerful greenhouse gas believed to be one of the most important determinants of climate. Ice cores provide a detailed and direct record of CO2 concentrations over the past 800,000 years, but not earlier. Tripati et al. (p. 1394, published online 8 October) report B/Ca measurements of planktonic foraminifera, from which they can infer atmospheric CO2 concentrations, for the past 20 million years. The concentration of atmospheric CO2 was similar to preindustrial values for the past 10 millions years, but between 15 and 20 million years ago, during the warm lower Miocene epoch, CO2 was more abundant, and major climate transitions toward cooler conditions occurred when CO2 decreased substantially. Changes in global sea level and atmospheric carbon dioxide levels were similar during the past 20 million years. The carbon dioxide (CO2) content of the atmosphere has varied cyclically between ~180 and ~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 ~3° to 6°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 pCO2 were apparently synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago) and Late Pliocene (~3.3 to 2.4 million years ago).

The Magnitude and Duration of Late Ordovician–Early Silurian Glaciation

Carbonate isotopes reveal a link between past ocean temperatures and mass extinction. 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.

Orbitally induced climate and geochemical variability across the Oligocene/Miocene boundary

To assess the influence of orbital-scale variations on late Oligocene to early Miocene climate and ocean chemistry, high-resolution (∼5 kyr) benthic foraminiferal carbon and oxygen isotope and percent coarse fraction time series were constructed for Ocean Drilling Program site 929 on Ceara Rise in the western equatorial Atlantic. These time series exhibit pervasive low- to high-frequency variability across a 5-Myr interval (20.5–25.4 Ma). The records also reveal several large-scale secular variations including two positive (∼1.6‰) oxygen isotope excursions at 22.95 and 21.1 Ma, suggestive of large but brief glacial maxima (Mi-1 and Mi-1a events of Miller et al. [1991]), and a long-term cyclical increase in the carbon isotopic composition of seawater (shift of ∼1.52‰) that reaches a maximum coincident with peak δ18O values at 22.95 Ma. Lower-resolution (∼25 kyr) records constructed from benthic and planktonic foraminifera as well as bulk carbonate at a shallower site on Ceara Rise (site 926) for the period 21.7–24.9 Ma covary with site 929 δ18O values reflecting changes in Antarctic ice-volume. Likewise, covariance among carbon isotopic records of bulk sediment, benthic, and planktonic foraminifera suggest that the low-frequency cycles (∼400 kyr) and long-term increase in δ13C values represent changes in the mean carbon composition of seawater ΣCO2. The time series presented here constitute the longest, most continuous, and highest-resolution records of pre-Pliocene climate and oceanography to date. The site 929 carbon and oxygen isotope power spectra show significant concentrations of variance at ∼400, 100, and 41 kyr, demonstrating that orbitally induced oscillations have been a normal characteristic of the global climate system since at least the Oligocene, including periods of equable climate and times with no apparent Northern Component Water production.

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

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 13C and 18O 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 13C-18O 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 °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.

Dinosaur Body Temperatures Determined from Isotopic (13C-18O) Ordering in Fossil Biominerals

Large dinosaurs had body temperatures similar to those of modern mammals and birds. 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.

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

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.

Constraints on glaciation in the middle Eocene (46–37 Ma) from Ocean Drilling Program (ODP) Site 1209 in the tropical Pacific Ocean

ocean foraminifera and seawater d 18 O reconstructions are less clear, and there are few high‐resolution d 18 O records. We present a new detailed record of benthic foraminiferal d 18 O from Site 1209 that exhibits variations (Dd 18 Obenthic) of 0.6‰–1.3‰. Different approaches have previously been used to interpret Dd 18 Obenthic, including (1) an a priori assumption of a 50% contribution of temperature, similar to what is reconstructed for the Last Glacial Maximum–recent change; (2) applying Oligocene calibrations between apparent sea level (ASL) and Dd 18 Obenthic; or (3) assuming temperature and seawater d 18 O contributions can be partitioned through comparison with benthic Mg/Ca. Using assumption 1, the record from Site 1209 indicates changes in seawater d 18 O of 0.3‰–0.7‰, equivalent to ∼33–72 m (m) of ASL (assuming mean ice d 18 Oo f∼−45‰). Using assumption 2 and two different end‐member calibrations, the d 18 Obenthic record implies changes in ASL of 23–50 m or 50–108 m. The third approach yields changes in seawater d 18 O of up to 0.6‰ to 1.4‰. We explore the compatibility of the results of each of these approaches with other studies that discuss evidence for ephemeral glaciations during the middle Eocene with variable ice storage at one or both poles.

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

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.