For at least a billion years of the distant past, planet Earth should have been frozen over but wasn't. Scientists thought they knew why, but a new modeling study from the Alternative Earths team of the NASA Astrobiology Institute has fired the lead actor in that long-accepted scenario.
Humans worry about greenhouse gases, but between 1.8 billion and 800 million years ago, microscopic ocean dwellers really needed them. The sun was 10 to 15 percent dimmer than it is today--too weak to warm the planet on its own. Earth required a potent mix of heat-trapping gases to keep the oceans liquid and livable.
For decades, atmospheric scientists cast methane in the leading role. The thinking was that methane, with 34 times the heat-trapping capacity of carbon dioxide, could have reigned supreme for most of the first 3.5 billion years of Earth history, when oxygen was absent initially and little more than a whiff later on. (Nowadays oxygen is one-fifth of the air we breathe, and it destroys methane in a matter of years.)
"A proper accounting of biogeochemical cycles in the oceans reveals that methane has a much more powerful foe than oxygen," said Stephanie Olson, a graduate student at the University of California, Riverside, a member of the Alternative Earths team and lead author of the new study published September 26 in the Proceedings of the National Academy of Sciences. "You can't get significant methane out of the ocean once there is sulfate."
Sulfate wasn't a factor until oxygen appeared in the atmosphere and triggered oxidative weathering of rocks on land. The breakdown of minerals such as pyrite produces sulfate, which then flows down rivers to the oceans. Less oxygen means less sulfate, but even 1 percent of the modern abundance is sufficient to kill methane, Olson said.
Glacial dropstones in the western Tethys during the late Aptian–early Albian cold snap: Palaeoclimate and palaeogeographic implications for the mid-Cretaceous
Authors:
Rodríguez-López et al
Abstract:
The late Jurassic–early Cretaceous is commonly considered the only cold climatic interval in Earth history without any direct evidence of polar ice. A newly discovered dropstone-bearing interval from the subtropical Iberian Basin (western Tethys) is described and provides evidence of contemporaneous polar glaciation. This interval is correlated laterally for 4.8 km and contains a boulder and two cobble-sized quartzite dropstones that are encased in mid-Cretaceous fissile black shales and fine-grained sandstones. Based on previously published dimensions of similar large clasts, only glacial dropstones and impact ejecta blocks reach the dimensions of the boulder-sized dropstone reported from Iberia. The dropstones show morphological features compatible with glacial transport and abrasion in a subglacial setting which closely resembles the features observed in recent glacial boulders exposed near the snouts of glaciers in Iceland. These Late Aptian dropstones from Spain correlate with many other similar erratics in the northern and southern palaeohemispheres, and suggest that ice sheets formed around the palaeo-North Pole during certain periods of the early Cretaceous. Our results and associated evidence such as the occurrence of glendonites, tillites, moderate- to high-amplitude sea-level oscillations worldwide, minimum pCO2 concentrations, variation in calcareous nannofossil assemblages from low and high latitudes and isotopic excursions suggest that during the mid-Cretaceous there were periods of ice growth and decay that influenced the palaeotemperature, palaeoecology and sedimentology of the marine realm. The new data from Iberia are supported by recent results from Arctic Canada that indicate cool shelves and a mid-Cretaceous cold snap that developed for ~ 6 Myr between 118 and 112 Ma. The late Aptian dropstones reported in eastern Iberia were likely transported from high northern latitudes towards subtropical ones in the western Tethys by an extreme iceberg drift similar to those occurring at the present day in the Atlantic Ocean. Icebergs released from a northern fringing ice sheet may have travelled southwards through the Greenland–Norwegian Seaway.
Mg/Ca in fossil oyster shells as palaeotemperature proxy, an example from the Palaeogene of Central Asia
Authors:
Bougeis et al
Abstract:
Fossil oyster shells are well-suited to provide palaeotemperature proxies from geologic to seasonal timescales due to their ubiquitous occurrence from Triassic to Quaternary sediments, the seasonal nature of their shell growth and their relative strong resistance to post-mortem alteration. However, the common use to translate calcitic oxygen isotopes into palaeotemperatures is challenged by uncertainties in accounting for past seawater δ18O, especially in shallow coastal environment where oysters calcify. In principle, the Mg/Ca ratio in oyster shells can provide an alternative palaeothermometer. Several studies provided temperature calibrations for this potential proxy based on modern species, nevertheless their application to palaeo-studies remains hitherto unexplored. Here, we show that past temperature variability in seawater can be obtained from Mg/Ca analyses from selected fossil oyster species and specimens. High-resolution Mg/Ca profiles, combined with δ18O, were obtained along 41 fossil oyster shells of seven different species from the Palaeogene Proto-Paratethys sea (Central Asia) found in similar as well as different depositional age and environments providing comparison. Suitable Mg/Ca profiles, defined by continuous cyclicity and reproducibility within one shell, are found to be consistent for specimens of the same species but differ systematically between species, implying a dominant species-specific effect on the Mg/Ca signal. Two species studied here (Ostrea (Turkostrea) strictiplicata and Sokolowia buhsii) provide an excellent proxy for palaeoclimate reconstruction from China to Europe in Palaeogene marine sediments. More generally, the protocol developed here can be applied to identify other fossil oyster species suitable for palaeoclimate reconstructions.
Crayfish burrows from the latest Cretaceous lower Cantwell Formation (Denali National Park, Alaska): Their morphology and paleoclimatic significance
Authors:
Fiorillo et al
Abstract:
Latest Cretaceous strata of the lower Cantwell Formation, Denali National Park, central Alaska Range, contain an abundance of megafloral remains and invertebrate and vertebrate trace fossils. Though dominated by herbivorous dinosaur footprints, the abundance and diversity of fossil bird tracks are unique. We present newly discovered crayfish burrows from several areas along a 50 km transect with Denali National Park. Most crayfish burrows from the lower Cantwell Formation are preserved only in cross-section and range from approximately 5–10 cm in diameter. Where preserved in full relief and terminations present, burrow depth is generally less than 50 cm. Burrow morphology is similar to burrow morphology of modern freshwater crayfish (Cambaridae).
The Cantwell Formation fills the Cantwell Basin, a 135 km-long and up to 35 km-wide, east–west trending basin, bracketed by the Hines Creek Fault to the north and the McKinley Fault to the south. Basin fill comprises up to 4000 m of continental deposits, interpreted as braided rivers, alluvial fans, floodplains, swamps, and ponds.
Crayfish burrows provide evidence of water table level, soil moisture fluctuations, as well as insight into mean annual temperatures at the time of deposition of the lower Cantwell Formation, a Late Cretaceous high-latitude paleoecosystem. Despite a relatively high latitudinal setting (~ 71°N paleolatitude), the Late Cretaceous (i.e., Campanian–Maastrichtian) mean annual temperature, based on the distribution of similar present-day crayfish burrows, was more like that of southernmost Ontario, Canada, where the northernmost burrowing crayfish are found today. The burrow depth suggests (1) no permafrost was present, and (2) the phreatic zone was ~ 30–50 cm below the paleo-ground surface. Based on the presence of these crayfish burrows, the paleoclimate is interpreted as humid continental (Köppen scheme), with average summer high temperatures between 25 °C and 28 °C and average winter low temperatures between − 6 °C and 0 °C. These estimates compare somewhat favorably with previous CLAMP estimates of a warm monthly mean temperature of 17.08 +/− 1.6 °C and a cold monthly mean temperature of − 2.31 +/− 1.9 °C.
Reconstruction of Late Cretaceous coastal paleotemperature from halite deposits of the Late Cretaceous Nongbok Formation (Khorat Plateau, Laos)
Authors:
Zhang et al
Abstract:
Cretaceous evaporites of the Maha Sarakhan Formation in Thailand (e.g., the Nongbok Formation, Laos) have been studied for almost a century as the huge potash deposits in the world. The consistently high local paleotemperatures should lead to huge salt deposits during the evaporation process. Primary fluid inclusions in halite can provide surface brine water temperatures directly and quantitatively. Until now, there have been no data published from paleotemperature of primary fluid inclusions of Cretaceous halite. The non-marine halite from the Cretaceous Nongbok Formation (Laos) precipitated from shallow brine waters with temperatures of 17.7–42.3 °C.
Quantitative temperature records of mid Cretaceous hothouse: Evidence from halite fluid inclusions
Authors:
Zhang et al
Abstract:
Quantitative temperature reconstructions for the mid Cretaceous are still rare although its typical greenhouse climate is regarded as one of the best analogues for future global warming. For the first time, mid Cretaceous temperatures were quantitatively reconstructed based on homogenization temperatures (Th) of halite fluid inclusions from the Lower Member of the Nong Bok Formation in the Thakhek mining area, Laos. Petrological features and primary textures of halite indicate a shallow water setting during halite formation that allows the use of homogenization temperatures as air temperature proxy. Obtained homogenization temperatures mainly range from 30 to 50 °C, probably representing local air temperature conditions of Laos during the mid Cretaceous. The determined temperatures are relatively similar to temperatures in many modern and ancient evaporative settings. The maximum homogenization temperature (ThMax) for the Nong Bok Formation of 62.1 °C is significantly higher than existing mid Cretaceous paleotemperature records from both continental and marine sediments and comparable with the temperature records from the contemporaneous Mengyejing Formation of Yunnan, Southern China. The correspondence of the temperature records from Laos and Yunnan implies that very high temperature conditions prevailed in the region. Our results and other reconstructed very high temperatures from potash deposits (e.g., 59 °C for the Silurian Michigan Basin and 58 °C for the Quaternary Lop Nur region) suggest that extremely high temperatures probably had an important impact on the formation of the large potash deposits of the world. However, more studies are needed to improve our understanding of the detailed and quantitative relationships between temperature conditions and potash formation.
The Paleocene-Eocene Thermal Maximum: How much carbon is enough?
Authors:
Meissner et al
Abstract:
The Paleocene-Eocene Thermal Maximum (PETM), ~55.53 million years before present, was an abrupt warming event that involved profound changes in the carbon cycle and led to major perturbations of marine and terrestrial ecosystems. The PETM was triggered by the release of a massive amount of carbon, and thus the event provides an analogue for future climate and environmental changes given current anthropogenic CO2 emissions. Previous attempts to constrain the amount of carbon released have produced widely diverging results, between 2000 and 10000 gigatonnes carbon (GtC). Here we use the UVic Earth System Climate Model in conjunction with a recently published compilation of PETM temperatures [14] to constrain the initial atmospheric CO2 concentration as well as the total mass of carbon released during the event. Thirty-six simulations were initialized with varying ocean alkalinity, river runoff, and ocean sediment cover. Simulating various combinations of pre-PETM CO2 levels (840, 1680, and 2520 ppm) and total carbon releases (3000, 4500, 7000, and 10000 GtC), we find that both the 840 ppm plus 7000 GtC and 1680 ppm plus 7000–10000 GtC scenarios agree best with temperature reconstructions. Bottom waters outside the Arctic and North Atlantic Oceans remain well oxygenated in all of our simulations. While the recovery time and rates are highly dependent on ocean alkalinity and sediment cover, the maximum temperature anomaly, used here to constrain the amount of carbon released, is less dependent on this slow acting feedback.
Warm water benthic foraminifera document the Pennsylvanian–Permian warming and cooling events — The record from the Western Pangea tropical shelves
Author:
Davydov
Abstract:
Shallow warm water benthic foraminifera (SWWBF), including all larger fusulinids (symbiont-bearing benthic foraminifera), are among the best indicators of paleoclimate and paleogeography in the Carboniferous and Permian. The distribution of benthic foraminifera in space and time constrain important tectonic, paleogeographic and climatic events at a global scale. The North American shelves during Pennsylvanian and Permian time – though geographically within the tropical belt – are characterized by temperate environments with significantly lower foraminifera diversification and rare occurrences of warm water Tethyan forms, that are in general appear in the region as a migration entities. Such environments allow documentation of warming episodes associated with sudden immigration of warm water and exotic forms of SWWBF that evolved elsewhere into the area. First occurrence datum (FOD) of the forms exotic to North America during warming episodes are always delayed in respect of their First Appearance Datum (FAD) elsewhere. The time of delay and taxonomic diversity of fusulinids in North America shelves depended on the scale and intensity of the warming episodes. Cooling events, on the other hand, are associated with decreased taxonomic diversity and appearances of endemic forms characteristic only of temperate water provinces. The occurrence of these forms in Boreal and North American provinces appears to be isochronous, as their environments are uniform and induce their uniform and isochronous distribution. Several warming and cooling episodes during Pennsylvanian–Permian time are recognized. The differences between taxonomic variations in each event could potentially be used for provisional estimation of the degree of climatic change. A strong link between biotic and climatic events in North American province and the similarity of biotic changes in the North American and other provinces suggests that paleoclimatic events in North American province were controlled by global factors.
A new study of three ice cores from Greenland documents the warming of the large ice sheet at the end of the last ice age – resolving a long-standing paradox over when that warming occurred.
Large ice sheets covered North America and northern Europe some 20,000 years ago during the coldest part of the ice age, when global average temperatures were about four degrees Celsius (or seven degrees Fahrenheit) colder than during pre-industrial times. And then changes in the Earth's orbit around the sun increased the solar energy reaching Greenland. Beginning some 18,000 years ago, release of carbon from the deep ocean led to a graduate rise in atmospheric carbon dioxide (CO2).
Yet past analysis of ice cores from Greenland did not show any warming response as would be expected from an increase in CO2 and solar energy flux, the researchers note.
In this new study, funded by the National Science Foundation and published this week in the journal Science, scientists reconstructed air temperatures by examining ratios of nitrogen isotopes in air trapped within the ice instead of isotopes in the ice itself, which had been used in past studies.
Not only did the new analysis detect significant warming in response to increasing atmospheric CO2, it documents a warming trend at a rate closely matching what climate change models predict should have happened as the Earth shifted out of its ice age, according to lead author Christo Buizert, a postdoctoral researcher at Oregon State University and lead author on the Science article.
The Holocene temperature conundrum
Authors:
Liu et al
Abstract:
A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198–1201]. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.
Global continental weathering trends across the Early Permian glacial to postglacial transition: Correlating high- and low-paleolatitude sedimentary records
Authors:
Yang et al
Abstract:
Time-equivalent Early Permian sedimentary successions from high-latitude Gondwana basins and from equatorial accumulations in North China, covering the glacial to postglacial transition, display correlatable trends in continental weathering intensity based on chemical index of alteration (CIA) values. The successions display a pattern of CIA values that varies with latitude, similar to modern estuarine suspended sediments. Based on the modern-day CIA-temperature correlation, the glacial Early Permian low- to high-latitude land surface temperature gradient is estimated as ∼20 °C, slightly higher than the calculated values for the contemporaneous sea-surface temperature gradient.
Expression of the late Aptian cold snaps and the OAE1b in a highly subsiding carbonate platform (Aralar, northern Spain)
Authors:
Millan et al
Abstract:
Cretaceous climate records provide evidence that major volcanic pulses with duration of 103 to several 105 yr triggered changes in climate and oceanography. Black shales of Oceanic Anoxic Event 1b (ca. 113–109 Ma) are regarded as signatures of late Aptian greenhouse pulses associated with volcanic episodes. New TEX86 paleotemperature record indicates that warm Aptian climate was interrupted by repeated cold snaps alternating with described greenhouse pulses. An extraordinary thick shallow-water carbonate succession of the Aralar Platform from the southeastern Basque–Cantabrian Basin in Spain provides a unique opportunity to test the hypothesis that cold phases interrupted warm climate in the Aptian. Platform evolution is traced through time of the proposed Aptian cold snaps alternating with greenhouse episodes. New high-resolution C-isotope records established in the studied upper Aptian platform carbonates serve as chemostratigraphic tool providing a link between carbonate-platform evolution, basinal black-shale formation and associated changes in open ocean palaeotemperatures. The late Aptian cold snaps find their expression in a remarkably reduced neritic succession, punctuated by several emersion horizons formed during sea-level lowstands. The studied section bears evidence for several episodes of choked carbonate production related to increased runoff at times of sea-level rise, during which orbitolinids bloomed in a sediment-loaden lagoonal setting. These intervals coincide with the deposition time of organic-rich Kilian and Leenhardt levels belonging to OAE1b.
Reconstruction of spring temperature on the southern edge of the Gobi Desert, Asia, reveals recent climatic warming
Authors:
Chen et al
Abstract:
We have reconstructed the spring (March-June) temperature from 1701 to 2008 for the Changling Mountains, the southern edge of the Gobi Desert, using mean earlywood density data of Pinus tabulaeformis. The reconstruction explains 41.7% of the actual temperature variance during the common period 1959–2008. The temperature reconstruction is representative of temperature conditions over a large area of Central and East Asia, especially in the Gobi Desert. Significant spectral peaks are identified at 2.0-2.8 and 26.9 years. Our temperature reconstruction successfully captured most recent warming and agreed in general with other temperature reconstructions from nearby regions, eastern China and Northern Hemisphere on a decadal timescale. Our reconstructed temperature is significantly correlated with sea surface temperature in the western tropical and northern Pacific Ocean. Preliminary analysis of links between large-scale climatic variation and the temperature reconstruction shows that there is a relationship between extremes in spring temperature and the Asian summer monsoon circulations.
