Ocean temperatures through the Phanerozoic reassessed

Back to an ice-free future: Early Cretaceous seasonal cycles of sea surface temperature and glacier ice

Global ice losses will likely continue with ongoing climate warming, culminating in an almost ice-free planet analogous to that which persisted throughout much of the Cretaceous. Despite extensive research, Early Cretaceous cryosphere responses to temperature and atmospheric PCO2 fluctuations over short, human, timescales remain uncertain. Here, we show rapid late Valanginian (~133 million years ago) seasonal fluctuations in sea surface temperature (SST) and δ18O mainly driven by atmospheric PCO2. Two distinctive features emerge: large seasonal variability of up to 15.9° ± 4.9°C in Southern Hemisphere mid-latitudes, comparable to that found today, a positive sea surface δ18O value related to evaporation (expressed as salinity increases), and the existence of polar ice. Model-predicted patterns of SST change match with high statistical confidence those derived from clumped isotopes in well-preserved oyster fossils from Madagascar and display consistent warm/cold seasonality. Given its relative coolness in a Cretaceous context, the late Valanginian is a valuable analog for Earth's future climate.

Phanerozoic icehouse climates as the result of multiple solid-Earth cooling mechanisms

The Phanerozoic climate has been interrupted by two long "icehouse" intervals, including the current icehouse of the last ~34 million years. While these cool intervals correspond to lower atmospheric CO2, it is unclear why CO2 levels fell, with hypotheses suggesting changes in CO2 degassing rates or modification of silicate weathering through changing continental lithology or paleogeography. Here, we construct an Earth System Model that integrates these proposed cooling mechanisms in detail. The model can reproduce the broad geologic record of ice cap expansion, allowing us to infer the primary drivers of long-term climate change. Our results indicate that recent icehouse climates required a combination of different cooling mechanisms acting simultaneously and were not driven by a single known process, potentially explaining why icehouses have been rarer than greenhouses over Earth history.

Mitochondria: great balls of fire

Recent experimental studies indicate that mitochondria in mammalian cells are maintained at temperatures of at least 50 °C. While acknowledging the limitations of current experimental methods and their interpretation, we here consider the ramifications of this finding for cellular functions and for evolution. We consider whether mitochondria as heat-producing organelles had a role in the origin of eukaryotes and in the emergence of homeotherms. The homeostatic responses of mitochondrial temperature to externally applied heat imply the existence of a molecular heat-sensing system in mitochondria. While current findings indicate high temperatures for the innermost compartments of mitochondria, those of the mitochondrial surface and of the immediately surrounding cytosol remain to be determined. We ask whether some aspects of mitochondrial dynamics and motility could reflect changes in the supply and demand for mitochondrial heat, and whether mitochondrial heat production could be a factor in diseases and immunity.

A 485-million-year history of Earth's surface temperature

A long-term record of global mean surface temperature (GMST) provides critical insight into the dynamical limits of Earth's climate and the complex feedbacks between temperature and the broader Earth system. Here, we present PhanDA, a reconstruction of GMST over the past 485 million years, generated by statistically integrating proxy data with climate model simulations. PhanDA exhibits a large range of GMST, spanning 11° to 36°C. Partitioning the reconstruction into climate states indicates that more time was spent in warmer rather than colder climates and reveals consistent latitudinal temperature gradients within each state. There is a strong correlation between atmospheric carbon dioxide (CO2) concentrations and GMST, identifying CO2 as the dominant control on variations in Phanerozoic global climate and suggesting an apparent Earth system sensitivity of ~8°C.

Hot and cold Earth through time

Reconstructing ancient Earth's temperature reveals a global climate regulation system.

Stochastic assessment of temperature-CO2 causal relationship in climate from the Phanerozoic through modern times

As a result of recent research, a new stochastic methodology of assessing causality was developed. Its application to instrumental measurements of temperature (T) and atmospheric carbon dioxide concentration ([CO2]) over the last seven decades provided evidence for a unidirectional, potentially causal link between T as the cause and [CO2] as the effect. Here, I refine and extend this methodology and apply it to both paleoclimatic proxy data and instrumental data of T and [CO2]. Several proxy series, extending over the Phanerozoic or parts of it, gradually improving in accuracy and temporal resolution up to the modern period of accurate records, are compiled, paired, and analyzed. The extensive analyses made converge to the single inference that change in temperature leads, and that in carbon dioxide concentration lags. This conclusion is valid for both proxy and instrumental data in all time scales and time spans. The time scales examined begin from annual and decadal for the modern period (instrumental data) and the last two millennia (proxy data), and reach one million years for the most sparse time series for the Phanerozoic. The type of causality appears to be unidirectional, T→[CO2], as in earlier studies. The time lags found depend on the time span and time scale and are of the same order of magnitude as the latter. These results contradict the conventional wisdom, according to which the temperature rise is caused by [CO2] increase.

Convergent evolution of giant size in eurypterids

Eurypterids-Palaeozoic marine and freshwater arthropods commonly known as sea scorpions-repeatedly evolved to remarkable sizes (over 0.5 m in length) and colonized continental aquatic habitats multiple times. We compiled data on the majority of eurypterid species and explored several previously proposed explanations for the evolution of giant size in the group, including the potential role of habitat, sea surface temperature and dissolved sea surface oxygen levels, using a phylogenetic comparative approach with a new tip-dated tree. There is no compelling evidence that the evolution of giant size was driven by temperature or oxygen levels, nor that it was coupled with the invasion of continental aquatic environments, latitude or local faunal diversity. Eurypterid body size evolution is best characterized by rapid bursts of change that occurred independently of habitat or environmental conditions. Intrinsic factors played a major role in determining the convergent origin of gigantism in eurypterids.

Massive genome reduction predates the divergence of Symbiodiniaceae dinoflagellates

Dinoflagellates in the family Symbiodiniaceae are taxonomically diverse, predominantly symbiotic lineages that are well-known for their association with corals. The ancestor of these taxa is believed to have been free-living. The establishment of symbiosis (i.e. symbiogenesis) is hypothesized to have occurred multiple times during Symbiodiniaceae evolution, but its impact on genome evolution of these taxa is largely unknown. Among Symbiodiniaceae, the genus Effrenium is a free-living lineage that is phylogenetically positioned between two robustly supported groups of genera within which symbiotic taxa have emerged. The apparent lack of symbiogenesis in Effrenium suggests that the ancestral features of Symbiodiniaceae may have been retained in this lineage. Here, we present de novo assembled genomes (1.2-1.9 Gbp in size) and transcriptome data from three isolates of Effrenium voratum and conduct a comparative analysis that includes 16 Symbiodiniaceae taxa and the other dinoflagellates. Surprisingly, we find that genome reduction, which is often associated with a symbiotic lifestyle, predates the origin of Symbiodiniaceae. The free-living lifestyle distinguishes Effrenium from symbiotic Symbiodiniaceae vis-à-vis their longer introns, more-extensive mRNA editing, fewer (~30%) lineage-specific gene sets, and lower (~10%) level of pseudogenization. These results demonstrate how genome reduction and the adaptation to distinct lifestyles intersect to drive diversification and genome evolution of Symbiodiniaceae.

Impact of global climate cooling on Ordovician marine biodiversity

Global cooling has been proposed as a driver of the Great Ordovician Biodiversification Event, the largest radiation of Phanerozoic marine animal Life. Yet, mechanistic understanding of the underlying pathways is lacking and other possible causes are debated. Here we couple a global climate model with a macroecological model to reconstruct global biodiversity patterns during the Ordovician. In our simulations, an inverted latitudinal biodiversity gradient characterizes the late Cambrian and Early Ordovician when climate was much warmer than today. During the Mid-Late Ordovician, climate cooling simultaneously permits the development of a modern latitudinal biodiversity gradient and an increase in global biodiversity. This increase is a consequence of the ecophysiological limitations to marine Life and is robust to uncertainties in both proxy-derived temperature reconstructions and organism physiology. First-order model-data agreement suggests that the most conspicuous rise in biodiversity over Earth's history - the Great Ordovician Biodiversification Event - was primarily driven by global cooling.

Isotopic history of seawater: the stable isotope character of the global ocean at present and in the geological past

After the atmosphere, the ocean is the most well-mixed and homogeneous global geochemical reservoir. Both physical and biological processes generate elemental and isotope variations in seawater. Contrasting geochemical behaviors cause elements to be susceptible to different fractionation mechanisms, with their isotopes providing unique insights into the composition and evolution of the ocean over the course of geological history. Supplementing the traditional stable isotopes (H, C, O, N, S) that provide information about ocean processes and past environmental conditions, radiogenic isotope (Sr, Nd, Os, Pb, U) systems can be used as time markers, indicators of terrestrial weathering, and ocean water mass mixing. Recent instrumentation advances have made possible the measurement of natural stable isotope variations produced by both mass-dependent and mass-independent fractionation for an ever-increasing number of metal elements (e.g. Li, B, Mg, Si, Ca, V, Cr, Fe, Ni, Cu, Zn, Se, Mo, Cd, Tl, U). The major emphasis in this review is on the isotopic composition of the light elements based on a comparatively large literature. Unlike O, H and S, the stable isotopes of C, N and Si do not have a constant isotopic composition in the modern ocean. The major cations Ca, Mg, and Sr fixed in carbonate shells provide the best proxies for reconstruction of the composition of the ocean in the past. Exhibiting large isotope enrichments in ocean water, B and Li are suitable for the investigation of water/rock interactions and can act as monitors of former oceanic pH. The bioessential elements Zn, Cd, and Ni are indicators of paleoproductivity in the ocean. Characteristic isotope enrichments or depletions of the multivalent elements V, Cr, Fe, Se, Mo, and U record the past redox state of the ocean/atmosphere system. Case studies describe how isotopes have been used to define the seawater composition in the geological past.