Cooler estimates of Cretaceous temperatures

Sellwood, B. W.; Price, Gregory D.; Valdest, Paul J.

doi:10.1038/370453a0

Cited by 133 publications

(70 citation statements)

References 25 publications

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“…These results confirm that, even though codes such as GADGET-2 are frequently called collisionless, they can still present significant collisional relaxation in the long-term evolution. Furthermore, this collisional relaxation is similar to that obtained with other techniques and equivalent softening lengths, as already demonstrated by Hernquist & Barnes (1990);Sellwood (2015).…”

Section: Substituting In Eq (28) We Obtainsupporting

confidence: 69%

The Arrow of Time in the Collapse of Collisionless Self-gravitating Systems: Non-validity of the Vlasov–Poisson Equation during Violent Relaxation

Silva

Pedra

Sodré

et al. 2017

ApJ

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The collapse of a collisionless self-gravitating system, with the fast achievement of a quasi-stationary state, is driven by violent relaxation, with a typical particle interacting with the time-changing collective potential. It is traditionally assumed that this evolution is governed by the Vlasov-Poisson equation, in which case entropy must be conserved. We run N-body simulations of isolated self-gravitating systems, using three simulation codes: NBODY-6 (direct summation without softening), NBODY-2 (direct summation with softening) and GADGET-2 (tree code with softening), for different numbers of particles and initial conditions. At each snapshot, we estimate the Shannon entropy of the distribution function with three different techniques: Kernel, Nearest Neighbor and EnBiD. For all simulation codes and estimators, the entropy evolution converges to the same limit as N increases. During violent relaxation, the entropy has a fast increase followed by damping oscillations, indicating that violent relaxation must be described by a kinetic equation other than the Vlasov-Poisson, even for N as large as that of astronomical structures. This indicates that violent relaxation cannot be described by a time-reversible equation, shedding some light on the so-called "fundamental paradox of stellar dynamics". The long-term evolution is well described by the orbit-averaged Fokker-Planck model, with Coulomb logarithm values in the expected range 10 − 12. By means of NBODY-2, we also study the dependence of the 2-body relaxation time-scale on the softening length. The approach presented in the current work can potentially provide a general method for testing any kinetic equation intended to describe the macroscopic evolution of N-body systems.

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Section: Substituting In Eq (28) We Obtainsupporting

confidence: 69%

The Arrow of Time in the Collapse of Collisionless Self-gravitating Systems: Non-validity of the Vlasov–Poisson Equation during Violent Relaxation

Silva

Pedra

Sodré

et al. 2017

ApJ

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“…Larson and Erba, 1999). This model fails to reconcile conflicting clay mineral data obtained from OAE 2 sequences, which suggest increased aridity rather than chemical weathering (Jeans et al,1991;Sellwood et al,1994). The sea-level transgression, leading up to OAE 2, probably played a significant role, potentially decreasing the erosional capacity of rivers due to higher base levels, flood and leaching nutrients on previous dry land and causing the encroachment of waters with low oxygen availability higher onto the shelf.…”

Section: Introductionmentioning

confidence: 68%

Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

Mort

Adatte

Keller

et al. 2008

Cretaceous Research

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a b s t r a c tWith a multi-proxy approach, an attempt was made to constrain productivity and bottom-water redox conditions and their effects on the phosphorus accumulation rate at the Mohammed Plage section on the Tarfaya coast, Morocco, during the Cenomanian-Turonian Anoxic Event (OAE 2). A distinct d 13 C org isotope excursion of þ2.5& occurs close to the top of the section. The unusually abrupt shift of the isotope excursion and disappearance of several planktonic foraminiferal species (e.g. Rotalipora cushmani and Rotalipora greenhornensis) in this level suggests a hiatus of between 40-60 kyrs at the excursion onset. Nevertheless, it was possible to determine both the long-term environmental history as well as the processes that took place immediately prior to and during OAE 2. TOC% values increase gradually from the base of the section to the top (from w2.5% to w10%). This is interpreted as the consequence of a longterm eustatic sea-level rise and subsidence causing the encroachment of less oxic waters into the Tarfaya Basin. Similarly a reduction in the mineralogically constructed 'detrital index' can be explained by the decrease in the continental flux of terrigenous material due to a relative sea-level rise. A speciation of phosphorus in the upper part of the section, which spans the start and mid-stages of OAE 2, shows overall higher abundances of P reactive mass accumulation rates before the isotope excursion onset and lower values during the plateau. Due to the probable short hiatus, the onset of the decrease in phosphorus content relative to the isotope excursion is uncertain, although the excursion plateau already contains lower concentrations. The C org /P total and V/Al ratios suggest that this reduction was mostly likely caused by a decrease in the available bottom oxygen content (probably as a result of higher productivity) and a corresponding fall in the phosphorus retention ability of the sediment. Productivity appears to have remained high during the isotope plateau possibly due to a combination of ocean-surface fertilisation via increased aridity (increased K/Al and Ti/Al ratios) and/or higher dissolved inorganic phosphorus content in the water column as a result of the decrease in sediment P retention. The evidence for decreased P-burial has been observed in many other palaeoenvironments during OAE 2. Tarfaya's unique upwelling paleosituation provides strong evidence that the nutrient recycling was a global phenomenon and therefore a critical factor in starting and sustaining OAE 2.

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“…It should be noted in passing that a recent revision of Cretaceous temperatures by Sellwood et al (1994) obtained 'minimum estimates' somewhat cooler than the Barron's lower limits. Sellwood et al, however, failed to consider the substantial Equator-to-Pole gradient in the background sea water value of oxygen isotope ratio (their proxy for temperature).…”

Section: Review Of Results and A Fundamental Criticismmentioning

confidence: 99%

Paleoclimate data constraints on climate sensitivity: The paleocalibration method

1996

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Abstract. The relationship between paleoclimates and the future climate, while not as simple as implied in the 'paleoanalog' studies of Budyko and others, nevertheless provides sufficient constraints to broadly confirm the climate sensitivity range of theoretical models and perhaps eventually narrow the model-derived uncertainties. We use a new technique called 'paleocalibration' to calculate the ratio of temperature response to forcing on a global mean scale for three key intervals of Earth history. By examining surface conditions reconstructed from geologic data for the Last Glacial Maximum, the middle Cretaceous and the early Eocene, we can estimate the equilibrium climate sensitivity to radiative forcing changes for different extreme climates. We find that the ratios for these three periods, within error bounds, all lie in the range obtained from general circulation models: 2-5 K global warming for doubled atmospheric carbon dioxide. Paleocalibration thus provides a data-based confirmation of theoretically calculated climate sensitivity. However, when compared with paleodata on regional scales, the models show less agreeement with data. For example, our GCM simulation of the early Eocene fails to obtain the temperature contrasts between the Equator and the Poles (and between land and ocean areas) indicated by the data, even though it agrees with the temperature data in the global average. Similar results have been reported by others for the Cretaceous and for the Last Glacial Maximum.

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Cooler estimates of Cretaceous temperatures

Cited by 133 publications

References 25 publications

The Arrow of Time in the Collapse of Collisionless Self-gravitating Systems: Non-validity of the Vlasov–Poisson Equation during Violent Relaxation

The Arrow of Time in the Collapse of Collisionless Self-gravitating Systems: Non-validity of the Vlasov–Poisson Equation during Violent Relaxation

Organic carbon deposition and phosphorus accumulation during Oceanic Anoxic Event 2 in Tarfaya, Morocco

Paleoclimate data constraints on climate sensitivity: The paleocalibration method

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