Thomas R. Worsley

Causes and consequences of extreme Permo-Triassic warming to globally equable climate and relation to the Permo-Triassic extinction and recovery

Permian waning of the low-latitude Alleghenian/Variscan/Hercynian orogenesis led to a long collisional orogeny gap that cut down the availability of chemically weatherable fresh silicate rock resulting in a high-CO2 atmosphere and global warming. The correspondingly reduced delivery of nutrients to the biosphere caused further increases in CO2 and warming. Melting of polar ice curtailed sinking of O2- and nutrient-rich cold brines while pole-to-equator thermal gradients weakened. Wind shear and associated wind-driven upwelling lessened, further diminishing productivity and carbon burial. As the Earth warmed, dry climates expanded to mid-latitudes, causing latitudinal expansion of the Ferrel circulation cell at the expense of the polar cell. Increased coastal evaporation generated O2- and nutrient-deficient warm saline bottom water (WSBW) and delivered it to a weakly circulating deep ocean. Warm, deep currents delivered ever more heat to high latitudes until polar sinking of cold water was replaced by upwelling WSBW. With the loss of polar sinking, the ocean was rapidly filled with WSBW that became increasingly anoxic and finally euxinic by the end of the Permian. Rapid incursion of WSBW could have produced ∼20 m of thermal expansion of the oceans, generating the well-documented marine transgression that flooded embayments in dry, hot Pangaean mid-latitudes. The flooding further increased WSBW production and anoxia, and brought that anoxic water onto the shelves. Release of CO2 from the Siberian traps and methane from clathrates below the warming ocean bottom sharply enhanced the already strong greenhouse. Increasingly frequent and powerful cyclonic storms mined upwelling high-latitude heat and released it to the atmosphere. That heat, trapped by overlying clouds of its own making, suggests complete breakdown of the dry polar cell. Resulting rapid and intense polar warming caused or contributed to extinction of the remaining latest Permian coal forests that could not migrate any farther poleward because of light limitations. Loss of water stored by the forests led to aquifer drainage, adding another ∼5 m to the transgression. Non-peat-forming vegetation survived at the newly moist poles. Climate feedback from the coal-forest extinction further intensified warmth, contributing to delayed biotic recovery that generally did not begin until mid-Triassic, but appears to have resumed first at high latitudes late in the Early Triassic. Current quantitative models fail to generate high-latitude warmth and so do not produce the chain of events we outline in this paper. Future quantitative modeling addressing factors such as polar cloudiness, increased poleward heat transport by deep water and its upwelling by cyclonic storms, and sustainable mid-latitude sinking of warm brines to promote anoxia, warming, and thermal expansion of deep water may more closely simulate conditions indicated by geological and paleontological data.

Global tectonics and eustasy for the past 2 billion years

Abstract Continental freeboard and eustasy, as gauged by the relative position of the world shelf break with respect to sea level, have varied by ± 250 m from todays ice-free shelf break depth of ∼ 200 m, during the past 600 Ma. Assuming constant or uniformly accreting continental crust and ocean water volume in an ice-free world, sea level fluctuations can be attributed to variation in the world ocean basin volume caused by changes in either its area or its depth relative to the world shelf break. An increase in volume and lowering of sea level occur as: (1) the world ocean floor ages, cools and subsides; (2) accreting continents collide, thicken and decrease in area; and (3) poorly conductive continental platforms become thermally elevated due to a size-induced stasis over the mantle. Conversely, a decrease in the age of the world ocean floor, attenuation of continental crust during rifting, and an increase in continent number and mobility, will reduce the world ocean basin volume and raise sea level. Theoretical sea level calculated from these principles correlates well with calibrated, first-order cycles of eustatic sea level change for the Phanerozoic. The record closely fits a simple model of retardation and acceleration of terrestrial heat loss during alternating periods of supercontinent accretion and fragmentation. Calibrated to sea-level highstands, successive first-order marine transgressions and orogenic “Pangea” regressions characterize a self-sustaining, ∼ 440 Ma plate tectonic cycle for the late Precambrian and Phanerozoic. The cycle can be recognized as far back as 2 Ga from the tectonic evidence of continental collision and rifting recorded in global orogenic peaks and mafic dike swarms, and may be related to major episodes of glaciation and evolutional biogenesis.

Late Miocene marine carbon-isotopic shift and synchroneity of some phytoplanktonic biostratigraphic events

A search for stable-isotopic signals and biostratigraphic events in Deep Sea Drilling Project (DSDP) cores to improve chronologic resolution with an aim to reconstruct the paleoenvironment of the preglacial and postglacial Miocene oceans has led to the recognition of an apparently global decrease in the benthic foraminiferal δ 13 C in the latest Miocene. This carbon-isotopic shift is consistently bracketed by the first evolutionary appearances of several taxa of phytoplankton the ages of which have been accurately estimated from paleomagnetically dated piston cores. The first appearance of nannofossils Amaurolithus primus and A. delicatus at 6.25 m.y. B.P. and the diatoms Thalassiosira praeconvexa and Nitzschia miocenica elongata at 6.10 and 6.00 m.y. B.P., respectively, and the carbon-isotopic shift itself (dated between 6.10 and 5.90 m.y. B.P.) provide convenient synchronous events to aid in the reconstruction of the late Miocene world ocean. Magnetostratigraphically estimated ages of other useful late Miocene nannofossil events include first appearances of Discoaster quinqueramus at 8.00 m.y. B.P., D. surculus at 6.40 m.y. B.P., Amaurolithus tricorniculatus s.s. at 5.70 m.y. B.P., A. amplificus at 5.65 m.y. B.P., and Ceratolithus acutus at 5.20 m.y. B.P., and the last appearances of D. quinqueramus at 5.45 m.y. B.P. and A. amplificus at 5.30 m.y. B.P.

Post-Archean biogeochemical cycles and long-term episodicity in tectonic processes

Long-term interactions between increasing and decreasing secular trends in Earth evolution have produced a geologic record of steady-state equilibrium that is punctuated by nonrecurring events and variations in Earth9s endogenic heat loss decipherable as ∼0.5-b.y. cycles in orogeny, rift magmatism, and platform sedimentation. A detailed record for the past 0.6 b.y. has permitted construction and testing of a heat-driven model of plate-tectonic episodicity that reproduces major tectonic, biogeochemical, and isotopic trends for the Phanerozoic. Application to the Proterozoic record reveals clues on the relationship of tectonic processes to the life-mediated history of Earth9s atmosphere, hydrosphere, and crust.

First-order coupling of paleogeography and CO2, with global surface temperature and its latitudinal contrast

The authors propose that for any geography, halving the amount of emergent land area will elevate CO{sub 2} levels enough to raise land surface temperature 10C and vice versa. They have evaluated this relation by specifying latitude and level of emergence for six end-member continental configurations. They show that a world with polar continents (capworld) will be warmest, whereas a world dominated by tropical ones (ringworld) will be coldest - a result superficially counterintuitive to established climate dogma. A meridional configuration (sliceworld) will have intermediate temperatures. The model is consistent with modern, Pleistocene maximum-emergence and mid-Cretaceous minimum-emergence climates. It also predicts a cool global climate for the half-emergent mid-Cambrian ringworld and a very warm, equable climate for the half-emergent mid-Silurian capworld. Furthermore, the relations among latitude, land area, temperature, and CO{sub 2} levels predict that a Late Proterozoic, equator-straddling land mass could have been glaciated. A strong point of the model is that it yields realistic results with no knowledge of paleolongitude, sea-floor-generation rates, or orogeny (or, by implication, degassing and erosion rates), non of which is obtainable for pre-Mesozoic paleogeographies.

Sea-Level Fluctuations and Deep-Sea Sedimentation Rates

Sediment accumulation rate curves from 95 drilled cores from the Pacific basin and sea-level curves derived from continental margin seismic stratigraphy show that high biogenous sediment accumulation rates correspond to low eustatic sea levels for at least the last 48 million years. This relationship fits a simple model of high sea levels producing lower land/sea ratios and hence slower chemical erosion of the continents, and vice versa.

Episodic Ice-Free Arctic Ocean in Pliocene and Pleistocene Time: Calcareous Nannofossil Evidence

Todays ice cover (2 to 4 meters thick) over the Arctic Ocean provides a shadow that prevents coccolithophorids (photosynthetic, planktonic algae) from living there. Sparse, low-diversity, but indigenous coccolith assemblages in late Pliocene to mid-Pleistocene (but not Holocene) sediments imply deep penetrating warm currents or an ice-free Arctic Ocean, or both, as those layers were being deposited.

A human-induced hothouse climate?

Hothouse climate has been approached or achieved more than a dozen times in Phanerozoic history. Geologically rapid onset of hothouses in 10–10 yr occurs as HEATT (haline euxinic acidic thermal transgression) episodes, which generally persist for less than 1 million years. Greenhouse climate preconditions conducive to hothouse development allowed large igneous provinces (LIPs), combined with positive feedback amplifiers, to force the Earth to the hothouse climate state. The two most significant Cenozoic LIPs (Columbia River Basalts and much larger Early Oligocene Ethiopian Highlands) failed to trigger a hothouse climate from icehouse preconditions, suggesting that such preconditions can limit the impact of CO 2 emissions at the levels and rates of those LIPs. Human burning of fossil fuels can release as much CO 2 in centuries as do LIPs over 10–10 yr or longer. Although burning fossil fuels to exhaustion over the next several centuries may not suffice to trigger hothouse conditions, such combustion will probably stimulate enough polar ice melting to tip Earth into a greenhouse climate. Long atmospheric CO 2 residence times will maintain that state for tens of thousands of years.

Carbon redox and climate control through earth history: A speculative reconstruction

Abstract Maintenance of the temperatures required for oceans at the Earths surface since the beginning of the geologic record almost 4 Ga ago has been accomplished in the face of the Suns increasing luminosity as a function of its evolution as a main sequence star by a progressive drawdown of the Earths atmospheric “greenhouse” gases (chiefly CO 2 ). Atmospheric CO 2 has decreased roughly a hundred fold in the past 3 Ga with a halving time of about 0.4 Ga. As a by-product of a portion of this drawdown, O 2 has increased about a thousand fold in the same interval with a doubling time of about 0.25 Ga. O 2 reached saturation at todays level some 0.25 Ga ago. This drawdown of greenhouse gases has been implemented in large part (about 80% today) by the carbonate-silicate cycle in which increases in surface temperatures are buffered by increased rates of acid rainout of atmospheric CO 2 and its reaction with crustal silicates to form carbonate (C carb ). Decreases in surface temperatures curtail this process and allow buildup of atmospheric CO 2 from volcanic sources. Photosynthesis and the burial of organic carbon (C org ) accounts for some 20% of todays CO 2 drawdown. For the early Earth, the negative feedback mechanism of the carbonate-silicate buffer alone is likely to have lowered surface temperatures despite a brightening Sun as CO 2 degassing rates declined and land areas grew. However, evidence of an icecap at c. 2.7 Ga and the increasing frequency of continental glaciation thereafter suggest that post-Archean Earth has remained cool despite stabilization of CO 2 degassing rates and continental growth: an unlikely consequence of purely negative feedback. Maintenance of constant and perhaps even slightly declining surface temperatures since the Archean may therefore involve the redox cycle of crustal carbon and apparently mandates a continuous, organism-mediated increase in the burial ratio of reduced carbon (C org ) to oxidized carbon (C carb ) with time via more efficient use of phosphorus (P org ) in C org burial. This path would require the δ 13 C value of C org burial to decrease with age if the bulk Earth δ 13 C value is to be preserved. Comparison with the δ 13 C record of C org is consistent with this conclusion. Furthermore, the record suggests that the Earths climatic evolution as preserved on the continents has been tectonically punctuated by alternating intervals of “greenhouse” drowning and “icehouse” emergence; respectively marked by episodes of C org isotope clustering and scatter. Icehouse intervals, which appear to coincide with either supercontinents or periods of maximum continent dispersal, typically correspond to episodes of biotic innovation, phosphate deposition and probable step-function increases in C org : P org burial ratios that have kept the Earths surface close to the freezing point despite increasing solar luminosity.

Cenozoic Sedimentation in the Pacific Ocean: Steps Toward a Quantitative Evaluation

ABSTRACT Computerized procedures have been developed for calculating ocean sediment accumulation rates in terms of mass per unit area. Using these procedures and a simplified plate tectonic model, palinspastic maps of sedimentation in the Pacific Ocean over the past 48 million years (m.y.) have been prepared. These maps show a relatively simple and persistent sedimentation pattern with maximum accumulation occurring around the perimeter of the ocean and along the equator. Sediment seems to be more uniformly distributed at times of low overall accumulation rates. The maps further show that the history of sedimentation in the Pacific over the past 48 m.y. can be divided into five broad time periods: 0-6 m.y., 6-15 m.y., 15-27 m.y., 27-42 m.y., and prior to 42 m.y. Detailed differences between the sediment accumulation patterns of these time periods are related to major reorganizations of Pacific Ocean circulation and climate.