Modulation of Late Cretaceous and Cenozoic climate by variable drawdown of atmospheric pCO2 from weathering of basaltic provinces on continents drifting through the equatorial humid belt

The small reservoir of carbon dioxide in the atmosphere (pCO2) that modulates climate through the greenhouse effect reflects a delicate balance between large fluxes of sources and sinks. The major long-term source of CO2 is global outgassing from sea-floor spreading, subduction, hotspot activity, and metamorphism; the ultimate sink is through weathering of continental silicates and deposition of carbonates. Most carbon cycle models are driven by changes in the source flux scaled to variable rates of ocean floor production, but ocean floor production may not be distinguishable from being steady since 180 Ma. We evaluate potential changes in sources and sinks of CO2 for the past 120 Ma in a paleogeographic context. Our new calculations show that decarbonation of pelagic sediments by Tethyan subduction contributed only modestly to generally high pCO2 levels from the Late Cretaceous until the early Eocene, and thus shutdown of this CO2 source with the collision of India and Asia at the early Eocene climate optimum at around 50 Ma was inadequate to account for the large and prolonged decrease in pCO2 that eventually allowed the growth of significant Antarctic ice sheets by around 34 Ma. Instead, variation in area of continental basalt terranes in the equatorial humid belt (5° S–5° N) seems to be a dominant factor controlling how much CO2 is retained in the atmosphere via the silicate weathering feedback. The arrival of the highly weatherable Deccan Traps in the equatorial humid belt at around 50 Ma was decisive in initiating the long-term slide to lower atmospheric pCO2, which was pushed further down by the emplacement of the 30 Ma Ethiopian Traps near the equator and the southerly tectonic extrusion of SE Asia, an arc terrane that presently is estimated to account for 1/4 of CO2 consumption from all basaltic provinces that account for ~1/3 of the total CO2 consumption by continental silicate weathering (Dessert et al., 2003). A negative climate-feedback mechanism that (usually) inhibits the complete collapse of atmospheric pCO2 is the accelerating formation of thick cation-deficient soils that retard chemical weathering of the underlying bedrock. Nevertheless, equatorial climate seems to be relatively insensitive to pCO2 greenhouse forcing and thus with availability of some rejuvenating relief as in arc terranes or thick basaltic provinces, silicate weathering in this venue is not subject to a strong negative feedback, providing an avenue for ice ages. The safety valve that prevents excessive atmospheric pCO2 levels is the triggering of silicate weathering of continental areas and basaltic provinces in the temperate humid belt. Excess organic carbon burial seems to have played a negligible role in atmospheric pCO2 over the Late Cretaceous and Cenozoic

Organic Carbon Burial following the Middle Eocene Climatic Optimum (MECO) in the central - western Tethys

We present trace metal geochemistry and stable isotope records for the middle Eocene Alano di Piave section, NE Italy, deposited during magnetochron C18n in the marginal Tethys Ocean. We identify a $\sim$ 500 kyr long carbon isotope perturbation event we infer to be the middle Eocene Climatic Optimum (MECO) confirming the northern hemisphere expression and global occurrence of MECO. Interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals ($\le$3\% TOC) and contemporaneous positive $\delta^{13}$C excursions. These two intervals are associated with increases in the concentration of sulphur and redox-sensitive trace metals, and low concentrations of Mn, as well as coupled with the occurrence of pyrite. Together these changes imply low, possibly dysoxic, bottom water O$_{2}$ conditions promoting increased organic carbon burial. We hypothesize that this rapid burial of organic carbon lowered global {\it p}CO$_{2}$ following the peak warming and returned the climate system to the general Eocene cooling trend

Latitudinal land–sea distributions and global surface albedo since the cretaceous

We estimate global surface albedo from the areal proportion of land to sea in climatically-significant latitudinal belts at ten million-year intervals for the Late Cretaceous and Cenozoic (120 million years ago to Present) using modern plate tectonic reconstructions and a composite apparent polar path designed to minimize known biases in the determination of paleolatitude. We find that global surface albedo stayed almost constant until it shifted 30% higher to the modern value of around 0.15 with the inception of the Late Cenozoic Ice Age 34 million years ago, reflecting polar ice-albedo amplification of global cooling resulting from the reduction of greenhouse gases below a critical threshold, most probably as the culmination of enhanced CO2 weathering consumption of continental mafic rocks in the tropical humid belt. The contribution from cloud cover toward a planetary albedo is unclear in the absence of measurable proxies but might eventually be gauged from the role cloudiness evidently plays in maintaining radiative balance with the increasing land bias between northern and southern hemispheres over the Cenozoic

Magnetostratigraphic confirmation of a much faster tempo for sea-level change for the Middle Triassic Latemar platform carbonates

New magnetostratigraphic data for the 3c470-m-thick Latemar carbonate platform, which includes 3c600 shallowing-upward bedding cycles, are consistent with litho- and biostratigraphic correlations of the section to a 3c10-m-thick interval in the basinal Buchenstein Beds that most likely represents only 3c1 m.y. of deposition according to published U-Pb single-crystal zircon dates. A reappraisal of reported cycle stratigraphic analyses of the Latemar suggests that the visibly obvious meter-scale bedding is not due to Milankovitch precessional forcing but rather reflects tempos an order of magnitude faster that may involve millennial-scale tidal amplitude variations

Magnetostratigraphy of a Lower/Middle Triassic boundary section from Chios (Greece)

The Marmarotrapeza Formation at Chios Island (northern Aegean Sea, Greece) is renowned for its Lower-Middle Triassic boundary sections in a marine Tethyan setting. Two sections have been sampled bed by bed to develop a magnetostratigraphic framework for the ammonoid and conodont biostratigraphy. The boundary sections occur within a lower normal (A+)-reverse (B-)-upper normal (C+) polarity sequence. The Lower-Middle Triassic boundary, placed at the first occurrence of the ammonoid genera Aegeiceras ugra Diener, Paracrochordiceras spp., Paradanubites depressus Fantini Sestini and Japonites sp., and close to the first appearance of the conodont species Gondolella timorensis Nogami, occurs in normal polarity zone Chios C+. The overall mean direction of the reversal-bearing characteristic component, whose early acquisition is suggested by a tilt test, is D = 271.2°, I = 33.2° (α95 = 11.7°, k = 112.5, N = 3). The inferred paleolatitude of the sampling sites is about 18°N, consistent with either an African or stable European affinity, although the declinations suggest large-scale counter-clockwise rotations with respect to Africa or stable Europe since the Early-Middle Triassic

Ecce Homo in Milan

Come ogni essere vivente anche l\u2019uomo ha colonizzato gli habitat solo nel momento in cui questi presentavano le condizioni favorevoli alla sua sopravvivenza. La colonizzazione del continente europeo \ue8 avvenuta molto probabilmente da Est verso Ovest e l\u2019ingresso in Italia pu\uf2 essere avvenuto solo dal punto pi\uf9 comodo in cui si poteva superare la catena Alpina, ossia passando dai Balcani (Figura 2). Dal record geologico si deduce il susseguirsi di periodi caldi a periodi freddi durante tutto il Pleistocene, tuttavia non ci sono evidenze di un passaggio ad un clima glaciale nel nostro paese prima del Pleistocene Superiore. Questo cambiamento ha come prima conseguenza l\u2019espansione delle calotte glaciali legata all\u2019intrappolamento dell\u2019acqua degli oceani e il conseguente abbassamento del livello del mare che provoca il passaggio di molte zone precedentemente sommerse prima ad un ambiente litorale e inne emerso. Questo \ue8 quanto si \ue8 vericato in Pianura Padana. Il cambiamento di ambiente ha provocato una variazione nel materiale che si stava depositando: prima si avevano limi e sabbie marine, poi ciottoli legati al trasporto uviale e materiale ne legato alle esondazioni dei umi. La dierenza nel materiale deposto \ue8 stata riconosciuta lungo tutta la Pianura Padana ed \ue8 stata nominata Discontinuit\ue0 R. La Discontinuit\ue0 R \ue8 quindi legata a un cambiamento nel clima, e all\u2019istaurarsi delle grandi glaciazioni nel continente europeo. Mediante lo studio di diverse carote prelevate in Pianura Padana \ue8 stato possibile creare un modello di et\ue0 che ha permesso di associare la Discontinuit\ue0 R al MIS22, datandola a circa 870.000 anni fa (Figura 1). Il MIS22 corrisponde inoltre alla ne della cosiddetta \u201cRivoluzione del Pleistocene Medio\u201d, un momento in cui si \ue8 assistito ad un cambiamento nella ora con la comparsa delle prime piante legate all\u2019ambiente glaciale, ma anche ad un cambiamento nella fauna legato all\u2019arrivo di specie come il mammuth (Mammuthus meridionalis), l\u2019elefante antico (Elephas antiquus), e il rinoceronte lanoso (Coelodonta antiquitatis). La nostra ipotesi \ue8 che l\u2019arrivo dell\u2019uomo in Europa e in Italia sia stato legato alla sua tendenza a seguire le onde migratorie degli animali che costituivano le sue principali prede (Figura 2), e che sia avvenuto in concomitanza con l\u2019instaurarsi delle grandi glaciazioni Pleistocenica, ossia attorno all\u2019et\ue0 del MIS22