Abstract
The breakup of Pangea and onset of growth of the Pacific plate led to several paleoenvironmental feedbacks, which radically affected paleoclimate and ocean chemistry during the Jurassic. Overall, this period was characterized by intense volcanic degassing from large igneous provinces and circum-Panthalassan arcs, new oceanic circulation patterns, and changes in heat and humidity transports affecting continental weathering. Few studies, however, have attempted to unravel the global interactions linking these processes over the long-term. In this paper, we address this question by documenting the global changes in continental drainage and surface oceanic circulation for the whole Jurassic period. For this purpose, we present 53 new neodymium isotope values (εNd(t)) measured on well-dated fossil fish teeth, ichthyosaur bones, phosphatized nodules, phosphatized ooids, and clastic sediments from Europe, western Russia, and North America.Combined with an extensive compilation of published εNd(t) data, our results show that the continental sources of Nd were very heterogeneous across the world. Volcanic inputs from a Jurassic equivalent of the modern Pacific Ring of Fire contributed to radiogenic εNd(t) values (−4ε-units) in the Panthalassa Ocean. For the Tethyan Ocean, the average surface seawater signal was less radiogenic in the equatorial region (−6.3), and gradually lower toward the epicontinental peri-Tethyan (−7.4), western Russian (−7.4) and Euro-Boreal seas (−8.6). Different Nd sources contributed to this disparity, with radiogenic Nd influxes from westward Panthalassan currents or juvenile volcanic arcs in open oceanic domains, and substantial unradiogenic inputs from old Laurasian and Gondwanan shields for the NW Tethyan platforms. Overall, the εNd(t) values of Euro-Boreal, peri-Tethyan, and western Russian waters varied quite similarly through time, in response to regional changes in oceanic circulation, paleoclimate, continental drainage, and volcanism. Three positive shifts in εNd(t) values occurred successively in these epicontinental seas during the Pliensbachian, in the Aalenian–Bathonian interval, and in the mid-Oxfordian. The first and third events are interpreted as regional incursions of warm surface radiogenic currents from low latitudes. The Aalenian–Bathonian shift seems linked to volcanic outbursts in the NW Tethys and/or circulation of deep currents resulting from extensional events in the Hispanic Corridor and reduced influences of boreal currents crossing the Viking Corridor. In contrast, the εNd(t) signals decreased and remained very low (<−8) during the global warming events of the Toarcian and Late Oxfordian–Early Tithonian intervals. In these greenhouse contexts, a latitudinal expansion of humid belts could have extended the drainage pathways toward boreal Nd sources of Precambrian age and increased the supply of very unradiogenic crustal-derived inputs to seawater. Finally, a brief negative εNd(t) excursion recorded in parallel with regional drops in seawater temperature suggests that southward circulation of cold unradiogenic Arctic waters occurred in the NW Tethys in the Callovian–Early Oxfordian. All these results show that changes in surface oceanic circulation resulting from the Pangean breakup could have regionally impacted the evolution of seawater temperatures in the NW Tethys.
Highlights
The Jurassic marked the onset of the Pangean breakup that led to our modern geography
In the Early Jurassic, this was reflected by the progressive openings of the Hispanic and Viking corridors, which led to the formation of Caribbean and North Atlantic oceanic crusts (Figure 1) (Doré, 1991; Labails et al, 2010)
It is noteworthy that, for all biogenic or authigenic materials combined, the longterm trends displayed by εNd(t) values of Euro-Boreal, peri-Tethyan, and Russian basins are very similar through time
Summary
The Jurassic marked the onset of the Pangean breakup that led to our modern geography. In the Early Jurassic, this was reflected by the progressive openings of the Hispanic and Viking corridors (i.e., trans-Pangean and transLaurasian seaways linking the Tethyan domain to the Eastern Panthalassan and Arctic areas, respectively), which led to the formation of Caribbean and North Atlantic oceanic crusts (Figure 1) (Doré, 1991; Labails et al, 2010). The incipient growth of the Pacific plate in the Middle Jurassic triggered compressive movements, increasing subduction, terrane accretion, and arc volcanism all around the Panthalassa Ocean (Bartolini and Larson, 2001). The best illustrations are the successive accretions of Quesnellia, Stikinia, and Wrangellia terranes to the North American Cordillera (Ricketts, 2008), the intra-oceanic subduction of the Pontus Ocean (van der Meer et al, 2012), and the progressive closure of the MongolOkhotsk Ocean suturing the Siberian and Asian cratons (Cogné et al, 2005) (Figure1)
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