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It has been suggested that a massive pulse of oceanic crustal production occurred during the Cretaceous Normal Superchron (CNS) and that this pulse is causally connected to a global sea-level highstand in the mid-Cretaceous. An alternative hypothesis is that the mid-Cretaceous sea-level highstand was caused by the "supercontinent break-up effect", which results in the creation of the mid-Atlantic ridge at the expense of old ocean floor in the Pacific. To test these hypotheses, we reconstruct paleo-oceans by creating "synthetic plates", the locations and geometry of which are established on the basis of preserved ocean crust (magnetic lineations and fracture zones), geological data, paleogeography, and the rules of plate tectonics. We use a merged moving hotspot (Late Cretaceous-present) and fixed hotspot (Early Cretaceous) reference frame, coupled with reconstructed spreading histories of the Pacific, Phoenix and Farallon plates, the plates involved in the Tethys oceanic domain and a revised reconstruction of the Arctic ocean. Based on this approach we have created a set of global oceanic paleo-isochrons and paleo-oceanic age grids.
The grids provide the first complete global set of paleo-basement depth maps, including now subducted ocean floor, for the last 140 million years based on a depth-age relationship. We show that the late-Cretaceous sea-level highstand and the subsequent long-term drop in sea-level was primarily caused by the changing age-area distribution of Pacific ocean floor through time, and to a much lesser extent by the "supercontinent break-up effect", which resulted in the creation of the mid-Atlantic and Indian Ocean ridges at the expense of subducting old ocean floor in the Tethys. Our models show that the enormous change in mid ocean ridge length from the Early to the Late Cretaceous, from less than 40,000 km at 140 Ma to over 60,000 km at 80 Ma was the primary driving factor for the long-term sea-level rise during the Cretaceous. Global average sea-floor spreading rates dropped rather than increased from 140 to 80 Ma. The expansive mid and late Cretaceous epicontinental seas, coupled with warm climates and oxygen-poor water masses, were ultimately driven by this vast increase in mid-ocean ridge length, and not by a super-plume as suggested previously. Therefore, mid-ocean ridge dynamics drove enhanced burial of organic carbon in the mid-Cretaceous oceans, leading to widespread black shale deposition and source rock formation. Our models show that the "Pacific break-up effect", forming the triangular Pacific Plate at a triple junction between the Izanagi, Farallon and Phoenix plates, was ultimately a more important driving force of global change than Pangaea break-up.
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