Tropical weathering of the Taconic orogeny as a driver for Ordovician cooling

Abstract

8The Earth's climate cooled through the Ordovician Period leading up to the 9 Hirnantian glaciation. Increased weatherability of silicate rocks associated with 10 topography generated on the Appalachian margin during the Taconic orogeny has been 11 proposed as a mechanism for Ordovician cooling. However, paleogeographic 12 reconstructions typically place the Appalachian margin within the arid subtropics, outside 13 of the warm and wet tropics where chemical weathering rates are highest. In this study 14 we reanalyze the paleomagnetic database and conclude that Ordovician constraints from 15 cratonic Laurentia are not robust. Instead, we use paleomagnetic data from well-dated 16 volcanic rocks in the accreting terranes to constrain Laurentia's position given that the 17Appalachian margin was at, or equatorward of, the paleolatitude of these terranes. To 18 satisfy these allochthonous data, Laurentia must have moved toward the equator during 19 the Ordovician such that the Appalachian margin was within 10° of the equator by 465 20Ma. This movement into the tropics coincided with the collision and exhumation of the 21Taconic arc system, recorded by a shift in neodymium isotope data from shale on the 22 Taconic 28 related increase in weatherability is a viable mechanism for lowering atmospheric CO 2 29 levels through silicate weathering contributing to long-term Ordovician cooling. 30

Arc-continent collisions in the tropics set Earth’s climate state

“…The sutures within the resultant shapefile were reconstructed from 520 Ma to the present along with the tectonic units utilizing the paleogeographic model of Torsvik and Cocks (23) in the spin axis reference frame (anchor plate ID of 1). This paleogeographic model was updated to include a revision to Ordovician Laurentia (21) and the Paleozoic of Asia, including the addition of the Kazakh terranes following the model of Domeier (28). We also compared this model with the Mesozoic and Cenozoic model of Matthews (32), which gave similar results for suture length through time.…”

“…Previous attempts to compare changes in CO2 sources or sinks to climate change through the Phanerozoic have relied on pCO2 or temperature estimates from proxy data (19,21,187) or geochemical box models (18,191). Although organic proxies coupled with oxygen and boron isotopes of benthic foraminifera provide consistent and relatively robust records of climate over the Cenozoic (24,26), prior to that, direct estimates of pCO2 rely on paleosols and leaf stomata with variable data density that is particularly lacking in the Paleozoic (24,192).…”

“…This lag may be due to warm background climate states and low global weatherability during the early Cenozoic and early Paleozoic, such that significant and prolonged increases in global weatherability were necessary to drive cooling and initiate glaciation. Indeed, oxygen isotope records from both the Eocene and Ordovician demonstrate long-term cooling trends prior to glaciation (21,26).…”

A Paleozoic age for the Tunnunik impact structure

“…Following 500 Ma, the angular distance of the Tunnunik VGP becomes progressively greater from the geographic poles implied by the running mean poles of Torsvik et al (2012) and the paleogeographic model of Torsvik and Cocks (2017) and its modification by Swanson-Hysell and Macdonald (2017) (Fig. 6A).…”

“…6. A) Angle between Tunnunik impact VGP and paleopoles as a function of time for the Torsvik et al (2012) APWP, the paleogeographic models of Torsvik and Cocks (2017) and its Ordovician modification by Swanson-Hysell and Macdonald (2017). B) Likelihood for a VGP to occur at an angular deviation from the paleopole equal or greater to that observed for the Tunnunik impact VGP for a Fisherian (k = 20) distribution of VGPs.…”