TACONIC LOW-LATITUDE ARC-CONTINENT COLLISION AS A DRIVER FOR ORDOVICIAN COOLING

Ordovician strata record the transition from an ice-free world to glaciation temporally associated with mass extinction. Several hypotheses have been proposed to account for the initiation of Late Ordovician glaciation: increased carbon burial and sequestration, paleogeographic isolation of Gondwana, increased weathering due to orogenic relief and exposure of fresh volcanic rocks, and aerosol release from intense volcanism. We present a new compilation of weathering proxy data and a new paleogeographic model linked to recent geological and geochronological constraints to evaluate the hypothesis that arc-continent collision associated with the closure of the Iapetus Ocean along an east-west oriented margin led to increased silicate weathering rates. We propose that the margin was at lower latitude than is typically reconstructed and that arc obduction at tropical latitudes contributed to CO₂ drawdown, which drove the Ordovician cooling that culminated in the Hirnantian glaciation.

Chemical weathering rates in the warm and wet tropics are approximately an order of magnitude higher than those at mid-latitudes. Although Ordovician paleogeographic models typically place the Appalachian margin of Laurentia southwards of 20°S in the relatively arid subtropics, this position is poorly constrained due to a paucity of reliable Ordovician paleomagnetic poles from cratonic Laurentia. We take the approach of using paleomagnetic data from well-dated volcanic rocks in the accreting terranes to constrain Laurentia’s position given that the Appalachian margin must have been at or equatorwards of their paleolatitude. To satisfy the allochthonous data, Laurentia must have moved toward the equator during the Ordovician such that the Appalachian margin was at ~10ºS by 465 Ma. This movement into the tropics coincides with: 1) obduction of the Taconic arcs marked by the appearance of detrital chromite in foreland basins; 2) a shift in Nd isotope data from shale on the Laurentian margin to more juvenile values; 3) a drop in seawater Sr isotopic values from conodonts; and 4) a trend to the higher values in the oxygen isotopic composition of both brachiopod carbonate and conodont phosphate. These data are all consistent with the tropical weathering of the Taconic arc terranes as a driver for Ordovician cooling.