Orbital-scale fluctuations in continental weathering flux and continental ice-volume during greenhouse and icehouse climate intervals : evidence from oxygen and neodymium isotopes
Rapid changes in global and local climate and weather, noticeable over our lifetimes, have spurred heated public and political debate over the cause of these climatic changes, whether or not we can reverse, stop or slow down this process, and how to create adequate models that can predict Earth's changing climate. To create the most accurate models of future climate, many researchers look to past greenhouse intervals for insight; intervals of time identified by high modeled atmospheric CO2, a lack of glacial deposits, and geologic evidence of globally high sea-levels--all of which suggest little to no glacial ice and a relatively monotonous climate. However, recent stratigraphic and geochemical evidence from the Cretaceous 'supergreenhouse' interval document compelling evidence to support dynamic greenhouse climates. The goals of the studies hereafter described are to document and describe orbital-scale (104-105 yr) climate fluctuations recorded by δ18O as a proxy for glacio-eustasy and sea surface temperature and µNd as a proxy for continental weathering and wet/warm or dry/cool climate during two global greenhouse intervals: the Late Silurian and Late Devonian. The first study (Chapter 1) explores whether glacio-eustasy was the driver for orbital-scale shallowing-upward cycles developed in Late Silurian and early Late Devonian tropical and subtropical greenhouse climates. Two intra-cycle δ18O conodont apatite trends were observed: asymmetric trends demonstrating progressive δ18O increases coincident with facies shallowing, or symmetric trends demonstrating initially decreasing, then increasing δ18O values. These isotopic trends and intra-cycle magnitudes support the hypothesis that these cycles developed in response to glacio-eustasy during glacial stages, implying that Late Silurian and Late Devonian greenhouse climates were more dynamic than traditionally assumed. Relationships between orbital-scale continental weathering flux and glacial-interglacial marine cycles was first assessed comparing intra-cycle µNd and δ18O values from Middle Pennsylvanian icehouse cycles (Chapter 2). Observed conodont δ18O trends support previous interpretations that sampled cycles were generated by glacio-eustasy (30-50 m magnitudes) combined with <1° sea surface temperature changes. µNd trends typically demonstrate lower µNd values during interglacial intervals and higher µNd during glacial intervals, supporting claims that increases in precipitation and/or air temperatures during interglacial intervals result in increased continental weathering rates and/or increased flux to marine basins. Using this initial continental weathering flux study and Pleistocene Nd-isotope studies as a model, we tested the phase-relationships of continental weathering flux and sea-level change in Upper Silurian and Upper Devonian greenhouse cycles (Chapter 3). Upper Silurian µNd demonstrates relatively uniform intra-cycle values, due to averaging out of far-field source variations in continental weathering flux, relatively uniform Late Silurian sub-tropical climate, and/or subdued continental weathering and flux due to the absence of upland and inland vascular land plants. Upper Devonian µNd demonstrate greater intra-cycle variation in µNd, which may be due to enhanced chemical weathering as a result of upland and inland colonization of land plants with large root systems and an intensified hydrologic cycle due to evapotranspiration. Observed µNd trends support the hypothesis that these greenhouse cycles record increases and decreases in continental weathering due to increases and decreases in precipitation and/or air temperature. µNd and 147Sm/144Nd values for Upper Silurian and Upper Devonian cycles support previous claims of far-field transport of Caledonian Mountain Belt material via trans-Laurentian fluvial systems.
isotopes, stratigraphy, greenhouse, climate, icehouse, Pennsylvanian, Late Devonian, Upper Devonian, Late Silurian, Upper Silurian, neodymium, Nd, oxygen, d18O, conodont, glacial, interglacial, sea-level
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