Rise of the Givetian (385 Ma) Forests, Northern Appalachian Basin, Catskill State Park, New York, U.S.A.

Abstract

Forestation on Earth may have profoundly changed fluvial sedimentary patterns, increased the release of bio-limiting nutrients to marine systems and increased the consumption of atmospheric CO₂ through biologic productivity and silicate mineral weathering. Evidence is presented for occurrences of forested ecosystems in swamplands in deltaic coastal settings, across a range of seasonally-drained floodplain overbank environments, and even in what were likely terraced interfluve landscapes during the Givetian in the Appalachian basin. Evidence is also presented that chemical weathering in paleosols on alluvial plains in the Appalachian basin may have increased from the Ordovician through the Middle Devonian. Temporal correlation of terrestrial and marine strata is essential in order to understand the potential cause and effect relationships between changes in Devonian weathering systems and marine environmental, ecological and geochemical conditions. Over 450 m of nearly continuous outcrop exposure along Plattekill Creek in West Saugerties, New York, was measured in order to develop a sequence-stratigraphic framework, using alluvial stacking-pattern analysis. The analysis reveals several orders of cyclicity that correspond with Acadian tectophases and previously documented marine depositional sequences. Paleo-precipitation for 37 paleosol profiles was estimated using the CALMAG proxy, a geochemical ratio from bulk soil material in vertic paleosols, which suggests dominantly wet paleoclimates throughout the Middle Devonian, an interpretation also supported by micromorphological data. The paleosol calcite paleobarometer was used to estimate atmospheric CO₂ decline related to the development of forested ecosystems, as well as determine the initial atmospheric pCO₂ at the onset of forestation. The timing of calcite precipitation in relation to the soil saturation state and soil-atmosphere connectivity was investigated in a modern Vertisol (smectitic, clay-rich soil, seasonally saturated) in Brazoria County, Texas, U.S.A., which is an excellent modern analog to forested paleosols in the Appalachian basin. A luminescent phase of calcite formed during the water-saturated portion of the year, negating its use for pCO₂ estimations. A non-luminescent phase formed during the well-drained portion of the year when atmospheric CO₂ mixed with soil-respired CO₂ and is therefore useful for pCO₂ estimation. From these results a model is presented that independently tests the saturation state of a paleosol at the time of pedogenic carbonate precipitation.