Multi-proxy evidence for constant hydrological sources and mild, wet climate in post-EECO Greater Green River Basin

Abstract:

The early Eocene climatic optimum (EECO), a warm period ~50 million years ago, is considered a model for warm-world conditions in a high emissions anthropogenic climate change scenario. The Greater Green River Basin, which accumulated sediment in a hypersaline lake (paleo-lake Gosiute) at the foot of the forming Rocky Mountains, is known for its high resolution, well-preserved records of the EECO from both floodplain and lacustrine sediments. Due to the basin’s proximity to multiple potential sources of moisture and transported sediment to the west, northeast and south, there is concern that inconsistent hydrological inputs may interfere with climate signals. The Bridger Formation (Southwest Wyoming, USA) records alluvial and fluvial sedimentation from ~50-45.5 million years ago and includes an extremely well-preserved and well-characterized flora at the Blue Rim fossil quarries (constrained using ⁴⁰Ar/³⁹Ar geochronological dating to record before 49.29 Ma to 48.29 Ma). Using multiple geochemical proxies and fossil evidence, we demonstrate that moisture and sediment sources in the basin throughout the >1 million years of post-EECO uplift of the Rocky Mountains stayed constant, and that variability in biogeochemistry and floral preservation is related to changes in climate and biota. Previous studies have found rainfall ranging from 1300 to 1900 mm yr^-1 in the region with temperatures between 14 and 15°C, consistent with the wet forest ecosystem demonstrated in preserved plant remains (dicotyledonous taxa including some climbers, some monocots, and ferns). Multi-proxy floral- and paleosol-based climate proxies (e.g. CIA-K, floral humidity province, Holdridge Life Zones) demonstrate that the rim of paleo-lake Gosiute was a wet forest with warm temperatures, high precipitation, moderate weathering, and sediments coming from consistent provenance. Robust geochemical data demonstrate that fluvial inputs stay constant and do not obfuscate climate signals in Blue Rim’s sediments.