Non-marine paleoclimatic response of Early-Middle Eocene "hothouse" interval evidence from a lacustrine record of the Green River Formation in Utah, USA

Reconstructions of extremely warm "hothouse" intervals are important testbeds to investigate the impact of climate change upon carbon cycling. The Early Eocene is characterized by the warmest interval (> 10 °C warmer than pre-industrial) of the Cenozoic with elevated atmospheric CO₂ levels (> 1000 parts per million). To reconstruct paleoclimatic change in the mid-latitude non-marine setting during the Early to Middle Eocene interval, we targeted the lacustrine sedimentary sequence of the Green River Formation in the central United States.

We examined an 850 m thick succession at Indian Canyon Uinta Basin in northern Utah. Alterations in lithofacies allow us to subdivide the succession into four paleoenvironmental stages: the fluvio-lacustrine stage; the fluctuating deep-lake stage; the evaporation-dominant stage; and the fluctuating shallow-lake stage, in ascending order.

Furthermore, semi-quantitative "Depth Rank" assignment based on color, grain size and sedimentary structure, arranged in order of increasing inferred water depth for reconstructing lake-level changes allow deduction of astronomically-paced signals embedded in the strata. Repetitions at ca. 2 m, 10 m and 40 m appear to correspond to orbital-scale lake level change, e.g., precession and eccentricity cycles. Major elemental composition by XRF analysis indicate an overall upward drying of paleoclimatic changes with orbital-scale variations.

Rhythmically alternating bedded chert occur in the fluctuating shallow-lake stage in the uppermost part of the section. Previous studies have interpreted that the evaporation process and/or alkalinity changes of lake water are related to chert formation; however, the formation mechanism is unclear. We performed microscopic observation and elemental mapping analysis. Fluorescent microscopic inspection revealed that dense accumulations of highly fluorescent organic-matter spheres in Si rich layer. It suggested that silica precipitation is possibly caused by decreased pH due to the decomposition of organic matter. Initial deposition of organic matter can be related to changes in the lake algal productivity. Therefore, periodic occurrences of chert-dolomite beds in the formation are possibly linked to centennial- to millennial-scale changes in lake algal productivity.