Evidence for a short-lived increase in atmospheric CO2 at the Oligocene/Miocene boundary

Abstract:

In the earliest Miocene the Antarctic ice sheet retreated substantially following the Mi-1 glaciation event. The relationship between pCO₂ and orbital scale climate variations at this time is poorly understood, due to the paucity of pCO₂ reconstructions with sufficient temporal resolution. Here, we report a pCO₂ reconstruction based on fossil leaf micromorphological properties and supported by δ¹³C measurements, that indicates that pCO₂ increased following the Mi-1 event and remained elevated for approximately 24 kyrs. The fossil leaves analyzed (Lauraceae) and δ¹³C measurements come from a drill core of annually laminated sediments recovered from a maar lake deposit in southern New Zealand spanning ~100 kyr across the Oligocene/Miocene boundary. The lake had a large and stable anoxic zone, allowing for remarkable preservation of organic material, including exquisitely preserved fossil leaves. The leaf stomatal/epidermal cell ratio (stomatal index) decreased for ~24 kyr during this time period, suggesting increased pCO₂. δ¹³C values of primarily terrestrially sourced lake organic matter decreased by ~4‰ across the same interval, providing further support for an abrupt 24kyr-long increase in pCO₂ at this time. By comparison with stomatal conductance and pCO₂-induced carbon isotope fractionation in modern land plants, we estimate that the magnitude of the pCO₂ increase was between 140 and 220 ppm. These results imply that dynamic variations in pCO₂ occurred at precessional timescales during the early Miocene. We are further constraining the magnitude of pCO₂ change and quantifying the pCO₂ levels by: 1) Analyzing micromorphology and δ¹³C of close ecological and taxonomical modern analogues to early Miocene New Zealand Lauraceae, to better quantify changes in gas conductance and carbon isotope fractionation in response to recent pCO₂ changes; 2) Directly measuring δ¹³C values and stomatal geometry of fossil leaves, to quantify pCO₂ values using a recently published model based on fundamental leaf conductance principles.