Assessing the Timescale of the Miocene Climate Optimum with U-Pb Zircon Geochronology

Abstract:

With elevated global temperatures and CO₂ levels, the Miocene Climate Optimum (MCO) presents a fruitful opportunity to better understand today’s anthropogenic climate change. Within the ~3 Ma duration of the event, there are shorter-term variations in δ¹³C, δ¹⁸O, and δ¹¹B records, and the timescale and drivers of these fluctuations require further study. However, MCO proxy records from deep-sea sediment cores have not been the subject of a high-precision geochronological study yielding absolute radioisotopic ages, which are infrequently obtained from ocean drill cores. Instead, many records are calibrated by relative means including biostratigraphy, chemostratigraphic correlation, and magnetostratigraphy. Age calibration through astronomical tuning has been carried out in only a minority of records, and these records arrive at different ages for magnetic reversals and thus highlight difficult to quantify uncertainties in tuned age models. A combination of all available dating and correlation techniques is likely required to obtain an accurate age model for the MCO.

ODP Site 1000 (Nicaragua Rise) has yielded a record of biostratigraphy, δ¹³C and δ¹⁸O across the MCO, along with biotite ⁴⁰Ar/³⁹Ar ages obtained from some of the hundreds of silicic ash layers interbedded with carbonates in the core. These ashes also contain zircons amenable to U-Pb CA-ID-TIMS geochronology, which produces ages with uncertainty of ~10 ka. Here we present the first high-precision zircon ages targeting the MCO from Site 1000. Our preliminary new ages improve upon the precision of prior ages by an order of magnitude, and show that the decline in δ¹⁸O, indicating the onset of warming at the site, occurred ~17.5 Ma, differing from other records that place the warming onset at 17-16.8 Ma. Possible explanations for this offset include global diachroneity of the MCO, errors in age calibrations of other MCO records, or alteration of the warming signal at Site 1000 through diagenesis. While large igneous province volcanism and CO₂ outgassing of the Columbia River Basalt Group (CRBG) has previously been invoked as the driver of the MCO, our zircon geochronology from the CRBG (Kasbohm & Schoene, 2018) compared with our new Site 1000 age model suggest that the CRBG was erupting during only a small portion (~750 ka) of the MCO.