Dr. Trevor Porter, U. Toronto
Local PK-7605, 7e étage, 201 ave. du Président-Kennedy
The Late Miocene was a dynamic period of global change, marked by a transition to a cooler mean climate relative to the Middle Miocene, glacier expansion and large-scale reorganization of ecosystems. Sea Surface Temperature (SST) records show a globally coherent Late Miocene Cooling (LMC, ~8-5.5 Ma), putatively linked to declining pCO2 (atm). However, the terrestrial expression of LMC is poorly described, especially at higher latitudes, where long records are lacking. This talk outlines recent efforts to reconstruct Late Miocene conditions in continuous detail based on a fossil pollen record from a ~400 m sediment core drilled from a paleolake bed at Fort Yukon near the Arctic Circle (66.5°N) in central Alaska. This talk focuses on the upper 350 m of core, representing the period ~11.9-4.9 Ma (age constraints: tephra- and detrital-zircon 206Pb/238U, meteoric 10Be and magnetostratigraphy). After 12 Ma, the pollen reveals a persistent decline in taxonomic diversity of hardwood and softwood types and a concomitant rise in graminoids into the Pliocene. Superposed on these trends are shorter-period variations in representation of warm- versus cold-adapted plant taxa. The Modern Analogue Technique was used to quantify summer climatic changes from the pollen data (σ = ±1.3°C). The reconstruction (mean time-step = 22 kyr) reveals a mostly warmer-than-present Late Miocene climate, except brief (<100 kyr) cool intervals around 10, 6.5 and 5.5 Ma when the mean climate was colder than today by as much as 2°C. The most recent cold episode coincides with the onset of Neogene glacial activity in coastal Alaska documented by ice-rafted debris in proximal marine sediments and confirms glacial mass balance in coastal Alaska was (and remains) sensitive to mean climate state. Another feature of the reconstruction is a thermal maximum at 8 Ma that was 3-4°C warmer than today and followed by sustained cooling to ~5.5 Ma. The magnitude and timing of the cooling trend closely matches LMC trends observed in Pacific SST records, suggesting continental Alaska and the broader Pacific region were tightly coupled climatically in the Late Miocene, as is true today. The Fort Yukon core offers the first continuous record of Late Miocene climatic change from a terrestrial Arctic borderland area and a key dataset needed to refine estimates of high-latitude climate sensitivity and future climate change.
Événement hybride / Hybrid event:
En personne / In person: Local PK-7605, 7e étage, 201 ave. du Président-Kennedy, Montréal QC
Zoom ID: 832 1632 7466
A continuous record of Late Miocene climate and vegetation change from continental Alaska