Early Pleistocene

Melting ice reveals two-million-year old peat

Posted by William Colgan on August 31, 2022
Climate Change, New Research / Comments Off on Melting ice reveals two-million-year old peat

     This week, in an open-access Boreas article, we describe a new Early Pleistocene peat deposit in Northwest Greenland. We discovered the deposit quite accidentally during fieldwork near Thule Air Base in September 2019. That year, there was extreme melt in North Greenland, which removed the seasonal snowpack as far as the eye. Anders Bjørk and I were working at the margin of Pingorsuit Glacier, when eagle-eyes Anders spotted some very dark, organic-rich, sediments. They were very understated tufts of peat, mostly covered in till, that were clearly being released from the melting landscape. Quick examination revealed large pieces of wood, and even a small pinecone. As this type of pine tree haven’t grown in Greenland during the Holocene, we knew this must be very old material! We opportunistically grabbed a few bags of peat to transport back to Copenhagen for Ole Bennike and colleagues to analyze.  

Figure 1Left: The pinecone that immediately told us the organic-rich deposit was pre-Holocene in age. Right: A dark peat tuff emerging from beneath glacier till by erosion from a supraglacial stream.

     It is quite tricky to provide absolute dates for Early Pleistocene formations, as they are radiocarbon dead. The published ages of these deposits ranges from 1.9±0.1 Ma at Store Koldewey to 3.4±0.5 Ma at Beaver Pond. Our just-published macrofossil analysis shows that the Pingorsuit Formation has broad similarities with the fossil assemblage observed in the Kap København Formation. We therefore suggest an age of about two-million-years old, or about the same as the Kap København Formation. The Pingorsuit flora reflects an open boreal forest ecosystem, with invertebrates typical of ponds or standing water. Some of these species are extinct during the Holocene. Ole provides the first description of an extinct waterwort species, which he has named Elatine odgaardii after the Danish naturalist Bent Odgaard. From the Pingorsuit species assemblage, we can estimate that its Early Pleistocene ecosystem likely had a mean July air temperature of >10°C. This is at least 9°C warmer than present-day, and approximately equivalent to the North Greenland climate anticipated in 2100 under the SSP5-8.5 (or “very high emissions”) climate scenario.

Figure 2 – Left: Pingorsuit Glacier in 1985. Right: Pingorsuit Glacier in 2019. The ice has retreated approximately 150 m during the 35 years. The Pingorsuit Formation sampled were located at the dot.

     Only seven organic-rich Early Pleistocene deposits have been discovered in the High Arctic. Located at ~480 m elevation, the Pingorsuit Formation is likely the only Early Pleistocene organic bed in Greenland to have remained above sea-level since its deposition. All other Early Pleistocene deposits in Greenland reflect deposition in marine conditions. This makes the Pingorsuit Formation rather unique. But it is also perhaps the most fragile Early Pleistocene deposit discovered to date. Unlike the km-scale outcrops of other Early Pleistocene marine deposits, we could only identify five small mounds with a total area of approximately 20 m2. These small mounds are all located immediately adjacent to the Pingorsuit Glacier margin. This suggests they are being eroded by flowing water within years of emerging from the subglacial to the subaerial environment. Simply put, the Pingorsuit Glacier appears to be preserving a two-million-year old formation beneath it. The anticipated loss of Pingorsuit Glacier under climate change therefore provides a compelling urgency to fully assess the Early Pleistocene time capsule with the next decade or two.

Bennike, O., W. Colgan, L. Hedenäs, O. Heiri, G. Lemdahl, P. Wiberg-Larsen, S. Ribeiro, R. Pronzato, R. Manconi and A. Bjørk. 2022. An Early Pleistocene interglacial deposit at Pingorsuit, North-West Greenland. Boreas.

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