A climate archive at the bottom of a Greenland fjord 

Climate change is especially pronounced in the Arctic, where rising temperatures are causing the rapid melt of the Greenland Ice Sheet. This results in increased freshwater discharge and coastal erosion, which in turn affect the transport of sediments, organic matter, and nutrients in the fjord systems. To understand environmental conditions before the advent of satellite monitoring and historical records, we rely on long-term paleorecords. One important archive is the seabed, where sediments accumulate year after year. These sediments preserve microscopic organisms such as algae, along with geochemical tracers, offering crucial insights into past climate and environmental conditions. 

In this study, we analysed a sediment core from Nuup Kangerlua, the fjord of Nuuk, formerly known as Godthåbsfjorden. The system is the largest in southwest Greenland and it  receives meltwater from several glaciers, three of which terminate into the fjord. Cold freshwater discharged from beneath these glaciers brings vital nutrients to the surface, supporting microscopic life such as phytoplankton, which forms the basis of all life in the fjord. At the fjord’s mouth, a shallow sill restricts the exchange between fjord and ocean waters, limiting  the intrusion of warm, saline Atlantic-derived waters from the adjacent shelf. The settings in Nuup Kangerlua support a rich and varied ecosystem, where algae, fish and whales thrive.

The sediment core spans the entire period since the fjord was deglaciated, the so-called Holocene epoch (approximately the last 11,700 years). This record offers a valuable baseline for understanding the long-term interactions between the Greenlandic ice sheet, the fjord environment, and broader oceanographic processes in the context of a changing climate.

The marine sediment cores analysed in our study were collected from Nuup Kangerlua, a fjord in southwest Greenland, aboard the Greenlandic research vessel Sanna

Reconstructing the past: a multi-proxy approach 

In 2013, an expedition aboard the Greenlandic research vessel Sanna ventured into the Nuup Kangerlua fjord system (also known as Godthåbsfjord) to collect marine sediment records. The team used a six-meter-long gravity corer, a weighted metal tube with a plastic liner designed to penetrate and extract layers of the seabed. Once retrieved, the sediment core was transported to Denmark, where it was subsequently sliced into thick slices of sediment, each recording a specific time in the geological history of the fjord. 

First, we needed to determine the age of as many samples as possible to construct a reliable age model for the sediment record. This was achieved by searching the mud for microscopic carbonate-shelled fossils called foraminifera, that could be radiocarbon dated. Next, we used various indicators as proxies, to extract information about past environmental conditions from the marine sediments. In this study, we combined the following proxies: 

• Grain size: The presence of larger grains within otherwise fine-grained sediments, suggests iceberg activity. These grains, known as ice-rafted debris or dropstones, were transported and deposited by melting icebergs.  
• Geochemical tracers: By measuring the concentration of biogenic silica and organic carbon, we could infer the abundance of siliceous and carbonate organisms in the past. 
• Dinoflagellate cysts: These microscopic algae were identified to the species level, providing valuable insights into historical sea-surface conditions.

Anna Bang Kvorning at the microscope, identifying and counting dinoflagellate cysts 

Our finding: An exceptional oceanographic regime during past warming

Typically we observe the same dinoflagellate species throughout a sediment record, though their abundance varies. However, to our surprise, one species displayed an unusual pattern of absence, presence, absence. From 7000 to 3000 years ago, Nematosphaeropsis labyrinthus appeared suddenly and in high abundance, standing out against its absence in earlier and later periods of the record. When comparing this to the species’ modern distribution, it became evident that such high concentrations of Nematosphaeropsis labyrinthus are associated with elevated salinity levels, which are linked to the presence of Atlantic-sourced water. This suggests that during this period, the fjord system experienced an exceptional influence of Atlantic water. Notable, this aligns with other studies indicating a minimum extent of the Greenland Ice Sheet during the same timeframe. 

The period between 7000 and 3000 years ago has no modern analog, as Atlantic-sourced water is currently restricted from entering the fjord by the presence of Southwest Greenland Coastal Water. Under near-future climate scenarios, it is expected that this coastal water mass will continue to dominate, further limiting the entertainment of Atlantic-sourced water. However, with continued warming and retreat of the ice sheet, it is possible that the conditions may once again allow for the entrainment of Atlantic-sourced water, similar to the mid-to-late Holocene. We will now test this hypothesis by combining our data with model simulations.