The global carbon cycle is complex, and the ocean plays an integral part in it. The ocean has a natural ability remove CO2 from the ocean-atmosphere system through the burial of organic carbon (OC) in sediments. In this way, marine sediments have controlled atmospheric CO2 and O2 concentrations over the past 500 million years (Figure 1). The question is: where do these processes occur, and just how much OC can these sediments store?
When you hear “marine sediments,” what do you think of? Perhaps you see massive river deltas, like that of the Mississippi, or tropical white sand beaches with lots of wave erosion. Most OC burial budget models reflect this way of thinking, and mostly consider these environments. But did you think of fjords? Fjords are not often thought of as suppliers of marine sediment, perhaps due to their rocky, mountainous terrain and lack of beaches. However, they have recently been suspected as “hot-spots” for OC burial; beneath their flat, high-latitude waters, massive amounts of terrestrial sediment are transported into the oceans each year.
Glacially-deposited sediments have been freshly eroded from inland up the fjord valleys. While glacial transport accounts for only ~6% of the world’s sediments transported to the oceans, glaciers transport an average of 813 Megatons of terrestrial sediment to the ocean per year (Smith et al., 2015); saying that is a lot of sediment is an understatement. After all, the land that had originally filled up the fjord valley had to go somewhere, and where it went is into the ocean. Fjords are geologically young features, and are therefore sites of net sediment accumulation, trapping terrestrial sediments and promoting OC burial. Fjord environments are additionally suitable for this process because of how deep they are – the steep valley walls can create inlets over 4,000 feet deep. At these depths, the cold waters can become anoxic – have no or very low O2 – which is chemically ideal for the OC preservation and long-term burial.
Modern OC burial budgets do not take into account these important fjord environments, but some believe there is enough evidence for them to be included in OC burial models as a standalone depositional environment. Scientists from various geologic and marine research departments around the world sought to fix this oversight, and to create and provide a revised model of OC burial in the ocean. In order to do this properly, they needed to measure just how much OC is taken up in fjord marine sediments each year.
To accomplish this, they looked at data from 573 fjord surface sediment samples and 124 sediment cores, from fjord systems in Europe, Greenland, Svalbard, Western Canada, Eastern Canada, Alaska, Chile, New Zealand, and Antarctica. In addition, they added to the dataset new geochemical information from Fiordland, a national park in New Zealand home to 14 fjords on the edge of a very active mountain-building range, with some of the highest erosion rates in the world due to both chemical and physical processes.
The study found that 18 Megatons of OC are buried in fjord sediments each year (Figure 2). This is equivalent to about 11% of the total annual marine carbon burial around the globe. 11% may not sound like a large percent; however, fjords bury twice the amount of organic carbon per unit area as the global ocean average. Additionally impressive is that terrestrial fjord sediments hold twice as much OC as biologically-derived sediments that lie beneath the upwelling regions of the ocean, where there is massive amounts of biological productivity due to high nutrient concentrations. Fjords are beating the ocean at its own game.
The researchers came to the logical conclusion that fjords play an important role in the carbon cycle, and may thus play an important role climate regulation. Another major take-away from this study is the undeniable importance of glaciers. If they continue to shrink as evidence shows, they will not transport as much sediment. Thus, they take up less carbon, putting even more CO2 into the ocean and atmosphere, creating a positive feedback cycle and perpetuating the impact of climate change.