The role of ancient carbon in the modern carbon cycle
Fossil fuel combustion is the major source of the excess CO2 in the atmosphere today that causes the observed rapid climate change which is threatening our planet’s climate and affecting the Arctic even more dramatically than the rest of the globe. Warming in these regions may, however, cause even more fossil carbon to escape into the atmosphere from its long-term reservoirs where it has resided for many thousands or millions of years. The destabilization of these long-term reservoirs is a major concern, as shifts from the long-term geological carbon cycle operating on time-scales of millions of years to the short-term cycle with effects on millennial or shorter time scales are the main cause for climate instability. Marine sediments are one of the large carbon reservoirs, in which both short-term carbon cycling takes place and which also acts as a long-term geological reservoir. Organic matter in marine sediments consists of a mix of materials from a variety of different sources. Along the ocean margins, terrigenous organic matter constitutes an important fraction of it, including organic matter eroded from ancient organic-rich rocks outcropping on land. Traditionally, the re-burial of rock-derived organic matter has been regarded to not affect the short-term carbon cycle, as carbon is moved from one geological reservoir, i.e. sedimentary rocks that have been formed millions of years ago, to another potential geological reservoir, the ocean sediments, which will potentially also develop into ancient sediments.
In recent times, however, an increasing number of studies has challenged this traditional view, positing that eroded rock-derived organic matter might indeed be more vulnerable to decomposition than previously thought. This suggests that changes in the rate at which rock-derived organic matter is eroded and delivered to locations where it might be oxidized could impact the carbon cycle also on shorter time scales. Today, rapid changes in the supply of finely ground eroded material from land to the ocean are observed in the high latitudes, where climate change causes glaciers to retreat at ever accelerating rates. If these glaciers also erode into organic-rich sedimentary rocks, the potential exists for an increasing supply of organic-rich debris to the adjacent marine depocenters.
Ancient carbon supplied by to Arctic fjords – may it escape burial?
Arctic fjords are known to be hotspots of organic matter burial, and they receive increasing amounts of eroded terrestrial materials as the glaciers retreat. In order to assess whether this process has a potential impact on short-term carbon cycling, we investigated whether marine microbial communities are capable of utilizing the ancient carbon supplied by this process for their metabolic processes. We used radiocarbon as a very sensitive tracer to identify the uptake of ancient (= radiocarbon-free) organic matter into living biomass. This was achieved by extracting membrane lipids of live microbial communities, so-called intact polar lipids, from marine sediments, cleaving off their headgroups, and purifying the alkanoic acids from the membrane lipids’ core. By determining the radiocarbon content in these purified core lipids and comparing the results with the radiocarbon levels expected in freshly produced marine biomass (high levels, modern) and in rock organic matte (no radiocarbon, radiocarbon-dead), we were able to estimate the percentage of ancient carbon used by the microbes.
We applied this approach to an Arctic fjord of the Svalbard Archipelago by sampling sediments at three locations in Spitsbergen’s southernmost fjord, the Hornsund. The glaciers terminating in this fjord have been retreating dramatically over the past century, and good documentation exists of this retreat based on aerial photographs. Sediment cores were taken at three locations: a) in the marine basin of the fjord, which was always open and receives relatively high proportions of freshly produced organic matter, b) in the inner fjord basin at a location that was passed by the receding glacier front in the middle of the 20th century, and c) near the current glacier front. All sediment cores were dated using radiometric methods (210Pb excess – a radioactive lead isotope with a short half-life), showing that all sediments were deposited within the last decades. As expected, the microbial membrane lipids revealed preferential consumption of freshly produced and easily degraded marine organic matter, in particular at the marine basin location. However, the scarcer this fresh marine organic matter became – either with increasing sediment depth or with decreasing distance from the glacier front, where more rock-derived carbon was supplied – the higher the percentage of ancient organic matter utilization was evident from the microbial membrane lipids. We were able to demonstrate that these sedimentary microbes may take up to 55% of their diet from ancient rock-derived organic matter. This is unexpected, as ancient organic matter locked up in rocks is regarded to be difficult to degrade, having survived millions of years of microbial attack during transport, deposition and burial, having been subjected to diagenesis and increased temperature and pressure conditions in the Earth’s crust.
Implications and what remains to be done
As we are expecting glacier retreat to accelerate over the coming decades, the supply of ancient carbon to marine sediments will likewise increase. Additional ancient carbon, which might be much more accessible to microbial degradation, will be supplied by coastal erosion and riverine discharge of thawed Pleistocene age organic rich permafrost deposits. Both processes may fuel microbial degradation processes in marine sediments, leading to even more fossil CO2 emissions. However, the rate at which these degradation processes occur and how much of the ancient carbon delivered to the sediments will be degraded, while the rest escapes microbial attack and is re-buried in marine sediments, remains to be determined in future studies.