Cryosphere-Carbon on Top of the Earth (CC-Top): Decreasing Uncertainties of Thawing Permafrost and Collapsing Methane Hydrates in the Arctic

Periodic Reporting for period 4 - CC-TOP (Cryosphere-Carbon on Top of the Earth (CC-Top):Decreasing Uncertainties of Thawing Permafrost and Collapsing Methane Hydrates in the Arctic)

Berichtszeitraum: 2021-05-01 bis 2021-10-31

The ERC-AdG project CC-TOP addressed one of the Grand Challenges in Earth System Science: “Re-awakening” (remobilization) of carbon from the huge frozen reservoirs contained in terrestrial and within and below subsea permafrost in the Eurasian Arctic. Thawing permafrost is one of the few processes that can cause a large net repartitioning of carbon as greenhouse gases to the atmosphere on decade-century timescales. CC-TOP reduced the uncertainties for this to aid societal mitigation to combat climate change.

To address the overall objective, to enable decreasing uncertainties in our understanding of Arctic thawing permafrost and its effects, the work is broadly structured into four core domains: Terrestrial Cryosphere Carbon ("Terra"), Shelf/Subsea Cryosphere Carbon ("Shelf"), Slope Hydrate Cryosphere Carbon ("Slope"), and Atmospheric constraints on CH4 sources ("Atmos").

Objective Terra-1
One illustrative break-through in this area is represented by our 2019 PNAS paper (Wild et al., 2019). It presents a first quantitative estimate of how much of the river organic carbon that comes from permafrost and peat deposits across the Eurasian Arctic. While there had been many Arctic river studies earlier, these long-term 14C results are the first one to pinpoint that only a small portion of DOC was from peat and permafrost (PP), whereas about half of the POC is released from PP. This is an essential component for meaningful predictions of the Arctic permafrost/peat carbon-climate feedbacks.

Objective Terra-2
The project produced about ten strong papers on the sources, transport and degradation of terrestrial carbon to and in the wide Siberian-Arctic shelf seas. Among other results, our 2018 Nature Comm. paper provided breakthrough on a long-standing challenge in the ocean carbon cycle: what is the time for cross-shelf transport, allowing unique constraint on ambient degradation rates and fluxes of different compound classes of translocated permafrost carbon (Bröder et al., 2018, 2019).

Objective Terra-3
Permafrost carbon remobilization during earlier rapid warming periods may give clues to what is in store with current climate warming. Here the project pioneered a whole new line of investigation by deciphering the historical record of changing permafrost-C input to the Arctic Ocean recipients in the Laptev Sea (Tesi et al., 2016, Nature Comm; Martens et al., Science Advances 2021), The East Siberian Sea (Keskitalo et al., 2017, Climate of the Past) and Chukchi Sea (Martens et al., 2019, Global Biogeochemical Cycles). Using advanced molecular and isotopic techniques, these CC-TOP studies document that there were massive remobilization of permafrost-C during the Younger Dryas – Preboreal warm spell some 11500 years ago, which is synchronous to a sudden increase in global atmospheric CO2.

Objective Shelf-1
The CC-TOP investigated 20-60 m drill cores of subsea permafrost from Laptev Sea. One major publication (Shakhova et al., 2017, Nature Comm.) provided true breakthrough: The thermal state of the subsea permafrost is now near 0degC, which is about 10degC warmer than neighboring permafrost cores on land – this is likely due to the warming of overlying seawater. Comparison with the thermal state of cores recovered by the Soviets in the 1970s at same location, we documented that the thaw horizon of the subsea permafrost was lowered by 4-5 m, corresponding to an average thaw-out of 14±3 cm/yr; this is an order of magnitude more rapid thawing than for land-based permafrost.
Objective Shelf-2
There is a large void of data on the composition of the subsea permafrost system. Our 2017 Nature Comm. study provided some first data on physical properties. A follow-up study in the ERC project documenting the composition around the subsea permafrost thaw horizon and its propensity to be converted microbially to methane is currently in re-review in a high-impact journal.

Objective Shelf-3
Samples for triple-isotope analysis of methane (dD, d13C, D14C) were obtained from the East Siberian Arctic Sea in 2014, 2018 and 2019. A method was advanced that enbabled high-precision triple isotope measurements of methane in seawater. The first study, from methane hotspots in Laptev Sea, using the triple-isotope approach revealed that a major source was a deep thermogenic pool of methane penetrating up through the subsea permafrost.

Objective Slope-1
The Arctic shelf slopes are believed to hold extensive marine hydrates, which may be vulnerable to destabilization through warming by the intermediate Atlantic Water mass - yet there are to date no published observations on this. We processed data from earlier expeditions and found evidence for methane releases from slope hydrates at some 3-4 transects (ms in prep.)

The CC-TOP project produced massive progress on most objectives . Some of the key breakthroughs were:

• The first quantitative estimate for the fluvial remobilization of organic carbon specifically from permafrost and peat deposits across the Eurasian Arctic (Wild et al., PNAS 2019): The long-term 14C results are the first ones to pinpoint how large a fraction of the total river carbon is really coming from peat/permafrost; surprisingly, only a small portion of DOC was from permafrost and peat, whereas about half of the POC originated from these sources with clear spatial trends across the Eurasian-Arctic climosequence. This is an essential component for meaningful predictions of the Arctic permafrost/peat carbon-climate feedbacks.

• The first quantitative constraint on cross-shelf transport (and degradation) time (Bröder et al., 2018 Nature Comm.): This allows deriving the expected degradation flux in this receptor 1000s of km away from the original point of permafrost thaw.

• The first observation-based estimate on the rate of thawing of subsea permafrost (Shakhova et al., 2017 Nature Comm.): The studies of the 20-60 m subsea permafrost drill cores revealed that this elusive Arctic cryosphere has recently reached a thermal state near or at the point of thawing. The thermal state in the top tens of meters is about 10 degC warmer than nearby land-based permafrost. The rate of lowering of the permafrost table (the interface between still frozen and thawed matter) is at least ten times faster than for surface permafrost on land. This CC-TOP finding has large repercussions on predictions of how methane may be released from vulnerable subsea permafrost.

• Discovery of massive permafrost-carbon remobilization during earlier geological periods of sudden warming and sea-level rise (e.g. Nature Comm., 2016, Science Advances, 2020): This includes the first paper to observationally document that there were massive release pulses of carbon from permafrost deposits during the three most recent sudden warming periods found in the Greenland Ice core record. By inferences, this likely contributed to the rapid rise in atmospheric CO2 during the sudden warming pulses in the transition from the ice age into the Holocene. The findings gives an idea of what may be in store now wrt cryosphere-carbon-climate feedbacks.

• Development and application of unique triple-isotope fingerprinting to pin down the relative importance of several potential sources of the massive releases of methane on the outer East Siberian Arctic shelf (PNAS 2021): It strongly points to a dominant role for deep thermogenic pools of methane penetrating through the subsea permafrost system.

A figure showing the CC TOP Study System and WPs

Periodic Reporting for period 4 - CC-TOP (Cryosphere-Carbon on Top of the Earth (CC-Top):Decreasing Uncertainties of Thawing Permafrost and Collapsing Methane Hydrates in the Arctic)

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