Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød - PubMed (original) (raw)

Permafrost thawing as a possible source of abrupt carbon release at the onset of the Bølling/Allerød

Peter Köhler et al. Nat Commun. 2014.

Abstract

One of the most abrupt and yet unexplained past rises in atmospheric CO2 (>10 p.p.m.v. in two centuries) occurred in quasi-synchrony with abrupt northern hemispheric warming into the Bølling/Allerød, ~14,600 years ago. Here we use a U/Th-dated record of atmospheric Δ(14)C from Tahiti corals to provide an independent and precise age control for this CO2 rise. We also use model simulations to show that the release of old (nearly (14)C-free) carbon can explain these changes in CO2 and Δ(14)C. The Δ(14)C record provides an independent constraint on the amount of carbon released (~125 Pg C). We suggest, in line with observations of atmospheric CH4 and terrigenous biomarkers, that thawing permafrost in high northern latitudes could have been the source of carbon, possibly with contribution from flooding of the Siberian continental shelf during meltwater pulse 1A. Our findings highlight the potential of the permafrost carbon reservoir to modulate abrupt climate changes via greenhouse-gas feedbacks.

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Figures

Figure 1

Figure 1. Relevant ice core and Δ14C data during Termination I.

Atmospheric Δ14C based on Tahiti corals (magenta circles, mean ±1_σ_) or IntCal13 (grey area, ±1_σ_ uncertainty band around the mean), the latter including linear trends with −0.04‰ per year or −0.10‰ per year; CO2 from EDC (blue filled circles); CH4 from EDC (blue) and Greenland (red); WAIS Divide (WD) _δ_18O (original (blue) and 100 years running mean (yellow)); stack of calculated temperature change ΔTANT from the five East Antarctic ice cores EDC, EPICA DML, Vostok, Dome Fuji and Talos Dome (original (black) and 100 years running mean (orange)); NGRIP_δ_18O. All Greenland records on GICC05 (ref. 23), all EDC records on AICC2012 chronology, WD and ΔTANT on their own independent chronology.

Figure 2

Figure 2. Data analysis of atmospheric Δ14C around 14.6 kyr BP.

Atmospheric Δ14C based on Tahiti corals (magenta circles) and IntCal13 (grey line, ±1_σ_ uncertainty band around the mean) are analysed for trends and compared with various other archives (speleothems from Bahamas (blue diamonds), Hulu Cave (dark green squares), marine sediments in Cariaco (light green triangles) plotted on revised Hulu2 age model, Lake Suigetsu (orange open squares)). All individual data points are plotted with ±1_σ_ in both age and Δ14C. IntCal13 was approximated by a linear trend with either −0.04‰ per year (black solid line) or −0.10‰ per year (black dashed line). Tahiti data were analysed for break points with two different models (see Methods). For the linear model (red lines), a statistical package was used. For the non-linear (NL) model (cyan lines), two data points at beginning of the Tahiti data Δ14C anomaly from the IntCal13 data and the eight points around the local minimum (black open circles) were averaged, plotted with ±1_σ_ in both age and Δ14C (bold large black open circles) and further analysed. The anomaly in the Tahiti Δ14C data following the linear model is Δ(Δ14C)=−54±8‰ in Δ(age)=207±95 years and following the NL model: Δ(Δ14C)= −58±14‰ in Δ(age)=258±53 years.

Figure 3

Figure 3. Ice core and simulated true atmospheric CO2 and _δ_13CO2.

(a) Ice core CO2 data (±1_σ_) from EDC on two different chronologies AICC2012 and Parrenin2013, Taylor Dome on revised age model, Siple Dome on own age model (top x axis), Byrd on age model GICC05 (refs 25, 28). Simulated true atmospheric CO2 in our best-guess scenario according to 14C data (black bold line), filtered to a signal that might be recorded in EDC (blue dashed line), shifted by 50 years to meet the EDC data (dashed red line). (b) Ice core δ_13CO2 data (±1_σ) from EDC, simulated true atmospheric _δ_13CO2 of our best-guess scenario and how it would have been recorded in EDC for either terrestrial or marine origin of the released carbon, implying a _δ_13C signature of −22.5 or −8.5‰, respectively.

Figure 4

Figure 4. Main carbon cycle simulation results.

The transient simulation results (left) showing the impact of a carbon release event on true atmospheric Δ14C and CO2 obtained with the carbon cycle model BICYCLE for the best-guess scenario are compared with the data. In sensitivity studies (right), the length of the release event and the radiocarbon signature Δ(Δ14C) of the released carbon are constrained by the data. (a) Atmospheric Δ14C data from Tahiti corals (magenta, mean ±1_σ_ in both age and Δ14C) and IntCal13 (ref. 9) (grey band, mean ±1_σ_) data. Black bold circles denote start and stop (±1_σ_) of carbon release in the non-linear model of the Tahiti data interpretation. The vertical black dashed line marks the estimated started of carbon release at 14.6 kyr BP based on a combination of different explanations. Best-guess simulation results of atmospheric Δ14C (blue) superimposed by a linear trend of either −0.04‰ per year (long dashed line) or −0.10‰ per year (solid line) (short dashed: no trend superimposed). (b) Atmospheric CO2. EDC ice core CO2 data (mean ±1_σ_) on two different chronologies AICC2012 and Parrenin2013. Simulated true atmospheric CO2 rise (black bold line), and how the signal might be recorded in EDC (dashed red line) after filtering for gas enclosure and shifted by 50 years to meet the data. (c) Simulated peak height in atmospheric Δ14C (grey areas) as function of length of carbon release and of the Δ14C depletion. (d) Simulated peak height in atmospheric CO2 (dark blue area) as function of length of carbon release. In c,d, simulations result with the AMOC in either a weak or a strong mode are combined spanning a range of results. Magenta square and circle in c,d mark results of our best-guess scenario for Δ14C and CO2, respectively. We colour coded the areas in the parameter space where simulation results agree with the EDC CO2 data (d, light blue) and with the interpretation of the Tahiti Δ14C data (c, black boxes). The latter are modified for background linear trends already contained in IntCal13 based on other processes.

Figure 5

Figure 5. Implication on the timing of abrupt climate change as obtained in various ice core records from Greenland and Antarctica.

Our results suggest that anomalies in Tahiti Δ14C and true atmospheric CO2 are caused by the same process. This information is used here as an independent age constraint. The onset of the abrupt rise in atmospheric CO2 (black bold, this study) is thus tied to 14.6 kyr BP. From previous ice core analysis, it is known that the rise in CO2 and CH4 (red circles, Greenland composite69) occur synchronously here. A new study on the NEEM ice core tied the CH4 rise to be near synchronous to Greenland temperature rise. This synchronicity of the start of the abrupt changes in atmospheric CO2, CH4 and Greenland temperature tied to 14.6 kyr BP led to the age alignments in CH4 and NGRIP _δ_18O (high (black thin line) and low (red line) resolution). We furthermore show some Antarctic climate records on their own independent chronologies to illustrate the temporal north–south offsets. WD_δ_18O, original (blue) and 100 years running mean (yellow) and stack of temperature change from five ice cores in East Antarctica, Δ_T_ANT, original (black) and 100 years running mean (orange).

Figure 6

Figure 6. Radiocarbon depletion of soil carbon of different age and high northern latitude climate change.

(a) Atmospheric Δ14C based on IntCal13 (ref. 9) over the last 50 kyr (blue, left y axis, mean ±1_σ_). Calculated radiocarbon depletion resulting in Δ(Δ14C) (mean ±1_σ_) of soil carbon released during the B/A as a function of its age (magenta, right y axis, upper x axis) and of atmospheric Δ14C during time of production. (b) NGRIP temperature reconstruction from 120 to 10 kyr BP. The time series is plotted in two different colours because of the break in the x axis scale at 50 kyr BP. Numbers label selected D/O events. Red labelled arrows highlight the time which past since NGRIP was similar as warm as during the B/A (32 kyr since D/O event 12) and since the previous significant warming before the B/A (13 kyr since D/O 3).

Figure 7

Figure 7. PMIP3 simulation results on the LGM permafrost extend.

Results show a polar projection of the NH from 20 °N northwards, are based on soil temperature and distinguish land with ice (dark blue), permafrost (blue), seasonal frozen (light blue) and not frozen (red). Present day coastlines are sketched in thin black lines. Magenta points mark potential core sites (Siberian Shelf, Black Sea, Caspian Sea, Sea of Okhotsk) from which future 14C measurements on terrigenous material might verify the age of permafrost possible thawed around 14.6 kyr BP (suggested green areas).

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