# Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate dissociation during the last glacial episode

T De Garidel-Thoron, L. Beaufort, F Bassinot, P Henry

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T De Garidel-Thoron, L. Beaufort, F Bassinot, P Henry. Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate dissociation during the last glacial episode. Proceedings of the National Academy of Sciences of the United States of America, 2004, 101 (25), pp.9187-9192. 10.1073/pnas.0402909101. hal-01460380

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# Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate dissociation during the last glacial episode

Thibault de Garidel-Thoron*†‡, Luc Beaufort*, Franck Bassinot§, and Pierre Henry¶

*Centre Européen de Recherche et d'Enseignement en Géosciences de l'Environnement/Centre National de la Recherche Scientifique/Université Aix-Marseille 3, B.P. 80, 13545 Aix-en-Provence Cedex 4, France; †Institute of Marine and Coastal Sciences, Rutgers, The State University of New Jersey, 71 Dudley Road, New Brunswick, NJ 08901; §Laboratoire des Sciences du Climat et de l'Environnement, Domaine du Centre National de la Recherche Scientifique, 91198 Gif/Yvette, France; and ¶Collège de France/Centre National de la Recherche Scientifique, 13545 Aix-en-Provence, France

Communicated by James P. Kennett, University of California, Santa Barbara, CA, April 25, 2004 (received for review February 23, 2004)

Past atmospheric methane-concentration oscillations recorded in polar ice cores vary together with rapid global climatic changes during the last glacial episode. In the "clathrate gun hypothesis," massive releases of deep-sea methane from marine gas-hydrate dissociation led to these well known, global, abrupt warnings in the past. If evidence for such releases in the water column exists, however, the mechanism and eventual transfer to the atmosphere has not yet been documented clearly. Here we describe a high-resolution marine-sediment record of stable carbon isotopic changes from the Papua Gulf, off Papua New Guinea, which exhibits two extremely depleted excursions (down to -9‰) at ≈39,000 and ≈55,000 years. Morphological, isotopic, and trace metal evidence dismisses authigenic calcite as the main source of depleted carbon. Massive methane release associated with deep-sea gas-hydrate dissociation is the most likely cause for such large depletions of δ¹³C. The absence of a δ¹³C gradient in the water column during these events implies that the methane rose through the entire water column, reaching the sea-air interface and thus the atmosphere. Foraminiferal δ¹⁸O composition suggests that the rise of the methane in the water column created an upwelling flow. These inferred emission events suggest that during the last glacial episode, this process was likely widespread, including tropical regions. Thus, the release of methane from the ocean floor into the atmosphere cannot be dismissed as a strong positive feedback in climate dynamics processes.

During the last glacial episode, rapid climatic changes recorded in ice-core records are closely associated with variations in concentration of atmospheric methane, a powerful greenhouse gas, showing a maximum amplitude of ≈200 parts per billion by volume (1). Until recently, the main source of atmospheric methane during the last glacial episode was attributed to low-latitude continental wetlands emissions (2, 3), whereas the potential role of deep-sea and lake-gas hydrates was largely ignored. The accepted hypothesis of a strong link between climate and methane emissions from wetlands was called into question recently by an exhaustive review of the timing and extension in the geological record of these wetlands (4), as well as by numerical simulations of the global carbon cycle (5). Finally, two independent geochemical proxies, the isotopic composition of foraminifera (6) and the molecular composition of the organic matter (7), shed light on the potential role of large methane releases from gas-hydrate dissociation as an important source in the oceanic global carbon cycle. These two records from the Santa Barbara Basin have shown that these inferred methane releases occurred during the last glacial episode in response to a warming of the intermediate waters and thus presumably of the deep-sea sediments. These deep-sea methane emissions occurred synchronously with rapid climate warnings associated with atmospheric methane increases and led Kennett et al. (6) to propose the "clathrate gun hypothesis," which postulates that deep-sea methane hydrates played a significant

role in late quaternary climate changes (4). Thus far, evidence for such methane releases in the glacial world is limited to a few regions (8, 9), and the mechanism by which the methane passed through the water column, and eventually to the atmosphere, has not yet been demonstrated. Here we document evidence that two massive methane releases from clathrate dissociation, of amplitude two to three times larger than those inferred in the Santa Barbara Basin, occurred during the last glacial episode in the western Pacific.

## Materials and Methods

We studied the IMAGES MD97-2134 piston core located in the Papua Gulf (depth of 760 m, lat 9°54′S, long 144°39′E), ≈150 km off the Fly River delta (Fig. 1). High terrigenous inputs from Papua New Guinea lead to high sedimentation rates of ≈25 cm-(thousand years)⁻¹ (ka) during the last glacial episode (Fig. 2). The core contains dark argillaceous muds with abundant coccoliths, foraminifera, and pteropods. Isotope measurements were conducted at Laboratoire des Sciences du Climat et de l'Environnement (Centre National de la Recherche Scientifique-Commissariat à l'Energie Atomique, Gif-sur-Yvette, France) by using a Kiel device coupled to a Finnigan MAT 251 mass spectrometer. The external precision was 0.05‰ for δ¹⁸O and 0.03‰ for δ¹³C analysis as determined by repeated measurements of a standard carbonate material, NBS19. Approximately 10–15 foraminifera shells were used for each isotopic measurement. Mg/Ca measurements were conducted by using a Rutgers Varian inductively coupled plasma optical emission spectrometer following the analytical protocol detailed by Rosenthal et al. (10). The chronostratigraphy was established by using 13 accelerator mass spectrometer ¹⁴C ages on the planktonic foraminifer *Globigerinoides ruber*. To account for past changes in ¹⁴C production, these ages were converted to calendar ages assuming 400 years of reservoir age and applying the INTCAL98 calibration curve for ages up to 24 ka (11) and a polynomial calibration based on coupled ¹⁴C/U/Th ages in corals for ages >24 ka (12). The Laschamp excursion (1,050 cm) is well marked in the magnetic record (C. L. Blanchet, N. Thouveny, and T.d.G.-T., unpublished results) and provides a chronological datum in marine isotope stage (MIS) 3 at 40 ka (13). The similarity of MD97-2134 paleointensity record with paleomagnetic reference stacks indicates that no hiatus interrupts the sediment record during the last 60 ky. The MIS 4/3 boundary in the oxygen isotope record at 1,600 cm was also tuned to the SPECMAP reference curve (14). The uncertainty of this age model is ≈2–3 ky.

Abbreviations: ka, thousand years; MIS, marine isotope stage; DIC, dissolved inorganic carbon.

†To whom correspondence should be addressed. E-mail: garidel@imcs.rutgers.edu.

© 2004 by The National Academy of Sciences of the USA
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Map location of the MD97-2134 core in the Papua Gulf. The black circles indicate the position of known bottom-simulating reflectors indicative of deep-sea gas hydrates (36, 37). The shaded rectangle highlights the area taken into account for methane-release estimates.</figcaption></figure>

## Results

We document past changes in surface hydrography by using stable isotopes measured in the calcite shells of the planktonic

foraminifera *G. ruber*, a species living primarily in the mixed layer (Fig. 2). The $\delta^{18}\text{O}$ record displays global glacial/interglacial changes associated with the growth and decay of the polar ice sheets. The $\delta^{13}\text{C}$ record of *G. ruber* exhibits glacial-interglacial changes of ≈1.2–1.5‰, reflecting changes in the surface productivity. However, two large anomalous negative excursions in this record reach –8.95‰ ($\Delta = -9‰$ from the background) and –5.62‰ ($\Delta = -5.7‰$) at 39 and 55 ka, respectively (Fig. 3). Associated with the negative $\delta^{13}\text{C}$ excursions, the $\delta^{18}\text{O}$ of *G. ruber* shows strong positive anomalies reaching –0.45‰ ($\Delta = +1.05‰$ from the background) and –0.6‰ ($\Delta = +0.7‰$ from the background) at 39 and 55 ka, respectively. Due to the thickness of the 39-ka excursion (25 cm), bioturbation has no significant dilution effect on its amplitude but may induce a smoothing on the 55-ka event (10 cm thick). To examine whether the entire water column was affected during these events, we analyzed stable isotopes in two other foraminiferal species: *Globorotalia truncatulinoides*, a deep-dweller planktonic species living close to the thermocline, and a benthic species, *Uvigerina sp.* (Fig. 3). The deep-dwelling planktonic species exhibits $\delta^{13}\text{C}$ anomalies at 39 ka reaching –5.8‰ ($\Delta = -5.4‰$) and at 55 ka reaching –3.12‰ ($\Delta = -2.6‰$) (Table 1). Likewise, the $\delta^{13}\text{C}$ of benthic foraminifera decreases to –9.4‰ ($\Delta = -8.5‰$) at 39 ka and to –6.22‰ ($\Delta = -5.2‰$) at 55 ka. These negative isotopic anomalies were found in replicate analyses of the foraminifera.

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Comparison of Greenland ice-core record of $\delta^{18}\text{O}$ and atmospheric methane concentration with foraminiferal $\delta^{18}\text{O}$ and $\delta^{13}\text{C}$ time series from the MD97-2134 sediment core located in the Papua Gulf. (a) Greenland Ice Sheet Project 2 (GISP2) ice-core record of $\delta^{18}\text{O}$ spanning the last 17 interstadials (38). (b) Atmospheric methane concentration (35). (c) MD97-2134 age-model tie points: circles indicate radiocarbon-calibrated dates, the star refers to the Laschamp paleomagnetic excursion, and the diamond refers to the MIS 4/3 boundary tuned to the SPECMAP stack. Records of $\delta^{18}\text{O}$ (d) and $\delta^{13}\text{C}$ (e) isotopic composition of *G. ruber* in core MD97-2134 are shown also. ppbv, parts per billion by volume; VPDB, Vienna Pee Dee Bee.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 3.** δ¹⁸O and δ¹³C isotopic records at two levels in the MD97-2134 core for three foraminiferal species: *G. ruber* (shallow-dwelling species), *G. truncatulinoides* (deep-dwelling species), and *Uvigerina sp.* (benthic species). δ¹³C values for *P. obliquiloculata* (open circle) and *N. dutertrei* (filled diamond), both subsurface planktonic-dwelling species, are shown. VPDB, Vienna Pee Dee Bee.</figcaption></figure>

Discussion

We examine different hypotheses that might explain the large negative δ¹³C anomalies observed in our record. Foraminifera δ¹³C is a function of the δ¹³C of dissolved inorganic carbon (DIC), air-sea exchange rate, and temperature. The latter two factors have only a minor influence on δ¹³C (15). First, we exclude the hypothesis of a change in the biological productivity, because the observed range in the modern and past foraminifera composition δ¹³C spans 0 to -2‰ (16). Four other ¹³C-depleted sources have been invoked to explain past excursions in the DIC isotopic composition. (i) Volcanic and hydrothermal activities release large amounts of CO₂ with δ¹³C of approximately -5‰ (17). This value, however, is too positive to explain the carbon isotope excursions seen in our record. (ii) There is exceptional freshwater flow from the Fly River with δ¹³C_DIC of approximately -10‰. Three arguments rule out this hypothesis. First, atmosphere-river CO₂ exchange tends to modify the DIC isotopic composition toward the equilibrium value of δ¹³C_DIC ≈ 1‰ (18). The “flow hypothesis” would also imply that the isotopic composition of the 700-m-thick water column reflects only the riverine pool, with no influence from the seawater pool. Moreover, the Fly River flow scenario is not supported by the positive ≈0.8‰ peak occurring simultaneously in the planktonic δ¹⁸O records. This shift corresponds to a minimal 2°C cooling and/or an increase in δ¹⁸O of seawater, whereas a massive increase of riverine input from the Fly River should have caused a negative δ¹⁸O excursion. (iii) Oxidation of the low δ¹³C

methane produced during the burial of organic matter (19), or enhanced carbon rain and carbon oxidation in the sediments (20), can also modify the surficial sediment pore water δ¹³C gradient. Although benthic infaunal foraminifera might record this steeper δ¹³C gradient, this process cannot have lowered simultaneously the surface ocean δ¹³C of DIC and thus the isotopic composition of planktonic shells. (iv) Finally, large methane hydrate releases with an average δ¹³C value of approximately -65‰ can be invoked to explain the amplitude of the carbon excursion (21). Bottom-simulating reflectors, which are associated with gas hydrate occurrences in deep-sea sediments, have been described in western Pacific marginal seas, although as yet not in the Gulf of Papua (Fig. 1). They suggest that conditions conducive to the formation of gas hydrates can occur on the tropical western Pacific margins.

We consider unlikely the possibility of significant diagenetic recrystallization in the sediment from pore waters isotopically modified by the *in situ* dissociation of methane gas hydrates, because such events are associated with strong disturbance of the sediments, which are lacking in core MD97-2134. The source of inferred methane releases during these events is thus not at the location of the core but was likely close to the site. This conclusion is supported by the depth of the MD97-2134 core, at which such releases can potentially occur, and because benthic foraminifera δ¹³C records these events. A species-dependent mass-balanced recrystallization of the calcite from pore waters linked to *in situ* gas-hydrate dissociation also cannot account for

<table><caption>**Table 1.** Isotopic values and Mg/Ca ratio for the different species analyzed in the δ¹³C minimum levels recorded in the MD97-2134 core</caption><thead><tr><th>Event age,<br>ka B.P.</th><th>Depth, cm</th><th>Species</th><th>δ¹⁸O,<br>‰ VPDB</th><th>δ¹³C,<br>‰ VPDB</th><th>Mg/Ca,<br>mmol/mol</th></tr></thead><tbody><tr><td rowspan="6">39</td><td>970</td><td>G. ruber</td><td>-0.54</td><td>-8.95</td><td></td></tr><tr><td>968</td><td>Pulleniatina obliquiloculata</td><td>0.08</td><td>-3.11</td><td>3.27</td></tr><tr><td>968</td><td>Neogloboquadrina dutertrei</td><td>-0.02</td><td>-3.52</td><td>2.59</td></tr><tr><td>972</td><td>G. truncatulinoides</td><td>1.59</td><td>-5.80</td><td></td></tr><tr><td>970</td><td>Uvigerina sp.</td><td>3.65</td><td>-9.34</td><td></td></tr><tr><td>1,530</td><td>G. ruber</td><td>-0.56</td><td>-5.62</td><td></td></tr><tr><td rowspan="3">55</td><td>1,530</td><td>G. truncatulinoides</td><td>1.39</td><td>-1.23</td><td></td></tr><tr><td>1,530</td><td>Uvigerina sp.</td><td>3.70</td><td>-6.24</td><td></td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>**Fig. 4.** Scanning electron microscope pictures of well preserved planktonic foraminifera from the ¹³C minimum level at 968 cm in sediment core MD97-2134. *G. ruber* umbilical (A) and extraumbilical (B and C) views by scanning electron microscopy at 968 cm in core MD97-2134, in the δ¹³C minimum. At higher resolution, the specimen (C) exhibits a typical external porosity (D). Calcite layers and inner porosity are also well preserved (F) of the crushed *G. ruber* (E). (Scale bars: A–C and E, 100 µm; D and F, 10 µm.)</figcaption></figure>

these excursions. To imprint a −8.5‰ excursion in the benthic foraminifer calcite δ¹³C and of −9‰ in the surface-dwelling planktonic foraminifera requires that a similar amount (≈12–15%) of the calcite in both taxa come from the δ¹³C hydrate pool of approximately −65‰. This percentage cannot be reconciled with the oxygen isotopic data that imply a differential species-dependant recrystallization of 60% and 25%, respectively, for these two taxa. For this calculation, we assume that the δ¹⁸O of the pore waters was of ≈1.4‰, accounting for (i) the ≈1‰ enrichment due to the glacial polar ice sheet that trapped depleted ¹⁸O (22) and (ii) an ≈0.4‰ increase linked to the gas-hydrate fractionation process (23).

The original ultrastructures of foraminiferal tests are well preserved across the negative carbon isotopic excursions, as revealed by visual observation and scanning electron microscope observations in the δ¹³C minimum at 968 cm (Fig. 4). At this level, inner and outer sectors of pores are well connected, exhibiting similar patterns to modern forms (24) and to another core level (945 cm) with normal δ¹³C values. The possible presence of diagenetic calcite that might not be visible in scanning electron microscope investigation has been evaluated further by determining Mg/Ca on two deep-dwelling foraminifera species: *Neogloboquadrina dutertrei* and *Pulleniatina obliquiloculata*. Mg/Ca in foraminiferal tests has been shown to be a sensitive marker of diagenetic alteration, with values up to 150–250 mmol/mol depending on the amount of authigenic calcite (25, 26). The paucity of *G. ruber* across the negative carbon excursion prevented trace metal analysis on this species. The δ¹³C composition of *N. dutertrei* and *P. obliquiloculata* are consistently depleted with values of −3.5‰ and −3.1‰, respectively, at 968 cm. At this level, preliminary analyses in inductively coupled plasma optical emission spectrometry of Mg/Ca indicate a value of 2.57 mmol/mol for *N. dutertrei* and 3.18 mmol/mol for *P. obliquiloculata*. These values are in the same range as the background values measured at 1,015 cm, with 2.69 mmol/mol for *N. dutertrei* and ≈2.34 mmol/mol in *P. obliquiloculata*, although slightly higher for the latter. This approach confirms that the existence of a ¹³C-depleted and

Mg/Ca-enriched crust is minimal in the interval studied. The lower carbon and higher oxygen isotopic values in the various foraminiferal taxa cannot be accounted for by authigenic recrystallization. These isotopic anomalies must reflect changes in seawater chemistry in agreement with the combined isotopic and molecular evidence previously shown for the Santa Barbara Basin (7).

The stability of gas hydrates varies with temperature and pressure (27). During MIS 3, sea level was lower by ≈80 m (28), reducing the hydrostatic pressure and enhancing the probability of hydrate dissociations (29). However, abrupt changes in sea level did not exceed ≈15-m amplitude and are unlikely, therefore, to have triggered methane releases (28). The largest excursion at 39 ka is marked by two successive steps. First, a slight δ¹⁸O decrease in benthic foraminifera, which might indicate a warming (≈1°C) of deep-sea waters preceding the inferred hydrate dissociation event. Although earthquakes or tsunamis might also have triggered dissociation of hydrates, this slight increase in temperature is in agreement with recent experimental studies emphasizing the critical role of temperature in the dissociation of hydrates (e.g., ref. 30). Second, this trend is interrupted by a significant 0.4‰ increase of the oxygen isotopic signal, in phase with the carbon isotopic anomaly (Fig. 3). That latter increase in δ¹⁸O is similar to the interstitial water anomaly observed during gas-hydrate dissociation on Blake Ridge (23) and in agreement with the shallow infaunal habitat of *Uvigerina sp.* The largest δ¹⁸O increase in the surface waters (Δ δ¹⁸O ≈ +1‰ in *G. ruber*) is by far too large to be attributed to the same process. This enrichment could be interpreted as a local cooling in upper surface waters (≈3–4°C), resulting from the mixing of the water column through upwelling of cold deep waters induced by the thermal dissociation of gas hydrates. The chronological succession is less clear for the 55-ka excursion. The lower sedimentation rate during this interval and thus a stronger effect of the bioturbation is likely to have removed small-scale details.

The amplitude of the benthic carbon isotopic excursions associated with warm intervals during MIS 3 observed in the Santa Barbara Basin is lower than the amplitude of the anom-
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