Evidence for massive methane hydrate destabilization during the penultimate interglacial warming

The stability of widespread methane hydrates in shallow subsurface sediments of the marine continental margins is sensitive to temperature increases experienced by upper intermediate waters. Destabilization of methane hydrates and ensuing release of methane would produce climatic feedbacks amplifying and accelerating global warming. Hence, improved assessment of ongoing intermediate water warming is crucially important, especially that resulting from a weakening of Atlantic meridional overturning circulation (AMOC). Our study provides an independent paleoclimatic perspective by reconstructing the thermal structure and imprint of methane oxidation throughout a water column of 1,300 m. We studied a sediment sequence from the eastern equatorial Atlantic (Gulf of Guinea), a region containing abundant shallow subsurface methane hydrates. We focused on the early part of the penultimate interglacial and present a hitherto undocumented and remarkably large intermediate water warming of 6.8 °C in response to a brief episode of meltwater-induced, modest AMOC weakening centered at 126,000 to 125,000 y ago. The warming of intermediate waters to 14 °C significantly exceeds the stability field of methane hydrates. In conjunction with this warming, our study reveals an anomalously low δ¹³C spike throughout the entire water column, recorded as primary signatures in single and pooled shells of multitaxa foraminifers. This extremely negative δ¹³C excursion was almost certainly the result of massive destabilization of methane hydrates. This study documents and connects a sequence of climatic events and climatic feedback processes associated with and triggered by the penultimate climate warming that can serve as a paleoanalog for modern ongoing warming.

Because ocean intermediate waters impinge on marine sediments that often contain potentially unstable shallow subsurface methane hydrates (1–3), better understanding is crucial about the factors that contribute to intermediate water warming and their potential extent, especially in context with ongoing global warming. Simulation studies have suggested warming of intermediate waters has been limited to ∼1.5 °C to 3 °C, and that such warmings were insufficient to significantly affect the stability of shallow subsurface methane hydrates (2–5). However, the magnitude of intermediate water warming can be significantly amplified by meltwater-induced weakening of atmospheric and ocean circulation (6–11), an amplification not considered in the simulations that examined potential gas hydrate destabilization (2–5). A recent simulation study estimates the contribution of weak Atlantic meridional overturning circulation (AMOC) at 0.3 °C to 0.4 °C to the warming of the intermediate waters for a business-as-usual scenario at the end of the 21st century (12), a modest contribution compared to observations in past climate studies (6–11). An accelerated mass loss of the Greenland ice sheet and the associated freshening of subpolar North Atlantic sea surface waters represents a robust proxy of ongoing rapid global warming (13). Causally linked to this freshening is increasing evidence of a steady weakening of the AMOC (14, 15). Warming of the intermediate waters by 3 °C to 5 °C in response to a meltwater-induced AMOC weakening is a robust feature of the last deglacial (6–11). This corresponds to pockmark formations on the ocean floor and extremely negative foraminiferal δ¹³C values in sediment sequences that reflect methane hydrate dissociation in response to intermediate water warming related to meltwater-induced weakening of AMOC and associated changes in atmospheric circulation during the last deglacial (10, 11, 16–18). A sequence of episodic, extremely low foraminiferal δ¹³C values observed in Late Quaternary sediments of Santa Barbara Basin led to the formulation of the “clathrate gun hypothesis” (10, 11). The hypothesis states that episodic warming of intermediate waters during the last glacial and early deglacial led to dissociation of shallow subsurface methane hydrates and release of methane, contributing to the observed increases of atmosphere methane concentrations, further contributing to climatic warming episodes (10, 11). The key findings of our study add to a growing body of observational findings strongly supporting the “clathrate gun hypothesis” (10, 11). The magnitude of intermediate water warming in response to AMOC weakening most likely is critically dependent on the existing mean climatic state. Importantly, the interval we have studied is marked by a mean climate state comparable to future projections of transient global climate warming of 1.3 °C to 3.0 °C (19). Our findings thus provide insights about major meltwater-induced intermediate water warming during warm episodes like the present with the potential to destabilize structural-type methane hydrates.In this study, we focus on the early part of the Eemian interglacial episode (128,000 to 125,000 y before present [ky BP]), the youngest episode when tropical oceans were warmer than the Holocene by up to 2 °C (20–22). We show that the combination of a warm mean climate state and a relatively brief and modest episode of meltwater-induced AMOC weakening produced an exceptionally large intermediate water warming that significantly exceeds the stability field of methane hydrates. Coincident with this warming, we demonstrate that the dissolved inorganic carbon across the entire 1,300-m water column was marked by an anomalously low carbon isotope ratio (¹³C/¹²C) which we interpret to indicate destabilization of shallow subsurface methane hydrates and ensuing methane oxidation.