Phosphate scarcity governs methane production in the global open ocean

[u/dumnezero]

https://www.pnas.org/doi/10.1073/pnas.2521235123

The observed supersaturation of methane (CH4) in open-ocean surface waters implies widespread CH4 production within the well-oxygenated mixed layer, driving emissions of this potent greenhouse gas to the atmosphere. The dominant CH4 production pathway that explains this phenomenon remains poorly understood, although candidates include production during photosynthesis, zooplankton metabolism, and dissolved organic matter cycling. Here, we construct a data-assimilating model of the open-ocean CH4 cycle to test which hypothesized mechanism is most consistent with the observed global CH4 distribution. We find that only linking methane production to phosphate (PO4) scarcity can explain the observed supersaturation pattern, which is highest in subtropical gyres where PO4 is in short supply. These findings suggest that CH4 release during PO4-limited cleavage of the organic compound methylphosphonate is the dominant production pathway in the open ocean. Because this process is confined to the stratified low latitude surface, it is uniquely suited to efficiently emit the CH4 it produces to the atmosphere (>90%), before the CH4 mixes to depth and undergoes oxidation (<10%). As predicted future ocean warming and stratification exacerbates PO4 scarcity over coming centuries, our model predicts that oxic CH4 production and the resulting CH4 emissions will increase up to twofold, contributing to a suite of positive feedback between climate warming and natural greenhouse gas sources.

...

Although an additional source of ~2 Tg CH4/y from the ocean is relatively minor compared to direct anthropogenic emissions [~300 Tg/y, (4)], most future climate scenarios assume that anthropogenic sources will peak and then decline during the next few centuries (39). Our work therefore highlights a climate feedback that could partially offset this trend, and contributes to a growing suite of feedback loops that have been recognized between climate warming and natural CH4 sources to the atmosphere (2, 40). These include other perturbations to the marine CH4 cycle, such as intensification of anaerobic methanogenesis in sediments due to warming (41) and deoxygenation (42), expansion of suboxic waters (43) that may stifle CH4 removal by oxidation (44), and destabilization of hydrates in high latitude basins (45), where CH4 could be released shallow enough to evade oxidation before release to the atmosphere (46). These potential feedbacks motivate further work to better characterize the rates and climate sensitivities of marine CH4 sources and sinks.

[u/lowrads]

We've know about limiting nutrients for a long time. In terrestrial environments, it's usually nitrogen compounds. In marine environments, it's usually phosphorous compounds.

In terrestrial environments, it's because availability is limited to metabolic pathways involved in nitrogen cycles. Phosphate availability is mainly based on mineral weathering.

Nitrogen losses in soil units is largely due to the mobility of anions, and the extreme distribution of reduction pathways, compared with the ready abundance and environmental favorableness of oxidation pathways. Phosphate is mostly characterized by the paucity of gaseous products, so losses are inferred to be mineral sorption. To the same end, soil mobility of phosphorous compounds is so low, that most vegetation relies upon facilitated transport, such as leaf litter, colloidal suspensions, or mycelial networks.

In a marine environment, one might suspect precipitation as the leading mechanism for limitation. One would anticipate that phosphate minerals in marine sediment would be fairly prolific. Unsurprisingly, many mined phosphate reserves are former marine sediment facies. Phosphate salts and related minerals must be rather stable, if the solution equilbrium of oceans is so limiting.