Methane dynamics in the southeastern Arabian Sea: Patterns of distribution and annual emission estimates

Abstract

The production pathways and sea-to-air fluxes of climate-relevant trace gases, including methane (CH₄), in the southeastern Arabian Sea (SEAS), are poorly addressed and suffer significant uncertainties due to their spatial and seasonal heterogeneity. We measured CH₄ in five coastal-oceanic transects along the SEAS during the southwest (SWM) and northeast monsoon (NEM) seasons to understand the distribution and sea-to-air fluxes.CH₄ concentrations ranged from 3 to 151 nM (13 ± 16 nM) in SWM and from 1 to 140 nM (16 ± 38 nM) in NEM, decreasing in a gradient towards the offshore, which indicated that freshwater influx and stratification were key processes controlling the CH₄ distribution. CH₄ concentrations were positively correlated with chlorophyll-a and turbidity during the SWM, indicating anaerobic methanogenesis via micro-niches. Although globally hypoxic zones are considered a significant source of CH₄, the SEAS exhibited relatively low CH₄ levels under hypoxia, possibly due to the low organic carbon (<5 %) in sediments. The sea-to-air CH₄ fluxes in the SEAS ranged from 0.1 to 278 μM m⁻² day⁻¹, and the estimated annual CH₄ emissions from the SEAS (∼0.006 Tg yr⁻¹) align with previous estimates. The global warming potential during SWM and NEM was 0.21 ± 0.45 and 0.6 ± 1.7 g CO₂ eq m⁻² day⁻¹, with net annual emissions of 0.18 Tg CO₂ yr⁻¹. The present study highlights the significance of SEAS's role as a hotspot for CH₄ release, emphasising that accurate and high-resolution measurements of climate-relevant trace gases from the marine environment are essential for improving future climate projections, global greenhouse gas budgets, and the development of mitigation strategies to achieve future climate goals.