The impact of the South-East Madagascar Bloom on the oceanic CO 2 sink

[摘要] We described new sea surface CO 2 observations in the south-westernIndian Ocean obtained in January 2020 when a strong bloom event occurredsouth-east of Madagascar and extended eastward in the oligotrophic IndianOcean subtropical domain. Compared to previous years (1991–2019) we observedvery low f CO 2 and dissolved inorganic carbon concentrations ( C T )in austral summer 2020, indicative of a biologically driven process. In thebloom, the anomaly of f CO 2 and C T reached respectively − 33  µ atm and − 42  µ mol kg −1 , whereas no change is observed foralkalinity ( A T ). In January 2020 we estimated a local maximum ofair–sea CO 2 flux at 27 ∘  S of − 6.9 mmol m −2  d −1  (ocean sink) and − 4.3 mmol m −2  d −1 when averaging the flux in theband 26–30 ∘  S. In the domain 25–30 ∘  S, 50–60 ∘  Ewe estimated that the bloom led to a regional carbon uptake of about − 1 TgC per month in January 2020, whereas this region was previouslyrecognized as an ocean CO 2 source or near equilibrium during thisseason. Using a neural network approach that reconstructs the monthly f CO 2 fields, we estimated that when the bloom was at peak in December2019 the CO 2 sink reached − 3.1 ( ±1.0 ) mmol m −2  d −1 inthe band 25–30 ∘  S; i.e. the model captured the impact of thebloom. Integrated in the domain restricted to 25–30 ∘  S, 50–60 ∘  E, the region was a CO 2 sink in December 2019 of − 0.8 TgC per month compared to a CO 2 source of + 0.12 ( ±0.10 ) TgC per month in December when averaged over the period 1996–2018.Consequently in 2019 this region was a stronger CO 2 annual sink of − 8.8 TgC yr −1 compared to −7.0  ( ±0.5 ) TgC yr −1 averaged over1996–2018. In austral summer 2019–2020, the bloom was likely controlled bya relatively deep mixed-layer depth during the preceding winter (July–September2019) that would supply macro- and/or micro-nutrients such as iron to the surfacelayer to promote the bloom that started in November 2019 in two large ringsin the Madagascar Basin. Based on measurements in January 2020, we observedrelatively high N 2  fixation rates (up to 18 nmol N L −1  d −1 ),suggesting that diazotrophs could play a role in the bloom in the nutrient-depleted waters. The bloom event in austral summer 2020, along with the newcarbonate system observations, represents a benchmark case for complexbiogeochemical model sensitivity studies (including the N 2  fixation processand iron supplies) for a better understanding of the origin and terminationof this still “mysterious” sporadic bloom and its impact on ocean carbonuptake in the future.

[学科分类] 大气科学