POC export fluxes in the Arabian Sea and the Bay of Bengal: A simultaneous 234Th/238U and 210Po/210Pb study

Abstract Seasonal variation in POC export flux, from the upper ocean, was studied during post southwest monsoon (October–November 2013) and spring intermonsoon (May 2014) at selected locations in the central and southern Arabian Sea and during northeast monsoon (January 2014) and spring intermonsoon (March 2014) in the northern Bay of Bengal. POC fluxes estimated from two particle-reactive radionuclide flux proxies ( 234 Th/ 238 U and 210 Po/ 210 Pb) were compared. Spatial and temporal variations were observed in the POC/radionuclide ratios, net deficit flux of radionuclides and the estimated POC export fluxes in both the basins. During post southwest monsoon, the 234 Th flux (1346 ± 101 dpm m − 2  d − 1 ), POC/ 234 Th ratio (5.96 ± 0.29 μmol dpm − 1 ) and associated POC flux (8.0 ± 0.6 mmol m − 2  d − 1 ) were high in the southern Arabian Sea than those in the central Arabian Sea (450 ± 95 dpm m − 2  d − 1 , 1.76 ± 0.07 μmol dpm − 1 and 0.8 ± 0.2 mmol m − 2  d − 1 , respectively). However, during spring intermonsoon these trends were similar in both the central and southern Arabian Sea with mean 234 Th fluxes, POC/ 234 Th ratios and 234 Th based POC export fluxes being 2762 ± 61 dpm m − 2  d − 1 , 1.45 ± 0.05 μmol dpm − 1 and 4.0 ± 0.14 mmol m − 2  d − 1 , respectively. On the other hand, the 210 Po based POC export fluxes in the southern Arabian Sea (3.37 ± 0.32 and 6.73 ± 0.47 mmol m − 2  d − 1 ) were higher by a factor of 2–3 than those in the central Arabian Sea (2.08 ± 0.16 and 2.12 ± 0.28 mmol m − 2  d − 1 ) during post southwest monsoon and spring intermonsoon, respectively. In the northern Bay of Bengal, the 234 Th based POC export fluxes were similar and low (0.6–1.2 mmol m − 2  d − 1 ) while the 210 Po based POC export fluxes were much higher (3.75–8.48 mmol m − 2  d − 1 ) during both the northeast monsoon and spring intermonsoon seasons. One reason could be that more production and carbon export occurring in months prior to the period preceding sample collection due to weeks to several months difference in the mean-lives of the isotopes. The computed POC export fluxes and inferred efficiency of the biological carbon pump in the Arabian Sea and the northern Bay of Bengal are differentiated by this different span in half-lives of 234 Th and 210 Po (weeks to months, respectively). Also these values are distinct due to their biogeochemical behavior and preferred affinity for specific types of particles. This would require more joint application of each tracer in time and space for various biogeochemical regimes.