Spatial variability of upper ocean POC export in the Bay of Bengal and the Indian Ocean determined using particle‐reactive ²³⁴Th

Abstract

The northern Indian Ocean is globally significant for its seasonally reversing winds, upwelled nutrients, high biological production, and expanding oxygen minimum zones. The region acts as sink and source for atmospheric CO2. However, the efficiency of the biological carbon pump to sequester atmospheric CO2 and export particulate organic carbon from the surface is not well known. To quantify the upper ocean carbon export flux and to estimate the efficiency of biological carbon pump in the Bay of Bengal and the Indian Ocean, seawater profiles of total 234Th were measured from surface to 300 m depth at 13 stations from 19.9°N to 25.3°S in a transect along 87°E, during spring intermonsoon period (March–April 2014). Results showed enhanced in situ primary production in the equatorial Indian Ocean and the central Bay of Bengal and varied from 13.2 to 173.8 mmol C m−2 d−1. POC export flux in this region varied from 0 to 7.7 mmol C m−2 d−1. Though high carbon export flux was found in the equatorial region, remineralization of organic carbon in the surface and subsurface waters considerably reduced organic carbon export in the Bay of Bengal. Annually recurring anticyclonic eddies enhanced organic carbon utilization and heterotrophy. Oxygen minimum zone developed due to stratification and poor ventilation was intensified by subsurface remineralization. 234Th‐based carbon export fluxes were not comparable with empirical statistical model estimates based on primary production and temperature. Region‐specific refinement of model parameters is required to accurately predict POC export fluxes.

Organic Nutrients Support High Primary Production in the Bay of Bengal

“…These authors used the same data set collected during Bay of Bengal process study when only three stations were occupied for primary production in the open sea region. On the other hand, Anand et al (2017) measured primary production in the range of 182 to 1261 (936 ± 350) mgC·m −2 ·d −1 during spring intermonsoon (March-April, 2014) in the BoB and it is consistent with the present estimations (758 ± 220 mgC·m −2 ·d −1 ). On the other hand, the depth-integrated primary production in the upper 100 m ranged between 411 and 920 (mean of 758 ± 220) mgC·m −2 ·d −1 in this study region that is significantly higher than earlier estimates (184 to 512 (344 ± 164) mgC·m −2 ·d −1 ; Gauns et al, 2005) during intermonsoon period and it is even less during other periods.…”

“…On the other hand, the depth-integrated primary production in the upper 100 m ranged between 411 and 920 (mean of 758 ± 220) mgC·m −2 ·d −1 in this study region that is significantly higher than earlier estimates (184 to 512 (344 ± 164) mgC·m −2 ·d −1 ; Gauns et al, 2005) during intermonsoon period and it is even less during other periods. On the other hand, sinking organic carbon fluxes at 100 m are almost equal in the Arabian Sea (22.8 to 84 mgC·m −2 ·d −1 ; Anand et al, 2018) and BoB (3.6 to 93.2 mgC·m −2 ·d −1 ; Anand et al, 2017) that further suggest that the primary production in the BoB was underestimated due to lack of enough coverage as measurements were conducted only at three stations during each season (Gauns et al, 2005). Recent primary production results from BoB (Anand et al, 2017 and this study) are comparable to that of the Arabian Sea (1,032 ± 260 mgC·m −2 ·d −1 , Barber et al, 2001; 924 ± 296 mgC·m −2 ·d −1 , Dickson et al, 2001; and 1752 ± 726 mgC·m −2 ·d −1 , Anand et al, 2018).…”

“…In contrast, the sinking organic carbon flux to bottom (~3,000 m) in the BoB (Ittekkot et al, 1991) is more than that in highly productive AS (Honjo et al, 1999;Nair et al, 1989). Recently, it was observed that sinking organic carbon fluxes at 100 m were higher in the AS (median: 39.0 [range: 22.8 to 84] mgC·m −2 ·d −1 , Anand et al, 2018) than in the BoB (median: 8.4 [range: 3.6 to 92.4] mgC·m −2 d −1 , Anand et al, 2017). Such low export production in the BoB is attributed to high utilization of organic matter by heterotrophs in the upper 100 m (Anand et al, 2017).…”

An investigation of basin-scale controls on upper ocean export and remineralization

“…The 234 Th and 238 U profiles resembled those from other areas of the Pacific Maiti et al, 2010) and these more typical 234 Th profile sequences (deficit-excess-equilibrium) were not observed again along either Arctic transect. This characteristic profile supports the reliability of the method used here, despite differences between the Arctic profiles and those from other ocean basins Buesseler et al, 2005;Subha Anand et al, 2017).…”

Mesoscale contribution to the long-range offshore transport of organic carbon from the Canary Upwelling System to the open North Atlantic

“…Third, the relevant share of C org found in the offshore region within mesoscale eddies, which are mostly laterally isolated structures (Chelton et al, Karstensen et al, 2017;Stramma et al, 2013), also tells us that a fraction of the offshore biological activity is fueled discontinuously. In particular, both the transit of an eddy, which is associated with enhanced vertical export (Subha Anand et al, 2017;Waite et al, 2016), and the death of an eddy provide a discontinuous, but substantial, input of carbon for the oligotrophic waters.…”