²³⁴Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean

Abstract

234Th (t1/2 = 24.1 d), present in seawater, is a naturally occurring particle‐reactive radionuclide formed through the radioactive decay of its parent, 238U (t1/2 = 4.47 × 109 years). The 234Th:238U disequilibrium is exploited to quantify fluxes of elements moving out of the euphotic zone by attaching on to sinking particles. Under the Indian GEOTRACES programme, high‐resolution sampling in the upper 300 m depth was carried out at 11 stations in the Arabian Sea and the Indian Ocean during April–May 2014 from 17°N to 16°S along 65°E transect to estimate the 234Th‐based particulate organic carbon (POC) export flux from the upper ocean. Average 234Th fluxes integrated to 100 m depth were 2,612 and 1,968 dpm m−2 d−1 for the Arabian Sea and the Indian Ocean, respectively. The estimated POC export fluxes ranged from negligible to 9.0 mmol m−2 d−1, and the 234Th‐based POC export efficiencies were <2 to 5%. For the same season, the Arabian Sea and the Bay of Bengal showed highly contrasting carbon export trends (mean: 4.0 and 0.8 mmol C m−2 d−1, respectively). The modeled POC export fluxes from in situ and satellite‐derived primary production are higher than the 234Th‐based values for the Laws and Dunne models and are comparable for the Henson model. The modeled POC fluxes that depend on surface temperature and primary production could be further refined for the seasonal cycle in biological productivity and associated differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity, and associated dissolved organic carbon (DOC) export.

Organic Nutrients Support High Primary Production in the Bay of Bengal

“…Therefore, it is possible that data may be biased due to lack of spatial coverage. 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). 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 ).…”

“…Gauns et al (2005) recorded a primary production in the range of 184 to 512 (mean: 344 ± 164) mgC·m −2 ·d −1 , which is lower than that found in recent studies (Anand, et al, 2018;182-1261 [mean: 936 ± 350] mgC·m −2 ·d −1 ). The latter results are on par with that primary production in the AS (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). Though DIN inputs from the rivers were reported to be low as most of them are utilized within estuaries and coastal regions (Krishna et al, 2016;Sarma, Krishna, et al, 2013; the cyclonic eddies and other mixing processes seem to supply required nutrients to the primary producers (Martin & Shaji, 2015;Narvekar & Prasanna Kumar, 2014;Vidya & Prasanna Kumar, 2013).…”

“…As a result, low primary production has been observed throughout the year in the BoB, based on a limited coverage, compared to the adjacent basin, the Arabian Sea (AS, Gauns et al, 2005). Gauns et al (2005) recorded a primary production in the range of 184 to 512 (mean: 344 ± 164) mgC·m −2 ·d −1 , which is lower than that found in recent studies (Anand, et al, 2018;182-1261 [mean: 936 ± 350] mgC·m −2 ·d −1 ). The latter results are on par with that primary production in the AS (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).…”

“…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).…”