Influence of physical and biological processes on the seasonal cycle of biogenic flux in the equatorial Indian Ocean

Abstract

Abstract. Seasonal cycle of biogenic fluxes obtained from sediment trap at two locations 5 • 24 N, 86 • 46 E (southern Bay of Bengal trap; SBBT) and 3 • 34 N, 77 • 46 E (equatorial Indian Ocean trap; EIOT) within the equatorial Indian Ocean (EIO) were examined to understand the factors that control them. The sediment trap data at SBBT was collected for ten years from November 1987 while that at EIOT was for a one year period from January 1996. The characteristic of biogenic flux at SBBT was the strong seasonality with peak flux in August, while lack of seasonality characterised the flux at EIOT. The high chlorophyll biomass at the SBBT during the summer monsoon was supported by a combination of processes such as wind-mixing and advection, both of which supplied new nitrogen to the upper ocean. In contrast, the elevated chlorophyll at EIOT during summer monsoon was supported only by wind mixing. High cell counts of phytoplankton (> 5 µm) at SBBT dominated by diatoms suggest the operation of classical food web and high carbon export. On the contrary, dominance of pico-phytoplankton and one-and-a-half time higher magnitude of micro-zooplankton biomass along with 2-fold lesser meso-zooplankton at EIOT indicated the importance of microbial loop. The substantial decrease in the carbon export at EIOT indicated faster remineralization of photosynthetically produced organic matter.

Secular and orbital-scale variability of equatorial Indian Ocean summer monsoon winds during the late Miocene

“…In Fig. 2 we show data from the deep traps (∼ 3000 m, ∼ 21 d sampling intervals) (Rixen et al, 2019), but seasonal patterns of biogenic particle flux are very similar in the shallow (∼ 1000 m) SBBT traps (Unger and Jennerjahn, 2009;Vidya et al, 2013). Data points show fluxes recorded in individual years (1987-1997, plotted against mid-time for the trap deployment), with monthly averages also shown.…”

“…1c) (Schott et al, 2009;Jensen, 2003). The Southwest Monsoon Current and associated eddies have been shown to increase chlorophyll concentrations and average phytoplankton size along their paths as far east as 88-90 • E, with the current's influence generally restricted to north of 6 • N at this longitude (Jyothibabu et al, 2015;Vinayachandran et al, 2004;Webber et al, 2018).…”

Evidence of Poor Bottom Water Ventilation during LGM in the Equatorial Indian Ocean

“…During winter monsoon, EICC reverses its direction and flows in southwest direction feeding low salinity water to the west flowing northeast monsoon current (NMC), which in turn contributes to northward flowing WICC (Schott and McCreary, 2001). Thus, these currents play a crucial role in nutrient distribution and chlorophyll concentrations in the equatorial regions (Vidya et al, 2013).…”

“…But the eastern Arabian Sea, the southern Bay of Bengal (SBBT), and the equatorial Indian Ocean (EIOT) traps are dominated by carbonate fluxes (Ramaswamy and Gaye, 2006). Strong seasonality in the biogenic flux has been observed at SBBT while EIOT remained deprived of such seasonal variations (Vidya et al, 2013). interval with non-metallic knife to avoid metallic contamination and kept in a clean ziplock bags till further analysis.…”

Drivers of late Miocene tropical sea surface cooling: a new perspective from the equatorial Indian Ocean

“…As a consequence, seasonal SST variability in the BOB is mainly controlled by the strength of (monsoonal) wind-driven mixing (e.g. Unger et al, 2003;Vidya et al, 2013) and by salinity stratification (barrier layer formation). In waters overlying Site U1443, strong southwesterlies result in maximum annual surface ocean wind stress, deeper wind-driven mixing and reduced stratification during the summer monsoon (Figure 1c).…”