Strong solar influence on multi-decadal periodic productivity changes in the central-western Bay of Bengal

Abstract

Oceanic productivity is a vital component of the global carbon cycle. The productivity in the tropical oceans is strongly modulated by the monsoon winds. The monsoon winds are driven by the temperature. Therefore, the oceanic productivity is most likely to be influenced by the increasing global warming. However, the projected response of oceanic productivity to global warming is ambiguous. The past records can help to better constrain the future response of oceanic productivity to different warming scenarios. Here, we reconstruct decadal monsoon and productivity changes from the central-western Bay of Bengal, covering the entire Holocene. We use both foraminiferal (relative abundance of Globigerina bulloides, stable oxygen isotopic ratio of surface dwelling Globigerinoides ruber) and geochemical [calcium carbonate (CaCO3), organic carbon (Corg), organic carbon/total nitrogen (Corg/TN)] proxies to reconstruct productivity and associated parameters. All the proxy data suggests a large variation in productivity and carbon burial during the Holocene. A very intense upwelling, concomitant with a phase of increased precipitation and CaCO3 burial is inferred during the beginning of Greenlandian (~12,000 - ~10,000 yrs BP). The intense upwelling, however, did not result in higher organic carbon burial. The beginning of Northgrippian (~8000-7000 yr BP) was a significantly wet phase followed by gradual decrease in precipitation. Incidentally, the reduced upwelling during the Northgrippian was synchronous with a large increase in organic carbon burial. The opposite trend in upwelling intensity and %Corg during the Meghalayan as well, suggests decoupling of upwelling induced productivity and organic carbon burial in the western Bay of Bengal. Towards the late Meghalayan (after ~1500 yr BP), we report a relatively enhanced precipitation with continual increase in CaCO3 but with reduced Corg burial. Further, these long-term variations were accompanied by significant short-term periodic changes, closely resembling Gleissberg cycle as well as Seuss cycle, suggesting a possible link with the solar variability.