Dynamics of primary productivity in the northeastern Bay of Bengal over the last 26 000 years

Xinquan Zhou,Franck Bassinot,Masa Kageyama,Christophe Colin,Stéphanie Duchamp-Alphonse,Luc Beaufort

doi:10.5194/cp-16-1969-2020

Abstract

Abstract. At present, variations of primary productivity (PP) in the Bay of Bengal (BoB) are driven by salinity-related stratification, which is controlled by the Indian summer monsoon (ISM). The relationships between PP, precipitation, and more generally climate in the past are not clearly understood. Here, we present a new record of PP based on the examination of coccolithophore assemblages in a 26 000-year sedimentary series retrieved in the northeastern BoB (core MD77-176). We compare our PP records to published climate and monsoon records, as well as outputs from numerical experiments obtained with the Earth system model IPSL-CM5A-LR, including the marine biogeochemical component PISCES, and with the transient climate simulation TraCE-21. Our results show that PP was most probably controlled by nutrient contents and distribution within the upper water column, which were predominantly influenced by (i) regional river systems between 26 and 19 ka, i.e. when sea level was relatively low and climate was relatively dry, and (ii) salinity-related stratification over the last 19 kyr, i.e. when sea level rose and more humid conditions prevailed. During that period, salinity and stratification were directly related to monsoon precipitation dynamics, which were chiefly forced by both insolation and Atlantic meridional overturning circulation (AMOC) strength. During Heinrich Stadial 1 and the Younger Dryas, i.e. when the AMOC collapsed, weaker South Asian precipitation diminished stratification and enhanced PP. During Bølling–Allerød, i.e. when the AMOC recovered, stronger South Asian precipitation increased stratification and subdued PP. Similarly, the precipitation peak recorded around the middle–early Holocene is consistent with a stronger stratification that drives PP minima.

Highlights

The climatology and oceanography of South Asia and the north Indian Ocean are dominated by the Indian monsoon, which is characterized by strong seasonal contrasts in wind and precipitation patterns (Shankar et al, 2002; Gadgil, 2003)
F. profunda largely dominates the assemblage (> 60 %) over the last 26 kyr, while E. huxleyi and Gephyrocapsa spp. never exceed 23 % (Fig. 2). Such relative contributions are coherent with coccolith distribution in sediment traps from the northern Bay of Bengal (BoB) (Stoll et al, 2007), which shows a high abundance of F. profunda due to a strong salinityrelated stratification and low surface nutrient concentration
Simulated annual and seasonal patterns of primary productivity (PP) (g C m−2 yr−1) are shown for the BoB and the Andaman Sea in Fig. 3, where the mid-Holocene experiment (MH) and LGMc simulations are compared to the CTRL and where the LGMf simulation is compared to the LGMc, highlighting the effects of the Atlantic meridional overturning circulation (AMOC) slowdown

Summary

Introduction

The climatology and oceanography of South Asia and the north Indian Ocean are dominated by the Indian monsoon, which is characterized by strong seasonal contrasts in wind and precipitation patterns (Shankar et al, 2002; Gadgil, 2003). Monsoon precipitation is directly associated with the position of the Intertropical Convergence Zone (ITCZ; Schneider et al, 2014), whose latitudinal displacement is paced by seasonal changes in insolation due to the obliquity of the Earth’s axis and precession, and it results from variations in the land–sea thermal contrast caused by differences in heat capacity of the continent and the ocean (Meehl, 1994, 1997; Webster, 1998; Wang et al, 2003) It is influenced by teleconnections with the El Niño–Southern Oscillation and the Indian Ocean Dipole, two climate modes of interannual. Zhou et al.: Dynamics of primary productivity in the northeastern Bay of Bengal variability that develop from air–sea interactions in the tropical Pacific and drive significant changes within the Indian Ocean (Ashok et al, 2004; Wang et al, 2008; Currie et al, 2013; Jourdain et al, 2013)

Methods

Results

Conclusion