Strong glacial-interglacial variability in upper ocean hydrodynamics, biogeochemistry, and productivity in the southern Indian Ocean

D Tangunan ,Aidan Starr,A Cartagena‐Sierra ,Francisco Jose Jiménez-Espejo ,José-Abel Flores ,J.j Coenen ,Oscar E Romero ,S R Hemming ,Mariem Saavedra‐Pellitero ,Melissa A Berke,Karl-Heinz Baumann,Leah J Levay,Jens Gruetzner,Ian R Hall

doi:10.1038/s43247-021-00148-0

Abstract

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Here we analyse a sedimentary record from the Agulhas Plateau, located close to the modern position of the subtropical front and use alkenones and coccolith assemblages to reconstruct oceanographic conditions over the past 300,000 years. We identify a strong glacial-interglacial variability in sea surface temperature and productivity associated with subtropical front migration over the Agulhas Plateau, as well as shorter-term high frequency variability aligned with variations in high latitude insolation. Alkenone and coccolith abundances, in combination with diatom and organic carbon records indicate high glacial export productivity. We conclude that the biological pump was more efficient and strengthened during glacial periods, which could partly account for the reported reduction in atmospheric carbon dioxide concentrations.

Highlights

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry
In the from 2013 to 2015. a Sea surface temperature (SST, °C) showing the study site and other core locations mentioned in the text, together with the Southern Ocean and the southern Indian Ocean fronts: Subtropical Front (STF), Subantarctic Front (SAF) and Polar Front (PF)[2,74], and the schematic representation of the average surface ocean circulation (Agulhas Current, AC; Agulhas Return Current, Agulhas return current (ARC); Agulhas Leakage, AL)75. b Chlorophyll-α concentrations. c Coccolithophores. d Diatoms
Cooling occurred at the glacial inceptions and coldest conditions are recorded before glacial maxima, with an early warming through the terminations (Fig. 2c)

Summary

Introduction

In the southern Indian Ocean, the position of the subtropical front – the boundary between colder, fresher waters to the south and warmer, saltier waters to the north – has a strong influence on the upper ocean hydrodynamics and biogeochemistry. Despite the numerous studies on the southern Indian Ocean linking STF migration to variations in primary productivity, understanding remains limited regarding how biotic communities have responded to changes in water column biogeochemistry (nutrients, productivity) and hydrodynamic characteristics (SST, stratification) with varying STF position over glacial-interglacial cycles.

Results

Conclusion