Last Interglacial to present heat, salt and nutrient variability in the Indian-Atlantic Ocean gateway from Eparses Islands (Indian Ocean) coral core geochemistry

Overview

Understanding the role of ocean circulation in shaping the patterns of ocean warming, salinity and nutrient transport remain key to predicting global and regional climate change, ocean nutrient cycling and ecosystem responses. Western boundary currents are vital bottlenecks modulating global heat and freshwater distribution, as well as nutrient dynamics. Currently it is not known to what extent anthropogenic warming and associated changes in the hydrological cycle and nutrient dynamics exceed natural variability linked to seasonal, interannual and (multi)decadal Indo-Pacific and Atlantic climate variability. In particular, we have little knowledge if anthropogenic climate change contributed to amplifying or dampening heat and freshwater transport along the southern Indian Ocean currents (South Equatorial Current; Mozambique Channel eddies) feeding the Agulhas Current (AC), one of the most powerful western boundary currents on the planet. Ultimately, the AC will feed into the Atlantic Meridional Overturning Circulation (AMOC) via Agulhas leakage into the South Atlantic and therefore is a critical joke point in the global thermohaline circulation affecting global climate (Beal et al., 2011; de Ruijter et al., 2004).

Given the importance of vigorous, eddy-driven western boundary currents for nutrient transport between the shelf and open ocean in the Mozambique Channel (MC) and AC sustaining highly productive and highly valuable fishing grounds (e.g. Tuna; Chassot et al., 2019), it is of paramount importance to study the long-term trends and the full suite of variability in nutrient dynamics coupled to thermohaline properties under varying climate background conditions influenced by orbital cycles (present to past climate states).

Our proposed study, therefore, will address this key knowledge gap by developing modern (~1850-2019 A.D.) and fossil (Last Millennium to Last Interglacial) coral paleoclimate records at unprecedented sub-monthly resolution obtained from key sites within the tropical, low-latitude MC inflow and the southern MC outflow feeding the globally important AC (Fig. 1). We will use state-of-the-art analytical approaches to develop coral geochemical records which will be paired with climate model-derived (Kiel Climate Model) and observational estimates to unravel the climate and ocean dynamics influencing the MC-AC western boundary flow on a variety of society-relevant time scales.

Sea surface temperatures (austral summer; map available from http://apdrc.soest.hawaii.edu/data) across the southwestern Indian Ocean with the major ocean currents forming part of the global surface thermohaline circulation route: SEC: South Equatorial Current, EMC: East Madagascar Current, NEMC: North East Madagascar Current, AC: Agulhas Current, ACR: Agulhas Current return flow, MCE: Mozambique Channel Eddies. The Europa and Glorieuses archipelago coral locations  and the region of dense SST observations for the Agulhas Current are indicated. The region of Agulhas ring shedding (AR) indicates Indian-Atlantic ocean exchange of heat and salt thought to influence the AMOC. Coral data available for regional comparisons are indicated (JdN: Juan de Nova; MAY: Mayotte, Zinke et al., 2008; Ifaty, SW Madagascar, Zinke et al., 2014).

Figure 1 – Sea surface temperatures (austral summer; map from http://apdrc.soest.hawaii.edu/data) across the southwestern Indian Ocean with the major ocean currents forming part of the global surface thermohaline circulation route indicated: SEC: South Equatorial Current, EMC: East Madagascar Current, NEMC: North East Madagascar Current, AC: Agulhas Current, ACR: Agulhas Current return flow, MCE: Mozambique Channel Eddies. The Europa and Glorieuses archipelago coral locations (large yellow dot) and the region of dense SST observations (rectangular box) for the Agulhas Current are indicated. The region of Agulhas ring shedding (AR) indicates Indian-Atlantic ocean exchange of heat and salt thought to influence the AMOC. Coral data available for regional comparisons are indicated by a small yellow dot (JdN: Juan de Nova; MAY: Mayotte, Zinke et al., 2008; Ifaty, SW Madagascar, Zinke et al., 2014).