Indian Ocean circulation and productivity during the last glacial cycle

Abstract

The Indian Ocean is an important part of the global thermohaline circulation system, receiving deep waters sourced from the Southern Ocean and being the location of upwelling and surface-ocean current flow which returns warm and salty waters to the Atlantic. It is also an ideal location to reconstruct the link between thermohaline circulation and deep-water nutrient contents. No mixing occurs between major deep-water masses along flow paths within the Indian Ocean, so changes in water-mass provenance reflect changes in deep-ocean circulation while nutrient contents reflect addition and dissolution of organic matter. Neodymium (Nd) and carbon (C) isotope records from an equatorial Indian Ocean core (SK129-CR2) spanning the last 150 kyr are presented. The Nd isotope record shows that an increased proportion of North Atlantic Deep Water (NADW) reached the Indian Ocean during interglacials (Marine Isotope Stages; MIS 1 and 5), and a reduced proportion during glacials (MIS 2, 4, and 6), and also that changes occurred during MIS 3. The magnitude and timing of deglacial and some MIS 3 variability is very similar to those in the RC11-83/TNO57-21 South Atlantic deep Cape Basin Nd isotope record. Since the deep Cape Basin is oceanographically upstream of the Indian Ocean from a deep water circulation perspective, the Nd isotopes suggest that Atlantic meridional overturning circulation changes were effectively propagated from the southeastern Atlantic into the central Indian Ocean via the Southern Ocean. Comparison of the Nd and C isotope records shows that deep-ocean circulation was decoupled from nutrient-content changes on glacial-interglacial timescales, in particular suggesting higher productivity during MIS 5. Increased equatorial productivity during MIS 5 is supported by benthic infaunal-epifaunal foraminiferal delta sup(13) C gradients, as well as benthic foraminiferal delta sup(13) C gradients along deep-water flow. Concurrent warming, indicated by planktonic foraminiferal Mg/Ca during MIS 5 at the site is consistent with changing thermocline temperature and may indicate a link to surface-ocean hydrographic changes.