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Hydrographic Changes in the Agulhas Recirculation Region During the Late Quaternary

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Hydrographic Changes in the Agulhas Recirculation Region During the Late Quaternary Clim. Past, 10, 745–758, 2014 Open Access www.clim-past.net/10/745/2014/ Climate doi:10.5194/cp-10-745-2014 © Author(s) 2014. CC Attribution 3.0 License. of the Past Hydrographic changes in the Agulhas Recirculation Region during the late Quaternary D. K. Naik1, R. Saraswat1, N. Khare2, A. C. Pandey3,*, and R. Nigam1 1Micropaleontology Laboratory, National Institute of Oceanography, Goa, India 2Ministry of Earth Sciences, New Delhi, India 3Allahabad University, Allahabad, India *presently at: Bundelkhand University, Jhansi, India Correspondence to: R. Saraswat ([email protected]) Received: 14 August 2013 – Published in Clim. Past Discuss.: 30 September 2013 Revised: 11 February 2014 – Accepted: 2 March 2014 – Published: 15 April 2014 Abstract. The strength of Southern Hemisphere westerlies, 1 Introduction as well as the positions of the subtropical front (STF), Ag- ulhas Current (AC) and Agulhas Return Current (ARC) con- trol the hydrography of the southwestern Indian Ocean. Al- The thermohaline circulation is responsible for distribution though equatorward migration of the STF and reduction in of heat across the world oceans. In the modern ocean, cold Agulhas leakage were reported during the last glacial pe- surface-ventilated water sinks to the bottom in the North riod, the fate of ARC during the last glacial–interglacial cy- Atlantic and around Antarctica, while the aged and gradu- cle is not clear. Therefore, in order to understand changes ally warmed water resurfaces in both the Indian and Pacific in the position and strength of ARC during the last glacial– Ocean. The warmer water is transported back to the North interglacial cycle, here we reconstruct hydrographic changes Atlantic (Talley, 2013). The southwestern Indian Ocean is in the southwestern Indian Ocean from temporal variation in the conduit for transport of about 70 Sv of warm and salty planktic foraminiferal abundance, stable isotopic ratio (δ18O) water from the Indian Ocean into the Atlantic Ocean, via and trace elemental ratio (Mg/Ca) of planktic foraminifera the eddy shedding by the Agulhas Current (AC) (Gordon, Globigerina bulloides in a core collected from the Agul- 1986; Bryden and Beal, 2001; Beal et al., 2011). A part of the has Recirculation Region (ARR) in the southwestern Indian AC retroflects off the southern tip of Africa and returns back Ocean. Increased abundance of G. bulloides suggests that to the Indian Ocean as the Agulhas Return Current (ARC) the productivity in the southwestern Indian Ocean increased (Quartly and Srokosz, 1993; Lutjeharms and Ansorge 2001; during the last glacial period which confirms previous re- Quartly et al., 2006). The retroflection depends on the in- ports of high glacial productivity in the Southern Ocean. The ertia of the AC off Africa, wind stress over this region and increased productivity was likely driven by the intensified the bottom topography (Lutjeharms and Ballegooyen, 1988; Southern Hemisphere westerlies supported by an equator- Le Bars et al., 2012). A distinct seasonality in the Agulhas ward migration of the subtropical front. Increase in relative Retroflection (AR) is also observed with earlier retroflection abundance of Neogloboquadrina incompta suggests season- during austral summer than in winter (Matano et al., 1998). ally strong thermocline and enhanced advection of southern A few sporadic, large eastward shifts of the AR, leading to source water in the southwestern Indian Ocean as a result a disruption of eddy shedding and thus a reduction in the of strengthened ARC, right through MIS 4 to MIS 2, during amount of water being transported from the South Indian to the last glacial period. Therefore, it is inferred that over the the South Atlantic Ocean, have also been observed (van Aken last glacial–interglacial cycle, the hydrography of the south- et al., 2013). A significant change in this inter-ocean water western Indian Ocean was driven by strengthened westerlies, exchange has also been reported over the geologic period ARC as well as a migrating subtropical front. (Rau et al., 2002), especially the glacial terminations (Peeters et al., 2004; Barker et al., 2009). As the global thermohaline Published by Copernicus Publications on behalf of the European Geosciences Union. 746 D. K. Naik et al.: Hydrographic changes in the Agulhas Recirculation Region during the late Quaternary circulation responds to the changes in the amount of water to reconstruct paleoclimatic changes from the southwestern transported from the Indian and Pacific Ocean to the South Indian Ocean, with an aim to understand changes in the Atlantic via the Agulhas Current in the southwestern Indian strength of ARC over the last glacial–interglacial cycle. Ocean (Knorr and Lohmann, 2003; Beal et al., 2011), it is Globigerina bulloides is abundant during periods of high possible that the changes in the southwestern Indian Ocean phytoplankton productivity (Schiebel et al., 1997). It has a may be a precursor to climate changes over the North At- wide temperature tolerance limit and has been reported from lantic (de Ruijter et al., 2005; Marino et al., 2013). The almost all possible sea surface temperature ranges in the strength of the ARC depends on the retroflection as well as world oceans (Bé and Hutson, 1977; Hemleben et al., 1989; the position of the subtropical front (STF), which marks the Sautter and Thunell, 1989). A several orders of magnitude transition between the tropical Indian Ocean and the South- higher abundance of G. bulloides is reported in the areas ern Ocean, and is distinguished as a sharp decrease in sea having a large phytoplankton population, as a result of up- surface temperature (Rintoul et al., 2001; Anilkumar et al., welling nutrient-rich cold water from deeper depths to the 2006). The latitudinal migration of STF and processes asso- surface (Peeters et al., 2002). Recently, Žaric´ et al. (2006) ciated with it affect transport of water from the southwestern and Fraile et al. (2008) modeled the global distribution of Indian Ocean to the Atlantic Ocean by the Agulhas Current planktic foraminiferal species, including G. bulloides, and (Flores et al., 1999; Simon et al., 2013). As far as the link found that this species is strongly correlated with highly pro- between westerlies and Agulhas leakage is concerned, con- ductive regions. High-productivity regions are generally as- trasting views have been proposed. While paleostudies sug- sociated with upwelling induced by seasonal strong winds gest reduced leakage associated with northward migration of (Wyrtki, 1971; McCreary et al., 1996). Thus the temporal westerlies and thus STF (Bard and Rickaby, 2009; Caley et variation in the relative abundance of G. bulloides in the In- al., 2012; Simon et al., 2013), the modeling studies suggest dian Ocean region has been suggested as an efficient tracer otherwise (Durgadoo et al., 2013). Even different paleostud- for the past changes in the surface productivity as a result ies provide contrasting evidence and there is no consensus of wind-driven upwelling associated with summer monsoon among the modeling studies either (Kohfeld et al., 2013). (Prell and Curry, 1981). A surface to near-surface habitat Further, the factors used to define the STF are also debated for G. bulloides in the southern Ocean was inferred based and it is suggested that bottom topography is also an impor- on δ18O of the specimens collected in sediment traps (King tant contributor in Agulhas leakage (De Boer et al., 2013; and Howard, 2005). In view of reported increased abundance Graham and De Boer, 2013), necessitating more and more of G. bulloides in waters with high surface productivity, it data from this region to understand Agulhas leakage dynam- has been widely used to infer paleo-upwelling and thus pa- ics. The response of ARC to the changes in the hydrog- leomonsoon changes in the Indian Ocean region (Gupta et raphy of the southwestern Indian Ocean over the glacial– al., 2003). Neogloboquadrina incompta prefers subsurface interglacial timescales is not clear yet. waters and its abundance is strongly correlated with high The physico-chemical state of the southwestern Indian chlorophyll a concentration in pycnocline (Kuroyangi and Ocean is also an important component of the monsoon sys- Kawahata, 2004; Bergami et al., 2009). Increased relative tem and modulates the intensity and timing of the monsoon abundance of N. incompta is also reported in upwelling ar- in India (Clemens et al., 1991), as well as in African re- eas including that around 40◦ S, though the effect of seawa- gions (Bader and Latif, 2003). Any change in global climate ter temperature was also observed (Žaric´ et al., 2006; Fraile will affect the thermal structure of the southwestern Indian et al., 2008). Though the coiling direction ratio of N. pachy- Ocean, which in turn may act as feedback for further cli- derma was proposed (Ericson, 1959) and frequently used as mate change. Therefore it is necessary to understand hydro- seawater temperature proxy, lately it was inferred that the graphic changes in the southwestern Indian Ocean from the differently coiled variants of N. pachyderma are in fact al- last glacial–interglacial transition, which will help to con- together different species, and further, that the right coiling strain the past climatic history of both the Indian monsoon morphospecies should be named as N. incompta (Darling et as well the southeastern Atlantic Ocean. Limited informa- al., 2006). tion is available on past climatic history of the southwest- The temperature-dependent replacement of Ca by Mg in ern Indian Ocean. Foraminifera – single-celled, preferen- both inorganically (Chave, 1954; Katz, 1973; Oomori et al., tially marine microorganisms with hard exoskeleton (test) – 1987) and organically precipitated carbonates (Lea et al., have ∼ 30 extant species, each of which inhabits different 1999; Rosenthal et al., 2000; Barker et al., 2005) lead to the depths of the water column. The foraminiferal tests accumu- application of Mg/Ca ratio of foraminiferal shells as sea- late on the seafloor, thus preserving the record of past water water temperature proxy.
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