Holocene evolution of summer winds and marine productivity in the tropical Indian Ocean in response to insolation forcing: data-model comparison

Abstract

Abstract. The relative abundance of Globigerinoides bulloides was used to infer Holocene paleo-productivity changes on the Oman margin and at the southern tip of India. Today, the primary productivity at both sites reaches its maximum during the summer season, when monsoon winds result in local Eckman pumping, which brings more nutrients to the surface. On a millennium time-scale, however, the % G. bulloides records indicate an opposite evolution of paleo-productivity at these sites through the Holocene. The Oman Margin productivity was maximal at ∼9 ka (boreal summer insolation maximum) and has decreased since then, suggesting a direct response to insolation forcing. On the contrary, the productivity at the southern tip of India was minimum at ∼9 ka, and strengthened towards the present.Paleo-reconstructions of wind patterns, marine productivity and foraminifera assemblages were obtained using the IPSL-CM4 climate model coupled to the PISCES marine biogeochemical model and the FORAMCLIM ecophysiological model. These reconstructions are fully coherent with the marine core data. They confirm that the evolution of particulate export production and foraminifera assemblages at our two sites were directly linked with the strength of the upwelling. Model simulations at 9 ka and 6 ka BP show that the relative evolution between the two sites since the early Holocene can be explained by the weakening but also the southward shift of monsoon winds over the Arabian Sea during boreal summer.

Orbitally-induced changes of the Atlantic and Indian monsoons over the past 20,000 years: New insights based on the comparison of continental and marine records

“…Results show that the early Holocene humid period corresponds to a decrease in coastal upwelling. This decrease in upwelling intensity off western Africa is out of phase, therefore, with the early Holocene increase in western Arabian sea upwelling [e.g., Bassinot et al, 2011]. Although no data is available to estimate and correct past changes in age reservoir at ODP 658, we expect that 14 C-derived chronologies of sites 74KL and ODP 658 may be off by several hundred years over specific intervals due to changes in upwelling intensity.…”

“…Several studies combining proxy data and climate model simulations have shown that changes in insolation controlled by the slow variation of the Earth orbital parameters (mainly precession), have been the major driver of Holocene changes in atmospheric circulation and in the hydrological budget, which are ultimately linked to the afro-asian monsoon system [e.g. COHMAP, 1988;Prell and Kutzbach, 1987;Joussaume et al, 1999;Liu et al, 2003;Marzin and Braconnot, 2009 a,b;Bassinot et al, 2011;Lézine et al, 2011a]. During the first half of the Holocene, orbital configuration resulted in enhanced seasonality in northern tropical areas compared to the late Holocene.…”

The late Pliocene Benguela upwelling status revisited by means of multiple temperature proxies

“…At ODP site 1082, core‐top planktonic foraminifera assemblages are dominated by the subpolar species G. bulloides and Neogloboquadrina pachyderma (right coiling) [ Giraudeau , ]. High abundance of G. bulloides at low latitudes is globally associated with upwelling environments [ Lombard et al ., ], one ecological characteristic used to reconstruct primary productivity of tropical upwelling systems by means of G. bulloides countings [ Gupta et al ., ; Bassinot et al ., ]. We hence anticipate the Mg/Ca‐derived SST reconstruction based on that species to be reflective of seasons with SST below the mean‐annual SST, i.e., when upwelling intensifies.…”

“…High abundance of G. bulloides at low latitudes is globally associated with upwelling environments bulloides tests for foraminifera dated at 2.23, 2.74, and 3.37, i.e., for times when high, intermediate and low opal MAR were recorded, respectively. , one ecological characteristic used to reconstruct primary productivity of tropical upwelling systems by means of G. bulloides countings [Gupta et al, 2003;Bassinot et al, 2011]. We hence anticipate the Mg/Caderived SST reconstruction based on that species to be reflective of seasons with SST below the mean-annual SST, i.e., when upwelling intensifies.…”

Vegetational and climatic variations during the past 3100 years in southern India: evidence from pollen, magnetic susceptibility and particle size data

“…The pollen sequences from the respective sites have also offered insights into the changing floristic patterns and the coeval climatic episodes. Besides pollen, other proxies like foraminifera (Nagender Nath et al 2012;Bassinot et al 2011;Banakar et al 2010;Khare et al 2008), foraminiferal isotopic and elemental data (Saraswat et al 2013), rock magnetic properties (Anil Kumar et al 2005;Thamban et al 2007), oxygen isotopes (Prabhu et al 2004), nitrogen isotopes (d 15 N; Agnihotri et al 2008), geochemical parameters (Chauhan et al 2010a), biomarkers (Kurian et al 2009), and clay mineral data (Chauhan et al 2010a) of marine sediments have also been employed in reconstructing the palaeoclimate of the region. However, most of these do not provide much information on the important climatic events of the Holocene.…”