Terrigenous input off northern South America driven by changes in Amazonian climate and the North Brazil Current retroflection during the last 250 ka

A Govin,D Heslop,T Hörner,C M Chiessi,M Zabel,A O Sawakuchi,Y Zhang,S Mulitza

doi:10.5194/cp-10-843-2014

Abstract

Abstract. We investigate changes in the delivery and oceanic transport of Amazon sediments related to terrestrial climate variations over the last 250 ka. We present high-resolution geochemical records from four marine sediment cores located between 5 and 12° N along the northern South American margin. The Amazon River is the sole source of terrigenous material for sites at 5 and 9° N, while the core at 12° N receives a mixture of Amazon and Orinoco detrital particles. Using an endmember unmixing model, we estimated the relative proportions of Amazon Andean material ("%-Andes", at 5 and 9° N) and of Amazon material ("%-Amazon", at 12° N) within the terrigenous fraction. The %-Andes and %-Amazon records exhibit significant precessional variations over the last 250 ka that are more pronounced during interglacials in comparison to glacial periods. High %-Andes values observed during periods of high austral summer insolation reflect the increased delivery of suspended sediments by Andean tributaries and enhanced Amazonian precipitation, in agreement with western Amazonian speleothem records. Increased Amazonian rainfall reflects the intensification of the South American monsoon in response to enhanced land–ocean thermal gradient and moisture convergence. However, low %-Amazon values obtained at 12° N during the same periods seem to contradict the increased delivery of Amazon sediments. We propose that reorganizations in surface ocean currents modulate the northwestward transport of Amazon material. In agreement with published records, the seasonal North Brazil Current retroflection is intensified (or prolonged in duration) during cold substages of the last 250 ka (which correspond to intervals of high DJF or low JJA insolation) and deflects eastward the Amazon sediment and freshwater plume.

Highlights

The Amazon River recently alternated between record floods in 2009 (Marengo et al, 2012) and 2012 (Satyamurty et al, 2013) and severe droughts in 2005 (Marengo et al, 2008) and 2010 (Lewis et al, 2011), both with severe socio-economic consequences
In light of the arguments made above, we propose that the %-Andes and %-Amazon variations recorded in the western tropical Atlantic (Fig. 6) mainly reflect changes in South American fluvial input, that is, in the relative amount of Amazon Andean material at 5 and 9◦ N and of Amazon material at 12◦ N
We present high-resolution geochemical records from four marine sediment cores located between 5◦ and 12◦ N along the northern South American margin for the last 250 ka

Summary

Introduction

The Amazon River recently alternated between record floods in 2009 (Marengo et al, 2012) and 2012 (Satyamurty et al, 2013) and severe droughts in 2005 (Marengo et al, 2008) and 2010 (Lewis et al, 2011), both with severe socio-economic consequences. Observations over the last decades show signs of a changing water cycle in the eastern and southern regions of the Amazon Basin linked to deforestation, land use and climate change Davidson et al, 2012). Anthropogenic impacts do not yet seem to surpass the magnitude of natural hydrologic cycle variability (Davidson et al, 2012), model projections suggest that the Amazon Basin is nearing the transition into a disturbance-dominated regime

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