Eastern Pacific background state and tropical South American climate history during the last 3 million years

Abstract

Nowadays reorganizations of tropical Pacific convection and wind variability respond to the zonal distribution of sea surface temperatures and have a profound effect on the Pacific Ocean s upwelling, thermocline depth, productivity, dust fertilization, riverine sediment loads, and continental dryness of the non-convective regions. Though much progress has been made in the understanding of the present dynamical interaction between the ocean and the atmosphere, many uncertainties remain concerning tropical reorganizations of convection under different climatic scenarios, such as future global warming and ice ages. The subject of this thesis is to gain deeper insights into the deposition of windblown and fluvially transported terrigenous material in deep-sea sediments of the easternmost tropical and subtropical Pacific Ocean and its relationship to the evolution of the eastern tropical Pacific background state and the continental climate of the west coast of South America over Plio-Pleistocene time. Part of this study is based on a set of surface sediment samples of the eastern tropical and subtropical Pacific (10°N - 25°S, 100°W - 70°W) retrieved from water depths between 192 and 4,622 m.b.s.I, mostly on the Cocos and Carnegie ridges. Down-core analyses were performed on ODP Sites 1239 (drilled approximately 120 km off the coast of Ecuador, close to the eastern crest of Carnegie Ridge at a water depth of 1,414 m), 1237 (located ~140 km off the southern Peruvian coast on the easternmost flank of Nazca Ridge at a water depth of 3,212 m), 1229 (located on the Lima Basin, Peruvian shelf, at 150 m water depth), and core MD02-2529 (collected off the Costa Rican margin in the Panama Basin at a water depth of 1619 m). In Chapter 4 a set of surface sediment samples was analyzed to define the location of the equatorial front in the Pacific, based on foraminifer s census and stable isotope data. The sample material was obtained by means of multi-corers, box-corers, gravity or piston corers. We propose that the ratio between G. menardii cultrata and N dutertrei abundances (Rc/d) as well as the oxygen isotopic difference between G. ruber and G. tumida (18OG.tumida-G.ruber) and between P. oliquiloculata and G. tumida (18OG.tumida-P.obliquiloculata) are useful paleoceanographic tools for reconstructing the latitudinal position of the eastern Pacific Equatorial Front in an area delimited by the Cocos and Carnegie ridges. In our core top dataset, high Rc/d values (>0.3) as well as high 18OG.tumida-G.ruber values are related to warm, low salinity TSW north of the EF, whereas cool, nutrient-rich ESW south of the front are characterized by lower Rc/d values and 18OG.tumida-G.ruber. Although we are currently not able to explain the latitudinal difference between modern EF position and the observed changes in 18OG.tumida-G.ruber, the application of the 18OG.tumida-G.ruberto a down-core record located south of the EF indicates nevertheless a potential value of this proxy. Chapter 5 provides a combined analysis of proxy data that allude to paleoceanographic changes in the EEP and concomitant continental paleoclimate variations onshore during the past 500 kyr. The proxy profiles are derived from samples obtained from sediment cores off the coast of Ecuador (ODP Site 1239) and in the Panama Basin (core MD02-2529). We find prominent glacial-interglacial changes of fluvial sediment input that reflects more humid conditions along the Ecuadorian coast during interglacials. A warmer interglacial EEP cold tongue and a southward shift of the EF-ITCZ system likely control these humid interglacial conditions. Conversely, reduced fluvial input during glacials suggests more arid conditions coinciding with larger tropical Pacific SST gradients and a more northward location of the EF-ITCZ system. The glacial-interglacial latitudinal shifts of the EF-ITCZ system suggested by our data may be restricted to the EEP and the coastal area of northwest South America. Glacial cooling is particularly pronounced in the Southeast Pacific, which suggests a possibility that ITCZ migration in the region may be controlled by the northward advection of cold waters with the Humboldt Current system. Over the South American continent, away from the coast, the Andes and Amazon Basin impact atmospheric circulation patterns, allowing larger southward migrations of the ITCZ during glacial periods.