The missing link to understand Plio-Pleistocene changes in southeast Pacific oceanography, productivity, and El Niño behavior – SE trade wind strength and its dust transport.

One of the fundamental missing links in understanding Pliocene and Pleistocene changes in southeast Pacific oceanography, productivity and El Niño behavior as well as associated variations in thermocline depth, upwelling and dust fertilisation is the knowledge of changes in southeast trade wind strength and dust transport on millennial and orbital time scales.

Although the trade winds are generally characterized by great steadiness, they demonstrate a considerable interannual variability in their strength and geographic extension that is, at least in the southeast Pacific, primarily related to the El Niño Southern Oscillation (ENSO). During El Niño years, both the southeast Pacific anticyclone and the southeast trade winds are considerably weakened. As a result the thermocline in the eastern Pacific is significantly deeper and coastal upwelling off Peru substantially weakens or even collapses. During such events, tropical rainfall increases over Ecuador and enhances fluvial runoff (e.g. Guayas River). During the other extreme (La Niña years), enhanced trade winds shoal the thermocline and intensify coastal upwelling, and coastal aridity in northern Peru is even more severe than usual.

Little is known on how the interaction and coupling of the southeast trade winds and associated oceanographic patterns changed in the southeast Pacific over geological time-scales. Therefore, our project aims to reconstruct millennial-scale to orbital-scale changes in tropical and subtropical S-American climate and southeast trade wind intensity at selected time intervals that were characterized by marked reorganizations in global climate and oceanography: the pronounced glacial/interglacial climate variability of the last 1 Ma, the Plio-Pleistocene transition from 2–1.5 Ma, the Pliocene intensification of the Northern Hemisphere Glaciation (NHG) from 2.4–3.1 Ma, and the Pliocene warm period from 4–5 Ma.

In a first step, we are currently reconstructing the modern spatial pattern of siliciclastic grain size variability in the southeastern Pacific from surface sediment samples to identify the eolian signal and regions of fluvial overprints. Though, the pattern of quartz distribution in southeast Pacific surface sediments (Molina-Cruz, 1977) clearly marks the modern supply of dust from the Atacama Desert, a detailed analyses of the regional grain-size pattern is a major prerequisite for the present project.

The subsequent paleostudies will focus on ODP Sites 1237 and 1239 drilled during Leg 202. Site 1237 is located at a water depth of 3212 m on a relatively flat bench of Nazca Ridge, about 250 km off the coast of Peru (Fig. 1) and Site 1239 is located in a water depth of 1414 m at Carnegie Ridge, about 200 km off the coast of Ecuador. Today, both sites underlie the path of eolian transport from the Atacama Desert in Chile and Peru. However, Site 1239 is likely affected by fluvial sediment input during El Niño years controlled by enhanced rainfall over Ecuador (Fig. 1). We plan to apply a multi-proxy approach in order to target the following major goals:

Establishing records of siliciclastic accumulation rates and grain sizes at Sites 1237 and 1239 to reconstruct changes in continental aridity and trade wind strength (Site 1237) as well as possible fluvial overprints at Site 1239, indicative of enhanced tropical rainfall.

Distinguishing between continental aridity and wind strength changes through comparing compositional (XRF scanning) and grain-size data of the terrigenous sediment input involving an end-member modelling approach. Are changes in continental climate and wind intensity in phase or decoupled?

Establishing vertical Pleistocene temperature profiles of the upper water column at Site 1239 to reconstruct changes in thermocline depth (Mg/Ca temperature records from deep- and shallow-dwelling planktonic foraminifers). Such records are available for the Pliocene.
Comparing trade wind strength and thermocline proxy records to paleoproductivity reconstructions. For this purpose, we plan to use shipboard bulk density records as a proxy for the opal content (these will be calibrated with opal measurements on discrete samples).
Assessing the forcing mechanisms (low versus high latitudes, NH contra SH) that control changes in South American aridity/humidity, trade wind intensity, thermocline depth, and ocean productivity on different time-scales (millennial to orbital-scale) during the key intervals outlined above.
Investigating the role of long-term ENSO changes on orbital time-scales as suggested by modelling studies (Clement and Cane, 1999; Philander and Federov, 2003). Can we even distinguish a millennial-scale signal in such changes?

: Fig. 1: A) Zones of high atmospheric dust concentrations inferred from mean annual equivalent aerosol optical depth (x1000) as measured by AVHRR from Husar et al., 1997. (B) Quartz distribution in surface sediments of the subtropical southeast Pacific, expressed as weight percentage (carbonate- and opal-free basis, reproduced from Molina- Cruz, 1977). Arrows indicate trade winds. Sites 1237, 1239 and 1241 are shown with their plate tectonic backtracks of the last 5 Ma. (C) The Gulf of Guayaquil receives runoff from 20 rivers. The major source is the Guayas River (blue) with a mean discharge of 1.4 Sv, enhanced during El Niño (Stevenson, 1981).