Paleoceanography of the Eastern Tropical North Pacific on millennial timescales

Abstract

The occurrence of large scale and rapid climate shifts at millennial time-scales (suborbital) remains an enigma between records from high and low latitudes spanning the Late Quaternary. This thesis studies such variations in the eastern tropical North Pacific (ETNP) using marine sediment cores retrieved from Mexico and Nicaragua. The main goals are to understand the nature of millennial timescale climate-changes in the Pacific low latitudes, to identify the atmospheric and oceanic teleconnections involved, to document the impacts on the biogeochemical cycles of carbon, nitrogen and silicon, and their potential to regulate Greenhouse Gas (GHG) concentrations during the last two glacial cycles (the last 240,000 years before present). In this thesis, we use a suite of multi-proxy records from the Core MD02-2519, which are compared to others records from adjoining regions to study the climatic history of the ETNP at millennial timescales. The Core MD02-2519, was retrieved from 955 mbsl off NW Mexico. It is strategically located within the North Pacific Intermediate Water (NPIW), underlying the coastal upwelling and denitrification zones of the ETNP. The paleoceanography of the region is studied using proxy records of productivity, denitrification, intermediate water circulation and radiocarbon activity, which are discussed in 5 separated chapters. In Chapter 1, we use records of organic carbon (%OC) and diffuse spectral reflectivity (DSRa*) to document changes in productivity, which are shown in phase with Northern Hemisphere (NH) timing at millennial scale, suggesting a direct atmospheric teleconnection with higher northern latitudes. In Chapter 2, reconstruction of nitrogen isotope records (δ15N) show that abrupt changes in denitrification are in phase with NH timing over the last glacial period; however, the advection of heavy nitrate from southern sources is also documented, possibly from the denitrification zone off Peru-Chile. Records of opal (%opal – Chapter 3) and carbon isotopes from benthic foraminifera (δ13C-Uvigerina – Chapter 4) support the inference of oceanic teleconnections between the ETNP and the South Pacific via subthermocline circulation. In Chapter 4, the δ13C records also suggest that intermediate water circulation changed over glacial periods and terminations, being the result of intrusion of southern component waters. In Chapter 5, the reconstruction of radiocarbon activity (Δ14C) records from surface (planktonic foraminifera) and intermediate water (benthic foraminifera) suggest oceanic degassing of old-carbon from the deep ocean during the last termination. In this way, the ETNP upwelling system could be an important locus of CO2 release at millennial timescales.