Diatoms in northeast Pacific surface sediments as paleoceanographic proxies C. Lopes a,b, ⁎ , A.C. Mix a , F. Abrantes b a College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvallis, OR 97331-5503, USA b Departamento de Geologia Marinha, Instituto Nacional de Engenharia, Tecnologia e Inovação-INETI, Aptdo. 7586, Estrada da Portela, 2721-Alfragide, Portugal Received 10 December 2005; received in revised form 20 February 2006; accepted 22 February 2006 Abstract Fossil diatom total abundances (# valves/g) in 54 surface-sediment samples from the northeast (NE) Pacific Ocean reflect the position of high primary production associated with coastal upwelling and that possible biases associated with dilution or dissolution are small. Diatom species assemblages, defined by Q-mode factor analysis in 30 samples with abundant diatoms, are related to modern oceanographic properties. Five statistical assemblages, given by five specific diatom species and/or groups, are related to upwelling (Chaetoceros spores), subtropical (Thalassionema nitzschioides), subarctic (Rhizosolenia hebetata), transitional (Neodenticula seminae) and freshwater (freshwater diatoms) ecological environments. These factors are significantly correlated with primary productivity, temperature, nutrient concentrations and salinity, although the strongest relationship is that between diatom assemblages and productivity. However, it is not possible to distinguish between coastal and open-ocean (curl- driven) upwelling based on Chaetoceros spores relative percentages by themselves or on the floral factors. © 2006 Elsevier B.V. All rights reserved. Keywords: diatoms; factor analysis; modern sediments; upwelling; California; Oregon 1. Introduction Marine export productivity plays an important role in natural CO 2 variations through the mechanism of the “biological pump” (Berger and Wefer, 1991). The processes that drive CO 2 sequestration are focused in specific regions. For example, about 5% of the total annual North Pacific uptake of atmospheric CO 2 (and as much as 100% of the summer uptake) occurs in the upwelling region off Oregon and California, even though this region comprises <2% of the total North Pacific, likely because the coastal upwelling system contains preformed nutrients (relative to its CO 2 content) and is efficient at exporting carbon to the deep sea (Hales et al., 2005). To understand the component of natural variations in atmospheric CO 2 that is driven by this biological pumping, we must understand the long-term history of these regional upwelling systems. Primary production is higher in upwelling areas where nutrients such as phosphate and nitrate are brought to the surface. As more nutrients (including micronutrients such as iron and other limiting com- pounds such as silica) are available in the photic zone, Marine Micropaleontology 60 (2006) 45 – 65 www.elsevier.com/locate/marmicro ⁎ Corresponding author. College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Admin. Bldg., Corvallis, OR 97331-5503, USA. Tel.: +1 541 737 9351; fax: +1 541 737 2064. E-mail address: clopes@coas.oregonstate.edu (C. Lopes). 0377-8398/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.marmicro.2006.02.010