Marine primary production changes recorded by algal biomarkers in sediments from West Iberian Margin and Gulf of Cadiz during the latest Miocene to mid Pleistocene

Abstract:

West Iberian Margin is known as an upwelling area caused by strong trade wind in late spring and summer. Also, the Azores Front (AF), in which upwelling locally occur, resides south of the Gulf of Cadiz in the present, and presumably penetrated to the Gulf of Cadiz with enhanced upwelling during the glacial period (Rogerson et al., 2004). Therefore, primary production in surface waters of West Iberian Margin and Gulf of Cadiz could sensitively vary against paleocenographic and paeloclimatic changes. Moreover, these areas are located directly outside of the Strait of Gibraltar, and influence the Mediterranean Outflow Water (MOW). In the present study, we investigate marine algal biomarkers including long chain alkenones (haptophyte origin) and diols (diatom and eustigmatophyte origin), and dinosteroids (dinoflagellate origin) in the sediment cores from the Gulf of Cadiz (IODP site U1387; 5.5-3.0 Ma) and west of the Iberian Margin (IODP site U1391; 3.5-1.3 Ma) for reconstructing variations in marine primary production from the latest Miocene to mid Pleistocene.

In studied samples, we mainly detected marine algal C₂₈ and C₃₀ 1,13-diols, 1,14-diols and 1,15-diols. Our results in the samples from the Gulf of Cadiz show elevated values in the diol index (DI; upwelling intensity proxy; Rampen et al., 2008) and higher concentration of dinosterol around 3.3 Ma. These results indicate high production around 3.3 Ma. The DI values in the sediments of West Iberian Margin were low during 3.5-2.1 Ma but increased after 2.1 Ma, indicating enhanced coastal upwelling. The timing of increasing production in Gulf of Cadiz (3.3 Ma) is similar to that of global cooling as the MIS M2. Naafs et al. (2010) suggested development of glacial-like surface circulation in the North Atlantic during this period. Thus, we interpret that high production during 3.3 Ma in the Gulf of Cadiz was associated with the AF penetration into the gulf. While, MOW circulation system at the present-day might have been established since 2.1 Ma (Hernadez-Molina et al., 2014), when major glaciation was expanded (Rohling et al., 2014). Hence, high DI trends after 2.1 Ma imply the ocean circulation change caused by the establishment of present MOW system, and/or global change of ocean / wind circulation related to glaciation.