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doi:10.2204/iodp.proc.339.202.2017

Introduction

Diatoms are unicellular algae that segregate into opaline silica frustules with two valves and constitute one of the major players in the basis of the trophic chain. Planktonic and benthic forms exist in both freshwater and marine environments and reflect the environmental conditions in such settings, making these organisms excellent indicators for past environments. In the modern ocean, diatoms are abundant in sediments underlying temperate continental shelves, where nutrient and photic conditions in the upper mixed oceanic layer are seasonally favorable for them to quickly divide and dominate the phytoplankton communities (the “spring bloom”) and in upwelling regions where nutrient input can be seasonal or perennial (coastal, equatorial, and high-latitude regions). Contrary to the small r-strategic species that dominate in those regions, in the open ocean, where conditions are typically oligotrophic, diatoms tend to be larger and in some cases capable of actively regulating their buoyancy to counter sinking rates (Dugdale and Wilkerson, 1998). However, open-ocean diatoms might also become very abundant through concentration along frontal regions or thermal wave instabilities (Yoder et al., 1994), which are recorded in the sediments as diatom mats (Kemp, 1995; Smetacek, 2000; Kemp et al., 2006).

The standard shipboard micropaleontological evaluation of the sites recovered during Integrated Ocean Drilling Program Expedition 339, Mediterranean Outflow (see the “Site U1387” chapter [Expedition 339 Scientists, 2013]), revealed the presence of diatoms (some pyritized) and radiolarians in core catcher samples from Site U1387 (36.8°N, 7.7°W; 559 m below sea level; Fig. F1), especially in Sample 339-U1387B-24X-CC. As a follow up, we investigated the total abundance (valves/gram) and diatom assemblage composition, as well as the presence of other siliceous microfossils in samples surrounding this level, in order to evaluate its ecological meaning.

Site U1387 is under the direct influence of the upper core of Mediterranean Outflow Water (MOW) (see the “Site U1387” chapter [Expedition 339 Scientists, 2013]); Voelker et al., 2015), and surface waters of subtropical origin are transported by the eastern branch of the Azores Current (far offshore) and the eastbound Gulf of Cadiz Slope Current (GCC), an open-ocean current located along the southern Iberian margin (Peliz et al., 2009) (Fig. F1). During the upwelling season (May–September), the filament formed off Cape São Vicente frequently extends eastward along the southern shelf break and slope of Portugal (Relvas and Barton, 2002), potentially influencing Site U1387. In addition, a local upwelling plume can form off Cape Santa Maria (Fig. F1) under westerly winds (Criado-Aldeanueva et al., 2006; Navarro and Ruiz, 2006).

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