Enhanced productivity on the Iberian margin during glacial/interglacial transitions revealed by barium and diatoms

The Portuguese margin is at a critical location for studies of the ocean's behaviour during glacial/interglacial climatic changes, and the rapid accumulation rates of the sediments enable high-resolution palaeoclimatic investigation. The sedimentary record of the past 350 ka has been investigated in a 35 m long core from 3.5 km water depth on the slope at 40°N by geochemical, isotopic and micropalaeontological techniques. The CaCO3 content of this core as a function of time contains significant Milankovitch orbital frequencies of 18.8, 23.7, 38.0 and 100.6 ka, but these are driven primarily by dilution by clay-flux variations rather than by CaCO3 productivity variations. The largest signals in the productivity indicators Corg, Ba/Al and diatom abundance are all observed as simultaneous peaks at the oxygen isotope stage boundaries 10/9 and 6/5, with the signal magnitude in the order 10/9>6/5 for all three indicators. Smaller coincident signals in Corg, Ba/Al but not diatoms are also observed at the oxygen isotope stage 2/1 boundary. Other less prominent peaks in the Corg and Ba/Al profiles occur elsewhere, including Heinrich Event horizons, but these are not always simultaneous and none contain evidence of the dissolution-prone diatom microfossils. The 10/9, 6/5 and 2/1 oxygen isotope stage transitions represent the three most extreme glacial/interglacial sea level rises in the past 350 ky, possibly in the same sequence of magnitude, when sea level rose rapidly by 120+m from glacial low stands to interglacial high stands. The productivity signals at these transitions are contained within