Holocene temperature and productivity variability at the Portuguese margin - evidence from planktonic foraminifera faunas

The Holocene, the current interglacial period covering the last ~11.5 ky, is generally characterized by warm sea surface temperatures (SSTs), high atmospheric CO₂ levels, and low ocean productivity in some regions. However, previous studies showed that the oceanographic conditions across the Holocene are highly variable, whereby regional patterns, such as in upwelling regions, are still poorly known.

The Portuguese margin is under influence of the Canary Eastern Boundary Upwelling System and is characterized by seasonal upwelling associated with cold waters and high primary productivity during spring-summer. During this period, are present the southward flowing, relatively cool and nutrient rich offshore Portugal Current (PC), and the nearshore Portuguese Coastal Current (PCC). During the fall-winter months, the PCC is substituted by the warm and nutrient poor Iberian Polar Current.

Considering that the planktonic foraminifera (PF) fauna is affected by water temperature and food availability, the present study reconstructs the Portuguese margin temperature and productivity, through a transfer function, across the Holocene using the PF fauna, at two sites under different oceanographic conditions: Shak-03-6K (37⁰42.45’N, 10⁰29.542’W, 3735m) recovered on the Sines margin and core MD03-2699 (39°02.20′N, 10°39.63′W, 1895m) from the Estremadura spur.

The fauna analysis at the Estremadura core revealed a total of 27 species and an estimated average summer SST of 18.9 °C. At the Sines core, we found 25 species, and a Holocene average SST of 21.2 °C. Our Holocene record starts with warm SSTs (above 20 °C) at both sites, followed by a cold event (starting at 10.5 ky), when the SST record at the Estremadura site abruptly dropped by 13 ºC, while at the Sines site the observed cooling was only 2 ºC. Two additional cold events, at 8.2 ky (~16 °C) and 5.2 ky (17.5 ºC) occurred during the Holocene Thermal Optimum (HTO, from 9.5 to 5.5 ky) at the Estremadura Site. According to previous studies, these short cold events are well marked in the North Atlantic, possibly caused by freshwater input from the melting Laurentide ice sheet. During these events, the relative abundance of polar and subpolar species (N. pachyderma and T. quinqueloba) increased, while the PC related species (N. incompta and G. inflata) decreased. However, this period is the warmest period of the Holocene when global glaciers reached their minimum volume extend, and in both sites the subtropical and tropical species increased.

During the Late Holocene (~4.2 ky to present), the SSTs at both sites remained constantly warm, with a high abundance of G. bulloides at the transition of HTO and this period. A feature that, most probably reflects enhanced upwelling influence, mainly at the Estremadura site. In fact, the Estremadura site records higher productivity than the Sines site, as that site is under a stronger influence of upwelling events and riverine nutrient supply from the Tagus River. Furthermore, occasional incursions of cold waters from the North are clearly marked on the Estremadura site, located close to the limit between subtropical and transitional surface and subsurface waters.