Response of benthic foraminiferal assemblages off NW Africa to climate change during the past 27.000 years

Benthic foraminifera (BF) typically constitute around 50% of the eukaryotic biomass of seafloor environments and are excellent recorders of bottom water environmental and geochemical changes in the past. In the last 27.000 years, major climatic oscillations including the Heinrich Stadial 1 (HS1), Bølling–Allerød (B-A) and Younger Dryas (YD) shaped the climate of a big part of the northern hemisphere. Although the response of the ocean surface to these events is well documented, information about the response of benthic ecosystems is still limited.

To better understand how BF responded to major climatic shifts in the last 27.000 years, we analyzed the benthic foraminifera content from core GeoB9512-5 (15°29.90'N/17°56.88'W, 793 m water depth) off NW Africa. Our high-resolution sediment record covers the last 27.000 years of the eastern North Atlantic, including the Heinrich Stadial 2 (HS2), Last Glacial Maximum (LGM), HS1, B-A and YD.

Taxonomic and quantitative analyses were used to reconstruct changes in bottom water oxygenation and organic matter fluxes and show that BF assemblages shifted in coincidence with the major climatic periods documented for the North Atlantic. After the LGM, Bottom water salinity, oxygenation and quantity/quality of organic matter played a major role in BF distribution and are linked to transient changes in BF diversity in the last 27.000 years.

The LGM showed no major diversity changes for thousands of years, while BF distribution shifted rapidly during HS1, B-A and YD. Low-diversity intervals during the HS1, B-A and the last 6.000 years are typically dominated by stress species in times of oxygen decrease and high organic matter content at bottom waters. These short intervals (typically lasting 500-1300 years) are commonly intercalated by low-duration high-diversity periods, associated with higher bottom water oxygenation and relatively lower organic matter content. Additionally, relatively abundant porcelaneous BF during HS1, LGM and HS1 indicate relatively higher salinity than the observed in the last 14.000 years.

Our results show that BF at intermediate depths at the NE Atlantic off NW Africa are strongly influenced by changes in bottom water paleoenvironmental conditions potentially linked to major climatic events. Bottom water oxygenation played a major role in BF diversity, observed by alternating low-diversity periods in times of low oxic conditions and high-diversity intervals in high oxic bottom waters. At the same time, bottom water salinity favored porcelaneous BF distribution during LGM and HS1 times and increasing hyaline-calcareous BF show decreased salinity in this part of the NE Atlantic after the end of the HS1.