Variations in sediment provenance during the past 3000 years off the Tagus River, Portugal

Variations in benthic foraminiferal assemblages in the Tagus mud belt during the last 5700 years: Implications for Tagus River discharge

“…This rendered the Holocene a suitable period to test benthic foraminifera as bio-indicators for past Tagus River discharge and environmental changes based on the living foraminiferal calibration developed by Dessandier et al (2016Dessandier et al ( , 2018 in the same location. Previous sediment cores in the area extended back to 2000 (Abrantes et al, 2005;Bartels-Jónsdóttir et al, 2006) or 3000 cal yr BP (Alt-Epping et al, 2009). The 10-m core retrieved in our study provided a record that dated to 5750 cal yr BP.…”

“…This suggests a deposition of finer sediments of terrestrial origin in the Tagus prodelta during this period. This important change in sedimentary conditions toward muddier and organic-rich sediment (Martins et al, 2006) was observed in the Portuguese Margin earlier, at around 2000 cal yr BP (Alt-Epping et al, 2009;Martins et al, 2007), and indicated the onset of the Tagus mud belt, which was mainly composed of sediment exported from the Tagus estuary (Jouanneau et al, 1998). This increase of organic compounds was also highlighted by the large increase in the Br/Cl ratio during this phase.…”

“…This increase of organic compounds was also highlighted by the large increase in the Br/Cl ratio during this phase. The reduction of bottom water currents driven by decreasing wind and more humid conditions have been postulated as the physical processes that triggered a strong Tagus River export of sediment and led to the formation of the mud belt around this time (Alt-Epping et al, 2009). The mud belt formation was responsible for the increased accumulation of organic matter, especially during Phase II, corresponding to the increased levels of Br/Cl, TOC, and TN content (Fig.…”

“…Climatic changes during the late Holocene have been well studied in the Portuguese Margin, especially for the last 3000 years (e.g., Desprat et al, 2003;Abrantes et al, 2005;Bartels-Jónsdóttir et al, 2006;Alt-Epping et al, 2009). This rendered the Holocene a suitable period to test benthic foraminifera as bio-indicators for past Tagus River discharge and environmental changes based on the living foraminiferal calibration developed by Dessandier et al (2016Dessandier et al ( , 2018 in the same location.…”

“…The Zr/Rb ratio indicates a bigger grain size, which had a strong positive loading on PC1 together with Ca/Ti. Ca was previously linked to grain size in the Portuguese Margin and thought to be associated with coarse reworked shells of macrobenthic organisms (Martins et al, 2007;Abrantes et al, 2008;Alt-Epping et al, 2009).…”