5.3 Palaeoenvironmental evolution, climate and sea level change implications
The integrated multi-proxies obtained in this study provides a link to two evolutionary time-bound stages of vegetation dynamics and sediment supply in the Niger Delta during the last 20 ka. These two succinct time-bound stages provide a resonating clue on the prevailing past environment, climate and sea level change as follows: stage 1 (advancing or prograding delta) and stage 2 (retreating or retrograding delta) (Figures 6-7)
During stage 1, the GC data confirm the emergence of the continental shelf in response to a lower than present day sea level (Figures 6-7). The littoral environment (mangrove and coastal swamp) is subaerially exposed, and sedimentation and fluvial transport are effected by mass movement triggered by the weak West African Monsoon (WAM) (Morley, 1995; Reijers, 2011; Zong et al. , 2009; Shannahan et al., 2015; Skonieczny et al., 2015; Adojoh et al., 2017; Höpker et al., 2019; Chadwick et al., 2020; Dai et al, 2021). The GC records indicate the predominance of hinterland pollen during stage 1 evolution of the Late Quaternary Niger Delta. The pollen types identified were transported beyond the littoral realm/coastal zone, representing a strong signal of the prevalent arid conditions and low sedimentation rates (Figures 3-5; 6-7). In addition, the dominance of hinterland pollen taxa and low abundance of planktonic foraminifera suggest that stage 1 was a distal offshore/lower shoreface prodelta environment (Adegoke, 1975; Murray, 1991; Morley, 1995) (Figure 7). On a regional scale, this stage was a period of an enhanced sediment transport, as observed in the settings of the Amazon, Senegal (Ogolian regression), and Congo Rivers (Figures 3-5; 7) (Barusseau, 1988; Barusseau et al. , 1995; Marret et al., 2001; Giosan et al., 2005; Bonne, 2014; Adeonipkekun and Sowunmi, 2019: Boyden et al., 2021).
Following stage 1 was a period of rapid sediment retreat in the Niger Delta attributed to sea level rise (Figures 6-7). This period of an onset of Early to mid-Holocene sea level rise coincides with an episode of shoreline transgression and fine-grained sediment suspension reflecting the proximity of turbidity currents (Peltier, 1994; Goodbred , 2003; Scourse et al., 2005; Joo-Chang et al ; 2015; Höpker et al., 2019; Dai et al, 2021). Increases in the values of mangrove pollen (e.g., Rhizophora ) and the Fe/S elemental data suggest a reducing environment with the potential of rapid post-dissolution of pyrite (FeS) minerals and organic-rich sulphur content during the warm climate (Fletcher, 2005; Mendoza, 2007) (Figures 3-5; 6-7). In addition, this stage suggests a proximal-upper shoreface (delta plain/delta front) palaeoenvironment based on the dominance of the planktonic foraminiferal (Globorotalia spp.) (Figure 7). Sediment supply and pollen deposition during stage 2 was principally driven by the interaction between the creation of accommodation space and density of mangrove vegetation spread across the near shore (Bonne, 2014) (Figures 6-7). On a regional extent, stage 2 correlates to the period of sea level rise (Nouakchottian transgression) as observed from the coastal margins of Congo, Senegal, and Mauritania (Barusseau, 1988; Barusseau et al. , 1995; Lézine 1997; Lézine and Denefle, 1997; Dalibard et al., 2014; Scourse et al., 2005; Höpker et al., 2019; Dai et al, 2021).