Using lipid biomarker signatures to reconstruct marine primary production in the Iberian Margin over the last millennium

Despite efforts to understand the response of marine ecosystems to the impact of climate variability in the Iberian Margin [1-6], the effects of warming on phytoplankton community composition and total primary production remain an open question. Here, we present a high-resolution record of sediment-preserved lipids from the Iberian Margin (IM) over the last millennium, with special attention to the modern rise in atmospheric CO₂. To address the temporal variation of marine primary production and terrestrial organic matter input, we studied GC-MS/FID-determined lipid compounds from cores PO287-06-2G (off the Douro River in the northern IM), PO287-26-3G (off the Tagus River in the central IM) and POPEI-VC2B (on the Algarve continental shelf in the southern IM). Lipids of typical marine origin (C₂₇, C₂₈, and C₃₀ sterols, phytol, and C₃₇ and C₃₈ alkenones) and of typical terrestrial origin (odd long-chain n-alkanes nC₂₁-nC₃₅ and even long-chain n-alcohols C_22OH-C_32OH) were evaluated. As proxies for specific phytoplankton groups, we used the signatures of 4α-23,24-trimethyl-5α-cholest-22(E)-en-3β-ol (30d²² sterol) for dinoflagellates, 24-methylcholesta-5,22-dien-3β-ol (28d^5,22 sterol) for diatoms and heptatriaconta-15E,22E-diene-2-one (C_37:2 alkenone) for coccolithophores. The results showed a significant difference between the northern, central, and southern sites of the Iberian Margin, caused by the different regional environmental factors in each area. Higher concentrations of lipids are observed off Douro River, there is a higher relative contribution of cholesterol off Tagus River, suggesting the importance of zooplanktonic production in the central area, and there is a higher contribution of coccolithophores lipid signatures in the Algarve continental shelf. In terms of temporal variability, there is an alternation between coccolithophores and dinoflagellates lipid signatures over the millennium, with significant diatom sterol signatures at specific ages of the Industrial Era (since 1850 CE) and in the early MCA (900-1100 CE) for all sites. The temporal variability is most likely driven by changes in local and hemispheric ocean circulation and coastal upwelling conditions in the Iberian Margin.

References: [1] Abrantes et al., 2017; [2] Abrantes et al., 2011; [3] Salgueiro et al, 2008; [4] Ribeiro and Amorim 2008; [5] Abrantes 2000; [6] Rodrigues et al., 2009