Mid‐Holocene Iberian hydroclimate variability and paleoenvironmental change: molecular and isotopic insights from Praia Rei Cortiço, Portugal

Abstract

At Praia Rei CortiS co (PRC), coastal Portugal, we analyzed compound-specific isotopes of plant wax-derived n-alkanes in combination with molecular distribution proxies and C/N ratios to reconstruct hydrologic and environmental change in a mid-Holocene wetland. During this relatively brief segment of the Holocene (6.6-5.4k cal a BP), substantial shifts in the stable hydrogen isotope composition of terrestrially sourced C 29 n-alkanes (dD n-C29 ) record significant multi-decadal changes in precipitation origin and storm trajectory. Wetland formation at PRC occurred during a humid interval (6.6-6.5k cal a BP) with a dominantly tropical precipitation source, likely the result of a persistent negative phase of the North Atlantic Oscillation, which permitted the establishment and dominance of Sphagnum moss. A subsequent decrease in precipitation significantly reduced Sphagnum abundance at PRC, which is evidenced by decreased sedimentary C/N ratios and reduced mid-chain n-alkane abundances. From 5.9 to 5.4k cal a BP, relatively low dD n-C29 values coincide with sand invasion along the Iberian coast and cooling in the North Atlantic. Strong correspondence between dD n-C29 values and the occurrence of ice-rafted debris recorded in deep sea sediment cores during this period illustrate a marked North Atlantic control on the trajectory of mid-latitude storms and precipitation regimes in Iberia.

Holocene centennial to millennial shifts in North-Atlantic storminess and ocean dynamics

“…On the contrary, the abrupt drop in storminess shown in our record around 6,500 B.P. was accompanied by a rapid onset of more humid conditions over Portuguese wetlands as well as by a shift from an extra-tropical to a tropical provenance of the humid air masses affecting the Iberian Peninsula 55 . Thus, we infer that the marked drop of storminess observed at this period was controlled by a southward relocation of the Northern Atlantic westerly wind belt.…”

Late Pleistocene site formation and paleoclimate at Lapa do Picareiro, Portugal

“…The composition of the organic fine sediment fraction is inferred from field evidence along with stable carbon and nitrogen isotopes, and carbon/nitrogen ratios, which are commonly used as indicators of organic matter types and sources, such as terrestrial versus marine organic compounds, vegetation communities, such as grassland versus forest, or trophic position in food webs (Finlay & Kendall, ; O’Leary, ; Peterson & Fry, ). Although bulk organic carbon and nitrogen isotopes from soils and sediments contain some information on organic matter cycling, a compound‐specific isotope approach would better resolve spatial and temporal patterns associated with particular biomarkers (Bi, Sheng, Liu, Li, & Fu, ; Collister, Rieley, Stern, Eglinton, & Fry, ; Diefendorf & Freimuth, ; Gleixner, ; Taylor, Benedetti, Haws, & Lane, ). Here we are limited to bulk analysis of organic δ 13 C and δ 15 N isotope ratios from fine sediment samples collected at reference profiles 1 and 2.…”

Fossil leaf wax hydrogen isotopes reveal variability of Atlantic and Mediterranean climate forcing on the southeast Iberian Peninsula between 6000 to 3000 cal. BP

“…Since Atlantic sourced precipitation in the study area is much more prominent during the winter season [ 30 , 48 ], changes in precipitation source likely account for the apparent correlation of monthly amount-weighted δD prec values with the monthly variability of atmospheric temperature and precipitation amount ( Fig 1 ). n -Alkane δD values were also considered to reflect changes in the precipitation source by previous paleoclimatic studies from the Iberian Peninsula [ 65 , 67 , 68 ]. Accordingly, we conclude that the dominant parameter driving hydrogen isotopic variability of individual n -alkanes in marine sediment core ODP-161-976A is the source of precipitation with low δD WMA values reflecting increasing Atlantic derived precipitation and high δD WMA values reflecting an increase in Mediterranean sourced precipitation.…”

“…resulting in an increasing ACL, when water availability is reduced [60]. Also, isotopic analyses of individual n-alkanes, have been used extensively during the last years in tropical [61][62][63][64] but also in Mediterranean regions [65][66][67][68] in order to assess terrestrial environmental and climatic parameters. Analysing individual n-alkane homologues for carbon isotopes (δ 13 C Cx ) provides important information on the distribution of C3 and C4 plants [69].…”

“…This is based on the observation, that plants produce on average longer n -alkane homologues resulting in an increasing ACL, when water availability is reduced [ 60 ]. Also, isotopic analyses of individual n -alkanes, have been used extensively during the last years in tropical [ 61 – 64 ] but also in Mediterranean regions [ 65 – 68 ] in order to assess terrestrial environmental and climatic parameters.…”