Stable isotopes in foraminiferal tests to study changes in the sedimentation in the NW Iberian Continental Margin since the late Pleistocene

Isotopic data of 13C and 18O in planktonic (Globigerina bulloides) and benthic (Cibicidoides wuellerstorfi) were used together with an age model based on five AMS 14C ages and modeled with the OxCal program and a Bayesian approach, in the study of a core (5 m long) collected at 2000 m depth off Galicia (latitude 42º34.32', longitude 9º41.4') containing a record from the last ~40 Ka BP. Textural (micro particle size), mineralogical (by X-ray diffraction, on oriented and non-oriented powders) and micropaleontological (benthic and planktonic foraminifera) data were also studied to assess changes in the water column and sedimentary environment on the NW Iberian continental margin related to climate change and sea level variations during the late Pleistocene and Holocene. This core is essentially made up of fine detrital sediments (fine fraction <63 µm), suggesting an environment characterized by low-intensity hydrodynamic conditions. However, there are significant changes in grain size and sediment composition. The foraminifera data allowed the observations of higher abundance of species associated to artic-subartic environmental conditions, suggesting the record of climatic events associated to ice-rifted debris (IRD), i.e. Heinrich Events and Dansgaard-Oeschger cycles/events. The 18O isotopic composition of the foraminifera tests is influenced by several factors such as temperature, salinity, and the water isotopic composition. Throughout the core, the 18O profile varied for planktonic and benthic species. The 18O isotopic profile of planktonic foraminifera seems to show a later change in values and trends with more frequent variation, since the surface ocean is a more unstable environment. The isotopic profile of benthic foraminifera shows smoother variations, since benthic species live in more stable conditions. The 13C isotopic composition of the foraminifera tests is influenced by several factors such as dissolved inorganic carbon (DIC) in the water and primary productivity. The 13C isotopic variation along the core profile of the planktonic specie presents a higher frequency variation, comparatively to the benthic one. The 13C variation along the profile might be mostly due to primary productivity, although, from the depth of 80 cm to the top of the core profile (i.e., between 7-1 Ka BP), the mineralogical data point to a greater occurrence of carbonates, which has certainly altered the availability of DIC, which is indeed noticeable in the isotopic profile of the planktonic species. The correlation matrix corroborated the hypothesis of two balanced systems along the core profile that can be differentiated into a detrital input/inflow and a more carbonate settlement. The correlation matrix made it possible to identify the relationship between detrital input (kaolinite, quartz, mica, other clastic minerals, with strong positive correlations between them), as opposed to carbonate elements (calcite, carbonate/detrital, foraminifer density, Shannon index, equitability, also with strong positive correlations between them). Another important observation along the correlation matrix is that these elements/systems are also strongly correlated to the 18O, that depends on the temperature, salinity, and the water isotopic composition. The textural data, together with the compositional data, provided evidence suggesting a variation between two main environments, one dominated by calmer waters, associated with the carbonate context, and a second, associated with detrital input, related to a period when the sea level was lower and the coastline closer to the study area. The study of this core should be improved, in order to better characterize the changes and evolution of the different scenarios over this interval of known climate change.