ULTRA-HIGH RESOLUTION ANALYSIS OF INTER-ANNUAL NATURAL AND ANTHROPOGENIC VARIABILITY USING FLUVIOGENIC FLOOD LAYERS OVER THE PAST 600 YEARS DOCUMENTED BY A PRODELTAIC DEPOCENTER IN THE GULF OF CADIZ, SPAIN

Mud depocenters (MDCs) on the continental shelf are composed of fine-grained fluvially sourced material and can provide an exceptional opportunity to examine natural and anthropogenic processes in ultra-high resolution. The prodeltaic record of the Guadalquivir River in the eastern Gulf of Cadiz is one such place to decipher the variability over the past centuries. This region has a rich history of anthropogenic disturbances, particularly from the Industrial Revolution, and is influenced by both the Atlantic and Mediterranean climate systems; both of which affect the Guadalquivir River basin and the regional coastal-marine system.

To determine the magnitude and temporal scale of changes and impacts, a 5-m long sediment core was taken at 23 m water depth from the Guadalquivir MDC spanning the past 645 ± 25 cal yrs CE. Both radiography images and XRF element distribution core scanning at 1 cm resolution suggest three types of sedimentary facies within the core: hemipelagic (83%), fluviogenic flood (15%), and storm layers (2%). Flood layers (10-year return period with ~50 events) were targeted for high-resolution grain size analysis at a 0.25 cm resolution to determine the frequency as well as the specific supply and transport dynamics of the flood events. In addition, end-member modeling analysis (EMMA) using a non-parametric fit of the mean grain size of all facies revealed three dominant modes describing 99.5% of the variance. EM1, which describes 83.0% of the total variance, illustrates the hemipelagic fine-silt sub-population (7.6 ± 1.5 μm) of the prodeltaic record. EM2 and EM 3 have mean grain sizes of 11.5 ± 2.3 μm (97.7% of the total variance) and 17.3 ± 3.1 μm (99.5% of the total variance), respectively, and contribute the highest percentage of abundance to the flood layers. The EMMA simulation, in combination with elemental proxies, organic C and N stable isotopes, and foraminifera assemblages, reveals both an oscillatory pattern in flood layer frequency and magnitude as well as an increased sedimentation rate and an overall upward-fining of the sediment record. These changes are likely the result of regional climatic changes and human-linked environmental impacts on the river system, its estuary, and the nearshore hydrodynamics from the Little Ice Age to modern times, particularly since the Industrial Revolution.