Continental shelves are highly complex depositional systems. Deposits are formed under specific combinations of hydrodynamics, physiography and sediment sources, which allow paleoenvironmental reconstructions of climatic and oceanographic changes. The continental shelf of Northern Gulf of Cadiz (SW Iberia) is important to understand Holocene climatic oscillations in the North Atlantic region. Guadiana River is the shelf main source of sediments, and drains a semi-arid region with high inter-annual variability of discharges. Its estuary is highly infilled and the deeper channel works as a bypassing zone of sediments to the coast.
To study the depositional system of the Northern Gulf of Cadiz several cores were collected from the Guadiana continental shelf. Grain-size and coarse fraction analysis were performed on vibrocore 16, located in the middle shelf Mud Patch (65-130 m water depth), off Guadiana River. Morphoscopic analysis of quartz was performed and radiocarbon dating obtained for several levels.
The sedimentary sequence is dominated by sand and gravel from base up to level dated 5270-4980 yr Cal. BP, a depositional pattern probably linked to Guadiana estuary infilling process, which trapped mostly fine sediments between ca. 9800-6500 yr Cal. BP. These older sediments are mostly terrigenous, with dominant quartz. Accessory elements, namely schist/greywackes abundant in the river basin, and non-used quartz were possibly deposited in middle shelf by direct river discharges. Paleochannels located in the Guadiana shelf have been associated with ancient courses of the Guadiana River, active prior and during Holocene transgression. This would place present distal shelf areas closer to fluvial discharges during this period. After ca. 5000 yr Cal. BP deposition shifted, with great quantities of fines and half sand content on the shelf. Sea-level stabilization around 5000-3500 yr Cal. BP led to estuary enclosure behind spit bars and consequent sand trapping. Schist/greywackes are almost absent and bioclasts increase to ca. 40%, showing a clear transition to conditions out of direct fluvial inputs. Between ca. 5000 and 700 yr Cal. BP, 50 cm of sediments were deposited, whilst 175 cm were deposited over the last 750-650 years. Early Holocene higher sedimentation rates followed by middle Holocene decrease have also been detected in the nearby Guadalquivir shelf. However, sedimentary discontinuities in the Guadiana shelf may mask real deposition rates. Environmental changes during Holocene climatic oscillations may have reduced/suppressed deposition in certain shelf areas. Around 700 yr Cal. BP deposition changed with the increase of fines up to 90%. Quartz and mica dominate terrigenous sand and benthic foraminifera dominate biogenic elements. Continuous imprisonment of sands inside the Guadiana estuary and exportation of fines to the shelf set the Mud Patch in the middle shelf. This new sedimentary dynamics is active until present day.