Material Budget and Contamination History of a Mud Depocenter System in the Gulf of Cadiz
Till Jens Joerg Hanebuth, Coastal Carolina University, Department of Marine Science, Conway, United States, Mary Lee King, Coastal Carolina University, Conway, SC, United States, Isabel Mendes, University of the Algarve, Faro, Portugal, Susana Lebreiro, IGME, Madrid, Spain and Francisco Lobo, IACT-CSIC, Armilla, Spain
Abstract:
Mud depocenters (MDCs) on continental shelves provide valuable information about the history of human land use. Using 2,040 km of subbottom profiles and 18 sediment cores, the aim of this study was to establish a robust chronostratigraphic framework for the MDCs in the Gulf of Cadiz, to decipher the depositional dynamics of these MDCs over time, to calculate a regional sediment budget, and to detect anthropogenic influences. The MDCs started growing during maximum flooding around 6.5 cal ka BP. Natural sedimentation rates of 2-35 cm/ka increased significantly around the Roman Warm Period (30-205 cm/ka) due to both a humid climate and mining and agriculture. Since 1.0 cal ka BP, sedimentation rates rose further (20-3,000 cm/ka), as a result of widespread land clearing during a cultural blossom (the Islamic period), coinciding with erosion-driving aridity of the Medieval Climate Anomaly. Unprecedentedly high sediment accumulation happened with the Industrial Era, particularly in the second half of the 20
th century. The sediment volume of the two major MDCs is 5.80 km
3 with a mass of 12,970 Mt. 512 Mt of carbon make this depocenter an important shallow-marine sink as part the regional carbon cycle.
The MDCs also recorded the contamination history (lead and zinc as suited indicators). Ore mining and metal processing industries on started 5,000 yrs ago, with a first peak during the Roman Period, detectable gulf-wide. The Industrial Era added a massive, shelf-wide heavy metal excursion. This contamination decreased in the 1990s and appears to be today limited to areas off the two major rivers. The peak heavy metal concentration of the Industrial Era is overlain by a sediment veneer just thick enough to cover the contaminant horizon. The contamination layer is, however, still within the reach of natural (storms) or human-induced (chronic bottom trawling) sediment mobilization events. Thus, a careful quantitative examination is the key for sustainable management decisions.
