Dating recent tidal marsh sediments using windborne giant particles of green petcoke – An example from the southwest coast of Portugal

Coastal systems are especially vulnerable to human activities resulting from a range of direct physical modifications of the ecosystems to the biological, chemical, and physical impacts of pollutants. For the most recent decades, these modifications can be accurately tracked by industrial development indicators, providing an exceptional record of historical changes. In return, when such changes are identified in the sedimentological record, they can serve as precise chronological markers. Here, we examined the potential of petcoke particles, a by-product of crude oil refining, as an emergent sediment dating tool. To this end, giant (63–∼200 μm) black spheroidal petcoke particles – present in cores retrieved in the salt marshes of the Sado estuary (SW Portugal) – were used. These anthropogenic particles were counted and identified according to their morphology, structure, and chemical composition, using complementary micro-beam techniques: Scanning Electron Microscope and Energy Dispersive Spectroscopy, and Micro X-Ray Diffraction. The analyses revealed shot-type green petcoke particles containing a set of elements, which are well-known metallic/non-metallic impurities in petcoke. Based on the available historical data on petcoke imports in the port of Setúbal (Sado estuary) and Portugal, their first appearance in sediments can be dated to 1996 ± 2, with maxima in 2006 ± 4. Both the presence and the historical evolution of the use of petcoke in the region were recorded in all sediment cores, with higher particle density in locations aligned with the prevailing winds. The combination of petcoke and 137Cs data provided accretion rates for the 1963–2006 period ranging from 0.1 to 0.3 cm yr−1 (average: 0.2 cm yr−1), while these rates significantly increased from 2006 onwards (avg. rate: 1.1 cm yr−1; range: 0.6–2.1 cm yr−1). The widespread handling of petcoke worldwide leads us to suggest petcoke particles as a reliable chronological marker at local and regional scales, to be used in combination with other dating methods to build more exact sediment chronologies.